Dielectric relaxation and molecular motion in the chiral main-chain liquid crystalline copolyester, BB-4*(2-Me)/BB-6

Abstract

Broad-band dielectric relaxation spectroscopy has been used to examine molecular motions in a main-chain liquid crystalline copolyester BB-4*(2-Me)/BB-6 consisting of a biphenyldicarboxylic acid and equimolar mixture of chiral 2-methylbutanediol and hexanediol. From 60 to 140 °C where molecules assume a chiral smectic C phase, BB-4* (2-Me)/BB-6 exhibits a Goldstone mode process whose relaxation strength rapidly decreases with increasing dc bias field. Combining dielectric relaxation time and strength allows estimate of interlayer rotational viscosity which is 10-100 times larger than that of low molecular weight liquid crystals reflecting methylene chains connecting smectic layers. Decreasing temperature leads to two additional processes, Vogel-Fulcher type and Arrhenius types. These processes are analogous to segmental and local mode motion characteristic of non-crystalline polymers. The former defines a Tg of 15 °C. The coexistence of the Goldstone mode and segmental mode indicates that BB-4*(2-Me)/BB-6 possesses both orientational order forming liquid crystalline phases and disorder required to undergo glass transition.