Heterogeneous nucleation in the crystallization of polyolefins: Part 2. Kinetics of crystallization of nucleated polypropylene

Abstract

The kinetics of isothermal crystallization of nucleated polypropylene has been determined by depolarization measurement or dilatometry. The depolarization measurement was preferred, being faster and more reliable than dilatometry. For the evaluation of the recorded crystallization curves a non-linear regression analysis program was used fitting the data into the Avrami equation. An indication is given of some limitations in the applicability of this equation. Various experimental conditions may cause deviations in the crystallization kinetics from this equation. It is shown that in the analysis according to Avrami induction times become irrelevant. The nucleation density is highly dependent on the temperature of crystallization. Although the nucleation is for the larger part instantaneous, the density increases roughly by a factor of 10 at an increase in the degree of supercooling of 4°C. The temperature of melting prior to crystallization has no influence on the nucleation density, provided orientations in the melt have been relaxed. The nucleation density found was not proportional to the content of nucleating agent in the concentration series used, probably owing to the methods of preparation.