Unexpected Crystallization Behavior of Polypropylene With Low Rubber Content

ABSTRACT This study investigates the crystallization behavior of low‐rubber‐content polypropylene (LRPP) from different initial phase morphologies. The employed polypropylene, produced by a two–reactor polymerization process, exhibits an upper critical solution temperature (UCST) at ~210°C, as revealed by combined rheological and scattering analyses. A counterintuitive finding shows that crystallization is markedly promoted from an initially homogeneous melt state compared to an equilibrated phase‐separated state. This enhanced crystallization should be attributed to the spatial distribution of the propylene‐propylene‐propylene triad sequence segments (PPP) of the ethylene‐propylene random copolymer. An equilibrium morphology results in a compatibilized interface that diminishes nucleation efficacy, whereas the non‐equilibrium morphology generated by direct cooling probably possesses relatively sharper interfaces that facilitate a higher nucleation density.