Gold Nanoparticles Incorporated Nematic Gel ... - ACS Publications

Oct 19, 2016 - Institut Curie, PSL Research University, CNRS, UMR 168, F-75005 Paris ... Crystal Materials Research Center, University of Colorado, Bo...
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Gold Nanoparticles Incorporated Nematic Gel Micropillars Capable of Laser Actuation at Room Temperature Xiyang Liu,† Xiaogong Wang,*,† Tao Liu,*,‡ and Patrick Keller*,§,∥,⊥ †

Department of Chemical Engineering, Key Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, P. R. China ‡ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren-ai Road, Suzhou 215123, P. R. China § Institut Curie, PSL Research University, CNRS, UMR 168, F-75005 Paris, France ∥ Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 168, F-75005 Paris, France ⊥ Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: Liquid crystal elastomers (LCEs) capable of rapid reversible shape change and ease of actuation at room temperature are highly desired. In this study, a photothermally actuated system based on LCE gel micropillars incorporated with gold nanoparticles is reported for the first time. The photothermal conversion effects of gold nanoparticles (AuNPs) have been studied with a focus on nanoparticle shape (rods and spheres) and loading content. Upon swelling in low molecular mass liquid crystal solvent, the formation of LCE and LCE/AuNP gel enables the reduction of nematic-to-isotropic phase transition temperature (TNI), which allows the gelled micropillar capable of photothermal actuation at room temperature. It has been demonstrated that the LCE gel micropillar with incorporation of 1.0 wt % of gold nanorods can achieve a rapid 30% change in volume within 3 s when under proper laser irradiation at room temperature. The observed discontinuous volumetric transition of the gel micropillars is in good agreement with the mean field theory prediction. A simple finite element analysis (FEA) model was developed to facilitate the understanding of the photothermal effect of LCE/AuNR gel micropillar upon laser irradiation.

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INTRODUCTION Smart materials responsive to external stimuli to result in the change of size/shape have been a rapidly growing subject. A wide range of sensing and actuating applications have been built upon these novel materials, such as artificial muscles,1 microswitches,2 micromotors,3 and responsive smart surfaces.4,5 Among different types of smart materials, liquid crystal elastomers (LCEs), in particular, nematic LCEs, are especially interesting, which rely on the intrinsic anisotropic/isotropic phase transition of uniformly oriented liquid crystal mesogenic units and the entropic elasticity of the rubbery backbone to induce the shape change.6−8 Various kinds of external stimulating signals have been attempted to induce the shape change of LCEs, such as thermal,9,10 optical,11,12 electrical,13 magnetic,14 and chemical15 stimuli. In comparison to different stimulating schemes, light-driven actuating has some inherent advantages in that it is convenient, portable, clean, remotely operable, and easy to be implemented. By simply dissolving azo dyes in a LCE matrix and taking use of the cis−trans isomerization of azobenzene groups, Camocho-Lopez et al.16 fabricated a LCE actuator with ultrafast response (80 ms) to laser irradiation. Besides the cis−trans isomerization, light irradiation induced photothermal conversion has also been © XXXX American Chemical Society

used as a method for actuating LCEs. Different photothermal agents have been used for this purpose. By incorporating 0.1− 0.2 wt % single-walled carbon nanotubes (SWCNT) into LCE films, Yang et al.17 have prepared LCE/SWCNT nanocomposite responsive to infrared light. Similarly, Yang et al.18 also prepared graphene-enabled LCE nanocomposites and demonstrated their large and reversible shape change upon IR irradiation. Gold nanoparticles (AuNPs) are different from the broad-band photothermal agents, e.g., carbon nanotubes and graphene, in that they rely on the localized surface plasmon resonance (LSPR) phenomenon to enable their unique photothermal conversion capacity. With incorporation of gold nanoparticles into LCEs, the resulted light-responsive nanocomposite possesses several advantages including wavelength selectivity and relatively high photothermal conversion efficiency. First, the LSPR of the gold nanoparticles makes the nanocomposite responsive to a narrow range of wavelength, and the peak-absorption wavelength can be readily adjusted by tailoring the gold nanoparticle shape (rods, spheres, etc.) and Received: September 3, 2016 Revised: October 9, 2016

