Role of Compatibilizer and Blend Ratio in Governing the Mechanical, Thermal, and Morphological Behavior of NR-g-N-(4-Hydroxyphenyl)Maleimide/Polypropylene Thermoplastic Vulcanizates

Abstract

Thermoplastic vulcanizates (TPVs) based on natural rubber grafted with N-(4-hydroxyphenyl)maleimide (NR-g-HPM) and polypropylene (PP) were developed via dynamic vulcanization to clarify the synergistic roles of HPM grafting, compatibilizer chemistry, and blend ratio on interfacial structure and macroscopic properties. Incorporation of HPM introduces phenolic and maleimide functionalities onto NR chains, enabling enhanced interfacial interactions with phenolic-modified polypropylene compatibilizers (PhHRJ-PP and PhSP-PP). Compared with PP-g-MA, phenolic compatibilization in the NR-g-HPM/PP (60/40) system produced higher plateau mixing torque, superior tensile strength, lower tension set, and significantly refined rubber morphology. The PhHRJ-PP-compatibilized TPV exhibited the finest rubber dispersion, with an average domain size of ≈2.2 µm, which is substantially smaller than that of the uncompatibilized control (≈4.0 µm) and the PP-g-MA-compatibilized system (≈6.1 µm). This refined morphology resulted in a higher storage modulus and reduced viscous dissipation. When varying the blend ratio, the 50/50 NR-g-HPM/PP composition delivered the optimum balance of properties, achieving the highest tensile strength (15.86 MPa), lowest permanent deformation, and finest rubber dispersion (≈2.0 µm). The results demonstrate that HPM grafting plays a decisive role in activating phenolic-assisted interfacial coupling, providing a new and effective pathway for designing high-performance NR-based TPVs with tailored morphology and durability.