Flattening of Latex Film Surface: Theory and Experiments by Atomic Force Microscopy

A model is proposed to describe the flattening of latex film surfaces when the film is annealed for various periods of time above the glass-transition temperature, T g, of the polymer. This model is based on the excess pressure produced by the curvature of the particles on the film surface, which acts as driving force in surface film flattening, and the polymer stress growth viscosity, μ(t), which is the resistance to particle deformation and acts against surface film flattening. To test the model, surfactant-free poly(butyl methacrylate) (PBMA) latexes were synthesized. Dry films were annealed at 70 °C, i.e., about 35 °C above the PBMA T g, for various periods of time, and for each annealing time the film roughness was measured by atomic force microscopy (AFM). The decrease of the film roughness versus annealing time measured by AFM is compared to the decrease predicted from the model. The model is also compared to data obtained with latex particles stabilized with sodium dodecyl sulfate which decreases the polymer surface tension.