SEPS Elastomer-induced β-Crystallization and the Role of Isothermal Crystallization on Enhancing the Low-Temperature Toughness of Polypropylene

Abstract

In the quest to develop high-performance polypropylene (PP) blends with enhanced toughness, a blend was prepared by integrating poly(styrene-ethylene/propylene-styrene) (SEPS), which functions not only as an elastomer but also as a β-nucleating agent. The dispersion with a smaller interparticle distance of SEPS and its formation of an entangled network within PP significantly enhance the material’s toughness, shifting the brittle-ductile transition temperature to lower values. Under natural cooling conditions, only a low amount of β-crystals forms. However, isothermal crystallization of PP with only elastomer added leads to a substantial increase in the quantity and aggregation of β-spherulites, resulting in enhanced intercrystalline connectivity and the development of “flower”-like aggregates. This unique microstructure, facilitated by SEPS, is highly effective in stress transfer and impact energy absorption. After isothermal crystallization, the relative content of β-crystals increases significantly, from 9.00% to 29.55%, leading to a remarkable 89.95% increase in the impact strength of PP/SEPS blends at -10°C. Importantly, the β-crystals produced through isothermal crystallization with SEPS as a nucleating agent exhibit greater stability compared to those formed using other β-nucleating agents.