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Programmed morphing of liquid crystal networks

Citation for published version (APA):Haan, de, L. T. (2014). Programmed morphing of liquid crystal networks. Eindhoven: Technische UniversiteitEindhoven. https://doi.org/10.6100/IR781577

DOI:10.6100/IR781577

Document status and date:Published: 13/11/2014

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https://doi.org/10.6100/IR781577

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PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op

donderdag 13 november 2014 om 16:00 uur

door

Laurens Theobald de Haan

geboren te Apeldoorn

Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt: voorzitter: prof.dr.ir. J.C. Schouten 1e promotor: prof.dr. A.P.H.J. Schenning 2e promotor: prof.dr. D.J. Broer leden: prof.dr.ing. C.W.M. Bastiaansen (Queen Mary University of London)

prof.dr. A.E. Rowan (Radboud University Nijmegen)

prof.dr. R.P. Sijbesma adviseurs: dr. T.J. White (US Airforce Research Laboratory) dr. C. Sanchez-Somolinos (University of Zaragoza)

A catalogue record is available from the Eindhoven University of Technology

Library ISBN: 978-90-386-3729-7

Copyright © 2014 by Laurens Theobald de Haan

Cover design: Proefschriftmaken.nl || Uitgeverij BOXPress

Printed & Lay Out by: Proefschriftmaken.nl || Uitgeverij BOXPress

The research described in this thesis has been financially supported by the

Netherlands Organization for Scientific Research, Chemical Sciences (NWO-CW),

file number 700.57.444

Table of contents

Chapter 1. Morphing of liquid crystal polymer networks

1.1 Stimuli-responsive materials 1

1.2 Liquid crystal networks 2

1.3 Alignment director variation through the thickness of the polymer film 6

1.3.1 Twisted and splayed alignment profiles: bending 6

1.3.2 Twisted and splayed alignment profiles: coils and helicoids 10

1.4 Director profiling in the xy-plane of the film 15

1.4.1 Continuous circular patterns 15

1.4.2 Discrete patterns 17

1.5 Director patterning in 3 dimensions 19

1.6 Aim and outline of the thesis 20

1.7 References 21

Chapter 2. Programmed deformation of uniaxially aligned actuators with

asymmetry in the molecular trigger

2.1 Introduction 27

2.2 Results and discussion 29

2.2.1 Film preparation, KOH treatment and analysis 29

2.2.2 Deformation behavior of the films 30

2.2.3 Complex folding 33

2.2.4 Curling 34

2.3 Conclusions and outlook 35

2.4 Experimental Section 36

2.5 References 37

Chapter 3. Actuators with a director pattern in the xy-plane of the film

3.1 Introduction 41


3.2.1 Preparation of photoaligned liquid crystal network films 43

3.2.2 Films based on continuous disclinations 44

3.2.3 Actuation of +1 disclination-based actuator films 46

3.2.4 Disclination-based, discrete patterns 48

3.2.5 Aperture patterns 49


3.4 Experimental section 52

3.5 References 55

Chapter 4. Folding actuators with 3-dimensional alignment patterns

4.1 Introduction 59


4.2.1 Alignment pattern design 60

4.2.2 Preparation and optical characterization of the polymer films 61

4.2.3 Actuation of striped twisted nematic films 62

4.2.4 Water-responsive actuators 65

4.2.5 Finite-element analysis 66

4.2.6 Checkerboard pattern 69

4.2.7 Finite element analysis of checkerboard pattern 69

4.2.8 Radimuthal films 70


4.4 Experimental Section 72

4.5 References 76

Chapter 5. Generating wrinkling patterns in metal/LC-polymer bilayer films

5.1 Introduction 77


5.2.1 Sample preparation and wrinkle formation 79

5.2.2 Measuring the wrinkle period and amplitude 82

5.2.3 Preparation and examination of various wrinkle patterns 83


5.4 Experimental section 86

5.5 References 87

Chapter 6. Technology assessment

6.1 Introduction 91

6.2 Soft robotics 91

6.3 Switchable sieves 95

6.4 Smart optics 96

6.5 References 98

List of symbols 101

Summary 103

Samenvatting 105

Curriculum Vitae 107

Publications 109

Acknowledgements 111

This chapter was based on a review: de Haan, L. T.; Schenning, A. P. H. J.; Broer, D. J., Programmed morphing of liquid crystal networks. Polymer, In Press

1

Chapter 1. Morphing of liquid crystal polymer

networks

1.1 Stimuli-responsive materials

In nature, the well-defined and precisely controlled organization of components on

a molecular, supramolecular and mesoscopic length scale is crucial for the complex

morphing processes found in organisms.1 This hierarchical structuring has been a

source of inspiration for materials scientists,2 and has led to the field of functional

supramolecular polymers.3 In order to mimic the systems found in nature, the

materials must have a hierarchical structure that can be tailored to achieve a target

actuation behavior, without the need to assemble separate mechanical components.

One of the focal points in functional supramolecular polymers is stimuli-responsive

polymer materials, which respond to stimuli from the environment by changing

their properties.4 An important class of these materials consists of those which

reversibly change their macroscopic shape upon exposure to a stimulus.5 In

particular, the ability to program a certain morphology change into the material

itself, which can be accessed at any time by applying the stimulus, will lead to

materials that can find applications ranging from moving elements in microfluidic

systems,6 such as shutters and flow controllers, to medical systems7 and robotics,5a, 8

where precisely controlled, complex movements are desired. Complex deformation

has previously been achieved using various techniques, for example by applying

ion-induced plastic strain to metal films9 or application of a dye for localized light

absorption.10 Material based on hydrogels are particularly popular for the

preparation of morphing materials, and complex deformation has been induced

Chapter 1 Morphing of liquid crystal polymer networks

2

using halftone gel lithography on polymer hydrogels,10 and photopatterning of

composite hydrogel sheets.11 However, such materials are limited to wet conditions.

Another promising group of materials that is a candidate for the development of

stimuli-responsive polymer materials is based on nematic liquid crystals which,

unlike hydrogels, can operate in both dry and wet conditions.

1.2 Liquid crystal networks

Liquid crystals (LCs) are a class of materials that possess a phase that has both

anisotropic and fluid-like properties.12 The material consist of rod-like molecules

with orientational alignment that can be described by the alignment director n and

the order parameter S, where S = 3cos θ 1 , with θn being the angular

deviation of the long molecular axis from n (0 < θn < π/2, brackets indicate an

average, see Figure 1.1a).13 Liquid crystals in the nematic phase typically have a

multidomain microstructure, but the alignment of the molecules can be controlled

using various alignment techniques, such as electric fields, magnetic fields, or

command surfaces, which allows excellent control over the molecular alignment

even at macroscopic length scales.

Figure 1.1 a) Schematic representation of the orientational alignment of rod-like molecules in the nematic liquid

crystalline phase. b) Deformation of a liquid crystal polymer network upon decreasing the order parameter S. As

the average θn increases, contraction (λ < 1) occurs parallel to the alignment director (L// λL//), and expansion

occurs perpendicular to it (L┴λ-νL

┴). c) A liquid crystal polymer film depicted as a coordinate system, with the

xy-plane parallel to the surface of the film and the z-axis from z =0 to z = h, where h is the film thickness. The

liquid crystals are shown in a planar alignment, with the alignment director parallel to the xy-plane.


3

When liquid crystals are equipped with polymerizable end groups, and a

crosslinker is present, they can be polymerized into molecularly well-ordered

polymer networks, commonly referred to as liquid crystal networks. UV initiated

radical polymerization is usually the chosen polymerization technique, which can

be performed on liquid crystals containing (meth)acrylate moieties. The polymer

network retains the molecular alignment of the monomers.12, 14 By combining such

reactive mesogens with alignment techniques, excellent control can be obtained

over the molecular arrangement within the material, though the restrictions

imposed by the alignment techniques often require the liquid crystal polymer

networks to be prepared as thin films. Depending on its glass transition

temperature, the polymer network can either behave as an elastomer or a glass at

room temperature, which have different mechanical properties. However, in both

cases the films undergo a shape deformation upon changing the temperature.15 An

increase in temperature lowers S and therefore increases the average θn. This

manifests itself as a contraction λlc (with λlc < 1) of the length of the polymer

network along the nematic director (L//) and an expansion λlc-ν of the length

perpendicular to the director (L┴), where ν (ν > 0) is the Poisson ratio that relates

the responses of L// and L┴ (Figure 1.1b).16

In the presence of molecules that respond to other stimuli, the order parameter of

the polymer network becomes susceptible to other triggers, and the dimensions

may change, for instance, upon contact with water or chemicals or by being

exposed to light of a specific wavelength. The ability to change shape by an

external trigger has made these materials of interest for their use as actuators. The

most popular liquid crystal actuators are those which contain a photoresponsive

molecular trigger, usually based on azobenzene. This building block will undergo a

trans-to-cis conformational change upon UV irradiation, which causes a reduction

of the order parameter and corresponding deformation similar to heating. This

allows for the preparation of light-responsive films, which are attractive since light,


4

when compared to heat or chemical stimuli, is easier to apply locally and without

contact.17

While the linear contraction and expansion behavior of liquid crystal actuators is

interesting,18 their true potential lies in the generation of bending motion to produce

three-dimensional shapes.19 A common strategy to achieve bending is by using a

bilayer consisting of a responsive liquid crystal network and a non-responsive

polymer layer. Alternatively, bending can be achieved by exposing the material to

a stimulus that is inhomogeneous along the z-axis of the film. For example, when a

UV-responsive material containing a UV absorbing dye is exposed to irradiation on

one side, an intensity gradient will develop through the material. This causes a

stronger deformation at one side, and bending takes place.20 However, it would be

more desirable to have access to single-layer, monolithic materials that undergo

complex, 3-dimensional shape deformations without depending on the

inhomogeneity of the stimulus, as this would greatly increase their versatility and

the number of possible applications. To accomplish this, the morphology change

has to be programmed into the molecular structure of the material itself, so that it

can be accessed at any time by application of the stimulus, without receiving any

information about the morphology from the stimulus. As this thesis will show, this

is possible with liquid crystal networks.

In the case of a uniaxially aligned liquid crystal polymer network, n is uniform

throughout the entire matrix (considering ideal circumstances). When a

homogeneous stimulus is used to actuate such a film, contraction and expansion do

not lead to a bending deformation. However, by applying certain alignment

techniques, films having a director variation in one or more directions can be

prepared and more complex deformations can be achieved. This allows the

development of programmable, complex shape deformation. The variation in

director is often referred to as the director profile. To describe the director profile, a

film can be pictured as a coordinate system, in which the z-axis run from z = 0 to

z = h, where h is the thickness of the film, and the xy-plane is positioned parallel to


5

the surface of the film (Figure 1.1c). Paragraphs 1.3 - 1.5 will give an overview of

previously reported director profiles and their corresponding deformations, both

theoretically and experimentally. Initially, profiles that only vary in the z-direction,

but are homogeneous in the xy-plane, are shown. Next, profiles are discussed that

are patterned in the xy-plane but do not vary in the z-direction. Finally, a director

profile that combines both characteristics is presented.


6

1.3 Alignment director variation through the thickness of the polymer film

1.3.1 Twisted and splayed alignment profiles: bending

A straightforward way to obtain bending deformations in liquid crystal networks is

by utilizing the twisted and splayed nematic alignment profiles (Figure 1.2). When

a twisted or splayed alignment is present, there is a 90 degrees change of the

director along the z-axis. This causes the expansion and contraction in the xy-plane

at z = 0 to be different compared to those in the plane at z = h, and this mismatch in

deformation causes bending. Therefore, such films are suitable for the preparation

of actuators that behave as bending cantilevers. The actuators are somewhat similar

in behavior to bilayer systems, except for the fact that instead of a sharp boundary

Figure 1.2 a) Molecular buildup and deformation behavior of a liquid crystal polymer film

with twisted nematic alignment upon decreasing the order parameter S. Initially, anticlastic

deformation leads to a saddle shape, but at higher strains the curvature along one of the

axis is suppressed, and the film bends in only one direction. b) Deformation of a film with

splayed nematic alignment upon decreasing the order parameter S. This film always bends

in only one direction, due to the deformation along one axis being cooperative between the

two faces of the film.


7

between the layers, there is a gradual transition between the two sides of the film.

For twisted films, at z = 0 contraction along the x-axis and expansion along the

y-axis takes place, while the opposite happens at z = h. This leads to an anticlastic

(saddle-shaped) deformation (Figure 1.2a).21 However, such curvature leads to

stretches due to the bending in the x-direction working against the bending in the

y-direction and vice versa. When these stretches become sufficiently large,

suppression of one of the principal curvatures becomes more favorable, which

leads to cantilever-like bending behavior. The strains at which suppression takes

place depend on the sample dimensions, but for the dimensions of a typical

experimental sample, a saddle shape is rarely observed. The direction in which

bending is suppressed is the direction with the lowest curvature, since that

curvature is easier to undo. Therefore, a long ribbon bends along its long axis while

suppressing the bending along the short axis.22 In addition, some out-of-plane

twisting deformation can be expected, due to the chirality of the twisted nematic

alignment,23 and this has also been observed in practice.24 These predictions do not,

however, take into account any outside influences. For example, in clamped

samples often formation of kinks can occur near the clamp, and such effects can

make the deformation of cantilevers with this alignment profile unpredictable and

hard to control.24

A better controlled bending deformation can be achieved in films with splayed

alignment.24 In these films, bending will only be in one direction, and therefore

there is no anticlastic deformation (Figure 1.2b). As the homeotropic side expands

in both the x and y directions, the bending direction of the cantilever is solely

determined by the alignment director on the planar side. It always bends in the

same direction as the alignment director on the planar side, independent of sample

dimensions. As there is no energy cost for anticlastic suppression to achieve

cantilever-like deformation, it is easier for these actuators to bend. The bending is

also less effected by clamping, if the clamp is placed at the edge of the film that

doesn’t bend, which allows better controlled deformations.


8

Twisted and splayed actuators may find applications in devices. For example,

small, inkjet-printed, light responsive actuators with splayed alignment were

prepared that mimic the movements of cilia found in microorganisms.25 A liquid

crystal mixture was inkjet-printed from dichloroethane. The splay alignment was

induced using one rubbed polyimide layer that induced a homeotropic alignment

with an 80 degrees pretilt angle on one side of the film, and a surfactant was added

to the liquid crystal mixture to induce planar alignment at the LC-air interface on

the other side (Figure 1.3a and 1.3b). This resulted in splayed alignment with a

uniform director on the planar side due to the pretilt angle. In a special case the

structures consisted of two domains: One domain contained the azobenzene

derivative A3MA, which undergoes a trans-to-cis isomerization when irradiated at

= 358 nm, while the other domain contained the azobenzene derivative DR1A,

which undergoes this transition at = 490 nm (Figure 1.3b). This allowed

independent actuation of the domains with light of different wavelengths, to induce

Figure 1.3 Light-responsive artificial cilia based on liquid crystal networks with splay

alignment. a) Schematic representation of the alignment, and SEM picture of the cross-section

of the actual film. b) Liquid crystals used to prepare the actuators. c) Schematic representation

of the artificial cilia. The red areas contain DR1A and respond to visible light, while the

yellow areas contain A3MA and respond to UV light. d) Pictures of the printed cilia in action.

Copyright Nature Publishing Group.24


9

asymmetric motion with a different forward and backward stroke, which should be

capable of creating flow in liquids (Figure 1.3c and 1.3d). Such flow generation

could be useful in microfluidic systems.25

By using hydrogen-bridged liquid crystal networks (nOBA), twisted and splayed

actuators were produced that respond to water vapor (Figure 1.4a). After

polymerization, water is not yet capable of penetrating the network, but when the

film is treated with a basic solution (usually KOH) the internal hydrogen bonds are

broken and a hygroscopic salt is formed. When water is absorbed the network

swells, the order parameter is reduced, and the anisotropic deformation results in

stronger expansion perpendicular to the alignment director. In this way, actuators

Figure 1.4 Liquid crystal films based on twisted and splayed alignment that

deform upon water swelling. a) Composition of the reactive liquid crystals used to

make the network. b) Base-treated films that deform based on humidity, at high

(85%) and low (15%) relative humidity. The films were treated with KOH for

various periods of time (0-20 seconds) prior to the experiment. Copyright Institute

of Electrical and Electronics Engineers.25b c) The base-treated liquid crystal film

shows different bending behavior in different solvents, in this case water and

acetone. Copyright Wiley-VCH25a d) The activated film de-swells when the pH is

lowered. Copyright Institute of Electrical and Electronics Engineers.25b


10

that respond to humidity have been prepared (Figure 1.4b). As different solvents

have different swelling capability, the polymer films also respond differently to

solvent polarity upon immersion (Figure 1.4c). The activation with basic solution

will be reversed in an acidic solution, and this means that the actuator will also

respond to solution pH (Figure 1.4d). All these films were pre-bent after

polymerization due to intrinsic stresses, which explains why they were not flat in

the non-swollen state.26

1.3.2 Twisted and splayed alignment profiles: coils and helicoids

Coil-shaped and helicoidal deformation can be achieved by cutting ribbons from a

90 degrees twisted nematic network in such a way that the angle θc between the

alignment director at z = 0 and the long axis of the ribbon is ±45 degrees. Like

twisted nematic actuators that bend, the coiling deformation is caused by a

mismatch in contraction and expansion in the xy-plane at z = 0 and at z = h. The

sign of the value of θc determines the handedness of the resulting coil; when the

angle is positive (clockwise from the long axis), the coil will be right-handed, and

when it is negative (counterclockwise from the long axis), the coil will be left-

handed (Figure 1.5). There is also a small influence of the twist handedness on the

formation of the coils,23 which will be discussed below.

Figure 1.5 Coil formation in ribbons made from twisted nematic films (helicoids

not shown). a) When the angle θc between the long axis of the ribbon and the

nematic director at z = 0 is positive, a right-handed coil is formed. b) When θc is

negative, a left-handed coil is formed.


11

It was shown, both experimentally and through finite-elements simulations, that

cutting ribbons with θc = ±45 degrees can lead to the formation of both helicoids

and coils. In this research, a mixture of a polymerizable, achiral mesogen

(A-6OCB), a nonreactive LC (6OCB), a nonreactive left-handed chiral dopant

(S-811), plus 7% of crosslinker (HDDA), was polymerized between rubbed

polyimide alignment layers with orthogonal alignment directors to create a polymer

network with a 90 degrees left-handed twist (Figure 1.6a). Upon removal of the

nonreactive components, anisotropic shrinkage caused the deformation into a chiral

shape. Ribbons were cut from this film with θc being 45 degrees or -45 degrees,

which were called S- and L-geometry, respectively. The length and thickness of the

Figure 1.6 a) Liquid crystal mixture used to make coil and helicoid ribbons. A-6OCB is a reactive mesogen,

6OCB is a nonreactive mesogen, HDDA is a crosslinker, S-811 is a nonreactive chiral dopant. b) Helicoids

formed by narrow ribbons (thickness 35.2 μm, width 0.23 mm) with a negative θc (L-geometry). c) Helicoids

formed by narrow ribbons (thickness 35.2 μm, width 0.22 mm) with a positive θc (S-geometry). d) Coils

formed by wide ribbons (thickness 35.2 μm, width 0.76 mm) with a negative θc (L-geometry). e) Coils formed

by wide ribbons (thickness 35.2 μm, width 0.83 mm) with a positive θc (S-geometry). f) Plot of the inverse

twist pitch (1/PT) of the helicoids as a function of temperature, T/TNI, where T is current temperature and TNI is

the nematic to isotropic transition point. A left-handed twist pitch is defined as positive and a right handed

twist pitch as negative. The lines indicate theoretical predictions. g) Plot of the inverse pitch (1/PH) and

inverse diameter (1/d) of the coils as a function of temperature. A left-handed twist pitch is defined as positive

and a right handed twist pitch as negative. The open symbols represent the pitch and the filled symbols

represent the diameter. The lines indicate theoretical predictions. Copyright National Academy of Sciences.26a


12

ribbons were kept constant while the width was varied. It was found that narrow

ribbons with a small width/thickness ratio twist around a straight central line to

form helicoids with no unidirectional bending (Figure 1.6b and 6c), while wide

films with a large width/thickness ratio will bend to form curls with strong bending

(Figure 1.6d and 1.6e). The dependence of the shape on the sample dimensions is

caused by the competition between bending energy cost and in-plane elastic energy

cost: Narrow films are easier to be stretched than to be bent, which leads to

helicoid formation, while wide films are easier to be bent than to be stretched, and

form coils. This is similar to the previously shown twisted nematic films, which

either form a saddle shape or a unidirectional bend, depending on whether

suppression of curvature takes place or not. It was found that the shapes uncurl

upon raising the temperature, go through a flat state (at temperature Tflat), and then

curl into the opposite handedness (Figure 1.6b-1.6e). The pitch was plotted as a

function of temperature, and was shown to fit well with the theoretical predictions

of the shape deformations (Figure 1.6f and 1.6g). It was also predicted that at

temperatures near Tflat, the wide ribbons should morph into a helicoid shape, but

this could not be verified in the experiments.27

The deformation of the same ribbons, in which θc was of a different value than 45

or -45 degrees, which was termed X-geometry, was also studied. Only the narrow

ribbons with small width/thickness ratios were considered in this study.

