Fibrillar Structure of Nafion: Matching Fourier and Real Space Studies of Corresponding Films and Solutions L. Rubatat, G. Gebel, and O. Diat* De ´ partement de Recherche Fondamentale sur la Matie ` re Condense ´ e, SI3M, Groupe Polyme ` res Conducteurs Ioniques, CEA Grenoble, 17 rue des Martyrs, 38054, Grenoble Cedex 9, France Received February 16, 2004; Revised Manuscript Received July 6, 2004 ABSTRACT: Using both scattering and microscopy techniques, we have characterized the complex Nafion structure over a large range of length scales. Analysis of experimental data from dry membrane to aqueous dispersion suggests an intrinsic fibrillar structure. The fibrils correspond to elongated polymeric aggregates surrounded with the ionic charges. In the Nafion membrane, the fibrils are entangled and collapsed with a degree of orientation at the mesoscopic scale. Upon swelling and temperature treatment, these aggregates are hydrated and dispersed in a colloidal suspension. Introduction In earlier papers, we have introduced evidence that cylindrical aggregate is an important motif in Nafion solution 1,2 and in highly swollen membranes. 3 In this article, we extend this study to hydrated membranes with lower water content. Using both microscopy and small angle scattering (SAS), we investigate the struc- ture of these aggregates and their spatial packing in both solution and membrane states. One of the main reasons for this work on Nafion membranes (produced by du Pont de Nemours) and solutions is that they are benchmark systems for low temperature (<100 °C) fuel cells based on polymer electrolytes: 4,5,6 these perfluoro- sulfonic acid (PFSA) membranes 7,8,9 have excellent proton conductivity and chemical stability in stationary operation. 10 When a new proton exchange membrane is developed, its physicochemical and electrochemical properties are always compared with those of Nafion. 11,14,16 The corresponding solution state 11,12 is necessary for volumetric electrodes processing where catalytic and carbon particles are mixed together with the Nafion dispersion for optimizing electrode-mem- brane interfaces. 13 In these PFSA membranes, structure and ion transport properties are strongly correlated; 11,17,18 the proton conductivity of native or recast membranes is attributed to the existence of a nanostructure with a sharp interface between hydrophobic and hydrophilic domains. The theoretical models developed to quantify the relationships between water content and ion con- ductivity are mainly based on the solvation of spherical ionic nanoclusters, usually called inverted micelles in analogy with surfactant systems. 19-27 In an aqueous medium, depending on the ionic charge content, these clusters swell, percolate, and grow as a coalescence process. 28 However, numerous problems emerge in implementing this picture. For example, the percolation threshold in water volume fraction determined in conductivity is quite low (<10%) or the existence and the distribution of some crystalline zones are not so well explained. Experiments also show some anisotropic properties, 29-31 which has to be taken into account in the models. These aspects can be taken into account by considering an ionic and aqueous cylindrical network 32,33 embedded in the polymeric matrix or developing planar ionic geometries. 34-38 However, these approaches leave open problems. For example, how can we describe physically a strong reorganization of the polymer ma- trix 39 in order to explain either the continuous swelling from dry to Nafion dispersion or the corresponding development of transport properties between a native and a annealed cast membrane? Finally, ultralow angle scattering techniques reveal a more complex structural organization, 40,45 at least up to the micrometer scale, that is not yet understood and that previous models do not take into account. For these reasons, this study was carried out over a large water content variation. Taking into account simultaneously the observations at all the different scales and those from our previous results, 3 it seems logical to consider that the rodlike structure preexists in the dry Nafion membrane. SAS is a very suitable technique to probe the topology of the hydrophilic/hydrophobic phase separation but only if it is applied over a range of scattering angles corresponding in the complete range of length scales exhibiting density modulation. However the lack of periodicity in these membrane systems prevents a simple analysis of the scattering data. Elliot et al. 46 have applied a powerful maximum-entropy method for the inversion of small-angle X-ray diffraction patterns. In this case, the electron density reconstruction depends on the wave vector range over which the fit procedure is applied. In Elliot’s work, this was done over 1 decade and half of the q-vectors while we have shown that the scattered intensity varies over more than 3 decades. Consequently, parts of the long-range correlation from intra and/or inter scattered particles were missed. Another approach is to perform a 2D chord analysis as in ref 47. However, this procedure encounters the same problem: the characteristic correlation lengths ex- tracted from this method do not allow to define unam- biguously the morphology of the ionic and hydrophobic domains and their spatial distribution in an apparent isotropic polymeric system. To avoid most of these difficulties, we performed simultaneous studies in both real and Fourier spaces as a function of the water content, from a dry membrane to a colloidal suspension. Our objective is to obtain a coherent picture of the Nafion structure at different length scales and in both spaces. Our model is based on the existence of elongated * Corresponding author: Telephone: 33 4 3878 9171. Fax: 33 4 3878 5691. E-mail: odiat@cea.fr. 7772 Macromolecules 2004, 37, 7772-7783 10.1021/ma049683j CCC: $27.50 © 2004 American Chemical Society Published on Web 09/08/2004