Evolution of Polystyrene Adhesion on a Si Substrate Coupled with Interfacial Structural Relaxation

Adhesion using polymers to assemble various materials has been attracting attention as an alternative to mechanical bonding. This is because polymer adhesion promotes lighter weight and higher toughness in structural materials, such as in mobility applications. Polymer adhesion is expected to depend on van der Waals forces and is considered to develop at the point of contact between the polymer and the adherend. However, it is also known empirically that adhesive force increases over time after contact. This is expected to be related to the structural relaxation of polymers, however, it is still open. In this study, with the aim of elucidating the polymer adhesion mechanism at the molecular level, we systematically changed the condition of the annealing process for polystyrene (PS) thin films on the adherend, Si substrates, and examined the effects of heat treatment on the adhesion properties. The adhesion strength drastically increased after thermal annealing at 50 K above the glass transition temperature for 1 h. In addition, the adhesive strength gently increased as the annealing time. This indicates that the adhesion strength depended on not only van der Waals forces but also the conformation of polymer chains at the interface.