Visualizing the Evolution of Heterogeneous Crystallization Driven by Complex Deformation Scenarios in Natural Rubber

Natural rubber (NR) has gained renewed attention due to its sustainability and exceptional mechanical properties, largely driven by strain-induced crystallization (SIC)—a unique self-reinforcing mechanism. Despite its long research history, the behavior of SIC under complex deformation conditions, frequently encountered in NR products, remains incompletely understood. This study visualizes the evolution of non-uniform SIC under heterogeneous deformation using a specially designed tensile geometry. This experimental configuration induces diverse local deformation modes, including uniaxial, planar and biaxial stretching, within a single tensile test. By integrating digital image correlation and high-speed infrared thermography, we successfully map the spatial distribution of strain and the associated crystallinity evolution across the specimen. The findings reveal that local SIC initiates at nearly the same critical longitudinal strain, regardless of local strain biaxiality, characterized by the lateral-to-longitudinal true strain ratio (m12 = -e2/e1). In contrast, the subsequent evolution of SIC is strongly influenced by local deformation characteristics, specifically the longitudinal true strain (e1) and m12. At a constant m12 (representing a fixed deformation mode), local crystallinity (c) increases with 1. Conversely, at a constant e1, c increases with m12, indicating that uniaxial stretching promotes higher crystallization when compared to other deformation modes at equivalent longitudinal strains. Classical models based solely on conformational entropy reduction fail to capture these strain-biaxiality effects, underscoring the necessity of incorporating m12 into predictive frameworks. The observed strain-crystallinity relationship is effectively described by a unified model, comprising of a quadratic dependence of e1 and a linear dependence of m12. This model enables a comprehensive visualization of crystallinity evolution during non-uniform deformation, providing deeper insights into SIC mechanisms. These findings guide the design and development of high-performance, sustainable rubber materials.