Consequences of Nonergodicity in Aqueous Binary PEO-PB Micellar Dispersions

Aqueous micellar dispersions of poly(ethylene oxide)-poly(butadiene) (PEO-PB) diblock copolymers were investigated using cryogenic transmission electron microscopy (cryo-TEM). A variety of binary blends were prepared by premixing (before dispersion) and postmixing (after dispersion) diblocks with varying composition and molecular weight. Cryo-TEM results establish there is no perceptible exchange of macromolecules between aggregates resulting in a nonergodic state, where overall equilibrium is never achieved. However, analysis of many microscopic images leads to the conclusion that these nonergodic micelles relax to a state of local equilibrium through the redistribution of block copolymers within the topological framework established during dispersion. Different average compositions and molecular weights exhibit varying degrees of structural complexity, which appears to peak in the vicinity of network formation. Away from this condition the micelle morphology is relatively insensitive to core or corona polydispersity. However, at compositions where the monomodal and nearly monodisperse amphiphiles produce branched wormlike micelles and a network, bimodal mixtures displayed intramicellar segregation leading to cylindrical undulations and octopus-like aggregates with cylindrical micelles emanating from a single bilayer core. These findings are discussed in the context of interfacial curvature and surfactancy.