Effect of Ion-Polymer Solvation Strength on Ion Diffusion in Model Diblock Copolymers

Salt-doped block copolymers, with one microphase domain that dissolves ions (allowing for ion conduction) and another that provides mechanical strength, are of interest as safe, robust battery electrolytes, among other applications. These materials are challenging to model due to the need to represent both strong, local ion correlations and much longer length scale features of the overall microphase separated morphology. Additionally, the microphase domains have significantly different local dielectric strengths and, relatedly, ions are strongly selectively solvated in the higher dielectric microphase; it is not clear how/whether the major physical impacts of these chemical features can be captured using simple, generic models that can easily access the time and length scales of interest (e.g. without resorting to atomistic simulations with polarizability). We work towards such a coarse-grained model, including only radially symmetric pairwise interactions. The model is implemented within a fluids density functional theory framework and in molecular dynamics simulations. Besides bonding interactions, Lennard-Jones potentials (less favorable for unlike monomers), and the Coulomb potential between ions (the strength of which is set by the higher dielectric microphase where the vast majority of ions exist), we include a phenomenological solvation potential for interactions with ions. The solvation potential is of form S/r⁴, where S can be different for the different ion-ion and ion-monomer interactions (this drives ions to the higher dielectric microphase). This form gives the proper scaling of solvation energy with ion size, among other advantages. In contrast to prior work without strong solvation, we find conditions under which the microphase domain spacing increases monotonically as salt is added, and we find that ion diffusion increases with polymer length for diblock copolymers but decreases for homopolymers.