Enhanced connectivity in hybrid polymers

We have studied the protonic conductivity of silica polymerized in situ wit hin a poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) matrix. T he macroscopic homogeneity of the membranes and the connectivity of the sil ica network strongly depend on the relative kinetics of inorganic condensat ion and PVDF drying. Upon base catalysis of the condensation reaction, the silica network forms faster than the organic polymer one. The conductivity linearly increases with the silica content, In contrast, acid conditions le ad to slow kinetics of silica condensation that forms nanoclusters disperse d within a polymer matrix. The conductivity remains very low up to 30-35 wt .% of silica, then rapidly increases up to sigma similar to 5 X 10(-8) Omeg a (-1) cm(-1). Although these values remain very low, they allow to estimat e the percolation threshold for the connectivity of the silica network, We report as well the growth of this network as inorganic filler in PVDF-co-HF P/SiO2/H3PO4 systems. Despite a loss of control in the inorganic polymer mo rphology, silica allows a better dispersion of H3PO4 in PVDF. The room temp erature conductivity of these membranes becomes significant for some applic ations (sigma similar to 5 X 10(-3) Omega (-1) cm(-1)), remains stable for relative humidities between 20% and 90% while the water content does not dr astically vary with water partial pressure. (C) 2001 Elsevier Science B.V. All rights reserved.