Interestingly, it was found that when θc was not ±45 degrees, the formation of

helicoids was not possible and only coils were observed. In other words, only S-

and L- geometry could produce helicoids. Simulations showed that for ribbons with

a small θc (between 3 degrees and 9 degrees), small variations in θc strongly

influenced the pitch, but did not influence the diameter of the coil. It was also

shown that upon gradually increasing θc from 5 degrees to 45 degrees, the shape

gradually changes from a coil into a helicoid (Figure 1.7a). Finally, simulations

showed that at θc = 0, the ribbon still has a non-zero pitch (Figure 1.7b). This is in

agreement with the earlier observation that such ribbons tend to twist out of plane

instead of showing unidirectional bending, and the effect is indeed caused by the


13

chirality of the director twist. However, the effect is very weak and can be

cancelled out by having θc deviate a few degrees from 0. In this case, achiral

bending can be obtained.23

UV-driven coiling actuators have also been prepared by adding azobenzene-based

mesogens to the liquid crystal networks. Ribbons cut at a 45 degrees angle from a

densely crosslinked network with twisted nematic alignment displayed UV-driven

deformation (Figure 1.8a).28 In a different research, curling deformations of UV

responsive, azobenzene-containing films with twisted and splayed alignment were

compared. However, the UV light that was used as the stimulus was polarized, and

the irradiation was carried out on one side of the film, which created an intensity

gradient over the thickness of the film. Therefore, the stimulus was not

homogeneous.29 UV-responsive curling actuators with a right handed domain, a left

handed domain, and a straight domain have been prepared by adjusting the cutting

method, and have been shown to be able to do work by moving the uncurled part

up and down upon switching between UV and visible light (Figure 1.8b).30

Figure 1.7 Simulations of the effect of the angle θc on the shape of narrow ribbons at

T = TNI, where TNI is the nematic to isotropic temperature. a) A series of ribbons with

increasing θc. At low values, a coiling ribbon is observed, which slowly transitions into

a helicoid upon approaching θc = 45 degrees. b) Chirality reversal of the ribbons at

values of θc around zero. At θc = 0, the ribbon still has a chiral shape. The open ring

shape lies around θc = -2.5 degrees. Copyright American Physical Society.22


14

Figure 1.8 a) Light-responsive coiling actuator based on a densely crosslinked

network. Copyright Royal Society of Chemistry.27 b) Coiling actuator with a

left-handed domain, a straight domain, and a right-handed domain. Upon irradiation

with UV or visible light, one of the coiling domains contracts and the other expands.

The liquid crystal mixture used is shown in Figure 6a. Copyright Nature Publishing

Group.29


15

1.4 Director profiling in the xy-plane of the film

1.4.1 Continuous circular patterns

Liquid crystal films that deform into 3-dimensional shapes can also be prepared

using continuous, circular director profiles. These profiles are based on

disclinations, which are the topological defects that occur naturally in planar

aligned nematic liquid crystals without a preferred orientation, and can be observed

as the centers of the brushes in a Schlieren texture when the material is analyzed

using polarized optical microscopy.31 They are described as singularities around

which the alignment director varies continuously. The strength of the disclination

m is defined as the number of times the director rotates when the singularity is

circumnavigated once,32 with the sign depending on whether the rotation is in the

same direction as the circumnavigation (+) or in the opposite direction (-). For

example an azimuthal (concentric circles) or radial (spokes) pattern would be

described as an m = +1 pattern, as the alignment director makes one full rotation

when the disclination is circumnavigated once (Figure 1.9a-c).

There have been numerous theoretical predictions on these systems. Consider, for

instance, a disk-shaped piece of liquid crystal polymer film with an azimuthal

director profile. As the alignment director throughout this film is always parallel to

Figure 1.9 Examples of disclinations, and the

corresponding picture between crossed polarizers. a)

m = +1 azimuthal disclination. b) m = +1 radial

disclination. c) m = +1/2 disclination.


16

the circumference C, this means C = L// = 2πr. Likewise, the radius r is always

perpendicular to the alignment director, so r = L┴. When this film is heated, it is

expected to adopt a new circumference C’ = λL// = λC and a new radius

r’ = L┴ = λ-νr due to anisotropic deformations. Because 0 < λ < 1 and ν > 0, we

know that C’ < 2πr’, which means that the circumference has become too small to

accommodate the radius but is still a circle. This can only be solved by a

deformation out of the plane of the film to form a cone shape. The circumference

of the base of this cone equals C’, and the radius of the base equals r’sinφ, where φ

is the cone opening angle (Figure 1.10). In the case of a disk with radial alignment,

C’ = L┴ = λ-νC and r’ = L// = λr, so C’ > 2πr’ which means that the circumference is

too large for the radius. This can be solved by buckling of the circumference into a

sinusoidal wave, which is positioned on a sphere with radius r’. This will generate

an “anticone” shape (Figure 1.10). At higher strains, the number of buckles can

increase, leading to a more crumpled anticone.16d

Figure 1.10 Shape deformation of films with an

m = +1 disclination pattern upon heating. A film

with an azimuthal pattern will contract along the

radius and expand along the circumference to

form a cone with a base radius r’sinφ, where φ is

the cone opening angle. A film with a radial

pattern will form an anticone shape with a

buckled circumference. Copyright Royal

Society.32


17

1.4.2 Discrete patterns

Discrete patterns can also be used to obtain complex shape deformation in liquid

crystal polymer films. Such patterns consist of multiple domains, with varying

alignment directors between the domains, which are joined by discontinuous

boundaries. To avoid incompatibility between the domains, the deformations on

both sides of the boundary have to match, which can be accomplished by having

the angle between the boundary and the alignment director be the same on both

sides. This is referred to as rank-1 connectedness. Keeping this rule in mind, it is

possible to design a two-dimensional director profile using a number of building

blocks, and predict the change in shape upon actuation. It has been shown that this

kind of blueprinting can be accomplished using a “vocabulary” consisting of six

different types of triangular building blocks.33 The first two types are triangles

which are an angular section from an m = +1 disclination (azimuthal or radial), the

second two types have the alignment director running parallel or perpendicular to

one of the edges, and the third two types have a rank-1 connected border bisecting

the triangle (Figure 1.11a). Upon actuation, the base angle θw of the wedge will

change. When multiple wedges are joined in such a way that the base angles form a

closed circle, the change in θw will create either an angular surplus or deficiency,

and shape deformation in the third dimension will occur (Figure 1.11b). This is

Figure 1.11 a) The six different triangular building blocks from which morphing films with discrete

alignment profiles can be constructed. Copyright SPIE.33b b) Four wedges are joined together to form a

pattern similar to the azimuthal pattern shown earlier. When smoothening is not taken into account,

this film is expected to deform into a pyramidal shape, but smoothening will cause the film to

resemble a cone instead. Copyright American Physical Society.33a


18

similar to the deformation of the films with a circular pattern as described in the

previous paragraph. The sharp boundaries are not expected to induce sharp folds,

as the curvature will be delocalized over the surface to reduce bend energy,

smoothening the surface. However, if more disclinations are present, this

smoothening will cause incompatibilities and will be suppressed to some extent.33

Several blueprint designs have been proposed. For example, two domains with

perpendicular alignment directors, which would normally be incompatible, could

be joined by a zigzag line of alternating points of positive and negative

disclinations. At low strains, such a film could deform by having one domain

crumple to accommodate to the deformation of the other domain. However, at

higher strains it is more likely that the buckling gets divided over the entire surface.

A likely solution is a shape with faceted cylindrical shells joined by an expanding

midsection (Figure 1.12a).33b A different example is a film with a pyramidal pattern

(as in Figure 1.11b), but with a single ±1/2 disclination pair located halfway on one

of the rank-1 connected boundaries. This film is expected to deform into a pyramid

with a spiraling, crumpled moat around it (figure 1.12b).33a

Figure 1.12 Deformation of films with discrete alignment patterns featuring discontinuous

boundaries. a) Two domains with orthogonal alignment directors are joined by a boundary with

disclinations of positive (blue) and negative (red) strength. The actuated shape is expected to

have a crumpled and a flat domain at low strains, and to resemble two faceted cylindrical shells

joined by an expanding midsection at high strains. Copyright SPIE.33b b) A pattern with an

m = +1 disclination at the center and an m = ±1/2 disclination halfway one of the boundaries.

This pattern is expected to result in a pyramid with a spiraling moat. Copyright American

Physical Society.33a


19

All the profiles with patterns in the xy-plane presented here, as well as their

deformations, are based on theoretical predictions. So far, besides the work

presented in this thesis, only one publication has been published that shows the

deformation behavior of liquid crystal polymer films with disclination-based

alignment profiles.34

1.5 Director patterning in 3 dimensions

The previous paragraphs described director patterns that vary either along the

z-direction or in the xy-direction of the film. To make a film which is patterned in

all three directions, these two types of alignment can be combined. Recently,

shape-memory materials with alternating bend and straight domains were prepared

by combining a substrate with a uniaxial alignment layer with a photoaligned

alignment layer with domains with orthogonal alignment directions. This way a

cell with alternating twisted nematic and uniaxial domains was created. Upon

heating, in a ribbon with this director profile the twisted nematic domains showed

bending, while the uniaxial domains remained flat (Figure 1.13).35

Figure 1.13 Ribbons with patterned twisted alignment. A ribbon shape-memory prepared

using a uniaxial planar alignment layer and a patterned alignment layer with orthogonal

domains. The film deforms upon heating by bending of the two outward domains while the

center part remains flat.35


20

1.6 Aim and outline of the thesis

As shown in this chapter, theoretical predictions have revealed that liquid crystal

polymer films with complex alignment profiles can lead to a multitude of exotic

shape deformations upon application of the appropriate stimulus. But despite these

exciting possibilities, these systems have only been sparsely explored in practice.

The only alignment profiles that have been extensively studied are those with a

varying director along the z-direction of the film: the twisted and splayed

alignment.

The aim of this thesis is to explore new ways to prepare liquid crystal polymer

films that reversibly deform into exotic shapes upon exposure to a stimulus, by

programming the molecular organization and orientation in the material. In

chapter 2, a simple method is presented that allows the preparation of a variety of

humidity-responsive actuators with an asymmetry in the molecular trigger by

selectively treating a uniaxially aligned liquid crystal polymer film with a

potassium hydroxide solution to alter the responsiveness to humidity and

selectively create the required molecular trigger. Chapters 3 and 4 discuss the

application of photoalignment to prepare liquid crystal polymer films with complex

alignment profiles in the xy-plane of the film, based on disclinations, which

deformed upon heating. Films were prepared with an azimuthal alignment profile

that deformed into cones, while a radial alignment profile resulted in an anticone

shape. A different alignment pattern, containing slits cut at pre-designed

disclination lines, deformed by opening the slits, resulting in remotely addressable

apertures. Chapter 4 deals with films with alignment director profiles that vary in

all three dimensions. These films possess either a discrete or continuous pattern in

the xy-plane, while also having a 90 degrees twisted nematic alignment. This lead

to films that contracted in a way similar to an accordion, and films that showed

polygonal folding. In chapter 5, it is shown that a patterned liquid crystal polymer

network can be used to steer the direction of surface wrinkles which are formed

when a thin layer of gold is sputter-coated on top of the film, followed by heating.


21

In the last chapter the technology assessment of soft actuators based on liquid

crystal networks is presented, which shows the potential applications of the

materials in soft robotics, switchable sieves, and smart optics, as well as the

challenges that still have to be overcome.

1.7 References

1. Harrington, M. J.; Razghandi, K.; Ditsch, F.; Guiducci, L.; Rueggeberg,

M.; Dunlop, J. W. C.; Fratzl, P.; Neinhuis, C.; Burgert, I., Origami-like unfolding

of hydro-actuated ice plant seed capsules. Nat. Commun. 2011, 2, 337.

2. (a) Forterre, Y.; Skotheim, J. M.; Dumais, J.; Mahadevan, L., How the

Venus flytrap snaps. Nature 2005, 433 (7024), 421-425; (b) Mahadevan, L.,

Polymer science and biology: structure and dynamics at multiple scales. Faraday

Discuss. 2008, 139, 9-19; (c) Meyers, M. A.; Chen, P.-Y.; Lopez, M. I.; Seki, Y.;

Lin, A. Y. M., Biological materials: A materials science approach. J. Mech. Behav.

Biomed. 2011, 4 (5), 626-657; (d) Zhang, S., Fabrication of novel biomaterials

through molecular self-assembly. Nat. Biotechnol. 2003, 21 (10), 1171-1178.

3. Aida, T.; Meijer, E. W.; Stupp, S. I., Functional supramolecular polymers.

Science 2012, 335 (6070), 813-817.

4. (a) Bashari, A.; Nejad, N. H.; Pourjavadi, A., Applications of stimuli

responsive hydrogels: a textile engineering approach. J. Text. I. 2013, 104 (11),

1145-1155; (b) Lee, J. H.; Koh, C. Y.; Singer, J. P.; Jeon, S. J.; Maldovan, M.;

Stein, O.; Thomas, E. L., 25th Anniversary article: Ordered polymer structures for

the engineering of photons and phonons. Adv. Mater. 2014, 26 (4), 532-568; (c)

Liu, K.; Kang, Y. T.; Wang, Z. Q.; Zhang, X., 25th Anniversary article: reversible

and adaptive functional supramolecular materials: "Noncovalent interaction"

matters. Adv. Mater. 2013, 25 (39), 5530-5548; (d) Qing, G. Y.; Li, M. M.; Deng,

L. J.; Lv, Z. Y.; Ding, P.; Sun, T. L., Smart drug release systems based on stimuli-

responsive polymers. Mini-Rev. Med. Chem. 2013, 13 (9), 1369-1380; (e)

Schattling, P.; Jochum, F. D.; Theato, P., Multi-stimuli responsive polymers - the

all-in-one talents. Polym. Chem. 2014, 5 (1), 25-36.


22

5. (a) Bauer, S.; Bauer-Gogonea, S.; Graz, I.; Kaltenbrunner, M.; Keplinger,

C.; Schwodiauer, R., 25th Anniversary article: A soft future: From robots and

sensor skin to energy harvesters. Adv. Mater. 2014, 26 (1), 149-162; (b) Kim, T.;

Zhu, L. Y.; Al-Kaysi, R. O.; Bardeen, C. J., Organic photomechanical materials.

ChemPhysChem 2014, 15 (3), 400-414; (c) Liu, Y. J.; Du, H. Y.; Liu, L. W.; Leng,

J. S., Shape memory polymers and their composites in aerospace applications: a

review. Smart Mater. Struct. 2014, 23 (2), 22.

6. (a) Harrison, C.; Stafford, C. M.; Zhang, W.; Karim, A., Sinusoidal phase

grating created by a tunably buckled surface. Appl. Phys. Lett. 2004, 85 (18),

4016-4018; (b) Ziolkowski, B.; Czugala, M.; Diamond, D., Integrating stimulus

responsive materials and microfluidics: The key to next-generation chemical

sensors. J. Intel. Mat. Syst. Str. 2013, 24 (18), 2221-2238.

7. Woltman, S. J.; Jay, G. D.; Crawford, G. P., Liquid-crystal materials find a

new order in biomedical applications. Nat. Mater. 2007, 6 (12), 929-938.

8. Murata, S.; Kurokawa, H., Self-reconfigurable robots. Robot. Autom. Mag.

2007, 14 (1), 71-78.

9. Chalapat, K.; Chekurov, N.; Jiang, H.; Li, J.; Parviz, B.; Paraoanu, G. S.,

Self-organized origami structures via ion-induced plastic strain. Adv. Mater. 2013,

25 (1), 91-95.

10. Kim, J.; Hanna, J. A.; Byun, M.; Santangelo, C. D.; Hayward, R. C.,

Designing responsive buckled surfaces by halftone gel lithography. Science 2012,

335 (6073), 1201-1205.

11. Thérien-Aubin, H.; Wu, Z. L.; Nie, Z.; Kumacheva, E., Multiple shape

transformations of composite hydrogel sheets. J. Am. Chem. Soc. 2013, 135 (12),

4834-4839.

12. Broer, D. J.; Crawford, G. P.; Zumer, S., Cross-linked Liquid Crystalline

Systems. CRC, Taylor and Francis Group, Boca Raton, FL: 2011.

13. Jakli, A.; Saupe, A., Phase transitions in one- and two-dimensional fluids:

properties of smectic, lamellar and columnar liquid crystals (condensed matter

physics), Taylor & Francis: BocaRaton, Florida, USA: 2006; pp 85-102.


23

14. Broer, D. J.; Boven, J.; Mol, G. N.; Challa, G., In-situ photopolymerization

of oriented liquid-crystalline acrylates, 3. Oriented polymer networks from a

mesogenic diacrylate. Makromolekul. Chem. 1989, 190 (9), 2255-2268.

15. (a) M. Warner; Terentjev, E. M., Liquid crystal elastomers. Oxford

University Press, Oxford: 2003; (b) Ohm, C.; Brehmer, M.; Zentel, R., Liquid

Crystalline Elastomers as Actuators and Sensors. Adv. Mater. 2010, 22 (31), 3366-

3387.

16. (a) Van Oosten, C. L., Responsive liquid crystal networks. Eindhoven

University of Technology, 2009, thesis; (b) Hikmet, R. A. M.; Broer, D. J.,

Dynamic mechanical properties of anisotropic networks formed by liquid

crystalline acrylates. Polymer 1991, 32 (9), 1627-1632; (c) Broer, D. J.; Mol, G.

N., Anisotropic thermal expansion of densely cross-linked oriented polymer

networks. Polym. Eng. Sci. 1991, 31 (9), 625-631; (d) Modes, C. D.; Bhattacharya,

K.; Warner, M., Gaussian curvature from flat elastica sheets. P. Roy. Soc. A-Math.

Phy. 2011, 467 (2128), 1121-1140.

17. (a) Ikeda, T.; Mamiya, J.-i.; Yu, Y., Photomechanics of liquid-crystalline

elastomers and other polymers. Angew. Chem. Int. Edit. 2007, 46 (4), 506-528; (b)

Serak, S.; Tabiryan, N.; Vergara, R.; White, T. J.; Vaia, R. A.; Bunning, T. J.,

Liquid crystalline polymer cantilever oscillators fueled by light. Soft Matter 2010,

6 (4), 779-783; (c) Shankar, M. R.; Smith, M. L.; Tondiglia, V. P.; Lee, K. M.;

McConney, M. E.; Wang, D. H.; Tan, L.-S.; White, T. J., Contactless,

photoinitiated snap-through in azobenzene-functionalized polymers. P. Natl. Acad.

Sci. USA 2013; (d) Yamada, M.; Kondo, M.; Mamiya, J.-i.; Yu, Y.; Kinoshita, M.;

Barrett, C. J.; Ikeda, T., Photomobile polymer materials: towards light-driven

plastic motors. Angew. Chem. Int. Edit. 2008, 47 (27), 4986-4988; (e) Yamada,

M.; Kondo, M.; Miyasato, R.; Naka, Y.; Mamiya, J.-i.; Kinoshita, M.; Shishido, A.;

Yu, Y.; Barrett, C. J.; Ikeda, T., Photomobile polymer materials-various three-

dimensional movements. J. Mater. Chem. 2009, 19 (1), 60-62; (f) Yu, H.; Ikeda, T.,

photocontrollable liquid-crystalline actuators. Adv. Mater. 2011, 23 (19), 2149-

2180; (g) Yu, Y.; Nakano, M.; Ikeda, T., Photomechanics: directed bending of a

polymer film by light. Nature 2003, 425 (6954), 145-145.


24

18. Li, M. H.; Keller, P.; Yang, J.; Albouy, P. A., An artificial muscle with

lamellar structure based on a nematic triblock copolymer. Adv. Mater. 2004, 16

(21), 1922-1925.

19. Sánchez-Ferrer, A.; Fischl, T.; Stubenrauch, M.; Albrecht, A.; Wurmus,

H.; Hoffmann, M.; Finkelmann, H., Liquid-crystalline elastomer microvalve for

microfluidics. Adv. Mater. 2011, 23 (39), 4526-4530.

20. (a) Jin, L.; Jiang, X.; Huo, Y., Light-induced nonhomogeneity and gradient

bending in photochromic liquid crystal elastomers. Sci. China Ser. G 2006, 49 (5),

553-563; (b) Jin, L.; Yan, Y.; Huo, Y. In Light-induced bending and smart control

of photochromic liquid crystal elastomers, 2007; pp 64230W-64230W-8; (c)

Warner, M.; Mahadevan, L., Photoinduced deformations of beams, plates, and

films. Phys. Rev. Lett. 2004, 92 (13), 134302.

21. Warner, M.; Modes, C. D.; Corbett, D., Curvature in nematic elastica

responding to light and heat. P. Roy. Soc. A-Math. Phy. 2010, 466 (2122), 2975-

2989.

22. Warner, M.; Modes, C. D.; Corbett, D., Suppression of curvature in

nematic elastica. P. Roy. Soc. A-Math. Phy. 2010, 466 (2124), 3561-3578.

23. Sawa, Y.; Urayama, K.; Takigawa, T.; Gimenez-Pinto, V.; Mbanga, B. L.;

Ye, F. F.; Selinger, J. V.; Selinger, R. L. B., Shape and chirality transitions in off-

axis twist nematic elastomer ribbons. Phys. Rev. E 2013, 88 (2), 022502.

24. Mol, G. N.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Thermo-

mechanical responses of liquid-crystal networks with a splayed molecular

organization. Adv. Funct. Mater. 2005, 15 (7), 1155-1159.

25. van Oosten, C. L.; Bastiaansen, C. W. M.; Broer, D. J., Printed artificial

cilia from liquid-crystal network actuators modularly driven by light. Nat. Mater.

2009, 8 (8), 677-682.

26. (a) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., A glassy bending-

mode polymeric actuator which deforms in response to solvent polarity. Macromol.

Rapid Comm. 2006, 27 (16), 1323-1329; (b) Harris, K. D.; Bastiaansen, C. W. M.;

Broer, D. J., Physical properties of anisotropically swelling hydrogen-bonded

liquid crystal polymer actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c)


25

Harris, K. D.; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J., Self-assembled polymer

films for controlled agent-driven motion. Nano Lett. 2005, 5 (9), 1857-1860.

