Developing a molecular-level understanding of adhesion using sum frequency generation vibrational spectroscopy

A better understanding of the molecular-level mechanisms of adhesion at the adhesive/adherend interface would facilitate the design of improved adhesives for challenging materials such as plastics. Sum frequency generation (SFG) vibrational spectroscopy is a nonlinear optical laser technique that is uniquely suited to study adhesion at buried interfaces. Here, SFG is employed to examine the adhesion of hydrosilylation-curing silicone elastomers to polymers such as poly(ethylene terephthalate) (PET). This research seeks to correlate adhesion to the molecular-level structures of polymer/silicone elastomer interfaces, as deduced by SFG, and to develop this technique into a non-destructive tool to evaluate adhesion in situ.; Organ osilane-based adhesion promoters are often used to improve the adhesion of silicones to many substrates but the mechanism of this enhancement is not yet fully developed. SFG was used to probe the interfaces between polymers and a number of silanes containing different functional groups. It was found that the conformation of interfacial silane molecules is dependent on both the surface-presenting groups on the polymer and the chemical groups on the silane. SFG spectra revealed that silanes that contain hydrogen bond donors, such as 3-aminopropyltrimethoxysilane, can hydrogen bond to the PET surface ester carbonyl groups.; A mixture of (3-glycidoxypropyl)trimethoxysilane and a methylvinylsiloxanol has been shown to enhance the adhesion of silicone elastomers to some plastics. The interfaces between this silane adhesion-promoting mixture (SAPM) and polymers such as PET, poly(methyl methacrylate), and polystyrene were studied by SFG. Insight was gained into the important coordinating roles played by oligomeric siloxanes and organosilanes in the adhesion of silicone elastomers to polymers.; After gaining a better understanding of the conformation, ordering, and interactions between organosilane-based adhesion promoters and polymeric surfaces, these adhesion-promoting molecules were incorporated into a hydrosilylation-curing silicone elastomer. The buried polymer/silicone elastomer interfaces were investigated by SFG and the interfacial segregation of the silane molecules was detected. By curing the polymer/silicone elastomer specimens under a linear thermal gradient, adhesion was directly correlated with the SFG spectra collected from these interfaces.