Spatial Inhomogeneity of Thermal Degradation of Semi-Crystalline Polymers

The thermal degradation of polymers is typically interpreted as a uniform chemical process involving molecular weight reduction and oxidation. However, the role of localized microstructural changes as defect sites that deteriorate polymer mechanical performance remains underexplored owing to the limitations of conventional bulk characterization techniques. We applied an isotope-labeled Raman mapping technique with molecular weight selectivity to visualize the structural evolution of semi-crystalline polyethylene during long-term thermal exposure under conditions in which the bulk molecular weight and carbonyl index remained nearly unchanged. In this material, mechanical deterioration is induced by spatially localized structural changes. Despite the negligible average chemical change, the ductility decreased drastically upon heating. High-resolution mapping revealed discrete domains of enhanced crystallinity and conformational ordering, which emerged with spherulite-scale correlation lengths that expanded with the aging time. These domains spatially coincide with the fracture-origin regions after tensile deformation. Molecular weight-resolved analysis showed that high-molecular-weight chains preferentially underwent degradation-induced crystallization (chemi-crystallization), suggesting that regions rich in tie molecules are the primary transformation sites. Our findings show that thermal exposure is a defect nucleation process governed by structural heterogeneity, providing a new framework for predicting the long-term reliability of semi-crystalline polymers. This mechanism provides a predictive foundation for a wide range of engineering thermoplastics.