Combined main-chain/side-chain liquid crystalline polymer with main-chain on the basis of 'jacketing' effect and side-chain containing azobenzene groups

摘要

Combining the concept of "flexible spacer" which can bring liquid crystalline (LC) properties to the side-chains and the side-group "jacketing" effect which can result in main-chain with rod-like conformation, we have synthesized a new combined main-chain/side-chain LC polymer based on radical polymerization, poly(2,5-bis{[6-(4-methoxy-4′- oxy-azobenzene)hexyl]oxycarbonyl}styrene) (denoted as P_1) with two azobenzene groups per repeating unit. The chemical structures of P_1 and the corresponding monomer were characterized using various techniques with satisfactory analysis data. The phase structures and transitions of P _1 were investigated using differential scanning calorimetry, polarized optical microscope, and one- and two-dimensional (1D and 2D) wide-angle X-ray diffraction. We identify that P_1 can form a hierarchically ordered structure with double orderings on both the nanometer and subnanometer length scales. Most likely, the thick main-chains of P_1 obtained by "jacketing" the central rigid portion of terephthalate side-chain to the polyethylene backbone construct a 2D centered rectangular scaffold, which is stable until the sample becomes completely isotropic. The packing of side-chains inside the main-chain scaffold undergoes the transitions of smectic B- (SmB-) like ? smecitc A (SmA)-like ? isotropic. The confinement arising from the scaffold induces the SmB-like packing and enhances the stability of SmA-like structure. The hierarchically ordered structure of P_1 renders a biaxial orientation with the side-chains perpendicular to the main-chains. We compared P_1 with an end-on side-chain LC polymer of poly(4-{[6-(4-methoxy-4′-oxy-azobenzene)hexyl]oxycarbonyl} styrene) (denoted as P_2). P_2 bearing one mesogenic group per repeating unit forms a monolayer SmA phase, with the transition temperature much lower than that of P_1. Upon UV irradiation, in contrast to that P_2 will become isotropic, P_1 can still exhibit LC behavior after the azobenzene groups adopt cis conformat