Reconfigurable LC Elastomers: Using a Thermally Programmable

Jul 25, 2018 - Reconfigurable LC Elastomers: Using a Thermally Programmable Monodomain To Access Two-Way Free-Standing Multiple Shape Memory ...
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Reconfigurable LC Elastomers: Using a Thermally Programmable Monodomain To Access Two-Way Free-Standing Multiple Shape Memory Polymers Zhibin Wen,†,‡,§ Matthew K. McBride,‡ Xingpeng Zhang,‡ Xun Han,‡ Alina M. Martinez,‡ Renfan Shao,§ Chenhui Zhu,∥ Rayshan Visvanathan,§ Noel A. Clark,§ Yuzhong Wang,† Keke Yang,*,† and Christopher N. Bowman*,‡ Downloaded via DURHAM UNIV on July 29, 2018 at 03:00:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China ‡ Department of Chemical and Biological Engineering, Materials Science and Engineering Program, University of Colorado Boulder, Boulder, Colorado 80309, United States § Department of Physics and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0390, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: This work details a novel polyurethane liquid crystal elastomer (PULCE) with exchangeable carbamate functional groups that enable programming of a uniformly aligned monodomain sample through the application of external stress and simultaneous activation of dynamic bond exchange of the carbamate. PULCEs were synthesized using a thiol-Michael addition reaction, and the reversion of the carbamate group was observed by real-time FT-IR and mechanical analysis. Two independent phase transitions (isotropic−nematic and nematic− smectic) were employed to actuate two-way autonomous strains and multiple shape memory effects in a single system. In addition, thermally activated bond exchange engineered into the network transformed the permanent configuration of the material into various complex shapes. The programmed network topology and bond exchange conditions controlled the twoway autonomous shape changes. Coupling the shape memory effect of the polymer network with the plasticity induced by the thermally activated dynamic covalent chemistry in shape changing and shape memory materials will expand their applications and capabilities in emerging multifunctional devices.

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INTRODUCTION Liquid crystal elastomers (LCEs) are of intense interest due to their unique anisotropic shape changing and mechanical properties. Characteristics, such as rapid actuation and high extension at break, make this class of materials good candidates for stimuli-responsive materials like shape memory polymers (SMPs) and two-way actuators.1−4 As such, shape programmable LCEs are desired for applications in optical devices, switchable surfaces, and soft actuators. One of the simplest and most commonly implemented methods to dictate alignment is currently twostep polymerization processes where an initial elastic network is formed followed by a second cross-linking step that locks in the desired alignment.5−7 Despite much success in implementation of this method, complex conditions and limited ordering of the mesogenic molecules inhibit this approach because the initial step requires a cross-linked network to facilitate © XXXX American Chemical Society

programming. Chemically cross-linked networks, broadly noted for shape stability, mechanical properties, and thermal stability, provide an excellent platform to build shape changing polymers because they provide translation of molecular order into macroscopic shape change and improve the shape fixity and recovery in SMPs. Although implemented broadly for the advantages listed above, thermosetting materials also present unique challenges such as the inability to recycle, reprogram, and reprocess these materials. To address the inability to process thermosetting materials, covalent adaptable networks (CANs) and vitrimers,8−10 polymer networks with dynamic covalent bond exchange, bridge Received: June 21, 2018 Revised: July 12, 2018

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Figure 1. (A) Illustration of the network formation approach, including molecular synthesis. Conditions: (1a) TEA, DCM, rt, overnight, with acrylate excess; (1b) 2-mercaptoethanol, rt, 5 h; (2) 2-isocyanatoethyl acrylate, C2H4Cl2, DBTDL, 70 °C, 3 h; stoichiometries: oligomer_1 (1:0), oligomer_2 (1:0.75), and oligomer_3 (1:0.85); (3) DCM, TMPTMP, TEA, overnight. (B) Mechanism and illustration of network’s plastic behavior: the windmill is folded at 25 °C and then heated to 150 °C for 1 h allowing the urethane bond exchange to adapt to the new configuration. (C) Digital photos show extension and shrinkage of the aligned film. A monodomain (optically tranparent area) was programmed by stretching the sample and holding it at 150 °C for 60 min. The two-stage unbiased reversible strains were actuated by isotropic−nematic (shape 3 to shape 2) and nematic−smectic phase transitions (shape 3 to shape 2) upon thermal cycling.

