A survey of the rheological properties, phase morphology, and crystallization behavior of PP‐POSS materials with weak phase separation

Abstract

AbstractPolypropylene (PP) compounds with varying amounts (0.4‐8.2 vol%) of tailored allyl‐isobutyl polyhedral oligomeric silsesquioxane (POSS) were prepared by melt‐blending. The dependence of the crystallization behavior and crystalline structure of PP on the melt‐state phase morphology of PP‐POSS materials is addressed. PP‐POSS systems were predicted to exhibit weak phase separation in both the molten and solid states based on Bagley plot using Hansen solubility parameters. Small‐amplitude shear rheometry analysis suggested that the highest loaded (8.2 vol%) PP‐POSS system behaves as a two‐liquid emulsion, whereas the low‐content POSS (0.4‐4.0 vol%) systems deviate from this model, resembling a polymer nanocomposite. Based on these findings we hypothesized the formation of a heterogeneous phase morphology in the molten state comprised of nano‐size “pseudo‐solid” POSS clusters and micrometer‐size “pseudo‐liquid” POSS droplets dispersed within the PP matrix, depending on the POSS content. Upon cooling, POSS droplets comingle, forming cube‐like micrometer‐sized crystal domains. The nonisothermal crystallization of PP is enhanced by the presence of POSS clusters. Small‐angle X‐ray scattering analysis revealed the formation of thinner and more heterogeneous folded chain PP lamellae in the PP‐POSS systems.