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Photo-Induced Anomalous Deformation of Poly(N-Isopropylacrylamide) Gel Hybridized with an Inorganic Nanosheet Liquid Crystal Aligned by Electric Field Takumi Inadomi, Shogo Ikeda, Yasushi Okumura, Hirotsugu Kikuchi, Nobuyoshi Miyamoto*

Poly-(N-isopropylacrylamide) (PNIPA) hydrogel films doped with uniaxially aligned liquid crystalline (LC) nanosheets adsorbed with a dye are synthesized and its anomalous photothermal deformation is demonstrated. The alignment of the nanosheet LC at the cm-scale is easily achieved by the application of an in-plane or out-of-plane AC electric field during photo-polymerization. A photoresponsive pattern is printable onto the gel with μm-scale resolution by adsorption of the dye through a pattern-holed silicone rubber. When the gel is irradiated with light, only the colored part is photothermally deformed. Interestingly, the photo-irradiated gel shows temporal expansion along one direction followed by anisotropic shrinkage, which is an anomalous behavior for a conventional PNIPA gel.

1. Introduction Anisotropic structures and multiple responses to stimuli are the key for the fabrication of high-performance soft materials such as living systems with high energy efficiency, fast response, and vectorial motion. Liquid crystal (LC) elastomers,[1] which consist of cross-linked polymer chains with LC moieties on the main- or side-chains, are potentially important for such purposes. Polymer gels composed of cross-linked polymer chains swollen by a

T. Inadomi, S. Ikeda, Prof. N. Miyamoto Department of Life, Environment and Materials Science, Graduate School of Fukuoka Institute of Technology, 3–30–1 Wajirohigashi, Higashiku, Fukuoka 811–0295, Japan E-mail: [email protected] Prof. Y. Okumura, Prof. H. Kikuchi, Prof. N. Miyamoto Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816–8580, Japan Macromol. Rapid Commun. 2014, 35, 1741−1746 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

solvent are also promising. Among them, poly(N-isopropylacrylamide) gel (abbreviated as PNIPA gel) is the most investigated because of its fascinating thermoresponsive swelling/deswelling property. Fundamental aspects and applications for soft actuators[2] and cell cultures[3] have been investigated by many researchers. However, fabrication of PNIPA gels showing anisotropic response, better response time and mechanical strength,[4] as well as responses to stimuli other than temperature (such as light[5]) are still challenging. We recently reported the facile synthesis of the cmscale anisotropic PNIPA gel[6] by utilizing inorganic nanosheet liquid crystals.[7] Similar anisotropic gels were also reported by several researchers.[8,9] Inorganic nanosheets[10] are obtained by exfoliation of inorganiclayered materials such as clays,[11,12] titanates,[13,14] perovskites,[15] and niobates.[16,17] The colloids of inorganic nanosheets form LC phases[11,14,17,18] through their spontaneous orientation induced by excluded volume

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DOI: 10.1002/marc.201400333

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effects.[19] Nanosheet LCs are valuable as new class inorganic LCs with electric, magnetic, and mechanical properties.[20] In the PNIPA gel hybridized with the LC clay mineral of fluorohectorite (FHT),[12] the anisotropic structure of the LC nanosheets is retained, so that the gel shows birefringence, anisotropic diffusion of substances, and anisotropic thermo-induced deformation. However, in that gel, the nanosheets are oriented in a tree-ring-like fashion, so that the detailed characterization, optimization of the anisotropic properties, and further applications are still difficult. Here, we demonstrate a photo-induced anomalous deformation behavior of the PNIPA gel as achieved by hybridization with uniaxially aligned LC inorganic nanosheets and a dye. The LC gels with macroscopically oriented monodomains are synthesized by alignment of the nanosheet LC mixed with NIPA monomer by an electric field, followed by photo-polymerization and patterned adsorption of a dye. Since the mesogenic nanosheet is much larger (several microns) than conventional LC molecules and the nanosheet colloids form a LC phase at a very low concentration, the LC phase shows a rather low viscosity, and the orientation of the LC nanosheets is attained more easily than for conventional LCs by an electric field,[8,21] magnetic field,[18] weak shear, and even with weak gravitational force.[17] Different from a normal PNIPA gel that only shows isotropic swelling/ deswelling induced by heat, the present gels show not only anisotropic shrinkage but also anomalous expansion in a certain direction upon photo-irradiation.

