Mechanisms Underlying Ion Transport in Lamellar Block Copolymer Membranes

Abstract

Recent experiments have reported intriguing trends for the molecular weight (MW) dependence of the conductivity of block copolymer lamellae that contrast with the behavior of homopolymer matrices. By using coarse-grained simulations of the sorption and transport of penetrant cations, we probe the possible mechanisms underlying such behavior. Our results indicate that the MW dependence of conductivity of homopolymeric and block copolymeric matrices arise from different mechanisms. On the one hand, the solvation energies of cations, and, in turn, the charge carrier concentrations, themselves, exhibit a MW dependence in block copolymer matrices. Such trends are shown to arise from variations in the thickness of the conducting phase relative to that of the interfacial zones. Moreover, distinct mechanisms are shown to be responsible for the diffusivities of ions in homopolymer and block copolymer matrices. In the former, diffusivity effects associated with the free ends of the polymers play an important role. In contrast, in block copolymer lamellae, the interfacial zone between the blocks presents a zone of hindered diffusivity for ions and manifests as a molecular weight dependence of the ionic diffusivity. Together, the preceding mechanisms are shown to provide a plausible explanation for the experimentally observed trends for the conductivity of block copolymer matrices.

Influence of molecular weight and degree of segregation on local segmental dynamics of ordered block copolymers

“…Motivated by the above results, in an earlier article we presented coarse‐grained simulations of the sorption, distribution and transport of penetrant cations in block copolymer systems. Our results for ion conductivities exhibited MW behavior similar to those seen in experimental results . Our analysis suggested that with increasing molecular weight of the block copolymer there was a reduction in the overlap of the ions with the interfacial zone of the block copolymer.…”

“…However, we note that ion conductivities in block copolymer electrolytes also depend on a number factors not accounted in the present study. For instance, the ion distributions in the lamella can depend on the molecular weight and the degree of segregation between the blocks (such effects were investigated in our earlier work). Moreover, the segmental dynamics themselves can be affected by the coordination between the ions and the polymer .…”

Poly(ethylene oxide)‐based block copolymer electrolytes for lithium metal batteries

“…They found that the ionic conductivity of the PS‐ b ‐PEO/Li electrolytes with a ratio of [EO]/[Li] = 10–50 increased with M n (PEO) until a plateau was reached; this behaviour was the opposite to that of the PEO homopolymer. Such a surprising behaviour was nevertheless theoretically predicted recently . Our recent experimental studies of PS‐ b ‐PEO‐ b ‐PS triblock copolymers undoped and doped with LiTFSI revealed the existence of a ‘dead zone’, excluded from ionic transport, at the PS/PEO interface .…”

“…Such a surprising behaviour was nevertheless theoretically predicted recently. 35 Our recent experimental studies of PS-b-PEO-b-PS triblock copolymers undoped and doped with LiTFSI revealed the existence of a 'dead zone', excluded from ionic transport, at the PS/PEO interface. 36,37 The thickness of this zone, , included about 4-5 EO units (ca 1.6 nm), and was the same as that of the exclusion zone for crystallization in undoped PS-b-PEO-b-PS triblock copolymers.…”

Simulation of local ion transport in lamellar block copolymer electrolytes based on electron micrographs

“…Within this model, the ionic conductivity of a block copolymer electrolyte, , is given by where f is the morphology factor, is the intrinsic conductivity of the bulk ion‐transporting microphase phase, and is the volume fraction of the ion‐transporting microphase . Others have proposed modifications to the model in eq to account for additional structural details such as tortuosity and the non‐conducting interfacial volume between micrphases . For randomly‐oriented lamellae in a bulk three‐dimensional material, f is 2/3; two out of three principle orientations lead to conduction.…”