Studies of the aging of Nafion (R)/silicate nanocomposites using Si-29 solid state NMR spectroscopy

Sol-gel reactions of tetraethylorthosilicate were conducted within the nano phase-separated morphology of Nafion (R) films to yield Nafion (R) /silicat e 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 Si-29 solid state NMR spectroscopy. In particular, the charact eristic chemical shifts and relative intensities of peaks that identify Q(n ) = (HO)(4-n)Si(OSi)(n) molecular subunits in silicate structures were dete rmined for samples exposed to these aging conditions. Shifts in the Q peak distribution reflected the evolution of degree of Si atom coordination abou t SiO4 units. The coordination states are always Q(4) and Q(3) although a s mall Q(2) population is occasionally present. Thus, a significant degree of silicate phase coordination can develop despite constraints posed by the N afion (R) medium. Percent Q(4) increases in all cases but then reaches a ma ximum after which it decreases, the greatest de-polymerization being for sa mples aged in water. The greatest increase in Q relative to the un-aged con trol occurred for samples heated at 100 degreesC. Exposure to ambient air p roduced changes in Q that were small, but not insignificant. While the init ial increase in Si atom substitution around SiO4 units can be rationalized in terms of gradual condensation reactions between residual SiOH groups, ac companied by the liberation of water molecules and their removal from the s ite of the reaction, the eventual decrease in coordination is not currently understood.