Abstract
The evaporation of a polymer solution droplet is important in solution-based polymer film fabrications, such as inkjet printing, spray coatings, and droplet casting, etc. In this work, we investigated the effect of droplet size, solvent evaporation rate, and concentration on the “coffee-ring” effect, crystal nucleation, polymorphism, and morphology of dried poly(vinylidene fluoride) (PVDF) solution droplets with the atomic force microscopy (AFM) and two-dimensional grazing incidence wide angle X-ray scattering (2D GIWAXS) method. We found that the crystal structure, morphology and crystal distribution in the center and edge regions of dried PVDF droplets were different due to the “coffee-ring” effect. The “coffee-ring” effect of dried PVDF droplets was mainly composited of accumulated crystals at the edge region of a droplet, which was mainly made by the crystallization of migrated chains. The interplay between the migration of chains and the crystallization and solidification of PVDF droplets significantly influenced the formation of the “coffee-ring”. In addition, our results showed that the decrease in droplet size and the controlling solvent evaporation rate were effective ways to improve the electroactive crystalline phases (β and γ-phases) nucleation and decrease the crystal size.
Highlights
The evaporation of polymer solution droplets is very important in some solutionbased polymer film fabrications, such as inkjet printing, spray coatings, and droplet casting, etc. [1]
To investigate the effect of droplet size on poly(vinylidene fluoride) (PVDF) nucleation and morphology, we prepared L and S sizes of the PVDF/DMF solution drops with DMF solvent
The crystals presented in the edge region of the PVDF L-droplet is denser than that in center region
Summary
The evaporation of polymer solution droplets is very important in some solutionbased polymer film fabrications, such as inkjet printing, spray coatings, and droplet casting, etc. [1]. In contrast with solution casting and blade coating processes, the evaporation of droplets generally generates a “coffee-ring” effect that generally occurs by an evaporation-induced outward capillary flow [3,4,5,6,7]. Hu et al [8] investigated the “spherulites ring” morphology and “coffee-ring” profile of a polyethylene oxide (PEO) film formed by drying the droplet at a glass substrate with a different heating rate. They found that the outward capillary flow is the dominant micro-flow inside the drying droplet of a PEO aqueous solution when the substrate temperature is below 50 ◦ C. As far as we know, the evaporation-induced crystal nucleation and the morphology of polymer solution droplets has not been fully understood far
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