Yumiko Naka

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.

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.

Real-time dynamic hologram in photorefractive ferroelectric liquid crystal with two-beam coupling gain coefficient of over 800 cm–1 and response time of 8 ms

The photorefractive effect in photoconductive ferroelectric liquid crystals (FLCs) that contain photoconductive chiral compounds was investigated. Terthiophene compounds with chiral structure were chosen as the photoconductive chiral compounds and mixed with an achiral smectic C liquid crystal. The mixture exhibited the ferroelectric chiral smectic C phase and photoconductivity. The photorefractivity of the mixture was investigated by two-beam coupling experiment. It was found that the FLC containing the photoconductive chiral compound exhibits a large gain coefficient of over 800 cm−1 and a fast response time of 8 ms. The high photorefractive performance was considered to be due to enhanced charge mobility.

Direct fabrication of photomobile polymer materials with an adhesive-free bilayer structure by electron-beam irradiation

Azobenzene liquid-crystalline (LC) polymers coated on a flexible polymer substrate were directly crosslinked by electron beams (EBs). We demonstrated that EB-crosslinked azobenzene LC polymers could successfully work as photomobile polymer materials with an adhesive-free bilayer structure.

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.

Photoresponsive and Holographic Behavior of an Azobenzene-Containing Block Copolymer and a Random Copolymer

A block copolymer containing an azobenzene (Az) moiety in the side chain was prepared by atom transfer radical polymerization (ATRP). For comparison, a random copolymer with a similar Az content was also designed. Their photoresponsive and holographic behavior was investigated in thin films. The block copolymer film showed microphase separation with cylinder microdomains after annealing, which was then irradiated at 366 nm. It was found that trans-cis photoisomerization in the block copolymer film was slower than that in the random copolymer film. This may be due to the restricted motion of the Az moieties in the Az blocks. The photoinduced birefringence (Δn) of the block copolymer film was higher than that of the random copolymer film upon irradiation with a linearly polarized beam. In holographic recording, similar results were obtained. A block copolymer with 22-mol% Az showed a liquid-crystalline (LC) phase, while a random copolymer with 24-mol% Az exhibited no LC phase. The block copolymer showed an LC phase at a lower Az content compared with the random copolymer, suggesting that the molecular alignment of Az mesogenic groups is enhanced by cooperative motion of LCs in block copolymer films.

Effect of Recording Time on Grating Formation and Enhancement in an Amphiphilic Diblock Liquid-Crystalline Copolymer

To precisely control the first-order diffraction efficiency of holographic gratings recorded in an amphiphilic diblock liquid-crystalline copolymer P40A37, effect of recording time on grating formation and enhancement was studied systematically. Several parameters such as surface relief, the first-order diffraction efficiency, the maximum transmittance under an optical polarizing microscope and order parameters of grating samples were evaluated before and after annealing. Then the enhancement coefficient with recording time was estimated. The best enhancement effect was obtained at 10-s recording upon nano-scaled phase separation. By precisely adjusting the recording time, the diffraction efficiency of holographic gratings in P40A37 can be finely controlled from 0.13% to 10%.

Photorefractive effect of photoconductive ferroelectric liquid crystalline mixtures composed of photoconductive chiral compounds and liquid crystal

The photorefractive effect in photoconductive ferroelectric liquid crystals (FLCs) that contain photoconductive chiral compounds was investigated. A series of novel photoconductive chiral compounds were synthesized. Terthiophene compounds with four chiral structures were chosen as the photoconductive chiral compounds and mixed with an achiral smectic C liquid crystal. The mixtures exhibit the ferroelectric chiral smectic C phase. The photorefractivity of the mixtures was investigated by two-beam coupling experiments. It was found that the FLCs containing the photoconductive chiral compound exhibit a large gain coefficient of 110 cm−1 and a fast response time of 5 ms.

Laser irradiation durability of photorefractive ferroelectric liquid crystal blends containing terthiophene photoconductive chiral dopants

Ferroelectric liquid crystal blends composed of a smectic liquid crystalline mixture, a photoconductive chiral dopant, and an electron trap reagent exhibit significant photorefractivity together with rapid responses. As such, they allow the dynamic amplification of moving optical signals. In the present work, the photochemical durability of a photorefractive ferroelectric liquid crystal blend sandwiched between two transparent electrodes was investigated. A series of photoconductive chiral dopants was prepared and the durability of blends incorporating these dopants during laser irradiation was examined. The photorefractivity of the liquid crystal blend with application of a DC electric field was found to decay on increasing the laser irradiation time. However, no evidence for a photochemical reaction of the photoconductive chiral dopant was obtained. The effect of the conduction of photogenerated ionic species on the photorefractivity decay was clarified. It was concluded that the observed decay of the photorefractivity was caused by the adsorption of photogenerated ionic species on the electrode.

Dismantlable Thermosetting Adhesives Composed of a Cross-Linkable Poly(olefin sulfone) with a Photobase Generator

A novel photodetachable adhesive was prepared using a photodepolymerizable cross-linked poly(olefin sulfone). A mixture of a cross-linkable poly(olefin sulfone), a cross-linking reagent, and a photobase generator functioned as a thermosetting adhesive and exhibited high adhesive strength on quartz plates comparable to that obtained for commercially available epoxy adhesives. The cured resin was stable in the absence of UV light irradiation but completely lost its adhesive strength upon exposure of glued quartz plates to UV light in conjunction with heating to 100 °C.