Self-Consistent Mean Field Theory for Micro-domain Structure in

H. J. Ploehn
University of South Carolina, US

Keywords: PEM Fuel Cell

Abstract:
ORAL The objective of this research is to develop statistical mechanical models that can be used to predict the phase behavior and micro-domain structure of perfluorinated ionomers. Specifically, we wish to understand how the ionomer architecture controls the equilibrium microstructure. To achieve this goal, existing statistical mechanical models for linear homopolymers have been extended in order to account for co-monomers and branched chain topology. We have implemented lattice-based self-consistent mean field (SCMF) theory, programmed using an object-oriented approach, to predict the equilibrium structure in Nafion® and Dow short-side-chain ionomers. With decreasing water content or temperature, we observe a phase transition: the spatial distributions of polymer and water transform from homogeneous to lamellar, with the domain size and coexistence conditions depending on the number of sulfonyl side-chains, their length, and their spacing along the fluorocarbon main chain. The primary phase transition occurs when the hydrophobic fluorocarbon segments segregate from the other, more hydrophilic segments. We see secondary transitions due to the segregation of ether and sulfonyl segments. The conditions for these transitions on the (concentration-temperature) phase diagram depend on the details of the molecular architecture. We must regard the results as qualitative because none of the interaction parameters have been set in accord with experimental data, nor do we account for the possibility of morphologies other than lamellar. Nevertheless, the results demonstrate that we have established the foundation to explore the relationship between polymer architecture and phase separated micro-domain structure in perfluorinated ionomers.

NSTI Nanotech 2003 Conference Technical Program Abstract