Thermotropic liquid-crystalline polyesters containing biphenyl mesogens in the main chain: The effect of connectivity

A homologous series of main-chain thermotropic liquid-crystalline polyesters containing-rigid biphenyl mesogen and flexible methylene spacers were synthesized with the AB-type self-polycondensation approach. The polyesters were characterized with H-1 NMR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, polarized light optical microscopy, and X-ray diffraction. These polyesters, containing trimethylene spacers on the acid side and various spacers on the alcohol side of the biphenyl mesogen, showed an odd- even effect in the transition temperatures and mesophase type. The even members showed higher transition temperatures than the odd ones. A normal smectic mesophase was observed for the even members, whereas the odd-membered counterparts exhibited a tilted smectic mesophase. To study the effect of connectivity, the mesophase characteristics of these polyesters were compared with those of the isomeric AB-type polyesters without any methylene spacer on the acid side of the biphenyl moiety. The mesophase characteristics were insensitive to whether the mesogen was connected to a carboxyl unit or a methylene unit. (C) 2004 Wiley Periodicals, Inc.