CHARACTERIZATION OF THE NETWORK STRUCTURE OF DEXTRAN GLYCIDYL METHACRYLATE HYDROGELS BY STUDYING THE RHEOLOGICAL AND SWELLING BEHAVIOR

This paper reports the results and the structural interpretation of th eological, swelling, and analytical sol fraction (w(s)) measurements p erformed on dextran glycidyl methacrylate (dex-gma) hydrogels as a fun ction of the dex-gma concentration (c) and the degree of gma substitut ion (DS). Besides the analytical determination, the sol fi action was also calculated from the elasticity of the hydrogels using a theoretic al model. This model takes into account the presence of sol chains and dangling ends, the polydispersity of the dex-gma chains, the function ality of the junctions, and the nonaffinelike behavior of the dex-gma network. It assumes the absence of intramolecular cross-links and of p hysical entanglements. Fundamental in the discussion is the reason why the w(s) values determined analytically are systematically lower than the w(s) values calculated using this theoretical model. Besides poss ible influences from the unknown functionality of the junctions and fr om a nonaffinelike deformation behavior of the network, the presence o f intramolecular cross-links (especially for hydrogels with a low dex- gma concentration and a high DS) may contribute to the observed differ ences between the measured and the calculated sol fraction. Structural information is also revealed from the influence of the DS on the elas ticity if plotted against the network concentration (i.e., (1 - w(s))c ) of the hydrogels. Clearly, a DS increase increases quantitatively th e network fraction. However, for high DS values, a DS increase does no t increase the number of intermolecular cross-links per unit of mass p resent in the network fraction. Also the abundant presence of intramol ecular cross-links, especially for dex-gma hydrogels with a high DS, m ay contribute to this phenomenom. Contrary to the elastic properties, considering hydrogels with the same network concentration, the DS does have a definite influence on the swelling properties of the network f raction of the gels. This was attributed to the dependence of the poly mer-solvent interaction parameter on the DS.