Kinetics of nucleation and growth of spherulites in homopolymers

Abstract

AbstractIn order to check secondary nucleation theories previously proposed by various authors, spherulitic growth rates of polypropylene, high density polyethylene, and polyoxymethylene were measured at temperatures relatively close to their respective polymer melting points in an air circulating system with temperature fluctuations of less than ±0.05°C. Only partial agreement between theory and experiment was obtained. Interpretation of the experimental data using the similar theories of Burnett and McDevitt, and Barnes and co‐workers, gave melt‐spherulite interfacial energies which to date have not been checked by independent experiment. The critical surface tension of wetting as defined and measured by Zisman and co‐workers was compared with the melt‐spherulite interfacial energy and found in all cases to be substantially larger. The theory of Hirai, which is partially based on the absolute reaction rate theory of Eyring, was also tested, and heats of fusion which were about twice as large as values reported in the literature were found. A major shortcoming of all three nucleation theories is that the intercepts of the theoretical plots differed by many orders of magnitude for the various polymers tested. These theories do not give an adequate theoretical explanation of this fact. Thermal history of the melt was found to have a marked effect on the nucleation rate in polyoxymethylene. This implies that the overall rate constant in the Avrami theory is also strongly dependent on melt history. The data indicated that the rate constant can be decreased about five hundred times by increasing the melt temperature by 20°C.