Electrical Control of Shape in Voxelated Liquid Crystalline Polymer

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Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites Tyler Guin, Benjamin A Kowalski, Rahul Rao, Anesia D Auguste, Christopher A. Grabowski, Pamela Lloyd, Vincent P. Tondiglia, Benji Maruyama, Richard A. Vaia, and Timothy J. White ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13814 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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ACS Applied Materials & Interfaces

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites Tyler Guin,†‡ Benjamin A. Kowalski,†‡ Rahul Rao,†ꜟ Anesia D. Auguste,† Christopher A. Grabowski,† § Pamela F. Lloyd,† § Vincent P. Tondiglia,†‡ Benji Maruyama,† Richard A. Vaia,† Timothy J. White†* †

Air Force Research Laboratory, Materials and Manufacturing Directorate, 3005 Hobson Way, Wright-Patterson AFB, Ohio, 45433-7750, USA ‡

Azimuth Corporation, 4027 Colonel Glenn Highway, Beavercreek, Ohio 45431, USA

§

UES, Inc., 4401 Dayton Xenia Rd, Beavercreek, Ohio 45432, USA

Keywords liquid crystal elastomer, nanocomposite, actuator, electromechanical, shape change

ACS Paragon Plus Environment

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Abstract Liquid crystal elastomers (LCE) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels (3-dimensional volume elements) via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a DC field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be triggered with an applied DC field. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein. 1. Introduction One-dimensional, high aspect ratio nanomaterials such as carbon nanotubes possess unique anisotropic electrical, photonic, and mechanical properties.1 The addition of these materials in polymer matrices yields composites that assimilate the processing and durability of polymers with the distinctive properties (such as superior electrical or thermal conductivity or mechanical reinforcement) of the nanoinclusion.2, 3 Enforcing alignment upon the nanoinclusion can further enhance the properties of the nanocomposite by facilitating cooperative interactions.4


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The facile preparation and alignment of nanoinclusions in polymer nanocomposites is complicated by entropy, which is minimized in the disordered state.3, 5 A number of techniques have been explored to orient one-dimensional nanomaterials.

The most common method

employed to date has been extrusion, which aligns the nanomaterial through rheological forces.6, 7

This approach, along with other methods that rely on mechanical forces or external fields are

limited to uniaxial alignment without spatial control of the local order.1, 4, 6 Liquid crystalline materials are inherently anisotropic and benefit from cooperative interactions when aligned.8,9 Liquid crystals are typified by long-range order (orientation) which can be locally controlled through electric fields, magnetic fields, or surface alignment.10 Of interest to the work presented here, liquid crystallinity can be retained in various polymeric forms. Here, we prepare liquid crystal elastomers (LCEs), lightly crosslinked polymers which retain the order of their liquid crystalline precursors.11,

12

These materials undergo large

reversible deformations in response to heat,12 light,13 or solvent.14 Arbitrary and complex control of the local orientation of the liquid crystalline director within LCEs has been recently demonstrated.15 Localized irradiation of the photoalignment surfaces direct the self-assembly of liquid crystal monomers to yield a “voxelated“ (3dimensionally pixelated) LCE upon polymerization.15 The localization of the mechanical response of the materials enables stimuli-responsive transformation from 2-d flat sheets to 3-d shapes.16 Much of the recent literature exploring shape programming in LCEs has focused on thermally or photo-induced mechanical responses.17

Here, we focus on realizing electrical

control of topologically imprinted LCEs, which could enable easier device integration and


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swifter response times. In this way, this work builds upon limited but seminal prior efforts reported on the electromechanical response of LCEs and LCE composites.

A number of

examinations of electrically induced mechanical responses of polymer nanocomposites have employed resistive heating.

Comparatively few reports detail direct electromechanical

transduction. The most compelling of these results are electromechanical effects observed in tilted smectic LCEs, where the mesogen unit of the LCE is free to rotate in an electric field, resulting in macroscopic shear.

