Effect of Substrate on Structural Phase Transition in a Conducting Polymer during Ion Injection and Water Intake: A View from a Computational Microscope

Conducting polymers operating in aqueous electrolyte represent mixed electron-ion conductors, where the ion injection and water intake can lead to structural and morphological changes that can strongly affect the material morphology and device performance. In the present paper, using molecular dynamics simulations, we provide an atomistic understanding of the structural phase transitions during electrochemical oxidation and ion injection in a conjugated polymer with glycolated side chains recently reported by Bischak et al. [J. Am. Chem. Soc., 2020, 142, 7434], where the polymer switched between two structurally distinct phases corresponding to different oxidation levels. To outline the structural changes, we calculated the polymer film morphology and X-ray diffraction patterns at different oxidation levels. We demonstrated that the observed phase transition arises due to interplay between several factors, including the effect of the substrate leading to the preferential edge-on arrangement of the chains and formation of lamellas; unzipping of the interdigitated polymer chains during oxidation and ion intake; and changes in the morphology when pi-pi stacking is absent at low oxidation level and forms at the high oxidation level facilitating the electron mobility and enabling the oxidation of the polymer film. Our calculations quantitatively reproduce the experimental data, which outlines the predictive power of the molecular modeling of the polymer systems that can be utilized for the design of materials and devices with improved performance.