To better understand the effects of interfacial molecular orientation on adhesion to plastics, the interfaces between poly(ethylene terephthalate) (PET) and different silane coupling agents were probed using sum frequency generation (SFG) vibrational spectroscopy. The polymer/air interface was dominated by the ester carbonyl, methylene, and phenyl groups. Upon contacting the PET film with the amino-functional silane 3-aminopropyltrimethoxysilane (ATMS), the ester carbonyl stretch shifted to a lower energy indicating the formation of hydrogen bonds between the polymer surface and the silane molecules. This shift was not observed when silanes that contained no hydrogen bond donors, such as (3-glycidoxypropyl)-trimethoxysilane and n-butyltrimethoxysilane, were placed into contact with the PET surface. Further evidence of silane ordering at the interface was observed as vibrational peaks attributed to the C-H stretching of the silane methoxy headgroups dominated the PET/silane spectra. It was determined that the conformation of the ATMS molecules at the interface was such that the amino endgroups were oriented toward the interface while the methoxy headgroups were directed toward the silane bulk.

Buried polymer/silane interfacial molecular structures were examined using sum frequency generation (SFG) vibrational spectroscopy. The silanes under investigation included n-octadecyltrichlorosilane (OTCS), n-octadecyltrimethoxysilane (OTMS), and (3-aminopropyl)trimethoxysilane (ATMS). Poly(methyl methacrylate) (PMMA) and polystyrene (PS) were among the polymers studied, and to avoid spectral confusion, deuterated polymers (d-PMMA and d-PS) were also employed. When OTCS or OTMS contacted a polymer surface, a stable and ordered interface immediately formed. It was found that the silane molecules could adopt different conformations at the interface, depending on the surface structure of the polymer. Such interfacial structures revealed evidence of interfacial molecular interactions at polymer/silane interfaces. Conversely, ATMS molecules tended to become disordered while interacting with the polymer surface, accompanied by the loss of polymer surface order. Through our SFG studies, we demonstrated that by varying the chemistries of both the silane and polymer molecules, the polymer/silane interfacial structures could be altered at the molecular level. Our observations of these different polymer/silane interfaces should further the understanding of the relationships between molecular interfacial structures and properties, such as adhesion.

Sum frequency generation (SFG) vibrational spectroscopy was used to probe the interface between poly(ethylene terephthalate) with deuterated ethylene glycol subunits (d4-PET) and a silane adhesion-promoting mixture (SAPM) comprised of (3-glycidoxypropyl)trimethoxysilane (gamma-GPS) and a methylvinylsiloxanol (MVS). Such a mixture has been found to improve the adhesion of an addition-curing silicone elastomer to a range of plastic and metal substrates. Our results demonstrated that at the interface between d4-PET and a SAPM with a gamma-GPS/MVS ratio of 1:1 (w/w), the silane molecules not only segregated to the interface but also the methoxy headgroups likely adopted a greater net orientational order along the surface normal than at the d4-PET/gamma-GPS interface. The effects of varying the silane/siloxane ratio and using different siloxane oligomers on interfacial structures were also examined. This study provides unique molecular-level insights into the prerequisite conditions for adhesion of curable silicone adhesives.