Low-Temperature Toughening of PPR Composites Based on Isothermal Treatment by Forming Hard-Shell Soft-Core Structure and Regulating β-Crystals

Achieving a superior toughness-rigidity balance in high-performance polymer composites at subzero temperatures is critical for extreme environment applications. However, current toughening strategies relying on high elastomer or nanofiller loadings inevitably suffer from severe stiffness degradation or limited low-temperature toughening efficiency. Herein, a combined strategy that integrates an in situ soft-core/hard-shell architecture with β-crystallization via isothermal annealing is proposed to achieve an exceptional low-temperature toughness-rigidity balance in polypropylene random copolymer/ethylene-propylene rubber/silica (PPR/EPR/SiO2) nanocomposites. The in situ formed EPR@SiO2 core–shell particles, together with the robust crystalline network, efficiently reinforce interfacial load transfer, thereby ensuring an exceptional retention of mechanical rigidity. Simultaneously, the reduced particle spacing, the formation of ductile β-crystals, and the relaxation of frozen orientation and internal microstresses in amorphous chain segments collectively enhance chain segment mobility, markedly driving down the brittle-ductile transition temperature (Tbd) to −40 °C. Compared with the unmodified counterpart, the composite with combined modifications achieves a superior low-temperature toughness. The impact strength at −20 °C attained its highest value of 68 kJ/m2, accompanied by a reduction of 8.8% in Young’s modulus relative to neat PPR. It is envisioned that this work establishes a promising design paradigm for polymer composites requiring simultaneous high low-temperature toughness and rigidity.