title: Atomistic Modeling of PEDOT:PSS Complexes II: Force Field Parameterization
author: Wesley Michaels
description: The conductive polymer complex poly­(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is integral to many next-generation polymeric devices. All-atom molecular dynamics (MD) is increasingly employed to probe film microstructure, but the commonly used Generalized AMBER Force Field (GAFF) parameters remain unvalidated for this system. Here, we develop all-atom force fields for both undoped and highly doped PEDOT compatible with GAFF. Molecular geometries, vibrational energies, torsional profiles, and conformational energetics calculated with density functional theory comprise the training data. The optimized force fields improve upon GAFF for all the training data, including notable improvements in torsional barriers, and capture the finite size of charge carriers in doped PEDOT. Although GAFF predicts similar behavior between undoped and doped polymers, our models identify meaningful differences in ground-state geometry, vibrational spectra, and torsional barriers between the two. Furthermore, vacuum MD simulations show that the neutral polymer is more flexible than the highly doped polymer, primarily due to the weaker inter-monomer dihedral potentials. The foregoing trends are directly attributable to the aromatic-to-quinoid transition PEDOT experiences upon doping. This coupling between doping and molecular flexibility highlights the importance of charge–geometry interactions in atomistic modeling and should be considered while modeling conjugated polymers beyond PEDOT.