Cyclic azobenzene-containing side-chain liquid crystalline polymers: Synthesis and topological effect on mesophase transition, order, and photoinduced birefringence

Article abstract of DOI:10.1021/ma100246c

A cyclic side-chain liquid crystalline polymer (SCLCP) bearing azobenzene mesogens, namely, poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate} (PAzoMA), was successfully synthesized by using "click" cyclization of the linear polymer precursor with alkyne and azide end groups. Samples of cyclic-PAzoMA of various molecular weights were prepared, characterized, and studied in comparison with their linear counterparts. The results show that the topological constraint arising from the tortuosity of the ring structure and the absence of chain ends in cyclic-SCLCPs affects profoundly the liquid crystalline (LC) phase transitions (temperature, enthalpy, and entropy) of mesogenic side groups and that this topological effect is more prominent for smaller SCLCP rings. Moreover, the photoinduced anisotropy in films as a result of the trans-cis photoisomerization of azobenzene mesogens was investigated, and cyclicPAzoMA was found to behave differently from linear-PAzoMA. On the one hand, the cyclic polymer exhibits a nonmonotonic rise and erasure of birefringence upon linearly and circularly polarized excitation (488 nm), respectively, in contrast with the linear polymer displaying monotonic changes. On the other hand, unlike the linear polymer, the photoinduced orientation of azobenzene mesogens in cyclic-PAzoMA cannot be enhanced upon annealing in the nematic phase. All these manifestations of the topological constraint suggest that cyclization offers a new way to change the coupling between mesogenic side groups and chain backbone of SCLCPs, and their interplay under the additional topological effect may generate new behaviors that are of interest to be explored.