A Study of Crystallization in Bisphenol-A Polycarbonate
Farmer, Robin Sandra
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The crystallization behavior of bisphenol-A polycarbonate (BAPC) was studied, focusing on the initial stage of crystallization and the secondary stage of crystallization. Bisphenol-A polycarbonate was the polymer chosen for this study because of its slow crystallization rate. With slow crystallization kinetics, the polymer morphology does not change when quenched below its glass transition temperature, enabling the study of different stages of crystallization through the frozen morphology. The study of the initial stages of crystallization pertained to crystallization times prior to the growth of detectable crystallinity. This study employed BAPC because of the long induction period, a direct result of the slow crystallization kinetics. During the induction period of polycarbonate crystallized at 190Â°C there was no evidence of polymer chain ordering that was seen in literature for other polymers. The length of the induction period determined by differential scanning calorimetry and wide-angle X-ray diffraction varied by over 6 hours because differential scanning calorimetry can detect a smaller amount of crystallinity than wide-angle X-ray diffraction. Signs of pre-ordering in the literature could be a result of experimental sensitivity. The study of the secondary crystallization dealt with the isothermal lamellar thickening of BAPC crystals during annealing, after crystallization for an extended period of time. Small-angle X-ray scattering and differential scanning calorimetry experiments were performed on bisphenol-A polycarbonate samples crystallized near 190Â°C for 8 days and annealed at either 223Â°C or 228Â°C for various times. The Gibbs-Thomson relationship, which can be defined using the experiments mentioned, yielded two thermodynamic constants, the equilibrium melting temperature and the surface free energy. Including data from literature in the determination of the constants, the equilibrium melting temperature and surface free energy of BAPC is 303Â°C and 36.6mJ/m2, respectively. Comparing the lamellar thickness measurements by small-angle X-ray diffraction with direct measurements by microscopy was difficult because the morphology of the polymer was not easily seen in the bulk using atomic force microscopy or scanning electron microscopy. Etching the sample was the most promising technique for future investigations of revealing the bulk morphology for direct lamellar thickness measurements. Crystallizing thin films of polycarbonate on calcite substrates allowed the measurement of lamellar thickness using scanning electron microscopy because the lamellae grow epitaxially to the substrate. The measurement of the long spacing in thin film samples was comparable to that of bulk samples.
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