Kinetic Pathways for Polyelectrolyte Coacervate Micelle Formation Revealed by Time-Resolved Synchrotron SAXS

The kinetic pathways for coacervation and micelle formation are still not fully understood. Driven by electrostatic interactions and entropically driven counterion release, complexation of oppositely charged macromolecules leads to the formation of micellar nanostructures. Here we study the coacervation process, from initial formation and growth of stable micelles, on a nanometric length scale using time-resolved small-angle X-ray scattering (TR-SAXS). The micellar coacervates are formed through the complexation of anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSSS) and cationic block-copolymer poly(ethylene oxide)-block-poly((vinylbenzyl)trimethylammonium chloride) (PEO-b-PVBTA). Mixing the polyelectrolytes in a stoichiometric 1:1 charge ratio resulted in the formation of stable spherical core-shell micellar-like coacervates consisting of a central core of complexed PSSS and PVBTA with a PEO corona. By use of synchrotron SAXS coupled to a stopped-flow mixing apparatus, the whole formation kinetics of coacervates could be followed in situ from a few milliseconds. The results of a detailed data modeling reveal that the formation of these polyelectrolyte coacervates follows a two-step process: (i) first, metastable large-scale aggregates are formed upon a barrier-free complexation immediately after mixing; (ii) subsequently, the clusters undergo charge equilibration upon chain rearrangement and exchange processes yielding micellar-like aggregates with net neutral charge that are pinched off to yield the final stable micelle-like coacervates. While the initial cluster formation is very fast and completed within the dead time of mixing, the subsequent rearrangement becomes significantly slower with increasing molecular weight of the PVBTA block. Interestingly, the overall kinetic process was essentially concentration independent, indicating that the rearrangement process is mainly accomplished via noncooperative chain rearrangement and chain exchange processes.