the LCE behavior, two independent phase transitions (TI−N and TN−Sm) present in these materials drove two-stage freestanding triple shape memory effects on a uniformly aligned sample (Figure 1C). In addition, the two-way autonomous actuation was programmed and subsequently reconfigured by changing the network’s topology during bond exchange. This straightforward approach to program the uniform alignment of PULCEs will expand LCE applications in multifunctional devices.

the divide between thermosets and thermoplastics to enable a variety of fundamentally new behaviors in otherwise covalently cross-linked networks. In LCEs the pioneering work of Ji and co-workers demonstrated programming of monodomain LCEs with the topological network rearrangement facilitated by dynamic chemistry. Since this initial demonstration, multiple groups have implemented various dynamic chemistries and activating stimuli into LCE networks.11−16 Several thermally activated dynamic covalent chemistries successfully enable dynamic exchange behavior in a variety of networks. These chemistries include Diels− Alder,17,18 transesterification,19 transcarbamoylation,20,21 transamination of vinylogous urethanes,20 boronic ester exchange reaction,22,23 olefin metathesis,24,25 and disulfide metathesis.26,27 Transesterification, in particular, has been successfully implemented by multiple groups to form monodomain LCEs. For example, the material developed by Ji and co-workers showed excellent reversible actuation and shape memory effects (SME), but it required sufficient ester cross-links and free hydroxyl groups to promote the exchange reaction.14 By expanding the scope of dynamic linkages, thermosetting polyurethanes are ubiquitously employed across industries including adhesives, optical materials, and coatings, among others.28 Traditionally, the dissociative reversion of carbamates to isocyanates and alcohols typically occurred at high temperatures (>200 °C);29 however, Xie et al. recently developed a PEG backbone polyurethane and observed bond rearrangement-induced plasticity by transcarbamoyalation as low as 130 °C.30 Described here, a polyurethane liquid crystal elastomer (PULCE) with an exchangeable carbamate bond was formed by a thiol-Michael addition reaction under mild conditions (Figure 1A). Real time FTIR elucidated the urethane bond reversion dynamics and confirmed the reversible nature of this linkage. In Figure 1B the urethane reversion stabilized and enabled reprogramming of a flat film into a 3D shape at temperatures where the bond exchange was activated. To employ

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EXPERIMENTS AND METHODS

Materials. 1,4-Di[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82) was purchased from Merck UK. 1,3-Propanedithiol, trimethylolpropane tris(3-mercaptopropionate), 2-mercaptoethanol, and trimethylamine (TEA) were purchased from SigmaAldrich, USA. 2-Isocyanatoethyl acrylate was purchased from Showa Denko, Japan. All chemicals were used as received without purification. Synthesis of Oligomer-Diol. In a 50 mL round-bottomed flask, excess RM82 (5.0 g, 7.44 mmol) and 1,3-propanedithol (0.68 g, 6.32 mmol) were dissolved in 20 mL of dichloromethane (DCM). TEA (3.0 g, 37.2 mmol) was added, and the mixture was allowed to stir overnight at room temperature. A 0.26 g amount of 2-mercaptoethanol (3.35 mmol) was added to react with excess acrylate forming diol. With 5 h of constant stirring, the reaction was diluted with 200 mL of DCM, washed with HCl (1 N, 200 mL), and saturated solution of brine (200 mL) in a separatory funnel. The combined organic was concentrated under reduced pressure and dried overnight in vacuum to obtain the product. Synthesis of Oligomer-Diacrylate. 0.5 mmol of the diol monomer was dissolved in 10 mL of dry 1,2-dichloroethane at 60 °C with N2. 1.5 mmol of 2-isocyanatoethyl acrylate with DBTDL (0.005 mmol, 1 wt % to monomer) were added. The mixture was stirred for 3 h. After cooling to room temperature, the mixture was precipitated into methanol and dried under vacuum at 40 °C overnight. Comparing the acrylate peaks (5.8−6.5 ppm) and phenyl peak at 8.2 ppm revealed Mn values of 3900, 2200, and 1110 g mol−1, respectively. All structures were analyzed by 1H NMR (Figures S1−S2). B

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Shape Memory Properties. The quadruple shape memory experiments were conducted as described previously,31,32 where the experiments were performed on a DMA Q800 (TA Instruments, USA) under controlled force mode. The testing program is available in the Supporting Information.

