The influence of solvent cage structure on ionic aggregate structure, and structural morphology in polyether-ester-sulfonate copolymer ionomers

All-atom molecular dynamics simulations are carried out to understand the solvation cage structure, ionic aggregate structure, and structural morphology of polyether-ester [poly(ethylene oxide), PEO, and poly(tetramethylene oxide), PTMO] sulfonate sodium copolymer ionomers. The pure PEO-Na ionomer shows weak ionic aggregates compared with pure PTMO-Na ionomer, which arises from the formation of a solvation cage structure that ethers oxygen atoms within PEO backbone chains encase ions to shield the coulombic interaction of ionic pairs. By analyzing the difference of solvation cage structure in pure PEO and PTMO-Na ionomers, the changes of ionic aggregate structure and structural morphology of copolymer ionomers upon the content of PEO and temperature are clarified. Specifically, the separated microdomains show a thermally driven mixing tendency as temperature is raised, which is attributed to thermally driven enhancing the ionic aggregates near the interface between PEO and PTMO microdomains, thereby promoting the effective compatibility of interface between PEO and PTMO microdomains. These findings provide a direct link between previous experiments about x-ray scattering [Macromolecules 45, 3962 (2012)] and rheology [Macromolecules 47, 3635 (2014)], which has significant implications for the design and optimization of single-ion conductor material properties.