18, 19

However, smectic LCEs, despite displaying impressive

reversible strains (>8%), are not amenable to command surfaces and thus are not applicable to topologically complex director orientations. Isotropic-genesis LCEs (polymerized above their transition temperature) also display large electromechanical deformations, expanding in-plane through induced Maxwell stresses (similar to dielectric elastomers).20 However, as these LCEs have no net orientation, the electromechanical deformation is uniform which precludes their use in voxelated systems.20 Nematic LCEs have also been mildly sensitized to electrical control (95% carbon. PAAD-22 was provided by Beam Co., and was diluted to 0.33 wt% in dimethylformamide before use. Elvamide was provided by DuPont, and was dissolved into methanol as a 0.15 wt% solution. 2.2 Sample Preparation RM2AE and RM82 were added at a 2:1 mol ratio to a glass vial and dissolved in warm acetone. SWNT loosely suspended in acetone was then added to the vial, and the mixture was sonicated in a Branson ultrasonic bath for 1 hour. At all times, the mixture was shielded from fluorescent lighting. The mesogenic monomers act as a mild dispersant24-26 and prevent the SWNT from aggregating after sonication. The mixture was then sonicated in an ultrasonic bath heated to ~95°C, which removed the majority of the solvent while keeping the SWNT dispersed, and then the mixture was placed in a vacuum chamber at 100°C for 10 min to remove the remaining solvent. Afterward, EDT (equivalent molar to RM2AE) was added to the mixture


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along with 1 wt% Irgacure 651, and the mixture melt-mixed a minimum of three times. We note that the SWNT did not stay stably dispersed in the nematic state, and would precipitate over the course of several hours, so the mixture was held in the isotropic state until filling. This is not unexpected - previous work has found that nanomaterials can preferentially migrate to defect sites in nematic liquid crystals, eventually precipitating.24, 27 The LC monomers that the formulation is based upon form a stable room temperature nematic mixture with an isotropic transition at 24°C (DSC traces of the monomer mixtures shown in Figure S8).8 The SWNT increases the nematic to isotropic transition temperature (TNI) to 37°C. We believe that the processing conditions, in which small but influential concentrations of solvent are likely retained in the mixture. The incorporation of SWNT led to an increase in viscosity,28 making it difficult to fill liquid crystal cells at loadings higher than 0.08 wt% (images of cells filled with various loadings of SWNT shown in Figure S1). The mixture was filled into liquid crystal cells via capillary action at 100°C, cooled to room temperature over 20 minutes, and then polymerized at room temperature under 365 nm UV light (~150 mW/cm2) for 20 minutes. The film was removed from the cell by soaking in deionized water for 16 hours, followed by separating the glass slides with a razor blade. 2.3 Liquid Crystal Cell Preparation Glass slides were initially cleaned by successive rinses in acetone and methanol, followed by a 10 minute plasma cleaning treatment (Branson Ultrasonic Cleaner). To apply the photoalignment layer, a photoalignment solution (PAAD-22, BEAM Co.) was spin-coated onto the glass slides at 4500 RPM, and then baked at 100°C for 10 minutes. To apply the Elvamide buffed alignment layer, Elvamide solution was spin-coated onto the glass slides at 4500 RPM,


