Chain Microstructure and Phase Behavior of Two Impact Copolymer Polypropylene With Similar Stiffness and Different Toughness: The Key Role of EbP Microphase Separation

ABSTRACT Impact polypropylene copolymer (IPC) is widely used commercially. Ethylene–propylene blocky copolymer (EbP) boosts IPC compatibility at the interface to realize an excellent rigidity‐toughness balance, but the impact of its complex distribution on IPC multiphase properties is rarely studied. In this work, two industrial resins (Samples A and B) with similar flexural modulus but significantly different impact strength were analyzed for chain microstructure and copolymer distribution. Combined atomic force microscopy‐infrared (AFM‐IR), fractionation, chain‐structure, and thermal analyses suggest that an EbP‐enriched EP‐copolymer interphase is preferentially formed in Sample A. In contrast, the stronger crystallization tendency of B100 (the 100°C elution fraction of Sample B) favors premature microphase separation and redistribution of the EbP fraction before crystallization of the isotactic polypropylene matrix, resulting in ethylene‐containing copolymer‐rich regions in both the dispersed phase and selected locations within the continuous PP phase. This less interface‐confined distribution may weaken the compatibilizing effect of EbP and contribute to the poorer mechanical properties of Sample B. The lower molecular weight ( M w ) and ethylene content of B100 facilitate the mobility, rearrangement, and crystallization of propylene segments. This work elucidates how the molecular characteristics and crystallization behavior of EbP influence its interfacial distribution and, consequently, IPC compatibility and toughness.