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854 Journal of Chemical Education Vol. 81 No. 6 June 2004 www.JCE.DivCHED.org

Examples of technologies that utilize the liquid crystal-line phase of matter are not difficult to find. The liquid-crystaldisplay is the most common application: twisted nematic liq-uid-crystal displays are found in digital wristwatches, micro-wave ovens, mobile phones, and video recorders. Worldwide,

sales of flat-screen liquid-crystal displays have been projectedto reach $47 billion in 2004 (1). With this in mind, intenseefforts by researchers in industry and in academia continueto be made into the synthesis of new liquid-crystal com-pounds and the characterization of their properties (2). Ap-plications of polymerizable liquid crystals have also been ofgreat interest since the discovery of Kevlar and Nomex byStephanie Kwolek at Du Pont. The casting of nanostructuredand nanocomposite materials such as periodic mesoporousmetals and metal alloys from lyotropic liquid crystals is an-other interesting materials science application of orderedmesophases (3, 4). Despite these advances, undergraduatechemistry students are still unlikely to encounter liquid crys-

tals in the course of their experimental studies (5).The experiment described here was developed for a labo-ratory course on nanostructures, as part of the undergradu-ate bachelor of science degree in nanotechnology at FlindersUniversity. The nanotechnology degree has been in place forfour years and has proven to be successful, attracting largenumbers of high-caliber students. In the second year of theirdegree, students specialize in either of two areas: (i) nano-structures and laser devices or (ii) biodevices. Where possible,experiments relevant to both areas (such as this liquid-crys-tal laboratory) were designed for this course. The experimentswere designed to demonstrate the relationship between mo-lecular order and the optical and dielectric properties of theliquid crystalline state and the application of these proper-ties to create a working display device. The laboratory is suit-able for second-year undergraduate students, particularly ifthey have some background in physical chemistry and possi-bly optics.

Molecules that possess a nematic liquid-crystal phase aretypically elongated. Depending upon the details of their struc-ture, many of these molecules pass through several liquid-crystal phases, corresponding to different degrees of molecularorder (6). For nematic liquid crystals, a rigid component inthe molecular structure, a permanent dipole moment, andvaried polarizability in the different planes of the moleculelead to weak intermolecular attractions that preferentiallyorient the molecules with respect to each other. This causes

the molecules to form highly associated fluids or mesophasesover certain temperature ranges (thermotropic liquid crys-tals) or in some solvents (lyotropic liquid crystals) (6). In theabsence of restrictive influences, domains of ordered fluid arerandomly disposed within the material. The high degree of

order and the anisotropy in shape of liquid crystals can beobserved via X-ray diffraction patterns and as an anisotropyin viscosity coefficients, electrical, optical, or other proper-ties. Using equipment that is readily available to an under-graduate teaching laboratory, a laser diode, a polarizer, and aphotodiode as a detector, we describe a simple and inexpen-sive experiment to directly measure the birefringence, or lin-ear dichroism, of a nematic liquid-crystal sample. Theexperiment can be extended to investigate the performanceof a twisted nematic liquid-crystal display.

Experiment 1

Construction of a Nematic Liquid-Crystal Cell

The students are expected to complete three projects overthe course of two three-hour laboratory sessions. The firsttask is to manufacture an aligned nematic liquid-crystal cellso they can measure the optical birefringence of the liquid-crystal sample and the nematicisotropic phase transition.The liquid-crystal cell is constructed from two microscopeslides each coated with an alignment layer of polyvinyl alco-hol (PVA). The alignment direction is established by rub-bing the polymer surface with a velvet cloth for 10 minutes.Thin mylar spacers are used to hold the cell apart, which isthen filled with liquid crystal by capillary action. The cell issubsequently sealed at the edges using Araldite adhesive. Prop-erties of nematic liquid crystal 4-pentyl-4-biphenylcarbonitrile (5CB) are investigated, although anycompound or mixture that is nematic liquid crystalline atroom temperature would be equally suitable (e.g., MBBA)(7). The cell containing 5CB is birefringent, a consequenceof homogeneous orientation of the 5CB molecules that comeinto contact with the aligned polymer molecules at the sur-face (8). This surface orientation is transferred throughoutthe ensemble of liquid crystal causing a different optical re-tardation of polarized light depending whether the light isplane-polarized perpendicular or parallel to the sample. Thisexperiment therefore demonstrates how surface nanostructurecan influence the bulk properties of a material through self-organization.

