X-ray scattering study of network structure of κ-carrageenan hydrogels with high concentrations

Structural features of κ-carrageenan hydrogels with high concentrations prepared by using subcritical water have been examined by X-ray scattering measurements from ultra-small through wide angles. It has been found that the high concentration hydrogels have the first- and second-order aggregation structures of crosslinks, being different from the conventional hydrogels prepared under atmospheric pressure. It has been evaluated that the higher-order structures become compact and distinct with concentration. Furthermore, the crosslinks of the high concentration hydrogels have been successfully approximated by a cylinder consisting of helical κ-carrageenan chains, which is significantly larger than those of the conventional hydrogels. The existence of the first-order structure and the cylindrical shape of the crosslinks have been confirmed by a butterfly pattern and an ellipse in the SAXS image respectively for κ-carrageenan hydrogels under uniaxial stretching. Comparison of WAXS peaks among the high concentration hydrogels indicates the enhancement of ordered aggregation of κ-carrageenan chains inside the crosslinks with increasing concentration. The mechanical behavior of the high concentration hydrogels as well as their thermal properties observed in our previous study can be explained by those revealed structural features due to the strong interchain aggregation.