Mesophase texture and defects in thermotropic liquid-crystalline polymers

Abstract

Two rigid/flexible thermotropic polyesters based on 1,10-decane bisterephthaloyl chloride with hydroquinone or methyl hydroquinone have been investigated by transmission electron microscopy. Oriented thin films were prepared by melt spreading on phosphoric acid. After thermal quenching to freeze-in the liquid-crystalline state, films were examined at room temperature using electron microscopy and electron diffraction. High local orientation within 10 µm diameter areas was observed by electron diffraction. Layer-line spacings showed that the molecules adopted a highly aligned conformation. Multiple equatorial reflections in the unsubstituted polymer and the formation of a single-crystal-like texture on annealing the frozen liquid-crystalline state in the methyl-substituted polymer suggest biaxial chain packing in the liquid-crystalline state. Bright-field images of the quenched films were essentially featureless, but equatorial dark field and polarized-light optical images revealed an alternating band structure. Annealing below the crystal-melting transition results in the growth of lamellae which decorate the overall pattern of molecular order in the precursor frozen liquid-crystalline state. The 200 Å thick lamellae provide high contrast and resolution for observation of characteristic mesophase texture and defects. The molecular trajectory across the transverse bands is found to be sawtoothed rather than serpentine. Two types of disclination lines have been observed: non-singular S=±1 disclination loops lying in the plane of the films with their loop axes oriented along the overall chain-axis direction and S=±½ disclination lines lying normal to the film surface.

The semicrystalline microstructure consists of thin lamellae separated by frozen liquid-crystal regions. The single-crystal-like texture of the oriented lamellae, together with the nature of the meridional scattering, suggests a biaxial smectic liquid-crystalline phase for these polymers. The liquid-crystalline state is not believed to consist of an intrinsic polydomain structure but rather a continuous liquid-crystalline medium containing various types of disclinations.