Effect of Long Chain Branches of High Melt Strength Polypropylene on Its Supercritical CO₂ Foaming Behavior

Polypropylene (PP) has a relatively low melt strength due to its linear structure, which seriously limits its supercritical CO 2 foaming performance. Introducing long‐chain branches (LCBs) via grafting can significantly enhance its melt strength. However, the relationship between the LCB level of high melt strength polypropylene (HMSPP) and its foaming behavior remains unclear. In this study, a series of HMSPP with different LCB levels was prepared using vinyl polydimethylsiloxane (VS) of varying viscosities as grafting monomers to investigate this relationship. Rheological analysis showed that the increase in viscosity of VS led to higher LCB levels in HMSPP. The melt strength of HMSPP increases with the increase of LCB levels, reaching up to 0.62 N, which is 13 times higher than that of the raw material. Supercritical CO 2 foaming results revealed that the expansion ratio of HMSPP first increased and then decreased with the increase of melt strength, reaching a maximum of 39.4 times. Combining experiments with simulations, the influence of LCB levels on the dissolution and diffusion behavior of CO 2 in HMSPP was clarified. This study deepens the understanding of the relationship between LCB and the foaming behavior of HMSPP, providing valuable insights for designing HMSPP with optimized foaming properties.