Relaxation of longitudinal PLCs

The relaxation behavior of nematic, longitudinal PLCs is greatly complicated by their multiphase character. The presence of both crystallites and a coexisting isotropic component has been reported [32]. Vectra A (poly(p-hydroxybenzoic acid-co-2,6-hydroxynaphthoic acid)), currently one of the more widely used and studied commercial polymers, exhibits, despite that fact that it is a statistical copolymer, a sizeable amount (10-60%) of crystallinity, either of the non-periodic layer type [121] or of a truly statistical type according to the model of Biswas and Blackwell [122]. It has become clear that the relaxation of oriented Vectra is to a substantial degree controlled by the thermal behavior of its tiny crystallites. 

Another less attractive feature of these polymers is that they may undergo transesterification reactions at the high temperatures used in the studies [67]. Recommended reading on transesterification reactions in polyesters is Porter et al. [124]. 

Samples of oriented Vectra A950 (poly(hydroxy-benzoic acid (73%) -co-hydroxy-naphthotic acid (27%))), obtained from the surface skin of injection molded specimens, were annealed at different temperatures and the order parameter was assessed by IR spectroscopy and X-ray scattering in a recent study of Wiberg and Gedde [127]. They found 

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