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scopic (TEM) images to confirm the successful formation of AuNS and AuNR. The TEM imaging analysis results in an estimation of the AuNS diameter as 15 nm and the AuNR aspect ratio as 2.5 (length ≈ 66.3 nm, diameter ≈ 26.5 nm). Preparation of LCE Precursor. The protocol established in our previous work22 was adopted here for preparing LCE/AuNR and LCE/AuNS composites. In brief, the as-prepared gold nanoparticle aqueous dispersion was centrifuged at 12 000 rpm for 15 min by a Hitachi Himac CR22g supercentrifuge. Such obtained precipitates were redispersed in warm ultrapure water under sonication. The dispersion was then centrifuged at 12 000 rpm for 15 min. This redispersion/centrifugation procedure was repeated several times to remove the excess CTAB as much as possible. Subsequent to CTAB removal, an appropriate amount of gold nanoparticles was dispersed in a LCE prepolymer solution under sonication. The LCE prepolymer solution was prepared by following the previously reported procedure,10 which was a mixture of the monomer 4-ADBB, the photoinitiator (2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone, J&K Chemical Ltd.), and the cross-linking agent (1,6-hexanediol diacrylate, J&K Chemical Ltd.) in THF at a molar ratio of 77:18:5. An Agilent 8453 UV−vis−NIR spectrophotometer was used to acquire the UV−vis−NIR spectra of the dispersion of AuNR/LCE prepolymer and AuNS/LCE prepolymer in THF to confirm the successful incorporation of gold nanoparticles into LCE prepolymer. After THF evaporation, a dried mixture of LCE prepolymer with gold nanoparticles (AuNR, AuNS) was obtained for later use in fabricating LCE/AuNR and LCE/AuNS composite micropillars. Fabrication of LCE, LCE/AuNR, and LCE/AuNS Composite Micropillars. The LCE, LCE/AuNR, and LCE/AuNS micropillar arrays were fabricated through a soft lithography process previously reported by us in refs 10 and 22. First, a small amount of the dried mixture of LCE prepolymer with or without gold nanoparticles was placed on a piece of cover glass that was supported by a permanent magnet (1 T NdFeB rare earth magnet). The entire setup was then heated in an argon environment to 100 °C to melt the mixtures. Upon melting, a PDMS mold with an array of holes of diameter of 20 μm was brought in contact with the melt and maintained at 100 °C for ∼10 min to allow for a complete mold filling of the melt. After the filling process, the temperature was lowered to 55 °C at a cooling rate of 1 °C/min and maintained at this temperature for 10 min to induce the isotropic to nematic phase transition of the mesogenic components of the mixture. Subsequently, the UV irradiation (high pressure mercury lamp, 300 mW/cm2) was applied for 2 h to photopolymerize and photo-cross-link the LCE prepolymer mixture. Followed by cooling and PDMS mold release, the LCE, LCE/AuNR, and LCE/ AuNS micropillar arrays were finally obtained. The isolated micropillars used for later swelling process were manually cut off with a blade from the micropillar arrays. Characterization of LCE, LCE/AuNR, and LCE/AuNS Composite Micropillars. To examine the dispersion states of AuNR and AuNS in LCEs, a Hitachi HT7700 electron microscope was applied to acquire TEM images of the LCE/AuNR and LCE/AuNS composites. The TEM samples of ∼100 nm thickness were prepared by a Leica EM UC6 ultramicrotome. For the same purpose, the UV−vis−NIR spectra of LCE/AuNR and LCE/AuNS thin films were also acquired. The LCE, LCE/AuNR, and LCE/AuNS samples were subjected to a differential scanning calorimetry (DSC) test to examine the effect of gold nanoparticles on the phase transition temperature of the LCE host. Thermogravimetric analysis (TGA) was performed to confirm the mass concentration of gold nanoparticles in the prepared LCE/ AuNR and LCE/AuNS composite samples. Both DSC and TGA test were carried out on a DSC Q2000 system in nitrogen at a heating/ cooling rate of 10 °C/min. Fabrication and Characterization of LCE, LCE/AuNR, and LCE/AuNS Gels. The LCE, LCE/AuNR, and LCE/AuNS samples prepared above were swollen by a liquid crystal solvent 4-cyano-4′pentylbiphenyl (5CB, J&K Chemical Ltd.) at different amounts in an 80 °C oven for 24 h to reach equilibrium. Thus, prepared LCE, LCE/ AuNR, and LCE/AuNS gels were subjected to TGA and DSC

size. Second, the optical absorption cross section (Cabs) of gold nanoparticles, especially gold nanorods (AuNR), is high, and it imparts high photothermal conversion efficiency. It has been reported that the AuNR with a size of 42 nm in length and 12.4 nm in diameter possesses a longitudinal Cabs of 66.2 × 10−18 cm2 per Au atom at 785 nm.19 This value is significantly higher than that of SWCNTs (∼1.0 × 10−18 cm2 per C atom).20,21 With consideration of the advantages of gold nanoparticles, in our previous work, we fabricated LCE/AuNR micropillars and demonstrated their rapid and reversible actuation ability upon laser irradiation.22 However, due to the relatively high nematic-to-isotropic transition temperature (TNI, ∼110 °C) of the LCE used in this study, the photothermal actuation of the LCE/AuNR micropillars had to be performed at an elevated temperature (80 °C). This makes their use for some room temperature applications difficult. Previous studies on liquid crystal gels23−25 have shown that by simply swelling LCE networks in a liquid crystal solvent, one can readily reduce its TNI transition temperature. The phase transition temperature of the LCE gel network has been well revealed experimentally and theoretically.26,37,41 Inspired by these previous findings, in the present work, we attempted a simple approach to develop a LCE gel/AuNP nanocomposite system that can be photothermally actuated at room temperature. This novel system involves the fabrication of LCE/AuNP micropillars and their subsequent fully swelling in a liquid crystal solvent 4-cyano-4′pentylbiphenyl (5CB). The LCE polymer network containing 5CB in it formed a uniform nematic phase, and the TNI is reduced to ∼55 °C. Thus, formed gel micropillars can be transformed in size and changed in volume around TNI through both thermal and photothermal actuation. The readily deformation of the size and volume of the LCE gel micropillars suggests their novel applications for micromotor and pump that will be studied and demonstrated in the future.

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EXPERIMENTAL SECTION

Preparation of the LCE Monomer. The LCE monomer 4″acryloyloxybutyl 2,5-di(4′-butyloxybenzoyloxy)benzoate abbreviated as 4-ADBB was synthesized according to the procedure disclosed in our previous article.10 Synthesis and Characterization of Gold Nanoparticles. Gold nanoparticles were synthesized according to the seed-mediated growth method reported by El-Sayed et al.27 but with some minor adaption. The seed solution was prepared first by mixing the aqueous solution of cetyltrimethylammonium bromide (CTAB, 5 mL, 0.10 M) with tetrachlorauric acid solution (HAuCl4, 25 μL, 50 mM). To this mixture, a freshly prepared ice-cold sodium borahydride solution (NaBH4, 300 μL, 10 mM) was added under mild stirring, which gradually turned into tea-like color. The as-prepared seed solution was kept at 25 °C for at least 2 h before use. The growth solution was prepared by adding an aqueous solution of ascorbic acid (150 μL, 100 mM) to a freshly prepared mixture of HAuCl4 (200 μL, 50 mM) and CTAB (10 mL, 0.10 mM). With addition of ascorbic acid, the mixture turned from yellowish to colorless. Immediately after the color change, an aliquot silver nitrate (AgNO3, 5 mM) solution was added for size and shape control of the formed gold nanoparticles.28 With no or 300 μL of silver nitrate addition to the growth solution, two different types of gold nanoparticles, namely, gold nanosphere (AuNS) and gold nanorod (AuNR), were synthesized, respectively. To either the AuNS or the AuNR growth solution, a 120 μL previously prepared seed solution was added under vigorously stirring. The mixed solution was then continually stirred mildly for over 24 h at 28 °C to result in either a dark purple AuNR aqueous dispersion or a pink AuNS aqueous dispersion. An Agilent 8453 UV−vis−NIR spectrophotometer and a Hitachi HT7700 electron microscope were respectively used to acquire the UV−vis−NIR spectra and transmission electron microB

