Investigation of the swelling response and drug loading of ionic microgels: The dependence on functional group composition

Spherical micron-sized (4-7 μm diameter) poly(methacrylic acid-co-nitrophenyl acrylate) microgels were synthesized by precipitation polymerization and selectively derivatized with carboxylic acid, glutamic acid, hydroxamic acid, sulfonic acid, and ethanol functional groups in five separate post-polymerization reactions. The pH and NaCl induced swelling response, drug loading, capacity (`capacity' = total number of functional ionic groups that bind protons), and density of the five different composition anionic microgels, each containing a different functional group as well as a baseline of carboxylic acid groups, were measured. Using the micropipet flow technique, it was found that the pH range of the swelling response for the five different microgel compositions shifted by an amount that was proportional to the solution pK_a's of their functional groups. The degree of drug loading increased in proportion to the microgels' capacity. However, the drug loading did not decrease proportionately when the capacity was lowered.