Influence of crosslinking network structure on silicone rubber foaming: A LF-NMR and rheological study

Silicone rubber foam is recognized for its heat resistance and lightweight properties, making it suitable for a variety of applications. However, the relationship between the formulation, crosslinking network structure, rheological properties and foaming behavior is not thoroughly understood. In this work, the contents of crosslinked chains, dangling chains and free chains were characterized by low-field nuclear magnetic resonance (LF-NMR), which revealed the transformation of dangling chains into crosslinked chains with increasing the content of crosslinking agent. Additionally, utilizing chain segment information and rheological results, it was found that the densely crosslinking network structure of silicone rubber results in a storage modulus significantly exceeding its loss modulus. This leads to a smaller cell diameter in silicone rubber foam compared to thermoplastic polymers such as PP and PET, consequently resulting in a lower expansion ratio for silicone rubber. This study also investigated the mechanical properties of samples with different cell morphologies. The findings provide insights into the differences in foaming behavior between silicone rubber and thermoplastic polymers.