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.

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We present results of adhesion studies of poly(dimethylsiloxane) (PDMS) crosslinked hemispheres, before and after extraction, carried out using contact mechanics (the JKR apparatus.) Three types of PDMS networks were prepared with increasing crosslinker-to-PDMS ratio (r ) 1.7, 2.2, 2.7). Adhesion hysteresis was observed only after the PDMS hemispheres were extracted (toluene, Soxhlet, 20-24 h.) The size of hysteresis increased systematically with the decreasing ratio (increasing percentage of extracted weight.) Brief extraction yielded smaller hysteresis, which disappeared after 12 days. It is concluded that the entanglement of tethered chains and their interdigitation with cross-linked PDMS network at the interface may be the dominating mechanisms for the observed adhesion hysteresis.

Adhesion of poly(dimethylsiloxane) cross-linked networks to silicon oxide surfaces was studied using the Johnson−Kendall−Roberts method of contact mechanics. An increase in adhesion hysteresis was observed in the order of increasing molecular weight between cross-links. The log/log plots of energy release rates, G, vs molecular weights (M n), for different crack propagation rates (−da/dt), gave a family of straight lines, with their slopes increasing with increasing −da/dt. When a plot of the slopes vs crack propagation rates was extrapolated to −da/dt = 0, it was found that the slope at zero velocity is equal to 0.47, in agreement with the 1/2 power predicted by de Gennes (C. R. Acad. Sci. Paris 1995, 320, 193). On the other hand, when the network was kept for 30 min at maximum load before unloading, the scaling factor was 1/3. This decrease in scaling factor is attributed to the molecular weight dependence of network relaxation rates. The results suggest that the polymer chain in a network, when in contact with a surface under conditions close to equilibrium, behaves like a chain in a polymer melt near a surface.

A study of acid−base interactions in the adhesion between solid surfaces is presented. Adhesion was studied between various OH-functionalized self-assembled monolayer (SAM) surfaces and poly(dimethylsiloxane) (PDMS) cross-linked networks using the contact deformation mechanics method. 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]. Systematic control of surface OH concentration revealed that the strength of adhesion is proportional to the number of surface OH groups but that the relationship is highly nonlinear, probably due to the collective nature of H-bonding between the PDMS chain and surface OH groups. Intermolecular H-bonding at the surface of phosphonic acid (11-mercaptoundecyl phosphonic acid SAMs) results in decreased adhesion.