Effect of thermal history on the time evolution of the structure of thermotropic liquid crystalline polyesters containing a halogen-substituted hydroquinone ring

Thermal properties of two main-chain thermotropic liquid crystalline polyesters: poly(chloro-1,4-phenylene-trans-1,4-cyclohexanedicarboxylate) (PClP-TCD) and poly(bromo-1,4-phenylene-trans-1,4-cyclohexanedicarboxylate) (PBrPTCD), were investigated using modified differential scanning calorimetry (MDSC), polarized optical microscopy (POM), wide angle X-ray diffraction (WAXD) and thermal gravimetric analysis (TGA). It was demonstrated that two thermotropic polyesters showed quite different thermal properties although they have similar molecular structures. For PClPTCD, triple endotherms were discerned in the MDSC scans and the melting crystal phase transition manifested strong dependence on the thermal history. For PBrPTCD, however, only one peak appeared in MDSC thermograms during the heating and cooling cycles and the thermal history showed a little effect on the phase transition behavior. The differences in phase transition behavior between two polyesters arise from their different crystal structures.