Journal of Membrane Science 389 (2012) 294–304 Contents lists available at SciVerse ScienceDirect Journal of Membrane Science j ourna l ho me pag e: www.elsevier.com/locate/memsci Investigation of ionomer structure through its dependence on ion exchange capacity (IEC) E. Moukheiber, G. De Moor, L. Flandin, C. Bas ∗ LEPMI, UMR 5279, CNRS – Grenoble INP – Université de Savoie – Université J. Fourier – LMOPS – Bât. IUT – Campus de Savoie Technolac, F-73376 Le Bourget du Lac Cédex, France a r t i c l e i n f o Article history: Received 25 August 2011 Received in revised form 26 October 2011 Accepted 30 October 2011 Available online 4 November 2011 Keywords: Polymer electrolyte fuel cells Reinforced composite polymer electrolyte membrane Nafion ® membrane Short-Side-Chain perfluorosulfonic acid (SSC PFSA) membranes Long-Side-Chain perfluorosulfonic acid (LSC PFSA) membranes Ion exchange capacity (IEC) a b s t r a c t The ion exchange capacity (IEC) of conventional ion-exchange perfluorinated membranes based on the so-called Long-Side-Chain (LSC) polymers and their Short-Side-Chain (SSC) Aquivion ® derivatives was determined using a series of experimental techniques. Newly developed and already used experimental analyses were compared in terms of sensitivity to the determination of the ion exchange capacity. The use of complementary techniques allows better determination of IEC with an uncertainty of about 3%. This IEC determination method was then applied to membranes with large chemical differences such as copolymers and reinforced membranes. In addition, based on these analyses, relationship between DMA, WAXS, TGA and conductivity parameters and either IEC or the molar number of tetrafluoroethylene (TFE) per comonomer unit are underlined, leading to a more comprehensive approach of architecture in perfluorosulfonic acid (PFSA) polymer membranes. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Perfluorosulfonic acid (PFSA) based membranes have been used in portable, stationary, and automotive commercial applications of PEM technology. In addition to providing an attractive combination of performance and reliability, these polymers demonstrated high durability. Much of the early literature describes the properties of conventional ion exchange ionomers based on the so-called Long- Side-Chain (LSC) monomer such as Nafion ® (DuPont) [1], Flemion ® (Asahi glass) and Aciplex ® (Asahi Kasei) [2]. More recently, reli- able information about the Aquivion ® polymer (Solvay Speciality Polymers) [3] based on the Short-Side-Chain (SSC) monomer and previously known as Hyflon ® has become available. This polymer is based on the same monomer as the experimental Dow polymer [4], which has been discontinued. Lately, Minnesota Mining and Manu- facturing (3 M) claimed the synthesis of another sulfonic monomer containing only a single ether linkage [5]. Perfluorinated ionomers are typically obtained by copolymer- ization of tetrafluoroethylene and a perfluorovinylether containing a sulfonyl halide such as the perfluoro(4-methyl-3,6-dioxaoct-7- ene) sulfonyl fluoride (PFSVE) and it can be melt-extruded or tape- cast into membranes [6]. Membranes are commonly characterized ∗ Corresponding author. Tel.: +33 4 79 75 86 24, fax: +33 4 79 75 86 14. E-mail address: corine.bas@univ-savoie.fr (C. Bas). by their thickness, ion exchange capacity (IEC, mmol SO 3 H/g poly- mer) or equivalent weight (EW (g/mol) = 1000/IEC). One of the most important properties of a PEM is its ability to provide an ionic path for protons to travel from the anode to the cathode. In the case of Nafion ® and other PEMs, water channels are believed to form due to phase separation between hydrophilic (ionic groups) and hydrophobic regions (polymer backbone). Pro- ton conduction is thought to occur through these channels [1,7], mediated by the sulfonic acid groups and water. If the level of pro- ton transport is insufficient, a resistive (Ohmic-like) loss is observed with a drastic impact on the performance of the fuel cell [8]. Fac- tors such as polymers structure, morphology, water content and sulfonic content IEC alter proton conduction. Acid–base titration seems to be the most direct and appealing method for the determination of IEC as well as for the calibration of other analytical methods. The major limitation of acid–base titration is the difficulty of drying the polymer to a known water content value. The drying procedures, mentioned in the literature [9], consist mainly in drying the polymer in the acid form or in the salt form under vacuum at higher temperature (70–140 ◦ C) for at least 12 h, the salt form ionomer being more readily dried than the acid form. However, a large dispersion of ion exchange capacity values for commercially available membranes was reported com- pared to that provided by the supplier. For instance, for a Nafion 117 (EW = 1100, IEC = 0.91), the value of IEC was found within 0.83–1.02 mequiv./g [10,11]. Besides the technique uncertainty, 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.10.041