The structure of thermotropic copolyesters

Abstract

Highly oriented polymeric products have been produced over the past fifteen years by two very different processing routes; from conventional polymers processed to highly oriented extended chain structures, and from “rod-like” polymers which exhibit liquid crystalline behaviour. Gel spun polyethylene is an example of such a conventional polymer. There are three main types of liquid crystalline polymers (LCP) which have high orientation and modulus: lyotropic aramids, such as poly(ρ-phenylene terephthalamide) (PPTA); lyotropic, aromatic heterocyclic polymers, or “ordered polymers”; and the family of thermotropic aromatic copolyesters. Extensive characterization of the thermotropic copolyesters has resulted in the delineation of a fibrillar, hierarchial structural model which accounts for the structures observed in a broad range of oriented fibres, extrudates and moulded articles. Three distinct fibrillar species are observed: microfibrils that are about 50 nm, fibrils about 500 nm, and macrofibrils about 5μm, in size. Superimposed on the structural hierarchy is a defect hierarchy, defined by the regular meander of the molecular chain and a localization of defects within a microfibril at about a 50 nm periodicity. Orientational variations, layering and skin core structures, in thick specimens, are the result of local flow fields on the basic structural units during solidification. The fibrillar textures appear to be present prior to any preparation for microscopy. A wide range of specimen preparation methods, i.e. fractography, sonication, microtomy and etching, and microscopic techniques, i.e. optical, scanning and transmission electron microscopy, were applied to the characterization of the aromatic copolyesters and PPTA. Interestingly, the same basic hierarchy is observed for both the lyotropic and the thermotropic LCPs and the microfibrillar structures of all the highly oriented polymers, including polyethylene, appear quite similar.