A molecular dynamics investigation of the stability of a charged electroactive polymer monolayer

In a molecular dynamics simulation a monolayer is investigated consisting of an amphiphilic polythiophene on a sodium chloride solution. When the monolayer is allowed to become chemically reduced the π-conjugated thiophene rings assume overall negative charges which are compensated by excess Na+ concentrations in the aqueous layer. One result of endowing the polymer with net negative charge is a loss of the planarity of the monolayer, leading to a buckling and eventual rupture. A second is the attraction of the 'excess' Na+ ions to the interface, to occupy sites near the thiophene rings. At low redox reduction levels of the polymer the ions in these sites are much more mobile than the Na+ ions in the NaCl solution layer and respond to applied electric fields by jumping between sites, showing energy barriers of 0.33 eV. Gouy-Chapman theory is applied to discuss the eventual instability of the polymer membrane.