27. (a) Sawa, Y.; Ye, F. F.; Urayama, K.; Takigawa, T.; Gimenez-Pinto, V.;

Selinger, R. L. B.; Selinger, J. V., Shape selection of twist-nematic-elastomer

ribbons. P. Natl. Acad. Sci. USA 2011, 108 (16), 6364-6368; (b) Teresi, L.;

Varano, V., Modeling helicoid to spiral-ribbon transitions of twist-nematic

elastomers. Soft Matter 2013, 9 (11), 3081-3088.

28. Harris, K. D.; Cuypers, R.; Scheibe, P.; van Oosten, C. L.; Bastiaansen, C.

W. M.; Lub, J.; Broer, D. J., Large amplitude light-induced motion in high elastic

modulus polymer actuators. J. Mater. Chem. 2005, 15 (47), 5043-5048.

29. Wie, J. J.; Lee, K. M.; Smith, M. L.; Vaia, R. A.; White, T. J., Torsional

mechanical responses in azobenzene functionalized liquid crystalline polymer

networks. Soft Matter 2013, 9 (39), 9303-9310.

30. Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; CornelissenJeroen, J.

L. M.; Fletcher, S. P.; Katsonis, N., Conversion of light into macroscopic helical

motion. Nat. Chem. 2014, 6 (3), 229-235

31. Dierking, I., Textures of Liquid Crystals. Wiley-VCH: Weinheim, 2003.

32. Trebin, H. R., The topology of non-uniform media in condensed matter

physics. Adv. Phys. 1982, 31 (3), 195-254.

33. (a) Modes, C. D.; Warner, M., Blueprinting nematic glass: Systematically

constructing and combining active points of curvature for emergent morphology.

Phys. Rev. E 2011, 84 (2), 8; (b) Modes, C. D.; Warner, M., The activated

morphology of grain boundaries in nematic solid sheets. P. Soc. Photo-Opt. Ins.

2012, 8279, 82790Q.

34. McConney, M. E.; Martinez, A.; Tondiglia, V. P.; Lee, K. M.; Langley, D.;

Smalyukh, II; White, T. J., Topography from topology: Photoinduced surface

features generated in liquid crystal polymer networks. Adv. Mater. 2013, 25 (41),

5880-5885.

35. Lee, K. M.; Bunning, T. J.; White, T. J., Autonomous, hands-free shape

memory in glassy, liquid crystalline polymer networks. Adv. Mater. 2012, 24 (21),

2839-2843.

This chapter was based on an article: de Haan, L. T.; Verjans, J. M. N.; Broer, D. J.; Bastiaansen, C. M. W.; Schenning, A. P. H. J., Humidity-responsive liquid crystalline polymer actuators with an asymmetry in the molecular trigger that bend, fold, and curl. J. Am. Chem. Soc. 2014, 136 (30), 10585-10588

27

Chapter 2. Programmed deformation of uniaxially

aligned actuators with asymmetry in the molecular

trigger

2.1 Introduction

In nature, various examples exist of systems that show shape deformation as a

response to changes in humidity. For example, in plants bending deformations in

the opening and closing of pine cones,1 folding and unfolding of ice plant seed

capsules,2 opening of the morning glory flower,3 and helical curling deformation of

Bauhinia Variegate seed pods,4 take place as a result of changes in humidity. Such

responsive bending, folding and curling events in these hierarchically ordered

materials are important for the functioning of natural systems. In materials science

there is an ongoing interest in and demand for fabricating environmentally-

responsive polymer materials that behave similarly to the systems found in nature.5

While light and temperature responsive materials have often been reported, the

fabrication of humidity responsive materials remains less explored. Humidity-

responsive actuators are interesting for many applications, such as power

generators,6 smart textiles,7 and artificial muscles.8 Bending and curling can be

mimicked using bilayers of materials with different swelling behavior, 3-4, 7-9 or by

subjecting only one side of a single-layer film to high humidity.10 However, from a

manufacturing point of view, it would be desirable to have a facile method to

fabricate soft actuators from a single piece of polymer sheet that can be tailored to

achieve bending, folding or curling.6

Chapter 2 Asymmetry in the molecular trigger

28

As shown in the introduction chapter, liquid crystal films which deform as a

response to changes in humidity have previously been prepared by immersion of a

liquid crystal network with hydrogen bonded carboxylic acid moieties in a

potassium hydroxide solution to convert it to a hygroscopic polymer salt.11 Bending

could be achieved in these materials by subjecting a film with planar alignment to

high humidity on only one side, or by subjecting a film with a twisted or splayed

alignment to a uniform humidity change. As such, bending in humidity responsive

LC actuators has been achieved based on either the asymmetry in the molecular

orientation, or by the presence of a gradient in the humidity.

The actuators presented in this chapter are prepared from a single sheet of liquid

crystal film with unidirectional, planar alignment, having no variation in the

alignment director. To induce the complex deformations, the material is locally

treated with a potassium hydroxide solution to create a hygroscopic carboxylic salt

that takes up water from the air and swells in a high humidity environment. In

contrast to previous research, the shape deformation is not induced by asymmetry

in either the stimulus or the alignment director, but by an asymmetry in the

presence of the responsive molecular trigger in the material. This results in soft

actuators that operate in a uniform humidity environment and without requiring a

twisted or splayed alignment director. These soft actuators can be tailored to either

bend, fold or curl in response to changes in humidity, while being prepared from

the same polymer sheet. The ability to create a broad range of very different shapes

using a single material and procedure allows a large variety in the preparation of

actuators. This versatility was demonstrated by preparing actuators that bend

towards or away from a water surface, actuators that show complex

humidity-responsive folding, and curling actuators with controlled handedness that

unwind upon increasing the humidity of the environment.


29

2.2 Results and discussion

2.2.1 Film preparation, KOH treatment and analysis

Humidity-responsive polymer films were prepared using a liquid crystalline

mixture containing hydrogen-bridged moieties (Figure 2.1a).11 This material was

given a planar, uniaxial alignment using a cell containing rubbed polyimide

alignment layers. After photopolymerization (Figure 2.1b), the film (thickness

18 μm) was retrieved from the cell and cut to the appropriate size. It should be

noted that at this point the films did not yet respond to water. Initially, a water-

responsive actuator with simple bending behavior was prepared by bringing only

one side of the ribbon in contact with a 0.1 M KOH solution, followed by rinsing

with water. The infrared spectrum, measured in reflection before base treatment,

clearly showed the hydrogen bridges as a peak at 1682 cm-1, which corresponds to

the C=O stretching in hydrogen bonded dimers. After the single-side base

treatment and subsequent drying, this peak disappeared and two new peaks

appeared at 1547 and 1395 cm-1, resulting from anti-symmetric and symmetric

COO- stretching, respectively. The spectrum measured at the non-exposed side

remained unchanged.

Figure 2.1 a) Chemical composition of the hydrogen bonded

LC mixture. b) Preparation procedure for making the liquid

crystal films.


30

The stability of the base-treated liquid crystal polymer networks was investigated

using IR spectroscopy. The films were exposed to various humidity conditions, and

the IR spectra were periodically measured over a period of 10 days. Spectra were

measured in triplo, baseline corrected and normalized. Two films were analyzed,

one which was kept at ambient humidity for the entire experiment, and one which

was kept in a petri dish with water present (the exact humidity was not measured).

Directly after activation, the peak corresponding to the presence of the potassium

salt was clearly present in both films. Over time, the peak in the high humidity film

decreased in intensity, while in the low humidity film it hardly decreased. The

height of the salt peak was plotted as a function of time (Figure 2.2). It was

observed that the signal belonging to the salt peak decreased asymptotically. This

means that the location of the salt moieties in the films is somewhat unstable when

kept in prolonged high humidity conditions, while being more stable in ambient

humidity conditions.

2.2.2 Deformation behavior of the films

During the exposure to the aqueous KOH solution and rinsing with water,

anisotropic swelling of the exposed side caused bending of the film towards the

non-exposed side. When the ribbon was cut with the alignment director

perpendicular to the long axis, bending along the long axis took place

Figure 2.2 Salt peak intensity as a function of time, in a film kept at high humidity and

one kept at low humidity.


31

(Figure 2.3a). A ribbon in which the alignment was parallel to the long axis showed

bending along the short axis while wet. This behavior is in good agreement with

earlier observations that the swelling should take place predominantly

perpendicular to the alignment director due to the anisotropic nature of the

material,11 and shows that the bending direction of such ribbons is strongly

influenced by the alignment director of the mesogenic units. Unexpectedly, when

the film was dried, bending towards the base-treated side took place. This implies

that the material, upon base exposure and subsequent drying, shrinks in comparison

to the non-exposed side. The shrinkage is anisotropic, and mainly takes place in the

direction perpendicular to the alignment director. Although the mechanism for this

shrinkage is unclear, it may be caused by increasing molecular order, or by a more

dense packing. Alternatively, material might be lost during activation, for instance

through hydrolysis of ester bonds.12 When the film was placed in water again, it

returned to the swollen state, in which it was bending towards the non-exposed

side.

Figure 2.3 Actuation behavior of a one-side base treated planar aligned LC polymer film.

a) Schematic representation of the bending behavior of the ribbon with the alignment

director perpendicular to the long axis, upon activation and when dried. b) Actuation in a

controlled humidity chamber c) Curvature (1/r) of the film as a function of humidity.

Squares indicate data points which were obtained while increasing the humidity, while

circles indicate deceasing humidity. d) Bending of the film when facing the water surface

with the base-treated side (top) or the non-treated side (bottom).


32

To study the bending behavior in response to environmental humidity, the one-side

base-exposed film with the alignment director perpendicular to the long axis

(Figure 2.3a) was placed in a chamber in which the humidity is homogeneous and

controllable. At high humidity (75%), the sample was straight, while at low

humidity (15%) it was strongly bent towards the base-exposed side (Figure 2.3b)

due to shrinkage caused by the evaporation of water from the base-treated side.

When the bending curvature (1/r) was plotted against the relative humidity

(Figure 2.3c), a linear relationship was observed. The curvature was found to range

from 0 mm-1 at high humidity to 0.6 mm-1 at low humidity. The humidity could not

be increased to a point where bending towards the untreated side took place

(>75 %), such as was observed when the actuator was rinsed with water after the

preparation process. To investigate the response to a humidity gradient, the film

was brought close to a water surface. As expected, bending towards the water

surface took place when the untreated side was facing the water (Figure 2.3d), and

the curvature increased when the distance to the water surface was decreased.

When the film was flipped, it was bending away from the water. This shows that

the direction of the bending is determined by the asymmetry in the responsive

molecular trigger in the polymer film and not by the presence of the humidity

gradient.


33

2.2.3 Complex folding

Next, complex folding was induced in the polymer films by selective base

treatment of specific areas on the film’s surface (Figure 2.4). An actuator with

three folds in an alternating pattern was prepared. To do so, the outer parts on one

side of the film were first exposed to the base, then the film was rinsed in water and

dried, and a second exposure was carried out on the center part of the film on the

other side (Figure 2.4a). The actuation of this film was tested in the humidity

chamber. The film displayed a folded shape in an environment with low humidity

(15%), while at high humidity (75%) a straight polymer film was again obtained,

which is similar to the actuator with only one base-treated domain. To demonstrate

that sharp, hinge-like folding could also be induced, three narrow parts of the film

were base-treated. A ribbon with three hinges, two on one side of the film and one

on the other side, was prepared (Figure 2.4b) and investigated in the humidity

chamber. At low humidity, three sharply bent hinges were observed and the

actuator was fully contracted. Again, at high humidity (75%) the actuator was

nearly straight.

Figure 2.4 Folded actuators, which are prepared through selective treatment of the film.

a) A ribbon in which the outer parts on one side are exposed to an alkaline solution, and

the center part on the other side (blue regions), actuated in a humidity chamber. b) A

ribbon with three narrow hygroscopic polymer salt bands (blue parts), actuated in the

humidity chamber by increasing the humidity. Arrows indicate the molecular director in

the polymer films.


34

2.2.4 Curling

Finally, humidity-sensitive curling actuators were prepared. In this experiment, a

ribbon was cut from a planar aligned polymer film in such a way that the nematic

director made a -45 degrees angle with the long axis of the film (Figure 2.5a). After

one-side exposure, the ribbon adopted a right-handed curl shape while wet, with

the base-treated side on the outside. When the ribbon was dried, the handedness of

the curl reversed to left handed, with the base-treated side on the inside. As seen

before, the polymer salt side swells when wet in comparison to the untreated,

non-responsive side, and this swelling predominantly takes place perpendicular to

the molecular director. Since the nematic director makes a -45 degrees angle with

the long axis of the film this will lead to right-handed curling in which the polymer

salt side is on the outside (Figure 2.5a). In the dry state, shrinkage dominates

perpendicular to the molecular director, causing the helix to reverse its handedness.

Figure 2.5 Curling actuators prepared by one-side base exposure of liquid crystal

films. a) Ribbon with the molecular director at a -45 degrees angle with respect

to the long axis of the film, and the resulting curling behavior after activation in

KOH solution, both in the wet and dry state. b) Curling and uncurling of the +45

degrees actuator at high and low humidity, respectively. The behavior of this

actuator is also shown in the humidity chamber with increasing humidity.


35

When a strip was cut in which the nematic director was at a +45 degrees angle with

the long axis of the film, the resulting curl was left-handed when wet and right-

handed when dry (Figure 2.5b), which shows that the handedness in these humidity

responsive curling actuators is fully directed by the orientation of the molecular

director with respect to the long axis of the film. This ribbon was investigated in a

humidity chamber (Figure 2.5b). At low humidity, the ribbon was tightly curled.

When the humidity was increased, the ribbon started to uncurl. At high humidity it

was straight, similar to the bending and folding actuators. It should be noted this

kind of deformation requires anisotropic strain, and would not be possible with

materials that swell or shrink isotropically.

2.3 Conclusions and outlook

In conclusion, a new, versatile method was developed for the preparation of

polymer actuators based on liquid crystals that can be tailored to bend, fold or curl

as a response to changes in humidity. In this material the deformation is fully

controlled by the asymmetry in the responsive molecular trigger present in the

polymer film. Using the simple procedure of locally exposing a monolithic,

uniaxially aligned hydrogen bonded polymer film to an alkaline solution,

hygroscopic regions are obtained on one side of the film. Anisotropic swelling and

shrinkage was observed on the polymer salt side, which caused a strong bending

deformation of the ribbons. Folding deformation was achieved by patterned base

treatment, and curling deformation could be achieved when the alignment director

of the mesogenic units was put at a -45 degrees or +45 degrees angle with the long

axis of the ribbon, in which the handedness was controlled by the molecular

director.

This approach provides a new method for the preparation of stimuli responsive

actuators from a single polymer sheet. In this novel principle the asymmetry in the

molecular trigger in the anisotropic polymer film play a dominant role, leading to


36

programmed deformation events in a versatile fashion. This method can in

principle be extended to other materials, such as anisotropically ordered hydrogels.

In the future, even more complex deformation may be achieved using inkjet

printing techniques for the local base treatment, and using photoalignment to

prepare complex three-dimensional director patterns (as is presented in the

following chapters).

2.4 Experimental Section

Preparation of the liquid crystal films: Cleaned glass substrates (3x3 cm) were

spincoated with polyimide (AL 1051, JSR Corp.) at 1000 rpm for 5 seconds, then

5000 rpm for 40 seconds, using a Karl Suss RC8 spincoater. After heating to 180 oC for 1 hour, the plates were rubbed and glued into cells, using 18 micron SP-218

Micropearl plastic spacers (Sekisui Chemical Co.) to secure the cell gap. A mixture

of 3OBA, 5OBA and 6OBA (Merck) was prepared in a 1:1:1 ratio by weight, then

12 w% RM82 (Merck), 0.5-2.0 w% photoinitiator (Irgacure 819, Ciba Specialty

Chemicals), and 200 ppm thermal inhibitor (tert-butyl hydroquinone, Fluka) were

added. The components were dissolved in CH2Cl2, then dried by heating to 50 oC

under air flow, followed by drying in vacuo. The cells were filled at either 95 oC

(nematic) or 105 oC (isotropic), cooled to 80 oC, and cured for 5 min using an

Omnicure S2000 UV source.

Preparation of the actuators: The films were removed from the cells using a razor

blade after immersion in hot water, cut to the right size, and treated with a plasma

asher (Emitech K1050X) for 4 min at 45 Watt to remove remaining traces of

polyimide. Conversion to the KOH salt was carried out by treating the film with an

aqueous 0.1 M KOH solution. When an entire surface was treated, the film was

floated on the surface of the KOH solution for 20 seconds, then rinsed in

demineralized water and dried. When localized activation was performed, a piece

of kitchen sponge was soaked in KOH solution, and the film was placed on top of


37

it until it visibly started bending, which took a few minutes. The film was then

flushed and dried, and in some cases a second exposure was carried out.

Analysis: ATR FTIR analysis was performed for some of the actuators using a

Varian 670 FTIR spectrometer. Humidity experiments were performed using a

home built humidity chamber, containing a Vaisala HMI41 humidity sensor.

2.5 References

1. Dawson, C.; Vincent, J. F. V.; Rocca, A.-M., How pine cones open. Nature

1997, 390 (6661), 668-668.

2. Harrington, M. J.; Razghandi, K.; Ditsch, F.; Guiducci, L.; Rueggeberg,

M.; Dunlop, J. W. C.; Fratzl, P.; Neinhuis, C.; Burgert, I., Origami-like unfolding

of hydro-actuated ice plant seed capsules. Nat. Commun. 2011, 2, 337.

3. Ma, Y.; Sun, J., Humido- and Thermo-Responsive Free-standing films

mimicking the petals of the morning glory flower. Chem. Mater. 2009, 21 (5), 898-

902.

4. Armon, S.; Efrati, E.; Kupferman, R.; Sharon, E., Geometry and mechanics

in the opening of chiral seed pods. Science 2011, 333 (6050), 1726-1730.

5. (a) Roy, D.; Cambre, J. N.; Sumerlin, B. S., Future perspectives and recent

advances in stimuli-responsive materials. Prog. Polym. Sci. 2010, 35 (1–2),

278-301; (b) Spruell, J. M.; Hawker, C. J., Triggered structural and property

changes in polymeric nanomaterials. Chem. Sci. 2011, 2 (1), 18-26; (c) Stuart, M.

A. C.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov,

G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov,

I.; Minko, S., Emerging applications of stimuli-responsive polymer materials. Nat.

Mater. 2010, 9 (2), 101-113.

6. Ma, M.; Guo, L.; Anderson, D. G.; Langer, R., Bio-inspired polymer

composite actuator and generator driven by water gradients. Science 2013, 339

(6116), 186-189.


38

7. Dai, M.; Picot, O. T.; Verjans, J. M. N.; de Haan, L. T.; Schenning, A. P.

H. J.; Peijs, T.; Bastiaansen, C. W. M., Humidity-responsive bilayer actuators

based on a liquid-crystalline polymer network. ACS Appl. Mater. Inter. 2013, 5

(11), 4945-4950.

8. (a) Islam, M. R.; Li, X.; Smyth, K.; Serpe, M. J., Polymer-based muscle

expansion and contraction. Angew. Chem. Int. Edit. 2013, 52 (39), 10330-10333;

(b) Ma, Y.; Zhang, Y.; Wu, B.; Sun, W.; Li, Z.; Sun, J., Polyelectrolyte multilayer

films for building energetic walking devices. Angew. Chem. Int. Edit. 2011, 50

(28), 6254-6257.

9. (a) Ionov, L., Biomimetic hydrogel-based actuating systems. Adv. Funct.

Mater. 2013, 23 (36), 4555-4570; (b) Ochoa, M.; Chitnis, G.; Ziaie, B., Laser-

micromachined cellulose acetate adhesive tape as a low-cost smart material. J.

Polym. Sci. Pol. Phys. 2013, 51 (17), 1263-1267; (c) Shen, L.; Fu, J.; Fu, K.;

Picart, C.; Ji, J., Humidity responsive asymmetric free-standing multilayered film.

Langmuir 2010, 26 (22), 16634-16637; (d) Stoychev, G.; Turcaud, S.; Dunlop, J.

W. C.; Ionov, L., Hierarchical multi-step folding of polymer bilayers. Adv. Funct.

Mater. 2013, 23 (18), 2295-2300; (e) Tokarev, I.; Minko, S., Stimuli-responsive

hydrogel thin films. Soft Matter 2009, 5 (3), 511-524; (f) Zou, Y.; Lam, A.;

Brooks, D. E.; Srikantha Phani, A.; Kizhakkedathu, J. N., Bending and stretching

actuation of soft materials through surface-initiated polymerization. Angew. Chem.

Int. Edit. 2011, 50 (22), 5116-5119.

10. (a) Douezan, S.; Wyart, M.; Brochard-Wyart, F.; Cuvelier, D., Curling

instability induced by swelling. Soft Matter 2011, 7 (4), 1506-1511; (b) Geng, Y.;

Almeida, P. L.; Fernandes, S. N.; Cheng, C.; Palffy-Muhoray, P.; Godinho, M. H.,

A cellulose liquid crystal motor: a steam engine of the second kind. Sci. Rep. 2013,

3, 1028.

11. (a) Broer, D. J.; Bastiaansen, C. M. W.; Debije, M. G.; Schenning, A. P. H.

J., Functional organic materials based on polymerized liquid-crystal monomers:

Supramolecular hydrogen-bonded systems. Angew. Chem. Int. Edit. 2012, 51 (29),

7102-7109; (b) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Physical

properties of anisotropically swelling hydrogen-bonded liquid crystal polymer


39

actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c) Harris, K. D.;

Bastiaansen, C. W. M.; Lub, J.; Broer, D. J., Self-assembled polymer films for

controlled agent-driven motion. Nano Lett. 2005, 5 (9), 1857-1860.

12. van Kuringen, H. P. C.; Eikelboom, G. M.; Shishmanova, I. K.; Broer, D.

