Studies of the aging of nafion®/silicate nanocomposites using 29Si solid state NMR spectroscopy

Abstract

AbstractSol‐gel reactions of tetraethylorthosilicate were conducted within the nanophase‐separated morphology of Nafion® films to yield Nafion®/silicate nanocomposites. The chemical aging of silicate phase molecular structure when the nanocomposites were exposed to (1) ambient air, (2) applied heat, (3) liquid water, (4) methanol reflux, and (5) heated sulfuric acid was explored by means of 29Si solid state NMR spectroscopy. In particular, the characteristic chemical shifts and relative Intensities of peaks that identify Qn = (HO)4‐n Si(OSi)n molecular subunits in silicate structures were determined for samples exposed to these aging conditions. Shifts in the Q peak distribution reflected the evolution of degree of Si atom coordination about SiO4 units. The coordination states are always Q4 and Q3 although a small Q2 population is occasionally present. Thus, a significant degree of silicate phase coordination can develop despite constraints posed by the Nafion® medium. Percent Q4 increases in all cases but then reaches a maximum after which it decreases, the greatest de‐polymerization being for samples aged in water. The greatest increase in Q4 relative to the un‐aged control occurred for samples heated at 100°C. Exposure to ambient air produced changes in Q4 that were small, but not insignificant. While the initial increase in Si atom substitution around SiO4 units can be rationalized in terms of gradual condensation reactions between residual SiOH groups, accompanied by the liberation of water molecules and their removal from the site of the reaction, the eventual decrease in coordination is not currently understood.