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Figure 1. (a) Comparison of the UV−vis−NIR spectra of AuNR in the as-synthesized aqueous dispersion, LCE/AuNR (1 wt %) composite film, and swollen LCE/AuNR (1 wt %) gel film. (b) Comparison of the UV−vis−NIR spectra of AuNS in the as-synthesized aqueous dispersion and LCE/ AuNS (1 wt %) composite film and swollen LCE/AuNS (1 wt %) gel film. (c) Theoretically calculated absorption cross section spectra for a AuNR (length = 66.3 nm, diameter = 26.5 nm) and a AuNS (diameter = 15 nm) dispersed in water.

that the UV−vis−NIR spectra for the gel film were acquired at a temperature above the phase transition of 5CB (35.5 °C) to avoid the complication caused by light scattering effect. Evidently, as compared to the aqueous dispersion, the absorption bands of the LCE/AuNR and gelled LCE/AuNR located at 532 and 670 nm, which are respectively due to the transverse and longitudinal surface plasmon resonance of gold nanorods,29,30 become broadening and shift to longer wavelength. A similar observation also applies to the absorption band at 520 nm for of AuNS in the as-prepared aqueous dispersion, LCE/AuNS composite film, and the gelled LCE/ AuNS composite film (Figure 1b). The shift of the plasmon resonance bands to the longer wavelength is a result of different dielectric environment of gold nanoparticles in aqueous solution and the LCE host polymer.33 Nevertheless, its broadening suggests that the AuNR and AuNS experienced some degrees of aggregation in the LCE host polymer induced by the composite preparation process. Considering that only ∼15% of the initial gold supply could finally convert to the resulting gold nanorods,31 we calculated the optical absorption cross section, Cabs, of the AuNR at 670 nm according to the UV−vis−NIR absorbance of the asprepared aqueous dispersion by Lambert−Beer’s law. The result is Cabs = 5.52 × 10−18 cm2/Au-atom. For comparison, a numerical simulation package MNPBEM developed by Hohenester et al. based on boundary element method40 was also applied to theoretically determine the absorption cross section of an AuNR with length = 66.3 nm and diameter = 26.5 nm randomly dispersed in water. The results are shown in Figure 1c. The theoretically estimated longitudinal plasmon resonance of the AuNR is located at 670 nm, which agrees well with the experimental result. However, the theoretically estimated absorption cross section at 670 nm Cabs = 8.68 ×

measurements to respectively determine the amount of 5CB in the gel and examine its effects on the phase transition temperature of the host LCE. A DSC Q2000 system was used to carry out both the DSC and TGA test at a heating/cooling rate of 10 °C/min in a nitrogen atmosphere. Thermal and Photothermal Actuation of LCE, LCE/AuNR, and LCE/AuNS Gel Micropillars. The gel micropillars were prepared by immersing the isolated LCE, LCE/AuNR, and LCE/ AuNS micropillars on a piece of cover glass in 5CB for at least 1 h to ensure the swelling equilibration and then used for examining their thermal or photothermal actuation behavior. In thermal actuation, the temperature of the sample stage was controlled by a Linkam CSS450 hot stage. A homemade setup, discussed in Results and Discussion, was used to investigate the photothermal actuation behaviors of the LCE, LCE/AuNS, and LCE/AuNR gel micropillars. The setup was composed of a laser irradiation system and an optical image/video recording system. The laser system was equipped with a 635 nm diode laser (Changchun New Industries Optoelectronics Tech, CO., Ltd., nominal power of 5 W) and several reflection mirrors and beam focus lenses. There was a shutter in the beam path, which could be manually operated to control the on and off of laser irradiation on the pillars. By adjusting the working current of the diode laser device, a maximum power density focused on the gel micropillars placed on the hot stage was determined to be ∼100 W/cm2. A Nikon LV 1000 POL microscope equipped with a Nikon DS-fi2 CCD camera was used to capture the optical images and videos of the LCE, LCE/AuNR, and LCE/AuNS gel micropillars to visualize their shape-changing behaviors upon thermal and photothermal actuation.

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RESULTS AND DISCUSSION Plasmon Resonance Behavior of Gold Nanoparticles in LCE, LCE/AuNR, and LCE/AuNS. Figure 1a compares the UV−vis−NIR spectra of AuNR in the as-prepared aqueous dispersion, LCE/AuNR composite film, and the gelled LCE/ AuNR composite film to examine the influence of composite formation on the AuNR dispersion states. It should be noted C

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Macromolecules 10−15 m2 = 4.63 × 10−17 cm2/Au-atom is significantly higher than the experimental value of Cabs = 5.52 × 10−18 cm2/Auatom. This discrepancy suggests that the 15% conversion for Au atom into AuNR31 might be overestimated. In Figure 1c, we also calculated the absorption cross section spectra for an AuNS with diameter of 15 nm. Again, the theoretically estimated plasmon resonance frequency of the AuNS (522 nm) agrees with the experimental result (520 nm). Similar to the case of AuNR, the theoretically estimated absorption cross section at 520 nm for AuNS (Cabs = 1.71 × 10−16 m2 = 1.64 × 10−17 cm2/ Au-atom) is also significantly higher than the experimental value of Cabs = 2.27 × 10−18 cm2/Au-atom. The overestimation of Au atom conversion has been attributed to cause such a discrepancy. Swelling and Thermal Behavior of LCE, LCE/AuNR, and LCE/AuNS Gel Micropillars. The micropillars of LCE, LCE/AuNR, and LCE/AuNS can be readily swollen by the liquid crystal solvent 4-cyano-4′-pentylbiphenyl (5CB). The swelling behavior was qualitatively examined by optical microscopy. Figures 2a (2d) and 2b (2e) respectively show

