Tomiki Ikeda

Photomechanics: Directed bending of a polymer film by light

Polymer solutions and solids that contain light-sensitive molecules can undergo photo-contraction, whereby light energy is converted into mechanical energy. Here we show that a single film of a liquid-crystal network containing an azobenzene chromophore can be repeatedly and precisely bent along any chosen direction by using linearly polarized light. This striking photomechanical effect results from a photoselective volume contraction and may be useful in the development of high-speed actuators for microscale or nanoscale applications, for example in microrobots in medicine or optical microtweezers.

Optical Switching and Image Storage by Means of Azobenzene Liquid-Crystal Films

Liquid crystals are promising materials for optical switching and image storage because of their high resolution and sensitivity. Azobenzene liquid crystals (LCs) have been developed, in which azobenzene moieties play roles as both mesogens and photosensitive chromophores. Azobenzene LC films showed a nematic phase in trans isomers and no LC phase in cis isomers. Trans-cis photoisomerization of azobenzene with a laser pulse resulted in a nematic-to-isotropic phase transition with a rapid optical response of 200 microseconds.

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

Photomodulation of liquid crystal orientations for photonic applications

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.

Photocontrollable Liquid‐Crystalline Actuators

The history of human civilization is accompanied by the development of novel materials, from the period of natural stone, the bronze and iron age, to the modern times of synthetic materials and promising advanced materials. Liquid crystals (LCs) are one of such kind of materials that greatly infl uence our daily life, and which are not limited to displays such as TVs and personal computers. Being an intermediate phase of matter, LCs show characteristics of both controllable mobility of an isotropic liquid and ordered regularity of a crystalline solid. Combined with their photoresponsive properties, photocontrollable LC actuators have enabled a variety of applications in various fi elds, which include fl at panel displays, photonics, photo-driven devices, and more recently nanotechnology. The LC’s unique features provide soft materials in a liquid-crystalline state with interesting properties such as 1) selfassembly, 2) fl uidity with long-range order, 3) molecular and supramolecular cooperative motion (MCM and SMCM), 4) large birefringence and anisotropy in various physical properties (optical, mechanical, electrical and magnetic), 5) alignment change induced by external fi elds at surfaces and interfaces, and 6) deformation of LC elastomers (LCEs) in response to stimuli. [ 1–4 ]

Photo-mechanical effects in azobenzene-containing soft materials

The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. The most ubiquitous natural molecule for reversible shape change is the rhodopsin-retinal protein system that enables vision, and this is perhaps the quintessential reversible photo-switch. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. This review focuses on the study and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but non-mechanical photo-switching or photo-chemistry, nor any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere.

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.

New polymeric biocides: synthesis and antibacterial activities of polycations with pendant biguanide groups.

Acrylate monomers with pendant biguanide groups were successfully synthesized, and their homopolymers and copolymers were prepared with acrylamide. These cationic disinfectants of polymeric forms exhibited high antibacterial activity against gram-positive bacteria, whereas they were less active against gram-negative bacteria. It was found that the activity of the polymeric disinfectants was much higher than that of the monomeric species, and the difference in activity between the polymers and the monomers was discussed on the basis of their contributions to each elementary process of the lethal action.

Interaction of a polymeric biguanide biocide with phospholipid membranes

Differential scanning calorimetry (DSC) and fluorescence polarization methods have been used to study the interactions between phospholipid membranes and a polymeric biocide, poly(hexamethylene biguanide hydrochloride) (PHMB). It was found that PHMB had very little effect on neutral lipids such as phosphatidylcholine (PC) and phosphatidylethanolamine (PE), whereas it greatly reduced the phase transition temperature of phosphatidylglycerol (PG), an acidic lipid found in bacteria. Although the corresponding monomeric biocide had a similar effect on the PG bilayer, the behaviour towards mixed lipid bilayers of PC and PG has been shown to be completely different for the polymeric and monomeric biocides: viz. the former can induce isothermal phase separation into a PHMB-PG complex domain and a PC-enriched domain, whilst the latter cannot. This may account for the great difference in bactericidal activity between them. It is suggested that PHMB interacts primarily with negatively charged species in the membranes, inducing aggregation of acidic lipids in the vicinity of the adsorption site, where higher fluidity and higher permeability are expected. The results have shown that two factors might be crucial in the cidal activity of such types of cationic disinfectants as biguanides: phase separation and interaction with the hydrocarbon interior of the membranes. Polymeric biocides could be particularly effective by virtue of their ability to combine hydrophobic character and multiple charges within a single molecule.

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 .