Interdiffusion in carboxylated styrene/butyl acrylate latex films studied by topographic and thermal analysis

Atomic force microscopy, AFM, was used to study polymer interdiffusion in styrene/butyl acrylate latex films in the presence and absence of a methacrylic acid copolymer. The interdiffusion monitored as the disappearance of particle contours was found to be dependent on annealing time and temperature and was strongly influenced by the type of neutralizing agent. In the presence of NaOH, AFM images suggested the formation of a salt shell around the particles, which retarded the interdiffusion. The concentration of surface acid groups determined from titration in combination with measured particle dimensions resulted in a calculated shell thickness of about 10 nm. This polyelectrolyte layer is claimed to have a higher glass transition temperature, Tg, than the measured bulk Tg of the copolymer, thus explaining the retarded particle-particle interdiffusion. Less severe retardation was found when using ammonia for neutralization, presumably due to evaporation of the volatile base. Appropriate diffusion coefficients calculated using literature data on similar polymers confirmed the experimental observations with a decrease by about one order of magnitude in the presence of a methacrylic acid shell and a further decrease when sodium hydroxide was used for neutralization.