Complex Spherical Micelle Packings in Aqueous Dispersions of Diblock Polymers

Aqueous self-assembly of diblock polymers enables access to a plethora of concentration- and temperature- dependent morphologies including lamellae, hexagonally-packed cylindrical micelles (H), and high-symmetry packings of spherical micelles. Herein, we describe the aqueous lyotropic phase behaviour of poly(ethylene–block–ethylene oxide) using small angle X-ray scattering. The temperature versus amphiphile concentration phase diagram exhibits multiple micellar packings, including well-known body- and face-centered cubic structures as well as the lower symmetry Frank-Kasper (FK) A15 phase. On heating, the A15 phase undergoes a transition into the H-phase. Rapidly quenching this sample from high temperature to 25 °C results in the nucleation and growth of a FK σ phase, which contains 30 quasispherical micelles situated at five different symmetry equivalent positions in a tetragonal lattice. We find that the σ phase is metastable with respect to A15, and that the metastability strongly depends on the rate of cooling from the H phase and the quench depth. We rationalize our findings based on a subtle energy balance which maximizes interchain cohesion while minimizing both variations in coronal chain stretching and the interfacial penalty for the hydrophobic/hydrophilic contacts.