Phase transitions and rheology of aramid solutions

Clearing temperatures of solutions of poly(4-4′-benzanilidylene-terephthalamide) in concentrated sulphuric acid are presented as a function of polymer concentration and average molecular weight. The orientational order parameter <P ₂> is obtained from birefringence measurements. The experimental results are explained by a mean-field type theory similar to the Maier-Saupe model for thermotropic liquid crystals. Molecular flexibility, concentration and molecular weight are taken into account by using simple scaling factors. The birefringence induced by shear flow in an isotropic solution of poly(para-phenylene-terephthalamide) shows a strong pretransitional behaviour. This indicates the occurrence of a flow-induced phase transition.