Reciprocal-Space Analysis of Structural Reorganization and Orientational Correlation in a Cellulose Nanofibril Suspension during Constant-Stress Creep

Cellulose nanofibril (CNF) suspensions exhibit yielding and viscoplastic flow because high-aspect-ratio fibrils form entangled networks through fibril entanglement and interfibrillar interactions. However, the time evolution of structural reorganization and orientational correlation under constant stress remains insufficiently understood. In this study, rheo-small-angle light scattering (Rheo-SALS) measurements were performed on a mechanically fibrillated CNF suspension during constant-stress creep at 10, 40, 60, and 200 Pa using parallel-polarized (HH) and cross-polarized (HV) configurations. In the HH mode, at 40-200 Pa, the characteristic correlation length estimated from the shoulder position of the scattering profile decreased sharply during the first 0-20 s and then changed more gradually. The anisotropy ratio over 20-100 s increased with stress and was largest at 200 Pa. In the HV mode, a 4-fold azimuthal pattern developed at 40-200 Pa, with maxima near 45°, 135°, 225°, and 315° and minima near 0°, 90°, 180°, and 270°. The mean peak intensity decreased, the mean trough intensity increased, and an azimuthal modulation index decreased rapidly during 0-20 s, indicating that the major change in orientational correlation was concentrated immediately after stress application. At higher stress, the lower azimuthal modulation index reflected a broader angular distribution of orientational correlation rather than a loss of anisotropy. These results show that the creep response of the CNF suspension under constant stress involves a stress-dependent hierarchical structural transition from an isotropically entangled network to anisotropic mesoscale organization and, at the highest stress, to flow-aligned smaller aggregate units.