Atomistic Modeling of PEDOT:PSS Complexes II: Force Field Parameterization

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.