Characterization of electrosterically stabilized polystyrene latex; implications for radical entry kinetics

Electrosterically stabilized polystyrene latexes with a poly(acrylic acid) hydrophilic layer with either perdeuterated core or perdeuterated hydrophilic layer were prepared in situ in a styrene/acrylic acid copolymerization, in a manner similar to that commonly employed industrially. Small angle neutron scattering (SANS) measurements were made over a range of contrasts for three latexes at high and low pH. Parameters obtained by fitting to standard core/shell models were consistent with the shell being highly hydrated (about 89% at low pH and about 94% at high pH). The core was found to contain about 3% acrylic acid. Doubling the proportion of acrylic acid in the recipe increased shell thickness by about 20%, slightly reduced particle size and slightly increased the proportion of acrylic acid incorporated into the core. The maximum degree of polymerization (DOP) of the entering (and therefore grafted) species was estimated from the shell thickness to be about 44 monomer units for 0.02 M acrylic acid and 66 for 0.04 M. The observed dependence of hairy layer (shell) thickness on the initial amount of acrylic acid suggests that the critical DOP for entry (and therefore true grafting) of the electrosteric stabilizer is thermodynamically (not kinetically) controlled.