Effect of chemical functionality on adhesion hysteresis

Studies of adhesion and adhesion hysteresis were carried out using cross-linked poly(dimethyl siloxane) (PDMS) semispherical surfaces and self-assembled model surfaces containing different chemical functionalities, using the JKR method, the contact mechanics of solids spreading their interfacial area under load. Hysteresis resulting from fast relaxation processes in the PDMS elastomer network was practically eliminated using stepwise loading and unloading protocols. The interfacial H-bonding between PDMS and both Si-OH and -COOH surfaces was shown to be an important chemical interaction causing significant adhesion hysteresis. The number of H-bonds between PDMS and Si-OH surfaces increased with contact time under compressive load, indicating pressure-induced reorganization of the PDMS network near the interface that increased the number of H-bonds. The interaction between PDMS and functionalized biphenylthiolate monolayers exhibited a smaller hysteresis, which is believed to be caused by dipolar interaction, whereas that between PDMS and nonpolar perfluorocarbon groups showed negligible hysteresis. The distinction in the behavior of the unloading data between H-bonding related interaction and dipolar interaction seems to indicate the difference in the nature between nonspecific (van der Waals, dipolar) and specific (donor-acceptor, H-bond, acid-base) interactions.