Polydomain films of liquid crystalline polymer (MACB- AB6) with low content (6 mol %) of azobenzene units were prepared and irradiated with linearly polarized light at 366 nm, thus inducing an alignment of the photochromic side groups as well as the nonphotoactive mesogens, the cyanobiphenyl groups. An induced order parameter, S, was obtained from polarized UV and IR spectroscopy, and it increased with increase in temperature at first and then decreased due to phase transition. The effect of light intensity on the alignment behavior has been investigated in detail. Alignment change can take place even for a low intensity (0.7 mW/cm2) of irradiation light. Irradiation light at high intensity induced alignment as well as a phase transition at relatively low temperature. A higher saturation order parameter and faster achievement of the order parameter were obtained, since a higher concentration of cis-azobenzene was produced when the film was exposed to polarized light with a higher intensity. This is due to the angular-dependent absorbance of irradiation light by the azobenzene moieties.

By irradiation with linearly polarized light at 366 nm, alignment change was generated in polymer liquid crystals (MACB-AB n , n = 3, 6, and 11) with different spacer lengths of the azobenzene units but the same enthalpic stability of the liquid-crystalline phase. Polarized IR spectra revealed that the polymer main chain did not reorient while the mesogens were aligned perpendicular to the polarization direction of the writing light. The effect of the spacer length on alignment behavior was investigated by measuring the transmittance through the polymer liquid-crystal film and the induced order parameters. Results show that alignment change is more difficult to be induced in a polymer liquid crystal, having a short spacer. The spacer length does not affect the trans − cis photoisomerization and consequent cis − trans thermal isomerization behavior. The restriction of the polymer main chain on azobenzene moieties through the spacer affects the alignment efficiency of azobenzene mesogens and, therefore, affects the alignment behavior of the polymer liquid crystals.

Photoinduced alignment behavior in a series of polymer liquid crystals containing different azobenzene moieties in the side chain was investigated. With increase of the strength of donor and acceptor at 4 and 4‘ positions of the azobenzene groups, the possibility of an alignment change in the polymer liquid crystals having the same methacrylic backbone decreased due to the slightly increased enthalpic stability of the mesophase and the significantly decreased concentration of cis-azobenzene, although the cis−trans isomerization rate increased. However, a high alignment efficiency was observed in polymer liquid crystal ACB-ABA6, which contained strong donor−acceptor pairs in the azobenzene moiety and exhibited low stability of mesophase, since both the rate of cis−trans isomerization and the mobility of mesogens are favorable for alignment change. Three different relaxation processes were observed in the aligned polymer liquid crystals.

The photochemical phase-transition behavior of polymer liquid crystals (LCs) containing azobenzene moieties with strong donor-acceptor substituents has been investigated. Copolymers of a mesogen and a donoracceptor azobenzene were prepared. Photoirradiation of a thin film of the polymer LCs (100-200 nm thickness) in the nematic (N) phase caused an N-to-isotropic (I) phase transition due to trans-cis photoisomerization of the azobenzene moiety, and the N phase recovered rapidly when photoirradiation was ceased because of cis-trans thermal back-isomerization. Time-resolved measurements with a laser pulse (532 nm; 10 ns, fwhm) revealed that the N-I phase transition took place in 300 µs. The thermal recovery of the N phase in the polymer LC with a donor-acceptor azobenzene, which stabilized the LC phase, occurred in 8 ms at 146°C. This response of the recovery is 10 3 times faster than that of non-donor-acceptor azobenzene LCs. In a sample that showed a smectic (S) phase, however, no photochemical phase transition was observed. These phenomena are interpreted in terms of the stability of the LC phase.