Molecular mechanism of the improved adhesion between PDMS adhesive and silicon substrate after thermal treatment

An interesting experimental observation recently found that thermal treatment of polydimethylsiloxane (PDMS) adhesives and the contact adhesion time can significantly improve the interfacial adhesion. However, the mechanism underlying such adhesion enhancement is unclear. To disclose the micro-mechanical mechanism of the adhesion improvement, contrast experiments are systematically carried out with PDMS adhesives respectively adhering on silicon and silicon dioxide substrates in the present paper. For the case of thermal treatment of PDMS adhesives while on the substrates, the adhesion strength of a PDMS adhesive on a silicon substrate is significantly stronger than that on a silicon dioxide substrate after thermal treatment at different temperatures, and the adhesion strength on both substrates increases with increasing thermal treatment temperature and contact time. For the case of first thermally treating the PDMS adhesive and then adhering to the silicon substrate, the adhesion strength is much smaller than that of thermally treating PDMS adhesives while on silicon substrates. This is mainly because the migration and restructuring of the PDMS molecular chains form intimate contact between the PDMS adhesive and substrate after thermal treatment. Meanwhile, high temperature could lead to the easy formation of hydrogen bonds at the interface between the PDMS adhesive and silicon substrate. The surface of silicon provides more favorable conditions to form hydrogen bonds than the silicon dioxide surface, leading to the stronger adhesion of PDMS adhesives on silicon substrates after thermal treatment. If the silicon dioxide surface is modified and grafted with hydroxyl (–OH) groups, which are one of the main groups forming hydrogen bonds with the PDMS material, the adhesion strength of the PDMS adhesive on the modified silicon dioxide substrate is approximately the same as that on the silicon substrate after thermal treatment. The results obtained in the present study could provide a new and convenient design strategy to achieve strong attachment without any surface microstructures.