Primary crystal nucleation and growth regime transition in isotactic polypropylene

We studied nucleation and crystal growth in two polypropylene samples by analyzing the number of spherulites, their size, and shape for various treatments in a hot-stage microscope. The consistency of nucleation and growth data were tested by using overall crystallization experiments and the Johnson-Mehl-Avrami-Kolmogorov equation. In isothermal conditions, for temperatures, TC, varying between 123° and 138°C, the number of crystallites per unit area does not depend on crystallization time and temperature. Instead, at small undercoolings (for TC > 138°C), the total number of nuclei per unit area remains independent of crystallization time, but decreases with increasing temperature. We discuss the implications of this finding for the final microstructure of the crystallized samples. It is noteworthy that the passage between these two nucleation behaviors coincides with the transition from regime III to regime II, observed in the crystal growth kinetic analysis. This remarkable correlation between primary nucleation and crystal growth regimes is discussed.