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.

Improved mechanical, thermal and flame resistant properties of flexible isocyanate-based polyimide foams by graphite incorporation

“…It possesses outstanding comprehensive performance and extensive range of applications. 1,2 Polyimide foams (PIFs) were prepared by DuPont and Monsanto first in the 1960s. 3,4 Owing to the special cellular structure of the PIFs and especially because of the benzene and imide rings on the backbone, 5,6 PIFs possess excellent performance, such as low density, great insulation, high fire resistance, good thermal stability, lower smoke generation and so on.…”

Spontaneous Chain Orientation of Aromatic Polyimides Evolved during Thermal Imidization from Shear-Oriented Glassy Liquid Crystalline Precursors

“…As seen in the wet spinning processes of aromatic polyamides, LC phases are useful to obtain highly oriented polymer materials, owing to the good fluidity and high susceptibility of the LC phases to external fields such as shear flow and electric and magnetic fields. A few thermotropic PIs based on semiflexible structures − and lyotropic PIs based on bulky aromatic dianhydrides and diamines having soluble side groups − have also been reported. However, molecular designs of PIs toward LC properties reduce the thermal and chemical stabilities of the PIs.…”