POLYELECTROLYTE BLOCK-COPOLYMERS AS EFFECTIVE STABILIZERS IN EMULSIONPOLYMERIZATION

Amphiphilic block copolymers consisting of a hydrophobic block of hydr ogenated polybutadiene and a partly sulfonated polyelectrolyte block o f poly(styrenesulfonate) are effective stabilizers in emulsion polymer ization. At low relative amounts of block copolymer (0.4 wt % as a fra ction of monomer weight) and salt-free conditions, well-defined and st able latices with particle diameters of ca. 100 nm and solid contents of 20 wt % are obtained. This high stabilization efficiency of optimiz ed block copolymer systems enables the formulation of latex systems wi th a relatively low remaining polarity in solid films and offers new i nteresting model systems with exclusively electrosteric stabilization. A comparison of polymerization in high or low ionic strength solution and the variation of the degree of sulfonation of the poly(styrenesul fonate) block shows an optimum stabilization of the latices at low ion ic strength during polymerization. Fully sulfonated polymers systems p resumably show a molecular orientation perpendicular to the particle s urface, whereas 50% sulfonated species take on a traillike conformatio n along the surface, which is explained by remaining hydrophobic inter actions. Due to this multiple surface particle contacts, the applicati on of partly sulfonated polymers leads to more effective stabilization . At higher stabilizer concentrations, aggregates are found, which can be redispersed by ultrasonification or addition of low molecular weig ht surfactant solution.