(63c) Assessment of Molecular Dynamics Force Fields for Conjugated Polymers Using Neutron and X-Ray Scattering

Conjugated polymers can be ideal materials for the design of efficient photovoltaic devices, batteries, thermoelectric cells, light emitting diodes and many new technologies. Moreover, a large flexibility in organic synthesis methods enables the efficient application of molecular design principles to produce superior materials. Yet, the use of computational methodologies for conjugated polymers is still limited by a lack of properly validated simulation force fields that can be used to model structures and temporal fluctuations through molecular dynamics simulations. This presentation outlines our use of neutron and x-ray scattering techniques for the development of improved molecular simulation force fields and structural parameters specifically produced for poly-3-alkyl-thiophene, a model conjugated polymer. Quasi-elastic neutron scattering (QENS) experiments are used along with MD simulations to quantitatively compare proposed force fields to extensive sets of experimental data. X-ray and polarized neutron diffraction are also used to correlate experimental and model-generated polymer structures. QENS validation of MD force fields presents a unique opportunity to increase the accuracy of highly uncertain parameters that are used in the simulation of conjugated polymers, including partial charges, Lennard-Jones and backbone torsion parameters. Many of these parameters are estimated from quantum mechanical calculations such as density functional theory but, unlike for force fields designed for small molecules, they are not often parameterized to experimental data. High variability is also observed in parameters for the small number of simulation force fields that have been proposed in the literature. A vision for the accelerated development of accurate force fields for these classes of materials is also proposed.