Change in the dispersion states of short-length-cellulose nanofibers upon dilution investigated by a time-domain nuclear magnetic resonance (TD-NMR)

A short-length cellulose nanofiber (CNF) aqueous sol, prepared by a high-pressure homogenizer, showed a rapid longer relaxation time (T₂) in the low-field ¹H-nuclear magnetic resonance (NMR) when diluted from 20 wt% to 1 wt%. Magnetic stirring for 30 min disentangled the fiber networks and the fragmented fibers appeared in the 1 wt% CNF sol. A decrease in the specific viscosity of the diluted sols changed the rheological behavior from exponential to linear below 1 wt%, suggesting a significant decrease in the inter-fibril interaction. The small angle x-ray scattering (SAXS) with the generalized indirect Fourier transformation (GIFT) also indicated similar changes in the fiber flocculation structure without a change in the fiber size. The increasing viscosity upon severe fiber fragmentation by a high-pressure homogenizer may be ascribed to tighter holding of the interfibril water molecules. The time-domain (TD)-NMR fully supported the estimation that the transverse relaxation time (T₂) showed consistently short for the 2 wt%, became shorter with the stirring time when diluted from 5 wt% to 2 wt%, and showed long upon dilution from 20 wt% to 2 wt%. Understanding the complex behavior of the highly viscous CNF sols during a simple dilution process may pave the way for developing CNF-related technology.