Figure 3. (a) DSC results of LCE, LCE/AuNR (1.0 wt %), and LCE/ AuNS (1.0 wt %) composites. (b) DSC results of LCE gel, LCE/ AuNR (1.0 wt %) gel, and LCE/AuNS (1.0 wt %) gel. (c) Mass loss of LCE gel, LCE/AuNR (1.0 wt %) gel, and LCE/AuNS (1.0 wt %) gel determined by TGA. The mass loss during 130−250 °C represents the decomposition of 5CB, and the mass loss beginning at 350 °C results from the decomposition of polymer matrix.

reduced to ∼55 °C. The reduction of phase transition temperature is owing to the presence of the miscible 5CB liquid crystal solvent that has a lower transition temperature.34,35 The study on the phase diagrams of LCE/liquid crystal solvents by Nwabunma et al.37 has well explained the transition temperature reduction effect. Along the same line, an empirical mixing-rule-like formula24 has been successfully used to correlate the nematic−isotropic phase transition temperature TNIG of the gel sample with respect to that of the dry sample (TNI*) and the LC solvent (TNIs), which is read as

Figure 2. (a) Optical image of the LCE micropillar array. (b) Isolated LCE micropillar separated from the array. (c) LCE gel micropillar swollen in 5CB. (d) Optical image of the LCE/AuNR (1 wt %) micropillar array. (e) Isolated LCE/AuNR (1 wt %) micropillar. (f) LCE/AuNR (1 wt %) gel micropillar swollen in 5CB. The scale bar in (b), (c), (e), and (f) is 50 μm.

the as-prepared or dried LCE (LCE/AuNR) pillar arrays and a representative isolated pillar cut from the parent array. The LCE and LCE/AuNR pillars have a length of ∼70 μm and a diameter of 25 μm in average. Upon immersion in a droplet of 5CB, the micropillars, both LCE (Figure 2c) and LCE/AuNR (Figure 2f), underwent an elongation in length and expansion in diameter to result in a volume increase. The swelling is anisotropic; i.e., the diameter expansion of the micropillar is much greater than the length elongation. This anisotropic swelling behavior is attributed to the globally oriented nematic director of the mesogenic components of the LCE polymer in the pillar. Figures 3a and 3b compares the exothermic DSC traces for dried as well as 5CB swollen LCE, LCE/AuNR, and LCE/ AuNS micropillars. Clearly, the nematic−isotropic phase transition temperature (TNI*) of the as-prepared LCE, LCE/ AuNR, and LCE/AuNS is in a range from 100 to 110 °C. With swollen in 5CB, the TNIG of the gel sample is significantly

TNIG = φ*TNI* + (1 − φ*)TNIS

(1)

where 1 − φ* represents the mass fraction of LC solvent in the gelled LCE network. This formula in conjunction with the value of φ* = 1/3 determined by TGA measurement (Figure 3c), TNI* = 110 °C measured by DSC, and the known value of TNIS = 34.5 °C for 5CB, we calculated TNIG as 59.7 °C, which agrees the experimentally determined TNIG (∼55 °C) result. According to the procedure established in our previous work,10 we studied the thermal actuation induced size change behavior of the gel micropillars by using POM at different temperatures. Figure 4a shows the length and diameter change of a LCE gel micropillar at different temperatures in a heating− cooling cycle. In the heating process, when the temperature was increased from 35.5 to 50 °C, the LCE gel micropillar contracted gradually in both length (L) and diameter (D). With D

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Figure 4. (a) Length and diameter deformation behavior of LCE gel micropillar in heating−cooling cycle. (b) Optical and (c) the POM images of LCE gel micropillar at 35.5 °C. (d) Optical and (e) the POM images of LCE gel micropillar at 60 °C; the scale bar in (b), (c), (d), and (e) is 50 μm. (f) Size change behavior of LCE/AuNR (1.0 wt %) gel micropillar. (g) Size change behavior of LCE/AuNS (1.0 wt %) gel micropillar.

further increasing the temperature up to 60 °C, one can observe that in a relatively narrow temperature range around 55 °C there is a sudden increase of the rate of length reduction with respect to temperature. In the same temperature range, the diameter presents a rapid expansion. The temperature-dependent length and diameter change behavior in the cooling cycle from 60 to 35.5 °C is similar to that observed in the heating cycle. The thermal actuation behavior of the LCE gel pillar shown in Figure 4a suggests its TNIG is around 55 °C, which agrees with the value determined by DSC and that calculated by using the empirical formula eq 1. Furthermore, the POM observation of the LCE gel pillar in the heating and cooling processes also suggested the TNIG is approximately 55 °C. As representative examples, the optical and POM images of the LCE gel pillars at 35.5 and 60 °C are respectively shown in In Figures 4b,c and 4d,e. Above TNIG, the pillars become isotropic and transparent as shown in optical and POM images. The results for the length and diameter change of the LCE/AuNS and LCE/AuNR gel micropillar at different temperatures in a heating−cooling cycle are respectively shown in Figures 4f and 4g, which are very similar to that of the LCE gel pillar. The swelling degree for a cross-linked polymer network is defined as VG/V0, where VG and V0 refer to the volumes of the sample in swollen and dry state, respectively. For LC gels, a mean-field theory24 has been established capable of predicting the dependence of VG/V0 on temperature, which, as shown below, quite nicely describes the experimentally observed volume change of the LCE gel micropillar with temperature. According to the experimental results shown in Figure 4, we calculated the temperature-dependent volume VG = πD2L/4 for gel micropillars in the range of 35.5−60 °C, and the results are shown in Figure 5. For the gel micropillar samples of LCE,

Figure 5. Temperature-dependent volumetric transition behavior of LCE, LCE/AuNR (1.0 wt %), LCE/AuNS (1.0 wt %), LCE/AuNR (0.1 wt %), and LCE/AuNR (5.0 wt %) gel micropillars.

