Crystallization Processes in Poly(Ethylene Terephthalate) / Polycarbonate Blends
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crystallization temperature, and then scanned at 10 'C/min to 280 °C to collect melting data. To avoid cumulative thermal history effects and degradation, a new sample was used for each crystallization experiment. All crystallizations were repeated three times. Dynamic crystallization from the melt was also studied: samples were scanned to 280 0C, held for 5 min, and then cooled at 5 °C/min to 25 °C measuring heat flow as a function of temperature. The effects of melt processing on crystallization behavior were examined for the 60/40 PET/PC blend. The samples were scanned to a chosen melt processing temperature, held for a given melt time, then quenched to 220 'C. Isothermal crystallization at 220 0C was observed, collecting data of heat flow as a function of temperature. Melt processing temperatures were 280 and 300 °C, and melt times were 1, 5, 10, 20, and 30 min. When crystallization was complete, fusion data for the crystallized sample were collected by scanning from 205 'C to 280 'C, measuring heat flow as a function of time. A new sample was used for each treatment/crystallization, and each set of conditions was repeated three times. RESULTS Glass Transition Temperature Scans of heat flow as a function of temperature show either two glass transition temperatures for PET and PC, or a single glass transition for PET and recrystallization of PET which masks the glass transition of PC. These distinct Tg's, summarized in Table I, indicate that PET and PC are immiscible over the range of compositions studied. In the blends, it is interesting that the T of PET is slightly higher than that of 100% PET, while the Tg of PC is lower than that of 100% PC. The same shifts in component Tg's have previously been observed in PET/PC blends [1]. Since the glass transition of PET takes place in the presence of glassy PC, the rigid PC "matrix" could contribute toward increasing the Tg of PET. Conversely, the glass transition of PC in the blend takes place in the presence of rubbery PET, which may effectively plasticize the PC. Also, it is possible that some transesterification had occured in the blend samples during processing. This would cause some convergence in the Tg values through homogenization introduced by the newly created copolymer. Isothermal Crystallization Kinetics of PET The crystallization half-time is determined from a crystallization exotherm as the time at which half the crystallization peak area has evolved. The crystallization rate can be defined as the inverse of the crystallization half-time. Figure 1 shows the crystallization rate of PET as a function of crystallization temperature in the systems studied. Crystallization rate in all systems decreases with increasing crystallization temperature, since the driving force for crystallization in this regime is the degree of undercooling from the melt. For blends containing less than 80 wt % PC, the crystallization rate of PET is enhanced by blending with PC. We would suggest that the surfaces Table I: Glass transition temperature of PET and PC in blend samples de
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