Preparation of Polyurethane Liquid Crystal Elastomer (PULCE). 0.5 g of oligomer-diacrylate and stoichiometric trimethylolpropane tris(3-mercaptopropionate) (TMPTMP) were dissolved in DCM. TEA (5 times mol to oligomer-diacrylate) and DBTDL (5 mol % to oligomer-diacrylate) were added. The mixture was placed into a horizontal Teflon dish. The cross-linking reaction proceeded at room temperature overnight. Finally, the network was obtained after 1 day in the oven at 70 °C. Preparation of Control Liquid Crystal Elastomer (LCE). 0.5 g of RM82 and 0.197 g of TMPTMP were dissolved in DCM. TEA (5 times mol to RM82) and DBTDL (5 mol % to RM82) were added. The mixture was placed into a horizontal Teflon dish. The crosslinking reaction proceeded at room temperature overnight. Finally, the network was obtained after 1 day in the oven at 70 °C. Characterization and Measurements. Nuclear Magnetic Resonance (NMR). A Bruker AV400 (Bruker, Switzerland) was used to record 1H NMR spectra at room temperature. Solvent: deuterated chloroform (CDCl3); internal reference: tetramethyl silane (TMS). Real-Time Fourier Transform Infrared (FT-IR) Spectroscopy. Polymerization kinetics were analyzed using a Fourier transform infrared spectroscopy (FT-IR) instrument (Nicolet 8700) to detect the conversions of the thiol and acrylate functional groups by monitoring the disappearance of the corresponding IR peaks. The thiol and acrylate oligomer mixtures were placed between NaCl plates, and the thiol peak was monitored in the range between 2545 and 2600 cm−1 while the acrylate peak was detected in the absorption range between 800 and 830 cm−1. The reversion of the carbamate bond was monitored by real-time FT-IR with a temperature control stage. The acrylate oligomer was placed between NaCl plates, and the characteristic isocynate peak was monitored at 2225 cm−1. Gel Permeation Chromatography (GPC). The molecular weight distribution (Mw, Mn, and polydispersity index Mw/Mn) of the oligomer was estimated using a TOSOH ECO SEC HLC-8320GPC, which was equipped with two polystyrene columns and UV and RI detectors. The UV detector was set at 260 nm, and dimethyl sulfoxide (DMSO) was used as the eluent with a flow rate of 0.35 mL min−1 at 50 °C. The molecular weight data were calibrated to poly(methyl methacrylate) standards. Dynamic Mechanical Analysis (DMA). Thermomechanical properties of the samples were carried out on RSA-2 (TA Instruments, USA), with a heating rate of 3 °C min−1 from −30 to 120 °C and a frequency of 1 Hz. Differential Scanning Calorimetry (DSC). DSC was recorded on a DSC-Q200 (TA Instruments, USA), over the temperature range −30 to 120 °C with a heating or cooling rate of 10 °C min−1. Wide-Angle X-ray Scattering (WAXS). WAXS was performed on a Forvis Technologies X-ray instrument. The source is a 30 W Genix 3D X-ray generator with Cu anode (wavelength, λ = 1.5405 Å and energy = 8.05092 keV). The detector was a Dectris Eiger R 1 M with a 0.075 × 0.075 mm2 pixel size. The LC films were placed inside an Instec hot stage and measured in transmission mode. AgBe was used as the calibrant, and the sample-to-detector distance was about 200 mm. The X-ray scattering patterns were analyzed and plotted in Igor Pro software. The d-spacing for each film was calculated using Bragg’s equation. Polarized Optical Microscopy (POM). LC textures of the PULCEs were obtained by a Nikon Fi1 polarizing optical microscope (POM) equipped with a hot stage. To determine the liquid crystal phase, the reaction solution was placed into an alignment cell during the preparation of the PULCE. The cell was kept at room temperature overnight for polymerization and 70 °C for 1 day before testing. Swelling Ratio and Gel Content of PULCEs. The samples were cut into small pieces, then swelled in DCM, and extracted by DCM at room temperature for 24 h. The initial mass (m0) and the dried extracted specimen (m2) were measured. The gel content (G (%)) was calculated according to the following formula: G(%) =

