The effect of a silane coupling agent on water adsorption at a metal/polymer interface studied by neutron reflectivity and angle-resolved X-ray photoelectron spectroscopy

Abstract

Neutron reflectivity and angle-resolved X-ray photoelectron spectroscopy (XPS) have been used to examine the adsorption of water to a molybdenum/polyurethane interface. The initial quasi-steady state interfacial D2O profile (obtained after ∼1 h exposure to D2O saturated air at ambient temperature) was determined on a nanometre scale by neutron reflectivity. The profile was obtained as a function of the concentration of a silane coupling agent mixed into the bulk of the polymer. A large concentration of D2O (> 80 vol %) was observed at the interface when no silane was present. This is interpreted as a partial delamination. Roughly 2 wt% silane led to a sharp reduction in the amount of D2O at the interface. This is attributed to the silane diffusing to the interface and promoting adhesion. Samples conditioned over longer periods of time and at elevated temperature (∼ 16 h, 100% humidity at 80 °C) were examined by angle-resolved XPS. For all the conditioned samples the appearance of an additional molybdenum oxidation state, and significant dissolution of the molybdenum oxide into the polymer layer were observed. These effects were readily apparent even for samples with 2 wt% silane, although the effects were slightly more pronounced with decreasing concentration of silane. Therefore, we conclude that the silane promotes adhesion of the polyurethane to the molybdenum oxide surface, but does not provide an impenetrable barrier to water or restrict its degradative effects.