Crystallization behavior and secondary nucleation model of PP/GN nanocomposites under supercritical N2

Abstract

Supercritical N2, as a physical blowing agent, is commonly used in microcellular foam injection molding to improve the crystallization kinetics and foaming performance of polymer-based composites. Herein, the crystal morphology and growth behavior of polypropylene/graphite nanosheet (PP/GN) nanocomposites under supercritical N2 were investigated using a self made in-situ high pressure microscope system. Due to the heterogeneous nucleation effect of GN, more irregular lamellar crystals inside the spherulites were generated and the stacking of lamellar crystals was more disordered. The intrinsic mechanisms of GN content on the growth rate of spherulites were investigated on the basis of the interfacial free energy induced by GN. The calculated results of the secondary nucleation model are in line with the results obtained from the isothermal crystallization experiments, indicating that the model can effectively predict the grain growth rate of PP/GN nanocomposites under supercritical N2. In addition, based on the crystallization kinetics of PP/GN composites, the action mechanism of GN on the foaming behavior of PP was explored. As expected, more cell nuclei were induced due to the refinement of grains and the increased of GN content.