Self-assembly of block copolymers via micellar intermediate states into vesicles on time scales from milliseconds to days

•A new 3D-hydrodynamic flow focusing microfluidic chip is designed that avoids wall-contact of reacting solutions.•With this design the structural evolution starting from singly dissolved block copolymers can be followed by in-situ SAXS.•Micelle formation is diffusion-limited and occurs on time-scales of milliseconds.•The subsequent formation of a lyotropic ordered phase takes about 1 s.

Block copolymer micelles and vesicles are mostly prepared by the solvent mixing method, where the block copolymer is first dissolved in a common solvent for both blocks, which is then mixed with a selective solvent, mostly water, to induce self-assembly into the desired structure. Using a combination of microfluidic flow-focusing and capillary interdiffusion experiments combined with in-situ small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM) we investigated the structural evolution during solvent mixing from single block copolymers into spherical and cylindrical micellar intermediate structures into vesicles. We find that micelle formation is very fast and diffusion-limited, occurring on time scales of a few milliseconds. The development of an ordered lyotropic micellar phase is completed within 1 s. The structural transformation into cylindrical micelles occurs over several hours, which subsequently evolve into vesicles over time scales of days. Whereas the first two steps are transport-limited, the two latter processes involve large activation energies related to micellar fusion against the sterically stabilizing micellar coronas, which corresponds to much longer time scales of self-assembly.

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