Structural relaxation of an oriented thermotropic liquid crystalline copolyester assessed by infrared spectroscopy and X-ray diffraction

Abstract

Samples of approximately uniform degree of orientation based on poly(hydroxybenzoic acid (73 mol%)- co-hydroxynaphthoic acid (27 mol%)), known as Vectra A950, were obtained by sectioning injection-moulded specimens. The degree of chain orientation was assessed by X-ray scattering and infrared spectroscopy. A comparison of the data obtained by the two methods enabled the angles between the chain axis and the transition moment vectors for the infrared absorption bands at 1474 cm −1 (22°) and 890 cm −1 (90°) to be assessed. The oriented samples showed, depending on the particular thermomechanical history used, either an increase or a decrease in chain orientation on annealing at temperatures below and above the crystal melting range. Constraining factors, the application of an external force to keep the samples at constant length or the presence of stabilizing crystallites, favoured an increase in chain orientation presumably through the annihilation of disclinations. Slow and gradual heating favoured recrystallization, i.e. the transformation of pseudohexagonal crystallites to the thermally more stable orthorhombic crystallites and possibly also to crystal modification II of poly(hydroxybenzoic acid). These form new physical cross-links which prevent shrinkage of the sample. Unconstrained oriented nematics containing no stabilizing crystallites showed a rapid approach (decrease for the highly oriented samples) to a universal value for the Hermans orientation function of ∼0.3 at temperatures above 290°C. This behaviour, distinctly different from that of small-molecule nematics, shows some similarity with the behaviour of oriented flexible-chain polymers.