Identification of phase-separated structures and polymer crystals inside polyolefin blends

Abstract

Polyolefins, including polyethylene (PE) and isotactic polypropylene (iPP), constitute approximately half of all plastic production and consumption. However, recycled PE/iPP mixtures tend to be brittle, necessitating modifications to enhance their physical properties for practical applications. A comprehensive understanding of the internal structure of PE/iPP blends, which is influenced by phase separation and crystallization, is essential for improving their physical properties. Conventional electron microscopy techniques face limitations in distinguishing phases and observing crystal morphology. In this study, we introduce a four-dimensional scanning transmission electron microscopy technique known as nanodiffraction imaging (NDI) to analyze the internal structure of polyolefin blends. Scanning a 1.4 nm-diameter electron beam at 5 nm intervals, we examined a high-density PE (HDPE)/iPP blend specimen with a 10:90 wt composition. Based on scattering vectors and azimuthal angles of the diffraction peaks, NDI effectively identifies phases inside the HDPE/iPP blend, maps the spatial distribution of HDPE and iPP crystals, and facilitates the orientation mapping of their molecular chains. Additionally, it enables analysis of the relationship between HDPE and iPP crystals near the interface. The demonstrated effectiveness of NDI in structural analysis represents a significant advancement in understanding how the internal structure of polyolefin blends correlates with their physical properties.