Thermodynamic and kinetic effects in the aggregation behavior of a poly(ethylene glycol-b-propylene sulfide-b-ethylene glycol) ABA triblock copolymer

We explore the effects of preparation protocol on the morphology and stability of aggregates from a poly(ethylene glycol-b-propylene sulfide-b-ethylene glycol) triblock copolymer, PEG₄₄-PPS ₇₆-PEG₄₄. Fluorescence spectra and excimer formation of the probe molecule pyrene elucidated the various stages of aggregate formation, and cryo-TEM yielded insight into aggregate morphology. When prepared by direct hydration of polymer films, an extraordinary variety of morphologies was formed, ranging from spherical micelles to wormlike micelles, Y-junctions, blackberry micelles, and vesicles. Aging produced more uniform structural ensembles, including wormlike micelles with undulations and eventually spherical micelles, indicating the nonequilibrium nature of the system as initially formed. On the contrary, preparation by dilution from organic solvent yielded only structures that were closer to equilibrium distributions.