(740b) Domain Spacing and Phase Behavior of Salt-Doped Block Copolymers from Fluids Density Functional Theory

Salt-doped block copolymers have potential as mechanically robust, nonflammable electrolytes in batteries, among other applications. Typically, one monomer type has a low dielectric constant but provides mechanical strength to the material, while the other phase has a higher dielectric constant and can transport lithium ions. The salt strongly prefers to exist in the higher dielectric constant phase, and adding salt is known to significantly alter the order-to-disorder transition temperature, interfacial width, and, in some cases, the preferred microphase morphology. All of these impact both ion conduction and mechanical properties, and better understanding of these trends would increase the ability for rational design within this class of materials. We model these materials at the coarse-grained level to more clearly delineate how basic molecular characteristics such as ion contact strength and ion solvation (or dielectric constant differences) drive their phase behavior and morphology trends.

In particular, we aim to capture monomer and ion local packing and correlations as well as overall polymer chain behavior in a coherent way by using fluids density functional theory (fDFT) with correlation functions calculated from liquid state theory. Within liquid state theory, we consider a restricted primitive model (a mixture of ions and solvent of identical size) that represents a reference fluid for the ion-containing microphase of the overall system. The ions have Coulomb interactions with each other as well as shorter ranged solvation interactions that are meant to account for the amount of preferential solvation of ions in the higher dielectric microphase (the strength of these interactions can be varied). With the Ornstein-Zernike equation and the hypernetted-chain closure, we find direct correlation functions that include features of local ion and solvated monomer packing that would not be captured at the mean field level. The fDFT uses these direct correlation functions and also accounts for bonding and interactions with the other type of polymer to predict the overall equilibrium structure. We capture the initial sharpening of the interface as a function of salt concentration as well as the slight broadening of the interface at high salt concentrations seen in some experiments. These trends depend significantly on the choice of solvation strength and ion contact energy.