Multiscale Characterization of Cellulose Nanocrystal Suspensions

Abstract

Cellulose nanocrystal (CNC) suspensions are hierarchical colloidal systems com posed of anisotropic nanorods. Their rheological and optical responses depend on concentration, aging time, and deformation history. During extrusion-based print ing, the confined nozzle flow can reorganize CNC rods, aggregates, and liquid crystalline domains, but bulk rheology alone cannot resolve how this structural response appears in the emerging filament. This thesis therefore combines steady shear rheology, polarized optical microscopy (POM), polarized light imaging (PLI), and in-situ polarization imaging (PI) to characterize 4, 8, and 10 wt% CNC sus pensions after 3 and 7 days of aging during pneumatic nozzle extrusion. Rheology and POM show that increasing concentration and aging promote stronger viscos ity, more pronounced birefringent texture, and a more structured initial suspension state. In-situ PI further reveals clear concentration-, pressure-, and aging-dependent post-extrusion optical responses. The 4 wt% suspension produces weaker retarda tion signals that generally decrease downstream from the nozzle exit, whereas the 8 wt% suspension shows the most heterogeneous response, including pronounced temporal variation and pressure-dependent retardation patterns. The 10 wt% sus pension exhibits a comparatively steadier optical response in the cases where stable extrusion is achieved, suggesting a more persistent structured filament after noz zle exit. Lateral PI profiles also reveal edge-localized high-retardation bands and secondary internal bands, indicating spatially heterogeneous post-extrusion optical anisotropy across the filament. Overall, the results indicate that CNC extrusion reflects the coupled effects of the initial suspension state, nozzle-induced deforma tion, and downstream structural evolution, while in situ PI provides spatially and temporally resolved optical information complementary to rheology and POM