Film formation of latex blends with bimodal particle size distributions: Consideration of particle deformability and continuity of the dispersed phase

Latex dispersions having a well-controlled, bimodal particle size distribution are gaining attention because they potentially enable control of the dispersion rheology, the film formation characteristics, and the final film properties. Here we study the film formation of dispersions with a bimodal particle size distribution (large:small size ratio of ca. 6:1) and with varying concentrations of the two particle sizes. We also compare the film formation of blends containing only deformable (i.e., "soft") particles with blends containing both soft and nondeformable (i.e.,"hard") particles. We use ellipsometry as a noninvasive tool for studying film morphology as film formation proceeds. We interpret our ellipsometry data using a physical model of the morphology based on atomic force microscopy observation. Electron microscopy of film cross sections provides information about the bulk morphology. We measure void content and surface roughness in blend films as a function of the concentration of large particles for three series of blends based on soft particles only and on the two combinations of hard and soft particles. When large and small soft particles are blended above a small particle concentration of ca. 16.5 wt % (corresponding to a number ratio of small:large of 43:1), the void concentration in freshly prepared films reaches a low value. This concentration approximately corresponds to the critical volume fraction (V-c) of small particles required to obtain a continuous phase of small particles surrounding the large particles. Below this value, there are not enough small particles to create a continuous phase, so more interparticle voids are present in the film. Surface roughness and void volume concentration in blends of large-soft and small-hard particles also reach a minimum at V-c. At higher concentrations of small-hard particles, void concentration increases because the continuous phase is non-film-forming. Finally, when largehard particles are blended with small-soft particles, film formation is hindered by the clustering of the large particles and the subsequent creation of voids. In this case, film formation at concentrations of small particles below about ca. 45 wt % cannot be achieved. A coherent film cannot be obtained at small particle concentrations nearer to or below V-c.