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 distribut ion are gaining attention because they potentially enable control of the di spersion rheology, the film formation characteristics, and the final film p roperties. Here we study the film formation of dispersions with a bimodal p article size distribution (large:small size ratio of ca. 6:1) and with vary ing concentrations of the two particle sizes. We also compare the film form ation of blends containing only deformable (i.e., "soft") particles with bl ends containing both soft and nondeformable (i.e.,"hard") particles. We use ellipsometry as a noninvasive tool for studying film morphology as film fo rmation proceeds. We interpret our ellipsometry data using a physical model of the morphology based on atomic force microscopy observation. Electron m icroscopy of film cross sections provides information about the bulk morpho logy. We measure void content and surface roughness in blend films as a fun ction of the concentration of large particles for three series of blends ba sed on soft particles only and on the two combinations of hard and soft par ticles. When large and small soft particles are blended above a small parti cle concentration of ca. 16.5 wt % (corresponding to a number ratio of smal l:large of 43:1), the void concentration in freshly prepared films reaches a low value. This concentration approximately corresponds to the critical v olume fraction (V-c) of small particles required to obtain a continuous pha se of small particles surrounding the large particles. Below this value, th ere are not enough small particles to create a continuous phase, so more in terparticle voids are present in the film. Surface roughness and void volum e 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 c oncentration increases because the continuous phase is non-film-forming. Fi nally, when largehard particles are blended with small-soft particles, film formation is hindered by the clustering of the large particles and the sub sequent creation of voids. In this case, film formation at concentrations o f small particles below about ca. 45 wt % cannot be achieved. A coherent fi lm cannot be obtained at small particle concentrations nearer to or below V -c.