Nafion (R)/ORMOSIL nanocomposites via polymer-in situ sol-gel reactions. 1. Probe of ORMOSIL phase nanostructures by Si-29 solid-state NMR spectroscopy

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Polymers and High Performance Materials


A series of Nafion((R))/ORMOSIL hybrids, generated by in situ sol-gel co-polymerizations of tetraethylorthosilicate (TEOS) and semiorganic R'Si-n(OR)(4-n) co-monomers (SOC), were developed to generate a spectrum of nanoscale chemical environments within the Nafion((R)) morphological template. The molecular structures of the ORMOSIL phases were analyzed by means of Si-29 solid-state NMR (SSNMR) spectroscopy. A high average degree of Si atom coordination about SiO4 molecular sub-units can be achieved, but a significant number of unreacted equivalent toSiOH groups on reacted Q = Si(O-1/2)(4) units is always present. The shifting, with relative ORMOSIL composition, of D = RR'Si(O-1/2)(2) or T = R"Si(O-1/2)(3) (R, R' and R" are organic moeities) peak envelopes for difunctional or trifunctional SOCs is suggested to reflect random co-condensation rather than distinct Q and D (or T) block formation. The numbers of membrane-incorporated Q and D (or T) units per fixed sulfonate group were calculated from the Si-29 SSNMR spectra for those particular hybrids that had a reasonably low noise/signal aspect. Spectra for hybrids based on in situ sol-gel reactions for TEOS (no SOC present) suggest that inserted hydroquinone molecules interfere with condensation reactions between (RO)(4-x)Si(OH)(x) molecules and silanol oligomers to yield silicate structures with lower average coordination. While earlier small angle X-ray scattering studies indicated that ORMOSIL structures can be grown within the polar regions of Nafion((R)), the results reported here address the specific compositions of these nanoscale structures. (C) 2002 Published by Elsevier Science Ltd.

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