Interfaces are important in many areas including adhesion, friction, coatings, nanocomposites, heat transfer, biomedical implants and cell biology. Most interfacial experiments involve force measurements and, consequently, molecular models to explain the force results. To make progress in this field, it is necessary to understand the structure of interfacial molecules in direct contact. Here, we have used the interface sensitivity of infrared-visible sum frequency generation spectroscopy (SFG) to probe the molecular structure of contact and sliding interfaces in-situ along with force measurements. SFG is a second order nonlinear optical technique that provides information on the chemical structure, orientation and concentration of molecules at interfaces.
Two types of interfaces are probed in the current study. The initial two studies focus on static interfaces and the remaining three on dynamic interfaces. In the first investigation, we have studied polar interactions at solid-liquid and solid-solid interfaces using SFG spectroscopy. The shift of the sapphire surface hydroxyl peak in contact with several polar and non-polar liquids and polymers is used to determine the interaction energy. The trend in the interaction energies cannot be explained by only measuring water contact angles. Molecular rearrangements at the sapphire interface, to maximize the acid-base interactions, play a dominant role and these effects are not accounted for in the current theoretical models.
The second investigation probes the interactions of polystyrene (PS)-poly(methyl methacrylate) (PMMA) blends with a sapphire surface. The acid-base interaction of carboxyl groups with surface hydroxyl groups is a strong driving force for segregation of PMMA next to the sapphire surface. Even with 0.005 weight fraction of PMMA in the blend, the concentration at the sapphire-blend interface is similar to that of bulk PMMA. This result is significant for understanding and controlling the interfaces responsible for reinforcement in composites and the durability of coatings.
The last three experiments deal with dynamic interfaces. 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 un-aged lens, indicating that these changes in tribological properties cannot be due to surface contamination or degradation. However, we observed that the SFG intensity of the PDMS Si-CH3 symmetric band is three 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 have important consequences in understanding adhesion hysteresis and friction.
We have also investigated the velocity dependence of rubber friction. Unlike rigid materials like steel or wood, the friction of rubber depends on sliding velocity. Schallamach proposed that the surface chains of an elastomer adhere to the counter surface and go through a series of stretching and relaxing cycles during sliding. The magnitude of stretching increases with increasing velocity. Interfacial SFG spectra reveal that the PDMS Si-CH3 symmetric band intensity increases with increasing sliding velocity. Interestingly, the relaxation time of this ordered peak also increases with increasing sliding velocity. These results may provide the first experimental proof of chain stretching and chain relaxation during sliding, as postulated by Schallamach in 1969.
The final study involved alkane lubricated sliding of a PDMS lens on a sapphire surface. The femtosecond SFG spectra of the PDMS-sapphire interface shows ordering of the sandwiched pentadecane molecules at the edges of the lens during sliding and retraction. Even though the experiments were conducted well above the pentadecane melting point, the spectra indicated order similar to that in a crystal. The intensity of the SFG peak maximized when the lenses were retracted - just before jumping-out of contact. These results suggest that the crack formation results in ordering and perhaps crystallization of alkanes at the crack edges. However, the broad temperature dependence of the relaxation of this ordered interface suggests glassy behavior rather than crystallinity.