Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

Abstract

The biphasic behavior and phase transitions of a thermotropic main-chain liquid crystalline polyester, the poly(oxytrimetilen-etilen glycol p,p′-bibenzoate), was studied by X-ray diffractometry, differential scanning calorimetry (DSC), and rotational rheometry. The liquid crystalline mesophase structure of the polymer evolves from Smectic C to Smectic A at around 100°C and changes to the isotropic state at about 200°C. The annealing of polymer samples at different temperatures within the smectic–isotropic transition gives rise to double endotherm and exotherm peaks on the heating and cooling DSC traces. These peaks are attributed to the preferential segregation of the lower molecular weight molecules from the liquid crystalline to the isotropic phase in the biphasic zone. The rheological studies are consistent with this result. The dynamic moduli measured at constant frequency in temperature ramps using polymer samples with different thermal histories reveal that the type of annealing processes applied in the biphasic zone generates different evolution of the rheological data.