Crystallization of poly(ethylene terephthalate) induced by carbon dioxide sorption at elevated pressures

Abstract

Experimental results are reported for the crystallization and sorption kinetics of poly(ethylene terephthalate) in the presence of CO 2 at 35 °C and pressures of 40, 50, and 60 atm. The results show that the time scale for crystallization is significantly greater than the time scale for CO 2 sorption at these conditions. Crystallization occurs uniformly throughout the polymer film in this kinetic regime, and the sorption isotherm is characterized by a maximum in CO 2 solubility as a function of time. The effect of increasing CO 2 pressure over the pressure range studied is to enhance both the sorption and crystallization kinetics, although larger rate enhancements are obtained for crystallization. These results suggest that crystallization may occur on a time scale less than that for sorption at even higher CO 2 pressures. Crystallization would not occur uniformly throughout the polymer in this kinetic regime, but would move as a front through the polymer film. The morphology of thin polymer films which undergo crystallization at higher pressure may therefore be substantially different from the morphology obtained by crystallization at the CO 2 pressures studied here. This work has general significance for dense gas or supercritical fluid extractions of low molecular weight components from polymeric materials. Our results show that the effect of pressure on the extraction can involve more than the solvent properties of the dense gas or supercritical fluid; the effect of pressure on the relative rates of crystallization and sorption, and on polymer morphology must also be considered. The results of this study also suggest that CO 2 pressure may be used to control the kinetics of crystallization and sorption in thin films of poly(ethylene terephthalate) and thereby provides the means to manipulate the morphology of this semi-crystalline polymer.