Ionomers contain a small amount of ionic comonomer, such as carboxylic acid groups neutralized with a metal cation. Aggregation of these metal salt groups induces a heterogeneity with a length scale of a few nanometers, as shown schematically below:
Because electrostatic interactions are strong relative to those normally encountered in polymers, even very small ionic aggregates (order 1 nm) are stable, yielding structures an order of magnitude smaller than crystallization produces. Crystallizability is easily “built into” a macromolecule side-by-side with ionic aggregation, allowing the rich structural diversity which comes from having multiple self-organizing mechanisms at work. The best-known (and commercially most important) ionomers are indeed crystallizable; they are derived from ethylene-methacrylic acid copolymers, by neutralizing some or all of the methacrylic acid units with a metal cation (e.g., Na+ or Zn2+). Selected materials in this category are marketed by DuPont under the tradename Surlyn®.