Characterization of the Network Structure of Carbodiimide Cross-Linked Gelatin Gels

Abstract

The network structure of native and carbodiimide cross-linked gelatin A and B gels was studied based on their rheological behavior. Gelatin A and B contain different numbers of carboxylic acid groups caused by different preparation conditions and had previously shown different characteristics in controlled release applications. It was evaluated to which extent chemical cross-linking densified the network structure of physical gelatin gels. After normalization of the equilibrium shear modulus ( G e ) with respect to swelling ( Q ), it was observed that the normalized G e values largely depend on the way gelatin is prepared from collagen. At an equal number of chemical junctions, chemically cross-linked gelatin B gels had a lower elasticity modulus than chemically cross-linked gelatin A gels. This seemed contradictory as gelatin B contains more carboxylic acid groups, available for cross-linking, but is related to a higher probability for intramolecular cross-linking, as was validated quantitatively by chemical and rheological analysis of the number of cross-links. Assuming an ideal network, the average molecular weight of the elastic network chains ( M c ) was calculated for physical and chemical gelatin A and B networks, and on the basis of M c the mesh sizes of the gels were estimated. The calculated mesh sizes were experimentally confirmed by lysozyme and albumin diffusion. Chemical cross-linking increased the resistance of the gels toward thermal degradation, resulting in a more gradual disintegration of physical cross-links upon heating. Moreover, chemical cross-linking prevented recombination of these cross-links upon cooling.