Origins and Mitigation of Defects in Polypropylene Blends Containing Fragile Low Molecular Weight Fractions

Mechanical recycling of polypropylene (PP) is a major sustainability challenge complicated by the presence of low molecular weight (MW) fractions, which reduce strain at break and toughness. Despite the formation of some cocrystals between high- and low-MW PP, low-MW inclusion in the blends still produces undesirable brittleness, and the molecular origin of this embrittlement has remained unclear. Here, we demonstrate that low-MW PP chains can span interlamellar amorphous regions despite having molecular weights below the entanglement threshold, forming taut, unentangled bridges that act as stress-concentrating defects rather than effective load-bearing ties. This study employs a thermal solvent immersion annealing method to engineer the semicrystalline morphology of low- and high-MW PP blends. The process is controlled by heat and solvent, which drive partial crystal melting and expansion of the amorphous phase, allowing mitigation of the defect tie chains and thus significantly improving mechanical performance and enabling the extended use of postconsumer, brittle PP blends.