Sub-3-Nanometer Domain Spacings of Ultrahigh-χ Multiblock Copolymers with Pendant Ionic Groups: Morphologies, Phase Diagram, and Applications

We investigated the temperature-dependent phase behavior and interaction parameter of polyethylene-based multiblock copolymers with pendant ionic groups. These step-growth polymers contain short polyester blocks with a single SO₃^-Li⁺ group strictly alternating with polyethylene blocks of x-carbons (PESxLi, x = 12, 18). At room temperature, these polymers exhibit layered morphologies with semicrystalline polyethylene blocks. Upon heating above the melting point, PES18Li shows two order-to-order transitions involving  gyroid and hexagonal morphologies. For PES12Li, an order-to-disorder transition accompanies the melting of the polyethylene blocks. Notably, a Flory-Huggins interaction parameter was determined from the disordered PES12Li using mean-field theory: χ (T) = 77.4/T + 2.95 (T in K) and χ (25°C) ~ 3.21. This ultrahigh-χ indicates that the polar ionic and non-polar polyethylene blocks are highly incompatible, and affords well-ordered morphologies with sub-3 nm domain spacings. The experimental phase diagram of these polymers contradicts the self-consistent field theory of neutral multiblock copolymers. Finally, the potential applications of these multiblock copolymers as ion-conducting membranes and templating nanoscale materials will be discussed.