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

A series of Nafion®/ORMOSIL hybrids, generated by in situ sol–gel co-polymerizations of tetraethylorthosilicate (TEOS) and semi-organic R′nSi(OR)4−n co-monomers (SOC), were developed to generate a spectrum of nanoscale chemical environments within the Nafion® morphological template. The molecular structures of the ORMOSIL phases were analyzed by means of 29Si 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 SiOH groups on reacted Q=Si(O1/2)4 units is always present. The shifting, with relative ORMOSIL composition, of D=RR′Si(O1/2)2 or T=R″Si(O1/2)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 29Si 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−xSi(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®, the results reported here address the specific compositions of these nanoscale structures.