Hanging time-dependent gel content evolution and its effect on strain-induced crystallization of natural rubber

Strain-induced crystallization (SIC) is a critical factor governing the mechanical properties and service performance of natural rubber (NR). Gel content evolution during the hanging process exerts a substantial regulatory effect on the SIC behavior of NR. To elucidate the relationship between hanging time-dependent gel content evolution and SIC behavior, this study systematically investigated the changes in gel content and molecular weight of raw NR over hanging periods of 0–7 days using toluene extraction and gel permeation chromatography. In situ wide-angle X-ray diffraction (WAXD) under uniaxial stretching was employed to characterize the SIC behavior of NR samples subjected to different hanging durations. The results revealed that moderate hanging (3 days) yielded the maximum gel content (26.8%), while prolonged hanging (≥5 days) led to partial scission of the gel network, decreasing the gel content to 24.0%. This microstructural evolution directly modulated the SIC behavior: NR with 3 days of hanging exhibited the highest tensile crystallinity (10.74%) at a draw ratio of 7.0. This may be attributed to gel domains acting as physical anchors that facilitate molecular chain orientation and crystal nucleation. In contrast, both shorter and longer hanging durations resulted in lower crystallinity due to insufficient molecular entanglement (short hanging) or structural degradation (long hanging). Correlation analysis further confirmed a strong positive linear relationship between gel content and tensile crystallinity (R = 0.9602), indicating that gel content is a key correlative parameter associated with SIC efficiency. This study provides a theoretical foundation for optimizing the processing parameters of NR and offers new insights into tailoring the SIC behavior and mechanical properties of NR by controlling gel content evolution during hanging. It also advances the understanding of the structure–property relationship of NR, which is of great significance for the high-performance industrial applications of NR-based materials.