Flow-Induced Micellar Morphological Transformation in Microfluidic Chips under Nonequilibrium State: From Aggregates to Spherical Micelles.

Self-assembly of block copolymers (BCPs) in microfluidic chips is a versatile yet effective route to produce micellar aggregates with various controllable sizes and morphologies. In this study, the morphological transformation of BCP of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) assemblies from irregular aggregates to multicompartment micelles, and ultimately to ordered spherical micelles is demonstrated in microfluidic chips. Our experimental and computational simulation results indicate that transverse diffusion of solvents plays an important role in the morphological transformation of PS-b-P4VP assemblies in the confined flow condition. We find that the mixing time (tmix) between BCP/tetrahydrofuran (THF) solution and water affects the morphological transformation. Micellar morphologies are intended to transform from aggregates to ordered spherical structures under a relatively long mixing time (tmix). In addition, it is observed that the size of the micelles decreases with the increase of flow velocity ratio by tuning the hydrodynamic conditions of the flows. Moreover, by adjusting initial polymer solution concentration, temperature, and the weight fraction of introduced homopolystyrene (hPS), which can affect the viscosity of the BCP solution, the flow diffusion in the microfluidic chip and the resulted micellar structures can be also readily adjusted. The current study provides a new flow-driven method to adjust the micellar ordered structural transformation under nonequilibrium state.