LCE/AuNR (1.0 wt %), LCE/AuNS (1.0 wt %), and LCE/ AuNR (0.1 wt %), the T−VG relation clearly shows a discontinuity transition at about 52 °C, which is closely related to the phase transition temperature of the LCE gel pillar, TNIG. This discontinuity separates the T−VG relation into two regimes. One (regime I) is for TNIS < T < TNIG, and the other E

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Macromolecules (regime II) is for T > TNIG. In regime I, the volume continuously shrinks with temperature, and in regime II, the volume shows little or negligible dependence on the temperature. At the transition temperature (discontinuity point of the T−VG relation), the LCE gel pillar enters from I to II and demonstrates an abrupt volume expansion. According to the mean-field theory24 and some experimental work,36−38 in regime I, the LCE nematic network and the absorbed LC solvent forms a single phase, which is in equilibrium with the same solvent outside the swollen LCE network. The LC solvent inside the network is in its anisotropic state, but the solvent outside the network is in its isotropic state. With increasing the temperature, a continuous nematic-to-isotropic phase transition of the LC solvent inside the network occurs, and the newly generated isotropic solvent molecules tend to migrate out of the network to cause a continuous decrease of the swelling degree and thus the volume reduction of the LCE gel pillar. When the temperature reaches TNIG, a nematic-to-isotropic phase transition of the LCE network takes place. At this point, the isotropic LC solvent outside the network tends to migrate into the isotropic gel network to induce a sharp volume expansion of the LCE gel micropillar. With further increasing the temperature, the entire system enters regime II (T > TNIG) in which no phase transitions are involved and both the LCE network and the LC solvent are in their isotropic state. In regime II, the volume increase due to thermal expansion shows little or negligible temperature dependence. The results shown in Figure 5 suggest that the presence of gold nanoparticles at low concentration has not intervened in the volumetric transition of LCE gels. However, this is not true when the concentration of AuNPs is high. As an example, we fabricated LCE/AuNR gel micropillars with gold concentration of 5.0 wt %, which showed negligible size and volume change during the heating process. The DSC results shown in Figure S1 indicate that there still exists the nematic structure in the LCE network for both the LCE/AuNR composite and gel sample (5.0 wt %). Therefore, a probable explanation for the negligible size and volume change of LCE/AuNR (5.0 wt %) gel micropillar is that the high gold loading reinforces/stiffens the elastomeric network of the LCE to an extent such that the micropillar loses its rubbery elasticity and the volumetric transition behavior. Photothermal Actuation of Gel Micropillars. A homemade setup shown in Figure 6a was used to investigate the photothermal actuation behavior of the LCE, LCE/AuNS, and LCE/AuNR gel micropillars. Before proceeding, we would like to point out that the LCE/AuNS and LCE/AuNR gel micropillars indeed can be photothermally actuated at room temperature (25 °C, as shown in Figure S2). Nevertheless, to avoid any complications caused by the phase transition of the surrounding 5CB solvent, we performed the photothermal actuation experiments at 35.5 °Ca value slightly above TNIS to investigate the shape changing behavior of the gel micropillars fully swollen in a 5CB bath. As a representative example, Figures 6b, 6c, and 6d respectively show the snapshots of a LCE/AuNR (1 wt %) gel micropillar when the laser irradiation was set at OFF, ON, and OFF states. In Figure 6e, the time-dependent micropillar size (length and diameter) responsive to laser irradiation ON/OFF settings is also shown. The results shown in Figure 6 clearly indicate that when the laser irradiation was switched on, the gel micropillar was able to quickly response, which manifested a contraction in length and expansion in diameter. The completion of the laser actuation

Figure 6. (a) Schematic diagram of the photothermal actuation system. M1 and M2: mirrors; L1 and L2: focus lens. (b), (c), and (d) show the gel size before laser irradiation (laser off), under laser irradiation (laser on), and in recovery (laser off), respectively. (e) Time-dependent length and diameter change behavior of LCE/AuNR (1.0 wt %) gel micropillar during photothermal actuation. Gel swollen in 5CB was kept at 35.5 °C. The arrows inserted show the time when laser irradiation was switched on/off.

took less than 2 s. Upon switching off the laser irradiation, the gel micropillar expanded in length and shrank in diameter immediately. It took less than 3 s for the pillar to recover to the original state. The successful laser actuation of the LCE/AuNR gel pillar is attributed to the surface plasmon resonance enhanced photothermal conversion of the AuNRs. Upon laser irradiation, the heat generated from AuNRs propagates throughout the gel micropillar rapidly to cause a temperature rise above TNIG and induce a nematic-to-isotropic phase transition and the corresponding size/volume change. In our previous work,22 the temperature rise of a dried LCE/ AuNP micropillar in the photothermal actuation process was estimated through a photothermal−thermal actuation superposition procedure. It has been identified that the length change L/L0 for a pillar at temperature Tphotothermal in photothermal actuation can be superposed with that obtained in thermal actuation at temperature Tthermal. The temperature difference Tthermal − Tphotothermal gives an estimate of the temperature rise induced by the photothermal effect. With this procedure, we have examined the temperature rise of the LCE, LCE/AuNS (1 wt %), and LCE/AuNR (0.1, 1, and 5 wt %) gel micropillars in photothermal actuation to evaluate the effect of gold nanoparticle shape (AuNS vs AuNR) and concentration (0, 0.1, 1, and 5 wt %). For all samples, the photothermal actuation was performed at Tphotothermal = 35.5 °C with laser power maintained 100 W/cm2. Figure 7a−e shows the snapshots of these gel micropillars before and after the laser irradiation was switched on. For each case, the length ratio L/ L0 was accordingly determined and compared with the results F

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The effect of laser irradiation intensity on the photothermal actuation behavior of the series gel micropillars we fabricated was also investigated. Figure 8 shows the results. With the

Figure 8. (□) LCE gel micropillar; (○) LCE/AuNR (1.0 wt %) gel micropillar; (▽) LCE/AuNS (1.0 wt %) gel micropillar; (△) LCE/ AuNR (0.1 wt %) gel micropillar; (◇) LCE/AuNR (5.0 wt %) gel micropillar. (a) Normalized gel micropillar length under photothermal actuation with different laser intensity. (b) Laser intensity-dependent temperature rising of gel micropillars.

