Motoi Kinoshita

Photomobile Polymer Materials : Towards Light-Driven Plastic Motors

As light is a good energy source that can be controlled remotely, instantly, and precisely, light-driven soft actuators could play an important role for novel applications in wideranging industrial and medical fields. Liquid-crystalline elastomers (LCEs) are unique materials having both properties of liquid crystals (LCs) and elastomers, and a large deformation can be generated in LCEs, such as reversible contraction and expansion, and even bending, by incorporating photochromic molecules, such as an azobenzene, with the aid of photochemical reactions of these chromophores. Herein we demonstrate new sophisticated motions of LCEs and their composite materials: a plastic motor driven only by light. If materials absorb light and change their shape or volume, they can convert light energy directly into mechanical work (the photomechanical effect) and could be very efficient as a single-step energy conversion. Furthermore, these photomobile materials would be widely applicable because they can be controlled remotely just by manipulating the irradiation conditions. LCEs show an anisotropic order of mesogens with a cooperative effect, which leads them to undergo an anisotropic contraction along the alignment direction of mesogens when heated above their LC-isotropic(I) phase transition temperatures (TLC-I) and an expansion by lowering the temperature below TLC-I. [1, 13–18] The expansion and contraction is due to the microscopic change in alignment of mesogens, followed by the significant macroscopic change in order through the cooperative movement of mesogens and polymer segments. It is well known that when azobenzene derivatives are incorporated into LCs, the LC-I phase transition can be induced isothermally by irradiation with UV light to cause trans–cis photoisomerization, and the I-LC reverse-phase transition by irradiation with visible light to cause cis–trans back-isomerization. This photoinduced phase transition (or photoinduced reduction of LC order) has led successfully to a reversible deformation of LCEs containing azobenzene chromophores just by changing the wavelength of actinic light. Although the photoinduced deformation of LCEs previously reported is large and interesting, it is limited to contraction/expansion and bending, preventing them from being used for actual applications. Herein we report potentially applicable rotational motions of azobenzene-containing LCEs and their composite materials, including a first lightdriven plastic motor with laminated films composed of an LCE film and a flexible polyethylene (PE) sheet. The LCE films were prepared by photopolymerization of a mixture of an LC monomer containing an azobenzene moiety (molecule 1 shown in Scheme 1) and an LC diacrylate with an azobenzene moiety (2 in Scheme 1) with a ratio of 20/ 80 mol/mol, containing 2 mol% of a photoinitiator in a glass cell coated with rubbed polyimide alignment layers. The photopolymerization was conducted at a temperature at which the mixture exhibited a smectic phase. The glasstransition temperature of the LCE films is at about room temperature, allowing the LCE films to work at room temperature in air, as the films are flexible enough at this temperature. We prepared a continuous ring of the LCE film by connecting both ends of the film. The azobenzene mesogens were aligned along the circular direction of the ring. Upon exposure to UV light from the downside right and visible light from the upside right simultaneously (Figure 1), the ring

Photomobile polymer materials—various three-dimensional movements

The composition of a crosslinked azobenzene liquid-crystalline polymer and a flexible polymer film can provide a variety of simple devices that can walk in one direction like an ‘inchworm’ and move like a ‘robotic arm’ induced by light.

Three‐Dimensional Photomobility of Crosslinked Azobenzene Liquid‐Crystalline Polymer Fibers

