[Perfluorosulfonate ionomer]/silicate hybrid membranes via base-catalyzed in situ sol-gel processes for tetraethylorthosilicate

Base-catalyzed sol-gel reactions for tetraethylorthosilicate were conducted within Nafion(R) sulfonate membranes. Silicate percent mass uptake, for al l counterion forms and imposed conditions, decreases with increasing pH. En vironmental scanning electron microscopy-EDAX studies of the distribution o f silicon across the Nafion(R) membrane thickness direction revealed rather uniform concentration profiles except for upturns or downturns in the near -surface regions and demonstrated that it is possible to incorporate silica te structures deep within these membranes. Si-29 solid-state NMR spectrosco pic studies revealed that, for Li+ and Na+ counterion forms, bonding in the silicate phase is such that there is more intra-molecular coordination at high pH. FT-IR spectroscopy uncovered all the signature peaks characteristi c of silicate structures in the near-surface regions. Fingerprints of Si-O- Si groups in cyclic and linear molecular substructures, as well as SiOH 'de fects' are present. The spectra of Li+ form hybrids reflect a silicate netw ork that becomes increasingly more interconnected with increasing pH, in ha rmony with the NMR results. The relationship between the spectroscopic info rmation, porosity of the silicate phase, and possible permselectivity based on a molecular-sieving effect, is discussed. (C) 2000 Elsevier Science B.V . All rights reserved.