Figure 7. (a−e) Photothermal actuation behavior of LCE, LCE/AuNR (1.0 wt %), LCE/AuNS (1.0 wt %), LCE/AuNR (0.1 wt %), and LCE/AuNR (5.0 wt %) gel micropillar, respectively. (f) T-dependent length deformation behavior of gel micropillars in thermal actuation; the inserted red marks on the blue line represent the L/L0 length ratio data points, and the temperature rising in the photothermal actuation ΔT are labeled.

length of the gel micropillar L(P) observed at laser intensity P normalized by its original length L0 (no laser irradiation), Figure 8a compares the laser intensity-dependent length change behavior. In Figure 8b, the estimated temperature rise for each case is correspondingly shown. As shown in Figure 8, for LCE/ AuNR (1.0 and 0.1 wt %) and LCE/AuNS (1.0 wt %) gel micropillars, the temperature increase with laser intensity is evident. The temperature increase with laser intensity is very significant for LCE/AuNR (1.0 wt %). When the irradiation intensity is higher than 77.4 W/cm2, the LCE/AuNR (1.0 wt %) sample can be heated up at least by 25 °C. To further understand the laser irradiation induced temperature rise in the photothermal actuation of the LCE/AuNR micropillar, we performed finite element analysis (FEA) to compute the temperature distribution for a LCE/AuNR (1 wt %) gel micropillar under photothermal actuation. The FEA model considers a gel micropillar immersed in a liquid crystal solvent bath and a uniformly distributed heat source induced by laser irradiation. The commercial FEA package COMSOL was applied for solving the problem. In the computation, the heat source due to the photothermal conversion of AuNR, Q (W/ cm3), was estimated by

acquired through thermal actuation process (Figure 7f) for estimating the temperature rise induced by photothermal effect. Such estimated temperature rises due to photothermal actuation for LCE, LCE/AuNS (1 wt %), LCE/AuNR (0.1 wt %), and LCE/AuNR (1 wt %) gel micropillars are estimated to be 2, 7, 7, and >25 °C, respectively. Table 1 summarizes the Table 1. Estimation of Temperature Upshift in Photothermal Actuation gel sample

LLaser/L0 ratioa

experimental ΔT/°C

LCE gel LCE/AuNR (1.0 wt %) gel LCE/AuNS (1.0 wt %) gel LCE/AuNR (0.1 wt %) gel LCE/AuNR (5.0 wt %) gel

0.93 0.70 0.93 0.97 0.99

2 >25 7 7

a LLaser/L0 represents the ratio of the gel micropillar length under photothermal actuation to that without laser irradiation.

length change and temperature rise results. On the basis of these results, we can conclude that AuNR is a better photothermal agent than AuNS. At the same loading, AuNR results in higher temperature rise. The reason for this is due to the much higher absorption cross section of AuNR than that of AuNS (Figure 1c). Additionally, from the photothermal conversion respect, the higher loading of AuNRs also facilitate high temperature rise. However, as discussed previously, when the AuNR loading is too high (such as 5 wt %), the reinforcement effect of the AuNR may lead to the loss of rubbery elasticity of the micropillar and its volumetric transition behavior under both thermal and photothermal stimulation. This is indeed observed for the case for the LCE/AuNR (5 wt %) gel micropillar, which showed negligible size change in both thermal and photothermal actuation experiments.

Q = 4P[1 − exp( −αD)]/πD

(2)

α = CabsNACρ /MAu

(3)

where P is the laser power density received by the pillar, D is the diameter of the gel micropillar, α is the optical absorption coefficient of the gel sample, Cabs is the optical absorption cross section, NA is Avogadro’s number, C is the mass concentration of incorporated AuNR, ρ is the gel density, and MAu is the atomic mass of Au. The values of the parameters P, D, Cabs, C, and ρ are taken as 100 W/cm2, 45 μm, 1.26 × 10−17 cm2, 3.75 × 10−4, and 1067 kg/m3, respectively. By taking the gel− solvent heat transfer into consideration, the heat transfer coefficient h was calculated according to the empirical equation developed by Nakai and Okazaki.39 The resultant h is 3740 W/ (m2 K), and the calculation is shown in the Supporting Information. According to the FEA simulating result shown in Figures 9a and 9b, the temperature rise of the LCE/AuNR (1.0 G

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Figure 9. Temperature upshift result of LCE/AuNR (1.0 wt %) gel micropillar in photothermal actuation performed by FEA, taking no account of the mass transfer of 5CB in the process. The gel sample was kept at 308.65 K with an irradiating laser power density of 100 W/cm2. (a) Temperature distribution in gel micropillar 3D model. (b) Temperature distribution in 2D cross section.

wt %) gel micropillar in the photothermal actuation is ∼38 °C. This value is larger than but supports that determined by the experiment (>25 °C). Given that the solvent mass transfer and the related convection heat transfer as well as the 30% gel volume contraction/expansion complication have not been taken into account, the current very basic FEA model, though not accurate, but indeed gives a reasonable temperature rise estimation to substantiate the observations reported in this article. The photothermal actuation of the LCE/AuNR micropillars shows the high repeatability and durability. The result obtained for LCE/AuNR (1.0 wt %) gel micropillar is discussed here as a typical case to show the repeatability and durability of the photothermal actuation. Under the typical actuation condition (the surrounding temperature was 35.5 °C, the laser intensity was 100 W/cm2), the photothermal actuation experiment was carried out repeatedly for at least three cycles of the actuation as shown in Figure 10. The ∼30% contraction in length and ∼20% expansion in diameter are observed in each actuation

cycle. The rapid and repeatable actuation is observed without any observable difference.

