RHEOLOGY OF RODRUN LIQUID-CRYSTALLINE POLYMERS

The rheological characterization of two commercial thermotropic liquid crystalline polymers based on poly(ethylene terephthalate) (PET) and para-hydroxybenzoic acid (PHB) is carried out. The thermal transitions determined by DMTA are explained by the random character of these cop olyesters, in comparison with nonrandom copolyesters synthesized by Ja ckson and Kuhfuss. The evolution of the dynamic viscoelastic functions with time in the nematic state is concave in shape for the 20%PET/80% PHB copolymer, a result that leads us to treat this system as a suspen sion of solid spheres (unmolten crystals) where the volume fraction of crystals increases with time according to an Avrami equation. The res ponse of 40%PET/60%PHB copolymer is similar to a chemical or physical gelation and the hypothesis that the polydomain structure gives rise t o a network is considered. Continuous flow, time-independent viscosity results reveal the existence of a three-region flow curve for 40%PET/ 60%PHB copolymer, but a Newtonian zone followed by a shear thinning re gion for 20%PET/80%PHB sample. At high temperatures the isotropization of the samples leads to a very strong decrease of the activation ener gy of flow, which becomes zero for 40%PET/60%PHB. (C) 1998 John Wiley & Sons, Inc.