Effects of poly(acrylic acid) electrosteric stabilizer on entry and exit in emulsion polymerization

A methodology developed to obtain rate coefficients for entry and exit (desorption) in emulsion polymerizations was applied to systems stabilized electrosterically by a copolymer of acrylic acid and styrene embedded in a styrene seed particle. This was grown as a second-stage procedure, by adding styrene and acrylic acid to a styrene seed and then polymerizing. Rate coefficients for entry (rho) and exit (k) for subsequent homopolymerization of the resulting latices with styrene were obtained from the time dependence of the approach to steady state using both chemical and gamma-radiolytic initiation; the latter was used in relaxation mode, which measures K directly. Compared to the same latices with an electrostatic stabilizer, at pH 7 the electrosteric stabilizer greatly reduced both rho and k. When ionic strength was increased, rho increased relative to that found for electrosterically stabilized latex in the absence of added electrolyte. For electrostatically-stabilized latices, entry is supposed to occur by aqueous-phase propagation to a critical degree of polymerization z which then undergoes irreversible entry; the present data for electrostatically stabilized latices support this model, including its prediction that rho be independent of particle size, all other things being equal. The decrease in rho in the electrosterically-stabilized latices is ascribed to a ''hairy'' layer through which diffusion of a z-mer is slow, so that it may be terminated prior to actual entry. For electrostatically-stabilized latices, exit is supposed to occur by transfer, resulting in a monomeric radical which exits by diffusing through the aqueous phase, this event competing with intraparticle propagation; the decrease in k in the electrosterically-stabilized latices (also seen in other polymerically-stabilized systems) can be interpreted by assuming that aqueous-phase diffusion is slower in the hairy layer.