Understanding molecular structures of silanes at buried polymer interfaces using sum frequency generation vibrational spectroscopy and relating interfacial structures to polymer adhesion.

The use of silane adhesion promoters to improve adhesion of elastomeric materials to polymers has become increasingly common in many industrial applications. However, little is understood about the molecular-level mechanisms of how adhesion promoters enhance adhesion. Here, sum frequency generation (SFG) vibrational spectroscopy was used to probe the buried interface between poly(ethylene terephthalate) (PET) and (3-glycidoxypropyl)trimethoxysilane (gamma-GPS), and the interface between PET and a mixture of gamma-GPS and a methylvinylsiloxanol (MVS), a known adhesion-promoting mixture. Furthermore, the interfaces between PET and uncured silicone with incorporated silane or silane mixture and the interfaces between PET and cured silicone with incorporated silane or silane mixture were studied. The gamma-GPS methoxy groups were found to order at the polymer interface and the presence of MVS increased the interfacial segregation and/or order of gamma-GPS. For comparison, two other silanes, N-octadecyltrimethoxysilane (OTMS) and (tridecafluoro-1,1,2,2-tetrahydroctyl)trimethoxysilane (TDFTMS), as well as their mixtures with MVS were also studied at the various interfaces, and were found to exhibit different interfacial behaviors than gamma-GPS and the known silane adhesion-promoting mixture of gamma-GPS and MVS. Further, X-ray photoelectron spectroscopy (XPS) was used to investigate the exposed PET surfaces resulting from peeling the PET/cured silicone elastomer with TDFTMS and with the TDFTMS/MVS mixture interfaces, and it was shown that the fluorinated silane does segregate to the polymer interface. When correlated to adhesion testing results, it is inferred that segregation and ordering of the silane methoxy groups at the polymer/silane and polymer/silicone elastomer interfaces is crucial for adhesion promotion in this system.