Stable Double Gyroid Structures in Ionic-Aliphatic Multiblock Copolymers

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Ion transporting polymers are key materials for energy applications and membranes. As an alternative to fast chain dynamics as the basis of efficient ion transport, ions may be arranged in suitable percolate aggregated structures. This can advantageously decouple the ion transport dynamics from the matrix dynamics, which may not only benefit transport rates but at the same time also provide structurally stable ordered matrix materials. Bi-continuous structures are of particular interest to this end, however, achieving such structured materials is challenging. The double gyroid (DG) morphology intrigues by its structural complexity, being composed of two interpenetrating networks of the minor component which together with the matrix represent a bi-continuous structure. Beyond improving transport properties, the percolated domains of DG morphologies can also enhance mechanical properties compared to simpler morphologies. In our preliminary work we established that sulfonated polyesters with long-chain aliphatic repeat units behave as (AB)n multiblock copolymers in their phase behavior. The highly incompatible polar ionic and apolar hydrocarbon segments segregate even when the combined length of two alternating blocks AB is as low as eighteen carbon atoms. Unexpectedly, such ionic-aliphatic multiblock copolymers (IAMBCs) in the melt can adopt DG structures, with desirable sub-10 nm length scales. However, as the polymers' aliphatic units were designed here to crystallize to target layered structures these DGs are not stable at ambient conditions. The objective of the research proposed is to design and access DG morphologies stable at ambient conditions. We will elucidate apolar non-crystallizable hydrocarbon repeat unit motifs that afford microphase phase separation in ionic-aliphatic multiblock copolymers into DG morphologies. For this purpose we will work out synthetic routes to monomers anticipated to provide high reactivity for polycondensation and to afford non-crystallizing repeat units also favorable for adapting to the surface curvature of the DG morphology. As a complimentary approach, crosslinking strategies to permanently fixate the DG morphologies will be explored. As a prerequisite an understanding of the correlation of degrees of polymerization and morphology will be established. Cross-linking methodology directly during polymerization or via post-polymerization cross-linking will be implemented. This cross-linking is also expected to enhance the robustness of the target materials, for example to enhance resistance towards solvents that can enhance ion mobility. First insights into the ion conducting and mechanical properties resulting from these morphologies will be gained. In a broader sense, our anticipated understanding of microstructure-morphology relationships of this exceptional system will be instrumental to rationally design novel materials with for example superior mechanical stability and ion conductivity.