Two-Level Optical Birefringence Created by Evaporation-Induced Polymer Crystallization in Sessile Droplets

Controlling deposited crystal structures in sessile droplets is the basis for applying recent droplet-making techniques in processing semicrystalline polymer solutions. But it remains a fundamental challenge to clarify the relationship between solvent evaporation and polymer crystallization. Here, we study the solvent evaporation-induced crystallization in poly(ethylene oxide) sessile droplet and reveal a two-level optical birefringence in the dried film. We find that the evaporation-induced shear flow promotes crystal growth in the radial direction, which generates anisotropic spherulites with a noncentrosymmetric orientation-sensitive birefringence, while the whole film in a droplet exhibits a centrosymmetric negative birefringence. The two-level optical birefringence is dependent on the molecular weight and polymer concentration in the morphology diagram and is proven a general phenomenon in more kinds of semicrystalline polymers. Careful analysis on evaporation dynamics indicates that the anisotropic crystal growth is a competitive result of evaporation, polymer relaxation, and crystallization. The findings in this study shed insight into the evaporation-induced crystallization in polymer droplets and offer a generic method to create anisotropic crystal patterns using droplet-processing techniques.