Quantifying the Effects of Monomer Segment Distributions on Ion Transport in Tapered Block Polymer Electrolytes
PM Ketkar and KH Shen and MD Fan and LM Hall and TH Epps, MACROMOLECULES, 54, 7590-7602 (2021).
Tapered block polymers, which contain modified monomer segment distributions at the chemical junction between homogeneous blocks, can offer improved local segmental mobility and ion transport in comparison to their non-tapered, conventional analogues. In this work, the effective Flory-Huggins interaction parameters (chi(eff)s) and distributions of conductive monomer segments, two factors that influence the segmental mobility, were investigated in salt-doped, self-assembled non-, normal-, and inverse-tapered polystyrene-block-poly(oligo- oxyethylene methyl ether methacrylate) (PS-b-POEM) electrolytes using X-ray reflectometry measurements and coarse-grained molecular dynamics simulations. The increased covalent S/OEM bonding in tapered specimens contributed to a decrease in chi(eff), but the increased S-ion contacts contributed to an increase in chi(eff), especially in the inverse- tapered system. More specifically, the OEM content closer to the PS/POEM domain interfaces, at which the most drastic reduction in POEM mobility was found, was minimized at high domain spacings and interfacial widths. This combination of characteristics was particularly apparent in the non- and normal-tapered polymers as the stretching of chains across a single domain enabled reduced relative ion and OEM content near the PS/POEM domain boundaries, resulting in favorable ion transport conditions. In contrast, most inverse-tapered chains bridged across multiple domains or folded across an interface, which decreased the segmental mobility. The normal-tapered electrolytes also demonstrated reduced local stretching of the POEM chains in comparison to non- and inverse-tapered electrolytes. Taken together, these results suggested that a high-molecular-weight normal-tapered electrolyte could have the desirable structural characteristics (i.e., low segregation strength relative to non-tapered systems, large domain spacings, wide interfaces, and minimal local POEM chain stretching) that maximize the ionic conductivity in nanostructure-forming block polymers.
Return to Publications page