Structure–Property Tailoring of Natural-Rubber-Based Polypropylene Thermoplastic Vulcanizates via Rubber Polarity and N-(4-Hydroxyphenyl)maleimide  Grafting

This study presents a molecular-design strategy for tailoring polypropylene (PP)-based thermoplastic vulcanizates (TPVs) through natural rubber (NR) type variation and N-(4-hydroxyphenyl)maleimide (HPM) grafting. The novelty lies in the direct comparison of NR, epoxidized natural rubber with 25 mol% (ENR-25), NR-g-HPM, and ENR-25-g-HPM as rubber phases in phenolic modified PP (PhHRJ-PP)-compatibilized TPVs, enabling the role of rubber polarity and HPM functionality to be correlated with mechanical, morphological, dynamic, solvent-resistance, and thermal properties. FTIR analysis confirmed the successful grafting of HPM onto ENR-25 through characteristic imide carbonyl, aromatic, and phenolic hydroxyl absorptions. Among all TPVs, NR-g-HPM/PP showed the best mechanical balance, with tensile strength increasing from 11.35 MPa for NR/PP to 13.25 MPa, elongation at break from 351.2% to 368.8%, and tension set decreasing from 30% to 28%. SEM analysis revealed the smallest vulcanized rubber-domain size for NR-g-HPM/PP, decreasing from 2.715 ± 0.557 μm for NR/PP to 2.109 ± 0.256 μm. In contrast, ENR-g-HPM/PP exhibited superior solvent and thermal resistance, with engine-oil swelling reduced to 3.70% and the highest first-step degradation temperature of 411.27 °C. These results demonstrate that NR-g-HPM is preferred for toughness and elasticity, whereas ENR-g-HPM is advantageous for oil-resistant and thermally stable TPVs.