RHEOLOGY OF LOW NEMATIC TRANSITION-TEMPERATURE THERMOTROPIC LIQUID-CRYSTALLINE COPOLYESTER HBA HQ/SA/

The rheology of a liquid crystalline copolyester of hydroxybenzoic aci d, hydroqui none, and sebacic acid (HBA/HQ/SA copolyester) was studied on both a rotational and a capillary rheometer. DSC studies show that the copolyester has a crystal-mesophasic and a broad mesophasic-isotr opic transition at 170 degrees C and 220 degrees C. Optical texture ob servations show the mesophase is characterized by line defect textures , which are characteristic of a nematic structure. At 220 degrees C, b oth isotropic and nematic phases coexist with the latter being the maj or. As temperature reaches 250 degrees C, a clear dominance of isotrop ic phase is observed. At this temperature, the nematic phase of irregu lar shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, si ngle nematic phase, nematic/isotropic biphase, and the near single iso tropic phase. Dynamic strain sweep measurements show that a linear vis coelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly st ructure dependent; it is >100% in the nematic phase, similar to 20% in the biphases, and only about 5% in the isotropic phase. The concurren ce of curves obtained at different temperatures in a Cole-Cole plot of G' vs. G '' indicates similar structures in the nematic phase and bip hases. Measurements of steady shear viscosity using a rotational rheom eter and a roundhole capillary rheometer show that in the nematic phas e the copolyester behaves as a shear thinning fluid for a wide shear r ate range of 1 similar to 10,000 s(-1), in which the power law index I s about 0.6 similar to 0.8, and the viscosity is <10 Pa.s at shear rat es >1 s(-1).