Hydrogels based on modified chitosan, 1 - Synthesis and swelling behavior of poly(acrylic acid) grafted chitosan

A large number of carboxylic groups were introduced onto chitosan by grafti ng with poly(acrylic acid) as an efficient way of modification. The reactio ns were carried out in a homogeneous aqueous phase by using potassium persu lfate and ferrous ammonium sulfate as the combined redox initiator system. The efficiency of grafting was found to depend on monomer, initiator, and f errous ion concentrations as well as reaction time and temperature. It was observed that the level of grafting could be controlled to some degree by v arying the amount of ferrous ion used as a co-catalyst in the reaction. Evi dence of grafting was obtained by comparison of FTIR spectra of chitosan an d the graft copolymer as well as solubility characteristics of the products . The swelling behavior of chitosan samples, grafted the 115% and 524% poly (acrylic acid), was studied as function of pH, in distilled water and in aq ueous NaCl solution. The sample that has 115% grafting swells considerably more both in distilled water (pH 5.8) as well as higher pH values than the sample with 524% grafting. In contrast, an inverse swelling behavior at pH 2 was observed. An unusual swelling behavior at pH 2 was found for the samp le with 524% grafting, which swells to a greater extent than in distilled w ater. This was attributed to the amphiphilic nature of the modified chitosa n from which complex inter- and/or intramolecular interactions could origin ate.