Hierarchical self-assembly of miktoarm star copolymers with pathway complexity

Abstract

The hierarchical self-assembly of molecular building blocks provides promising opportunities toward the exploration of functional soft materials with structural programmability. Here, we report the hierarchical self-assembly behaviors of amphiphilic miktoarm star copolymers with pathway complexity. In our experiments, supramolecular miktoarm star copolymers with a cluster core of [α-SiW₁₂O₄₀]⁴⁻ and four polystyrene-block-poly(ethylene glycol) cations (PS_n-b⁺-PEG_m, n = 17, 26, 39, 57, 81; m = 45, SEW-1–5) are used as a model of amphiphilic miktoarm stars. When dispersed in the selective THF/methanol and toluene/methanol mixture solvents, the miktoarm stars of SEW-2–5 self-assemble to form bundled fibers, sheet-like assemblies, and hollow spheres. These complex structures are packed by reverse cylindrical or spherical micelles having [α-SiW₁₂O₄₀]⁴⁻/PEG₄₅ cores and PS_n coronas, wherein the micelle building blocks are originally formed by SEW-2–5 in the nonselective solvents THF and toluene. These hierarchically self-assembled structures do not resemble micelle-like nano-objects formed by amphiphilic copolymers in selective solvents. The mechanism behind such unconventional aggregates is presumably due to intra- and inter-micelle van der Waals attractions occurring under poor solvent conditions for the PS_n coronas. The difference is that SEW-3–5 self-assemble into normal vesicles with a PS_n core and a [α-SiW₁₂O₄₀]⁴⁻/PEG₄₅ corona in the chloroform/methanol mixture solvent. The reverse cylinders, rices, and spheres originally fabricated from SEW-3–5 in chloroform experience molecular reorganization for such normal vesicles in the present selective solvents. Moreover, SEW-1 forms normal lamellae in all the methanol based mixture solvents. The results presented herein not only enable us to reconsider the self-assembly behaviors of amphiphilic miktoarm stars in solution, but also provide opportunities for constructing advanced functional materials with high-level structural hierarchy.