Block-random copolymer stabilisers for semi-batch emulsion polymerisation

Polystyrene-b-[polystyrene-r-poly(acrylic acid)] block-random copolymers (BRCs) were used to stabilise polystyrene latexes prepared using a semi-batch emulsion process. The representative BRC used in this work was effective in stabilising the semi-batch emulsion polymerisations, yielding stable latexes with particle diameters as low as 45 nm and a stabilised particle surface area per BRC up to 200 nm(2). Unlike classic block copolymers (e.g., PS-b-PAA), the BRC could be used to achieve a high solids content (50 wt%) while maintaining a viscosity below 100 mPa s. The nucleation mechanism is dependent on the pH with only a single primary nucleation stage observed when pH is greater than 8.5 while a two-stage nucleation process is observed for pH below 8.5. In the latter case, a seeded mechanism due to an in situ second order self-assembly process is triggered, increasing polymerisation rates, decreasing mean particle size, and allowing the facile generation of either unimodal or bimodal particle size distributions through judicious control of the pH and monomer feeding rate.