Studies of adhesion to molecularly engineered surfaces using contact mechanics methods

We synthesized a number of poly(dimethylsiloxane) (PDMS) polymers with different molecular weights, and prepared model PDMS networks with a systematic change of cross-link density, and therefore elastic constant (K). Self-adhesion studies of PDMS hemispheres using the JKR method - the contact mechanics of solids spreading their interfacial area under load - reveal that hysteresis which results from relaxation processes in the elastomer can be practically eliminated using stepwise loading and unloading protocols. The adhesion of crosslinked PDMS surfaces to self-assembled monolayers with different chemical functionality was investigated. Interfacial H-bonding was shown to be an important chemical interaction causing significant adhesion hysteresis. The number of H-bonds between PDMS and silanol groups on SiO₂/Si surfaces increased with time of the contact under a constant load, indicating pressure-induced reorganization of the PDMS network near the interface. The interaction between PDMS and carboxylic acid groups showed somewhat smaller hysteresis which suggests weaker H-bonding strength. The interaction between PDMS and functionalized biphenyl groups exhibited small 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 non-specific (van der Waals, dipolar) and specific (donor-acceptor, H-bond, acid-base) interactions.