Human skeletal muscles are composed of many bundles of fibers and their crucial function to convert chemical energy into mechanical work is achieved by generating smooth motion and inducing high stress by external stimuli. Recently, there has been a considerable effort to develop artificial muscles or actuators that can mimic muscle performance, and various materials that resemble human muscles have been reported such as shapememory alloys, polymer gels, conducting polymers, carbon nanotubes, and dielectric elastomers. To achieve smooth motion as in human muscles, it is most desirable to use soft materials with high mechanical flexibility. Crosslinked liquid-crystalline polymers (CLCPs) are unique materials with properties of both of liquid crystals (LCs) and elastomers and especially promising for applications in actuators due to the self-organization nature of LC systems. CLCPs responding to external stimuli in the form of fibers were also reported for artificial muscles. By incorporating photochromic molecules such as azobenzenemoieties into CLCPs, largemotions can be induced by photochemical reactions of these azobenzene chromophores. Soft actuators driven by light could play an important role for novel applications in a wide range of industrial and medical fields, because light is a clean energy source and can be controlled rapidly and remotely. In our previous work, we have developed photomobile materials with CLCPs containing azobenzene moieties. A bending of the CLCP films composed only of azobenzene mesogens has been observed by irradiation with UV light. The CLCP films can generate surface deformation caused by a change in alignment of LCs upon exposure to UV light, which contributes to the bending. We have also demonstrated new threedimensional movements of the CLCP and their composite materials driven only by light: a light-driven plastic motor, an inchworm walk, and a flexible robotic armmotion. They can convert light energy directly into mechanical work without the aid of batteries, electric wires, or gears. With CLCP fibers containing azobenzene moieties, one may expect the change in alignment of LC mesogens upon exposure to UV light. In this Communication, we report a precise directional control of photomobility in the CLCP fibers. We were able to induce three-dimensional movement of the CLCP fibers only by light. The structures of LCmonomers (A6AB6 andA6AB6OH) and a crosslinker, 4,40-methylenebis(phenyl isocyanate) (MDI) used in this study are shown in Figure 1a. A6AB6 was synthesized according to a procedure similar to that in the literature. The CLCP fibers were prepared by two-step reactions, as previously reported. Firstly, the LC monomers were polymerized by radical polymerization. Then the obtained copolymers were mixed with MDI, and the mixtures were formed into fibers by dipping a tip of a toothpick into the mixture and pulling the mixtures with the toothpick as quickly as possible. Thermal and optical properties of the CLCP fibers were investigated by differential scanning calorimetry (DSC), IR absorption spectroscopy, and polarizing optical microscopy (POM). By DSC measurements, it was found that the CLCP fibers exhibited a glass-transition temperature (Tg) of around 60 8C. In IR spectra of the CLCP fibers, the absorption band corresponding to the N H stretch of the urethane bond was observed at around 3500 cm .

Effect of concentration of photoactive chromophores on photomechanical properties of crosslinked azobenzene liquid-crystalline polymers

We studied the effects of concentration and location of azobenzene chromophores on the photoinduced deformation of crosslinked liquid-crystalline polymers (CLCPs). The concentration of azobenzene chromophores in CLCP affects the degree of isomerization of azobenzene moieties and the macroscopic deformation behaviour of the films while the location of azobenzene moieties determines the contraction force and length.

Photomechanical properties of azobenzene liquid-crystalline elastomers

We prepared homogeneously aligned azobenzene liquid-crystalline elastomer (LCE) films with low T g and explored their photomechanical properties. Upon irradiation with UV light, the films bent toward a light source at room temperature. The mechanical force generated upon exposure to UV light was evaluated by thermomechanical analysis. It was found that the mechanical force generated by photo-irradiation increased with an increase in the cross-linking density. In particular, an LCE film containing 80 mol% azobenzene cross-linker produced a force of over 1 MPa by photo-irradiation, which enabled the film to lift an object 20 times heavier than itself. The degree of contraction by photo-irradiation increased with an increase in temperature and light intensity. Bending could be brought about by more than 5000 times with periodic irradiation. Furthermore, it was found that the LCE films exhibited bending and unbending behaviour by irradiation with sunlight.

Molecular engineering versus energy level alignment: Interface formation between oligothiophene derivatives and a metal substrate studied with photoemission spectroscopy

Two series of thin films of oligothiophene derivatives grown on Ag substrates have been studied with photoelectron spectroscopy. The oligothiophenes were end-capped with either electron-deficient (dismesitylboryl) or electron-rich (diphenyltolylamine) moieties to create molecules with electron-accepting or -donating properties, respectively. The position of the highest occupied molecular orbital (HOMO) at the metal/organic interface is found to be strongly dependent on the effective π-conjugation length of the oligothiophenes capped with dimesitylboryl groups, whereas in the oligothiophenes capped with diphenyltolylamine, the position of the HOMO is independent of the molecular length. The difference in the observed HOMO characteristics is attributed to the different make-up of the frontier orbitals in the two molecular series. This will particularly affect the overall energy barrier for charge injection at the conductor/organic interface in a device structure, such as an organic light-emitting diode, uti...

