Buckling-induced structural transition during the drying of a polymeric latex droplet on a solid surface

The structural evolution in the center of a droplet of a dialyzed styrene-n-butyl acrylate copolymer latex dispersion on a solid surface during drying was studied by means of an in situ synchrotron ultra small-angle X-ray scattering technique. During water evaporation, as the droplet shrinks the shell yields and thickens, as evidenced by the appearance of diffraction peaks at the early stage combined with TGA measurement. As the droplet dried further, colloidal crystalline structure transformation is identified. The transition is attributed to the collapse of the thin shell made of densely packed particles at the surface of the droplet caused by capillary forces that drive the deformation of the shell. As there is a small amount of water remaining beneath the surface layer, charged particles are able to explore new configurations to reestablish a stable structure after leaving the initially formed face-centered cubic structure. Finally, a torus-shaped solid film with a central flat thin film is produced