Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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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 F. Lloyd,†,§ Vincent P. Tondiglia,†,‡ Benji Maruyama,† Richard A. Vaia,† and Timothy J. White*,† †
Air Force Research Laboratory, Materials and Manufacturing Directorate, 3005 Hobson Way, Wright-Patterson AFB, Ohio 45433-7750, United States ‡ Azimuth Corporation, 4027 Colonel Glenn Highway, Beavercreek, Ohio 45431, United States § UES, Inc., 4401 Dayton Xenia Rd, Beavercreek, Ohio 45432, United States S Supporting Information *
ABSTRACT: Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) 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 electrically triggered. Initial experiments indicate that the SWNT−polymer interfaces play a crucial role in enabling the electrostriction reported herein. KEYWORDS: liquid crystal elastomer, nanocomposite, actuator, electromechanical, shape change
1. INTRODUCTION
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 directs the self-assembly of liquid crystal monomers to yield a “voxelated” [three-dimensionally (3-D) pixelated] LCE upon polymerization.15 The localization of the mechanical
One-dimensional (1-D), high aspect ratio nanomaterials such as carbon nanotubes possess unique anisotropic electrical, photonic, and mechanical properties.1 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 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 1-D 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, is limited to uniaxial alignment without spatial control of the local order.1,4,6 © XXXX American Chemical Society
Received: September 11, 2017 Accepted: December 14, 2017 Published: December 14, 2017 A
DOI: 10.1021/acsami.7b13814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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 along with 1 wt % Irgacure 651, and the mixture was 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 [dynamic scanning calorimetry (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, account for this low TNI. 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 min, and then polymerized at room temperature under 365 nm UV light (∼150 mW/cm2) for 20 min. The film was removed from the cell by soaking in deionized water for 16 h, 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 min plasma cleaning treatment (Branson Ultrasonic Cleaner). To apply the photoalignment layer, a photoalignment solution (PAAD22, BEAM Co.) was spin-coated onto the glass slides at 4500 rpm and then baked at 100 °C for 10 min. To apply the Elvamide buffed alignment layer, Elvamide solution was spin-coated onto the glass slides at 4500 rpm and allowed to dry under 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 UVcurable epoxy (Epofix 68) and glass spacers. Alignment of the photopatterning layer was achieved by using either a vector vortex waveplate (Beam) to produce a +1 defect or 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 818UV photodiode and a 600 nm filter attached to the microscope, as described in previous reports.30 Shape change of homogeneous planar films, floating on silicone oil and 5 μm glass spacers, as a function of temperature was also determined using POM. DSC (TA Instrument Q1000) was performed under nitrogen from −40 to 100 °C for monomer mixtures and −40 to 250 °C for cured films in hermetically sealed pans. The nematic transition was 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 (100× objective, 600 nm spot) from 514.5 to 633 nm laser excitation sources was used to excite the samples at various spots. The polarization of the incident laser was rotated every 5° from −90 to 90° to obtain angle-dependent Raman scattering from the LCE and the SWNTs. Furthermore, polarized Raman spectra were 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 literature31,32
response of the materials enables stimuli-responsive transformation from two-dimensional (2-D) flat sheets to 3-D shapes.16 Much of the recent literature exploring shape programming in LCEs has focused on thermally or photoinduced mechanical responses.17 Here, we focus on realizing electrical control of topologically imprinted LCEs, which could enable easier device integration and 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 h. At all times, the mixture was shielded from fluorescent lighting. The mesogenic monomers act as a mild dispersant24−26 and prevent the B
DOI: 10.1021/acsami.7b13814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces 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 (WAXS) 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 SWNT−LCE nanocomposite films (0.02 wt %) 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. Seventy-five nanometers thick section was collected onto a 400 hex Cu mesh grid and allowed to dry. Imaging was captured using an FEI CM200 transmission electron microscope at 200 kV. Digital images were captured with a chargecoupled device camera and a 4Pi system. Fourier transform infrared (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 to 3200 cm−1 every 0.5 s for 30 min. Gel fraction was performed by extraction in acetone for 24 h 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 the uniaxial strain mode at 2% strain/minute. Samples dimensions were 5 × 2 × 0.015 mm, and all tests were performed a minimum of five times with five different samples. Electromechanical measurements of a homogeneous 15 μm films were imaged using POM while the film floated in silicone oil between the two indium tin oxide (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 homemade ITO heat stage and a surface thermocouple. Electromechanical actuation of photopatterned films was measured using a Keyence optical profilometer 3D scanner. Twelve micrometers thick films, in air, were placed between the ITOcoated glass slides spaced 1 mm apart, and the field applied through the film thickness. The temperature was controlled using a homemade ITO heat stage. Temperatures were maintained below 120 °C for electromechanical measurements due to the 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 to 2 V/μm in 0.1 V intervals, and the current was measured after 2 s equilibration. This sweep was performed three times, and the data reported are 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 the temperature was ramped from 25 to 240 °C at a rate of 0.5 °C/min. Permittivity was measured at discrete frequencies, swept over the range of 0.5 Hz to 1 MHz at an ac driving voltage of 1 V; a new scan was initialized every 100 s.
