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The impact of high energy crosslinking on the network structure of gelatin hydrogels was investigated in comparison to physically entangled gels by small-angle X-ray scattering (SAXS). Physically entangled gelatin of increasing concentration exhibited a nearly constant correlation length of several nanometers. These gels had scattering behavior close to that of polymer coils swollen in a good solvent, as evidenced by the Porod exponent of 1.8. The mass fractal dimension decreased towards 1, indicating increased formation of semiflexible gelatin triple helices and rod-like structures as a function of the gelatin concentration. In contrast, electron irradiation lead to a decrease in the correlation length at doses above 20 kGy. Covalent crosslinking induced by electron irradiation lead to increased branching and formation of globular structures, as observed by a steady increase of both the Porod exponent and mass fractal dimension. Furthermore, the network mesh size systematically decreased from approximately 45 nm to under 20 nm with both additional physical and chemical crosslinking. These mesh sizes as obtained by SAXS were used to estimate the network shear modulus using several polymer models and were compared to macroscopic rheology measurements. Finally, SEM images of freeze-dried samples revealed changes in the microstructure of the irradiated hydrogels. Overall, fundamental differences in the network structures stemming from the crosslinking method were observed across a wide range of length scales.
Wisotzki et al. (Sun,) studied this question.