GEL-LIKE BEHAVIOR OF PH DEPENDENT LATEX FILMS

Gel-like properties of soft latex particles, made of styrene butyl acr ylate 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 sup posed to play the role of crosslinks. The concentration of these carbo xylic:acid groups, hereafter called kappa(COOH), was varied from 100 t o 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 fract ion or of the swelling ratio of latex films were performed as a functi on of the kappa(COOH) variable to attempt to show that this concentrat ion determines the state of gelation. The gel fraction was was shown t o obey an experimental master curve which was empirically described as an exponential function of kappa(COOH), and an estimate of the thresh old of gelation was given by kappa(COOH)(C) = 20 mu mol/g, at a pH equ al to 7.5; the threshold was lowered by raising the pH of the emulsion s. Using toluene as a good solvent, the swelling ratio was also found to obey an empirical master curve represented as a function of the inv erse of the (kappa(COOH) - kappa(COOH)(C)) variable; the power law ass ociated with this curve suggests that the state of maximum swelling co rresponds to a set of closely packed coils.