Crystallization kinetics of Poly (ethylene oxide)

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Title: Crystallization kinetics of Poly (ethylene oxide)
Author: Segouin, Frances
Abstract: The three main goals of this project are to determine the molecular weight dependence of polyethylene oxide) on the 1) morphology of crystalline material, as seen with the polarizing light microscope, 2) nucleation of the crystalline phase, as demonstrated with an annealing study, and 3) parameters of a mathematical model for crystallization kinetics. Bulk crystallization kinetics of polyethylene oxide) from the melt have been studied for a large molecular weight range, 1500 g/mol to 7,000,000 g/mol. The effects of molecular weight and cooling rate on the width and degree of supercooling of DSC crystallization exotherms have been studied to determine the type of nucleation involved and to determine the relationship between the DSC signals and the morphology of a crystalline sample. As the molecular weight increases, at a cooling rate of twenty degrees per minute, the width-at-half- height of the exothermic DSC peak decreases from seven degrees to approximately three degrees for the low molecular weight samples and increases from three to seven degrees for the high molecular weight samples. The degree of supercooling at which the peaks reach a maximum generally increases from twelve degrees Celsius to above twenty degrees Celsius. The slower the cooling rate the narrower the width at half height of the crystallization peak and the lower the degree of supercooling at the maximum point of the crystallization exotherms. For all of the molecular weight samples, the crystallization peak temperatures increase in the range of one to ten degrees of supercooling from a one-degree-per-minute cooling rate to a twenty degrees-per-minute cooling rate. The average width-at-half-height increases from three to six- degrees Celsius from the one degree-per-minute cooling rate experiment to the twenty degreesper- minute cooling rate experiment. The morphology of the samples varied from a coarse non-interconnected lamellae formation below the critical molecular weight to an interconnected lamellae formation, also known as a spherulite, above the critical molecular weight. The critical molecular weight for polyethylene oxide) is 3400 g/mol. Based on the expression for the linear crystal growth rate, reported by Cheng et al.1, a mathematical model was created to model the DSC crystallization curves. This model, which incorporates heat transfer effects, can model the width and shape of the DSC curves, but the degree of supercooling that the model predicts is generally smaller than the experimental degree of supercooling. While modeling the DSC crystallization curves, it was found that the activation energy for reptation motion, U*, has less of an effect on the model crystallization curves than does the surface energy term, Kg, of the Cheng growth rate expression.
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Date: 1994-08

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