GEL-LIKE BEHAVIOR OF PH DEPENDENT LATEX FILMS

Gel-like properties of soft latex particles, made of styrene butyl acrylate statistical copolymers, were observed after forming thin films; polymer chains were not covalently linked to one another. Acrylic acid groups, inserted at random on chain skeletons of copolymers, were supposed to play the role of crosslinks. The concentration of these carboxylic:acid groups, hereafter called kappa(COOH), was varied from 100 to 800 mu mol/g; the pH of emulsions, prior to the formation of films, was given three values: 2.5, 5, and 7.5. Measurements of the gel fraction or of the swelling ratio of latex films were performed as a function of the kappa(COOH) variable to attempt to show that this concentration determines the state of gelation. The gel fraction was was shown to obey an experimental master curve which was empirically described as an exponential function of kappa(COOH), and an estimate of the threshold of gelation was given by kappa(COOH)(C) = 20 mu mol/g, at a pH equal to 7.5; the threshold was lowered by raising the pH of the emulsions. Using toluene as a good solvent, the swelling ratio was also found to obey an empirical master curve represented as a function of the inverse of the (kappa(COOH) - kappa(COOH)(C)) variable; the power law associated with this curve suggests that the state of maximum swelling corresponds to a set of closely packed coils.