J.; Schenning, A. P. H. J., Responsive Nanoporous Smectic Liquid Crystal Polymer

Networks as Efficient and Selective Adsorbents. Adv. Funct. Mater. 2014, 24 (32),

5045-5051

This chapter was based on: de Haan, L. T.; Sanchez-Somolinos, C.; Bastiaansen, C. M. W.; Schenning, A. P. H. J.; Broer, D. J., Engineering of complex order and the macroscopic deformation of liquid crystal polymer networks. Angew. Chem. Int. Edit. 2012, 51 (50), 12469-12472; Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J., Angular deficits in flat space: remotely controllable apertures in nematic solid sheets. P. Roy. Soc. A.-Math. Phy. 2013, 469 (2153), 20120631

41

Chapter 3. Actuators with a director pattern in the

xy-plane of the film

3.1 Introduction

As described in the previous chapters, the morphing behavior of liquid crystalline

networks has so far been studied mainly in systems with uniaxial alignment, or in

those with a molecular director that varies in only in the z-direction, that is, twisted

nematic and splayed configurations that bend1 or curl2 after applying the stimulus.

In contrast, this chapter describes the preparation of polymer networks which are

patterned in the xy-plane of the film, which respond to temperature increase with

reversible, exotic shape deformations. In contrast to the previously shown

examples, these actuators do not require a gradient in the alignment director,

molecular trigger, or the stimulus in the z-direction to form 3-dimensional shapes.

While theoretically predicted (see Chapter 1),3 the deformation behavior of liquid

crystal networks with disclination alignment profiles had never been

experimentally confirmed.

Initially, circular patterns were prepared based on +1 disclinations: the azimuthal,

radial, and spiral pattern. Networks based on disclinations of +0.5 and +1.5

strength were also prepared. The deformation of the azimuthal and radial films was

studied by heating them up using an IR lamp. Then, discrete patterns based on

disclinations were also prepared, using the “vocabulary” as described in the

introduction chapter, and the deformation of these films was also investigated.

Chapter 3 Director pattern in the xy-plane of the film

42

Finally, a pattern was shown in which slits were cut, which allow in-plane

deformation that leads to opening of the slits. This can be used to prepare sieves or

filters that can be opened and closed through heating.

To create liquid crystal polymer films with a non-uniform director profile in the

xy-plane, an alignment technique that can facilitate the preparation of such

alignments in the reactive mesogens is required. Polyimide layers are favored for

the alignment of liquid crystals, but while advanced techniques for polyimide

buffing are capable of generating non-uniform director profiles,4 the most versatile

technique for the generation of complex director profiles is photoalignment.5 To

prepare photoaligned alignment layers, a photoresponsive material is irradiated

with polarized UV light to cause a permanent or reversible chemical

transformation. The molecules are more likely to absorb the light, and undergo the

transformation, if the transition dipole moment is parallel to the electric vector of

the polarized UV light. This creates anisotropy in the alignment layer, which can

then be transferred to the reactive mesogens that come into contact with it. There

are various photoalignment materials available. A popular class of materials are

based on azobenzene. These materials undergo a reversible trans to cis

rearrangement when they absorb UV light, which causes the molecules to reorient.

As the absorption is more efficient for molecules in which the transition dipole

matches the polarization direction of the light, the alignment of the molecules is

driven towards the direction perpendicular to the polarization direction, and

anisotropy is created. Another class of alignment materials are the linearly

photopolymerizable polymers (LPP’s), which rely on a [2+2] cycloaddition

reaction between the photosensitive side groups of the polymer, which results in

crosslinking of these groups upon exposure to polarized UV light. As the groups

will only crosslink when the transition dipole moment is in the right direction with

respect to the polarization direction of the UV light, anisotropy is created in the

alignment material. The anisotropic layer will align liquid crystals that come in

contact with it, either parallel or perpendicular to the polarization of the UV light

used to prepare it, depending on the material used.6 In the experiments described in


43

this thesis, only alignment materials that align parallel to the polarization direction

were used (Staralign 2100 and ROP-108).


3.2.1 Preparation of photoaligned liquid crystal network films

For the preparation of the photoalignment layers, a thin film of LPP spincoated

onto glass substrates was used. Two of these substrates were glued together with a

small (~18 μm) gap between them to obtain an alignment cell. When irradiated

with linearly polarized UV light, the alignment material forms an alignment layer,

with the alignment director oriented parallel to the polarization direction of the UV

light. A photomask was used to selectively expose areas of the cell to allow the

creation of various patterns (Figure 3.1a). The cells were filled with a mixture of

polymerizable liquid crystals in the isotropic phase, and after cooling the liquid

Figure 3.1 a) Setup for the preparation of patterned alignment cells. b) Procedure for

making a patterned liquid crystal polymer network from a patterned alignment cell. c)

Liquid crystal mixture for identification of the alignment patterns. d) Liquid crystal mixture

for the preparation of liquid crystal actuators.


44

crystalline samples to the nematic phase, photopolymerization was carried out

(Figure 3.1b). Two different nematic liquid crystal mixtures were used. One

mixture consisted of two mesogens, which were combined in a 1:1 ratio to reduce

the clearing point and allow curing at room temperature (Figure 3.1c).7 It contained

a blue colored dichroic dye (Rhodamine 700) that allowed the determination of the

alignment direction of the resulting polymer network when it is placed between

parallel polarizers. The second mixture consisted mostly of mesogens with a single

acrylate group, as the low crosslink density of a film prepared with these

monomers is known to result in a strong shape deformation response upon heating.8

It contained an IR absorbing dye (Lumogen 788 IR) to obtain an actuator which

could be heated by applying IR irradiation (Figure 3.1d). To both mixtures, a

radical initiator (Irgacure 184 or Irgacure 819, respectively) was added to allow

photopolymerization.

3.2.2 Films based on continuous disclinations

Liquid crystal polymer films with director profiles based on m = +1 disclinations

were prepared first: the azimuthal, radial and spiral pattern. The alignment layers

were made by slowly rotating the cells containing the LLP layers while exposing to

linearly polarized UV light through a wedge-shaped photomask (Figure 3.2a).

Different patterns could be prepared by using different polarization directions for

the UV light. When the polarization direction was set to be perpendicular to the

long axis of the wedge, azimuthal alignment layers were obtained. When the

polarization direction of the UV light was set parallel to the wedge, radial

alignment layers were created, while a polarization direction at 45 degrees

produced spiral alignment layers.


45

The cells were filled with the mixture containing the dichroic dye (Figure 3.1c),

photopolymerized at room temperature in the nematic phase, and the resulting

films were investigated between polarizers. Between crossed polarizers, the

azimuthal, spiral, and radial films showed no light transmission in the areas where

the mesogenic units are positioned either parallel or perpendicular to the

polarization direction of the incoming polarized light, while the transmission

gradually increased towards the intermediate areas where the long molecular axis is

at a 45 degrees angle (Figure 3.2b). This manifested itself as a dark fan-shaped area

in a bright field. Between parallel polarizers, the areas where the liquid crystals are

aligned parallel to the polarization direction appeared in a deeper shade of blue

compared to those aligned in a different direction (Figure 3.2b). This coloration is

caused by the blue colored dichroic dye, which aligns with the liquid crystals and

shows more absorption when it is aligned parallel to the polarization direction of

the incoming light. Blue colored regions are observed at different areas in the

polymer films, which is compatible with the azimuthal, radial and spiral alignment

director profiles. As expected, the black fan-shaped sections are stationary when

Figure 3.2 a) Photoalignment setup for the preparation of continuous circular alignment

cells. b) Optical characterization of the films with azimuthal, spiral and radial alignment

viewed between crossed polarizers and parallel polarizers. The gray triangles in the

schematic representation of the alignment patterns indicate non-transmissive areas in the

crossed polarizers setup. The blue triangles indicate areas with no transmission between

crossed polarizers, and blue transmission between parallel polarizers. c) Film prepared

using a rotating substrate in combination with the use of circular masks. d) Optical

characterization of films based on disclinations of other strength, aligned using alignment

cells which were prepared using UV light with a rotating polarization direction. The

substrate was rotated counterclockwise.


46

the samples are rotated. A polymer film with even higher complexity was prepared.

A structure was made in two steps with an azimuthal alignment on the inner circle

and a radial alignment on the outer circle, which was termed the sun alignment.

The alignment of the resulting film was verified between crossed and parallel

polarizers, and showed the expected properties (Figure 3.2c).

To further explore this method, alignment patterns based on disclinations of +1.5

and +0.5 strength were also prepared. This was done by continuously rotating the

polarization direction of the UV light during the rotation of the cell. Two patterns

were created with the polarization direction rotating at half the speed of the

substrate rotation. In one case the polarization direction and substrate were rotating

in the same direction, while in the other case they were rotating in opposite

directions. The resulting polymer films made using these cells showed the expected

features under crossed and parallel polarizers (Figure 3.2d). As expected, in

contrast to the azimuthal, spiral and radial films, the black fan-shaped sections in

these asymmetric samples were rotating upon rotating the sample between the

polarizers.

3.2.3 Actuation of m = +1 disclination-based actuator films

To show the functionality of the materials, the actuation behavior of freestanding

films upon heating by IR irradiation was investigated. To this end, new films were

prepared using the second liquid crystal mixture (Figure 3.1d). In this case,

photopolymerization was carried out at elevated temperature in the nematic phase

to obtain a polymer film. The actuation behavior of the azimuthal and radial films

upon heating was investigated. A piece of polymer film was cut in a circular shape

from the azimuthal actuator around the center, followed by irradiation with an IR

lamp while holding it in place with a suction gripper at the center. Prior to heating,

the actuator was only slightly bent (Figure 3.3a). Upon switching on the lamp,

deformation was observed into a conical shape within seconds, with the cone apex


47

located at the center and pointing downward (Figure 3.3a). When the distance

between the lamp and the actuator was varied, the upward bending angle changed

accordingly (Figure 3.3b), which demonstrates that the degree of deformation

depends on the intensity of IR irradiation. The sample is symmetrical, and the

symmetry is broken to favor the apex-down cone, which is probably caused by the

vacuum gripper pulling on the tip. This was further affirmed by the observation

that when a weaker vacuum was used, the apex-up cone was favored instead, in

which case the symmetry was broken by gravity. When the lamp was switched off,

the film returned to its original shape. The actuation experiment was repeated for

the actuator with a radial alignment pattern. When the IR lamp was switched on,

the film deformed into an anticone shape (Figure 3.3c). The degree of actuation

could again be varied by changing the distance from the IR source.

Figure 3.3 Actuation of films with azimuthal and radial alignments. a)

Actuation behavior of an azimuthal polymer film upon heating with an IR

lamp. b) Relationship between the distance of the sample to the lamp and the

upward bending angle. c) Actuation behavior of a radial polymer film. The

arrows along the radius and along the azimuth indicate the direction of

deformation.


48

The behavior of these liquid crystal networks is in agreement with the theoretical

models for mechanical responses of disclinations in nematic crosslinked networks

presented in the introduction chapter.3a In case of an azimuthal alignment pattern, a

reduction of the liquid crystal order upon temperature increase leads to contraction

along the azimuthal direction and an expansion along the radial direction (Figure

3.3a). These strains cannot be accommodated within the sheet plane, causing

deformation of the flat sheet out of the plane into a cone. In the case of a radial

alignment pattern, the strains are in the opposite directions (Figure 3.3c). This

results in an anticone, which is recognizable as a saddle shape.

The actuation behavior of the films with director profiles based on other

disclinations was not investigated. However, in a study performed by a different

research group the deformation of large number of UV-responsive films with

patterns based on disclinations was shown.9

3.2.4 Disclination-based, discrete patterns

Alignment patterns based on the previously described disclinations were also

prepared as discrete patterns. The photoalignment procedure used to generate these

patterns is comparable to the method used for the continuous domains. However,

instead of irradiating a small angular section of a rotating substrate, a static

substrate is irradiated through a photomask in a two-step fashion, changing the

polarization direction in between the two steps. This leads to patterns that have

areas with a single, non-varying alignment director, and are separated by sharp

boundaries. Alignment cells based on these patterns were then filled with the

reactive liquid crystal mixture with a lower crosslinker content (Figure 3.1d), and

after photopolymerization the deformation behavior of the resulting films was

examined. To properly design the discrete patterns, it is important to use the

“vocabulary” for multidomain pattern design as described in the introduction

chapter, containing only rank-1 connected boundaries to avoid incompatibilities.3b, 3c


49

Using this procedure, an alignment cell with a four-domain discrete pattern

resembling a m = +1 azimuthal disclination was produced, and a liquid crystal

polymer film was made using this cell as described above. When the film was

heated using an IR lamp, it deformed into the expected conical shape (Figure 3.4a).

The pattern could also be tiled multiple times in a single film, while maintaining

rank-1 connectedness. A film containing roughly thirty of these domains was

prepared and actuated. It strongly deformed into a dented surface, which somewhat

resembled the expected array of pyramids (Figure 3.4b). It is not clear, however,

why the shape is irregular.

3.2.5 Aperture patterns

Discrete alignment patterns based on disclinations can also be utilized to prepare

films with apertures that can be opened and closed when a stimulus is applied.

Such films could lead to an interesting application, where a filter or sieve is

prepared with pores that can be altered in size, for example to unclog them or to

Figure 3.4 a) Discrete m = +1 azimuthal pattern, and

the resulting shape deformation upon IR heating. b)

Tiled azimuthal pattern (partially shown) and the

resulting deformation upon IR heating.


50

allow larger particles to pass on command. As previously shown, a film with an

azimuthal +1 disclination will contract along its circumference and expand along

its radius. The new circumference and radius are incompatible, and the only

solution is to form a cone shape. However, when such a film would be cut from the

edge to the center, the strains can instead be relieved by widening the cut. This

solution is favored over formation of the cone shape, as it will stay flat and does

not experience the stretches involved with bending into the third dimension. Using

this principle, a complex director profile was designed to prepare a film with

apertures which could be opened and closed by applying heat. The profile

consisted of two half m = +1 disclinations, with an m = -1 disclination directly in

between. A slit was cut between from the center of one m = +1 disclination to the

center of the other, slicing the m = -1 disclination in half. This allows all

disclinations to release their strains by deforming in the plane of the film, by

opening the angles at the outer edges of the slit and closing the angles at the center

of the slit. The final solution is the slit opening. This profile was then tiled multiple

times in a single film (Figure 3.5a).

Figure 3.5 a) Schematic representation of the alignment

pattern with slits (thick lines) cut between the two positive

disclinations, crossing the negative disclination. The

opening of the slits upon temperature increase is also

shown. b) The actual film, floating on a water bath. A close-

up of one of the apertures is shown before and after heating.


51

The film was prepared in the same way as the other films with discrete alignment

profiles. The slits were carefully cut at the correct positions using a razor blade.

The film was then put onto a water bath so it would float, and the water was

gradually heated up to 85 oC using a hot plate. It was observed that the apertures

are indeed opened at this temperature (Figure 3.5b). The response was found to be

reversible upon cooling of the water bath to room temperature. This pattern

resembled a sieve in which the opening and closing of the pores can be controlled

with temperature, although the openings were still large (about 1 cm in length).


In conclusion, the results presented in this chapter show that patterned

photo-alignment layers can be used to align polymerizable liquid crystals and

prepare freestanding polymer films with complex order in the xy-plane, based on

disclinations of various strengths. It was demonstrated that m = +1 disclination

patterns can be prepared using a photoalignment layer in combination with a

wedge-shaped mask to irradiate a rotating substrate. Films with patterns based on

m = +1.5 and m = +0.5 disclinations were also prepared. The m = +1 disclination

films with azimuthal and radial patterns reversibly deformed into cone and saddle

shapes, respectively, upon heating with IR irradiation, as was predicted in earlier

studies. It was also shown that disclination-based patterns could be prepared using

discrete, stepwise patterning. These patterns showed deformations similar to their

continuous counterparts, but in addition could be tiled to have multiple

disclinations present in a single film.

The materials shown comprise only a small number of the possible structures that

can be designed and prepared, and it is to be expected that, by using different mask

sizes and patterns, the possible dimensions and shapes can be greatly expanded. As

an example of the applications that could emerge from these systems, a pattern

with slits was prepared, and it was shown that the slits could be opened and closed


52

by heating and cooling, respectively. The range of possible applications could be

further expanded by looking at different shapes. Additionally, materials can be

fabricated that respond to different stimuli, and the composition of the liquid

crystal mixtures can be varied in order to change other properties of the material,

leading to new and powerful methods for the fabrication of stimuli-responsive

polymer materials with a complex order having novel optical, mechanical, and

electronic properties.

3.4 Experimental section

All materials were used as received from Merck (RM82 (C6M), RM105, RM23,

RM257 (C3M)), BASF (Lumogen 788, LC756), Ciba Specialty Chemicals

(Irgacure 819, Irgacure 184), Huntsman (Staralign 2100,), Rolic (ROP-108),

Lambdachrome (Rhodamine 700), and Aldrich (t-butyl hydroquinone,

cyclopentanone, dichloromethane). The alignment cells were prepared using 20

micron glass rod spacers and adhesive (Norland NOA63).

The photoalignment material Staralign 2100 or ROP-108 was spincoated onto glass

substrates using a Karl Suss RC8 spincoater. Photoalignment was carried out using

an Ominicure S2000 UV lamp with 320-390 nm filter. A Knight Optical HNBP

sheet polarizer mounted in a Thorlabs PRM1/MZ8 rotation mount was used to

polarize the UV light in various directions, and the substrate was rotated using a

Thorlabs CR1/M-Z7 rotation stage. The rotation of the polarizer and stage was

controlled with Thorlabs TDC001 controllers, which were programmed with APT

software (Thorlabs).

Photopolymerization of the liquid crystal mixtures was carried out using a Philips

PL-S 9W UV lamp for the first mixture (C6M, C3M, Irgacure 184, Rhodamine

700), while an Ominicure S2000 UV lamp with 320-500 nm filter was used for the

second mixture (RM82, RM105, RM23, Irgacure 819, Lumogen 788). For the IR


53

actuation experiments a Heraeus Noblelight Shortwave surface irradiator (type

“Duo”, 250 Watt, 57.5 Volt) with gold reflector was used.

Preparation of liquid crystal cells with continuous alignment patterns: A Staralign

2100 solution was spincoated onto two cleaned glass plates, which were heated to

120 ºC for 10 minutes to evaporate the solvent. The LPP substrates were assembled

into cells using glue containing 20 micron diameter glass rod spacers to fix the cell

air gap. In order to generate the desired alignment pattern, the LC cells were

exposed to linearly polarized UV light at 5 mW/cm2. Care was taken to have the

center of the cell coincide with the axis of rotation of the rotating stage. Exposure

was carried out through a photomask whose transmission area was a 9 degrees

angle sector of the circle of rotation, which was placed in such a way that the

intersection of the radii was at the axis of rotation of the sample stage. The sample

stage was rotated counterclockwise at a speed of 0.022 degrees per second. In case

of the patterns without rotational symmetry, the motorized polarizer was rotated at

half this speed in either clockwise or counter clockwise.

Preparation of liquid crystal cells with discrete alignment patterns: ROP-108 was

spincoated onto two cleaned glass plates, which were heated to 115ºC for 10

minutes to evaporate the solvent. The LPP substrates were assembled into cells

using glue containing 18 micron diameter glass rod spacers to fix the cell air gap.

In order to generate the desired alignment pattern, the LC cells were exposed to

linearly polarized UV light through a photomask, then the sample was rotated to

expose the previously unexposed parts, and a second exposure was carried out with

the polarization direction rotated by 90 degrees.

Preparation of densely crosslinked liquid crystal networks: Liquid crystal cells with

continuous alignment patterns were filled at 95 ºC with a mixture of C6M and

C3M in a 20/80 ratio, to which 1%wt of photoinitiator Irgacure 184 and 0.5%wt. of

Rhodamine 700 were added. After filling the cell with the LC mixture, it was

slowly cooled down to room temperature, and the alignment of the LC was


54

checked by observing the cell between crossed polarizers. Photopolymerization

was carried out at room temperature by UV irradiation for 30 min.

Preparation of actuator films: RM82, RM105, and RM23 were put into a screw-cap

vial in a 2 : 3 :1 ratio by weight. A small amount of tert-butyl hydroquinone

dissolved in CH2Cl2 was added as a thermal inhibitor. 0.5% w/w Lumogen 788 and

1% w/w Irgacure 819 were added, then more CH2Cl2 was added and the mixture

was stirred at elevated temperature until all components were mixed and the

solvent had evaporated. An alignment cell with an azimuthal or radial pattern was

brought to 80 oC, and the cell gap was filled with the mixture though capillary

action. When the cell was completely filled, it was cooled down slowly to 60 oC.

Then UV polymerization took place by UV irradiation for 5 minutes. Finally, the

cell was opened with a razor blade to retrieve the freestanding film.

Investigation of IR actuation: The films with radial and azimuthal alignment were

actuated by holding them in place with a suction gripper which was mounted on a

height-adjustable clamp, with the IR lamp placed above it. The proximity of the

sample to the lamp was varied by changing the height of the clamp.


55

3.5 References

1. (a) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., A glassy bending-

mode polymeric actuator which deforms in response to solvent polarity. Macromol.

Rapid Comm. 2006, 27 (16), 1323-1329; (b) Harris, K. D.; Bastiaansen, C. W. M.;

Broer, D. J., Physical properties of anisotropically swelling hydrogen-bonded

liquid crystal polymer actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c)

Ohm, C.; Brehmer, M.; Zentel, R., Liquid crystalline elastomers as actuators and

sensors. Adv. Mater. 2010, 22 (31), 3366-3387.

2. (a) Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. L.

