In situ visualization of crystal nucleation and growth behaviors of linear and long chain branched polypropylene under shear and CO2 pressure

Abstract

Crystallization and foaming behaviors of a semi-crystalline polymer in conditions comparable to those found in polymer processing, where the polymer melt experiences shear under elevated pressures, are key for modeling polymer processes and predicting the final structure and mechanical properties of polymer products. We investigate the crystallization behaviors of two different high-melt-strength polypropylene (PP) resins using a novel high-pressure visualization system that was developed to capture the crystallization process of a plastic specimen under high pressure and controlled shear. In situ visualization results under isothermal conditions demonstrate that both linear and long chain branched (LCB) PP exhibit crystal growth rates and nucleation densities that increase with higher applied shear stress but reach an optimum value with increasing CO2 pressure. By selectively choosing processing conditions, we can enhance the formation of observable critical size nuclei and the exclusion of CO2 from the crystal growth front. Therefore, overall crystallization kinetics can be easily controlled through the effect of induced-shear stress and the presence of pressurized CO2.