Latex Films with In-Plane Composition Gradients Caused by Lateral Drying

Making use of a pH-sensitive dye contained in the aqueous phase of drying latex films, it was found that lateral drying can cause pH-gradients in the final film. The effect requires the presence of a thickener, which stabilizes a coupled network of latex spheres at a time, when there is still a significant amount of flow in the liquid phase. The thickener decouples the movement of the liquid from the movement of the latex spheres. If the glass temperature of the polymer is much below the drying temperature, the pH of the final film is low at the edge of the film, which can be explained with the coffee ring effect. The opposite behavior—meaning protons being carried to the center of the film—is found for films with a glass temperature close to the drying temperature and above. When there is a flow of protons toward the center of the film, one also finds the thickness to be increased at the center. Composition gradients can be exploited to generate films with spatially variable properties, examples being color (as demonstrated here), tack, etch resistance, contact angle, or swellability in solvent vapor.