Correlation between Ionic Mobility and Microstructure in Block Copolymers. A Coarse-Grained Modeling Study.

Ion transport in a generic model of block copolymers (A-b-B) was studied using Molecular Dynamics simulations. The design parameters of the block copolymer, the ions and the ion-polymer interactions, were all systematically varied to unveil correlations between microstructure and ion mobility (μ). It is found that two key microstructural features had a significant effect on ion transport: the extent of interpenetration between the microdomains (β) and the local fluctuations in the density (ρ) of the polymer matrix. While the β effect has been previously studied in some detail, the effect of density non-homogeneities has received much less attention. To be able to control the local fluctuations in ρ, a polymer design variant is explored that incorporates a second conductive block (A’) that is incompatible with the other two blocks (A’-b-A-b-B). It is found that increasing the fraction of A’ beads, increases the frequency and amplitude of the local ρ depleted regions within the conductive domain, resulting in an increase in μ. More generally, polymer designs are advocated that enhance ion transport by leveraging the natural density and composition fluctuations associated with block copolymer ordering transitions.