| Abstract
| - Micrometer-sized (4−7 μm diameter) poly(methacrylic acid) (PMAA) hydrogel microsphereswere synthesized by precipitation polymerization. Individual microspheres were held in a micropipetand visualized by interference contrast microscopy. They were characterized with regard to their mass,density, water content, electrophoretic mobility, and apparent pKa. Equilibrium changes in volume weremeasured as functions of the pH and NaCl concentration of the suspending solution. The maximumreduction in the microsphere equilibrium volume (Vrmax) at pH 3.0 was 0.28, where Vr was the ratio of themicrosphere volume at the test pH to its volume at pH 6.6. A Donnan-based thermodynamic model,modified to include counterion binding because of the high fixed charge density in the microspheres (3.0M), was applied to determine the difference in the ion concentration between the interior and exterior ofthe gel. The ion concentration differences (which were related to the osmotic pressure) predicted by themodel were proportional to the microsphere equilibrium volume with changing pH and salt concentration.This supported the hypothesis that the equilibrium volume of the microspheres was set by a force balancebetween the osmotic pressure and the elasticity of the hydrogel matrix. Microspheres changed from theirmaximum equilibrium volume at pH 6.6 to their minimum equilibrium volume at pH 3.0 in 300 ms.This indicated that diffusion of the polymer matrix and not diffusion of ions into and out of the microspherewas the rate-limiting factor in determining a microsphere's swelling rate.
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