Synergetic effect of microfluidic flow and ultrasonication on chains pre-ordering in conjugated polymer solution

Abstract

Multi-physical fields solution processing strategy provides a universal and facile way to prepare various conjugated polymer (CP) films and high-performance devices, whereas the hierarchical structures evolution and crystallization of the polymer chains are elusive particularly under the far-from-equilibrium state in solution. In this work, we employed a combined microfluidic flow and ultrasonication strategy (FU) for CP solution processing and found a pronounced synergetic effect of these fields in promoting the pre-ordering of chains in solution. This conformation order and anisotropy of the solution were revealed by using UV–Vis absorption, polarized optical microscopy (POM) and cryogenic transmission electron microscopy (Cryo-TEM) characterizations. A non-classical nucleation model for the conjugated polymer crystallization in the procedure of non-equilibrium solution processing was confirmed. The roles of microfluidic flow and ultrasonication on the chain aggregation and crystallization were addressed with the aim of a multi-physical simulation based on pressure acoustics streaming, fluid heat transfer and thermoviscous boundary layer impedance. Compared to pristine solutions, the FU strategy showed improved solution anisotropy and crystallization kinetics, substantially giving rise to a larger crystallinity in films and much increased mobilities for the organic field-effect transistor (OFET) devices by more than an order of magnitude. With an appropriate balance between the solvation and interchain interactions, the FU processing strategy is universal for regulation of chains conformation and aggregation in conjugated polymer solution.