INFLUENCE OF SOLUBLE PARTS IN GELATIN WATER GELS ON THEIR NETWORK STRUCTURE IN AN ULTRACENTRIFUGAL FIELD/

If gels are investigated in an analytical ultracentrifuge by means of sedimentation equilibrium experiments, so called soluble parts which a re originally not incorporated into the network influence the swelling pressure equilibria of the gels. For gelatin/water, it can be shown t hat an irreversible process takes place which is dependent on the rota tional speed of the experiment. Experimental evidence is given that so luble parts which are partly associated and aggregated to the polymer network can act as a branching and/or crosslinking agent. A model for this process is given. In such a case an application of the results of the swelling theory, e. g. that of Flory and Huggins assuming a const ant number of network junctions, cannot be applied anymore. A universa l procedure is described how swelling pressure equilibria can be ident ified, although the crosslinking density is changed. It can be shown t hat the gradient of the soluble parts, respectively the additional str uctural changes of the gel by the soluble parts, is reproducible and w ith it the swelling pressure equilibria. Furthermore the radial distri bution of dyed soluble parts inside the gel phase is presented as a fu nction of time during a sedimentation equilibrium run with a gelatin/w ater gel. It can be shown that the back diffusion process of the solub le parts in the gel is very slow and does not lead to the sedimentatio n-diffusion equlibrium of free soluble parts in the considered time in terval of 4 months. In combination with the detected fast sedimentatio n of the soluble component in the gel phase this gives further evidenc e for the association of the soluble parts to the gel network forming a gradient gel. The predicted formation of the anisotropic gel could b e proved by a swelling experiment.