Effects of epoxidation degree on strain-induced crystallization and mechanical properties of epoxidized natural rubber

The epoxidation degree (ED) plays a critical role in shaping the microstructure and macroscopic performance of epoxidized natural rubber (ENR). Through the application of Synchrotron Radiation-Wide Angle X-ray Diffraction (SR-WAXD), polarized Fourier Transform Infrared (FTIR) spectroscopy, and Atomic Force Microscopy (AFM), the effects of different ED on the strain-induced crystallization (SIC) behavior, mechanical properties, and molecular chain orientation of ENRs were systematically investigated. The findings reveal that at a given strain there is an optimum ED with regard to crystallinity and mechanical performance. Notably, ENR10 (with an ED of 10 %) demonstrated the highest tensile strength of 30.8 MPa and tear strength of 35.9 kN/m, corresponding to improvements of 27.8 % and 40.2 %, respectively, compared to NR. The influence of the ED is most pronounced in the orientation and crystallite growth. For ENR with a low ED, epoxy groups enhance molecular chain orientation and SIC, facilitating the formation of more crystallites. The increased crystallite number allows for the rapid establishment of a crystallite network, promoting continuous crystallite growth and cross-linking. Consequently, significant increases in tensile and tear strength are observed. In contrast, ENR with a high ED reduced crystallite growth due to steric hindrance and increased interactions among the densely packed epoxy groups. This effect impairs crystallite formation and diminishes SIC capability, resulting in a notable decline in both crystallinity and mechanical properties.