DIELECTRIC-RELAXATION STUDIES OF PERFLUOROCARBOXYLATE POLYMERS

Dielectric studies of Nafion(double dagger) perfluorocarboxylate polym ers in the methyl ester, potassium and sodium salt, and carboxylic aci d forms over an equivalent weight (EW) range of 950-1334 (15.6-9.8 mol % vinyl ether) uncovered alpha, alpha', beta, gamma, and delta dielec tric relaxations. The alpha relaxations occurred between -20 and + 100 -degrees-C with activation energies (E(a)) between 35 and 1 10 kcal/mo l indicative of the motion of the polar vinyl ether side chains. The b eta relaxation, observed between -80 and -10-degrees-C, was characteri zed as the ester rotation with an E(a) = 13-26 kcal/mol. The amplitude of the dissipation factor of both the alpha and the beta peaks increa sed linearly with decreasing equivalent weight indicating that a large r number of side chains (lower EW) resulted in a more intense dielectr ic response. Furthermore, the area under the beta relaxation curves (a measure of the total population of ester groups) increased monotonica lly with the side-group concentration. The beta peak location did not shift as a function of EW but did change depending on the chemical for m of the polymer; i.e., lowest temperature for the salt and highest te mperature for the acid. A gamma relaxation, the crankshaft rotational movement of nonlinear -CF2- groups along the polymer backbone similar to that in PTFE, was hidden under the more dominant beta peak but bega n to emerge at around -100-degrees-C at frequencies below 10 Hz with a n E(a) between 10 and 19 kcal/mol; in the acid form, however, the gamm a peak dominated the beta peak. The delta relaxation (E(a) = 3-5 kcal/ mol), occurring between -200 and -150-degrees-C, was attributed to the highly polar O-CH3, O-K, and O-H groups at the end of the side chain.