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CONCLUSIONS By incorporating gold nanoparticles (AuNS and AuNR) into side-on liquid crystal elastomer and upon a subsequent swelling in low molecular weight liquid crystal solvents 5CB, LCE, LCE/AuNR, and LCE/AuNS gel micropillars were successfully fabricated. Both thermal and photothermal actuation behavior of LCE gel micropillars with the two loading particle shapes (AuNS and AuNR) and different AuNR concentrations (0, 0.1, 1, and 5 wt %) was studied. The effects of laser intensity on the photothermal actuation were investigated. It was found that with incorporation of an appropriate amount of gold nanoparticles in particular gold nanorods the LCE/AuNR gel micropillars with nanoparticle loading content of 1.0 wt % show rapid size and volume change/recovery upon turning on/off of the laser irradiation at room temperature, which has not been achieved previously.22 The newly developed LCE/AuNR gel micropillars can be useful for some novel applications, such as micromotors and micropumps.

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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.6b01930. Heat transfer coefficient calculation in FEA, thermal phase transition behavior of LCE/AuNR (5.0 wt. %) gel micropillar, and photothermal actuation of LCE/AuNR (1.0 wt. %) gel micropillar at 25 °C (PDF)

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AUTHOR INFORMATION

Corresponding Authors

Figure 10. Relative sizes (LActuation/L0 and DActuation/D0) of LCE/ AuNR (1.0 wt %) gel micropillar during the photothermal actuation process. LActuation/L0 and DActuation/D0 represent the relative length and diameter change, respectively, where L0 and D0 are the micropillar length and diameter without laser irradiation; LActuation and DActuation are those during the photothermal actuation. Three repeated cycles of the laser on/off actuation are shown in the figure.

*(X.W.) E-mail: [email protected]. *(T.L.) E-mail: [email protected]. *(P.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. H

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(19) Orendorff, C. J.; Murphy, C. J. Quantitation of Metal Content in the Silver-Assisted Growth of Gold Nanorods. J. Phys. Chem. B 2006, 110, 3990−3994. (20) Liu, T.; Xiao, Z.; Wang, B. The Exfoliation of SWCNT Bundles Examined by Simultaneous Raman Scattering and Photoluminescence Spectroscopy. Carbon 2009, 47, 3529−3537. (21) Islam, M. F.; Milkie, D. E.; Kane, C. L.; Yodh, A. G.; Kikkawa, J. M. Direct Measurement of the Polarized Optical Absorption Cross Section of Single-Wall Carbon Nanotubes. Phys. Rev. Lett. 2004, 93, 037404. (22) Liu, X.; Wei, R.; Hoang, P. T.; Wang, X.; Liu, T.; Keller, P. Reversible and Rapid Laser Actuation of Liquid Crystalline Elastomer Micropillars with Inclusion of Gold Nanoparticles. Adv. Funct. Mater. 2015, 25, 3022−3032. (23) Urayama, K.; Okuno, Y.; Kawamura, T.; Kohjiya, S. Volume Phase Transition of Liquid Crystalline Gels in a Nematic Solvent. Macromolecules 2002, 35, 4567−4569. (24) Urayama, K.; Okuno, Y.; Nakao, T.; Kohjiya, S. Volume Transition of Nematic Gels in Nematogenic Solvents. J. Chem. Phys. 2003, 118, 2903−2910. (25) Okuno, Y.; Urayama, K.; Kohjiya, S. Influence of Cross-Linking Density on Volume Phase Transition of Liquid Crystalline Gels in a Nematogenic Solvent. J. Chem. Phys. 2003, 118, 9854−9860. (26) Urayama, K. Selected Issues in Liquid Crystal Elastomers and Gels. Macromolecules 2007, 40, 2277−2288. (27) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 1957−1962. (28) Buguin, A.; Li, M. H.; Silberzan, P.; Ladoux, B.; Keller, P. MicroActuators: When Artificial Muscles Made of Nematic Liquid Crystal Elastomers Meet Soft Lithography. J. Am. Chem. Soc. 2006, 128, 1088−1089. (29) Hornyak, G. L.; Martin, C. R. Optical Properties of a Family of Au-Nanoparticle-Containing Alumina Membranes in Which the Nanoparticle Shape is Varied from Needle-Like (Prolate) to Spheroid, to Pancake-Like (Oblate). Thin Solid Films 1997, 303, 84. (30) Link, S.; Mohamed, M. B.; El-Sayed, M. A. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant. J. Phys. Chem. B 1999, 103, 3073−3077. (31) Nigar, M.; Chvalun, S. N.; Blackwell, J. Modeling the Structure of the Hard Domains in HMDI-Based Polyurethane Elastomers. Acta Polym. 1998, 49, 27−34. (32) Su, J.; Zhang, Q. M.; Kim, C. H.; Ting, R. Y.; Capps, R. Effects of Transitional Phenomena on the Electric Field Induced Strain− Electrostrictive Response of a Segmented Polyurethane Elastomer. J. Appl. Polym. Sci. 1997, 65, 1363−1370. (33) Prescott, S. W.; Mulvaney, P. Gold Nanorod Extinction Spectra. J. Appl. Phys. 2006, 99, 123504. (34) Urayama, K.; Arai, Y. O.; Takigawa, T. Volume Phase Transition of Monodomain Nematic Polymer Networks in Isotropic Solvents Accompanied by Anisotropic Shape Variation. Macromolecules 2005, 38, 3469−3474. (35) Warner, M.; Wang, X. J. Phase Equilibria of Swollen Nematic Elastomers. Macromolecules 1992, 25, 445−449. (36) Benmouna, F.; Peng, B.; Ruhe, J.; Johannsmann, D. Phase Diagrams of Phenyl Benzoate Side Group Liquid Crystal Polymers and Similar Low Molecular Mass Liquid Crystals. Liq. Cryst. 1999, 26, 1655−1661. (37) Nwabunma, D.; Kyu, T. Phase Behavior of Mixtures of Low Molar Mass Nematic Liquid Crystal and in Situ Photo-Cross-Linked Polymer Network. Macromolecules 1999, 32, 664−674. (38) Matsuyama, A.; Kato, T. Discontinuous Elongation of Nematic Gels by a Magnetic Field. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2001, 64, 010701. (39) Seiichi, N.; Takuro, O. Heat Transfer form a Horizontal Circular Wire at Small Reynolds and Grashof Numbers-I. Int. J. Heat Mass Transfer 1975, 18, 387−396.