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and allowed to dry in ambient conditions. The Elvamide alignment layer was then rubbed in one direction 30 times with a felt cloth. Two glass slides were then glued together using UV-curable epoxy (Epofix 68) and glass spacers. Alignment of the photopatterning layer was achieved by either using a vector vortex waveplate (Beam Co.) to produce a +1 defect or by using a spatial light modulator to produce pixelated patterns of linearly polarized light, as detailed in ref. 29. In brief, the PAAD-22 photoalignment layer orients orthogonally to the polarization of the light, which then induces alignment of the liquid crystal mixture through the cell thickness. 2.4 Material Characterization Phase transitions, birefringence, and film quality were measured using polarized optical microscopy (POM) (Nikon) in transmission mode, and the temperature was controlled by a Mettler Toledo HS82 heat stage. Birefringence of films was measured at a wavelength of 600 nm using a Newport 818-UV photodiode and 600 nm filter attached to the microscope, as described in previous reports.30 Shape change of homogenous planar films, floating on silicone oil and 5 µm glass spacers, as a function of temperature was also determined using POM. Dynamic scanning calorimetry (DSC) (TA Instrument Q1000) was performed under nitrogen from -40°C to 100°C for monomer mixtures and -40°C to 250°C for cured films in hermetically sealed pans. The nematic transition determined from the peak of the heat flux trace on second cooling, and the glass transition was determined from the peak of the derivative of the heat flux trace. Polarized Raman spectroscopy was performed on 15 µm thick samples with a Renishaw inVia confocal Raman microscope. Focused light (100x objective, 600 nm spot) from 514.5 and 633 nm laser excitation sources was used to excite the samples at various spots. The polarization of the incident laser was rotated every 5 degrees from -90 to 90 deg. to obtain angle-dependent Raman scattering from the LCE and the SWNTs. Furthermore, polarized Raman spectra were


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collected at various confocal depths through the thickness of the films in the case of the twisted nematic films. Procedure for calculating order parameter from Raman spectroscopy is shown in prior literature,31,32 but in brief, is derived from the peak intensity at the LCE and G’ band peaks as a function of incoming laser polarization. Wide angle X-ray scattering was performed using a Rigaku Ultrax and Cu Kα radiation on a 15 µm thick sample with uniaxial alignment. Order parameter was calculated as shown in previous work.33 0.02 wt% SWNT-LCE nanocomposite films were fixed in OsO4 and then embedded into flat molds with Epofix resin so that the cutting direction would be either parallel or perpendicular to the LCE / SWNT orientation direction. The blocks were polymerized overnight at 60°C, and then the blocks were hand trimmed with razor blades to form a trapezoid face.

Then the blocks were ultramicrotomed using an RMC Ultracut microtome with a 35°

Diatome diamond knife. 75 nm thick section were collected onto a 400 hex Cu mesh grid and allowed to dry. Imaging was captured using an FEI CM 200 transmission electron microscope at 200 kV. Digital images were captured with a CCD camera and a 4Pi system. FTIR spectra of monomer polymerization between two 1 cm thick NaCl slides were collected using a Bruker FTIR in transmission mode. Upon exposure to UV light, four scans were taken from 400 cm-1 to 3200 cm-1 every 0.5 seconds for 30 minutes. Gel fraction was performed by extraction in acetone for 24 hours, and drying at 45°C under vacuum. The gel fraction of neat LCE and SWNT-LCE sample was found to be 0.78 ± 0.1. Mechanical measurements were performed at 25°C on an RSA3 TA Instruments tensile tester in uniaxial strain mode at 2% strain/minute. Samples dimensions were 5 x 2 x 0.015 mm, and all tests were performed a minimum of five times with five different samples.


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Electromechanical measurements of a homogeneous 15 µm films were imaged using POM while the film floated in silicone oil between two ITO-coated glass slides, spaced 100 µm apart. Strain was found by measuring the area of the film as a function of time using a homemade MATLAB program. The temperature was controlled using a home-made ITO heat stage and surface thermocouple. Electromechanical actuation of photopatterned films was measured using a Keyence optical profilometer 3D scanner. 12 µm thick films, in air, were placed between ITO-coated glass slides spaced 1 mm apart, and the field applied through the film thickness. The temperature was controlled using a home-made ITO heat stage. Temperatures were kept below 120°C for electromechanical measurements due to limitations of the testing apparatus and oil. Electrical conductivity of 0.02 wt% SWNT-LCE films was performed in an ITO-glass liquid crystal cell with homogeneous Elvamide alignment layers. Voltage was swept from 0-2 V/µm in 0.1 V intervals, and the current measured after 2 second equilibration. This sweep was performed three times, and data reported is from the second sweep. Dielectric impedance and permittivity measurements were conducted on ITO-glass liquid crystal cells with homogeneous Elvamide alignment layers using a Novocontrol Alpha Analyzer. The LC cells were placed inside an oven (Memmert), where temperature was ramped from 25240°C at a rate of 0.5°C/min. Permittivity was measured at discrete frequencies, swept over the range 0.5 Hz to 1 MHz at an AC driving voltage of 1 V; a new scan was initialized every 100 seconds.

