Relationships of Multiphase Structure with Flexibility and DC Insulation Performance in Impact-Resistant Polypropylene Copolymers

Softening impact-resistant polypropylene (PP) copolymers (ICPs) may be accompanied by reduced breakdown performance; coordinating mechanical and electrical properties is closely related to crystalline-framework and rubber-rich-phase organization. Homopolymer PP matrix HICP and random copolymer PP matrix RICP formed the primary comparison, with high-phase-continuity CICP as a reference, through composition, fractionation, crystalline structure, morphology, viscoelasticity, and charge analyses. Relative to HICP, RICP matrix randomization extended crystallizability distributions toward lower temperatures, refined the crystalline framework, and expanded interphase regions. Tensile and flexural moduli decreased by 38.8% and 38.7%, respectively, while Weibull characteristic breakdown strength increased from 226.2 to 310.8 kV/mm. CICP, containing 23.3 wt% ethylene, comprised a continuous low-crystallinity/rubber-rich phase with dispersed PP crystalline domains. Its tensile modulus was 89.4 ± 4.9 MPa; it retained partial break in notched impact testing at −20 °C and a characteristic breakdown strength of 384.4 kV/mm. Thermally stimulated depolarization current, conduction, and space-charge results indicated that local trapping environments and multiphase transport pathways jointly regulate charge migration and accumulation, providing a charge-behavior basis for maintaining high insulation performance as flexibility increases. The relationships linking molecular sequences, crystalline organization, phase connectivity, and mechanical–electrical responses provide a basis for designing multiphase PP-based DC cable insulation.