Polymer, Vol.43, No.8, 2311-2320, 2002
Nafion (R)/ORMOSIL nanocomposites via polymer-in situ sol-gel reactions. 1. Probe of ORMOSIL phase nanostructures by Si-29 solid-state NMR spectroscopy
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.