m2 × 100% m0

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RESULTS AND DISCUSSION Preparation of the Diacrylate Oligomer. The synthetic route for the formation of the diacrylate LC oligomer is depicted in Figure 1A. First, the oligomer-diol is prepared via a thiolMichael addition reaction by reacting RM82, a chain extender, 1,3-propanedithiol, and an end-capping agent 2-mercaptoethanol, which was subsequently reacted with 2-isocyanatoethyl acrylate to form the oligomer-diacrylate. This approach is readily adaptable to commercially available acrylic mesogens and provides a platform to form chemically defined, functional stepgrowth LC oligomers of various composition and functionality. Because of its step-growth nature, the thiol-Michael addition reaction enabled control over the molecular weight and functionality simply by altering the stoichiometry, resulting in Mn values of 1110, 2200, and 3900 g mol−1, respectively, for stoichiometric ratios of 1:0; 1:0.75; and 1:0.85 (Table S1). Polymerization Kinetics. To study the network forming reaction, real-time Fourier transform infrared (FT-IR) was used to monitor the peaks associated with the acrylate (800− 830 cm−1) and thiol (2545−2600 cm−1) in the thiol-Michael system during the polymerization process of the diacrylate oligomer and the thiol cross-linker, TMPTMP, at room temperature (Figure 2). The consumption of both the acrylate and thiol increase with time. Finally, the step-growth reaction is almost complete after 60 min, and the ultimate conversion is about 90% based on FT-IR analysis of the acrylate and thiol peaks. Extraction tests revealed that all three networks have high gel contents 97.8%, 96.7%, and 92.0% for PULCE_1, PULCE_2, and PULCE_3, respectively. This high gel content is indicative of a relatively high degree of cross-linking and complete reaction of the polymerizable functional groups. The relatively high degree of cross-linking and the corresponding gel fraction would inherently limit the ability of the network to rearrange or be reconfigured in the absence of dynamic covalent chemistry. Microstructural Analysis. The liquid crystalline behavior was characterized to elucidate the potential temperatures for inducing shape changing transitions (Figure 3). Taking PULCE_3 as an example, differential scanning calorimetry (DSC) shows a glass transition (Tg) at −8 °C and an LC clearing temperature (Tcl) at 80 °C (Figure 3A). Figure 3B illustrates a similar result determined by dynamic mechanical analysis (DMA) (1, 42, and 85 °C). Another independent Tcl is captured in PULCE_3 at 42 °C, but the peak was too weak to observe in DSC. Additionally, the nematic phase (N) and smectic A phase (SmA) were confirmed using POM and 2D WAXS. In detail, Schlieren and focal conic textures were observed on cooling from the isotropic phase to 65 and 25 °C, respectively (Figure 3C). Meanwhile, strong anisotropy was detected in the X-ray scattering patterns on the stretched films (Figure 3D). The spot patterns at room temperature in small angle XRD show that the layer normal is along the molecular long axis (SmA). The disappearance of spots reflects the SmA-N phase transition upon heating above 42 °C. Based on these results, the isotropic−nematic and the nematic−smectic A phase transitions are separated by a large temperature gap (>40 °C) enabling multiple SMEs. Moreover, phase transition temperatures are readily adjusted by controlling the oligomer length. With a

(1) C

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Figure 2. Thiol and acrylate functional group conversions versus time. Conditions: stoichiometry (thiol: acrylate) = 1:1, rt.

Figure 3. Liquid crystal properties of PULCE_3. (A) Two phase transition temperatures are detected by DSC upon cooling and heating at 10 °C min−1. (B) Storage modulus as a function of temperature when heated at a rate of 3 °C min−1 and 1 Hz frequency. Compared to DSC curves, an additional phase transition temperature is obtained at ∼42 °C. (C) POM images at 65 °C (Schlieren texture) and 25 °C (focal conic texture). All scale bars are 100 μm, thickness: ∼5 μm. (D) 2D WAXD images of the stretched film at 65 °C (nematic phase) and 25 °C (smetic phase).

