We have studied acid-base interactions at solid-liquid and solid-solid interfaces using interface-sensitive sum frequency generation (SFG) spectroscopy. The shift of the sapphire hydroxyl peak in contact with several polar and nonpolar liquids and polymers was used to determine the interaction energy. The trend in the interaction energies cannot be explained by measuring only water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role, and these effects are not accounted for in the current theoretical models. These results provide important insights into understanding adhesion, friction, and wetting on solid interfaces.

Infrared-visible sum frequency generation spectroscopy (SFG) was used to measure the interfacial concentrations of poly(methyl methacrylate) (PMMA)/polystyrene (PS) blends next to a sapphire substrate. The acid-base interactions of carbonyl groups of PMMA with the hydroxyl groups on the sapphire drive the interfacial segregation of PMMA next to the sapphire substrate. Using the shift of sapphire surface OH peaks, we have determined the difference in interfacial energy between the PMMA/sapphire and the PS/sapphire to be ~44-45 mJ/m(2). These results highlight the importance of acid-base interactions and their role in controlling the interfacial segregation next to solid substrates in polymer blends.

We have observed an unusual increase in adhesion hysteresis and frictional forces for poly-(dimethylsiloxane) (PDMS) lenses sliding on smooth glassy surfaces after a period of aging in a laboratory environment. X-ray photoelectron spectroscopy, contact angle, and in situ surface-sensitive sum frequency generation spectroscopy (SFG) measurements show no differences between an aged and unaged lens, indicating that these changes in tribological properties cannot be due to surface contaminant or degradation. Instead, we observed that the SFG intensity of the PDMS Si-CH 3 symmetric band is 3 orders of magnitude higher after sliding the aged lenses. Such a large increase in the SFG signal can only arise from very well-ordered PDMS molecules induced by sliding and has important consequences in understanding adhesion hysteresis and friction.

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