Oscillatory shear flow behaviour of a thermotropic liquid-crystalline polymer

Abstract

The oscillatory shear flow properties, dynamic storage and loss moduli ( G′ and G″), of a thermotropic liquid-crystalline polymer (TLCP) in both the isotropic and nematic regions were measured as functions of angular frequency (ω), using a cone-and-plate rheometer. For the study, an aromatic polyester, poly[(phenyl sulfonyl)- p-phenylene 1,10-decamethylene-bis(4-oxybenzoate)] (PSHQ10), was synthesized in our laboratory. The PSHQ10 was found to have (1) a glass transition temperature of 88°C, (2) a melting point of 115°C, and (3) a nematic-to-isotropic transition temperature of 175°C. We found that the oscillatory shear flow properties of PSHQ10 in the nematic region were strongly dependent on its thermal history (i.e. annealing temperature and the duration of annealing) and shear history. Thus, in order to completely erase the thermal history associated with polymerization and sample preparation, a solvent-cast PSHQ10 specimen was first heated to the isotropic region (e.g. 190°C), sheared there at a very low shear rate (0.0085 s −1) for ∼5 min, and then cooled very slowly down to a predetermined temperature (130, 140, 150, 160 or 170°C) in the nematic region. We found that in the isotropic region time-temperature superposition holds and plots of log G″ versus log G″ give rise to a temperature-independent correlation. On the other hand, we found that in the nematic region preshearing has a profound influence on the oscillatory shear flow properties and log G″ versus log G″ plots show temperature dependency. This suggests that time-temperature superposition would not hold in the nematic region of PSHQ10 (and also of other types of TLCPs). We thus conclude that log G′ versus log G″ plots are very sensitive to variations in the morphological state of PSHQ10 as the temperature varies in the nematic region. We found that the Cox-Merz rule holds for PSHQ10 in the isotropic region and the morphological state of a specimen plays an important role in determining whether or not the Cox-Merz rule holds in the nematic region; specifically an unsheared specimen does not follow the Cox-Merz rule, whereas a sheared specimen does.