H-1 spin-spin relaxation time of water and rheological properties of cellulose nanofiber dispersion, transparent cellulose hydrogel (TCG)

A new type of hydrogel, transparent cellulose hydrogel (TCG), is the aqueous dispersion of cellulose nanofibers (microfibrils) 10nm in width and several hundreds nanometers in extended fiber length, and shows unique rheological properties leading to unique applications. The rheological properties of TCG, especially their dependences on temperature were investigated through the spin-spin relaxation time (T-2) analysis in H-1 NMR for water in the systems. Viscosity under low shear stress and T-2 of water (being very short for T-2 value) were proved to be constant in a wide range of temperature (ca. 30-80 degreesC). These results may be explained by two considerations that network in TCG gradually grows with increasing temperature but collapses by adding weak shear stress and that TCG gel has large amount of bound water. It was also confirmed that the ionic strength such as pH and NaCl concentration sensitively influences on rheological parameters. With increasing ionic strength, the network formation and the successive aggregation of microfibrils occur and both should be interpreted in terms of the electrostatic interaction between negative charge on a cellulose surface and cationic aqueous layer around it (i.e., electric double layer).