Crystallization behaviour and morphological features of LARC-CPI

Abstract

This work describes the crystallization behaviour and morphological features of the semicrystalline polyimide known as LARC-CPI. The study considers several features affecting crystallization behaviour such as inherent viscosity, crystallization temperature, melt temperature and time in the melt. Crystallization kinetics data were analysed using an Avrami analysis in conjunction with differential scanning calorimetry. Complications encountered when evaluating the crystallization exotherm for this analysis are also discussed. Morphological features were investigated using several techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS). A permanganic etching technique combined with SEM successfully revealed morphological detail further supported by TEM results. This technique is also used to estimate the isothermal linear crystalline growth rate, G. The SEM and TEM data lend support to the Avrami data obtained by crystallization kinetics. SAXS results reflect the influence of crystallization temperature on the long spacing. SAXS also reveals the presence of a broad second scattering peak for semicrystalline samples which appears in the same position regardless of crystallization temperature or inherent viscosity. Molecular modelling predicts a low-energy helical conformation with a near-periodic repeat distance corresponding to that of the second SAXS peak. The periodic character of this conformation is offered as a hypothesis for that peak. This work also considers the effect of potential nucleating agents on the recrystallizability of a higher inherent viscosity LARC-CPI. One of the several nucleators investigated appears effective in enhancing the bulk crystallization rate.