ACKNOWLEDGMENTS The financial support from the NSFC under 51233002 is gratefully acknowledged.

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REFERENCES

(1) Foroughi, J.; Spinks, G. M.; Wallace, G.; Oh, J.; Kozlov, M.; Fang, S.; Mirfakhrai, T.; Madden, J.; Shin, M.; Kim, S.; Baughman, R. Torsional Carbon Nanotube Artificial Muscles. Science 2011, 334, 494−497. (2) Smalyukh, I. I.; Lansac, Y.; Clark, N. A.; Trivedi, R. P. ThreeDimensional Structure and Multistable Optical Switching of Triple Twist Toron Quasiparticles in Anisotropic Fluids. Nat. Mater. 2010, 9, 139−145. (3) Zhang, X.; Yu, Z.; Wang, C.; Zarrouk, D.; Seo, J. W.; Cheng, C.; Buchan, A. D.; Takei, K.; Zhao, Y.; Ager, J. W.; Zhang, J.; Hettick, M.; Hersam, M. C.; Pisano, A. P.; Fearing, R. S. Photoactuators and Motors Based on Carbon Nanotubes with Selective Chirality Distributions. Nat. Commun. 2014, 5, 2983−2990. (4) Wu, Z. L.; Buguin, A.; Yang, H.; Taulemesse, J.-M.; Le Moigne, N.; Bergeret, A.; Wang, X.; Keller, P. Microstructured Nematic Liquid Crystalline Elastomer Surfaces with Switchable Wetting Properties. Adv. Funct. Mater. 2013, 23, 3070−3076. (5) Elias, A. L.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J.; Brett, M. J. Photopatterned Liquid Crystalline Polymers for Microactuators. J. Mater. Chem. 2006, 16, 2903−2912. (6) Lebar, A.; Cordoyiannis, G.; Kutnjak, Z.; Zalar, B. The Isotropicto-Nematic Conversion in Liquid Crystalline Elastomers. In Liquid Crystal Elastomers: Materials and Applications; de Jeu, W. H., Ed.; Springer: Berlin, 2012; pp 147−185. (7) de Jeu, W. H.; Ostrovskii, B. I. Order and Disorder in LiquidCrystalline Elastomers. In Liquid Crystal Elastomers: Materials and Applications; de Jeu, W. H., Ed.; Springer: Berlin, 2012; pp 187−234. (8) Warner, M.; Terentjev, E. M. Liquid Crystal Elastomers; Oxford University Press: 2003; pp 95−115. (9) Yang, H.; Buguin, A.; Taulemesse, J. M.; Kaneko, K.; Mery, S.; Bergeret, A.; Keller, P. Micron-Sized Main-Chain Liquid Crystalline Elastomer Actuators with Ultralarge Amplitude Contractions. J. Am. Chem. Soc. 2009, 131, 15000−15004. (10) Wei, R.; Zhou, L.; He, Y.; Wang, X.; Keller, P. Effect of Molecular Parameters on Thermomechanical Behavior of Side-On Nematic Liquid Crystal Elastomers. Polymer 2013, 54, 5321−5329. (11) Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warner, M. A New Opto-Mechanical Effect in Solids. Phys. Rev. Lett. 2001, 87, 015501. (12) Yu, Y.; Nakano, M.; Ikeda, T. Photomechanics: Directed Bending of a Polymer Film by Light. Nature 2003, 425, 145−145. (13) Spillmann, C. M.; Naciri, J.; Martin, B.; Farahat, W.; Herr, H.; Ratna, B. R. Stacking Nematic Elastomers for Artificial Muscle Applications. Sens. Actuators, A 2007, 133, 500−505. (14) Haberl, J. M.; Sánchez-Ferrer, A.; Mihut, A. M.; Dietsch, H.; Hirt, A. M.; Mezzenga, R. Liquid-Crystalline Elastomer-Nanoparticle Hybrids with Reversible Switch of Magnetic Memory. Adv. Mater. 2013, 25, 1787−1791. (15) Michal, B. T.; McKenzie, B. M.; Felder, S. E.; Rowan, S. J. Metallo-, Thermo-, and Photoresponsive Shape Memory and Actuating Liquid Crystalline Elastomers. Macromolecules 2015, 48, 3239−3246. (16) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.; Shelley, M. Fast Liquid-Crystal Elastomer Swims into the Dark. Nat. Mater. 2004, 3, 307−310. (17) Yang, L.; Setyowati, K.; Li, A.; Gong, S.; Chen, J. Reversible Infrared Actuation of Carbon Nanotube−Liquid Crystalline Elastomer Nanocomposites. Adv. Mater. 2008, 20, 2271−2275. (18) Yang, Y.; Zhan, W.; Peng, R.; He, C.; Pang, X.; Shi, D.; Jiang, T.; Lin, Z. Graphene-Enabled Superior and Tunable Photomechanical Actuation in Liquid Crystalline Elastomer Nanocomposites. Adv. Mater. 2015, 27, 6376−6381. I

DOI: 10.1021/acs.macromol.6b01930 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (40) Hohenester, U.; Trugler, A. MNPBEM − A Matlab Toolbox for the Simulation of Plasmonic Nanoparticles. Comput. Phys. Commun. 2012, 183, 370−381. (41) Hayata, Y.; Nagano, S.; Takeoka, Y.; Seki, T. Photoinduced Volume Transition in Liquid Crystalline Polymer Gels Swollen by a Nematic Solvent. ACS Macro Lett. 2012, 1, 1357−1361.

J

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