[Perfluorosulfonate ionomer]/[SiO2-TiO2] nanocomposites via polymer-in situ sol-gel chemistry: Sequential alkoxide procedure

In situ sol-gel chemistry was used to create inorganic/perfluoro-organic hybrids wherein titanium oxide outer regions of SiO_21—x/4](OH)_x nanoparticles, which were preformed in Nafion® membranes, were created by postreaction with tetrabutyltitanate (TBT). U-shaped Si and Ti distributions across the membrane thickness direction were determined via x-ray energy dispersive spectroscopy. Ti/Si ratio profiles are also U-shaped, indicating more Ti relative to Si in near-surface regions. IR spectroscopy verified structural bonding of TiO₄ units onto SiO₂ nanoparticles and indicated that alkoxide hydrolysis is not complete. Reacted silicon oxide nanophases retain the topological unconnectedness possessed by the corresponding unreacted phase. IR bands signifying molecular loops and linear fragments of Si(SINGLE BOND)O(SINGLE BOND)Si groups are seen. ²⁹Si solid-state NMR spectroscopy indicated that, for an inorganic uptake of 16.3 wt %, the Q³ state of SiO₄ is most populated although Q⁴ is only slightly less prominent and Q² and Q¹ are either small or absent. The silicon oxide component, although not being predominantly linear, retains a measure of uncondensed SiOH groups. Tensile stress vs. strain analyses suggested that TBT postreaction links nanoparticles, causing them to be contiguous over considerable distances. This percolative intergrowth occurs in near-surface regions generating a glassy zone. © 1996 John Wiley & Sons, Inc.