Abstract
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 P1) with two azobenzene groups per repeating unit. The chemical structures of P1 and the corresponding monomer were characterized using various techniques with satisfactory analysis data. The phase structures and transitions of P1 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 P1 can form a hierarchically ordered structure with double orderings on both the nanometer and subnanometer length scales. Most likely, the thick main-chains of P1 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 P1 renders a biaxial orientation with the side-chains perpendicular to the main-chains. We compared P1 with an end-on side-chain LC polymer of poly(4-{[6-(4-methoxy-4′-oxy-azobenzene)hexyl]oxycarbonyl}styrene) (denoted as P2). P2 bearing one mesogenic group per repeating unit forms a monolayer SmA phase, with the transition temperature much lower than that of P1. Upon UV irradiation, in contrast to that P2 will become isotropic, P1 can still exhibit LC behavior after the azobenzene groups adopt cis conformation.
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