Tailoring Insulation and Mechanical Properties of Isotactic Polypropylene via Thermal History‐Dependent Crystalline Morphology

ABSTRACT Polypropylene (PP) has shown considerable promise as a recyclable alternative to crosslinked polyethylene (XLPE) in high‐voltage cable insulation applications. However, its widespread adoption has been hindered by inadequate breakdown strength and mechanical toughness. This study investigates the influence of thermal history‐controlled crystalline morphology on the dielectric and mechanical properties of isotactic polypropylene (iPP). By tailoring cooling rates and annealing temperatures, we demonstrate that iPP with nodular crystalline structures, obtained via rapid quenching and annealing, possesses a superior breakdown strength of up to 580.0 kV/mm and enhanced toughness compared to conventional spherulitic iPP. Multi‐scale characterization reveals that, despite its lower crystallinity and smaller crystallite sizes, the nodular morphology suppresses spherulitic boundary defects and enhances charge trapping at nanoscale interfaces. Conversely, the dielectric strength of melt‐crystallized iPP, though also influenced by lamellar thickness, is consistently limited by spherulite‐related defects and fails to surpass that of the nodular morphology. These findings establish a clear correlation between the insulating properties of iPP and its microstructure, particularly crystalline morphology, and may provide theoretical guidance for developing iPP‐based capacitor films or cable insulation layers.