CHEMICAL MODIFICATION OF A NAFION(R) SULFONYL FLUORIDE PRECURSOR VIA IN-SITU SOL-GEL REACTIONS

The melt-processible sulfonyl fluoride precursor of a Nafion(R) ionome r was utilized as a sol-gel reaction medium for 3-aminopropyltriethoxy silane (APrTEOS). The diffusion-mediated reaction of APrTEOS with SO2F groups can be controlled with high degree of reaction. Fourier transf orm infra-red/attenuated total reflection studies show that sulfonamid e linkages are formed and condensation reactions of SiOR groups provid e covalent crosslinking of chains. Formic acid treatment plus high tem perature plus long time resulted in a high degree of polymer crosslink ing as seen in Si-29 solid state nuclear magnetic resonance spectra. M echanical modulus and strength increase, and elongation-to-break decre ases with increasing filler. Hybrids with <18% uptake accumulate crack s with crosslinked outer layers, each event signalled by a drop in str ess 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. Dif ferential scanning calorimetry (d.s.c.) revealed a broad, weak endothe rmic event peaking at 67 degrees C for the precursor and shifting to h igher temperatures while broadening with increasing filler content, in dicating progressively-restrictive glass-transition-temperature-relate d molecular motions within an increasingly nonhomogeneous environment. For the unreacted precursor, this d.s.c. transition occurs at a tempe rature just above a glass transition detected by dynamic mechanical me ans. (C) 1997 Elsevier Science Ltd.