Nafion/ORMOSIL hybrids via in situ sol-gel reactions .3. Pyrene fluorescence probe investigations of nanoscale environment

Pyrene (Py) photophysical probes were used to interrogate structural diversity within [1] Nafion/[silicon oxide] and [2] Nafion/[ORMOSIL] hybrids. The interior of the silicon oxide phase in [1] has the most polar environment wherein Py is trapped in silicon oxide cages in the vicinity of -SO3- groups. Polar/nonpolar interphase regions in [1] are next in order of decreasing polarity. The interior of the ORMOSIL phase in [2] displays lowest polarity, reflecting CH3 groups in Py-encapsulating ORMOSIL cages, and the interphase ranks somewhat higher in polarity. Water uptake is ordered: Nafion/ORMOSIL < unfilled Nafion-H+ < Nafion/[silicon oxide]. The hydrophilicity of unfilled Nafion-H+ is adjustable by tailoring the organic content of the incorporated sol-gel-derived nanophase as polarity determined from fluorescence emission and water uptake correlate well. Fluorescence emission of Py monitored nanostructural polarity shifts that result from secondary in situ gel growth induced by annealing. Additional condensation of SiOH groups and liberation of volatiles is promoted by increasing temperature/time, as evidenced by diminishing polarity. Polarity decrease with annealing is more profound for Nafion/[ORMOSIL incorporating Py] than for Nafion/[silicon oxide incorporating Py] as polar solvents are removed more easily from the less polar ORMOSIL nanophase.