M.; Fletcher, S. P.; Katsonis, N., Conversion of light into macroscopic helical

motion. Nat. Chem. 2014, 6 (3), 229-235; (b) Sawa, Y.; Urayama, K.; Takigawa,

T.; Gimenez-Pinto, V.; Mbanga, B. L.; Ye, F. F.; Selinger, J. V.; Selinger, R. L. B.,

Shape and chirality transitions in off-axis twist nematic elastomer ribbons. Phys.

Rev. E 2013, 88 (2), 022502; (c) Sawa, Y.; Ye, F. F.; Urayama, K.; Takigawa, T.;

Gimenez-Pinto, V.; Selinger, R. L. B.; Selinger, J. V., Shape selection of twist-

nematic-elastomer ribbons. P. Natl. Acad. Sci. USA 2011, 108 (16), 6364-6368; (d)

Wie, J. J.; Lee, K. M.; Smith, M. L.; Vaia, R. A.; White, T. J., Torsional

mechanical responses in azobenzene functionalized liquid crystalline polymer

networks. Soft Matter 2013, 9 (39), 9303-9310.

3. (a) Modes, C. D.; Bhattacharya, K.; Warner, M., Gaussian curvature from

flat elastica sheets. P. Roy. Soc. A-Math Phy. 2011, 467 (2128), 1121-1140; (b)

Modes, C. D.; Warner, M., Blueprinting nematic glass: Systematically constructing

and combining active points of curvature for emergent morphology. Phys. Rev. E

2011, 84 (2), 021711; (c) Modes, C. D.; Warner, M., The activated morphology of

grain boundaries in nematic solid sheets. P. Soc. Photo-Opt. Ins. 2012, 84 (2),

021711.

4. (a) Chen, Y. D.; Fuh, A. Y. G.; Liu, C. K.; Cheng, K. T., Radial liquid

crystal alignment based on circular rubbing of a substrate coated with poly(N-vinyl

carbazole) film. J. Phys. D-Appl. Phys. 2011, 44 (21), 215304; (b) Varghese, S.;


56

Narayanankutty, S.; Bastiaansen, C. W. M.; Crawford, G. P.; Broer, D. J.,

Patterned alignment of liquid crystals by µ-rubbing. Adv. Mater. 2004, 16 (18),

1600–1605.

5. (a) Bechtold, I. H.; Oliveira, E. A., Liquid-crystal alignment on anisotropic

homeotropic-planar patterned substrates. Mol. Cryst. Liq. Cryst. 2005, 442 (1),

41-49; (b) Chen, J.; Johnson, D. L.; Bos, P. J.; Sprunt, S.; Lando, J.; Mann, J. A.,

Radially and azimuthally oriented liquid crystal alignment patterns fabricated by

linearly polarized ultraviolet exposure process. Appl. Phys. Lett. 1996, 68 (7),

885-887; (c) Jeng, S.-C.; Hwang, S.-J.; Horng, J.-S.; Lin, K.-R., Electrically

switchable liquid crystal Fresnel lens using UV-modified alignment film. Opt.

Express 2010, 18 (25), 26325-26331; (d) Yaroshchuk, O.; Reznikov, Y.,

Photoalignment of liquid crystals: basics and current trends. J. Mater. Chem. 2012,

22 (2), 286-300.

6. (a) Kim, C.; Trajkovska, A.; Wallace, J. U.; Chen, S. H., New insight into

photoalignment of liquid crystals on coumarin-containing polymer films.

Macromolecules 2006, 39 (11), 3817-3823; (b) Schadt, M.; Schmitt, K.; Kozinkov,

V.; Chigrinov, V., Surface-induced parallel alignment of liquid-crystals by linearly

polymerized photopolymers. Jpn. J. Apl. Phys. 1992, 31 (7), 2155-2164.

7. (a) Broer, D. J.; Crawford, G. P.; Zumer, S., Cross-linked Liquid

Crystalline Systems. CRC, Taylor and Francis Group, Boca Raton, FL: 2011; (b)

Mol, G. N.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Thermo-mechanical

responses of liquid-crystal networks with a splayed molecular organization. Adv.

Funct. Mater. 2005, 15 (7), 1155-1159; (c) van Oosten, C. L.; Corbett, D.; Davies,

D.; Warner, M.; Bastiaansen, C. W. M.; Broer, D. J., Bending dynamics and

directionality reversal in liquid crystal network photoactuators. Macromolecules

2008, 41 (22), 8592-8596.

8. Liu, D. Q.; Bastiaansen, C. W. M.; den Toonder, J. M. J.; Broer, D. J.,

Photo-switchable surface topologies in chiral nematic coatings. Angew. Chem. Int.

Edit. 2012, 51 (4), 892-896.


57

9. McConney, M. E.; Martinez, A.; Tondiglia, V. P.; Lee, K. M.; Langley, D.;

Smalyukh, II; White, T. J., Topography from topology: photoinduced surface

features generated in liquid crystal polymer networks. Adv. Mater. 2013, 25 (41),

5880-5885.

This chapter was based on: de Haan, L. T.; Gimenez-Pinto, V.; Konya, A.; Nguyen, T. S.; Verjans, J. M. N.; Sanchez-Somolinos, C.; Selinger, J. V.; Selinger, R. L. B.; Broer, D. J.; Schenning, A. P. H. J., Accordion- like Actuators of Multiple 3D Patterned Liquid Crystal Polymer Films. Adv. Funct. Mater 2014, 24 (9), 1251-1258; Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J., Mechanical frustration and spontaneous polygonal folding in active nematic sheets. Phys. Rev. E. 2012, 86 (6), 060701

59

Chapter 4. Folding actuators with 3-dimensional

alignment patterns

4.1 Introduction

As shown in the introduction chapter, most three-dimensionally morphing liquid

crystal polymer films which are reported in literature bend or curl upon application

of a stimulus because they possess a molecular director that varies along the

z-direction: the twisted nematic and splayed configurations.1 Chapter 3 describes

more exotic deformations of liquid crystal polymer films as a result of a

disclination-based director profile, which is patterned in the xy-plane of the film

but uniform through the sample thickness, and thus essentially 2-dimensional. The

next step towards even more complex deformation is to combine these two profiles

to impose a 3-dimensional director profile on the film, with both variation along

the z-direction and in the xy-plane of the polymer film. This can be achieved by

clever design of the alignment cells, which should be similar to the cells as

described in the previous chapter in that they have a patterned alignment profile,

but have orthogonal alignment directions between the two substrates to force the

liquid crystals into a twisted nematic alignment, eventually supported by small

concentrations of chiral material.2

Initially, a striped pattern with a twisted nematic director profile which has an

orthogonal alignment director between domains was prepared, and was shown to

Chapter 4 Folding actuators with 3-dimensional alignment patterns

60

deform into an accordion-like shape upon heating. This pattern was also used in

combination with water-responsive liquid crystalline systems which showed the

same deformation behavior in a high-pH environment.3 A similarly aligned

chessboard pattern was prepared as well, which showed deformation similar to the

film with multiple m = +1 disclinations as presented in chapter 3. Finite-element

analysis reproduced the deformation behavior of the films. Finally, a circular piece

of film with a continuous circular pattern and a twisted nematic alignment was

prepared and actuated, and was shown to fold into a square shape.


4.2.1 Alignment pattern design

The first design was an actuator consisting of a ribbon with a nematic director

which was patterned to have four discrete horizontal stripes with a twisted nematic

director of alternating orientation (Figure 4.1a). Stripes 1 and 3 have the director at

z = 0 oriented along the film’s short axis, while at z = h the director was oriented

along the film’s long axis. Stripes 2 and 4 have the director at z = 0 oriented along

the film’s long axis and oriented along the film’s short axis at z = h. The twist in all

stripes is 90 degrees and has uniform handedness.

If the domains would not be joined together, upon decreasing the order parameter,

each domain would initially deform into a saddle shape, followed by repression of

bending in one direction. Each domain is inverted with respect to its neighbors, and

they will bend in opposite directions. As the domains are joined together, this

causes a conflict to arise at the border. Therefore, it is expected that bending in that

direction is repressed, regardless of the dimensions of the domains, and only

bending along the long axis of the film takes place. The final shape is expected to

have four bends, in alternating directions, and will contract along its long axis in a


61

way similar to an iconic musical instrument, the accordion. This kind of

deformation is potentially interesting, as it achieves a large lateral displacement

between the endpoints of the film with only small strains.

4.2.2 Preparation and optical characterization of the polymer films

The alignment cells were prepared using photoalignment. However, instead of

irradiating a complete cell as described in the previous chapter, the two plates were

irradiated separately, and then an alignment cell was constructed in such a way that

the alignment of the opposing substrates was orthogonal. To make a striped pattern,

a striped photomask was used in a two-step procedure: one exposure was carried

out through the photomask, and then the substrate was rotated 180 degrees to

Figure 4.1 a) Alignment pattern of the accordion-shaped actuator, top view and side view. Arrows

indicate the contraction and expansion directions along the length of the film. Each domain is 5 mm

wide and 7.5 mm long. b) Two-step procedure for the preparation of the alignment layers. The black

arrows indicate the polarization direction of the UV light. Before the second exposure step the

polarization of the light is rotated 90 degrees while the sample is rotated 180 degrees. c) Liquid

crystal mixture used to make a heat responsive actuator (RM82:RM105:RM23 = 2:3:1), including an

IR absorber (Lumogen 788 IR), a radical initiator, and a chiral dopant (RM756). d) Optical analysis

of a four-lane alternating twisted nematic actuator. The film was analyzed between crossed polarizers

(left) and parallel polarizers (middle). To show the difference in orientation between the lanes, it was

also viewed between crossed polarizers with a quarter wave plate on top of the sample in two

different orientations (right).


62

expose the non-irradiated parts, and the polarization direction was rotated 90

degrees (Figure 4.1b). A cell was made, and the liquid crystal mixture (Figure 4.1c)

was then fed into the cell and cured, using the same procedure and liquid crystal

mixture that was used to prepare the low-crosslink films for the examination of

deformation as described in chapter 3, including the IR absorber.

After retrieving the film from the alignment cell, it was optically analyzed by

placing it between linear polarizers (Figure 4.1d). Twisted nematic liquid crystals

rotate the polarization direction of linearly polarized light by 90 degrees, causing

the film to be bright between crossed polarizers and dark between parallel

polarizers. Furthermore, when the film was placed between crossed polarizers with

a quarter wave plate on top of the sample at a 45 degrees angle, a difference in

brightness between the domains was observed. This is explained by the slow axis

of the wave plate being either parallel or perpendicular to the mid-plane director of

the nematic twist, which adds to the optical retardation or compensates for it,

respectively. The sample was more closely investigated using polarized optical

microscopy, and was found to contain no significant defects aside from the narrow

lines between the patterned domains, which are caused by a slight mismatch of the

alignment layers. These lines are expected not to influence the deformation

behavior of the actuators, as they are positioned in the direction in which bending

will be repressed, and because they represent a negligible fraction of the total

surface area. Surface profilometry and interferometry were used to measure the

final thickness of the film, which was found to range from 12 μm to 19 μm

between multiple samples.

4.2.3 Actuation of striped twisted nematic films

To perform the actuation experiments, a strip of 30 x 5 mm was cut from the

freestanding film with four alternating stripes, which resulted in an aspect ratio of

roughly 1650 : 275 : 1. The film was clamped on its short end and actuated through


63

heating by exposure to an IR source. It should be noted that the film was already

slightly bent before heating was applied, which was probably caused by stresses

induced during the photopolymerization of the material at elevated temperatures in

the flat state. Upon cooling down to room temperature, the anisotropy of the

system increases, which may have caused the material to deform. Upon heating the

vertically positioned film, the four areas of the film deformed by bending in

opposite directions. A four-bend accordion shape (Figure 4.2a) was clearly

observed, although the deformation of the area closest to the clamp was somewhat

suppressed. The deformation reversed upon switching off the IR source, and the

speed and magnitude of the deformation could be influenced by varying the

intensity of the IR irradiation.

The experiment was repeated for a 12-stripe film of the same dimensions. This film

was both actuated while being held vertically in front of the IR source (Figure

4.2b) and while lying horizontally on a glass surface (Figure 4.2c). Again, the

Figure 4.2 a) Deformations of the actuator (aspect ratio 1650 : 275 :1) upon

irradiation with an IR source for time t in seconds after switching on the light. b)

Deformation of a 12-lane patterned film, while being held vertically in front of the

IR source. c) Deformation of a 12-lane patterned film, while lying horizontally on a

surface. d) Fully contracted “teardrop” state of the twelve-lane polymer actuator. In

the bottom picture, a line drawing of the teardrop folds is overlaid on a section of

the photograph for increased clarity.


64

accordion shape was observed with the correct number of bends, but the amplitude

was smaller compared to the four-stripe actuator. Interestingly, upon heating the

film on the surface, sharp folds were observed (Figure 4.2c at t = 38 seconds). One

explanation for this is that due to the tips of the folds receiving more direct IR

irradiation, these areas have a higher temperature and bend more sharply. However,

the effect was not observed in the vertical experiments, and might also be caused

by friction between the sample and the glass substrate. In addition to the expected

shape change, a secondary deformation was observed in both experiments

involving the twelve-stripe actuator. In the vertical experiment, a right-handed

twist over the entire length of the film was observed (Figure 4.2b), while in the

horizontal experiment a sideways bending deformation took place. These two

observations are probably different manifestations of the same effect, which might

be related to the twist handedness of the twisted nematic moieties. Finally, it was

found that if the actuator was heated above a certain point it showed a strong

contraction, and the folds obtained a "teardrop" shape, resulting in an overall shape

not unlike a so-called “ruff collar”4 (Figure 4.2d). No further deformation was

observed after this shape was obtained. In this state, the folds would touch and

stick together, so the film did not revert back upon cooling unless the folds were

separated by hand.

Using the same method, a 2-stripe actuator film was prepared. From this film a

strip was cut at 45 degrees angle (25 mm x 5 mm, aspect ratio ~1300 : 260 : 1) to

the long axis of the stripes (Figure 4.3a). When this film was exposed to IR

irradiation, it formed a coiled shaped consisting of a left-handed coil and a right-

Figure 4.3 a) Cutting method for an alternating twisted nematic

coiling actuator. b) Actuation of the alternating twisted nematic

coiling actuator.


65

handed coil (Figure 4.3b). As with the previous actuators, the coil closest to the

clamping position was slightly suppressed.

4.2.4 Water-responsive actuators

To show that this exotic type of deformation can also be induced by other stimuli,

an actuator was prepared which responds to changes in the pH of an aqueous

environment. To this end, the reactive mesogen mixture with internal hydrogen

bonds presented in chapter 2 (Figure 4.4a) was used to make the liquid crystal

network.3 Upon immersion of such a film in a basic solution, the internal hydrogen

bonds are converted into a carboxylic salt. This causes an anisotropic swelling,

having a larger expansion in the direction perpendicular to the alignment director

Figure 4.4 a) Liquid crystal mixture used to make a pH responsive polymer actuator

(3OBA:5OBA:6OBA = 1:1:1, RM82 12 w%), including the initiator and chiral

dopant. b) Deformation of a pH responsive alternating twisted nematic actuator.

The film is flat at pH 6, and obtains the accordion shape when the pH is increased

to 11. When the pH is then reduced to 3, the flat shape is recovered. c) Normalized

FTIR spectra of the pH responsive film at various stages during the experiment. The

peak at 1682 cm−1 correspond to the C=O stretching in hydrogen bonded dimers,

while the peaks at 1547 and 1395 cm−1 are from antisymmetric and symmetric

COO− stretching, respectively.2


66

and a smaller expansion parallel to it. A ribbon of freestanding film with four

domains of alternating twisted alignment, prepared using the same alignment cells

as used before (Figure 4.1b), was immersed in demineralized water, with a 40 mg

metal weight attached to the far end to keep the film from floating to the surface or

crumpling up against the clamp. (Figure 4.4b). The pH of the medium was

monitored using a pH meter, and was determined to be 6.1 for demineralized water.

Also, the FTIR spectrum of the material was recorded prior to the experiment to

keep track of the presence of the internal hydrogen bonds (Figure 4.4c). A 0.1 M

KOH solution was added to increase the pH of the environment to 11.4. At this pH

the internal hydrogen bonds of the network were converted to the corresponding

carboxylic potassium salt, which was confirmed in the FTIR spectrum. The

induced hygroscopic behavior in the material caused it to take up water from the

aqueous medium, which in turn caused a decrease in anisotropy, and an accordion

shape was observed (Figure 4.4b). As observed before, the bends close to the

clamped end were somewhat suppressed, but the middle regions were bending as

programmed. After being exposed to the basic solution, the curvature of the

bending domains no longer increased, and the activation process was considered

complete. At this stage, four bends could be identified, which corresponded with

the four alignment domains. This bending is less pronounced when compared to

the heat responsive actuators due to the presence a weight on one end of the film.

When in a next step the pH was decreased to 2.3, the hydrogen bonds were

recovered and the FTIR spectrum reverted back to show the original peaks. As

expected, the film returned to its original flat shape.

4.2.5 Finite-element analysis

To examine the mechanism driving these shape transformations, a finite element

elastodynamics analysis was carried out by the group of Prof. Robin Selinger at

Kent state University. Thin film samples were meshed in three dimensions, rather

than treating them as having infinitesimal thickness, which was commonly done in


67

earlier simulations to reduce the demands on computing power. The Hamiltonian-

based model used includes the system’s kinetic energy, elastic energy, and

coupling between strain and nematic order. The scalar order parameter at the time

the material is cross-linked is defined by the parameter So. As the temperature

increases, the nematic order parameter S decreases, inducing a contraction of the

sample along the director and an elongation in the two directions perpendicular to

it. It was assumed that the director field is strongly anchored by cross-linking of the

polymer network such that it rotates with the body but does not otherwise reorient

in response to strain. This is the characteristic behavior of a nematic glass, and is a

reasonable approximation for this experimental system below its glass transition

temperature and where there is no external applied mechanical stress.

To represent a change of temperature, the scalar order parameter S was gradually

raised (to represent cooling) or lowered (to represent heating) and was then held

fixed until the sample reached elastic equilibrium. Finite element simulations of

Figure 4.5 Finite element simulation of the shape deformation of a nematic

network with alternating twisted domains when its nematic order decreases on

heating. For stripe width/thickness ratio in the range of 4–25, smooth, sinusoidal

deformation was observed; a) four stripes (ratio = 12.5) and b) eight stripes (ratio

= 6.25). c) Amplitude of resulting undulations increases as a function of stripe

width/thickness ratio; solid line represents the analytical prediction and square

points are simulation data. d) Simulation of a longer sample with stripe

width/thickness ratio = 50 and a larger temperature change (∆S = –0.8) shows

formation of teardrop-shaped accordion folds.


68

samples with aspect ratio 50 : 10 : 1 and with stripe widths of different sizes are

shown (Figure 4.5a-b), starting with a sample with four stripes of alternating twist

orientation (Figure 4.1a and Figure 4.5a). As temperature increased, formation of

undulations with two crests and two valleys was observed. The handedness of the

director twist induced a slight chiral bend within each stripe; resulting in a small

zigzag deformation along the sample borders. The simulation was repeated for a

sample of the same aspect ratio, divided into 8 stripes (Figure 4.5b). Simulations

were carried out for samples with 2 to 12 stripes, keeping the sample aspect ratio

constant, and it was found that larger stripe width drives larger undulation

amplitude (Figure 4.5c). This is consistent with the experimental finding that a

4-stripe actuator shows larger amplitude compared to a 12-stripe actuator. For a

large enough stripe width to thickness ratio, a transition from a sinusoidal

undulation to the formation of teardrop-shaped accordion folds was observed

(Figure 4.5d). The central axis of the ribbon formed approximately circular arcs of

alternating orientation. Finite element simulation also predicted that the ribbon

flexes across its short axis, with orientation alternating along each circular arc. A

slight out-of-plane twist, similar to that seen in the practical experiments, was also

observed (as seen in Figure 4.2b).

Using a mathematical model, the same group also estimated the amount of work

these actuators can output. For the experimental sample, the dimensions were set to

30 mm × 5 mm × 15 μm and the Young’s modulus was set to 1 GPa (below the

glass transition). The maximum work that can be done by contraction was

estimated to be approximately 3 × 10 −6 J. For comparison, the mass of the sample

is approximately 3 × 10 −6 kg, so Wmax is about three times the work needed to lift

its own mass by its own length.


69

4.2.6 Checkerboard pattern

To extend the fabrication method to an even more complex deformation, a film was

prepared with two orthogonally superimposed stripe masks to make a checkerboard

pattern. Four irradiation steps were required, while rotating the substrate 90

degrees after each step, to expose the whole substrate area (Figure 4.6a). Again, an

alignment cell was prepared and filled with the mixture of reactive mesogens

(Figure 4.1b), which was cured through UV irradiation, producing a director

pattern with twisted domains (Figure 4.6a). After removal of the film from the cell

(23 x 23 mm, aspect ratio roughly 1810 : 1810 : 1), the actuation experiment was

carried out for the checkerboard film (Figure 4.6b). Upon heating, the film

deformed from a mostly flat shape to form a periodic square pattern of peaks,

depressions, and saddle points. Like the 12-stripe accordion, a deformation over the

entire surface of the film was observed. The corners of the film bent inwards to

give the film an overall dome-shaped structure.

4.2.7 Finite element analysis of checkerboard pattern

To gain an understanding of this actuation pattern, a finite element analysis

simulation of a 3 x 3 checkerboard actuator was performed by the group of Prof.

Robin Selinger, showing shape evolution with increasing temperature (Figure 4.7).

A mesh with aspect ratio of 50 : 50 : 1 was used. The film distorted into an

Figure 4.6 a) Preparation procedure of the checkerboard alignment layers, and top view of the

final pattern (only 4×4 squares shown) b) Deformation upon IR heating of a film with a

checkerboard pattern (aspect ratio 1810 : 1810 : 1).


