Developing multiscale models of conjugated polymer morphology and transport
Keywords: Chemical Engineering.

Conjugated polymers (CPs), polymers with backbones of alternating single and double bonds, can conduct electricity yet remain lightweight, flexible, optically transparent, and biocompatible. With this attractive combination of properties, CPs have the potential to transform devices for clean energy generation and storage, computing, and personalized health. However, CPs cannot currently meet the full range of performance requirements in many of these demanding applications, while the intrinsic disorder in CP morphology frustrates experimental efforts to characterize film microstructure and to elucidate the structure-function relationships necessary for rational device design. Computational and theoretical techniques have the resolution to probe these microstructural details, but the reliance on generic models, which do not capture important chemical details underlying CP flexibility, has limited their ability to investigate CP morphology. I seek to extend generic models to capture these details, creating a reliable foundation to study the influence of CP structure on device morphology and transport. Generic molecular dynamics (MD) models commonly used to study CP film and solution microstructure were not trained on CPs, yet their extrapolation to CP systems has not been validated. Density functional theory (DFT) calculations can reliably model CP flexibility, but the plethora of density functionals (DFs) are empirically formulated and are not all accurate for every system. Thus, I completed two projects to improve MD models of a popular CP system, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). First, I benchmarked several DFs against high-accuracy ab initio predictions of CP flexibility. I found that electron localization induced by Hartree-Fock (HF) exchange systematically influences these predictions: DFs which balance HF with other contributions to the DFT Hamiltonian, such as ωB97x-D, provide better predictions. Then, I used ωB97x-D to parameterize an MD force field (FF) for PEDOT in undoped and highly doped states. These FFs are more accurate than the generic models and capture the effect of doping on CP flexibility. Accurately representing PEDOT:PSS microstructure proves to be foundational to understanding charge transport (CT) mechanisms in the material. In experiment, organic electrochemical transistors (OECTs) with a PEDOT:PSS channel exhibited an exponential decay of the mobility with increasing PSS loading, which existing percolation-based and effective-medium CT models could not explain. In a collaborative effort, I helped develop a model proposing that holes tunnel between isolated PEDOT chains in the PSS-rich phase, giving rise to macroscopic CT. Our multiscale CT model, using the optimized MD FF to represent film microstructure, replicates the observed mobility scaling. Thus, the ion-conductive phase in OECTs, traditionally understood as electrically insulating, is in fact a barrier to tunneling and therefore plays a critical role in CT. We demonstrate that the choice of ion-conducting material influences tunneling rates, suggesting a new relationship to guide future design of OECTs and other CP devices. Statistical polymer models, such as the wormlike chain (WLC), permit the study of polymer behavior over longer length and time scales than atomistic simulation, but existing models fail to capture the anisotropic bending present in CP chains and CP melt morphology. To introduce this anisotropy, I created a generalized WLC model called the ribbon-like chain (RLC) and solved the RLC free-chain Green's function. The Green's function solutions can calculate single-chain statistics such as tangent autocorrelations, the radius of gyration, and chain shape, and clearly demonstrate the influence of anisotropic bending on these properties. When the RLC model is applied to MD simulations of CP melts, the chain statistics also highlight anisotropic bending and are reflective of CP structure. Thus, the RLC model can better examine the relationship between CP structure and melt morphology. Altogether, this dissertation advances models of CP morphology and transport by incorporating chemical details influential on the conformational behavior of CPs. These models lay a foundation for more robust computational and theoretical studies of CPs, supporting experimental improvements of these promising materials