(346d) Penetrant and Ion Dynamics in Model Microphase Separated Copolymers

Nanostructured AB block copolymers have been of great interest for transport applications such as lithium ion battery electrolytes and CO₂ separation membranes. Various nanostructures (lamellae, cylinder, gyroid, sphere) form as functions of the fraction of A monomer, A-B interaction strength, and molecular weight, and one can choose the A monomer to allow transport of the target molecules while the B monomer provides mechanical robustness. Here, we study both diblock and tapered AB block copolymers (TBCs) where a â??taperedâ? mid-block is inserted between two pure blocks of A and B monomers. The taper has a gradient in composition from pure A to pure B (or from pure B to pure A for an inverse taper) and taper length can be used an additional tuning parameter to control the nanostructure. A recent experiment shows a certain length of taper can improve both polymer and ion dynamics for battery electrolyte systems [1]. To provide physical insight into such results and predict which systems may improve transport, we perform molecular dynamics simulations using a simple coarse-grained bead-spring model. Monomer-sized penetrants with favorable interactions with one type of monomer are added to lamellar structures of normal and inverse TBCs at a range of taper length from 0% (diblocks) to 100% (gradient copolymer). We observe both polymersâ?? and penetrantsâ?? dynamics increase as taper length increases for normal TBCs but non-monotonically change for inverse TBCs. For the inverse TBCs, nonintuitive trends in the dynamics arise from competing effects of local A-B mixing and chain conformations. We have also considered salt-doped systems (with full long-ranged Coulomb interactions) to explore how ion transport is different than that of uncharged small penetrant molecules.

[1] W.-F. Kuan, R. Remy, M. E. Mackay, and T. H. Epps, III, RSC Adv. 5, 12597 (2015).