EPR Studies on Film Formation of Colloidal Dispersions, 2. Drying, Film Annealing, and Film Rewetting

Abstract

Electron paramagnetic resonance spin probe techniques are used for the characterisation of structure and dynamics of selected microdomains of polyacrylate latices and films obtained from them. Analysis of the spectral line shapes provides quantitative data on the localisation and mobility of ionic and polar non-ionic low-molecular additives during the drying process. Such additives are highly mobile down to bulk water contents of only 5%. In the dried dispersions, ionic additives form aggregates, in which their local concentration exceeds their bulk concentration by at least a factor of ten. These aggregates can be “melted” by rewetting and the ionic additives can be partially washed out if the drying is performed below the minimum film forming temperature (MFT). Surfactants in the initial dispersions are partially anchored with their hydrophobic tails in the polymer particles. The fraction of such anchored surfactant molecules increases continuously with decreasing water content. If the drying is performed only slightly below the MFT, the surfactants reorganise to inverse micelles. In films obtained by drying at a temperature significantly below the MFT, somewhat larger surfactant aggregates are formed during annealing.