Tohei Yamamoto

Photoinduced alignment control of photoreactive side-chain polymer liquid crystal by linearly polarized ultraviolet light

Alignment control of mesogenic groups in a photocrosslinkable polymer liquid crystalline film was achieved by linearly polarized (LP) ultraviolet (UV) irradiation followed by heating. The alignment direction was parallel to the electric vector of the incident LPUV light and the tilt angle of the mesogenic groups could be controlled by a one-step slantwise LPUV irradiation. The photoinduced birefringence dn was 0.07 and the order parameter of the mesogenic group was 0.28.

Optical anisotropy of a photoreacted side-chain liquid-crystalline polymer induced by Linearly polarized UV light

Linearly polarized (LP) UV photoreaction of a photo-crosslinkable side-chain liquid-crystalline polymer (SLCP) containing photoreactive cinnamoyl and biphenyl mesogenic groups (1) was studied. The optical anisotropy of the polymer film was induced by the LP-UV photoreaction and was investigated by the temperature-controlled polarized UV absorption spectroscopy and polarized FT-IR measurements. The reorientation of the nonreacted mesogenic groups along to the E direction of the incident LP-UV light during the photoreaction occurred at the LC temperature range of the polymer, and the induced birefringence Δn was about 0.02. Because of the high-density photo-crosslinking, the LP-UV photoreacted film showed orientational stability up to 160°C.

Photoregulated Liquid Crystal Alignment on Photoreactive Side-Chain Liquid-Crystalline Polymer

We describe the anisotropic photoreaction of photoreactive side-chain liquid-crystalline polymers (SLCPs), containing biphenyl mesogen and photoreactive cinnamoyl group, by irradiation with a linearly polarized (LP) UV light, and the alignment behavior of the low-molecular-weight (LMW) liquid crystal (LC) on them. The photo-crosslinked SLCP thin film photoreacted at room temperature showed very small negative dichroism, while that photoreacted in the LC temperature range of the film exhibited positive dichroism. The photoreacted SLCP film can align LMW-LC director \tilden both parallel and perpendicular to the incident \hatE vector of the LP-UV light. The alignment direction of the LMW-LC was controlled by the amount of photoreaction of the cinnamoyl group.

Linearly polarized ultraviolet photoreaction of photocrosslinkable polymers comprising the p-phenylenediacrylate group and photoalignment control of liquid crystals on the resultant film

We present (1) the linearly polarized (LP) ultraviolet (UV) photoreaction of block copolyesterethers comprising a hard segment of polyester and soft segment of poly(tetramethylene ether) (PTMG) and (2) the alignment behavior of liquid crystals (LCs) on the resultant photoreacted polymer films. Two kinds of copolyesterethers with different polyester segments of poly(hexamethylene p-phenylenediacrylate) (1a) or poly(hexamethylene p-phenylenediacrylate)-co-poly(hexamethylene 1,4-dibenzoate) (1b) were used in this study. The LP-UV irradiation resulted in a negative optical anisotropy for both polymer films, and a homogeneous photoalignment of LCs was achieved on the photoreacted polymer film. The LC alignment was perpendicular to the electric vector of the incident polarized light for 1a, whereas a reversion of the alignment from parallel to perpendicular was observed for 1b, depending on the irradiation doses.

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.

Holographic Bragg grating generation in photorefractive polymer-dissolved liquid-crystal composites

A large optical nonlinearity of n2=0.57 cm2/W in photorefractive mesogenic composites was achieved with low applied dc fields (0.30 V/μm) and a fringe spacing of 2.8 μm. The resultant diffraction efficiency from the Bragg gratings, which were written by a less intense laser (frequency-doubled Nd-YAG laser with a 532 nm wavelength), was around 39%. The limiting factors underlying these nonlinearities and their dynamics are also discussed.

Photoalignment of Liquid Crystals on Photo-Cross-Linkable Polymer Films with Anthracene at the End of the Side Group.

Three kinds of photo-cross-linkable polymethacrylate films with an anthracenemethoxybiphenyl side group or an anthracenemethyl side group were irradiated by a linearly polarized (LP) UV light, and the resultant films were applied to the alignment layer for liquid crystals (LC). The photoinduced optical anisotropy of the film was negative and similar among the polymers. Uniform LC alignment could not be obtained on the polymer without the biphenyl group, but the LC aligned homogeneously on the polymers with a biphenyl group. When the absorptive direction of the anthracene group was parallel to the long axis of the biphenyl group, the alignment direction was parallel to the electric vector (E) of the LP-UV light at the beginning of the irradiation and it changed to perpendicular with increase in the exposure dose. In contrast, when the absorptive direction of the anthracene group was not parallel to the long axis of the biphenyl group, the LC alignment direction was perpendicular to E. These types of alignment behavior suggest that the LC alignment direction is controlled by the direction of the biphenyl group in the photo-cross-linked side group.

Photo-alignment of low-molecular liquid crystals on photo-crosslinkable polymer liquid crystalline film by linearly polarised UV light

Effects are described of the intensity and irradiation temperature of a linearly polarized (LP) UV light on a photoreaction of a photo-crosslinkable polymer liquid crystal (PLC) film comprising a cinnamoylethoxybiphenyl side group, and the photo-alignment behavior of nematic low-molecular (LM) liquid crystals (LCs) on a PLC film. Both homeotropic and homogeneous alignments of the LM-LC were possible on the photoreacted PLC film when the film was irradiated at room temperature. However, a homeotropic alignment was not observed when the film was exposed in the liquid crystalline temperature range of the PLC. When the light intensity was strong, the homogeneous alignment direction can be controlled both parallel and perpendicular to the electric vector (t E) of the incident LP-UV light, while it needed a large amount of irradiation energy to obtain a perpendicular alignment for the weak LP-UV irradiation. The LM-LC alignment behavior can be explained by a change of the interaction among the LM-LC, the photo-crosslinked mesogenic group of the PLC film and the photoinduced alignment of the photoreacted PLC film. Copyright

FUNCTIONALIZED MESOGENIC COMPOSITE FOR PHOTOREFRACTIVE APPLICATIONS

A mesogenic composite, which consists of a functionalized copolymer, low-molar-mass liquid crystals (L-LCs) and a sensitizing dye, was developed for photorefractive applications. The polymer dissolved L-LC composite (PDLCC) exhibited a mesophase although the copolymer itself did not exhibit a LC phase. A high gain coefficient of over 200 cm-1 was achieved with low applied dc fields (0.34 V/µm).

Coplanar alignment of mesogenic moieties in a photocrosslinked liquid crystalline polymer film containing cinnamoyl groups

Coplanar alignment of mesogenic groups using polymer films of a methacrylate comprising a photocrosslinkable 4′-(2-cinnamoyloxyethoxy)-biphenyl group and a photoinactive 4-cyanobiphenyl group was realized by slantwise ultraviolet (UV) irradiation. Slantwise absorption measurement revealed that an orientational plane of the mesogenic groups was perpendicular to direction of the UV light propagation. The orientational angle and the ellipsoid of refractive index of the film could be regulated by changing the irradiation angle and degree of polarization of the UV exposure, respectively.