2. Results and Discussion The orientation of the LC nanosheets in the gel is highly and facilely controlled by applying AC electric fields parallel or perpendicular to the cell surface during the gel synthesis. Figure 1A shows the photographs and its microscopic images of the film-shape gel, which is synthesized in the cell with a thickness of 1 mm by applying an inplane electric field. Observation is made with crossed polarizers and a wave plate. With this setup, interference color of blue or yellow is observed if the nanosheet plane is parallel or perpendicular, respectively, to the fast axis of the wave plate.[6] Since the uniform interference colors of blue or yellow are observed in the Figure 1A, it is confirmed that the nanosheets are unidirectionally oriented at the cm-scale along the shorter axis of the gel, which points along the applied electric field. When we synthesize the gel without the electric field, the nanosheets are oriented by shear along the long axis. Thus, the perpendicular orientation is only achieved with the electric field. To elucidate the detailed structure, we observe the cut gel from three directions with a polarized optical

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microscopy. In the top- and side-images of the in-planealigned gel (Figure 1B), uniform interference colors of blue or yellow are observed, again confirming that the nanosheet LC is aligned along the applied electric field. In the cross-section image with the fast axis of the wave plate set perpendicular to the gel surface, we observe yellow color near the surface and blue color at the center position. Thus, as schematically shown in Figure 1B, the nanosheets are aligned perpendicular to the cell surface in the center, while they are aligned parallel to the cell surface near the surface. On the other hand, it was already confirmed that the nanosheets are homogeneously dispersed in the gel, retaining the liquid crystalline lamellar structure with the basal spacing of 10–100 nm, as observed by smallangle X-ray scattering.[6] The nanosheet orientation is also controlled by the out-of-plane electric field. In this case (Figure 1C), uniform interference color is observed only in the cross-sectional image, while randomly oriented domains of blue and yellow with the size of ca. 100 μm are observed in the top-view image. These observations confirm that the nanosheets are perfectly aligned perpendicular to the cell surface, while the orientation is not controlled in the inplane direction as schematically shown in Figure 1C. Onto the unidirectionally oriented FHT/PNIPA gel films, we can print a photoresponsive pattern with a cationic dye in the resolution of several tens of microns (Figure 2a). The FHT/PNIPA gel containing 2 wt% of FHT synthesized with out-of-plane electric field is used for this demonstration. When a cationic dye solution is made contact with the gel, the dye diffuses into the gel forming sharp boundary between colored and uncolored parts. This behavior is different from the case for a PNIPA gel without nanosheets, where gradient boundary is observed. This occurs because the nanosheets embedded in the gel strongly immobilize cations on their surface due to their negative charge (ca. 1.5 e− • nm−2). Since the dyes diffuse into the gel at the rate of ca. 0.3 mm h−1,[6] we observe homogeneous coloration across the axis normal to the film surface after 2 d of the dye adsorption process. When we use a multivalent dye such as TMPyP4+, the pattern is sharper and more stable compared with the monovalent ones such as rhodamine 6G, because of the stronger electrostatic interaction. When the FHT/PNIPA gel (FHT conc. is 1 wt%) patterned with the dye is irradiated with visible light, only the colored part is quickly heated and deforms (Figure 2b and Movie S1, Supporting Information) as induced by the well-known thermoresponsive volume-phase transition of PNIPA gels. As exemplified in Figure 2b, we can design various photo-induced anisotropic and asymmetric shapes thanks to the printable patterns of the dye and anisotropic deformation due to the oriented nanosheets

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Figure 1. A) Photographs and B,C) microscopic images and schematic structures of the FHT/PNIPA gels containing 1 wt% of FHT synthesized with (A,B) in-plane and (C) out-of-plane electric field. The images are observed with crossed polarizers and a wave plate.

inside the gel described below. Compared with the previously reported photoresponsive gels synthesized by polymerization of dye-functionalized monomers[5a] or by incorporation of carbon nanotubes,[5b] the present system is advantageous because the patterning of the photoresponsive part is very easy and we can potentially design many kinds of photo-induced shapes. Interestingly, the FHT/PNIPA gels show anisotropic deformation and even anomalous size expansion by photo-irradiation, in contrast with a conventional PNIPA gel without nanosheets. The microscopic images of Figure 3A and the Movie S2 (Supporting Information)

