Main-chain side-chain combined liquid crystalline polymers bearing azobenzene and biphenyl-based mesogens – segregation of the mesogens within two distinct layers

Abstract

ABSTRACT Two different mesogenic units, namely biphenyl and azobenzene, were installed centrally within the polymer backbone and pendant alkylene segments; the purpose was to examine if chain folding of the backbone occurs so as to segregate the azobenzene and biphenyl units in adjacent layers of a smectic-type ordering. Two sets of isomeric main-chain side-chain liquid crystalline polymers (MCSC-LCPs), namely SC-AzoCx-MC-BPCx and SC-BPCx-MC-AzoCx (x=6 or 10), were synthesised; the former series has biphenyl in the backbone and azobenzene in the side-chain, while their locations are switched in the latter. It was observed that all four polymers were semicrystalline at room temperature and exhibited a smectic mesophase upon melting. DSC and X-ray scattering studies revealed that switching the locations of the azobenzene and biphenyls had little effect on; the phase transition temperatures and smectic layer spacings. A second series, namely SC-AzoCx-MC-BPCy and SC-BPCx-MC-AzoCy, where the length of alkylene spacers in the side-chain (Cx) and backbone (Cy) is different, was also studied; this was done to examine the influence of unequal alkylene segment lengths on the mesophase structure and interlayer spacing, and importantly, whether the zigzag folding-induced formation of a layered structure is retained, despite their unsymmetrical nature.