Liquid sorption and stability of polystyrene latices

摘要

The surface structure of the colloidal polymer particles and the liquid structure in the dispersion medium strongly affect the stability of a latex dispersion. Two kinds of polystyrene latices, stabilised by sulfonate groups, were synthesised by emulsion polymerisation. The reactions were started by a redox starter of potassium peroxodisulfate and sodium bisulfite. Type 1 latices have a smooth surface and were prepared from styrene. The type 3 latices were prepared by a two-step mechanism. In the first step, seed particles were prepared. In the second step, the seed particles were grown by further polymerisation of styrene and sodium styrene sulfonate. By this method oligomers of styrene and sodium styrene sulfonate accumulate in the outer spheres of the particles; thus a hairy surface is formed. The stability of the dispersions and the selective liquid sorption were investigated for both kinds of latices: selective liquid sorption from 1-propanol(1)/water(2) mixtures on the colloidal polymer particles with different surface structure was obtained by the reduced surface excess. The isotherms were analysed in order to determine the mole fraction of the adsorbed layer. The different surface structure of the particles leads to different stability behaviour of the dispersions. The critical coagulation concentrations (c_k) for barium perchlorate in 1-propanol/water mixtures were obtained by test-tube experiments. At low 1-propanol concentrations the stability of the type 3 latices increases, exhibits a maximum stability at about x_1 ≈ 0.1 and decreases upon further addition of 1-propanol. The decreased c_k values for type 3 latices in water-rich mixtures are explained by swelling of the hairy surface. Imbedding 1-propanol/water clusters between the oligomers causes swelling of the outer sphere of the particles. Formerly not dissociated surface groups in the oligomer region are able to dissociate. This leads to a vaster double layer and thus higher c_k values. By further addition of 1-propanol, counterions are pushed back to the surface and the stability decreases.