Molecular behavior of silicone adhesive at buried polymer interface studied by molecular dynamics simulation and sum frequency generation vibrational spectroscopy.

Silicones have excellent material properties and are used extensively in many applications, ranging from adhesives and lubricants to electrical insulation. To ensure strong adhesion of silicone adhesives to a wide variety of substrates, silane-based adhesion promotors are typically blended into the silicone adhesive formulation. However, little is known at the molecular level about the true silane adhesion promotion mechanism, which limits the ability to develop even more effective adhesion promoters. To understand the adhesion promotion mechanism of silane molecules at the molecular level, this study has used sum frequency generation vibrational spectroscopy (SFG) to determine the behavior of (3-glycidoxypropyl)trimethoxy silane (γ-GPS) at the buried interface between poly(ethylene terephthalate) (PET) and a bulk silicone adhesive. To complement and extend the SFG results, atomistic molecular dynamics (MD) simulations were applied to investigate molecular behavior and interfacial interaction of γ-GPS at the silicone/PET interface. Free energy computations were used to study the γ-GPS interaction in the sample system and determine the γ-GPS interfacial segregation mechanism. Both experiments and simulations consistently show that γ-GPS molecules prefer to segregate at the interface between PET and PDMS. The methoxy groups on γ-GPS molecules orient toward the PDMS polymer phase. The consistent picture of interfacial structure emerging from both simulation and experiment provides enhanced insight on how γ-GPS behaves in the silicone - PET system and illustrates why γ-GPS could improve the adhesion of silicone adhesive, leading to further understanding of silicone adhesion mechanisms useful in the design of silicone adhesives with improved performance.

Headgroup effect on silane structures at buried polymer/silane and polymer/polymer interfaces and their relations to adhesion.

Sum frequency generation (SFG) vibrational spectroscopy was used to study the effect of silane headgroups on the molecular interactions that occur between poly(ethylene terephthalate) (PET) and various epoxy silanes at the PET/silane and PET/silicone interfaces. Three different silanes were investigated: (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-glycidoxypropyl) methyl-dimethoxysilane (γ-GPMS), and (3-glycidoxypropyl) dimethyl-methoxysilane (γ-GPDMS). These silanes share the same backbone and epoxy end group but have different headgroups. SFG was used to examine the interfaces between PET and each of these silanes, as well as silanes mixed with methylvinylsiloxanol (MVS). We also examined the interfaces between PET and uncured or cured silicone with silanes or silane-MVS mixtures. Silanes with different headgroups were found to exhibit a variety of methoxy group interfacial segregation and ordering behaviors at various interfaces. The effect of MVS was also dependent upon silane headgroup choice, and the interfacial molecular structures of silane methoxy headgroups were found to differ at PET/silane and PET/silicone interfaces. Epoxy silanes have been widely used as adhesion promoters for polymer adhesives; therefore, the molecular structures probed using SFG were correlated to adhesion testing results to understand the molecular mechanisms of silicone-polymer adhesion. Our results demonstrated that silane methoxy headgroups play important roles in adhesion at the PET/silicone interfaces. The presence of MVS can change interfacial methoxy segregation and ordering, leading to different adhesion strengths.

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.