70

arrangement of peaks, depressions, and saddle points corresponding with the

square domains. The simulation also shows an overall dome-shape superposed over

the smaller wavelength undulation, in agreement with experimental findings.

4.2.8 Radimuthal films

In a similar way, a heat-responsive network with a continuous pattern in the

xy-plane and a 90 degrees twist along the z-direction was prepared. To do so, an

alignment cell was constructed from two glass substrates, one having a radial

alignment layer and the other one an azimuthal alignment layer. The resulting

polymer film obtained from this cell possessed a liquid crystal configuration with a

gradual transition from azimuthal to radial through the thickness of the film, which

can be described as a circular twisted nematic alignment, and was termed the

“radimuthal” alignment. It is patterned in all three dimensions, as it has both a

circular director variation in the xy-plane and a gradual variation along the

z-direction (Figure 4.8a). It showed the optical properties between parallel and

crossed polarizers that can be expected for twisted nematic systems, namely full

transmission between crossed polarizers and partial transmission between

perpendicular polarizers (Figure 4.8b).

When a radimuthal film was heated, it deformed by folding four of its outside flaps

inward to form a square shape (Figure 4.8c). This behavior can be explained as

follows: when heated, the film experienced forces similar to the twisted nematic

films discussed in the introduction, and tries to deform into a 3-dimensional shape.

Figure 4.7 a) Top view of the director microstructure of a 3 × 3 checkerboard

actuator. b) Finite element simulation of the 3 × 3 checkerboard actuator when

heated to the isotropic state.


71

However, suppression of curvature by bending in a single direction is not possible

in this system due to geometric constraints. This can be envisioned by dividing the

film into a number of angular sections that behave as twisted nematic actuators and

fold towards the side with radial alignment (Figure 4.8d). However, due to these

domains being joined together, bending can only take place in the outer areas of the

domains. The film therefore deforms into a polygon with curled edges and a flat

center. The number of domains, and as such the number of edges of the polygon, is

determined by a tradeoff between the total amount of bending surface and the

deviation from the preferred bending direction. When the number of domains is

small, more surface area is allowed to bend to release the stress, but as the domains

are larger the bending direction close to the borders of the domains deviates more

strongly from the preferred bending direction (Figure 4.8e). The optimal shape can

be found by minimizing the internal energy of the system (Figure 4.8f), and was

found to have four domains, which corresponds to a square shape.

Figure 4.8 a) Schematic representation of the “radimuthal” alignment profile, including cross sections

at z = 0, z = 0.5h, and z = h. b) The actual film, as seen between crossed polarizers (left) and parallel

polarizers (right). c) Deformation behavior of the film on a hot plate, as well as a schematic drawing of

the deformation. The film folds four flaps inward to form shape resembling a square. d) An angular

section of the film, with the alignment directors and the preferred bending direction. e) Division of a

circular film into four and five sections. The areas that are not joined, and can therefore bend, are

shown in gray. f) Internal energy of the system (U) as a function of the number of sections (mp). The

energy minimizes at mp = 4.


72


A new family of actuators with a 3D-patterned director profile that show exotic

deformations was prepared and analyzed. The results show that exciting new

deformations in liquid crystal networks can be achieved by 3D structuring. This

could eventually lead to new ways to perform self-folding in plastic materials.

Striped actuators showed accordion-like deformations upon exposure to an

appropriate stimulus, which, depending on the chosen chemical composition of the

mixture of reactive mesogens, could either be a temperature or a pH variation. This

kind of out-of-plane actuation achieves a larger total displacement for a given

temperature change than could be accomplished by contracting a monodomain

sample of the same material with uniform nematic director. It was also shown that

these deformations can be predicted with a mathematical simulation based on finite

element analysis. Such actuators are potentially useful in applications where large,

complex displacements are valued above strength and power.

4.4 Experimental Section

Materials: All materials were used as received from Merck (RM82, RM105,

RM23), Philips Research Organics (3-OBA, 6-OBA), Synthon Chemicals

(5-OBA), BASF (Lumogen 788 IR, LC756), Ciba Specialty Chemicals (Irgacure

819), Rolic (ROP-108), and Aldrich (t-butyl hydroquinone, dichloromethane). The

alignment cells were prepared using 18 micron SP-218 Micropearl plastic spacers

(Sekisui Chemical Co.).[25]

Layers of the photoalignment material (ROP-108) were spincoated onto glass

substrates using a Karl Suss RC8 spincoater. Photoalignment was carried out using

an Ominicure S2000 UV lamp. A Newport 10LP-UV precision linear polarizer,

mounted in a Thorlabs PRM1/MZ8 rotation mount, was used to polarize the UV

light in various directions, and the substrate was rotated using a Thorlabs CR1/M-

Z7 rotation stage (irradiation intensity ~13 mW cm-2 in the UVA range). The


73

rotation of the polarizer and stage was controlled with Thorlabs TDC001

controllers, which were programmed with APT software. Photopolymerization of

the liquid crystal mixtures was carried out using an Omnicure S2000 UV lamp with

320-500 nm filter (~25 mW cm-2). To remove the alignment material from the

surface of the pH-responsive actuators, an Emitech K1050X plasma asher was

used.

The thickness of the actuator films was measured using both a Fogale nanotech

zoomsurf 3D interferometer and a Veeco Dektak 150 surface profilometer. Slide-

on ATR IR spectra of the pH-responsive films were recorded using a Varian 670

FTIR spectrometer. For the IR actuation experiments, a Heraeus Noblelight

Shortwave surface irradiator (type “Duo”, 250 Watt, 57.5 Volt) with gold reflector

was used. During the pH experiments, the pH of the solution was monitored using

an inoLab pH meter.

Preparation of alternating twisted alignment cells: 3x3 cm glass plates were

cleaned with ethanol. ROP-108 was spincoated onto the plates at 1500 RPM for 30

seconds. The plates were heated briefly to 115 oC to evaporate the solvent.

Irradiation of each LPP substrate was carried out through a photomask with

polarized UV light for 50 seconds. When a 4- or 12-stripe pattern was prepared,

two irradiations were carried out on each substrate. After the initial irradiation step,

the substrate was rotated 180 degrees to expose the non-irradiated areas, while the

polarizer was rotated 90 degrees, and a second irradiation was carried out. When a

chessboard pattern was prepared, the substrates were irradiated four times and

rotated 90 degrees between each irradiation step. In all cases, the plates were glued

into cells, using 18 micron plastic spacers to secure the cell gap. Care was taken to

glue the cells in an orthogonal fashion to obtain cells that force the LC material

into a twisted nematic conformation.

Preparation of “radimuthal” alignment cells: These cells were prepared in the same

way as the alternating twisted alignment cells, except that the plates were irradiated


74

through a wedge-shaped mask on a rotating substrate (as described in the previous

chapter).

Preparation of heat-responsive, patterned, twisted nematic actuators: RM82 (201.1

mg), RM105 (302.2 mg), and RM23 (101.5 mg) were put into a screw-cap vial. A

1.5 mg ml-1 solution of the thermal inhibitor tert-butyl hydroquinone and the chiral

dopant LC756 in CH2Cl2 was added (~ 90 μL), as well as the radical initiator

Irgacure 819 (3.3 mg) and the IR absorber Lumogen 788 IR (0.8 mg). The

components were dissolved in CH2Cl2, which was then evaporated by stirring the

solution at 80 oC. An alignment cell was brought to 80 oC, and the cell gap was

filled with the mixture though capillary action. When the cell was completely

filled, it was cooled down slowly to 60 oC. Then UV polymerization took place in

the nematic phase by UV irradiation for 5 minutes. Finally, the cell was opened to

retrieve the freestanding polymer film. The film thickness was measured using

surface profilometry and interferometry.

Preparation of pH-responsive, alternating twisted nematic actuators: 3-OBA (99.5

mg), 5-OBA (99.8 mg), 6-OBA (99.9 mg), RM82 (42.2 mg), and Irgacure 819 (6.9

mg) were put into a screw-cap vial. Tert-butyl hydroquinone (0.071 mg) and

LC756 (0,35 mg) were added as a solution in CH2Cl2. The components were

dissolved in CH2Cl2, the resulting solution was stirred while heating with a heat

gun, and the solution was vortex-stirred for 5 minutes. The solvent was removed by

heating the solution to 30 oC under argon flow, followed by drying in vacuo for one

hour. An alignment cell was brought to 90 oC, and the cell gap was filled with the

mixture though capillary action. When the cell was completely filled, it was cooled

down slowly to 80 oC. Then UV polymerization took place by UV irradiation for 5

minutes. Finally, the cell was opened to retrieve the freestanding film.

Investigation of IR actuation: Strips were cut from the alternating twisted nematic

films. The following dimensions were used: 4-stripe and 12- stripe actuator:

30 mm x 5 mm, chessboard actuator: 23 mm x 23 mm. The films were actuated


75

either in the vertical or horizontal position. When actuating in the vertical position,

they were held with a tweezers in front of the IR lamp until actuation took place.

When horizontal actuation was required, the actuator was placed on a glass slide

which was covered with sand to reduce friction. The material was then irradiated

from above.

Investigation of pH actuation: The film was subjected to oxygen plasma ashing for

4 minutes at 45 Watt to remove any remaining alignment material from the surface.

An IR spectrum of the film was measured, and a 40 mg weight was attached to one

end. The film was immersed in demineralized water, which was brought to pH 11.4

by slowly adding a 0.1 M KOH solution, and kept at that level for 17 minutes. The

film was removed from the medium, dried, and an IR spectrum was recorded. The

film was placed back, and the pH was decreased to 2.3 by the slow addition of a

1 M HCl solution. The film was removed from the medium, dried, and an IR

spectrum was recorded.

Finite Element Elastodynamics Simulation: In order to implement finite element

elastodynamic simulations, each thin film sample was discretized into an

unstructured 3-D tetrahedral mesh. Meshes were generated using Salome, an open

source tool downloaded from http://www.salome-platform.org. A mesh of aspect

ratio 50-10-1 with 66,647 tetrahedra and 14,274 nodes was used. In figure 4d a

mesh of aspect ratio 200-10-1 with 270,549 tetrahedra and 52,850 nodes was used.

The sample stripe width/thickness ratio in the experimental studies was

approximately 130. However due to hardware limitations, the largest stripe

width/thickness achievable in the simulations was 50, considering the size of the

required mesh. This ratio was sufficiently high to drive formation of teardrop-

shaped accordion folds. The checker board actuator was modeled using a mesh of

aspect ratio 50-50-1 with 268246 tetrahedra and 52780 nodes.


76

4.5 References

1. (a) Mol, G. N.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Thermo-

mechanical responses of liquid-crystal networks with a splayed molecular

organization. Adv. Funct. Mater. 2005, 15 (7), 1155-1159; (b) Warner, M.; Modes,

C. D.; Corbett, D., Curvature in nematic elastica responding to light and heat. P.

Roy. Soc. A-Math. Phy. 2010, 466 (2122), 2975-2989; (c) Warner, M.; Modes, C.

D.; Corbett, D., Suppression of curvature in nematic elastica. P. Roy. Soc. A-Math.

Phy. 2010, 466 (2124), 3561-3578.

2. Lee, K. M.; Bunning, T. J.; White, T. J., Autonomous, hands-free shape

memory in glassy, liquid crystalline polymer networks. Adv. Mater. 2012, 24 (21),

2839-2843.

3. (a) Broer, D. J.; Bastiaansen, C. M. W.; Debije, M. G.; Schenning, A. P. H.

J., Functional organic materials based on polymerized liquid-crystal monomers:

Supramolecular hydrogen-bonded systems. Angew. Chem. Int. Edit. 2012, 51 (29),

7102-7109; (b) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J., Physical

properties of anisotropically swelling hydrogen-bonded liquid crystal polymer

actuators. J. Microelectromech. S. 2007, 16 (2), 480-488; (c) Harris, K. D.;

Bastiaansen, C. W. M.; Lub, J.; Broer, D. J., Self-assembled polymer films for

controlled agent-driven motion. Nano Lett. 2005, 5 (9), 1857-1860.

4. Muller, M. M.; Ben Amar, M.; Guven, J., Conical defects in growing

sheets. Phys. Rev.Lett. 2008, 101 (15), 156104.

77

Chapter 5. Generating wrinkling patterns in

metal/LC-polymer bilayer films

5.1 Introduction

This chapter describes a way in which liquid crystal polymer films with a patterned

director profile in the xy-plane can be used to prepare complex microstructures. In

this case, no use is made of the anisotropic expansion and contraction of the film to

create exotic actuators; instead, the anisotropy of the modulus is used to control the

formation of surface wrinkles in a bilayer system consisting of a liquid crystal

network and a thin layer of gold.

Over the past decade, the elastic instability of thin sheets, such as surface

wrinkling, has gained significant interest from the scientific community.1

Controlled wrinkle formation can provide a quick and easy method for the

spontaneous generation of a variety of microstructured surfaces. Wrinkling can be

induced in bilayer systems consisting of a thin film of a relatively stiff material on

top of a foundation of a softer material as a result of a strain mismatch between the

layers, usually induced by mechanical or heat-driven contraction of the foundation

or expansion of the top layer. It is a well-studied and well-understood process.2

Many applications for wrinkled surfaces have been shown, including preparation of

superhydrophobic3 and antifouling surfaces,4 actuators,5 generation of strain-related

colors,6 alignment of living cells,7 optical focusing,8 fabrication of microlens

arrays,9 fabrication of patterned electrodes,10 stretchable conductors,11 particle

separation,12 and strain-sensitive diffraction gratings.13 Achieving precise control

over the wrinkling properties is vital for the further development of these

applications. While the amplitude and period of the wrinkles can be easily tuned by

changing the thickness of the thin top layer,14 and the preparation of

Chapter 5 Generating wrinkling patterns in metal/LC-polymer bilayer films

78

homogeneously wrinkled surfaces is rather straightforward,2c, 13b, 14a, 15 it has proven to

be far more challenging to generate complex patterns in the wrinkle direction.

There are many examples of research where some control over the wrinkle

direction was obtained, for example by introduction of plateaus and ridges,2a local

photostiffening of the foundation,16 creating areas of reduced adhesion in the

substrate,17 two-step plasma exposure,18 and controlled diffusion of a solvent.19

However, these techniques are restricted to a limited number of patterns that can be

produced, often apply only to small surface areas, and may require extensive and

minute local surface modifications. Arbitrary wrinkling patterns have also been

prepared using either patterned PDMS molds, or path-guided wrinkling by locally

modifying the modulus of the top layer with a laser.19a However, this requires direct

“writing” of the desired pattern instead of allowing the patterns to form

spontaneously, which in a way defeats their purpose as a quick access to a large

variety of microstructured surfaces. This chapter describes a new solution to this

challenge: a simple, contactless method for the spontaneous generation of complex

wrinkle patterns in a bilayer system consisting of a thin layer of gold on an LC

polymer network foundation. The wrinkling is driven by stresses which are induced

during the deposition of the gold layer, which can be released by heating the

system to lower the modulus of the foundation. Wrinkling will take place in the

direction of the lowest foundation modulus, which is perpendicular to the

alignment director. Therefore, the wrinkles will always follow the alignment

director (Figure 5.1a). This allows patterning of the wrinkle direction using

photoalignment.


79


5.2.1 Sample preparation and wrinkle formation

Photoalignment of the polymerizable LC was carried out in the same way as

described in chapters 3 and 4. Alignment cells were prepared by combining two

glass plates coated with a LPP (ROP-108), which was photoaligned by exposure to

polarized ultraviolet light through a variety of photomasks (Figure 5.1b and 5.1c).

The cells were filled with the same mixture of polymerizable liquid crystals that

was previously used to make actuator films (Figure 5.1d). Filling of the cells was

Figure 5.1 a) Mechanism of wrinkle formation. Heating causes the liquid crystal

network to go through the glass transition, which allows release of the stress in the gold

layer through wrinkling, which takes place in the direction with the lowest modulus. b)

Photoalignment setup used for the preparation of discrete patterns. c) Photoalignment

setup used for the preparation of continuous circular patterns. It consists of a UV lamp,

a polarizer, a photomask, and the substrate bearing the photoalignment layer. d)

Polymerizable liquid crystals that were used. e) Preparation procedure to make the

wrinkle patterns out of the patterned alignment cells.


80

carried out at 80 oC in the isotropic phase, and photopolymerization was carried out

at 60 oC in the nematic phase. The cell was opened by removing one of the glass

plates, and a gold layer was sputter coated on top of the solid LC film (Figure

5.1e). Putting the material on a hotplate at 60 oC then caused wrinkles to appear in

a few minutes, which could be readily detected as a transition from a reflective to a

diffractive surface. The presence of the wrinkles was confirmed through optical

microscopy (Figure 5.2a), and it was clear that the wrinkles always follow the

alignment director of the underlying liquid crystal network. This is in contrast to

previous research using a crosslinked liquid crystal network as the foundation,

where the wrinkles always ran perpendicular to the alignment director when

formed upon heating,20 or parallel to the director when formed upon cooling.14b, 20a, 20b

In these latter cases, the anisotropic contraction of the liquid crystal network, which

takes place parallel to the director upon heating or perpendicular upon cooling, was

responsible for the formation of wrinkles.

To test if anisotropic deformation in the liquid crystal network was required for

wrinkle formation in our samples, an isotropic liquid crystal network was prepared

by polymerization at a temperature above the nematic-to-isotropic transition. When

a gold layer was applied to this network, wrinkling still took place after heating,

but the wrinkles were randomly oriented (Figure 5.2b). These random wrinkles

were similar to the aligned wrinkles in period and amplitude (see also Figure 5.4e),

which indicates that their formation is strictly caused by the changing materials

properties during heating, and the anisotropy only serves to steer the wrinkling in a

specific direction. This means that compressive stress induced in the system during

the sputter coating process21 is the driving force for wrinkling. At room

temperature, this stress δ is below the critical stress δc required to induce wrinkling

of the gold film according to , where Ep and Em are the elastic

moduli of the polymer layer and the metal film, respectively.22 This creates a

metastable state in which wrinkling does not yet take place. Upon heating the

liquid crystal network foundation above the glass transition temperature, the


81

modulus Ep decreases, lowering the critical stress below the compressive stress and

allowing wrinkling to occur. The modulus of the LC network is anisotropic, with

the lowest modulus perpendicular to the alignment director. This means that δc is

lower in that direction, dictating that the film will buckle perpendicular to the

alignment director, and as a result the wrinkles will always follow the alignment

director (Figure 5.1a). Once grown, these parallel wrinkles rigidify the structure

and prevent the formation of wrinkles along another direction, an effect that is

reminiscent of the high rigidity observed for corrugated cardboards.

Figure 5.2 a) Optical microscopy image of a wrinkled surface. b) Optical microscopy

picture of a wrinkled surface on an isotropic liquid crystal foundation, showing

randomly oriented wrinkles. c) Typical surface profile of the wrinkled gold surface. d)

Zoom-out microscopy picture of the second order, larger wrinkles in which the smaller

wrinkles are nested. e) The same surface profile, viewed over a longer distance to show

the second-order wrinkles. f) AFM image of the wrinkled surface.


82

5.2.2 Measuring the wrinkle period and amplitude

The wrinkled surfaces were further studied using surface profilometry: a typical

profile, viewed over a 15 μm distance, is shown in Figure 5.2c. Surprisingly, when

viewed over longer distances it became clear that the profiles show a strong

periodic variation in peak height, forming what seems to be a second-order

hierarchically wrinkled structure (Figure 5.2d and 5.2e). Hierarchical wrinkling has

been previously reported in literature, and takes place when the amplitude of the

wrinkles reaches a maximum, causing the wrinkled layer to act as a much thicker

“effective layer” which then wrinkles with a much longer period.12 However, in this

case the smaller wave has varying amplitude, which is not expected for hierarchical

Figure 5.3 a) Plot of the measured average wrinkle period as a function of

sputtered gold layer thickness as estimated from standard sputter coater

deposition rate. The pattern was measured over a 400 μm length, then the

average distance between each peak and valley was calculated. For each

sample 3-4 measurements were carried out. b) Average wrinkle amplitude as a

function of average wrinkle period. The pattern was measured over a 400 μm

length, then the average distance between each peak and valley was

calculated. For each sample 3-4 measurements were carried out. c) Images

taken of samples using the profilometer with (left) 55 nm and (right) 225 nm

sputtered gold layers.


83

wrinkling. This implies that it is an oscillating amplitude instead. It is not clear why

the amplitude oscillates. AFM was used to confirm the profilometry measurements,

and showed a wrinkled surface with similar characteristics (Figure 5.2f).

A series of films with varying gold layer thicknesses was prepared by varying the

sputter time during the application of the gold. It was observed that the wrinkle

amplitude and period increased with thicker gold layers (Figure 5.3a and 5.3b),

which was expected due to the relations ~ , where λw is the wrinkle

period and h is the thickness of the metal film, and ~ , where A is the

wrinkle amplitude. From these relations, it is expected that the wrinkle period

increases linearly with gold layer thickness, and the wrinkle amplitude increases

linearly with increasing period.22 However, at higher values the error in the

measurements increased substantially, so it was difficult to verify this. It was also

observed that when thicker gold layers were used, the surface became more

irregular (Figure 5.3c), which could explain the high error in the measurements.

5.2.3 Preparation and examination of various wrinkle patterns

Alignment cells with various alignment patterns (Figure 5.4a) were used to prepare

a variety of different alignment profiles within the liquid crystal networks. Discrete

patterns, such as lines (patterns I and II), squares (pattern III), and text (pattern VII)

were prepared using photomasks (Figure 5.1b). Continuous circular patterns, such

as azimuthal (pattern IV), radial (pattern V), and an even higher complexity pattern

(pattern VI) were prepared through photoalignment on a rotating substrate

(Figure 5.1c). The samples diffract light perpendicular to the alignment direction of

the wrinkles, which allows visual characterization of the patterns and results in

striking optical effects. The patterns were also more closely analyzed with

microscopy, which showed the relatively smooth and localized boundary transition


84

between two domains (Figure 5.4b and 5.4c). Of special interest is pattern VI

(Figure 5.4d), which would be especially difficult to prepare using previously

reported wrinkling techniques, without the use of pre-patterned molds or laser

writing on the gold surface.

