Published: May 24, 2011 r2011 American Chemical Society 4901 dx.doi.org/10.1021/ma200937w | Macromolecules 2011, 44, 4901–4910 ARTICLE pubs.acs.org/Macromolecules Densely Sulfophenylated Segmented Copoly(arylene ether sulfone) Proton Exchange Membranes † Nanwen Li, ‡ Doo Sung Hwang, ‡ So Young Lee, § Ying-Ling Liu, || Young Moo Lee,* ,‡,§ and Michael D. Guiver* ,‡,^ ‡ WCU Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea § School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea ) Department of Chemical Engineering, R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan 32023, Taiwan ^ Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario K1A 0R6, Canada ’ INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) are promising energy conversion devices because of their high fuel utilization efficiency and environmentally clean operation. 1À3 Perfluorinated ionomers such as Nafion are the most widely used PEM materials in fuel cell applications due to their high proton conductivity and chemical stability, but well-known limitations such as high cost, high fuel crossover, restricted operation temperature, and humidity conditions prevent their widespread use in PEMFCs. 4À6 Acid-functionalized aromatic polymers have been investigated intensively over the past decade as alternative PEMs. 7À9 Despite these efforts, the performance of many hydrocarbon PEMs, especially proton conductivity under con- ditions of low relative humidity, is still inferior to that of per- fluorinated PEMs. Consequently, the understanding of PEM properties that result from different polymer architectures is essential for the further improvement of hydrocarbon PEMs. The proton conductivity of PEMs is usually closely related to several parameters such as acidity, number and position of ionic groups, main chain and/or side chain structures, composition and sequence of hydrophilic and hydrophobic components, and membrane morphology. 7,10,11 Among these, acidity of ionic groups and membrane morphology appear to be crucial, and they are inter- related. Kreuer et al. 12 reported that typical sulfonated aromatic polymers are unable to form defined hydrophilic domains, as the rigid aromatic backbone prevents the formation of continuous conducting channels and ionic clustering from occurring. Thus, several approaches have been examined to improve proton con- ductivity under conditions of low humidity and high temperature. One approach to enhance PEM performance is to induce phase separation between the hydrophilic sulfonic acid-contain- ing regions and the hydrophobic polymer main chain, by positioning the sulfonic acid groups on side chains grafted onto the polymer main chain. 13 If the polymer architecture is such that it contains flexible pendent side chains linking the polymer main chain and the sulfonic acid groups, nanophase separation between hydrophilic and hydrophobic domains may be improved. 14,15 For example, Jannasch and co-workers reported PEMs prepared by Received: April 23, 2011 Revised: May 13, 2011 ABSTRACT: Segmented copoly(arylene ether sulfone) membranes having densely sulfonated pendent phenyl blocks were synthesized by the coupling reaction of phenoxide-terminated oligomers with bis(4-hydroxyphenyl) sulfone and decafluoro- biphenyl (DFBP), followed by postpolymerization sulfonation of the blocks containing pendent phenyl substituents. The coupling reaction was conducted at relatively low temperature by utilizing highly reactive DFBP to prevent any possible trans-etherifica- tion that would randomize the hydrophilicÀhydrophobic sequences. Segmented copolymer molecular weights were reasonably high, as determined by viscosity measurements. Postsulfonation occurred selectively on the pendent phenyl substituent to yield hydrophilic blocks that were highly sulfonated in regular sequence on the linked phenyl rings. The resulting polymers gave transparent, flexible, and tough membranes by solution casting. Morphological observation by transmission electron microscopy (TEM) and atomic force microscopy (AFM) showed that the high local concentration and regular sequence of pendent sulfonic acid groups within the hydrophilic blocks enhanced nanophase separation between the hydrophobic and hydrophilic blocks. A comparison of copolymers with similar ion exchange capacities (IECs) indicated that proton conductivity and water uptake were strongly influenced by the hydrophilic block sequence lengths. Proton conductivity and water uptake increased with increasing block length, even at low relative humidity (RH). The ionomer membrane with X20Y20 (X and Y refer to the number of hydrophilic and hydrophobic repeat units, respectively) and 1.82 mequiv/g of IEC had a proton conductivity of 3.6 Â 10 À2 S/cm at 80 °C and 50% RH, which is comparable to that of perfluorinated ionomer (Nafion) membrane (4.0 Â 10 À2 S/cm).