The Segregation of Surfactant upon Film Formation of Latex Dispersions: an Investigation by Energy Filtering Transmission Electron Microscopy

We investigated the local distribution of emulsifier in dried films cast from latex dispersions. Energy filtering transmission electron microscopy (EFTEM) allowed the detection of heteroatom-containing domains in unstained samples by electron spectroscopic imaging and elemental mapping. If the samples were prepared at temperatures above the minimum film-forming temperature (MFT), separated domains of surfactant were observed in detachment replicas and ultrathin sections. If the preparation was carried out below MFT the emulsifier was retained at the particle surface and similar observations could not be made. It is suggested that the mobility of the emulsifier at the surface of the particles is increased upon deformation and coalescence in the process of film formation, and that this mobility is a prerequisite for its segregation. Both processes do not take place upon drying below MFT, owing to lower mobility. The correlation between the process of film formation and the segregation of surfactant is discussed for both a dispersion of poly(vinylacetate) and one of polyacrylate.