3. Results and Discussion


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The nanocomposites were prepared by incorporating pristine SWNTs into a liquid crystal monomer mixture (chemical structures shown in Figure 1A).

A variety of monomeric

formulations were considered. We selected a thiol-ene-acrylate formulation that is amenable to surface alignment8 and sufficiently compatible with SWNT.24, 25, 34, 35 Notably, the incorporation of SWNT increases the viscosity of these mixtures3 making it difficult to fill liquid crystal cells at SWNT concentrations exceeding 0.08 wt% (Figure S1). After filling and subsequent cooling to the nematic state, the SWNT-LCE mixture took on the surface alignment of the cell and retained the orientation after photopolymerization (Figure 1B). To illustrate the ability to impart spatially complex orientation patterns into the nanocomposite, an image of an F-14 fighter jet was imprinted into an alignment cell, filled with the pre-polymerization mixture composed with 0.02 wt% SWNT, and photopolymerized to prepare a SWNT-LCE nanocomposite that retains this image (Figure 1C). SWNTs exhibit a very strong Raman signal, which can enable accurate determination of their orientation via polarized Raman spectroscopy.36 The radial breathing modes (RBMs) and the G and G’ bands of the SWNT are apparent in the Raman spectra in Figure S2. Due to overlap in the Raman signals from the G band of the SWNTs and the LCE composition, we chose the G’ band at 2670 cm-1 to represent the long axis of the SWNT (Figure 1D). The strength of the G and G’ Raman signals are dependent on the orientation of the SWNT to the incident polarization of the Raman probe. The signal at 1700 cm-1 was selected to represent LC mesogen orientation. The average orientation of the SWNT and the LC mesogen was determined by adjusting the orientation the laser polarization and comparing the relative intensities of the normalized peak areas.32, 37 Evident in Figure 1D, both the SWNT and the LC mesogens take on the surface alignment of the cell.

The orientation parameter31 of the LC mesogens was


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calculated from these data to be 0.46 ± 0.08, while the SWNT orientation was found to be slightly higher, 0.51 ± 0.08. This orientation parameter is in good agreement with the orientation parameter for the liquid crystalline elastomers measured by wide-angle X-ray scattering (Figure S3). Notably, the orientation parameter of the LCE host (without SWNT) was nearly identical (0.46), indicating that the SWNT are not disrupting the mesogen alignment. To confirm the association of the SWNT and LCE in more complex topologies, a +1 radial defect was imprinted into an LCE containing 0.02 wt% SWNT via a photoalignment surfaces. The SWNT orientation matched the expected director profile (Figure 1E). These results confirm that previously developed methods to voxelate orientation in LCEs can be extended to arbitrarily align onedimensional nanomaterials in a monolithic polymer matrix. The formulations employed here to prepare the nanocomposites were intended to generate elastomers.8 Both the LCE and SWNT-LCE have glass transition temperatures (Tg) at 4°C. Despite similar glass transitions, the thermally-induced mechanical deformation in these materials are distinguished. In Figure 2A, the incorporation of 0.02 wt% SWNT into the LCE reduces the thermally induced contraction from 140% to 60%. To clarify whether the difference is attributable to an influence on the thermotropic nature of the materials, the birefringence of both the neat LCE and SWNT-LCE nanocomposites were examined over a range of temperatures (Figure 2B). The temperature dependence of the neat LCE sample and the SWNTLCE nanocomposite are effectively identical with the exception of a slight retention of residual birefringence for the SWNT-LCE sample at elevated temperatures (Figure 2B inset). Many nanocomposites exhibit enhanced mechanical properties, such as stiffness, in part derived from the anisotropy introduced by nanoinclusions6,

