Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA

The rheology of a liquid crystalline copolyester of hydroxybenzoic acid, 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-isotropic transition at 170 degrees C and 220 degrees C. Optical texture observations show the mesophase is characterized by line defect textures, which are characteristic of a nematic structure. At 220 degrees C, both isotropic and nematic phases coexist with the latter being the major. As temperature reaches 250 degrees C, a clear dominance of isotropic phase is observed. At this temperature, the nematic phase of irregular shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, single nematic phase, nematic/isotropic biphase, and the near single isotropic phase. Dynamic strain sweep measurements show that a linear viscoelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly structure dependent; it is >100% in the nematic phase, similar to 20% in the biphases, and only about 5% in the isotropic phase. The concurrence of curves obtained at different temperatures in a Cole-Cole plot of G' vs. G '' indicates similar structures in the nematic phase and biphases. Measurements of steady shear viscosity using a rotational rheometer and a roundhole capillary rheometer show that in the nematic phase the copolyester behaves as a shear thinning fluid for a wide shear rate range of 1 similar to 10,000 s(-1), in which the power law index Is about 0.6 similar to 0.8, and the viscosity is <10 Pa.s at shear rates >1 s(-1).