Comments on isothermal crystallization kinetics of polymers: Polypropylene at high supercooling

A robust procedure for isothermal kinetic evaluation of Differential Scanning Calorimetry (DSC) measurements of polymers using the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation is proposed. This procedure takes into account the experimental uncertainties for short (initial deflection) and long (secondary crystallization) measurement times. This procedure is applied to the isothermal crystallization of various commercially available isotactic polypropylenes (iPP) in the temperature range between 10 degrees C and 120 degrees C. Further measurements are performed on unfilled iPP and iPP highly filled with calcium carbonate. The measurements were performed using Fast Differential Scanning Calorimetry (FDSC). Evaluation of the crystallization kinetics indicates homogeneous nucleation in the temperature range of mesophase formation below 60 degrees C. With decreasing temperature, the crystallization is more affected by diffusion control and/or termination of growth at an early crystallization stage During alpha-phase formation (above 60 degrees C), crystallization is heterogeneously nucleated. Depending on the sample, growth occurs at a constant rate or is diffusion controlled. Polypropylene highly filled with calcium carbonate shows an additional alpha-phase crystallization process. The KJMA evaluation indicates that this process is homogeneously nucleated.

Automated summary

A robust procedure for isothermal kinetic evaluation of DSC measurements of polymers is proposed, considering experimental uncertainties. The procedure is applied to the crystallization of different polypropylenes and reveals the presence of homogeneous nucleation at lower temperatures, with diffusion control becoming more prominent at lower temperatures.