Effect of Thermally Induced Self-assembly of β Nucleating Agents on Space Charge in Isotactic Polypropylene

Phase behavior of semi-crystalline polymers is closely related to topological structure evolution of nucleating agents (NAs). In this study, thermally induced self-assembly of TMB-5 $\beta$-NAs was prepared and its effects on dielectric properties and space charge characteristics in isotactic polypropylene (iPP) samples were investigated. The results show that $\beta$-NAs self-assembled from point-like shape into fibril when the heat-treated temperature reached $240^{\circ}C$, and into three-dimensional dendritic structure when reaching $280^{\circ}C$. Dielectric loss tangent and conduction current decrease and then increase with the rise of heat-treated temperatures, while DC breakdown strength is opposite. Self-assembly characteristics and changes of $\beta$-crystal content have been analyzed by polarizing microscopy and X-ray diffraction patterns. Moreover, influence of thermally induced self-assembly of $\beta$-NAs on charge evolution and field distortion has been discussed as well. This study provides a new way to simultaneously enhance dielectric property and suppress space charge in iPP via tailoring the hierarchical structure of $\beta$-NAs.