Revealing the impact of biphase structure on electrical and mechanical properties of isotactic polypropylene via crystallization regulation

Abstract

Isotactic polypropylene (iPP) has emerged as a promising insulating material owing to its exceptional dielectric properties, recyclability, and processability. However, its semicrystalline nature leads to a complex biphase structure (crystalline and amorphous phases), in which disparities between the two phases critically influence electrical insulation and mechanical performance. This study aims to elucidate the intrinsic relationship between the biphase structure of iPP (regulated by crystallization conditions) and its properties, providing theoretical insights for optimizing dielectric and mechanical performance without introducing foreign additives. By adjusting crystallization temperatures (T_c​), five iPP samples with distinct biphase structures were prepared. Structural characterization techniques—including DSC, WAXD, and SEM—revealed that increasing T_c​​ enlarged spherulite sizes (from 32–104 μm) while maintaining constant crystallinity (~ 48–49%) and lamellar thickness (~ 17.6–17.8 nm). Dynamic mechanical analysis (DMA) quantified phase disparities (ΔT_{α2​−β​}, from 127.0 to 138.3 °C), which intensified interfacial electric field distortions and reduced dielectric breakdown strength (93 to 42 MV/m). Finite element simulations (COMSOL) confirmed higher ΔTα2​ − β​ amplified electric field inhomogeneity at phase boundaries. Uniaxial stretching tests at 25 °C and 140 °C demonstrated that increased biphase disparity weakened spherulite boundaries, lowering yield stress (37.0 to 31.0 MPa at 25 °C) and elongation at break (1207% to 20% at 25 °C). This study investigated the relationship between the biphase structure of iPP and its electrical insulation and mechanical properties. It provides valuable theoretical insights for manufacturing high-voltage cable insulation and energy storage devices such as capacitors, where enhanced dielectric stability is crucial.