Role of Water in Gelation of Curdlan/Dmso/Water Ternary System

We investigated the structure and viscoelastic properties of ternary gels of curdlan in mixed solvents of dimethyl sulfoxide (DMSO) and water as a function of the weight fraction of water phi(w) to reveal the gelation mechanism. Dynamic viscoelasticity measurements demonstrated that the gelation begins to be induced by the addition of 1 wt % of water (i.e., phi(w) = 0.01) and that the plateau modulus increases with increasing phi(w) in the phi(w) range up to 0.15. Spin lattice relaxation time T-1 measurements using H-1-NMR technique revealed that there exist at least two different types of water with different T-1 values in the ternary gel. By comparing those T-1 values with that of the water proton in the mixed solvent, the water with shorter T-1 (Water-1) was ascribed to the water molecules dispersed in the mixed solvent contained in the ternary gel. On the other hand, the water with the longer T-1 (Water-2) was presumed to exist under the situation where water-water interaction is dominant. We hypothesized that Water-2 is localized around the vicinity of cross-linking points and is more relevant to the formation of cross-linking points. Almost a linear relation was observed between the plateau modulus G, and the weight fraction of Water-2 phi(Water-2), indicating that the number of cross linking points increases in proportion to phi(Water-2) in a constant volume of the ternary gel. Further, the distance between neighboring cross-linking points and the size of the region of Water-2 (i.e., equivalent spherical radius) was estimated at 15-9.3 nm and 2.1-2.5 nm depending on phi(Water-2), respectively. All these results support the validity of our hypothesis about Water-2.