controllable electrostriction of polyurethane film
TRANSCRIPT
Controllable Electrostriction of Polyurethane Film
Masae Kanda 1, a, Kaori Yuse 2,b , Daniel Guyomar 2,c and Yoshitake Nishi 1,d
1 Department of Materials Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa,
259-1292, JAPAN
2 LGEF, INSA Lyon, Bat. Gustave Ferrié, 69621, Villeurbanne Cedex, FRANCE
a [email protected], b [email protected], c [email protected],
Keywords: electrostriction, electro active, crystalline periodicity, controllable, polyurethane
Abstract
Although their experimental errors can be observed, pure polyurethane (PU) elastomers are one
of the most important class of polymers due to some remarkable electromechanical characteristics
such as large electric field induced strain, high specific energy and fast speed of response. In order
to obtain the large strain at low electric field, a dependence of the solidification condition on strain
was investigated for pure polyurethane films. Optimum solidification condition to get thin film with
19 µm thickness remarkably enhanced the strain at high electric field at high electric filed, although
they show the low strain at low electric field at low electric filed. The starting point of the
convergence occurred at a lower electric field for the solidification condition to get thick film with
150 µm thickness as opposed to for the optimum condition to obtain the thin film with 19 µm
thickness. Based on results of crystalline volume fraction and crystalline periodicity, strongly
attributed to not only polarization, but also electrostriction, the strain was controlled by the
solidification condition. The optimum solidified samples do not have convergence until 20 MV/m.
Based on the prediction and experimental results, the electrostriction of PU films depended on its
solidification condition.
Introduction
Polymer actuators demonstrate numerous advantages, such as soft actuation, easy manufacturing,
being lightweight and, especially, presenting a large deformation. In general, a high driving energy
is required. [1] The strain level of piezoelectric materials, which are the most conventional actuator
materials, is quite small (< 0.2 %), but they require only a low electric driving field (< 5 MV/m).
Large strains can be generated in dielectric polymer actuators since electric dipoles are assumed
to be present within the polymer. In many research investigations, large electroactive strains up to
10-100 % have been readily observed. [2, 3] Since the strain of human muscles is limited to around
20 %, these materials would be quite interesting for fabricating artificial muscles.
Electroactive strain values up to 380 % have even been obtained when a silicone rubber was
utilized as the matrix material. [1] Although such extraordinary results have drastically increased
the potential of EAPs, it should be noted that these data were obtained under quite high values of
the driving electric field, i.e., 120 MV/m. This limits the use of EAPs since few portable electric
components can stand to be used together at such high electrical input. A reduction of the required
energy is thus desired for a widespread use of EAPs.
Dielectric EAPs are able to generate a large electroactive strain (> 30 %) while requiring only a
low electric field (< 20 MV/m). [4] Polyurethane (PU) is one of the most versatile materials in the
world today. Their many uses range from flexible foam in upholstered furniture, to rigid foam as
insulation in walls, roofs and appliances to thermoplastic pure PU used in medical devices and
Materials Science Forum Vols. 783-786 (2014) pp 2429-2432Online available since 2014/May/23 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.783-786.2429
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footwear, to coatings, adhesives, sealants and elastomers used on floors and automotive interiors.
Over the last two decades, the field of electrically controllable polymer actuators has developed
significantly because their performances are comparable to those of natural muscles. However, their
experimental errors can be observed. Pure PU elastomers are one of the most important class of
polymers due to some remarkable electromechanical characteristics such as large electric field
induced strain, high specific energy and fast speed of response. [5-9] This makes the material very
attractive for many electromechanical applications. Many electroactive strain properties of the PU
were investigated but the fundamental mechanisms which are responsible for the electrostriction
have not been yet well understood. [10]
We found that the thermal absorption on melting of differential scanning calorimetry (DSC)
analysis often corresponds to data of electrostriction. [11] Crystalline volume fraction may
contribute to the electrostriction. If the volume fraction of surface crystallization on solidification is
controlled by solidification thickness, the experimental error can be explained to obtain the
reproducible data. Thus, the influence of solidification thickness on electrostriction has been
investigated for pure PU before making composites to confirm the standard to prepare the
composite films. The purpose of the present paper is to report the influence of solidification
controlling on electrostriction of the pure PU films.
Experimental procedure
Pure polyurethane (PU) film was prepared by a simple solution cast method. [11-13] One gram of
PU granules (Noveon Estane 58888 NAT021, Lubrizol Corporation, Wickliffe, OH, USA) was
dissolved in approximately 20 ml of N,N-dimethylformamide (DMF) at 85 ºC for 45 min. The
solution was poured onto a glass plate and dried at 60 ºC at atmospheric pressure for 1 day. The
obtained films were removed from the plate with ethanol. Subsequently, they were placed in a
ventilated oven at 130 ºC for 4 h in order to eliminate residual solvent. The thicknesses of the films
varied from 19 to 150 µm. For the electromechanical characterization measurements, metal
electrodes were placed on both sides of disc-shaped specimens (25 mm in diameter).
