Dielectric, nuclear magnetic resonance and electron spin resonance studies of relaxation processes in a liquid-crystalline polyester

Abstract

The molecular dynamics of the polyester (—OC—[Ph]₃—CO—O—[CH₂–CH₂—O—]₄)_n, which has a liquid-crystalline smectic C (S_C) phase, have been investigated by the e.s.r. spin-probe, dielectric relaxation and carbon-13 solid-state n.m.r. techniques. The low-temperature, high-frequency γ relaxation preserves the characteristics of the local in-chain motions of polyethers. The apparent activation energy corresponds to a very simple flexible structure. The most likely mechanism of this γ relaxation might be the local tg⁺t ↔ tg^–t transition in the central ‘ether’ units. Two β relaxation processes were found: the β_L relaxation is believed to be caused by diffusional segmental motion of the flexible spacers located in isolated amorphous isotropic regions of the material and the β_U process is associated with the ‘ether’ sequences located in the glassy S_C domains. The increasing rate of rotation of the phenyl rings about their C₁—C₄ axis is related to the crystal ↔ S_C transition. Note that this motional process of the phenyl rings does not alter the mean orientation of the mesogenic groups.