Phase behavior, rheology, and morphology of binary blends of semiflexible main-chain thermotropic liquid-crystalline polymers

The phase behavior, rheology, and morphology of binary blends of semiflexib le main-chain thermotropic liquid-crystalline polymers (TLCPs) were investi gated. Specifically, binary blends consisting of poly [(phenylsulfonyl) p-p henylene 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, PSHQ 11, and their blends are glassy thermotropic polymers, exhibiting only glas s-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 degreesC in the biphasic region whe re PSHQ11 forms an isotropic phase and PSHQ5 forms a nematic phase and (ii) at 130 degreesC in the nematic region where both PSHQ5 and PSHQ11 formed t he nematic phase. It was found for such PSHQ5/PSHQ11 blends that the steady -state shear viscosity at 130 degreesC (in the nematic region) is lower tha n that at 160 degreesC (in the biphasic region). however, the: first normal stress difference at 130 degreesC exhibits a very large overshoot followed by an oscillatory decay until reaching a steady state, while it is virtual ly zero at 160 degreesC. The time evolution of morphology for the PSHQ5/PSH Q11 blends, upon shear startup and also upon cessation of shear flow, was i nvestigated 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 ble nd, and a shear-induced isotropic-to-nematic transition is observed from a mixture of isotropic phases containing two TLCPs.