To determine if the domains sizes of the different patterns had any effect on the

period and/or amplitude of the wrinkles, an in-depth analysis of patterns I, II, and

III generated in the 55 nm gold layer was performed, and the average wrinkle

height and period were determined for all three alignments (Figure 5.4e). For the

two samples with rectangular domains (pattern I and II), the results were grouped

based on whether the wrinkle direction in the measured domain was parallel or

Figure 5.4 a) Overview of the different patterns, both schematically and visually. In pattern VII, the

alignment is vertical in the text and horizontal around it. b) Microscopy image of the domain border

in pattern I. c) AFM image of the domain border in pattern II. d) Microscopy image of the center of

pattern VI. The four “lobes” in the pattern are clearly visible. e) Table showing the average wrinkle

period and height of patterns I-VI, as measured with profilometry on a 400 μm trajectory at multiple

locations. For patterns with rectangular domains (I and II), the data is divided between measurements

parallel and perpendicular to the long axis of the domain. The amplitude of the wrinkles on the

isotropic network is also shown, although the period could not be accurately measured.


85

perpendicular to the long axis of the domain. It appears that wrinkles in the

domains perpendicular to the long axis are quite similar in period and height

compared to domains parallel to the long axis. A series of samples were measured

for patterns displaying 1, 2, 4, 12, and 36 domains per side (with 1, 4, 16, 144, and

1296 individual ‘squares’ produced on LC layers of between 7 and 30 μm thick:

the variations were not large and there was no obvious correlation between LC

layer patterning and the period and amplitude of the generated wrinkles. Thus, at

least at the distance scales of these experiments, there is little impact of the

proximity of the domain boundaries on either the height or period of the wrinkling.

This allows reproducible wrinkling over a range of pattern sizes and forms.

Using the same technique, the films with curved patterns (Figure 5.4a, patterns

IV-VI) were also examined. From these pictures, it became clear that these

wrinkles also follow the curved alignment of the liquid crystal network.

Profilometry measurements were performed on pattern VI (Figure 5.4e) at

locations far from the central region, as the strong curvature of the wrinkles near

the center distorted the results. The wrinkle amplitude and depth were comparable

to patterns I and II. This implies that continuous circular patterns behave similarly

to discrete patterns. Thus, the photopatterning technique allows generation of a

myriad of different wrinkling alignments without adversely affecting the

periodicity or height of the wrinkling pattern.


In conclusion, a simple, contactless method to generate well-defined, controlled,

and complex wrinkling patterns has been demonstrated, which involves

sputtercoating a thin layer of gold on photoaligned liquid crystal networks,

followed by heating. The wrinkling always takes place perpendicular to the

alignment director of the LC network, due to the lower modulus in that direction.

Interestingly, a periodic oscillation in the amplitude of the wrinkles was observed.


86

There appears to be little or no dependence of wrinkle period and height on the

specific pattern, suggesting that the proximity of pattern boundaries has little

impact on the wrinkle formation. The method is versatile, straightforward, and does

not require a complex setup. The process requires no physical contact with the

substrate or fabrication of expensive masters or masks, and could be applied over

large surfaces, if desired. The preparation of complex diffraction gratings has been

shown using this technique, but it could be easily extended to any of the

applications mentioned earlier, such as alignment of cells and patterned electronics.

5.4 Experimental section

A photoalignment material (ROP-108, Rolic) was spin coated on clean 30 x 30

mm2 glass substrates at 1500 RPM for 30 seconds, followed by brief heating to 115 oC to remove the solvent. The substrates were glued into cells using 18 μm spacers.

The completed cells were then subjected to irradiation with polarized UV light via

mask exposure in various ways to create the patterns (Figure 5.1b and 5.1c). The

cells were filled at 80 oC with a mixture of reactive liquid crystals (RM82, RM105

and RM23 in a 2 : 3 : 1 ratio, Merck) containing a radical initiator (Irgacure 819,

Ciba) to allow photopolymerization, and an inhibitor (tert-butyl hydroquinone,

Fluka) to prevent thermal polymerization. The filled cell was allowed to cool to 60 oC, after which the mixture was cured for 5 minutes using UV irradiation. After

cooling to room temperature one plate of the cell was removed, and the polymer

film was sputter coated with gold at room temperature in an Emitek K575X sputter

coater operated at 65 W, where the sputtering duration determined the thickness of

the gold layer. The gold coated LC films were heated on a hot stage at 60 oC until

wrinkling became visible due to the optical effects caused by the generation of

diffractive structures. The entire procedure for producing the films is depicted in

Figure 5.1. The surface of one of the films was observed through an optical

microscope (Leica DM-6000M)), and a profilometer (Veeco Dektak 150, Bruker)

was used to characterize the surface of all the samples.


87

5.5 References

1. (a) Chen, C. M.; Yang, S., Wrinkling instabilities in polymer films and

their applications. Polym. Int. 2012, 61 (7), 1041-1047; (b) Chung, J. Y.; Nolte, A.

J.; Stafford, C. M., Surface wrinkling: A versatile platform for measuring thin-film

properties. Adv. Mater. 2011, 23 (3), 349-368; (c) Genzer, J.; Groenewold, J., Soft

matter with hard skin: From skin wrinkles to templating and material

characterization. Soft Matter 2006, 2 (4), 310-323; (d) Grinthal, A.; Aizenberg, J.,

Adaptive all the way down: Building responsive materials from hierarchies of

chemomechanical feedback. Chem. Soc. Rev. 2013, 42 (17), 7072-7085; (e) Li, B.;

Cao, Y.-P.; Feng, X.-Q.; Gao, H., Mechanics of morphological instabilities and

surface wrinkling in soft materials: a review. Soft Matter 2012, 8 (21), 5728-5745;

(f) Singamaneni, S.; Tsukruk, V. V., Buckling instabilities in periodic composite

polymeric materials. Soft Matter 2010, 6 (22), 5681-5692.

2. (a) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides,

G. M., Spontaneous formation of ordered structures in thin films of metals

supported on an elastomeric polymer. Nature 1998, 393 (6681), 146-149; (b) Brau,

F.; Vandeparre, H.; Sabbah, A.; Poulard, C.; Boudaoud, A.; Damman, P., Multiple-

length-scale elastic instability mimics parametric resonance of nonlinear

oscillators. Nat. Phys. 2011, 7 (1), 56-60; (c) Cerda, E.; Mahadevan, L., Geometry

and physics of wrinkling. Phys. Rev. Lett. 2003, 90 (7), 074302; (d) Diamant, H.;

Witten, T. A., Compression induced folding of a sheet: An integrable system. Phys.

Rev. Lett. 2011, 107 (16), 164302; (e) Huang, J.; Juszkiewicz, M.; de Jeu, W. H.;

Cerda, E.; Emrick, T.; Menon, N.; Russell, T. P., Capillary wrinkling of floating

thin polymer films. Science 2007, 317 (5838), 650-653; (f) King, H.; Schroll, R.

D.; Davidovitch, B.; Menon, N., Elastic sheet on a liquid drop reveals wrinkling

and crumpling as distinct symmetry-breaking instabilities. P. Natl. Acad. Sci. USA.

2012, 109 (25), 9716-9720; (g) Pocivavsek, L.; Dellsy, R.; Kern, A.; Johnson, S.;

Lin, B.; Lee, K. Y. C.; Cerda, E., Stress and fold localization in thin elastic

membranes. Science 2008, 320 (5878), 912-916.


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3. (a) Kim, S.-J.; Park, H.-J.; Lee, J.-C.; Park, S.; Ireland, P.; Park, S.-H., A

simple method to generate hierarchical nanoscale structures on microwrinkles for

hydrophobic applications. Mater. Lett. 2013, 105, 50-53; (b) Kim, Y. H.; Lee, Y.

M.; Lee, J. Y.; Ko, M. J.; Yoo, P. J., Hierarchical nanoflake surface driven by

spontaneous wrinkling of polyelectrolyte/metal complexed films. ACS Nano 2012,

6 (2), 1082-1093; (c) Li, Y.; Dai, S.; John, J.; Carter, K. R., Superhydrophobic

surfaces from hierarchically structured wrinkled polymers. ACS Appl. Mater. Inter.

2013, 5 (21), 11066-11073.

4. Efimenko, K.; Finlay, J.; Callow, M. E.; Callow, J. A.; Genzer, J.,

Development and testing of hierarchically wrinkled coatings for marine

antifouling. ACS Appl. Mater. Inter. 2009, 1 (5), 1031-1040.

5. Watanabe, M.; Shirai, H.; Hirai, T., Wrinkled polypyrrole electrode for

electroactive polymer actuators. J. Appl. Phys. 2002, 92 (8), 4631-4637.

6. Xie, T.; Xiao, X.; Li, J.; Wang, R., Encoding localized strain history

through wrinkle based structural colors. Adv. Mater. 2010, 22 (39), 4390-4394.

7. (a) Greco, F.; Fujie, T.; Ricotti, L.; Taccola, S.; Mazzolai, B.; Mattoli, V.,

Microwrinkled conducting polymer interface for anisotropic multicellular

alignment. ACS Appl. Mater. Inter. 2012, 5 (3), 573-584; (b) Yang, P.; Baker, R.

M.; Henderson, J. H.; Mather, P. T., In vitro wrinkle formation via shape memory

dynamically aligns adherent cells. Soft Matter 2013, 9 (18), 4705-4714.

8. Li, R.; Yi, H.; Hu, X.; Chen, L.; Shi, G.; Wang, W.; Yang, T., Generation

of diffraction-free optical beams using wrinkled membranes. Sci. Rep. 2013, 3,

2775.

9. Chan, E. P.; Crosby, A. J., Fabricating microlens arrays by surface

wrinkling. Adv. Mater. 2006, 18 (24), 3238–3242.

10. Wu, H.; Menon, M.; Gates, E.; Balasubramanian, A.; Bettinger, C. J.,

Reconfigurable topography for rapid solution processing of transparent conductors.

Adv. Mater. 2014, 26 (5), 706-711.

11. Lacour, S. P.; Wagner, S.; Huang, Z.; Suo, Z., Stretchable gold conductors

on elastomeric substrates. Appl. Phys. Lett. 2003, 82 (15), 2404-2406.


89

12. Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan, L.;

Genzer, J., Nested self-similar wrinkling patterns in skins. Nat. Mater. 2005, 4 (4),

293-297.

13. (a) Bowden, N.; Huck, W. T. S.; Paul, K. E.; Whitesides, G. M., The

controlled formation of ordered, sinusoidal structures by plasma oxidation of an

elastomeric polymer. Appl. Phys. Lett. 1999, 75 (17), 2557-2559; (b) Harrison, C.;

Stafford, C. M.; Zhang, W.; Karim, A., Sinusoidal phase grating created by a

tunably buckled surface. Appl. Phys. Lett. 2004, 85 (18), 4016-4018.

14. (a) Chen, Z. B.; Kim, Y. Y.; Krishnaswamy, S., Anisotropic wrinkle

formation on shape memory polymer substrates. J. Appl. Phys. 2012, 112 (12),

124319; (b) Kang, S. H.; Na, J.-H.; Moon, S. N.; Lee, W. I.; Yoo, P. J.; Lee, S.-D.,

Self-organized anisotropic wrinkling of molecularly aligned liquid crystalline

polymer. Langmuir 2012, 28 (7), 3576-3582; (c) Serrano, J. R.; Xu, Q. Q.; Cahill,

D. G., Stress-induced wrinkling of sputtered SiO2 films on

polymethylmethacrylate. J. Vac. Sci. Technol. A 2006, 24 (2), 324-327.

15. (a) Ahn, S. H.; Guo, L. J., Spontaneous formation of periodic

nanostructures by localized dynamic wrinkling. Nano Lett. 2010, 10 (10),

4228-4234; (b) Chen, Y. C.; Crosby, A. J., Wrinkling of inhomogeneously strained

thin polymer films. Soft Matter 2013, 9 (1), 43-47; (c) Okayasu, T.; Zhang, H. L.;

Bucknall, D. G.; Briggs, G. A. D., Spontaneous formation of ordered lateral

patterns in polymer thin-film structures. Adv. Funct. Mater. 2004, 14 (11),

1081-1088; (d) Watanabe, M.; Mizukami, K., Well-ordered wrinkling patterns on

chemically oxidized poly(dimethylsiloxane) surfaces. Macromolecules 2012, 45

(17), 7128-7134.

16. Huck, W. T. S.; Bowden, N.; Onck, P.; Pardoen, T.; Hutchinson, J. W.;

Whitesides, G. M., Ordering of spontaneously formed buckles on planar surfaces.

Langmuir 2000, 16 (7), 3497-3501.

17. Vandeparre, H.; Léopoldès, J.; Poulard, C.; Desprez, S.; Derue, G.; Gay,

C.; Damman, P., Slippery or sticky boundary conditions: Control of wrinkling in

metal-capped thin polymer films by selective adhesion to substrates. Phys. Rev.

Lett. 2007, 99 (18), 188302.


90

18. Ding, W. L.; Yang, Y.; Zhao, Y.; Jiang, S. C.; Cao, Y. P.; Lu, C. H., Well-

defined orthogonal surface wrinkles directed by the wrinkled boundary. Soft

Matter 2013, 9 (14), 3720-3726.

19. (a) Guo, C. F.; Nayyar, V.; Zhang, Z.; Chen, Y.; Miao, J.; Huang, R.; Liu,

Q., Path-guided wrinkling of nanoscale metal films. Adv. Mater. 2012, 24 (22),

3010-3014; (b) Vandeparre, H.; Desbief, S.; Lazzaroni, R.; Gay, C.; Damman, P.,

Confined wrinkling: Impact on pattern morphology and periodicity. Soft Matter

2011, 7 (15), 6878-6882.

20. (a) Agrawal, A.; Luchette, P.; Palffy-Muhoray, P.; Biswal, S. L.; Chapman,

W. G.; Verduzco, R., Surface wrinkling in liquid crystal elastomers. Soft Matter

2012, 8 (27), 7138-7142; (b) Agrawal, A.; Yun, T. H.; Pesek, S. L.; Chapman, W.

G.; Verduzco, R., Shape-responsive liquid crystal elastomer bilayers. Soft Matter

2014, 10 (9), 1411-1415; (c) Greco, F.; Domenici, V.; Romiti, S.; Assaf, T.;

Zupancic, B.; Milavec, J.; Zalar, B.; Mazzolai, B.; Mattoli, V., Reversible

heat-induced microwrinkling of PEDOT:PSS nanofilm Surface over a

monodomain liquid crystal elastomer. Mol. Cryst. Liq. Cryst. 2013, 572 (1), 40-49.

21. Thornton, J. A.; Hoffman, D. W., Stress-related effects in thin-films. Thin

Solid Films 1989, 171 (1), 5-31.

22. D., L. L.; M., L. E., Theory of Elasticity. Vol. 7. Pergamon Press: 1970.

91

Chapter 6. Technology assessment

6.1 Introduction

In this thesis a range of liquid crystal polymer actuators are described, which have

been fabricated by controlling the supramolecular organization in the materials in

three dimensions in order to open up potential applications in a variety of fields. It

has been shown that both through chemical patterning and photoalignment, many

new, exotic shapes can be created in these materials. Their versatility makes these

materials suitable for the development of advanced devices, as the deformations

can be tailored to achieve specific needs and requirements. Such small-scale, well

controlled actuation could find a variety of applications, for example in

microfluidics, soft robotics and medical devices.

However, during the course of this research it has also become clear that, for such

practical applications, further research is required. This research should proceed in

two directions: improving the mechanical properties of the developed materials to

do work, and towards functional shapes, such as switchable optics and

remote-addressable sieves. In addition, it would be useful to obtain actuators that

respond to other stimuli, such as UV-irradiation and electric fields, which are

attractive stimuli from an application point of view.

6.2 Soft robotics

During the last few decades, soft robotics have gained increased interest. Soft

robotics differentiates itself from hard robotics in that the actuation is distributed

over the whole actuator instead of being located at the joints.1 Such devices can be

used to handle fragile objects or to move across difficult terrain. A good example

Chapter 6 Technology assessment

92

consists of elastomers with pneumatic networks, which have been used to make

walking robots that can cross obstacles, grippers2 and tentacles3 capable of accurate

manipulation, and elastomeric origami.4 However, there are still challenges to

overcome in this field, such as miniaturization, in which liquid crystal actuators

could play an important part, as they can be fabricated on microscale.5

For many applications actuators should have a large end-to-end displacements,

while many of the reported materials only expand and contract by a few percent.6

This can be solved by using the striped director profiles with alternating twisted

nematic alignment, which will deform in an accordion-like manner. This allows the

actuators to reach a significantly larger end-to-end displacement compared to

uniaxial actuators of the same material, as strains of a few percent can cause the

actuator to contract to only a fraction of its original length. This way, the

displacement can be significantly amplified, which makes the actuators more

useful than their uniaxial counterparts in applications that require such large

displacements. For example, the actuator could be used to move a shutter that

blocks a channel, which can be moved out of the way by applying a stimulus,

enabling smart flow control. The actuators could also be used in medical devices to

perform operations where a small device, which fits into the body, has to achieve a

large displacement.

For such applications a certain mechanical strength is required, and actuators

should be capable of performing work. Of course, increasing the displacement will

also lead to a dramatic reduction in the amount of weight that the actuator can

displace. Some experiments were carried out to estimate how much work the

accordion actuator with 12 stripes can perform (see Chapter 4). A setup was

designed in which the actuator pulled a weight up a ramp, of which the incline

could be adjusted to change the effective weight pulled (Figure 6.1a). The

temperature of the ramp could gradually be increased until the film had reached its

maximum contraction. The maximum work the film could perform was determined

by performing a series of experiments with inclines of increasing steepness. Note


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that the friction between the sample and the surface was ignored. Two films with a

12 stripe twisted nematic alignment profile where tested: One consisting of the LC

mixture that was used for heat-responsive actuators in previous chapters, and a

100% crosslinked network prepared from a mixture of only diacrylates. From this

experiment it was found that the less densely crosslinked film could perform

1.41 x 10-6 J of maximum work (Figure 1b), while the fully crosslinked film could

perform 2.40 x 10-6 J of maximum work (Figure 1c). Although the amount of work

that can be performed is limited, these results show that a higher crosslink density

leads to an increase in maximum work performance.

Besides increasing the crosslink density of the networks, there are several ways in

which the chemistry of the materials could be improved to increase the ability of

the actuators to do work. For example, it is likely that a network of main-chain

liquid crystal polymers would generate a larger force, and therefore be capable of

Figure 6.1 Experiments to estimate the ability of accordion-shaped actuators to

do work. a) The experimental setup. The actuator is clamped on one side while

positioned on a ramp that can be heated, and a weight is attached to the other

side. b) The chemical composition of the less densely crosslinked soft actuator,

and the resulting measurements. c) The chemical composition of the fully

crosslinked soft actuator, and the resulting measurements.


94

displacing more weight, leading to an increase in maximum work output. Such a

network can be prepared using reactive liquid crystals with thiol and vinyl

functional groups instead of acrylates. Such groups can undergo a step-growth

polymerization when free radical species are introduced, forming head-to-tail thiol

ethers that result in a main-chain liquid crystal network. Another possibility to

improve the mechanical properties of the actuators is by combining the liquid

crystal polymer network with a different, non-LC species. Carbon nanotubes have

been successfully blended with elastomers to reinforce the polymer, leading to

enhanced mechanical properties.7 Unfortunately, mixing carbon nanotubes with

other molecules is difficult due to the strong tendency of the nanotubes to

aggregate. Another option would be to prepare an interpenetrating network (IPN)

between an LC polymer and a non-LC polymer. This can be done by adding a

non-reactive molecule to the reactive LC mixture, which can be removed from the

matrix after polymerization by evaporation or dissolution. Then the network can be

swollen with a reactive species, which is polymerized to form the second network.

In addition to changing the chemistry of the actuators, another possibility would be

to look into a different alignment profile. The twisted nematic alignment is not the

ideal alignment profile for this type of actuator, as the anticlastic suppression of the

separate domains to bend in only one direction works against the lateral

displacement, and the chirality of the twisted nematic alignment leads to

out-of-plane twisting of the actuator. These effects are not present when a splayed

alignment is used, making this the alignment of choice for any bending actuator.

However, an alternating striped alignment profile is significantly more difficult to

prepare using photoalignment, as it requires substrates that alternate between

planar and homeotropic alignment. This has previously been achieved in

coumarin-based LPP alignment layers by only irradiating the parts of the substrate

where planar alignment was desired, followed by a treatment of the substrate with a

thiol species bearing a long alkyl tail, in presence of a radical initiator.8 The thiol

reacted with the double bond present in the non-irradiated areas, inducing a


95

homeotropic alignment. In such way it would be possible to fabricate an alternating

splayed patterned actuator.

