Chemical modification of a Nafion ® sulfonyl fluoride precursor via in situ sol-gel reactions

Abstract

The melt-processible sulfonyl fluoride precursor of a Nafion ® ionomer was utilized as a sol gel reaction medium for 3-aminopropyltriethoxysilane (APrTEOS). The diffusion-mediated reaction of APrTEOS with SO 2F groups can be controlled with high degree of reaction. Fourier transform infra-red/attenuated total reflection studies show that sulfonamide linkages are formed and condensation reactions of SiOR groups provide covalent crosslinking of chains. Formic acid treatment plus high temperature plus long time resulted in a high degree of polymer crosslinking as seen in 29Si solid state nuclear magnetic resonance spectra. Mechanical modulus and strength increase, and elongation-to-break decreases with increasing filler. Hybrids with < 18% uptake accumulate cracks with crosslinked outer layers, each event signalled by a drop in stress followed by stress recovery. While there are sharp visual material fronts inward from both surfaces, EDAX (energy dispersive analysis of X-rays) showed that there are no sharp Si composition boundaries. Differential scanning calorimetry (d.s.c.) revealed a broad, weak endothermic event peaking at 67°C for the precursor and shifting to higher temperatures while broadening with increasing filler content, indicating progressively-restrictive glass-transition-temperature-related molecular motions within an increasingly nonhomogeneous environment. For the unreacted precursor, this d.s.c. transition occurs at a temperature just above a glass transition detected by dynamic mechanical means.