Adhesion studies using contact mechanics

We report adhesion studies using the JKR contact mechanics method and poly(dimethylsiloxane) (PDMS) cross-linked networks. When such networks are extracted, interdigitation of tethered chains into extraction zones at the network-network interface results in a significant enhancement of adhesion. When these networks come into contact with a silicon oxide surface, an increase in adhesion hysteresis is observed in the order of increasing molecular weight between cross-links, with a scaling factor of 1/2. Subjecting the system to maximum load for 30 min before unloading results in a scaling factor of 1/3, probably due to molecular weight dependent network relaxation rates. PDMS cross-link networks form H-bonds with surface Si-OH groups that result in significant and time-dependent adhesion increase. Systematic studies of interfacial H-bonding and its effect on adhesion have been carried out using self-assembled monolayer (SAM) surfaces. An increase in adhesion strength was observed in the order of increasing acidity of surface OH protons [Au/S(CH₂)₁₁OH < Au/S(C₆H₄)₂OH < Au/S(CH₂)₁₅COOH < Au/ S(CH₂)₁₁PO(OH)₂]. When surface OH concentration was controlled, the strength of adhesion was found to be proportional to the number of surface OH groups, with nonlinear relationship, probably due to the collective nature of H-bonding between the PDMS chain and surface OH groups. Isotope effect studies confirmed the formation of interfacial H-bonding.