Structure–Property Relationship of Diamine Cross-Linked Acrylate-Modified Natural Rubber: Improving Mechanical Properties, Shape Memory Behavior, and Thermal Stability of the Vulcanizates

In this work, poly(methyl methacrylate)-grafted natural rubber (MGNR) was cross-linked with various diamine cross-linkers. After curing, it was demonstrated that a three-dimensional network of MGNR was formed through amide linkage formation between ester groups of MGNR and –NH2 groups of cross-linkers. The curing rate and mechanical and thermal properties strongly depended on the diamine type. The diamine-containing benzene rings enhanced stiffness and reduced elongation at break, whereas long-chain aliphatic diamine provided better flexibility. Thermal analysis revealed that cross-linking with diamine did not affect the Tg of the natural rubber (NR) phase, whereas the Tg of grafted PMMA domains changed with diamine content. At the same time, diamine-cured MGNR also demonstrated good shape fixity and recovery due to the vitrification of grafted PMMA and cross-link networks. The thermal decomposition temperature and retention of the mechanical properties after thermal treatment were significantly enhanced for diamine-cured MGNR, particularly for MGNR cured with diamine-containing benzene rings. The findings pave the way for developing high-performance MGNR composites with tailored properties for various applications. The specific diamine structures play a role in the degree of property enhancement, offering opportunities for optimizing the composites for particular needs.