Molecular weight effects on phase transitions of side-chain liquid crystalline poly(vinyl ether)s with terminal alkoxybiphenyl groups

Abstract

Two vinyl ether monomers with alkoxybiphenyl groups, 2-(4′-ethoxy-4-biphenyloxy)ethyl vinyl ether (EtOVE) and 2-(4′-hexyloxy-4-biphenyloxy)ethyl vinyl ether (HexOVE), were polymerized by living cationic polymerization reactions using either the hydrogen iodide/iodine ( HI I 2 ) or the hydrogen iodide/zinc iodide ( HI ZnI 2 ) initiator systems, or both. These initiators yield polymers with narrow molecular weight distributions (MWDs) and for comparison broad MWD polymers were also prepared by using the BF 3OEt 2 initiator. The thermal properties and phase transitions of these polymers were determined by differential scanning calorimetry, by visual observations of samples on a hot stage on a polarizing microscope, by polarized light transmission intensity measurements and by wide angle X-ray diffraction analysis. The polymer from EtOVE, P(EtOVE), with a weight average molecular weight, M ̄ w , of less than about 8000 formed both smectic and nematic liquid crystalline, LC, phases after one heating cycle. In contrast, the polymer from this monomer which had an M ̄ w of more than about 8000 exhibited only a nematic LC phase. For the polymer from HexOVE, P(HexOVE), both the narrow and broad MWD samples showed only a nematic LC phase over the M ̄ w range from 2600 to 7300. The phase transitions of both types of polymers are discussed in relation to the molecular weight and MWD of the samples. The effect of the terminal group attached to the biphenyl group in the polymer is considered in relation to its possible steric effects.