Phase Behavior, Rheology, and Morphology of Binary Blends of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers

Abstract

The phase behavior, rheology, and morphology of binary blends of semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs) were investigated. Specifically, binary blends consisting of poly[(phenylsulfonyl)-p-phenylene alkylene-bis(4-oxybenzoate)]s (PSHQn) having five methylene groups (PSHQ5) and 11 methylene groups (PSHQ11) were prepared by solvent casting. It was found from differential scanning calorimetry (DSC) that PSHQ5, PSHQ11, and their blends are glassy thermotropic polymers, exhibiting only glass-to-nematic and nematic-to-isotropic (N−I) transitions. Approximate phase diagrams were constructed for PSHQ5/PSHQ11 blends based on DSC data. Using a cone-and-plate rheometer, transient shear flow experiments were conducted for the PSHQ5/PSHQ11 blends (i) at 160 °C in the biphasic region where PSHQ11 forms an isotropic phase and PSHQ5 forms a nematic phase and (ii) at 130 °C in the nematic region where both PSHQ5 and PSHQ11 formed the nematic phase. It was found for such PSHQ5/PSHQ11 blends that the steady-state shear viscosity at 130 °C (in the nematic region) is lower than that at 160 °C (in the biphasic region). However, the first normal stress difference at 130 °C exhibits a very large overshoot followed by an oscillatory decay until reaching a steady state, while it is virtually zero at 160 °C. The time evolution of morphology for the PSHQ5/PSHQ11 blends, upon shear startup and also upon cessation of shear flow, was investigated using a specially designed optical microrheometer equipped with a polarizing optical microscope. Contrasting observations are reported for the case of nematic PSHQ5 in isotropic PSHQ11 when compared to the nematic PSHQ5/nematic PSHQ11 blend. Shearing of a nematic/nematic blend induces a much larger birefringence change than does shearing a nematic/isotropic blend, and a shear-induced isotropic-to-nematic transition is observed from a mixture of isotropic phases containing two TLCPs.