High-resolution electron microscopic investigation of frustrated packing of a semiflexible liquid crystalline polyester

Abstract

The structure and morphology of a crystallizable thermotropic main-chain polyester, with a decamethylene flexible spacer, has been investigated by X-ray scattering, electron diffraction (ED), and high-resolution electron microscopy (HREM). Analysis of the electron and X-ray diffraction patterns from quenched specimens suggests a frustrated packing model of smectic order, Somprising bundles of chains. This conclusion is supported by HREM imaging of the 17 (002) and 24 Angstrom (011) layer spacings, which confirm two-dimensional ordering. Further insight in the molecular packing is gained by analysis of crystalline structures. Annealing results in one of two different crystal structures, depending on temperature. Low-temperature crystallization is kinetically limited and does not alter the axial packing of the smectic mesophase significantly. Annealing at temperatures near to the nematic phase, however, produces a markedly different crystal Structure, which yields a 33 Angstrom (001) reflection. rn the latter, more stable phase, the flexible spacers and aromatic mesogens are microphase separated from one another, whereas the former packing represents a frustration of microphase separation and packing strain. Small bundles, of molecules in registry, are shifted axially in the smectic state so as to relax the strain induced by the difference in cross section between the two portions of the molecular repeat unit. Images combining die-action and phase contrast allow simultaneous visualization of the mesomorphic and semicrystalline phases in annealed specimens. Bright field images of the 33 Angstrom (001) spacing of the high-temperature form enable the crystal size, shape, and relative orientations to be determined. Disclinations have also been examined.