Synthesis and characterization of a series of azobenzene-containing side-chain liquid crystalline polymers

A series of poly{4'- (X-methacryloyloxyalkylene)methylaminol-4-nitroazobenzene} (pXMAN, where X is the number of methylene units and varies from 2 to 12) polymers with liquid crystalline (LC) properties was synthesized. When annealed above their T-g values, the whole series of polymers exhibits a stable smectic A mesophase while only the material with the shorter spacer, p2MAN, presents a smectic phase followed by a nematic phase. The spacing of the smectic layers was determined by X-ray diffraction, revealing intermolecular reorganization by interdigitation of the side-chain mesogens since the smectic layer spacing was larger than a fully extended side chain but less than two extended chains. This behavior is confirmed by temperature dependent aggregation as observed by UV-visible spectroscopy during the transition from an amorphous state to a liquid crystalline phase. The stability of the mesophase is enhanced by intermolecular interactions as argued by enthalpic and thermochromic considerations. Systematic photoinduced birefringence measurements on the various states reveal that the maximum and remnant levels of birefringence decrease with increasing the spacer length and with decreasing the glass transition temperature of the considered liquid crystalline polymer.