110 °C via the formation of isocyanate as indicated by the infrared peak at 2225 cm−1 that appears upon heating, and the isocyanate amount continued to increase with increased temperature. After cooling, the reaction shifted back toward the formation of the urethane, and the isocyanate peak completely disappeared. With this information, it was concluded that, at least in part, disassociation and reversion of the carbamate bond are responsible for the observed network relaxation and reorganization. Stress relaxation experiments on PULCE_3 were investigated at different temperatures to explore its dynamic behavior. The relaxation constant τ* (G/G0 = e−1), calculated from the graph in Figure 4B, was used to calculate the activation energy (Ea) from the Arrhenius relationship (110 kJ mol−1) (for details see Supporting Information Figure S4).33−35

shortened backbone, the Tg increases slightly while Tcl decreases dramatically and TI−N (isotropic−nematic phase transition) is close to TN−Sm (nematic−smectic phase transition). Only one LC phase was detected in PULCE_1 (Figure S3). Reversion of the Carbamate Bond. As shown in Figure 1B, the topological structure of the material rearranged upon the application of an external stress that was relieved via the reversible addition reaction of the carbamate bond at high temperature in the isotropic phase. The newly molded shape replaced the original shape because the polymer network was able to adopt a new polymer network topology. To evaluate the reversibility of the reaction, real-time FT-IR coupled with a heating stage was employed to detect the presence of reverse reaction products, particularly isocyanates (Figure 4A). Significant dissociation of the carbamate oligomer is detected beginning at D

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Figure 4. (A) Reversion of the carbamate bond as detected by FT-IR. The isocyanate peak at 2225 cm−1 was detected upon heating (left) and the peak area increased with increasing temperature. The isocyanate peak disappears upon cooling due to the reformation of the urethane (right). (B) Thermal stress relaxation in the PULCE_3 film at different temperatures with a strain of 10%. Catalyst: DBTDL (5 mol %). G and G0 represent the instantaneous stress and the initial relaxation modulus, respectively.

As previously identified, both ester and urethane bonds are dynamic covalent bonds in polymers.36,37 A control experiment was used to determine the difference in the dynamics of the ester and urethane bonds in our system. An analogous film was prepared without the carbamate bond by directly reacting RM82 and TMPTMP under otherwise similar conditions making the primary difference between these two materials the presence/absence of the carbamate structure while preserving the ester. Minimal stress relaxation was observed at 170 °C in the films without carbamates (Figure S5). As such, it was determined that the exchange dynamics of the urethane bond are the most significant contribution to stress relaxation in our system at 150 °C with DBTDL as the catalyst. Shape Memory Effects. In previously reported LCEs, both TI−N and TI−Sm have been used to program shape memory behavior.31,32 Here, three independent transition temperatures (Ttrans = Tg, TI−N, and TN−Sm, respectively) were employed to explore quadruple shape memory effects. Due to the thermomechanical and LC phase behavior of PULCE_3, −18, 22, 65, and 102 °C are chosen as actuating temperatures. Due to the Arrhenius’ behavior of the reversible addition reaction, Rf (fixity ratio) and Rr (recovery ratio) will potentially be negatively impacted by bond exchange, with the characteristic relaxation time at 102 °C being approximately 180 min. Due to the long characteristic times at this temperature, the effect can be ignored in the short time in this system when programming the SME; however, longer programming times on the order of 100s of minutes could negatively impact the recovery. Figure 5A

Figure 5. Shape memory effects in PULCE_3 films. (A) Quadruple SMEs recorded by DMA. Three fixed recovered states are achieved corresponding to TI−N, TN−Sm, and Tg. (B) Digital photos. Three complex shapes are folded and fixed. The temporary shapes are recovered step by step upon heating to the programmed temperatures. (C) The two-way reversible SME of a polydomain sample under constant external load. This sample demonstrates the two-stage elongation/contraction cycle under constant load according to the stress-induced isotropic−nematic and nematic−smectic phase transitions that arise upon heating or cooling.

illustrates the quadruple shape memory behavior achieved in this material. As expected, TI−N-based shape S1 and TN−Smbased shape S2 have a high fixity ratio (Rf) (84.5% and 88.6%, respectively) as well as an outstanding recovery ratio (Rr) (>99%, both) through 2−4 cycles. Similar to other Tg-based shape memory materials, ∼100% Rf and Rr are achieved when using the glass transition. The four shape memory cycles without a visible strain shift are indicative of the robustness and cross-linked nature of the material (Figure S6). Additionally, the four bladed windmill was deformed and fixed at 102, 65, 22, and −18 °C followed by recovery of each shape, step by step, at the appropriate temperatures (Figure 5B). Similar to other LCE materials under constant stress, this material exhibits reversible shape change around the nematic and smectic phase transition due to the LC phase change.38,39 E

