Microstructural Evolution of a Silicon Oxide Phase in a Perfluorosulfonic Acid Ionomer by an In Situ Sol-Gel Reaction .2. Dielectric Relaxation Studies

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Polymers and High Performance Materials


Microcomposite membranes were produced via the in situ diffusion-controlled and acid-catalyzed sol-gel reaction for tetraethoxysilane in prehydrated and methanol-swollen Nafion perfluorosulfonic acid films. The storage and loss components of the complex dielectric constants of these modified membranes were determined over the frequency range 5 Hz to 13 MHz as a function of invasive silicon oxide content and temperature. The large storage components displayed by these microcomposites suggest the action of an interfacial polarization which, in turn, suggests the persistence of a microphase-separated, i.e. clustered, morphology after incorporation of the silicon oxide structures. Long-range motions of charge, tentatively attributed to the intercluster hopping of protons, is strongly manifest on the loss spectra. A parameter, n, which is extracted from the isothermal loss component vs frequency spectra, is reflective of the degree of connectivity of underlying charge pathway networks. n vs temperature or silicon oxide content relationships are then viewed as coarsely indicative of the evolution of the microcomposite morphological texture with the variance of these two factors. The observed range of n is broad, which suggests considerable morphological differentiation, and definite trends in this parameter are seen. While n vs filler level at constant temperature exhibits organized but complex behavior, n vs temperature at constant filler level plots appear as distinctly increasing curves except at the highest loading. A structural-mechanistic assignment of three absorption peaks detected beyond the dc conduction region for the microcomposites as well as for the unfilled acid precursors is not evident at this time, although the lowest frequency peak might be due to the relaxation of polarization across clusters.

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