The field-induced thickness strain was measured by a laser interferometer (Agilent 5519A) with a
precision on the order of 5 nm. The induced electric field was a sawtooth wave for 2-cycles at 0.1
Hz. Its maximum was varied with an upper limit at 20 MV/m. The film samples were placed on a
horizontal brass disc (20 mm in diameter) in order to avoid measuring a parasitic flexural motion,
and a second brass disc placed on the upper side of the film rendered it possible to apply a bipolar
electric field. A function generator (Agilent 33220A) delivered the corresponding bipolar voltage
amplified by a factor of 1000 through a high-voltage lock-in amplifier (Trek 10/10B). The ground
current between the sample holder and the ground was measured using a current amplifier (Stanford
Research Systems SR570).
Results
The starting point of the convergence occurred at a lower electric field for the thick films as
opposed to for the thin films. Optimum solidified thin film with 19 µm thickness does not have
convergence until 20 MV/m. Although the thin film shows the high strain value, the strain
saturation can not be found until 20 MV/m. The thin film with 19 µm thickness enhances the strain
at high electric field, although the thick film with 150 µm thickness exhibits the lower strain at high
electric field. On the contrary, thinning the films reduce the strain at low electric field, whereas
thick films apparently exhibited the higher strain of low electric field. Namely, thinning the films
remarkably enhance the large strain more than 27% at high electric field of 20 MV/m, whereas thick
films apparently exhibit the higher strain at low electric field of less than 4 MV/m. The strain
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saturation occurs at low electric field for thick sample. We conclude that thinning the PU thickness
is a useful tool to obtain the large strain at low electric field.
Discussion
Differential scanning calorimetry (DSC: 131Evo, SETARAM, France) analysis was used to
confirm the volume fraction of crystalline of pure PU films with different thicknesses. The
endothermic heat is usually generated by transformation from crystal to liquid on melting. It
corresponds thus to volume fraction of crystalline form in material. The fusion enthalpy values were
obtained by area of endothermic peak. DSC analysis was carried out to see the fusion enthalpy
value of thin and thick films. Endothermic heat from crystal to liquid on melting of thick pure PU
film is smaller than that of thin pure PU film. The fusion enthalpy value of thin pure PU film with
28 µm thickness is 1.24 times higher than that of thick pure PU film with 140 µm thickness. [11]
The thin pure PU film shows high strain at 20 MV/m. The results show that the strain of films
depends on thickness. Thus, it is clear that the high electrostriction of pure PU thin films can be
contributed by high volume fraction of surface crystalline. Although the crystalline volume fraction
strongly contributes to the electrostriction, the large change in electrostriction quantitatively cannot
correspond to the volume fraction. Thus, the crystalline periodicity should be considered.
X-ray diffraction (XRD: D8 ADVANCE, BRUKER) was used to confirm the internal structures
of periodicity of pure PU films with different thicknesses. The peak width corresponds to the
periodicity perfection. Most of the sharp peaks are related to thin pure PU film, whereas the broad
peak is related to the thick pure PU film. The big broad peaks from 12 to 29 degree for thick film
and from 16 to 26 degree for thin film were found at about 20 degree for soft segments. [14] The
sharpness with intensity and width of the peak corresponded to an enhancement of the periodicity.
An additive effect to compressive electrostriction could be gotten, when the periodicity
enhancement raised the polarizability. Namely, the angle width of thin pure PU film is smaller than
the thick one. The angle width corresponds to the periodicity of soft segments. In addition,
remarkable sharp peaks at 28.7 and 26.7 deg of hard segments were found in thin films, whereas
they were not observed in thick film. [15, 16] Thus, it is possible that thin film enhances the molar
volume and/or their preferred orientation ratio of hard segments and probably enhances the
polarizability, resulting in an additive effect to compressive electrostriction.
The pure PU (thin and thick) films show thickness effect. Furthermore crystallized volume
fraction and crystal perfection were confirmed to have changed by thickness difference. In other
terns, we can say that constituting thin film presents same differences to the material of thick film.
These films were fabricated by solution cast method. The films were solidified on a clean surface.
After the films were removed from the glass plate, they were dried again in high temperature. Two
surfaces were in contact with heated air at that time. Explanation can be given. One is the thermal
conductivity in material and the other is the oxidation from the surfaces. It is possible that formation
of a crystal starts from the surfaces due to the thermal gradients. A property gradient takes place
(For instance ratio soft/hard segment). Outside of film can have different morphology to the inside
one. In other terms, film can have skins. Neither DSC nor XRD can see the different of layers in
material, but the results do not against to this hypothesis. The crystallized volume fraction and
crystalline perfection of thin film are higher then thick film.
Materials Science Forum Vols. 783-786 2431
Conclusion
In order to obtain the large strain at low electric field, a dependence of the solidification thickness
on strain was investigated for pure polyurethane films.
1: Optimum solidified thin film with 19 µm thickness remarkably enhanced the strain at high
electric field at 20 MV/m.
2: Thick films with 150 µm thickness apparently exhibited the higher strain at low electric field of
less than 4 MV/m.
3: The starting point of the convergence occurred at a lower electric field for the thick films as
opposed to for the thin film, which does not have convergence until 20 MV/m.
4: Based on results of crystalline volume fraction and crystalline periodicity, the solidification
thickness dependent strain was explained.
5: Effects of making composite on strain were probably contributed by the polarization
enhancement induced by increasing the capacitance.
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