Equilibrium swelling of copolymerized acrylic acid- methacrylated dextran networks: Effects of pH and neutral salt

In this study, pH-responsive dextran hydrogels were prepared by radical copolymerization of methacrylated dextran (MA-dextran) with acrylic acid (AAc) in aqueous solution, using ammonium peroxydisulfate and N,N,N',N'-tetramethylethylenediamine as an initiation system. The AAc content in hydrogels was determined quantitatively by FTIR. The swelling response of hydrogels to changes in pH/ salt concentration was characterized by quantitative evaluation of ionic osmotic performance according to the Donnan equilibrium theory, taking account of the Debye-Huckel effect on the apparent acidity constant of AAc groups and the ionic binding of acrylate anions with counterions within hydrogel. A close-to-linear relation between the equilibrium swelling of hydrogels and the pH-induced ionic osmotic pressure is observed. The accompanied change of the polymer-solvent interaction parameter (chi) is obtained from free energy balance between ionic osmotic pressure, the elastic retractile force, and the interaction of polymer network with water molecules (the mixing contribution) and is governed by the concentration of fixed ionized AAc units within hydrogels. For hydrogels having varying AAc contents, the difference in swelling under identical osmotic pressure results from the variation in the effective network density of hydrogels. The neutral salt effect on the pH-induced swelling of the same hydrogel follows changes in the ionic osmotic pressure and the degree of dissociation of AAc units associated with the Debye-Huckel theory. The change of the degree of dissociation of AAc units in response to the ionic strength effect dominates and describes well the sensitivity of the swelling behavior of hydrogels to the ionic osmotic pressure.