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show the process of the photo-induced deformation of the FHT/PNIPA gel adsorbed with a cationic dye synthesized with the in-plane electric field. Immediately after turning on the photo-irradiation, the gel size expands quickly along the axis parallel to the nanosheet planes, while the gel shrinks along the perpendicular axis. Within 5.6 s, the parallel size reaches the maximum (3.0% larger than the original size), while the perpendicular size shrinks by 5.3%, giving the aspect ratio of 108.7%. After this stage, both the parallel and perpendicular sizes shrink. The gel size is then quickly recovered by turning off the photo-irradiation in ca. 2 s. The deformation and

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Figure 2. Patterned coloration of the FHT/PNIPA gels with TMPyP dye and its partial photoresponsive deformation: a) photograph (left) and microscopic image (right) of the gel printed with the dye pattern; b) photo-induced partial deformation of the gel partially adsorbed with the dye as observed by optical microscopy. The red arrow indicates the direction of the oriented nanosheet planes.

recovery are repeated many times by turning on and off the irradiation. To investigate the detail of the anomalous deformation, the size change of the FHT/PNIPA gel in the equilibrium state at varied temperature is evaluated. First, the swelling of the as-prepared gel, which was synthesized with the in-plane electric field (Figure 1A,B), to the equilibrium swelling state at 25 °C was examined. The gel swells by 5% and 30% along the in-plane axes parallel and perpendicular to the aligned nanosheet plane, respectively, while the gel swells by 14% along the outof-plane axis. The swelling ratio along the out-of-plane axis is in between the two in-plane axes, probably due to the imperfect orientation along this axis as shown in Figure 1B. Next, the temperature dependence is examined with the dye-adsorbed FHT/PNIPA gel (Figure 3B). As the temperature increases, the equilibrium gel size gradually decreases and then drastically decreases at around 34 °C, the LCST of a PNIPA gel. At 40 °C, the gel size shrinks anisotropically by 19% and 40% relative to the gel at 25 °C, along the in-plane axes parallel and perpendicular to the nanosheet planes, respectively. On the other hand, the gel aligned by out-of-plane direction (with 3 wt% nanosheets) also shows anisotropic deformation. While the as-prepared gel swells to the equilibrium swollen state at 25 °C by 54% and it shrinks by 42% along the inplane direction, the thickness only swells and shrinks by 27% and 13%, respectively. Thus, at the equilibrium

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state, although anisotropic deformations are observed, no expansion is observed. From these observations, we conclude that the photo-induced expansion of the gel is a transitional state. We explain the anomalous deformation behaviors as follows. The anisotropic volume transition in the equilibrium state is rationalized by the anisotropic elastic modulus of the gel and anisotropic repulsive force in the gel, both induced by the aligned LC nanosheets. Since the inorganic nanosheets are more elastic than the swollen polymer gel, the elastic modulus of the hybrid FHT/PNIPA gel should be much larger along the axis parallel to the aligned nanosheet plane; this situation induces smaller deformation along the aligned nanosheet plane. It is also possible that the large repulsive force along the normal of the negatively charged nanosheets hampers the shrinkage along the normal direction. The anomalous photo-induced expansion of the FHT/ PNIPA gel can be rationalized by the presence of less responsive microdomains in the anisotropic gel. It is known that a polymer gel synthesized from a mixture of a monomer and a cross-linker generally contains inhomogeneity of the cross-linking density and of the average degree of polymerization between the cross-link points. The nanosheets have polydispersity in lateral size. Hence, the space distribution of the nanosheets in the gel is not perfectly homogeneous. Since the dye is immobilized only on the nanosheets, the photo-induced heating is

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initiated locally at the dye-adsorbed nanosheet surface in the initial stage of photo-irradiation. Thus, although we observe, on the macroscopic scale, homogeneous distribution and orientation of the nanosheets and homogeneous coloration by the dye, the gel can have the microdomains. Upon photo-irradiation, the highly responsive microdomains first shrink, inducing compressive stress to the less responsive microdomains that have not yet been deformed. Since the shrinkage is anisotropic, the stress is also anisotropic and is larger along the axis perpendicular to the nanosheet planes. Generally, when a uniaxial compressive stress σ// is impressed to an elastic object, the object is compressed along the parallel axis, while it expands along the perpendicular axis; the strains ε// and ε⊥ are associated as ε// = −n•ε⊥, where n is Poisson coefficient. Thus, the less responsive microdomain expands along the parallel axis temporarily in the early stage. After this anomalous period, the gel shrinks along both directions, although the shrinkage is anisotropic due to the presence of the orientated nanosheet LC. We stress that the local heating of the microdomains in a PNIPA gel, which leads to the anomalous temporal size expansion behavior, is achievable only by the photo-induced heating of the dye/nanosheet/polymer composite system as demonstrated in the present study; this is not achievable by the conventional temperature control of a PNIPA gel. The real-time SAXS measurements under controlled temperature and photo-irradiation are now underway to elucidate the detailed structure change during the expansion and shrinkage, which will further elucidate the mechanism of this anomalous behavior.

