Thermal Transitions in Liquid Crystalline Polymers

摘要

Vectra (and Vectran, the fiber and film variants) is a 73/27 wholly aromatic liquid crystalline co-polyester made of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). This material exists in the nematic state above its melting point. As several analytical techniques indicate, unusually high molecular orientation can be achieved for this copolymer as a result of fiber melt spinning and film extrusion. Transitions of Vectra (flex bars) and Vectran (films and fibers) were studied by DSC, TMA and DMA. Vectra can be extruded into a film or a fiber having very high strength and modulus. Heat treatment improves significantly the tensile properties of fiber and film variants. Tg of Vectran is around 100-120°C depending on the technique used, the melting point is 300°C or 330, depending on the heat treatment of the polymer. The crystal structure of Vectra is not clear, therefore there are a lot of questions about its melting behavior. The crystallization is well defined due the presence of the nematic phase, although some supercooling can be observed. The nematic?isotropic transition cannot be achieved due to degradation.