Single-chain and aggregation properties of semiconducting polymer solutions investigated by coarse-grained Langevin dynamics simulation

A coarse-grained (CG) model and Langevin dynamics scheme are proposed to investigate the material properties in dilute solution of a model semiconducting conjugated polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). While the intra- and intermolecular potentials for the CG particle (currently, a monomer unit) were determined from the molecular dynamics (MD) simulation of a united atomistic model, fluctuation-dissipation forces arising from the treatment of a solvent field were self-consistently constructed from the measured particle diffusivity in a given solvent (i.e., chloroform or toluene) through the atomistic MD simulation. It is shown that the resultant Langevin dynamics simulation, which is substantially more efficient than the counterpart MD simulation of the same CG model, is able to capture the dynamic (such as center-of-mass diffusivity) as well as the structural (such as radius of gyration) features of the investigated polymer solutions. Essential material properties that can now be directly studied include the following: Scaling exponents for estimating the exact solvent qualities were, for the first time, determined for the two solvent systems investigated; the persistence length obtained was also noted to be in excellent agreement with early experimental estimations. Preliminary observations on the supramolecular aggregation properties were in good agreement with the general observations from a wide range of recent experiments, and shed light on the essential impact of solvent quality on the supramolecular aggregation structures.