LETTERS A New Insight into Nafion Structure Anne-Laure Rollet, Olivier Diat,* and Ge ´ rard Gebel De ´ partement de Recherche Fondamentale sur la Matie ` re Condense ´ e, SI3M, Groupe Polyme ` res Conducteurs Ioniques, CEA-Grenoble 17, rue de Martyrs, 38054 Grenoble Cedex 9, France ReceiVed: January 28, 2002 The structure of hydrated Nafion membranes neutralized by N(CH 3 ) 4 + ions has been investigated by the small angle neutron scattering technique. First, the spectra show that the counterions are condensed at the interface between hydrophilic and hydrophobic phases. More importantly, by applying a contrast variation method, the normalized scattered intensity of membranes swollen in several D 2 O/H 2 O mixtures was analyzed. The N(CH 3 ) 4 + counterions were used as a probe to identify the nature of the scattering entity that is polymeric aggregates surrounded by ionic groups and water molecules, contrary to the generally accepted model. This new insight into the Nafion structure will allow one to reconsider the swelling process and the degree of mesoscopic orientation in such perfluorinated systems. Introduction The Nafion (E. I. du Pont de Nemours), whose chemical formula is indicated below is the most studied ionomer membrane thanks to its good performance in electrochemical devices. 1,2 Its structure, characterized by a microphase separa- tion, has been widely studied using small angle scattering with neutrons (SANS) and X-rays (SAXS). 3-8 The range of transfer momentum q from 0.01 to 0.4 Å -1 has been particularly investigated because of the occurrence of the so-called “ionomer peak” at around 0.11 Å -1 which is the most striking feature of these spectra. Two other main features are observable: at lower q values (q ≈ 0.02 Å -1 ), a shoulder which is up to now not really simulated but associated to the degree of cristallinity of the polymer and, at higher q values (q > 0.2 Å -1 ), the q -4 decrease of the scattered intensity, characteristic of the so-called Porod law. Moreover, at very low q values (q < 10 -2 Å -1 ), a small angle upturn has been also observed and analyzed considering large scale density inhomogeneities. 9 Despite these numerous studies, Nafion structure is still subject to controversy and several models have been proposed. The most common is the Gierke’s one: 10 the polymer is assumed to form reverse micelles (about 20 Å radius) connected by small cylindrical pores (10 Å length, 5 Å radius). The Dreyfus model, 11 designed to analyze SANS and SAXS data, is derived from the Gierke model, with the micelles being organized on a diamond lattice but with a short range order. Cooper et al. 12,13 and Fujimura et al. 14 considered different distributions of the spherical aggregates of sulfonated groups. Other models, that do not consider connected spherical cavities, have also been suggested to describe the structure of the hydrophilic phase of Nafion. For example, Litt, 15,16 analyzing the shift of the ionomer peak with the water volume fraction, described the Nafion structure as a lamellar organization of planar clusters. As well, Verbrugge et * To whom correspondence should be addressed. E-mail: odiat@cea.fr. Fax: 33 4 38 78 56 91. © Copyright 2002 by the American Chemical Society VOLUME 106, NUMBER 12, MARCH 28, 2002 10.1021/jp020245t CCC: $22.00 © 2002 American Chemical Society Published on Web 03/03/2002