Processing Path-Dependent Complex Micelle Packings of Hydrated Diblock Polymer Amphiphiles

Water drives the self-assembly of short diblock polymer amphiphiles into spatially periodic lyotropic liquid crystalline (LLC) mesophases, including lamellae, polycontinuous networks, hexagonally-packed cylinders (HEX), and 3D sphere 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 weight of the constituent amphiphile, 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 at 22 °C to revert to the original 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 molecular-level rate of particle size reconfiguration and mesoscale rate of spatial rearrangement of the micelles. These findings highlight the importance of processing path-dependence on the observed mesophases of self-assembled soft materials.