Mechanical Properties and Swelling Behaviors of Ultrathin Chemically Cross-Linked Polybutadiene Films

With the advancement of nanoscience and nanotechnology, understanding the unique mechanical properties of ultrathin polymer films is becoming increasingly important. We report the mechanical properties and swelling behaviors of ultrathin chemically cross-linked polybutadiene films. Upon reducing the film thickness, the Young’s modulus increased, and an unusual stress-relaxation behavior was observed. After stress relaxation, the fully relaxed modulus was significantly higher than that expected from the amount of added cross-linker and the entanglement degree. The out-of-plane swelling of the ultrathin films prepared on a substrate was comparable to that in the bulk, whereas the in-plane swelling of films floating on water was significantly suppressed. Such suppression of the in-plane deformation may be attributed to the effects of interface-induced orientations of the chains and the chain segments trapped at the interfaces acting as pseudo-cross-linking points. This study contributes to an understanding of the physical properties of ultrathin cross-linked elastomer films.