Figure 1. (A) Chemical structures of the liquid crystal monomers and the dithiol employed to produce the LCE. (B) Polarized optical micrographs of 15 μm thick films prepared with and without SWNT. The scale bar is 50 μm. (C) A 12 μm thick 0.02 wt % SWNT−LCE film was prepared, retaining an image that was imprinted into the photoalignment surfaces. The scale bar is 500 μm. (D) Normalized Raman intensity as a function of polarization angle for identifiable Raman signals of the LCE and SWNT in a 15 μm thick 0.02 wt % SWNT−LCE film with planar alignment. (E) Average orientation of the SWNT (determined by Raman) in a SWNT−LCE film when imprinted with a radial +1 defect (birefringent pattern inset) as a function of the expected orientation.
3. RESULTS AND DISCUSSION 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 the SWNT.24,25,34,35 Notably, the incorporation of SWNT increases the viscosity of these mixtures,3 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 F14 fighter jet was imprinted into an alignment cell, filled with the prepolymerization mixture composed of 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 and the G and G′ bands of the SWNT are apparent in the Raman spectra in Figure S2. Because of the overlap in the Raman signals from C
DOI: 10.1021/acsami.7b13814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces 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 is 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 of 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 calculated from these data to be 0.46 ± 0.08, whereas 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 WAXS (Figure S3). Notably, the orientation parameter of the LCE host (without SWNT) was nearly identical (0.46), indicating that the SWNT is 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 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 1-D 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 is distinguished. In Figure 2A, the incorporation of the 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 was examined over a range of temperatures (Figure 2B). The temperature dependence of the neat LCE sample and the SWNT−LCE nanocomposite is 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 mechanical properties of the LCE. The deformation of the LCE in the planar orientation parallel to the director orientation exhibits 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 the predicted modulus increase for aligned SWNT composites.40 However, as evident in the representative stress−strain curves
Figure 2. (A) Shape change of the films along the director as a function of temperature. (B) Birefringence of neat and 0.02 wt % SWNT−LCE films as a function of temperature. (Inset) Photographs of 2 mm × 4 mm 15 μm thick films floating in oil at 250 °C. (C) Uniaxial tensile testing curves of 15 μm thick neat and 0.02 wt % SWNT−LCE films at 25 °C.
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 3D (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 the SWNT into the LCE sensitizes the nanocomposite to an electrical stimulus. Experiments were undertaken with 0.02 wt D
DOI: 10.1021/acsami.7b13814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. (A) Relative length change along the director (L/Lo) of a square film composed of a planarly aligned 0.02 wt % SWNT−LCE 15 μm film floating in oil. (B) Relative length change as a function of time at applied dc fields from 0.9 to 4.5 V/μm, at 80 °C. (C) Length change of the film at 90 °C in a 0.5 Hz 2.5 V/μm electric field. (D) Twelve micrometers 0.02 wt % SWNT−LCE film photopatterned such that the director field is described as a +1 defect. The polarized micrograph is overlaid with contour lines shown to represent localized LCE and SWNT orientation. The square represents the boundary of the harvested film examined in (E). (E) (i) Topographical features within the film before heating, (ii) after heating to 90 °C, and (iii) after a 1.2 Vdc/μm field applied. Images collected with a Keyence optical profilometer.
% SWNT−LCE films that were floated on oil between the two ITO-coated slides (unless otherwise noted). The electromechanical responses reported in Figure 3 are the result of an applied dc field across the sample thickness. The shape change of the film was monitored between crosspolarizers. Evident in the Supporting Information Movie S1, the films primarily constrict along the director orientation achieving an 18% reduction in the measured length at 100 °C and 5 V/μm (Figure 3A). Because of slight heterogeneity in the dispersion of the SWNT across the sample thickness [transmission electron microscopy (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 1 s (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 3E(ii)).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 deformation (Figure 3E(iii)). A conelike 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 3E(ii). While these results deviate slightly from prior reports of thermal and photoinduced 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 the 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 E
DOI: 10.1021/acsami.7b13814 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (A) Loss tangent (tan δ) plotted as a function of frequency and temperature of neat and 0.02 wt % SWNT−LCE films, with the peak maximum denoted as a dashed white line. (B) Real permittivity (ε′) at 1 Hz as a function of temperature for neat and 0.02 wt % SWNT−LCE films. (C) Steady-state electrical conductivity of a 15 μm thick 0.02 wt % SWNT−LCE film as a function of dc voltage.
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 that these accumulated charge complexes only occur at elevated temperatures, potentially because of a mismatch of strain at the SWNT−LCE interfaces. To determine if the SWNT themselves reorient 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 (
