Underwater stress relaxation studies of nafion (perfluorosulfonate) ionomer membranes

Abstract

In an effort to elucidate the nature of ionic aggregation and its effect on the primary relaxation (α process) in the Nafion polymers (EW = 1200), underwater and undermethanol stress relaxation tests were conducted at various temperatures (20–70°C). Both semicrystalline and amorphous Nafion-Na relax faster in the presence of water than in the dry state. In marked contrast to the behavior observed for Nafions in the dry condition, the kind of countercations is insignificant in the underwater stress relaxation. The great similarity in the underwater stress relaxation curves for different cations is attributable to the reduction of ionic interaction between the bound anions and unbound cations, probably associated with the water shielding around the ionic species. This leads to the suggestion that the primary α relaxation may be relevant to the ionic groups rather than to the matrix T_g, representing a reversal of the original assignment of the mechanical α and β relaxations. Increased swelling in methanol shows a modulus being one order of magnitude lower than that in the underwater runs. However, little or no difference is seen in the rates of stress relaxation in the underwater and undermethanol stress relaxation experiments. It is postulated that methanol also interacts with the interracial regions or with the fluorocarbon matrix in the Nafions, not just with the ionic regions.