Alternating Multiblock Polyethylenes With Associating Groups: Self-Assembled Nanoscale Morphologies And Ion Transport
Mechanics of Materials
Nanoscience and Nanotechnology
Single-ion conductors based on block copolymers (BCPs) are promising solid-state electrolytes for energy storage systems. Their ability to self-assemble into distinct nanostructures can provide both high ion transference numbers and strong mechanical integrity. Connecting the microphase-separated morphologies to the ion transport properties in BCP electrolytes as well as designing polymers to produce specific ion-conducting domain remain a critically important challenge. Combining non-conducting polyethylene (PE) blocks that alternate with short strongly-interacting ionic blocks leads to a wide range of intriguing nanoscale phase-separated morphologies. Depending on the PE block lengths, these alternating multiblock copolymers exhibit amorphous or semicrystalline morphologies and their ionic aggregation behaviors are further tunable via the modification of ionic block chemistry. When the PE blocks are long, such as 21 or 46 methylene units, the alternating multiblock PEs are semicrystalline and chains fold close to the short ionic blocks with the ionic groups aggregating into layers embedded in the crystalline regions, as determined both in bulk and solution-growth crystals. Remarkably, the ion transport in such semicrystalline layered morphology is decoupled from the polymer segmental motion even above these polymer glass transition temperatures. These layered ionic aggregates can transform upon heating into bicontinuous gyroid, hexagonal or disordered phases as controlled by the volume fraction and electrostatic interactions of the ionic blocks. At the same temperature and composition, the 3D interconnected gyroid structure exhibits higher ionic conductivity than the layered or hexagonal phases. Shortening the PE block lengths to 6 or 12 methylene units prohibits crystallization and produces nanoscale ionic domains with the connectivity of ionic aggregates influenced by total ion content. Increasing the conducting domain connectivity to form a percolated structure facilitates decoupling ion transport from the polymer segmental motion. In contrast to conventional salt-doped BCP electrolyte systems, the ion-conducting nanoscale domains in these alternating multiblock PEs mainly consist of ionic groups and are inherently favorable for fast ion conduction. Producing semicrystalline layered or highly interconnected ion-conducting domains that decouple the ionic conductivity from polymer segmental dynamics provides promising design principles toward efficient solid-state polymer electrolytes.