Exploiting Immiscibility to Induce Fibrillar Structure in Polyethylene/Polypropylene Blends through Two-Dimensional Multilayer Coextrusion and Thermal Drawing

Abstract

Polyethylene (PE) and polypropylene (PP) blends are widely encountered in mixed plastic waste streams, yet their inherent immiscibility typically leads to poor mechanical performance without prior separation. In this work, immiscibility is instead exploited as a processing advantage to induce fibrillar reinforcement through two-dimensional multilayer coextrusion and thermal drawing. A 50/50 PE/PP blend and its corresponding unblended system were processed using a 6X3 multilayer architecture and subjected to controlled draw ratios to investigate structure–processing–property relationships. Mechanical testing revealed substantial draw-direction dependent increases in modulus and tensile strength with draw ratio accompanied by substantial reductions in strain at break and toughness, with blended systems outperforming unblended films at higher deformation levels. Wide-angle X-ray scattering (WAXS) showed that drawing induces cooperative orientation in both PE and PP phases, accompanied by crystallographic redistribution and reflection-dependent crystalline reorganization. Unlike unblended materials, the blends exhibited non-monotonic orientation behavior and dynamic intensity ratio evolution, indicating interphase coupling during deformation. Thermal analysis confirmed moderate increases in crystallinity; however, mechanical enhancement was disproportionately large relative to crystallinity alone. Morphological analysis revealed significantly finer fibrillar structures in the blends, consistent with enhanced microfibrillation driven by phase incompatibility. These results demonstrate that multilayer coextrusion combined with mechanical deformation enables the transformation of immiscible polyolefin blends into hierarchically structured, higher draw-direction modulus, and tensile strength materials without compatibilization, providing a potentially scalable pathway for the upcycling of mixed plastic waste.