ABSTRACT
Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received 1 October 2003; accepted 20 January 2004
Abstract-It has been found that mixing of two kinds of water-soluble phospholipids polymers, such as poly(2-methacryloyloxyethyl phosphorylcholine-co-methacrylic acid) (PMA) and poly(2methacryloyloxyethyl phosphorylcholine-co-w-butyl methacrylate) (PMB), spontaneously forms a hydrogel in aqueous medium at room temperature without any chemical treatment. However, the mechanism of spontaneous gelation has not been clarified yet. The purpose of this study is to investigate the gelation mechanism of the hydrogel. Moreover, effects of ions on gelation and dissolution behavior were observed. We investigated the mechanism of the hydrogel formation by spectroscopic techniques and a rheological method with attention to the interactions between polymer chains. Both Raman spectroscopic analysis and FT-IR analysis revealed that carboxyl groups in methacrylic acid (MA) formed dimer when two polymer solutions were mixed, and the results of the rheological study showed dissociation of carboxyl groups caused dissolution of the hydrogel. Thus, the gelation occurred due to the formation of dimers by hydrogen bonding which acts as a physical cross-linking of polymer chains. The hydrogel dissolved in a large amount of aqueous medium. We also observed the addition of inorganic salts during the preparation of the hydrogel affected the gelation and dissolution behaviors by a rheological and a weight measuring method, respectively. The gelation period became longer in the presence of NaCl and CaCl2 compared with that in the absence of these salts. NaCl and CaCl2 disturbed the formation of hydrogen bonding between carboxyl groups by stabilization of carboxylate anion of the MA units. On the other hand, addition of FeCb made the gelation period shorter and stabilized the hydrogel in the aqueous medium. This is because FeCl3 can suppress dissociation of the carboxyl groups by acidic condition of FeCb aqueous solution and cross-link the carboxylate anions in the PMA effectively.
