Study on in situ visualization crystallization behavior and secondary nucleation model of polypropylene under supercritical N2

Abstract

The crystal morphology and crystallization kinetics of polymers under supercritical N2 have an important impact on the polymers’ melt strength as well as the microstructure and properties of microcellular plastic parts. Herein, the self-made in-situ high pressure microscopy system was employed to study the spherulite morphology and crystal growth behavior in high pressure N2. It was found that PP spherulites were significantly refined under supercritical N2. As the N2 pressure was augmented to 20.69 MPa at 130 °C, the spherulite size was reduced to 48.41 μm, refined by 35.84%, indicating that the crystallization behavior of PP can be controlled by pressure regulation. The new growth pattern for spherulite under supercritical N2 was discovered by atomic force microscope (AFM) for the first time. In addition, the spherulite growth rate exhibited a phenomenon that was inhibited first and then promoted. To explore the re-promoting effect of supercritical N2, a secondary nucleation model that considered the effect of additional free energy induced by high-pressure N2, was established. The re-promotion of spherulite growth rate at elevated N2 pressure is attributed to the augment in translational free energy induced by N2, i.e., entropy-driven crystallization. The calculated results of secondary nucleation model achieved a good agreement with the experimental results, and the variation trend of PP spherulite growth rate could be well predicted under supercritical N2.