The rheological characterization of a series of thermotropic liquid crystalline polymers

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Abstract
Dynamic rheological experiments were performed on a series of two copolymers and a homopolymer based on units of terephthaloyl chloride and isophthaloyl chloride at 90/10 and 75/25 mole ratios combined with 1,10‐bis(4‐hydroxyphenyl)‐decane. Optical microscopy and wide angle X‐ray diffraction (WAXD) confirmed that all of the polyesters in the present series formed nematic liquid crystals with nematic‐to‐isotropic temperatures in the range of 270 to 320°C with increasing terephthaloyl unit content. Broad nematic‐to‐isotropic transitions observed by differential scanning calorimetry (DSC) were indicative of biphasic regions where the nematic and isotropic phases coexist. The rheological behavior of each polymer was more complex in the nematic phase than in the isotropic phase with shear thinning occurring in the former but Newtonian behavior in the latter. There were also some indications that nematic flow behavior could be induced in these polymers by dynamic oscillatory shear flow above the nematic‐to‐isotropic transition, Ti. A form of hysteresis was observed with the homopolymer in that measurements of the dynamic viscosity, η*, taken with ascending frequency sweeps were higher than those taken with descending frequency sweeps.

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