3. Conclusions

Figure 3. Anisotropic deformation of the FHT/PNIPA gel film adsorbed with the dye. The nanosheets are aligned along the direction shown by the red arrows. The plots (a) and (b) in both the panels show the relative gel sizes along the axis parallel and perpendicular to the oriented nanosheet plane, respectively. The dotted lines in the microscopic images show the form of the gel before deformation. The aspect ratio (parallel to perpendicular) of the gel size is shown as the plot (c). Panel (A) shows the time course of the relative gel size upon photo-irradiation. The dashed lines are guide for eyes. Panel (B) shows the gel sizes at the equilibrium state at varied temperature.

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We demonstrated that the well-known PNIPA gel is further functionalized to show photoresponsive anomalous deformation by hybridizing with uniaxially aligned LC nanosheets and patterned dye adsorption. The alignment is achieved by an electric field which is of low cost and is universally applicable to any materials with or without magnetic properties. By optimizing the system by using various functional nanosheets, varying the parameters such as nanosheet concentration, and combinations with functional molecules, we expect further optimization and functionalization of the present gels and various applications such as sensing, drug delivery, micro-electromechanical systems, and molecular robots.[22]

4. Experimental Section NHT-B2 sol donated by Topy Inc. is purified and condensed by centrifugation, followed by dilution with water to obtain the liquid

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crystalline aqueous colloid of fluorohectorite (FHT), Na0.46Mg2.60 Li0.46Si4O10F2.00.[10] The monomer, N-isopropylacrylamide (NIPA), and the chemical cross-linker, N,N-methylenbisacrylamide (BIS), are recrystallized from hexane/acetone mixture, while other chemicals are used as-received. For the typical gel synthesis, 1.29 mol of NIPA, 0.65 mol of BIS, and 0.61 × 10−9 mol of the photoinitiator,2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone,are dissolved in 2 g of the FHT colloid (1–3 wt%), followed by stirring and bubbling with N2 for 1 h. The mixture is then sealed in a cell made of a silicone–rubber spacer with the thickness of 1 mm sandwiched by two glass plates. For the in-plane electric field application, thin aluminum foil is used as the electrode; the gap between the electrodes is set to 5 mm. For the out-of-plane electric field application, indiumtin-oxide-coated glass plate is used instead of the glass plates. In-plane or out-of-plane AC electric field (10 V, 10 kHz) is applied for 0.5 or 45 min, respectively, followed by UV light irradiation for 5 min for photo-polymerization. The electric field is kept applied also during the irradiation. The obtained gel is stored in distilled water to be an equilibrium swelling state. The gels are adsorbed with cationic dyes, Rhodamine 6G or 5,10,15,20-tetrakis(N-methyl4-pyridyl)-21H,23H-porphine (TMPyP), by making contact with the dye solutions. The gels are observed with a digital camera or the optical microscopy (Olympus BX51) equipped with temperature controller (Linkam 10021). In some observations, crossed polarizers and a wave plate with the retardation of 530 nm are used. The 1-mmthick gel synthesized with the in-plane electric field is cut along the gel surface, so that the gel film with the thickness of 0.3 mm with the perfect alignment of the nanosheets is used for the deformation test.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: N.M. thanks financial supports by Grantin-Aid for Scientific Research (No. 24104005) on Innovative Areas of Molecular Robotics from the Ministry of Education, Culture, Sports, Science, and Technology, Japan; Canon Foundation; and Electronics Research Laboratory of Fukuoka Institute of Technology. Received: June 13, 2014; Revised: July 14, 2014; Published online: September 16, 2014; DOI: 10.1002/marc.201400333 Keywords: hydrogels; inorganic nanosheets; layered materials; liquid crystals; poly(N-isopropylacrylamide)s

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Photo-Induced Anomalous Deformation of Poly(N-Isopropylacrylamide) Gel Hybridized with an Inorganic Nanosheet Liquid Crystal Aligned by Electric Field.

Poly-(N-isopropylacrylamide) (PNIPA) hydrogel films doped with uniaxially aligned liquid crystalline (LC) nanosheets adsorbed with a dye are synthesiz...
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