Structure and swelling of poly(acrylic acid) hydrogels: effect of pH, ionic strength, and dilution on the crosslinked polymer structure

The network formation of crosslinked polymer hydrogels made via a free radical polymerization mechanism is significantly influenced by the polymerization conditions. In particular, the crosslinked structure of ionic networks like poly(acrylic acid) copolymers is affected by the monomer concentration, the pH, and ionic strength during the polymerization. In this work experimental data as well as theoretical analysis are used to investigate how these factors control the degree of crosslinking and primary cyclization during the network formation of multifunctional monomers. It was found that the amount of water present during the polymerization increases primary cyclization rates, and this change affects the subsequent swelling behavior of the acrylic acid hydrogel. The effects of ionic strength and pH on the network structure are interrelated. An increase in the pH decreases the degree of primary cyclization while an increase in the ionic strength increases cyclization. To investigate further the effect of pH, a cationic polymer was formed that contained a monovinyl amine monomer and a novel diamine crosslinking agent synthesized in our laboratory. The combined effect of the ionizing backbone chain and crosslinking agent cause the degree of primary cyclization in this material to be extremely sensitive to the pH during polymerization. This result confirms the significant role of pH on the network formation in ionic materials.