Polymer crystallization and optical birefringence modulated by solvent evaporation in sessile droplets

Solvent evaporation is a dynamic process associated with mass transfer, heat exchange and complex internal flow . Their complicated effects on crystallization remain unclear in evaporating sessile droplets. Here, we study crystallization in evaporating poly(ethylene oxide) (PEO) sessile droplets and obtain three kinds of spherulites with monochromatic, anisotropic or negative birefringence . Monochromatic spherulites are generated at very slow solvent evaporation and low molecular weight , while high anisotropic spherulites are created under flow field at rapid solvent evaporation and large molecular weight . Ordinary negative spherulites appear at intermediate conditions. The temperature variation is verified during solvent evaporation, but plays an insignificant thermodynamic effect on crystallization. The experimental findings are further validated in different solvents and polymer systems . This work establishes the relationship between solvent evaporation and polymer crystallization in sessile droplets and also provides a guideline for modulating optical properties of deposited films using droplet processing techniques.