Phase Separation Behaviors and Correlated Crystallization of β-Nucleated Polypropylene Random Copolymer Composites for Regulating Impact Toughness

Polypropylene random copolymer (PPR) is a two-phase polymer composed of a crystallizable propylene-propylene-propylene (PPP) phase and an ethylene-propylene (EP) rubbery phase evolved through phase separation. Traditionally, it aims to solve the poor nucleation efficiency of beta-nucleating agent (beta-NA) toward PPR to improve the impact toughness by synthesizing chemical compounds highly selective for beta-modification, with the correlation between phase separation and crystallization behaviors inadequately elucidated. In this work, phase separation behaviors and correlated crystallization of beta-nucleated PPR were investigated and their interplay on impact strength were researched. The structures and temperature-dependent phase separation behaviors of neat PPR were resolved via 13C nuclear magnetic resonance spectroscopy (NMR), rheological analysis, and scanning electron microscopy (SEM). The solubility at different final heating temperatures (T f) of beta-NA in PPR was observed via polarized optical microscopy (POM), and the effect of solubility on phase separation behaviors was illustrated via SEM and dynamic time sweep in linear rheology. The crystallization behaviors of beta-nucleated PPR composites cooling from different T f were then analyzed via in situ two-dimensional wide-angle X-ray scattering (2D-WAXS). It was found that both a higher T f and the dissolve of beta-NAs favored the phase separation process, witnessed by a size increase of EP rubbery phases, but decreased relative beta-crystal content (K beta). A high degree of phase separation with a K beta of similar to 0.38 improved low temperature toughness, while a high K beta with small rubbery particle sizes benefited room temperature toughening. This work illustrates phase separation behaviors and the correlated crystallization of beta-nucleated PPR composites and provides a new perspective on toughening PPR using beta-NAs.