Unraveling the long-chain branching-gel interplay in raw natural rubber during storage: Implications for strain-induced crystallization

Natural rubber (NR) derived from Hevea brasiliensis is an indispensable industrial biopolymer, whose exceptional mechanical performance is intrinsically tied to strain-induced crystallization (SIC). While storage of raw NR drives complex structural evolution, the interplay between long-chain branching (LCB) and gel formation, and its impact on SIC, remains poorly understood. This study systematically investigates the concurrent evolution of LCB and gel content in raw NR during controlled storage (0–9 days) and elucidates their synergistic influence on vulcanization, SIC, and mechanical properties. Quantitative analyses reveal that moderate short-term storage (3 days) significantly enhances LCB index and gel content, creating a hierarchical network that acts as a template for a denser crosslinked network during vulcanization. This pre-structuring modulates curing kinetics, reducing scorch time while elevating final crosslinking density. In-situ WAXD and mechanical testing show that storage-induced evolution lowers the SIC strain threshold, accelerates crystallization, and increases final crystallinity by up to 22% relative to unstored NR (NR-0). A strong linear correlation (R = 0.9790) between crosslinking density and crystallinity at draw ratio λ = 7 confirms that storage-modulated network topology directly governs SIC efficiency. Prolonged storage (≥6 days) leads to oxidative chain scission, reducing LCB and gel content, compromising crosslinking efficiency, and diminishing SIC capacity and mechanical performance. Mechanistically, LCB acts as physical crosslinks to facilitate chain orientation, while gel networks provide nucleation sites for crystallization, synergistically enhancing SIC. These findings establish a quantitative framework linking storage conditions to molecular architecture and performance, offering science-based guidelines for optimizing NR storage protocols to balance processability and mechanical properties in industrial applications.