Journal of Polymer Science Part B: Polymer Physics, Vol.34, No.5, 873-882, 1996
(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 SiO2[1-x/4](OH)(x) nanoparticles, which were preformed in Nafion(R) 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-O-Si groups are seen. Si-29 solid-state NMR spectroscopy indicated that, for an inorganic uptake of 16.3 wt %, the Q(3) State of SiO4 is most populated although Q(4) is only slightly less prominent and Q(2) and Q(1) 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.