7

From these prior studies, it is

expected that the incorporation of SWNT will modify the LCE mechanical properties. The


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deformation of LCE in the planar orientation parallel to the director orientation exhibit classical linear stress-strain response of an elastomer. When the director orientation of the LCE is orthogonal to the stretch direction, these materials display nonlinear elastic behavior, known as “soft elasticity”.12, 38 The plateau in the stress-strain curve is associated with the rotation of the mesogens in the stress field.39 Evident in Figure 2C, SWNT-LCEs display a comparatively higher (and reinforced) modulus parallel to the director compared to an LCE without SWNT. Not surprisingly, there was no measurable modulus difference perpendicular to the director. The modulus increase with the inclusion of just 0.02 wt% SWNT is ~24% (Z=5, P < 0.001) and falls well within predicted modulus increase for aligned SWNT composites.40 However, as evident in the representative stress-strain curves presented in Figure 2C, the maximum elongation (strain to failure) of the SWNT-LCE is reduced when the director is orthogonal to the stretch direction. Considerable recent research has reported on the ability to transform a variety of material systems from 2-d (flat) into 3-d (objects). Shape programming in LCEs has leveraged advances in the ability to imprint various topologies and relevant tessellations into these material systems to transform the films into shapes when exposed to heat, solvent, or light.11, 15, 16, 41 These stimuli disrupt the order of the liquid crystalline material and accordingly produce spontaneous and anisotropic dimensional changes defined by the director orientation.11 Potential end use applications of these materials as actuators will require response times not yet achievable with heat or light stimuli. As such, electric fields have long been acknowledged as a desirable stimulus to trigger shape change or actuation. Here, we demonstrate that the incorporation of SWNT into the LCE sensitizes the nanocomposite to an electrical stimulus. Experiments were undertaken with 0.02 wt% SWNT-LCE films that were floated on oil between two ITO-coated slides (unless otherwise noted). The electromechanical responses reported in Figure 3 are the


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result of an applied DC field across the sample thickness. The shape change of the film was monitored between cross-polarizers.

Evident in Supporting Movie S1, the films primarily

constrict along the director orientation achieving an 18% reduction in measured length at 100°C and 5 V/µm (Figure 3A). Due to slight heterogeneity in the dispersion of SWNT across the sample thickness (TEM micrographs shown in Figure S4), some bending is also observed (Figure S5).a The electromechanical responses reported in Figure 3A are strongly temperature dependent. The nanocomposite prepared with this composition is not responsive to applied DC field at room temperature. Figure 3B examines the electromechanical response of the SWNT-LCE composite at 80°C as a function of voltage.

The nanocomposites constrict rapidly, reaching maximum

deformation in less than one second (Figure 3B) and return to their original state quickly (4.5 V/µm), the films show signs of unrecoverable deformation. Neat LCE films (prepared without SWNT) do not display any electrostrictive response. SWNT-LCE nanocomposites, aligned such that the director field can be described as a +1 radial defect (Figure 3D), were placed in a 1 mm thick, ITO cell to simulate free-standing actuation without the need for flexible electrodes or electrical contacts. Upon heating to 90°C, the films began to form a very shallow cone (Figure 3Eii). 42 At this temperature, a 1.5 V/µm field (below the breakdown voltage of wet air) was applied to electrically induce a rapid shape

a

We note that electrostriction was observed in samples with SWNT concentrations up to 0.08 wt%, but that consistent sample preparation (and therefore consistent electrostriction) was complicated at higher loadings due to the increased monomer viscosity (Figure S1).


