Influence of charge density on the swelling of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels

The volume phase transition of colloidal poly(N-isopropylacrylamide-co-acry lic acid) microgels depends in a complex way on the effective charge densit y within the polymer network. A series of monodisperse PNIPAM/AAc microgels with different content of acrylic acid were synthesized by surfactant-free emulsion polymerization employing sonication instead of a conventional sti rring technique. Subsequently, the colloids were characterized by dynamic l ight scattering and electron microscopy. Potentiometric titrations provided the amount of carboxyl groups incorporated into the copolymer. The effecti ve charge density was systematically controlled by the: content of acrylic acid monomers, the pH value of the suspension, and the salt concentration. The hydrodynamic dimensions of the microgels have been measured by dynamic light scattering. The swelling/deswelling behavior is determined by the del icate balance between hydrophobic attraction of NIPAM and the repulsive ele ctrostatic interactions of the carboxylate group of the acrylic acid moieti es. Compared to their macroscopic counterparts the charged microgel particl es show a significantly different swelling/deswelling behavior. This manife sts in the occurrence of a two-step volume phase-transition process with in creasing acrylic acid content. Hydrogen bonding has to be considered to und erstand this two step volume phase transition uniquely observed for colloid al microgels. Another interesting phenomenon presented here is the reversib le formation of well-defined aggregates at low pH and under high salt condi tions. (C) 2000 Elsevier Science B.V. All rights reserved.