PROBING THE SURFACE OF POLYMER COLLOIDS BY CONDUCTOMETRIC SURFACTANT TITRATION

摘要

Morphological characterisation of latex particles is carried out by a variety of techniques. This paper describes the use of conductometric surfactant titration as a relatively simple tool to gain information on the nature of the colloidal polymer surface. In a binary latex blend, a non-linear dependence of the surfactant adsorption density on the fractional polymer/water interfacial area is observed if at least one of the polymer dispersions is not completely free of surfactant. A quadratic fit to the data reduces the error in the determination of the adsorption density specific to the latex particle surface. Application of the technique to a series of poly(MMA-co-BA) latexes shows an enrichment of the particle surface with the more polar MMA, likely due to a composition drift taking place during the reaction. When one of these latexes is used to generate a composite system with polystyrene, it is shown that the latter, more hydrophobic, polymer has a strong tendency to bury into the particle. For weight fractions up to 75 % of polystyrene, the particle surface essentially consists of the initial MMA-BA copolymer. However, a substantial enhancement in minimum film formation temperature occurs at relatively low polystyrene contents, indicating that polystyrene may be located close to the surface e.g., in an occluded core-shell morphology. Finally, an inter-laboratory comparison of surfactant titration results on model composite latexes shows that despite some systematic deviations between the absolute values of the surfactant adsorption area, the predictions for the relative surface compositions are similar.