Latex film formation studied with the atomic force microscope: Influence of aging and annealing

The surface structure of latex dispersion films was examined with an atomic force microscope. All measurements were done in air on latex films having a minimum film formation temperature of 12°C and a glass transition temperature of 18°C. One aim of this study was to follow structural changes during film formation. Three minutes after spreading the film, its surface layer dried. Afterwards, the structure of the film did not change anymore. Only after 4 months could structural changes be observed: Though individual latex particles could be identified, the particles partly melted into one another.After annealing films at 50° or 60°C for 4 h, the latex particles partly melted into one another, but individual particles could still be identified. When annealing at or above 80°C, no individual latex particles were visible anymore. With increasing temperature the film roughness decreased from 3 nm without annealing to 0.8 nm at 100°C annealing temperature. In addition, islands of 2–4 nm thickness appeared on the film surface. These islands could be scraped off the film by increasing the force between tip and sample, indicating that they are composed of surfactant which was squeezed out of the film.