PHOTOCHEMICALLY INDUCED ISOTHERMAL PHASE-TRANSITION IN POLYMER LIQUID-CRYSTALS WITH MESOGENIC CYANOBIPHENYL SIDE-CHAINS

Photochemically induced isothermal phase transition in polymer liquid crystals (PLC) with side-chain mesogenic cyanobiphenyl has been explored. The PLC samples studied are poly[4-[(acryloyloxy)alkoxy]-4’-cyanobiphenyl] (PACBn), in which the alkyl spacer length (CH2)n was varied as n = 3, 5, 6, and 11, and the copolymers of PACB3 with 4’-methoxy-4-[(acryloyloxy)alkoxy]azobenzene (AAZOm) where m = 3, 6, and 11 (copolymer n-m). In the homopolymers, a photoresponsive molecule, 4-butyl-4’- methoxyazobenzene (BMAB), was doped to induce the photochemical phase transition. Photoirradiation of the PACB5/BMAB (5 mol %) film at 460 nm caused trans → cis isomerization of BMAB, which induced simultaneously nematic (N) isotropic (I) phase transition of the host PLC. This process was reversible, and photoirradiation at 525 nm, which induced cis → trans isomerization of BMAB, restored the system to the initial state (N). The photochemical phase transition was strongly dependent on the spacer length (n and m) and the molecular weight of the host PLC's. Among the PLC samples examined, the photochemical phase transition was observed in PACB3, PACB5, copolymer 3-3, and copolymer 3-6. On the basis of the calorimetrie studies and the order parameters, the efficiency was found to be closely correlated to the orientational ordering of the initial state of the host PLC's. The photochemical N → I phase transition took place more effectively in a system with a less ordered N state. The photochemical phase-transition behaviors of the present PLC systems are discussed in relation with those of PLC's with mesogenic phenyl benzoate side chains previously reported. © 1990, American Chemical Society. All rights reserved.