Liquid-Crystal Displays: Fabrication and Measurement Wof a Twisted Nematic Liquid-Crystal Cell

Eric R. Waclawik*

School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD, 4001,Australia; *[email protected]

Michael J. Ford

Institute of Nanoscale Technology, University of Technology, PO Box 123, Broadway, NSW, Sydney, 2007, Australia

Penny S. Hale, Joe G. Shapter, and Nico H. Voelcker

School of Chemistry, Physics and Earth Sciences (SoCPES), Flinders University of South Australia, GPO Box 2100,Adelaide, SA, 5001, Australia
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Birefringence Measurements

The birefringence measurements of the sample are simpleand inexpensive to perform. A polarizer, a rotatable samplestage, a HeNe laser, or laser diode light source, a photo-diode detector, and an oscilloscope are required. The experi-ments described here use the 670-nm output of a 3-mW laserdiode purchased from Power Technology Inc. (Model PM03

111). Using this polarized, monochromatic light source theexperiment is effectively performed with a crossed polarizerarrangement. The vertically polarized laser output passesthrough the center of the polarizer (often referred to as theanalyzer). The analyzer is then rotated into a horizontal align-ment with respect to the laser polarizations direction so thatno signal is detected at the photodiode. The aligned nematicliquid-crystal cell is then placed between the polarizers onthe rotatable stage. Photodiode signal is digitized and storedon computer for different angles between the polarizationvector of the incident beam and the net orientation of theliquid crystal using the analog-to-digital converter channelof a National Instruments NI-DAQ 6024E data acquisitionboard and software written in Labview 6.1. Although the re-sults could just as easily have been read directly from the os-cilloscope display, in a separate part of their course studentshad been given instruction in basic electronics and program-ming in Labview. This project is intended to highlight theuse of these previously acquired skills towards data analysisin their experimental coursework.

According to the molecularstatistical theory of the nem-atic phase developed by Maier and Saupe, the overall align-ment of the 5CB sample is described by a single-orderparameter, S(6, 8), according to,

S = 1

2

3 12

cos (1)

where is the angle between the long-axis of the moleculeand the preferred overall alignment direction of the liquid-crystal sample (often called the director, n) and the bracketsindicate an ensemble average. Rotation of the cell (and hence5CB director) modulates the laser intensity at the photodiode,yielding results typical of those displayed in Figure 1. Twomaxima and two minima appear for 360 rotation corre-sponding to the 5CB-director being 45 to, or aligned withthe polarization direction of the laser light.1

The birefringence of the nematic liquid crystal is givenby the difference in refractive index measured when planepolarized light is parallel or perpendicular with respect to the

director; n=nn. A useful property of liquid crystals isthat they have a large n, typically ranging between 0.05 to0.5 over the infrared, near infrared, optical, and ultravioletregions of the spectrum (9). Any polarization state can beproduced by a combination ofn and sample thickness d.For monochromatic light of wavelength , the phase shift isgiven by

d n = 2

(2)

The intensity of transmitted laser light through the sampledepends upon this phase shift and 0, the angle between the

polarization vector of the incident beam and the orientationof the director, given by,

I I= 22

2 2sin ( ) sin

0 0 (3)

where I0 is the intensity of plane polarized light incident uponthe cell (9). Students are asked to compare their results to aplot ofI= constant sin2(20) (represented by the solid linein Figure 1) and to calculate the thickness of sample required

to create a quarter-wave plate out of 5CB material.

Experiment 2: Nematic-to-Isotropic Phase TransitionMeasurements

The second task for the students to perform is the mea-surement of the nematic-to-isotropic phase transition of 5CBusing the same crossed-polarizer arrangement. This experi-ment requires temperature control of the liquid-crystalsample. The liquid-crystal sample cell is clipped to a metalblock that is maintained at a precise temperature using aPeltier element. The metal block is machined from mild steelto snugly fit the completed cell. A hole is drilled through themounts center to allow laser light to pass through themounted cell. The cell-mount is glued to the Peltier elementusing conductive glue to maintain a good thermal contactand the blocks temperature is monitored with a thermistorthat controls the operation of the Peltier element in a con-tinuous feedback system.2 Using this apparatus, studentsmeasure the change in optical transmission through thesample and crossed polarizer as the temperature of the liq-uid-crystal sample is increased above the nematicisotropictransition temperature. As the sample temperature increases,the nematicisotropic transition temperature is reached wherethe intermolecular forces that orient the 5CB molecules areovercome. At this critical temperature, all cooperative effectsin the sample are lost and the liquid crystal spontaneously

Figure 1. Transmitted light intensity through an aligned sample of5CB placed between crossed polarizers as a function of directorangle. Signal maxima are observed when the sample director is45 with respect to the polarization axes of the laser and analyzer.Signal minima are observed when the sample director is parallelto one of the polarizer axes (0 and 90). The line drawn in thefigure corresponds to the sine-squared distribution given by eq 3,normalized to the maximum laser intensity.

