Self-stratification during film formation from latex blends driven by differences in collective diffusivity.

Coatings with vertical gradients in composition were produced by drying an aqueous polymer dispersion containing both charged and neutral particles. After drying, the neutral component was enriched at the film/air interface. The spontaneous vertical segregation between the two types of particles goes back to a difference in collective diffusivity. As the film dries, a layer enriched in polymer develops at the top. Due to their mutual repulsion, charged spheres escape from this layer more quickly than their neutral counterparts. Provided that the total time of drying is between the times of diffusion for the two types of particles (approximately H(0)(2)/D(c) with H(0) the initial film thickness and D(c) the collective diffusivity of the respective species), a concentration gradient persists after the film has turned dry. This effect can be used to create a functionally graded material (FGM) in a single coating step.