© 2010 Macmillan Publishers Limited. All rights reserved. Enhancement of anhydrous proton transport by supramolecular nanochannels in comb polymers Yangbin Chen 1 , Michael Thorn 2 , Scott Christensen 3 , Craig Versek 2 , Ambata Poe 1 , Ryan C. Hayward 3 * , Mark T. Tuominen 2 * and S. Thayumanavan 1 * Transporting protons is essential in several biological processes as well as in renewable energy devices, such as fuel cells. Although biological systems exhibit precise supramolecular organization of chemical functionalities on the nanoscale to effect highly efficient proton conduction, to achieve similar organization in artificial systems remains a daunting challenge. Here, we are concerned with transporting protons on a micron scale under anhydrous conditions, that is proton transfer unassisted by any solvent, especially water. We report that proton-conducting systems derived from facially amphiphilic polymers that exhibit organized supramolecular assemblies show a dramatic enhancement in anhydrous conductivity relative to analogous materials that lack the capacity for self-organization. We describe the design, synthesis and characterization of these macromolecules, and suggest that nanoscale organization of proton-conducting functionalities is a key consideration in obtaining efficient anhydrous proton transport. E fficient and selective transport of protons is critical both in biological contexts 1 and in materials for renewable energy 2 . In biological systems, nature has optimized proton conduction on a nanometre scale by using secondary and tertiary structures of proteins to arrange precisely the appropriate side chains of amino acids, for example in the membrane protein M2 (refs 3–5). Although control of proton transfer on this scale is adequate for most biological processes, it is essential that efficient proton conduc- tion be obtained on a micron scale for clean-energy applications 6,7 . In hydrogen fuel cells, for example, after oxidation of molecular hydrogen at the anode, the resulting protons must be transported across a selective membrane to reach the cathode and complete the conversion of chemical energy into electrical energy. The proton conductivity of this membrane, often called the proton- exchange membrane or the polymer electrolyte membrane (PEM), has been one of the bottlenecks to achieving affordable fuel-cell technology. Nafion, a poly(tetrafluoroethylene)-based polymer with sulfonic acid groups arranged at intervals along the backbone, is one of the most widely used materials for this membrane 8 . The key to proton transport in Nafion is thought to be nanochannels of sulfonic acid groups, through which ‘hydrated’ protons can pass efficiently 9–11 . Although a good proton conductor for hydrated protons, Nafion suffers from poor conductivity in unassisted proton transfer, that is Grotthuss or anhydrous proton transfer 12,13 , which results in low conductivities at temperatures above the boiling point of water. PEMs with high proton conductivities at tempera- tures of 120–200 8C are desirable, because operation at higher temp- eratures can increase fuel-cell efficiency, reduce cost, simplify heat management and provide better tolerance of the catalysts against poisoning 14 . One approach to address this issue is to use amphoteric func- tional groups that allow anhydrous proton transport 15,16 , for example imidazole, which is a common motif in biological proton transport in the form of the amino acid histidine. Several groups have studied synthetic polymers that contain such amphoteric func- tional groups as candidates for high-temperature proton transfer 17–22 . Although a number of interesting candidate materials were identified, one avenue that was not explored in these anhydrous proton-conducting systems is the role of supramolecular organiz- ation in nanoscale ion-conducting channels. This is surprising because, in the context of hydrated proton-conducting systems, such as Nafion 9–11 , and sulfonated block copolymers 12,23–26 , as well as lithium-ion conducting supramolecular assemblies 27–29 , it is well-established that the formation of nanoscale domains enriched in the ion-conducting materials is critical to the resulting macro- scopic ionic conductivity. In this paper, we describe the molecular design and synthesis of a novel class of comb polymers with amphoteric proton-transfer functionalities that can self-assemble into organized supramolecular structures. We also show that very subtle changes in the monomer and analogous polymer provide solid-state structures that lack such nanoscale organization. By comparing these polymers, we show that the self-assembled structures yield a dramatic increase in proton conductivity (by as much as three orders of magnitude), presumably because of the locally increased concentration of proton-transport functionalities within the nanophase-separated domains. These results suggest that a careful consideration of macromolecular architecture and nanoscale assembly is critical to optimizing anhydrous proton transport in new materials for PEMs. Results To prepare polymers that form supramolecular assemblies with proton-transporting functionalities concentrated within nanoscale domains, we made use of comb polymer architectures (Fig. 1). One of our groups recently used this architecture to prepare amphi- philic comb polymers by attaching lipophilic and hydrophilic func- tionalities at the meta-positions of the benzene ring of each styrenic repeat unit 30,31 . Such polymers were shown to form assemblies of a micelle type in aqueous milieu and of an inverse-micelle type in apolar organic solvents. Thus, we hypothesized that similar poly- mers would also form nanoscale assemblies in the melt state. For this purpose, we designed a series of styrenic comb polymers in which one of the meta-positions contained a polar N-heterocyclic functionality capable of proton transport, and the other contained 1 Department of Chemistry, 2 Department of Physics, 3 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA. *e-mail: thai@chem.umass.edu; tuominen@physics.umass.edu; rhayward@mail.pse.umass.edu ARTICLES PUBLISHED ONLINE: 25 APRIL 2010 | DOI: 10.1038/NCHEM.629 NATURE CHEMISTRY | VOL 2 | JUNE 2010 | www.nature.com/naturechemistry 503