Effect of mesogenic density on liquid-crystalline behaviours of polymethacrylates bearing azobenzene mesogen

Abstract

Three sets of novel side-chain liquid-crystalline polymers, poly[3-(4′-methoxy-4-oxy hexyloxy azobenzene) benzyl methacrylate] (PHABM), poly[3, 5-di(4′-methoxy-4-oxy hexyloxy azobenzene) benzyl methacrylate] (PDHABM) and poly[3, 4, 5-tri(4′-methoxy-4-oxy hexyloxy azobenzene) benzyl methacrylate] (PTHABM), with different mesogenic densities were successfully designed and synthesised. The chemical structures of all the monomers were confirmed by 1H nuclear magnetic resonance (NMR), and 13C NMR spectroscopy and elemental analysis, and the characterisation of the polymers was performed by 1H NMR spectroscopy and gel permeation chromatography. Their phase structures and transitions were investigated using differential scanning calorimetry, polarising optical microscopy and single-angle X-ray scattering in order to study the effect of mesogenic density on liquid-crystalline behaviour. Each of the PHABMs exhibited only a glass transition and a smectic phase during the heating and cooling process, while PDHABMs and PTHABMs showed a glass transition and broader temperature range liquid-crystalline behaviour, including both nematic and smectic phases. In addition, the clearing temperatures of the three types of polymers were found to be in the order: PTHABM > PDHABM > PHABM, and the liquid crystal temperature range had the same order.