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Figure 6. (A) Plasticity-SME on PULCE_3 films. Plastic behavior is observed when the sample is heated to 150 °C which activates the carbamate bonds. Below that temperature, the carbamate bonds are not dynamic and conventional SME is observed. (B) Two-way free-standing SME on monodomain samples following stress-relaxation-based programming (prestretched strain: 10%; temperature: 150 °C). The two-stage elongation/contraction cycle actuates based on the isotropic−nematic and nematic−smectic phase transitions that arise upon heating and cooling.

phase (Figure S8). This stress appears because the equilibrium chain conformation becomes isotropic when heated above the clearing temperature causing the chains to retract from the anisotropic state. The anisotropic state was stable in the LC phase because the entropic penalty for order is offset by the LC phase behavior. Thus, transitioning from the LC state to the isotropic state will drastically change the mechanical properties of the materials.

The stress-induced polydomain−monodomain transition took place at 65 °C (nematic phase) and 22 °C (smectic A phase) with the stress plateau in one system (Figure S7). Thermal cycling under constant stress leads to a polydomain to monodomain transition, and the transition does not require prior programming. Here, by taking advantage of TI−N and TN−Sm, two-stage reversible strains were achieved under constant stress, with 60% elongation at 65 °C (nematic phase) and an additional 30% at 22 °C (smectic A phase) (Figure 5C). The stress guided the formation of the monodomain along the strain direction. When heated into the isotropic phase, the PULCE contracts along the stress direction as entropic elasticity became dominant instead of the LC order. Plasticity and SME. In order to obtain free-standing reversible strain, activating the reversible addition reaction of the urethane bond programmed a uniformly aligned monodomain LCE when stretched and heated to enable the exchange. The plasticity enabled by the bond exchange was coupled with the SME effect producing a material with the capability to both permanently fix a new shape and temporarily program another (Figure 6A). In the first process, stress is completely relaxed at 150 °C within 1 h. This permanent programming step aligned the polymer backbone along the stretching direction, thus causing the isotropic shape to be permanently altered. Subsequently, the LC phase transitions enabled programming of the SME in a second, separate step. Of note, the bond exchange did not adversely affect the fixity and recovery of the programmed SME (Figure 5A). After monodomain formation due to bond exchange under 10% strain at 150 °C, the PULCE film exhibited a bias-free two-way SME during thermal cycling with 84% reversible strain (Figure 6B). In addition, two-way reversible actuation was achieved by reprogramming the network’s topology. After completely releasing the strain, the reversible strain was much greater than the applied constant strain before stress relaxation indicative of a templating effect. This templating effect was explained by improved fixation of the polymer backbone in a more anisotropic state as larger deformation led to better alignment. The maximum reversible strain was about 120% strain which is comparable to other programmable shape changes (Table S2). When holding the length constant during heating, stress was detected when the expanded films were heated into the isotropic

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CONCLUSION In the present work, a uniformly aligned monodomain PULCE was formed by external stress relaxation via the reversible addition reaction of the urethane bond, which is the characteristic unit in polyurethane materials. The reversion of the urethane structure was directly determined by real-time FT-IR by monitoring the formation of isocyanates. By implementing these materials as shape changing materials, a system with twostage stress-free reversible strains was demonstrated based on two phase transitions (TI−N and TN−Sm) as well as having multiple shape memory effects. Complex 3D shapes were developed at a desired temperature. This approach represents a simple, robust method for straightforward programming for the uniform alignment of LCEs of large size and scale.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01315. Calculation of activation energy, testing program of SME, supplementary figures and tables (PDF) Movie of SME (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.N.B.). *E-mail: [email protected] (K.Y.). ORCID

Keke Yang: 0000-0002-7019-6059 Christopher N. Bowman: 0000-0001-8458-7723 Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS The authors gratefully acknowledge use of facilities and instrumentation supported by the National Science Foundation (NSF) (DMR-1310528) and by the Soft Material Research Center under an NSF MRSEC Grant (DMR1420736). This work was supported financially by the National Science Foundation of China (51773131, 51721091) and the International S&T Cooperation Project of Sichuan Province (2017HH0034). Z.W. acknowledges support from China Scholarship Council (No. 201606240142). Z.W. also acknowledges Dr. Joseph E. Maclennan and Dr. Matthew A Glaser for discussions.

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DOI: 10.1021/acs.macromol.8b01315 Macromolecules XXXX, XXX, XXX−XXX