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.

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Using fluorescence spectroscopy, we have determined the extent of preferential solvation of the dye PRODAN (6propionyl(VV,7V-dimethylamino)naphthalene) in binary mixtures where one component is capable of hydrogen bonding. In mixtures of methanol with acetone, acetonitrile, or pyridine the spectroscopically determined mole fraction of the PRO• DAN-methanol complex was much higher than the methanol mole fraction. The amount of preferential solvation follows the trend acetonitrile > acetone > pyridine with corresponding stoichiometries of 1.1,1.2, and 0.93 and equilibrium constants of 12.0,7.9, and 1.7. The amount of preferential solvation can be correlated to the heat of mixing of the solvent pair making up the mixture or to the donor number of the non-hydrogen-bonding solvent. This former correlation exists because preferential solvation of the probe involves unmixing of the solvents. For mixtures where hydrogen bonding between the probe and the cosolvent do not occur, we find no evidence of preferential solvation due to dielectric enrichment when the dielectric properties of the mixtures are taken into account.

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.

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