Angle/deg

In

tensity

(arb

unit)

1.0

0.8

0.6

0.4

0.2

0.00 10 20 30 40 50 60 70 80 90
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melts to form an isotropic liquid. This arrangement yields anematicisotropic transition temperature of32 C (the nem-aticisotropic transition of 5CB is 35 C) (10).3 The transi-tion is shown in Figure 2.

A useful adjunct to this experiment might be the mea-surement of liquid-crystal order parameter as a function oftemperature by means of its photometric absorbance. Suchan experiment has been described using a guestdye moleculedissolved in a nematic liquid-crystal host. The dye is alignedby the nematic host and hence a difference in absorptivity isobserved when the long axis of the dye is oriented parallel tothe polarized light source compared to a perpendicular ori-

entation (5, 11).

Experiment 3: Twisted Nematic Liquid-Crystal Cell

In the second three-hour session, students manufactureda twisted nematic liquid-crystal cell to demonstrate theelectrooptic effect. The twisted nematic cells were con-structed using ITO conductive glass plates.4 The conductivesides of the conductive glass were identified with an ohm-meter before masking off a 4-mm section at the edge of eachslide using transparent tape. As previously, slides were coatedwith an alignment layer of PVA. The PVA was allowed tocure at room temperature for 10 minutes, after which timethe tape was peeled from the coated slides. The slides wereplaced in an oven at 60 for 20 minutes to complete the cur-ing process. The uncoated section of each conductive slidecould be readily located. An alignment was established onthe slides by rubbing with velvet cloth for 10 minutes as be-fore. Care was taken to ensure that students establish thealignment direction of one slide perpendicular to the un-coated section while the alignment (i.e., rubbing) directionof the second slide was parallel. The twisted nematic liquid-crystal cell was constructed from the coated ITO glass slidesby placing the conducting side of one slide facedown towardsthe conductive side of the second slide upon which two thin23-m strips of insulating mylar spacers had been laid. Twobulldog-clips were used to fasten the cell together before fill-

Figure 3. Optical transmission of a student-built twisted nematic cellbetween parallel polarizers. Voltages corresponding to 10%, 50%,and 90% transmission are marked giving steepness parameters p10= 0.4 and p50= 0.5.

U90

U50

U10

PotentialDifference/V

Trans

mittance(%)

100

90

80

70

60

50

40

30

20

10

0

0 5 10 15 20

ing it with a sample of 5CB (obtained from Aldrich). Sincethe surface orientation at the coated slides was 2 with re-spect to each other, a twisted nematic liquid-crystal cell wasthereby created. Connection of an external circuit to the con-ductive sections of the glass completed the twisted nematiccell.

The optical response of this student-built liquid-crystalcell to an applied electromagnetic field was compared to theperformance of a commercial twisted nematic liquid-crystaldisplay (TNLCD) extracted from an old calculator. When apotential difference is placed across certain compounds suchas liquid crystal 5CB, the long-axis of the molecules align

themselves with respect to the electromagnetic field. This ef-fect is sometimes referred to as electrically-controlled bire-fringence (ECB). An ac voltage output of a signal generatoris used to control the birefringence in these experiments. Thecell can be operated in a crossed polarizer arrangement orbetween parallel polarizers. The intensity of transmitted lightat the detector is monitored when a voltage is applied to thecell. Students first measure the threshold voltage at whichtheir twisted nematic liquid-crystal cell function. The thresh-old voltage for operation of the cell could then be comparedto the commercial TNLCD. The transmission-voltage curve(TVC) for a student-built twisted nematic cell that had beenplaced between parallel polarizers is shown in Figure 3.

The steepness of the TVC is an important factor for atwist cell defined by the steepness parameterp(8), for crossedpolarizers,

p50= (U50 U90) 1; p10= (U10 U90) 1 (4)

where U10, U50, and U90 correspond to 10%, 50%, and 90%optical transmission.

The speed with which the liquid-crystal cell respondedto an applied voltage was also measured (see Figure 4). Therise time (tr)decreases with decreasing applied voltage, as doesthe decay time (tdecay). The bounce occurring on the decaycurves is due to backflow, where rotation of the director wasaccompanied by movement of the liquid. This occurs for high

Figure 2. Typical results for optical transmission versus tempera-ture for a liquid-crystal director aligned 45 with respect to the la-ser and polarizer.