6.3 Switchable sieves

A sieve with remotely addressable openings, which are opened and closed on

demand, would be a useful device as it would enable adjustable size selectivity and

unclogging. Discrete, disclination-based alignment profiles can be used to prepare

a film with slits that can be opened and closed through heating and cooling,

respectively. However, the pores that were shown in Chapter 3 are still too large

for this application to work, and miniaturization of both the domains of the

alignment profile and the apertures themselves is required. For the preparation of

small sized sieves, photoalignment on a small scale is required. Direct-writing of

the alignment pattern onto the substrate bearing the photoalignment material, using

a focused UV laser beam, can be used to prepare such smaller features. To do so,

an LPP substrate is placed onto an xy translation stage, and the polarization

direction of the beam is regulated using a polarization modulator. The movement

of the substrate stage and the polarization of the beam can be programmed to

generate arbitrary patterns, with micro-scale accuracy.9 The biggest difficulty in the

preparation of the addressable apertures is the cutting of the slits. In the

experiments presented earlier, the slits were cut using a razor blade. However, on

the length scales at which the direct-writing procedure operates, this is no longer an

option. A solution to this problem is to use laser cutting to apply the slits.

Using a direct laser writing setup, an aperture pattern, as presented in Chapter 3,

was prepared on an LPP-bearing substrate, in which the slits were 1 mm wide. A

liquid crystal mixture was spincoated from solution on top of the substrate and

cured. Then the slits were cut in the film using the direct laser writing setup at

maximum focus and energy. The resulting film had the correct pattern, but the slits

were often present in a slightly offset position. As the film was thin, and weakened


96

due to the presence of the slits, it could only be delaminated by framing it with

pieces of tape to prevent it from collapsing. The heat response of the slits was

investigated using optical microscopy in combination with a heating stage

(Figure 6.2). It seemed that the slits indeed opened at elevated temperatures. It

must be noted, however, that the framing of the film also caused buckling when

heated, which might cause the slits to appear open even while they were not. This

could be solved by preparing thicker films, which have less tendency to buckle and

do not required a frame to be handled.

6.4 Smart optics

In optics, shape is one of the most important parameters determining how an object

interacts with light. Therefore, adjustable, complex shape deformation could find

applications in optical devices. For example, the actuators with an azimuthal

alignment profile shift from a flat sheet to a cone, which could be used as a

switchable lens. In case of IR heating, the cone can be smoothened to better

resemble a lens by exposing the sheet only at the outer radius, which leads to an

inside-out temperature gradient. This has been tested and found to work. Another

example would be an accordion-shaped actuator with a thin cholesteric layer on top

of it. A cholesteric layer reflects light based on the angle of incidence, which

means that the reflective properties change when the film deforms from the flat

shape to the contracted shape. Finally, the patterned wrinkle surfaces have been

shown to possess diffractive properties. These systems presented here are non-

reversible, but reversible wrinkling in LC-based systems have been shown in the

literature. The procedure of photoalignment could be applied to such materials,

which would allow the preparation of gratings with complex, switchable

diffraction.

For this application, it would be useful to look into actuators that respond to

different triggers then the ones presented in this work, as the presence of high


97

temperatures or humidity might not always be desired in electronic devices. A

stimulus that is often used is UV light. To make the films responsive to UV

irradiation, azobenzene-based liquid crystals can be added to the polymerizable

mixture.10 The actuators could also be made to respond to electricity. This can

potentially been done using dielectric molecules with smectic C* alignment.11

Additionally, the addition of carbon nanotubes can give the polymer network

dielectric properties.12

In contrast to the previously mentioned applications in soft robotics, in optics the

ability to do work is of little importance. However, there is a different problem that

has to be addressed: the anchoring of the film. For most applications, the film has

to be attached inside a device. This will hamper the deformation of the film,

leading to a different shape or no deformation at all. For example, a film with an

azimuthal alignment profile could be glued to a metal or plastic ring to place it into

a device. When heated, it cannot contract along its circumference, so the cone

shape will not be observed, or will be repressed. This problem could be solved by

attaching the film to a substrate which can deform with it, or by attaching the film

Figure 6.2 Polarized optical microscopy images of

an aperture film with 1 mm slits, shown at varying

temperatures.


98

while already deformed and then allow it to relax into a different shape. However,

in both cases the film might respond by crumpling instead of deformation into a

smooth shape. Other solutions could be to attach the film to springs, or to have it

float on a liquid. Of course, in the latter case the surface tension of the liquid might

have a significant effect on the final shape.

6.5 References

1. Bauer, S.; Bauer-Gogonea, S.; Graz, I.; Kaltenbrunner, M.; Keplinger, C.;

Schwödiauer, R., 25th Anniversary article: A soft future: From robots and sensor

skin to energy harvesters. Adv. Mater. 2014, 26 (1), 149-162.

2. Ilievski, F.; Mazzeo, A. D.; Shepherd, R. F.; Chen, X.; Whitesides, G. M.,

Soft robotics for chemists. Angew. Chem. Int. Edit. 2011, 50 (8), 1890-1895.

3. Martinez, R. V.; Branch, J. L.; Fish, C. R.; Jin, L.; Shepherd, R. F.; Nunes,

R. M. D.; Suo, Z.; Whitesides, G. M., Robotic tentacles with three-dimensional

mobility based on flexible elastomers. Adv. Mater. 2013, 25 (2), 205-212.

4. Martinez, R. V.; Fish, C. R.; Chen, X.; Whitesides, G. M., Elastomeric

origami: Programmable paper-elastomer composites as pneumatic actuators. Adv.

Funct. Mater. 2012, 22 (7), 1376-1384.

5. (a) van Oosten, C. L.; Bastiaansen, C. W. M.; Broer, D. J., Printed artificial

cilia from liquid-crystal network actuators modularly driven by light. Nat. Mater.

2009, 8 (8), 677-682; (b) van Oosten, C. L.; Harris, K. D.; Bastiaansen, C. W. M.;

Broer, D. J., Glassy photomechanical liquid-crystal network actuators for

microscale devices. Eur. Phys. J. E 2007, 23 (3), 329-336.

6. Broer, D. J.; Crawford, G. P.; Zumer, S., Cross-linked Liquid Crystalline

Systems. CRC, Taylor and Francis Group, Boca Raton, FL: 2011.

7. Bokobza, L., Multiwall carbon nanotube elastomeric composites: A

review. Polymer 2007, 48 (17), 4907-4920.

8. Wilderbeek, H. T. A.; Teunissen, J. P.; Bastiaansen, C. W. M.; Broer, D. J.,

Patterned alignment of liquid crystals on selectively thiol-functionalized

photo-orientation layers. Adv. Mater. 2003, 15 (12), 985-988.


99

9. Miskiewicz, M. N.; Escuti, M. J., Direct-writing of complex liquid crystal

patterns. Opt. Express 2014, 22 (10), 12691-12706.

10. Ikeda, T.; Mamiya, J.-i.; Yu, Y., Photomechanics of liquid-crystalline

elastomers and other polymers. Angew. Chem. Int. Ed. 2007, 46 (4), 506-528.

11. Jakli, A.; Toth-Katona, T.; Scharf, T.; Schadt, M.; Saupe, A.,

Piezoelectricity of a ferroelectric liquid crystal with a glass transition. Phys. Rev. E

2002, 66 (1), 011701.

12. Courty, S.; Mine, J.; Tajbakhsh, A. R.; Terentjev, E. M., Nematic

elastomers with aligned carbon nanotubes: New electromechanical actuators.

Europhys. Lett. 2003, 64 (5), 654-660.

101

List of symbols

λ wavelength of light

λlc contration of a liquid crystal network along the director

λw wrinkle period

δ stress

δc critical stress required for wrinkling

θn angular deviation of liquid crystal molecules from the alignment director

θc angle between the alignment director at z = 0 and the long axis of a rbbon

θw base angle of a wedge-shaped building block

ν Poisson ratio

φ cone opening angle

A wrinkling amplitude

C circumference

d coil diameter

Ep elastic modulus polymer foundation

Em elastic modulus metal layer

h film thickness

L// length parallel to the alignment director

L┴ length perpendicular to the alignment director

m disclination strength

mp number of inward folding flaps of a heated radimuthal film

n alignment director

PT helicoid pitch

PH coil pitch

r radius

S order parameter

S0 initial order parameter

T temperature

Tflat temperature at which a coiling actuator is flat

TNI temperature of nematic to isotropic phase transition

t time

U internal energy of a radimuthal film

x y z coordinate system axis

103

Summary


This thesis describes the development of polymer films that show programmed

shape deformations upon exposure to a stimulus, such as heat, light, or humidity.

The stimuli-responsive materials are based on liquid crystal polymer networks,

which can be prepared by photopolymerization of reactive liquid crystals. Using

photoalignment, alignment layers that force the molecules into complex alignment

director patterns can be prepared, and this allows the programming of shape

changes in the material. Exotic shape deformations were achieved in materials with

alignment director variations of the molecules in one, two, and three spatial

dimensions.

The liquid crystal actuators with the least complex alignment pattern are those

which have uniaxial planar alignment, which means the molecules are aligned

parallel to the surface of the film in one direction. Such networks contract and

expand when a stimulus is applied, but can be made to bend by making one side of

the film more responsive to the stimulus compared to the other side. Selective

treatment of hydrogen-bonded liquid crystal networks with a basic solution was

used to create areas on the film which shrink when dry and swell when wet, which

resulted in patterned actuators that respond to changes in humidity by forming

exotic shapes.

Films with a complex director profile in the plane of the film were also prepared.

Using a circular azimuthal pattern, a film could be prepared that, upon heating,

contracts along the circumference and expands along the radius, which leads to a

cone-shaped deformation. Likewise, a film with a radial pattern experiences the

opposite forces and deforms into an anti-cone, which manifests as a saddle shape.

By including an IR-absorbing dye, the films could be heated with IR irradiation to

104

allow contactless activation of the shape change. Using a similar pattern, a film

with slits that open and close upon heating and cooling was prepared.

A film with director variation in all three dimensions was prepared using

photoalignment of a film with twisted nematic alignment, which has a continuous

director variation through the thickness of the film. This technique was used to

create films with stripe patterns that deform into accordion-like shapes upon

heating. Finally, it was shown that the wrinkling behavior of a bilayer system of

liquid crystal polymer and gold could be controlled by photoalignment, as the

wrinkles run parallel to the alignment director of the liquid crystals.

In conclusion, a number of new, exotic shape deformations of liquid crystal

networks were programmed and demonstrated. These results can lead to

applications in various fields, such as microfluidics, microrobotics, and medical

systems.

105

Samenvatting

Programmeerbare vervorming van vloeibaar kristallijne netwerken

Dit proefschrift beschrijft de ontwikkeling van polymeerfilms die een

ingeprogrammeerde vormverandering ondergaan wanneer ze worden blootgesteld

aan een stimulus zoals warmte, licht of vocht. Deze stimulus-responsieve

materialen zijn gebaseerd op vloeibaar kristallijne polymeernetwerken, welke

kunnen worden vervaardigd door het polymeriseren van reactieve vloeibare

kristallen. Door gebruik te maken van foto-uitlijning kunnen uitlijnlagen worden

gemaakt die de moleculen dwingen om complexe patronen te volgen, en dit maakt

het mogelijk om de vervorming in het materiaal zelf te programmeren. Dit heeft

geleid to exotische vormsveranderingen in materialen met variaties in de uitlijning

in een, twee en drie ruimtelijke dimensies.

De vloeibaar kristallijne actuatoren met de minste complexiteit hebben een

uniaxiale planaire uitlijning, wat betekent dat de moleculen parallel aan het

oppervlak van de film zijn uitgelijnd, en allemaal in dezelfde richting liggen. Dit

netwerk wordt korter in een richting en langer in de andere richting wanneer een

stimulus wordt toegepast, maar kan alleen buigen als een kant sterker op de

stimulus reageert dan de andere kant. Door middel van selectieve behandeling van

waterstof-gebrugde vloebaar kristallijne netwerken met een basische oplossing

zijn delen van de film gevoelig gemaakt voor water, zodat ze krimpen in een droge

omgeving en zwellen in een vochtige omgeving. Op deze manier zijn

gepatroneerde actuatoren verkregen die reageren op de luchtvochtigheid door

exotische vormsveranderingen te ondergaan.

Verder zijn er films met een complex uitlijnpatroon in het vlak van de film

vervaardigd. Door gebruik te maken van een azimuthaal patroon kon een

cirkelvormige film worden gemaakt die op verwarming reageert door langs de

106

omtrek samen te trekken en langs de radius uit te zetten, wat uiteindelijk leidt tot

een kegelvormige vormsverandering. Een film met een radieel patroon vervormt op

de omgekeerde manier, wat leidt tot een vorm die wordt beschreven als een

anti-kegel, welke eruitziet als een zadelvorm. Door een IR-absorberende kleurstof

toe te voegen konden de films worden verhit door middel van IR straling, zodat de

vormsverandering kon worden geactiveerd zonder contact te maken met de film.

Door gebruik te maken van een soortgelijk patroon kon ook een film worden

gemaakt met openingen die open en dicht gaan door te verhitten en af te koelen.

Een film met een variatie in de uitlijning in alle drie de ruimtelijke dimensies kon

worden vervaardigd door het foto-uitlijnen van een film met een gedraaide

nematische uitlijning die een doorlopende variatie door de dikte van de film heeft.

Door middel van deze techniek kon een film gemaakt worden met een

strepenpatroon, die bij verhitten deformeert op een manier die wel iets weg heeft

van de beweging van een accordeon. Als laatste is gedemonstreerd dat het

rimpelgedrag van een bilaag-systeem, bestaande uit een vloeibaar kristallijn

polymeernetwerk en een laagje goud, kon worden gestuurd door middel van foto-

uitlijning. Dit was mogelijk doordat de rimpels de uitlijnrichting van de vloeibare

kristallen volgen.

Uiteindelijk kan worden geconcludeerd dat een aantal nieuwe, exotische

vormsverandingen in vloeibaar kristallijne netwerken zijn geprogrammeerd. Deze

zijn gedemonstreerd door een stimulus toe te passen. De resultaten kunnen leiden

tot toepassingen in verschillende onderzoeksgebieden, zoals lab-on-a-chip,

microrobotica en gezondheidszorg.

107

Curriculum Vitae

Laurens Theobald de Haan was born on the 30th of May, 1986 in Apeldoorn, the

Netherlands. After completing his secondary education at the Udens College in

Uden, he studied Molecular Life Sciences at the Radboud University in Nijmegen.

During his first master’s project, he worked on the preparation of polymer

networks with azide side groups using RAFT polymerization to obtain coatings

that could be functionalized using click chemistry. His second master’s project

involved the synthesis of ionic liquid crystals and the study of their use as a

reaction medium for enzymes. After obtaining his master’s degree, he started his

PhD project on complex liquid crystal polymer actuators under the supervision of

prof. Albert Schenning and prof. Dick Broer. The results of this project are

presented in this thesis.

Laurens Theobald de Haan werd geboren op 30 mei 1986 in Apeldoorn, Nederland.

Na afronding van de middelbare school aan het Udens College in Uden begon hij

zijn studie Moleculaire Levenswetenschappen aan de Radboud Universiteit in

Nijmegen. Tijdens zijn eerste masterproject werkte hij aan het vervaardigen van

polymeernetwerken met azide zijgroepen door middel van RAFT polymerisatie,

die vervolgens gefunctionaliseerd konden worden middels clickchemie. Zijn

tweede masterproject bestond uit de synthese van ionische vloeibare kristallen en

het bestuderen van de geschiktheid van deze materialen als een reactiemedium

voor enzymen. Na het verkrijgen van zijn masterdiploma begon hij aan zijn

promotieproject, met als onderwerp complexe actuatoren gebaseerd op

gepolymerizeerde vloeibare kristallen, onder begeleiding van prof. Albert

Schenning en prof. Dick Broer. Het resultaat van dit project is beschreven in dit

proefschrift.

109

Publications

Publications related to this thesis

de Haan, L. T.; Leclère, P.; Damman, P.; Schenning, A. P. H. J.; Debije M. G., On

demand wrinkling patterns in thin metal films generated from self-assembling

liquid crystals. Adv. Funct Mater., Submitted.

de Haan, L. T.; Schenning, A. P. H. J.; Broer, D. J., Programmed morphing of

liquid crystal networks. Polymer, In Press.

de Haan, L. T.; Verjans, J. M. N.; Broer, D. J.; Bastiaansen, C. M. W.; Schenning,

A. P. H. J., Humidity-responsive liquid crystalline polymer actuators with an

asymmetry in the molecular trigger that bend, fold, and curl. J. Am. Chem. Soc.

2014, 136 (30), 10585-10588.

de Haan, L. T.; Gimenez-Pinto, V.; Konya, A.; Nguyen, T. S.; Verjans, J. M. N.;

Sanchez-Somolinos, C.; Selinger, J. V.; Selinger, R. L. B.; Broer, D. J.;

Schenning, A. P. H. J., Accordion- like actuators of multiple 3D patterned liquid

crystal polymer films. Adv. Funct. Mater 2014, 24 (9), 1251-1258.

Dai, M.; Pico, O. T.; Verjans , J. M. N.; de Haan, L. T.; Schenning, A. P. H. J.;

Peijs, T.; Bastiaansen, C. W. M., Humidity-responsive bilayer actuators based on a

liquid-crystalline polymer network. ACS Appl. Mater. Inter. 2013, 5 (11),

4945-4950.

Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J.,

Angular deficits in flat space: remotely controllable apertures in nematic solid

sheets. P. Roy. Soc. A.-Math. Phy. 2013, 469 (2153), 20120631.

110

Modes, C. D.; Warner, M.; Sanchez-Somolinos, C.; de Haan, L. T.; Broer, D. J.,

Mechanical frustration and spontaneous polygonal folding in active nematic sheets.

Phys. Rev. E. 2012, 86 (6), 060701.

de Haan, L. T.; Sanchez-Somolinos, C.; Bastiaansen, C. M. W.; Schenning, A. P.

H. J.; Broer, D. J., Engineering of complex order and the macroscopic deformation

of liquid crystal polymer networks. Angew. Chem. Int. Edit. 2012, 51 (50),

12469-12472.

Other publications

Fernandez, A. A.; de Haan, L. T.; Kouwer, P. H. J., Towards room-temperature

ionic liquid crystals. J. Mater. Chem. A. 2013, 1 (2), 354-357.

Canalle, L. A.; van Berkel, S. S.; de Haan, L. T.; van Hest, J. C. M., Copper-free

clickable coatings. Adv. Funct. Mater. 2009, 19 (21), 3464-3470.

111

Acknowledgements

During my time as a PhD candidate, I had the pleasure of working together with

many great people. Here, I would like to thank everyone who contributed to the

completion of this thesis by providing help or support, for engaging in stimulating

discussions or for providing useful advice.

I would like to thank my promoters, Albert Schenning and Dick Broer, for giving

me the opportunity to work in their amazing group. Albert, I want to thank you for

your daily support, your enthusiasm and positive outlook, and for allowing me to

choose my own directions within my research project. I wish you the best of luck

as the new professor of the SFD group. Dick, I could always count on your

extensive knowledge of liquid crystal polymers, which has been of great help,

especially when both me and Albert were still new to this field.

I would like to thank the members the reading committee, who took the time to

read and comment on the thesis. Cees Bastiaansen, Alan Rowan, Rint Sijbesma,

Carlos Sanchez-Somolinos, and Timothy White, thank you for your interest in my

thesis, and your corrections.

During this project I have had a number of collaborations with research groups

abroad. From Kent State University, I would like to thank Vianney Gimenez-Pinto,

Andrew Konya, Than-Son Nguyen, Jonathan Selinger, and Robin Selinger for their

work on finite-element analysis of my patterned twisted nematic films, and the

great paper we wrote together. From North Carolina State University I would like

to thank Michael Escuti and Matthew Miskiewicz for inviting me over to their lab

to do some experiments. It was a great experience to visit the United States, and I

think we accomplished a lot in the lab. I have also worked closely with Carl Modes

and Mark Warner from the University of Cambridge, who showed in their papers

the theoretical predictions of the experiments described in chapter 3. I would like to

112

thank you for the interesting and inspiring discussion we had, for your advice, and

for allowing me to contribute to two of your publications. From the University of

Mons I would like to thank Pascal Damman and Phillipe Leclère for their help with

the gold wrinkle patterns. You helped us understand a topic that nobody in our

group had much knowledge of, and we couldn’t have written our paper without

this. I especially want to thank Carlos Sanchez-Somolinos from the University of

Zaragoza. Carlos, thank you for your work on this project, both before and after I

started my PhD program, so I was able to quickly have some great results. Also,

thank you for having me visit you three times, and for allowing me to take the

photoalignment setup with me to Eindhoven. You have been a great collaborator,

and friend.

I also had the pleasure of supervising a number of students who worked on my

project. Jullien Verjans, Jelle Lendemijer and Stefan van Uden, thank you for your

hard work and for the many things you have taught me about supervising students.

I would also like to thank Dylan Davies, who worked as a student on my project

before I stared, and did some great preliminary experiments.

During my time at SFD I have worked with a number of amazing colleagues.

Michael Debije, thank you for all the great discussions we had, about work and

other things, and for inviting me to your board game sessions. Amol, Altug,

Antonio, Anne Hélène, Baran, Berry, Casper, Claudia, Danqing, Debarshi, Derya,

Dirk Jan, Felix, Hilal, Hitesh, Huub, Indu, Irén, Ivelina, Jelle R., Jelle S., Jeroen,

Joost, Jurica, Jurgen, Koen, Kamlesh, Lihua, Luc, Marjolijn, Michiel, Mian,

Monali, My, Natalia, Nicole, Paul, Pauline, Peter, Pim, Pit, Robert, Shufen, Sander,

Stephanie, Tamara, Tom, Wanyu, Xiao, Xiaoran, Yang, Youseli, Zeno, thank you

for the great times at work and outside of it.

I would like to thank my family and friends for their love and support. I thank my

father, Theo de Haan, and my brother, Martin de Haan, for always being there for

me.

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Finally, I want to thank my mother, Emmy de Haan van Randwijk. I cannot

describe how much you have done for me, my brother, and my father. You were

always there when I needed someone to talk to, you always showed interest in

everything I did, and you will always be the strongest person I have ever known.

I miss you.

Rest in peace.

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