Role of electrostatic forces in cluster formation in a dry ionomer.

This simulation study investigates the dependence of the structure of dry Nafion-like ionomers on the electrostatic interactions between the components of the molecules. In order to speed equilibration, a procedure was adopted which involved detaching the side chains from the backbone and cutting the backbone into segments, and then reassembling the macromolecule by means of a strong imposed attractive force between the cut ends of the backbone, and between the nonionic ends of the side chains and the midpoints of the backbone segments. Parameters varied in this study include the dielectric constant, the free volume, side chain length, and strength of head group interactions. A series of coarse-grained mesoscale simulations shows the morphology to depend sensitively on the ratio of the strength of the dipole-dipole interactions between the side-chain acidic end groups to the strength of the other electrostatic components of the Hamiltonian. Examples of the two differing morphologies proposed by Gierke and co-workers [J. Polym. Sci., Polym. Phys. Ed. 19, 1687 (1981); Macromolecules 15, 101 (1982); J. Membr. Sci. 13, 307 (1982)] and by Gebel [Fuel Cells 5, 261 (2005); Macromolecules 37, 7772 (2004)] emerge from our simulations.