Preparation and swelling properties of poly(NIPAM) "minigel" particles prepared by inverse suspension polymerization

The characterization of temperature- and pH-sensitive poly-N-isopropylacryl amide (poly-NIPAM) microgel particles, produced by surfactant-free emulsion polymerization, has been extensively reported. In the work described here poly(NIPAM) gel particles, cross-linked with N-N'-methylenebisacrylamide (B A), have been produced using inverse suspension polymerization. These parti cles have been termed "minigels" here since they are somewhat larger than c onventional microgels, Results suggest that minigel particles are formed as a dilute suspension, within the aqueous dispersed (droplet) phase. The hyd rodynamic diameter of the minigel particles produced in this work is less t han or equal to 2.5 mu m, at 25 degrees C. The effects of temperature and p H changes, variation in cross-linker concentration, and incorporation of a charged comonomer (methacrylic acid, MAA) have been investigated. Both poly (NIPAM-BA) and poly(NIPAM-BA-MAA) minigel particles are temperature sensiti ve with swelling behavior consistent with comparable microgels, Variations in pH were found to effect the size of minigels containing ionizable groups (such as a carboxylate) by a mechanism of increased electrostatic repulsio n of charged groups with increasing pH. Overall, the production of temperat ure- and/or pH-sensitive polymers by inverse suspension polymerization resu lts in particles with swelling characteristics similar to those produced by emulsion polymerization, albeit with differing particle sizes. (C) 2000 Ac ademic Press.