Effect of Flexible Spacers on the Rheological Behavior of Main-Chain Thermotropic Liquid-Crystalline Polymers Having Bulky Pendent Side Groups

Abstract

The effect of flexible spacers on the rheological behavior of main-chain thermotropic liquid-crystalline polymers having bulky pendent side groups, poly[(phenylsulfonyl)-p-phenylenealkylenebis(4-oxybenzoate)]s (PSHQn), was investigated. For the study we synthesized a homologous series of PSHQn polymers containing methylene groups, varying from 3 to 12, as the flexible spacer and then investigated their transient, steady-state, and oscillatory shear flow behaviors using a cone-and-plate rheometer. We identified the factors which give rise to, during experiment, time-dependent rheological behavior by taking wide-angle X-ray diffraction (WAXD) patterns of the specimen right before and after each rheological measurement. The initial conditions (i.e., the initial morphology) for transient shear flow were controlled by first heating an as-cast specimen to the isotropic region, shearing there at a rate of 0.1 s-1 for ca. 10 min, and then cooling slowly to a preset temperature in the nematic region. Only positive values of steady-state first normal stress difference were observed in all the PSHQn polymers synthesized, except for PSHQ3 having 3 methylene groups of flexible spacer, at all temperatures and shear rates investigated. Interestingly enough, we found that PSHQ3 exhibits Newtonian behavior at low shear rates over the entire range of temperatures tested, showing no evidence of liquid crystallinity from a rheological point of view. This observation is consistent with our earlier findings based on differential scanning calorimetry and WAXD that PSHQ3 is a glassy, non-liquid-crystalline homopolyester. When steady-state shear viscosity (η) in the nematic region is plotted against T − TNI, where T is a measurement temperature and TNI is a clearing temperature, we found that PSHQn moieties with odd numbers of methylene groups have higher values of η in the nematic region compared to PSHQn moieties with even numbers of methylene groups of the flexible spacer.