Pronounced effect of strain biaxiality on high-temperature behavior of strain-crystallizing elastomers.

We investigate the impact of strain biaxiality on strain induced crystallization (SIC) at elevated temperatures in natural rubber (NR) and synthetic isoprene rubber (IR). By comparing uniaxial (U) and pseudo-planar (P) stretching under different lateral contraction conditions, we find that the upper-limit ambient temperature for SIC-induced reinforcement in the P-geometry is more than 20 °C lower than in the U-geometry. Similarly, the melting temperature of SIC crystallites in the P-geometry is reduced by over 30 °C compared to the U-geometry. These findings demonstrate that finite lateral stretch significantly suppresses both the onset temperature and thermal stability of SIC-induced reinforcement. Our results reveal that strain biaxiality plays a pivotal role in SIC not only at room temperature, as previously recognized, but also under high temperature conditions. These strain biaxiality effects are more pronounced in IR than in NR. Furthermore, elevated-temperature fracture experiments reveal a non-linear crack propagation pattern in the P-geometry: local deformation transitions from planar to pseudo-uniaxial toward the specimen edge, where higher crystallinity forms a barrier. Cracks bifurcate to circumvent these regions, highlighting the critical role of spatial SIC heterogeneity in fracture resistance. Our results offer valuable insights into SIC mechanisms and contribute to the development of SIC-rubber materials with enhanced durability under complex deformation and high-temperature conditions.