Multilayered Core-Shell Structure in an Impact Polypropylene Copolymer Investigated by Atomic Force Microscopy-Infrared

The balanced mechanical properties of impact polypropylene copolymer (IPC) are largely attributed to the core-shell structure of its dispersed rubber particles, yet experimental observation of the outer shell interface between the rubber phase and the polypropylene (PP) matrix is challenging. In this article, atomic force microscopy-infrared (AFM-IR) was employed to study a commercial IPC to determine its phase structure. Quantitative analysis of the nanodomain composition in situ by AFM-IR in combination with the chain structure of the copolymers obtained ex situ by fractionation and NMR revealed a core surrounded by a rubber layer, comprising the ethylene-propylene segmented copolymer (EsP) and ethylene-propylene random copolymer (EPR), respectively, which suggests the existence of an outer shell for the particle composed of the ethylene-propylene block copolymer (EbP). The EbP fraction in the IPC was then replaced by an ethylene-deuterated propylene diblock copolymer (EbDP), which was then melt-blended with all other fractions to reconstruct the IPC. Both AFM-IR spectroscopic analysis and imaging of the nanodomains in the reconstructed IPC showed that the EbDP molecules are located at the interface between the rubber phase and the PP matrix, forming an outer shell for the particle. The results provide direct and unambiguous experimental evidence for the multilayered particle structure in the IPC. Mechanical test results further demonstrated that the outer shell for the rubber particle was beneficial to the tensile and impact properties of the alloy.