Processing Path-Dependent Complex Micelle Packings of Hydrated Small Molecule Amphiphiles

Water drives the self-assembly of (non)ionic diblock oligomers into spatially periodic lyotropic liquid crystals (LLCs), including lamellae, networks, hexagonally-packed cylinders (HEX), and 3D micelle packings. Beyond high symmetry body-centered cubic (BCC) and cubic close-packed micellar phases, diblock oligomers also form tetrahedrally closest-packed Frank-Kasper (FK) A15 phases. In spite of the low molecular weights of the constituent amphiphiles, we recently demonstrated that judicious thermal processing of an A15 LLC enables formation of a surprisingly long-lived, non-equilibrium state. Specifically, heating an A15 phase drives transitions to BCC and HEX phases at elevated temperatures. Quenching these LLCs unexpectedly drives formation of a remarkably well-ordered, tetragonal FK sigma phase comprising 30 quasispherical micelles per unit cell as a metastable state, which takes ~150 days of quiescent annealing at 22 °C to recover the equilibrium A15 structure. The formation and metastability of the sigma phase is contingent on sample quench rate, quench depth, and annealing temperature. These slow order-order phase transformation kinetics stem from a complex interplay of temperature-dependent phase nucleation and growth rates, which are coupled to the rates of micelle size reconfiguration by interparticle chain exchange and of spatial rearrangement of the micelles. These findings highlight the importance of processing path-dependence on the observed mesophases of self-assembled soft materials.