Electrosteric stabilization with poly(acrylic) acid in emulsion polymerization: Effect on kinetics and secondary particle formation

A kinetic study of seeded emulsion polymerization using seed latices electrosterically stabilized with poly(acrylic acid) was performed. Data were obtained at 50 degrees C for the steady-state rate of polymerization initiated with persulfate and for the non-steady-state relaxation initiated using gamma-radiolysis; conditions were chosen so the system obeyed "zero-one" kinetics (when entry of a radical into a particle already containing a growing radical leads to:instantaneous termination). The relaxation data directly measure the rate coefficient k for radical exit (desorption) from particles, which in combination with the steady-state rate then yields the rate coefficient for radical entry, rho. Electrosterically stabilized particles were synthesized from well-characterized seed (made with ionic surfactant) by second-stage polymerization of a small amount of styrene and acrylic acid. Two degrees of surface coverage were employed using 5 and 15 wt % acrylic acid. The low-coverage particles were much more colloidally stable than the polystyrene latices stabilized by initiator fragments and adsorbed surfactant but the behavior of k and rho was the same as that of the ionically stabilized seed. The colloidal stability of the high-coverage latices was strongly pH-dependent, with high stability at high pH. These latices exhibited strongly reduced k (by a factor of about 3) at low pH but rho was only slightly lower compared to the ionically stabilized seed. At high and neutral pH, kinetic parameters could not be determined for the highly covered particles because of secondary nucleation; however, a decreased rate was observed with increasing pH, despite an increase in particle number, indicating a reduced rho. Thus, extensive electrosteric stabilizer seems to reduce entry and exit rate coefficients, although to a lesser extent than reported in an earlier preliminary study (Coen et al., Macromolecules 1996, 29, 5128). For sparsely electrosterically stabilized particles, the dependences of rho and k on particle size and (for rho) initiator concentration were in accord both with those observed for ionically stabilized ones and with the values predicted from models for these processes (transfer diffusion for k and "propagation to z-mer" for rho).