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deformation (Figure 3Eiii). The cone-like shape appeared but is accompanied with some asymmetry (Figure S6A41). When the field was released, the film quickly returned to the shallow cone evident in Figure 3Eii. While these results deviate slightly from prior reports of thermal and photo-induced deformation of this topological profile in LCEs, an electric field is shown to generate a complex and curved shape by simply applying a small DC field. This shape change is smoothly varying across the film (Figure S6B). Our current hypothesis is that this shape is a combination of conical deformation expected of this topological director profile as well as bending attributable to the heterogeneous dispersion of SWNT across the sample thickness (TEM micrographs shown in Figure S4). The heterogeneity not only affects the electrical susceptibility but also influences the local stiffness. We hypothesize that the electromechanical response observed in Figure 3 is attributable to a combination of rotation of the SWNT ascribed to dielectric mismatch between the SWNT and the LCE43 and mesogen rotation via interfacial polarization between the nanoinclusions and the LCE.44 Previous work examining shape deformation induced with resistive-heating (Joule heating) of LCE nanocomposites produced responses that are transient and comparatively slow.45 We can reject the possibility that the observed deformation of the SWNT-LCEs produced here is electro-thermal in nature, as the films deform and recover in less than 1 s. Further, resistive heating can be induced with either an AC or a DC field. The SWNT-LCE prepared and examined here are only responsive to low frequency AC fields with 5 Hz or less. To further confirm that the deformation is not electrothermal in nature, the temperature of a homogeneous planar film was monitored via a surface contact thermocouple as a DC electric field was applied, and the SWNT-LCE heated by less than 3°C over 60 seconds. The specifics of the electric field


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susceptibility of the electromechanical response of the SWNT-LCE indicates that the interface of the SWNT and the LCE host may be critical. To further explore the electrical properties of these materials, we employed dielectric relaxation spectroscopy (DRS) to isolate if the LCE dipoles rotate more freely in the electric field in the presence of SWNT. The neat LCE displayed no dipole relaxation from 0.5-106 Hz until heated above 100°C.

In contrast, the SWNT-LCE films displayed a strong dipole

relaxation in the same frequency range by 65°C (Figure 4A). At all measured temperatures, the SWNT-LCE dipole relaxation occurred at a much higher frequency than the neat LCE film. This indicates that the LCE dipoles experience much less resistance to motion in an electric field and are able to relax faster.46, 47 The SWNT-LCE samples display no relaxation of the dipoles (and therefore motion) at room temperature, mirroring the lack of electrostriction. The permittivity of a dielectric material is increased both from the dipole rotation, which effectively screens the field, 48 and from additional free surface charge accumulation.49 From 50-150°C, the permittivity of the SWNT-LCE is much greater than the permittivity of the neat LCE, which indicates a significant increase in either dipole mobility, charge accumulation, or both (Figure 4B).47-50 Additionally, the dipole rotational activation energy can be determined from the relaxation peak as a function of temperature (Figure S7),48 and we find that the SWNT lowered the dipole rotational activation energy by 15% when the material was below the TNI. Previous work in polymer nanocomposites prepared with carbon nanotubes found that the large interfacial area, coupled with the formation of electron donor-acceptor complexes at the interfaces of the nanoparticle, greatly enhances the permittivity of the nanocomposite due to accumulated interfacial charges.50 These accumulated charges produce extremely high localized fields, which induce a strong polarization (and therefore motion) of the surrounding polymer.43 It is possible


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that these accumulated charge complexes only occur at elevated temperatures, potentially due to a mismatch of strain at the SWNT-LCE interfaces. To determine if the SWNT themselves are reorienting in the electric field, the electrical conductivity through the film thickness was measured as a function of field strength. If the SWNT are sufficiently aligned through the film thickness, then an increase in electrical conductivity should be apparent.7 Evident in Figure 4C, the films display highly non-Ohmic conductivity, increasing in conductivity with increasing DC electric field. At high field strength, the film became modestly semiconductive (

Electrical Control of Shape in Voxelated Liquid Crystalline Polymer

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