FTIR equivalent weight determination of perfluorosulfonate polymers

Abstract

Fourier transform infrared (FTIR) and attenuated total reflectance (ATR) spectroscopy studies of the sulfonyl fluoride, potassium salt, and sulfonic acid forms of long and short side chain perfluorosulfonate polymers revealed bands indicative of the sidegroup and backbone compositions, endgroups on the main chain, water content, monomer concentration, and degree of salt hydrolysis. The equivalent weight (EW) of the polymer was obtained by titration and NMR measurements which were then calibrated to either the C-F/C-O-C absorbance band ratio for thin (<1.1 mil) films or to a C-F/SO2F absorbance band ratio for thick films (5 to 25 mils). An FTIR measurement of the film thickness based on the C-F group concentration was found to be both a function of the actual thickness and the EW; a method for compensating for this EW dependence is described. Esterification and fluorination of the polymers yielded FTIR measurements of the endgroup compositions on the polymer backbone which were shown to consist of -COF, -COOH, -CO2CH3, and -CF=CF 2 groups. Thermogravimetric Analysis Infrared (TGA-IR) spectroscopy of the acid form polymers indicates that degradation begins by the decomposition of the -SO3H group at 320°C followed by bulk deterioration above 400°C. The FTIR techniques detailed herein have been developed for accurate, reproducible, and rapid compositional measurements of Nafion® and other perfluorosulfonate polymers. © 2010 Wiley Periodicals, Inc.