ESR studies of photosensitized degradation of poly(L-lactic acid) via photoionization of dopant

The photosensitized degradation of poly(L-lactic acid) (PLA) via an anionic reaction process was studied using spectrophotometry, electron spin resonance (ESR), and gel permeation chromatography (GPC) measurements. PLA film doped with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) was irradiated at 77 K using UV light (λc = 356 nm) by which the PLA matrix itself cannot be directly excited. After photoirradiation, a new broad absorption band appeared over the original spectrum due to TMPD+ ·, which was produced by two-photon ionization. The ESR spectrum of the irradiated sample indicated the presence of the TMPD+ · radical and main-chain scission radical of PLA. During the thermal annealing at 0 °C, the latter radical changed to another radical species by dehydrogenation of the alpha hydrogen of the PLA main chain. TMPD+ · was extremely stable at room temperature for 7 d. However, by thermal annealing at 40 °C, all the radicals decayed due to the enhanced molecular motions near Tg of PLA (58.7 °C). Spectral simulation for the obtained ESR spectra revealed the relative amounts of four radicals: TMPD+ ·, a main-chain scission radical, a main-chain tertiary radical, and an unknown radical. The last one was tentatively assigned to the PLA radical anion because of its short decay time. GPC measurements clearly indicated a decrease in the molecular weight of PLA after irradiation.

Photoinduced Deformation Behavior of Crosslinked Azobenzene Liquid-Crystalline Polymer Films with Unimorph and Bimorph Structure

The use of a crosslinked azobenzene liquid-crystalline polymer (CALP) system for micro-integrated actuators becomes especially attractive. CALPs can deform along the alignment direction of mesogens upon irradiation of UV/visible light owing to photochemical phase transition. In this study, to improve the bending speed, CALP films with a homogenous alignment on one surface and a homeotropic alignment on the opposite surface (hybrid alignment) were prepared and their bending behavior was investigated. The films showed photoinduced bending whose direction was determined by the surface alignment treatment, and the bending speed was greatly enhanced upon irradiation from both surfaces of the film at the same time.

Facile strain analysis of largely bending films by a surface-labelled grating method

Mechanical properties of flexible films, for example surface strain of largely bending films, are key to design of stretchable electronic devices, wearable biointegrated devices, and soft microactuators/robots. However, existing methods are mainly based on strain-gauge measurements that require miniaturized array sensors, lead wires, and complicated calibrations. Here we introduce a facile method, based on surface-labelled gratings, for two-dimensional evaluation of surface strains in largely bending films. With this technique, we demonstrate that soft-matter mechanics can be distinct from the mechanics of hard materials. In particular, liquid-crystalline elastomers may undergo unconventional bending in three dimensions, in which both the inner and outer surfaces of the bending film are compressed. We also show that this method can be applied to amorphous elastomeric films, which highlights the general importance of this new mechanical evaluation tool in designing soft-matter-based electronic/photonic as well as biointegrated materials.

White emission from liquid-crystalline copolymers containing oxadiazole moieties in the side chain

A liquid-crystalline polymer in the side chain was synthesized through copolymerization of a bipolar carrier-transporting monomer with a liquid-crystalline monomer containing oxadiazole moieties substituted with trifluoromethyl groups. A single-layer light-emitting diode of indium tin oxide (ITO)/copolymer/MgAg emitted white light with a maximum luminous efficiency of 0.1cd∕A. The origin of the white emission in the copolymer is the electroplex between bipolar carrier-transporting moieties and strong electron-withdrawing moieties. Furthermore, a simple multilayer device with configuration of ITO/poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)/copolymer/MgAg device showed white emission with CIE 1931 chromaticity coordinates (x,y): (0.30, 0.33).