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 φ_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., φ_w = 0.01) and that the plateau modulus increases with increasing φ_w in the φ_w range up to 0.15. Spinlattice relaxation time T₁ measurements using ¹H-NMR technique revealed that there exist at least two different types of water with different T₁ values in the ternary gel. By comparing those T₁ values with that of the water proton in the mixed solvent, the water with shorter T₁ (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 T₁ (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_c and the weight fraction of Water-2 φ_Water-2, indicating that the number of crosslinking points increases in proportion to φ_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 φ_Water-2, respectively. All these results support the validity of our hypothesis about Water-2.