Perspective: Ionomer Research and Applications Longhe Zhang, Nicole R. Brostowitz, Kevin A. Cavicchi,* R. A. Weiss* Ionomers are polymers with bonded ionic species that are used under conditions where the salt groups are in a condensed state. This Feature Article discusses the state of our understanding of ionomers and the historical applications of these nanostructured polymers. It also discusses modern methods for synthesiz- ing new ionomers and describes a number of relatively new applications for ionomers and the potential use of these materials in contemporary technologies, including, shape memory and self- healing materials and supramolecular polymer systems. 1. Ionomers: Historical and Structural Perspective The term ionomer was first used by Rees from E. I. Dupont de Nemours & Co., Inc., to describe, statistical copolymers of ethylene and methacrylic acid [1–3] (M-PEMA, where the first M denotes the metal cation) that were partially neutralized with sodium or zinc ion, Figure 1. The M-PEMA ionomers were first introduced commercially in 1964 by DuPont under the trade name Surlyn. Although methacrylic acid is used as the comonomer in the commercial Surlyn, the seminal patent in the field by Rees [4] is much broader with regard to the choice of the carboxylic acid comonomer, ‘‘a,b-ethylenically unsaturated carboxylic acids having from 3 to 8 carbon atoms, said copolymers having from 10 to 90% of the carboxylic acid groups.’’ In the 50 years since the development of Surlyn by DuPont, other compa- nies have commercialized poly(ethylene-co-acrylic acid) ionomers, e.g., Iotex by Exxon-Mobil and Amplify by Dow Chemical. The key feature of ionomers is that a relatively modest concentration of acid or ionic groups bonded to a low dielectric constant polymer can provide substantial changes in the physical, mechanical, optical, dielectric, and dynamic properties of a polymer. For acid functional- ization, interchain, physical crosslinks are formed by hydrogen bonding of two acid groups. If the acid groups are fully or partially neutralized to form an ionic compound, i.e., a salt, dipole–dipole interactions between two or more ionic groups also form physical crosslinks that significantly alter the material properties. Ionomers are nano-structured materials, similar to block copolymers and nanocomposites. The comparison to block copolymers is emphasized by the theories of Nyrkova et al. [5] and Semenov et al., [6] who consider ionomers as block copolymers with very short ionic blocks that attract each other. This leads to a new regime for describing the phase segregation in block copolymers that they term a ‘‘superstrong segregation regime.’’ Those theories predict the formation of nanodomains for a random ionomer where the ionic species is distributed randomly along the polymer chain, i.e., the ionic blocks have a degree of polymerization of one. Small angle X-ray scattering [7] and electron micro- scopy [7,8] experiments provide a vague picture of the microstructure, but there has yet to be a definitive characterization of the microstructure of ionomers. Kirkmeyer et al. [9] have provided some of the best visual evidence for a spherical nanostructure of ionomers using scanning transmission electron microscopy (STEM), L. Zhang, N. R. Brostowitz, K. A. Cavicchi, R. A. Weiss Department of Polymer Engineering, The University of Akron, Akron, OH 44325-0301 E-mail: kac58@uakron.edu; rweiss@uakron.edu Feature Article ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mren.201300181 81 Macromol. React. Eng. 2014, 8, 81–99 wileyonlinelibrary.com