NAMS 2002 Workshop - ICOM 2008

samples aged at 80%RH, water concentration decreases from 15% to 6.5% up to
80 days and remains constant beyond. With aging, Nafion absorbs less water: It
becomes less hydrophilic.
Pristine and aged samples are analysed in parallel by infrared spectroscopy
(transmission mode). The most spectacular phenomenon is the appearance of a
new band at 1440 cm -1 which is clearly observed in all IR spectra of aged
samples. The intensity of this absorbance band increases with aging and
remains constant after exposure times very close to that observed from DVS
measurements.
19F NMR spectra show that aged Nafion backbone is not chemically altered by
aging but that the environment of chemical functions located on pendant chains
is slightly modified.
To follow the influence of aging on the sulfonic groups of Nafion, 1H NMR MAS
spectra of aged samples are observed in parallel of the 19F NMR spectra. In its
original state, Nafion pristine membrane displays only one protonated site. 1H
NMR spectra of Nafion aged in a climatic chamber display a second peak at
3.4ppm with aging. Heteronuclear correlation NMR experiments 1H- 13C did not
point out any interaction of these protons with the carbonated structure of Nafion.
This peak does not result from a chemical degradation of the polymer.
The same observations are done for samples aged at 80°C, 0%RH except that it
evolves more slowly (stabilization over 200 days).
4. Discussion
One of the most interesting features of Nafion aging is the decrease of Nafion
water uptake. After exposure at 80°C, Nafion becomes less hydrophilic as shown
by DVS. Aged Nafion isotherms do not display Langmuir contribution anymore:
water is no longer trapped by sulfonic acid end-groups as in pristine Nafion.
The apparition of an infrared absorption band at 1440 cm -1 strongly suggests
anhydride formation and will be considered as a degradation tracer. The
mechanism proposed here is a condensation of two sulfonic acids creating a
cross-link S-O-S between two side groups which is accompanied by loss of one
water molecule.
Modifications observed in 19F NMR reveals a change of the chemical
environment of the pendant chains as expected by the anhydride formation. With
aging, a new 1H NMR peak located at 3.4ppm appears after exposure at 0 or
80%RH. According to its chemical shift, this peak is attributed to non- acidic
water molecules located around anhydrides. The observation of the non-acidic
water peak at 3.4 ppm, on the 1H NMR spectra, can be considered as a tracer of
the polymer aging.