Carboxymethyl cellulose: Polyvinylamine complex hydrogel swelling

The swelling behavior of the polyelectrolyte complex films made from polyvinylamine (PVAm) or poly(vinylformamide-co-vinylamine) copolymers and carboxymethyl cellulose (CMC) was studied as a function of polymer composition, pH, and salt concentration. Swelling is determined by the balance between ionic cross-link density, opposing swelling, with solvation and Donnan effects promoting swelling. Films containing excess ammonium groups gave the greatest swelling at low pH, whereas excess carboxyl groups enhanced swelling at high pH. Films with balanced charge stoichiometry (CMC:PVAm 3:1 w:w) gave the minimum swelling. High concentrations of sodium chloride increased swelling due to the decomposition (screening) of ionic cross-links, whereas calcium ion decreased swelling possibly because of calcium dicarboxylate cross-links. A simple swelling model based on the ionization behavior of dilute polymers and ionic cross-linking predicted the major swelling behaviors, including the effects of pH and polymer composition.