Kinetics of Fast and Slow Transitions in a Liquid Crystalline Polyimide

Abstract

Liquid crystal polymers (LCPs) are known to exhibit slow and fast phase transitions upon heating and cooling. The kinetics of both types were investigated for an liquid crystal (LC) polyimide derived from pyromellitic dianhydride (PMDA) and 1,3-bis[4'-(4''-aminophenoxy) cumyl] benzene (BACB) (PMDA/BACB). The kinetics were compared with our previous results on poly(bistrifluorethoxyphosphazene) (PBFP), which also shows LC and non-LC transitions, and with nylon PA6, a non-LC reference sample. Transition kinetics were studied upon cooling using non-isothermal differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and polarization microscopy. The DSC experiments were performed by cooling our samples from 400 to 100 °C using rates between 2.5 and 100 K min-1. We observed that the LC transitions show a linear dependency on the cooling rate. These transitions were solely governed by super-cooling and these observations are congruent with our earlier results on PBFP. Apparently, we may conclude that all LC transitions show this linear dependence. For PMDA/BACB, the fast LC transitions were related to the formation of a smectic phase. WAXD did not reveal a crystalline phase in PMDA/BACB. The non-LC transition was related to a thermally activated process, having an apparent activation energy, EA, of ≈ 360 kJ mol -1, indicating a re-arrangement of chains previously randomly oriented. Crystallization could proceed after cooling from the melt to room temperature followed by annealing based on a thermally activated process, with E A ≈ 160 kJ mol-1.