A Comparison of Mechanical and Electrical Percolation during the Gelling of Nafion Solutions

Abstract

Rheological and impedance measurements are performed on Nafion solutions at relatively high concentration. The viscosity scales with the Nafion volume fraction as ηr ∝ φ0.59, up to φ = 2.5%. At higher fractions the viscosity diverges. At φ = 2.5% the storage modulus exhibits scaling behavior with the frequency (G‘ ∝ ω0.81), and the solution behaves as a gel. The results for the viscosity seem to fit the Zimm, rather than the Rouse, model. For the sodium form of Nafion the conductivity behaves qualitatively the same as the viscosity, with σ ∝ φ0.89, and can be described by a percolation network of conductors and superconductors. Above φ = 2.5% a sharp increase is observed, and the conductivity scales with the frequency as σ ∝ ω0.75. For Nafion in the proton form at volume fractions higher than 2.5% no sharp increase is observed in the conductivity, and the conductivity is independent of frequency. This indicates that in the proton form no electrical percolation takes place. It is discussed how these results can be used to improve the properties of recast membranes.

Molar mass distributions and viscosity behavior of perfluorinated sulfonic acid polyelectrolyte aqueous dispersions

“…The results may also indicate a network structure through electrostatic interactions in which low shear rates only partially distort the structure. In support of the latter, it has been reported that the storage modulus becomes frequency‐independent at or below 15 °C; this is indicative of a network,16 and the temperature is not substantially lower than the temperature herein. The evidence may suggest a model for PFSA polyelectrolyte liquid dispersions in which charge‐stabilized, anisotropic, and potentially overlapping particles exist on a deformable three‐dimensional lattice, with the particle centers of mass at the lattice points.…”

Structure-property relationship in ionomer membranes

“…is consistent, at least qualitatively, with the of non-freezable water molecules in a PFSA membrane is reported findings of conductivity and sorption behavior of PFSA membranes to correspond to l w 6-7, independent of the initial water content in this regime ð15 < l < 22Þ [20,35]. The conductivity of the PFSA of the membrane at room temperature [36,38,39,46,62], with the membranes increases during water uptake [6,8,32,[34][35][36][37][38][39]59], remaining water, if any, being freezable. Thus, the higher the probably due to larger hydrophilic domains [2,[6][7][8]19,35,39,45,59].…”

“…The conductivity of PFSA membranes, k c , increases with water uptake (or swelling) [3,5,6,8,[32][33][34][35][36][37][38][39] and is assumed to be related to the water volume fraction according the equation, k c ff s w where the exponent, s, is empirically determined to be between 1.0 and 1.95 [8,33,35,36]. This relationship can be attributed to the growth of water-swollen ionic domains: both the interconnecting channels and spheres expand during water sorption [6,10,21,35,[39][40][41].…”

A Computer Simulation Study of the Mesoscopic Structure of the Polyelectrolyte Membrane Nafion

“…To investigate the structure and swelling behavior of the hydrated Nafion membranes, numerous experimental efforts have been concentrated on neutron, wide and small angle X-ray scattering, infrared (IR) and Raman spectroscopy, and transmission electron microscopy (TEM) techniques. [1][2][3][4][5][6][7][8][9][10][11][12][13] It has been well established that hydrated Nafion membranes have two phases on a nanometer scale, a hydrophobic phase containing the backbone and a hydrophilic phase containing sulfonic acid groups and water. Several models for these structures such as the interconnected spherical water clusters (the cluster-network model) have been proposed for the interpretation of the scattering patterns.…”

“…Several models for these structures such as the interconnected spherical water clusters (the cluster-network model) have been proposed for the interpretation of the scattering patterns. [1][2][3][4][5][6][7] However, these models are still under discussion concerning the size and shape of the water clusters. It is meaningful to clarify the structure of the membranes for analytical study and to improve the mechanical and transport properties of the materials.…”