DOI: 10.1002/adma.200702131 Exfoliated Sulfonated Poly(arylene ether sulfone)–Clay Nanocomposites By Yeong Suk Choi, * Tae Kyoung Kim, Eun Ah Kim, Sang Hoon Joo, Chanho Pak, Yoon Hoi Lee, Hyuk Chang, and Doyoung Seung Among the various types of proton exchange membranes (PEMs) for fuel cells, [1] arylene-based polymers are attracting much attention as alternatives to perfluorinated polymer membranes, such as Nafion or Aciplex, because arylene-based PEMs have advantages over the perfluorinated membranes in terms of costs, safety of the monomers, and structural diversity. [2] The arylene-based PEMs can achieve high proton conductivities by introducing proton-conductive groups, i. e. sulfonic acids, into arylene segments via electrophilic sub- stitution reactions. Introducing proton-conductive moieties to polymers, however, deteriorates the mechanical strength and permeability of PEMs, because the conductive moieties, forming proton-transporting channels from anodes to cathodes through aggregation of the conductive moieties, make the PEMs highly swollen or dissolved in aqueous/alcoholic solutions. This persistent problem demanded a new method or concept: polymer–inorganic hybrids, or nanocomposites. [3] Since the last decade, polymer–smectite clay nanocompo- sites have intensively been investigated as novel composites for vehicles or electronic devices, because the nanocomposites display a high modulus, good tensile strength, dimensional stability and flame-retarding properties at small loading amounts of clay. The unique properties of polymer–clay nanocomposites arise from nanoscale dispersion of clay layers in the polymer matrices. [4] From a morphological point of view, exfoliated polymer–clay nanocomposites rather than inter- calated nanocomposites are considered as the ideal structures for high-performance composites, because exfoliated struc- tures provide composites with large surface areas to allow interaction between clays and polymers. Due to the strong interactions between the clay layers, most smectite clays have a basal space of less than 2 nm, which is too narrow for high molecular-weight polymers to penetrate due to the large gyration radii of the polymers. Consequently, hydrophobic polymers show limited penetration into the interlayer spaces of hydrophilic clays. To overcome the hydrophobic/hydrophilic discrepancies between the clays and polymers, the clays were modified with oniums, more specifically alkylammoniums, [5] which possess hydrophobic surfaces and expanded basal spaces. Pristine smectite clays, however, can imbibe metal ions, polar or neutral molecules, having expanded interlayer spaces, resulting from the well-known, characteristic proper- ties: cation exchange, intercalation of molecules, and swelling in solvents. [6] The properties of clays indicate that polymer– clay nanocomposites show potential for improving the mechanical strength of highly sulfonated arylene-based PEMs without sacrificing ion conductivities. In this Communication, we report for the first time the exfoliated structure of sulfonated poly(arylene sulfone) (sPAS)–clay nanocomposites, which display drastic improve- ments in mechanical strength and methanol permeability at high proton conductivity; these favorable properties allow the application of (sPAS)–clay nanocomposite membranes in direct methanol fuel cells (DMFCs), which showed an improved performance with the novel nanocomposite mem- branes. The objective of this work has been the preparation of exfoliated sPAS–clay nanocomposites without organic modi- fiers, because most organic modifiers have lower chemical and mechanical stability than matrix polymers and organic modifiers are expected to deteriorate the electrochemical stability and mechanical strength of the composites. The dispersion state of clay layers in the sPAS matrix is, as discussed previously, the key parameter for improving the mechanical strength and methanol permeability without diminishing ion conductivity, because exfoliated clay nano- composites possess much higher surface areas for interaction with the sPAS molecules and display barrier properties through the exfoliated layers. The strategy for fabricating exfoliated sPAS–clay nano- composites was based on the solution intercalation method, which included the following steps: expanding the basal space of the clay with a polymerization solvent, adding the clay dispersion into a polymerization reactor containing the Na-sPAS (sulfonated PAS, sodium salt: salt form polymer), stirring the mixture to uniformly distribute the clay layers in Na-sPAS, and, finally, precipitating the polymer–clay nano- composites from non-solvents. The driving force behind the solution intercalation method is the entropy gain of solvent molecules that are desorbed from the clay surface into the COMMUNICATION [*] Dr. Y. S. Choi, E. A. Kim, Dr. S. H. Joo, Dr. C. Pak, Y. H. Lee, Dr. H. Chang, Dr. D. Seung Energy and Environment Laboratory P.O. Box 111, Suwon, 449-600 (Korea) E-mail: yeongsuk.choi@samsung.com T. K. Kim Research Institute of Chemical & Electronic Materials Cheil Industries Inc. Uiwang-si, 437-711 (Korea) Adv. Mater. 2008, 20, 2341–2344 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2341