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

Abstract

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.

Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface

“…The peak centered around 3000–3010 cm –1 is assigned to the epoxy C–H stretching of γ-GPS, indicating the presence of the epoxy end group of γ-GPS at the silicone/d4-PET interface with order . The sharp peak at 2840–2845 cm –1 is assigned to the symmetric C–H stretching of the methoxy head group in γ-GPS. , The peaks at 2875 and 2925–2935 cm –1 are assigned to the symmetric and anti-symmetric −CH 2 stretching, respectively, from the backbone methylene groups of DMMVS and γ-GPS as well as the curing agent.…”

“…Previously, we have carried out systematic studies to reveal the adhesion promotion mechanism of alkoxysilane-siloxane mixtures on aliphatic group-deuterated polyethylene terephthalate (d4-PET) using SFG. − ,− The interfacial ordering of the three methoxy end groups of (3-glycidoxypropyl)­trimethoxysilane (γ-GPS, which is an alkoxysilane, Scheme ) was found to play important roles in determining adhesion strength at the silicone/PET interface. The addition of DMMVS (hydroxyl-terminated dimethyl methylvinyl co-siloxanol, Scheme ) caused more ordering of the methoxy groups at the d4-PET/silicone interface before curing and, therefore, promoted the adhesion by the interfacial chemical reactions between methoxy groups and hydroxyl end groups of PET during the curing process. , …”

Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

“…Two different silanes, n -octadecyltrimethoxysilane (OTMS, CH 3 (CH 2 ) 7 Si(OCH 3 ) 3 ) and (tridecafluoro-1,1,2,2-tetrahydroctyl)trimethoxysilane (TDFTMS, CF 3 (CF 2 ) 5 (CH 2 ) 2 Si(OCH 3 ) 3 ), and their mixtures with MVS in contact with cured PDMS have also been examined using SFG. Molecular dynamics simulation was used to confirm SFG results [161]. …”

Investigating Thin Silicone Oil Films Using Four-Wave Mixing Spectroscopy and Sum Frequency Generation Vibrational Spectroscopy

“…There is a need to develop an ultrasensitive technique which could be used to study the distribution of ultrathin silicone oil layers on surfaces. In recent decades, second-order nonlinear optical spectroscopic techniques such as sum frequency generation (SFG) vibrational spectroscopy and second-harmonic generation (SHG) have been applied to study molecules (including proteins) on surfaces and at interfaces. − Such research demonstrated that second-order nonlinear optical spectroscopic techniques are very promising in detecting monolayer and even submonolayer molecules at interfaces in situ. − SFG has been applied to examine the molecular structures of surfaces and interfaces involving solid silicone and silicone oil to understand important interfacial properties such as adhesion, friction, antifouling, and fouling release. − In particular, SFG has been applied to study thin films of silicone on water surfaces . SFG provides chemical and orientational information on the molecular structure of chemical groups at interfaces.…”