Thermal transition behaviors in a liquid crystalline polyesterimide

The crystallization and melting behaviors of the polyesterimide, derived from N,N′-hexane-1,6-diylbis(trimellitimides), 4,4′-dihydroxybenzophenone and p-hydroxybenzoic acid, were investigated by using polarized light microscopy (PLM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The nematic texture of the polyesterimide was observed on raising temperature to 265 °C, and the nematic phase was found to convert to isotropic melt beginning from about 300 °C, the ordered nematic micro-domains still surviving after 320 °C. Isothermal crystallization of the samples was performed at 180 °C after heating samples at various temperatures in the range of 265-360 °C, and a completed crystallization peak can appear on DSC curves up to the heating temperature of 360 °C in the presence of the nematic phase and the ordered nematic micro-domains. Non-isothermal crystallization of the samples at different cooling rate was carried out, and the melting of the resulting crystals exhibits double endotherms. It is indicated that a fast crystallization in the nematic phase forms relatively more ordered crystals, which melt at higher temperature, and a slow crystallization in the isotropic phase or in the biphasic melt produces poor crystals, which melt at lower temperature. The crystallized polyesterimide was annealed, which has a minor effect on the high-melting peak but leads to a continual shifting of the low-melting peak to higher temperature with increasing annealing temperature or annealing time. WAXD patterns indicated that the structural transform was not found during annealing process.