Masters Theses

Ahmed Hammami

Date of Award

5-1990

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Committee Members

P. J. Phillips, R. A. Benson

Abstract

The quiescent isothermal and nonisothermal crystallization kinetics for ten polypropylenes with different molecular weights, tacticity and copolymerization were investigated by light depolarizing microscopy (LDM) and differential scanning calorimetry (DSC). The isothermal studies were carried out at temperatures between 117 and 139 °C; whereas the nonisothermal investigations were conducted at various constant cooling rates that ranged from 2 to 80 °C/min. Direct microscopic studies via LDM on various resins showed that in all cases, isothermal crystallization proceeded, at least at the higher temperatures, by spherical growth from a fixed number of nuclei, apparently arising from impurities in the polymers. With decreasing crystallization temperature, the number of nuclei increased remarkably and appeared almost always instantaneously with very small time dependence (heterogeneous nucleation). Three empirical formulations with increasing degree of complexity were proposed to model the nucleation density as a function of temperature. Accordingly, the nucleation density was found to be inversely proportional to the degree of undercooling. Spherulitic growth rate data of the same selected resins were analyzed in terms of the established theory for regime III crystallization from the melt and were found to agree with the literature. The recently developed steady state reptation model was tested for three non-commercial homopolymers and was found to apply in regime III crystallization where it was anticipated to fail. Bulk crystallization data were analyzed in terms of the Avrami equation as well as the half-time approach and a comparison between DSC and LDM results was carried out. The Avrami exponent obtained was found to depend on the experimental technique. In much the same way, markedly slower crystallization half-times for the various resins were measured by DSC than by LDM. Possible reasons for such discrepancies were presented. In both cases however, the copolymers crystallized at a much slower rate than the homopolymers. Likewise, high molecular weight samples crystallized relatively slower than lower molecular weight resins of the same category except for one copolymer group. Nonisothermal crystallization results were fairly consistent with the general trend observed during isothermal studies. Under nonisothermal conditions however, the overall rate of crystallization depended essentially on temperature; the effects of molecular weight and chain defects (ethylene co-units) on such a rate were rather weak. The nonisothermal crystallization data were modeled using isothermally obtained parameters according to three existing models. The different predictions were discussed and criticized in terms of the underlying assumptions set forth by a given model. The best formulation was used to extrapolate the nonisothermal crystallization results to very low temperatures (high cooling regimes) and the concept of Continuous Cooling Transformation (CCT) curves was adopted.

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