In Situ Observation and Control of P3HT Solution Crystallization under Microfluidic Interfacial Solvent Exchange

Solution crystallization plays a central role in determining the multiscale organization and functional properties of conjugated polymers. It depends not only on the final solvent composition but also on the pathway by which supersaturation develops. However, resolving how supersaturation history influences the evolution from dissolved chains to ordered structures remains challenging because supersaturation develops rapidly and heterogeneously during conventional solvent mixing. In this work, microfluidic interfacial solvent exchange is employed to generate a spatially resolved metastable region for poly(3-hexylthiophene) (P3HT) under well-defined nonequilibrium conditions. Acetone diffusion across a stable laminar interface continuously decreases the local solvent quality, while the downstream position provides a measure of the nominal residence time experienced by the evolving solution. Reflected bright-field imaging combined with numerical simulation characterizes the solvent interdiffusion field, and fluorescence microscopy distinguishes the polymer response from the solvent-induced optical background and tracks its downstream evolution. A detectable transition occurs at nominal inlet acetone fractions of approximately 10-15 vol %, compared with 15-20 vol % in bulk mixing. Fluorescence imaging directly reveals the emergence of low-intensity, red-shifted regions associated with crystalline ordering. Spectroscopic, microscopic, and electron diffraction analyses confirm the formation of anisotropic crystalline P3HT fibers.