Molecular Mechanism of Mechanical Breathing in Organic Mixed Ionic-Electronic Conductors

Mechanical breathing of organic mixed ionic-electronic conductors (OMIECs) is a structural response upon redox reactions. The breathing strain is often in the range of a few percentages to a few hundred percentages in different OMIECs operated in different chemical environments. Such mechanical activation needs to be tailored such as to be maximized in actuators and to be minimized in increasing the device reliability. We perform atomistic modeling to understand the molecular mechanism of mechanical swelling of OMIECs immersed in various electrolytes and at different oxidation states. We study poly(3,4-propylenedioxythiophene) (PProDOT) which is widely used in organic electrochromic devices as a model system and compare its swelling behavior in electrolytes of different salt concentrations, solvents, and anions. PProDOT deforms more in the electrolyte of lower salt concentration and larger anions. Mass transport of the electrolyte, especially the organic solvent, is dominant in regulating mechanical swelling of PProDOT compared to the electrostatic interactions. We examine the evolution of microstructural features and local bonding environments associated with the mixed conduction upon oxidation. We calculate the diffusion coefficients of the cation, anions, and solvents in the mixture of PProDOT and the electrolyte, which inform the swelling kinetics of PProDOT and the solvation structure in the electrolyte. The results are further validated by different self-aggregated PProDOT configurations and modeling protocols. The finding is in good agreement with the experiments and provides fundamental understanding of the molecular motifs underpinning the breathing strain in OMIECs.