Temperature/C

Intensit

y

(arb

unit)

1.0

0.8

0.6

0.4

0.2

0.0

0 10 20 30 40 50
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driving voltages U> 2Uth,where Uth is the threshold voltagefor reorientation (9). Invariably the response of the student-built cell was slower than the commercial TNLCD. Studentswere required to speculate as to the reasons for the slowerresponse. The important factors are (i) the quality of the align-ment polymer layer and (ii) the thickness of the sample. Athicker sample results in a higher threshold voltage beforeswitching of the device occurs and a slower response (repre-sented by the steepness parameterp) to the applied electricfield.

Hazards

The liquid crystal 4-pentyl-4-biphenylcarbonitrile is hy-groscopic and should be kept in a dry sealed container. Whilelimited evidence of harmful effects owing to exposure exists,students were required to wear gloves when handling thischemical as a precaution.

Lasers with peak power less than 5mW are classified asClass IIIa laser products. Avoid direct eye exposure to thebeam. Any unnecessary specular and diffuse reflections shouldbe eliminated. For the purposes of the experiments describedherein, a laser with peak power less than 1 mW, a Class IIlaser, is preferred.

DiscussionThe experiment was usually performed by groups of

three students. The groups were free to decide which tasksthey wished to perform and the order with which the taskswere carried out. Responses to this set of experiments werepositive. Students appeared to enjoy constructing and test-ing the twisted nematic cell in particular. This introductionto the liquid-crystal state of matter and the connection be-tween concepts learned in other topics in their course wasappreciated. While the three experiments described herein(optical birefringence measurements, phase transition mea-surements, and display performance measurements) were per-formed over two three-hour sessions, each experiment could

easily be adapted to a three-hour laboratory as an introduc-tion to the properties of liquid crystals. Students were giventhe opportunity to explore the relationship between molecularstructure and intermolecular interactions that leads to prop-erties observed at the macroscopic scale. The observed prop-erties of the bulk material (electric and optical birefringence)were then used in construction of a useful technological de-vice, a twisted nematic liquid-crystal display.

Summary

Students were given the opportunity to explore the re-lationship between molecular structure and intermolecularinteractions that leads to properties observed at the macro-scopic scale. The observed properties of the bulk material(electric and optical birefringence) were used to construct ofa useful technological device, a twisted nematic liquid-crys-tal cell.

Acknowledgments

The authors wish to acknowledge the financial supportof the Flinders University Faculty of Science and Engineer-ing. We also thank Peter Palffy-Muhoray for his helpful sug-gestions on making an effective twisted nematic liquid-crystaldisplay.

Notes

1. If time permits, groups could perform an additional ex-periment using unrubbed PVA slides to emphasize the importanceof polymer surface structure and the effect of rubbing to create asingle ordered liquid-crystal domain. In contrast to Figure 1, un-changed transmission would be observed.

2. The thermoelectric cooler and temperature control unitswe re bo th purc ha se d fr om Oa tl ey Elec tron ic s, http://www.oatleyelectronics.com (accessed Mar 2004). A circuit design forthe temperature control kit can be downloaded from http://www.oatleyelectronics.com/pdf/k140.pdf (accessed Mar 2004). ThePeltier device used was a Melcor brand PolarTEC (PT) series ther-moelectric cooler are also available from Melcor, 1040 Spruce Street,Trenton, NJ 08648 NJ, USA; phone 609/393-4178; fax 609/393-746; http://www.melcor.com.

3. Students typically measured the clearing temperature towithin 13 Cof the literature value of 35 Cusing the student-built temperature stage. Appropriate placement of the thermocouple

wire on the metal mount and good thermal contact between thesample cell and heatercooler stage is essential to obtain an accu-

rate measurement of the clearing temperature.4. Conductive (tin dioxide coated) transparent glass; precut

commercial (2.5 cm 2.5 cm) TEC 10 or TEC 15 glass can bepurchased from one of several suppliers: Hartford Glass Co. Inc.,P.O. Box 613, Hartford City, IN 47348; phone 765/348-1282; fax765/348-5435; email: [email protected] or Pilkington LibbeyOwens Ford, 811 Madison Ave., P.O. Box 799, Toledo, OH 43697-0799; phone 419/247-4517.

WSupplemental Material

Instructions for the students and notes for the instruc-tor are available in this issue of JCE Online.

Figure 4. Response time for the twist effect. Two lines represent thedecay and the rise oscillograms for U90 for a student-built twistednematic display placed between parallel polarizers. The other linesrepresent oscillograms for U75.
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