Effects of acrylic acid on preparation and swelling properties of pH-sensitive dextran hydrogels

Dextran hydrogels were obtained by radical copolymerization of methacrylated dextran (MA-dextran) with acrylic acid (AAc) using ammonium peroxydisulfate (APS) and N,N,N′,N′-tetramethylethylenediamine (TMEDA) as an initiation system in an aqueous solution. The AAc content in hydrogels was determined by FTIR. Copolymerization of MA-dextran with AAc increased the cross-linking density of hydrogels by the bridging effect of AAc and, to a certain extent, facilitated the formation of hydrogels from MA-dextran with a low degree of MA substitution (DS). For hydrogels with a low DS (5.9), the swelling at pH 7.4 initially decreased and then increased with increasing AAc. The swelling of hydrogels with high DS (11.4 and 22.4) increased gradually with AAc. This discrepancy was explained by the differences in the chemical potentials of water outside and inside of the hydrogels as a function of AAc. Further increases of AAc, however, led to a reduction in polymerization conversion and even incomplete formation of hydrogel. The reduction in polymerization yield was primarily a consequence of the pH reduction and salt formation of AAc with TMEDA.