Molecular Structures of the Buried Interfaces between Silicone Elastomer and Silane Adhesion Promoters Probed by Sum Frequency Generation Vibrational Spectroscopy and Molecular Dynamics Simulations

Silane adhesion promoters are commonly used to enhance the adhesion of elastomeric materials to polymers in many industrial applications. However, it is difficult to study the molecular-level mechanisms underlying adhesion promotion because adhesion occurs at the boundary between two layers, a buried interface that is difficult to probe with most techniques. Here, a surface/interface-sensitive optical technique, sum frequency generation vibrational spectroscopy, was used to probe the buried interfaces between the silicone elastomer and (3-glycidoxypropyl)trimethoxysilane (γ-GPS) as well as a known silane adhesion-promoting mixture of γ-GPS and methylvinylsiloxane (MVS). The γ-GPS methoxy groups were found to order at the silicone interface both in the neat silane and in the mixture with MVS. The interfacial structures between the silicone elastomer and two other silanes not used as adhesion promoters, n-octadecyltrimethoxysilane (OTMS) and (tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane (TDFTMS), and their mixtures with MVS, were also compared to those of γ-GPS and the γ-GPS/MVS mixture. It was found that these silanes behaved differently than the known adhesion-promoting mixture. Further, molecular dynamics simulations confirmed that all silanes showed broad, random orientation distributions at the silicone interface. Because only the known adhesion-promoting mixture of γ-GPS and MVS exhibited methoxy order at the silicone interface, as well as at the poly(ethylene terephthalate) interface, as shown in a previous publication, it is inferred that this ordering may be a necessary condition for adhesion promotion.