PH AND ION-TRIGGERED VOLUME RESPONSE OF ANIONIC HYDROGEL MICROSPHERES

Micrometer-sized (4-7 mu m diameter) poly(methacrylic acid) (PMAA) hyd rogel microspheres were synthesized by precipitation polymerization. I ndividual microspheres were held in a micropipet and visualized by int erference contrast microscopy. They Were characterized with regard to their mass, density, water content, electrophoretic mobility, and appa rent pK(a). Equilibrium changes in volume were measured as functions o f the pH and NaCl concentration of the suspending solution. The maximu m reduction in the microsphere equilibrium volume (V-rmax) at pH 3.0 w as 0.28, where V-r was the ratio of the microsphere volume at the test pH to its volume at pH 6.6. A Donnan-based thermodynamic model, modif ied to include counterion binding because of the high fixed charge den sity in the microspheres (3.0 M), was applied to determine the differe nce in the ion concentration between the interior and exterior of the gel. The ion concentration differences (which were related to the osmo tic pressure) predicted by the model were proportional to the microsph ere equilibrium volume with changing pH and salt concentration. This s upported the hypothesis that the equilibrium volume of the microsphere s was set by a force balance between the osmotic pressure and the elas ticity of the hydrogel matrix. Microspheres changed from their maximum equilibrium volume at pH 6.6 to their minimum equilibrium volume at p H 3.0 in 300 ms. This indicated that diffusion of the polymer matrix a nd not diffusion of ions into and out of the microsphere was the rate- limiting factor in determining a microsphere's swelling rate.