Investigation of monomer segment distributions, chain conformations, and lithium salt solvation in self-assembled, tapered block polymer electrolytes

Tapered block polymers (TBPs) contain modified monomer segment distributions (e.g., gradient or random copolymer regions) at the chemical junction between two homogeneous blocks. Nanostructured polystyrene-block-poly(oligo-oxyethylene methacrylate) (PS-b-POEM) TBP electrolytes have exhibited improved ionic conductivities, shear moduli, and processabilities in comparison to their conventional block polymer analogues. In this work, we studied the microscopic characteristics of TBPs that impart these enhanced properties. The monomer segment distributions of lithium salt-doped normal-, inverse-, and non-tapered PS-b-POEM TBPs were obtained via X-ray reflectivity, and these distributions also were successfully modeled through coarse-grained molecular dynamics simulations that included strong ion solvation effects. This combined experimental-computational approach allowed the segregation strengths, chain conformations, and ion solvation energies of the salt-doped TBPs to be quantified as a function of taper sequence and salt concentration. By understanding how these polymer assembly and ion solvation behaviors affect ion transport, we can guide the rational design of higher-performance polymer electrolytes.