Broadened glass transition in a liquid-crystalline polymer studied by thermally stimulated currents,AC dielectric,dynamic mechanical,and DSC methods

Thermally stimulated currents (TSC), a. c. dielectric, dynamic mechanical (DMTA), and differential scanning calorimetry (DSC) methods were used to study the glass transition in a thermotropic liquid-crystalline copolyester. All the techniques were consistent in the determination of the main glass transition temperature. Using the high sensitivity of the TSC thermal sampling method, it was shown that cooperative glass transition-like relax-ations occur down to 100°C below the main glass transition. DSC was sensitive only to a broadening of the glass transition to about ca. 30°C, so it was concluded that the thermal sampling method is sensing a very small fraction of cooperatively relaxing species which cannot be detected by DSC. Ac dielectric measurements and DMTA also indicated that the glass transition was broad, but difficulties with overlapping transitions prevented quantitative determination of the breadth of the glass transition. The results suggest that the broad glass transition, in this mostly amorphous LCP, is due to chemical heterogeneity of the copolyester chain. Other evidence indicates that the broadening is not due to the oriented nature of the glassy state. Some discussion is presented concerning how the heterogeneous nature of the LCP glass leads to compensation of the Arrhenius curves obtained by the thermal sampling method. © 1993 John Wiley & Sons, Inc.