Microcomposite membranes were produced via the in situ diffusion-controlled, acidcatalyzed sol-gel reaction for tetraethoxysilane in prehydrated and methanol-swollen Nafion perfluorosulfonic acid films. The thermal behavior of these microcomposites was investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). It was found by TGA that significant thermochemical degradation begins at around 350 "C and then takes place in at least three stages for both filled and unfilled membranes. Two strong thermal transitions, identified with the polar cluster and microcrystalline domains, in the predegradative temperature regime above 30 "C, were identified by DSC for all the microcomposite as well as the unfilled membranes. However, upon annealing at a temperature just below the "cluster transition temperature" and then quenching, this transition becomes suppressed for all membranes. It appears that both the polar and microcrystalline domains remain quite intact in spite of the in situ growth of the silicon oxide phase at the filler levels investigated. This observation reinforces our earlier hypothesis of the three-dimensional template action of Nafion's natural microphase-separated morphology in directing the final morphology of the inorganic phase.
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Stefanithis et al. (1990) studied this question.