Nanostructure of Nafion® membranes at different states of hydration -: An IR and Raman study

117 and 112 grades of H+ Nafion (R) membranes were studied using near-IR FT Raman scattering, IR absorption and ATR techniques. Tested membranes were as received, n water molecules per -HSO3 groups (n(117(I)) similar to 10, n(112(I)) similar to 2), equilibrated in water at RT (n(117(II)) similar to 20) or at 100 degreesC (n(112(II)) similar to 16), air dried at 80 degreesC (117(III) grade, n similar to 1) and 110 degreesC (117(IV) grade, n similar to 3 x 10(-1)) and completely dried over P2O5 (117(V)). Infrared (ATR) and Raman spectra show that whatever the water uptake, Nafion (R) (117 and 112) skeleton retains the helical zigzag conformation of the polytetrafluoroethylene (PTFE) and that the (-CF2-CF-) links prevail on the fluorocarbon part of the side-pendant chains. Comparison with PTFE allows for the calculation of the crystallinity degree of Nafion (R) membranes. Two v(2) bending OH vibrations are observed at 1650 (water, acid water) and 1720 cm(-1) (weakly-hydrated H3O+ ion). After drying, H3O+...SO3- interactions become visible and explain why the OH stretching massif shifts towards smaller wave numbers. At 80 degreesC (117(III)), sub-maximum at 2980 cm(-1) is assigned to H3O+ hydrogen bonded to SO3- groups. The acid water v(2) band disappears while the H3O+ band persists. In Nafion (R) 117(IV), almost all the water content is eliminated and many sulfonic groups are undissociated. Formation of the hydrated cluster follows from the helical conformation of the (CF2)(n) backbone and from the relative position of adjacent chains. (C) 2001 Elsevier Science B.V. All rights reserved.