Electrosteric Stabilization with Poly(Acrylic) Acid in Emulsion Polymerization:  Effect on Kinetics and Secondary Particle Formation

Abstract

A kinetic study of seeded emulsion polymerization using seed latices electrosterically stabilized with poly(acrylic acid) was performed. Data were obtained at 50 °C for the steady-state rate of polymerization initiated with persulfate and for the non-steady-state relaxation initiated using γ-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, ρ . 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 ρ 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 ρ 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 ρ . 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 ρ and k on particle size and (for ρ ) 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 ρ ).