Coagulation process-dependent long-chain branching in natural rubber: Relationships with strain-induced crystallization and mechanical properties

Abstract

The coagulation process significantly influences the molecular architecture of natural rubber (NR), particularly the formation of long-chain branching (LCB), which plays a crucial role in determining its mechanical properties and strain-induced crystallization (SIC) behavior. This study explores how three distinct coagulation methods — acid-induced, natural, and enzymatic coagulation — shape the LCB structure of NR, and further investigates their cascading effects on the crosslinking network, mechanical performance, and SIC characteristics. Through an integrated approach combining Rubber Processing Analyzer measurements, wide-angle X-ray diffraction (WAXD), crosslink network characterization, and mechanical testing, we found that varying coagulation processes yield NR with distinct LCB features, and LCB exhibits a linear correlation with Mooney viscosity during mastication. A higher LCB index in both raw rubber and compounds was observed to facilitate the initiation of SIC at lower strains, thereby enhancing NR’s self-reinforcement effect. Among the samples, enzyme-coagulated NR exhibits the highest LCB index; its tensile strength (31.54 ± 0.46 MPa) and crystallinity (10.01 %) outperform those of acid-coagulated (24.81 ± 0.31 MPa, 9.38 %) and naturally coagulated (23.47 ± 0.56 MPa, 7.87 %) samples. Furthermore, this work clarifies the regulatory role of coagulation processes in NR network evolution and establishes quantitative relationships between crystallization at a strain of 7, tensile performance, and the LCB index of raw rubber and compounds, offering novel insights into NR molecular design. These findings highlight the pivotal role of coagulation methods in tailoring NR’s LCB structure, paving the way for performance optimization in high-demand applications such as tire manufacturing and industrial elastomer production.

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