Preparation and Swelling Properties of Poly(NIPAM) "Minigel" Particles Prepared by Inverse Suspension Polymerization.

The characterization of temperature- and pH-sensitive poly-N-isopropylacrylamide (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 (BA), have been produced using inverse suspension polymerization. These particles have been termed "minigels" here since they are somewhat larger than conventional microgels. Results suggest that minigel particles are formed as a dilute suspension, within the aqueous dispersed (droplet) phase. The hydrodynamic diameter of the minigel particles produced in this work is </=2.5 µm, at 25 degrees C. The effects of temperature and pH 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 sensitive 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 repulsion of charged groups with increasing pH. Overall, the production of temperature- and/or pH-sensitive polymers by inverse suspension polymerization results in particles with swelling characteristics similar to those produced by emulsion polymerization, albeit with differing particle sizes. Copyright 2000 Academic Press.