Photoinduced Alignment of Polymer Liquid Crystals Containing Azobenzene Moieties in the Side Chain. 2. Effect of Spacer Length of the Azobenzene Unit on Alignment Behavior

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.