Cellulose nanocrystals in aqueous suspensions: rheology of lyotropic chiral liquid crystals

Abstract

A thorough rheological study of the behavior of cellulose nanocrystals (CNC) in water was undertaken, focusing on the main characteristics of rod-like suspensions that allign under flow and can display liquid crystal behavior at rest, in at least a range of concentrations. The effect of adding a dispersing agent (gum arabic, GA) to the suspension was analyzed, and different surface treatments were also performed on the CNC using TEMPO, chloro(dodecyl)dimethylsilane and trimethoxysilyl propylmethacrylate, named as TCNC, DCNC and MCNC respectively. The CNC, TCNC and the suspensions prepared with added GA formed stable suspensions with a concentration range in which isotropic and anisotropic (self-organized) phases coexisted in equilibrium. On the other hand, the aqueous suspensions of the two silanized CNCs only displayed flow-induced ordering, with a similar type of rheological behavior to the other nanocrystal suspensions, although with obvious quantitative differences. These latter ones were not stable suspensions and precipitated after a few hours of rest. The rheological tests included steady shear, dynamic oscillations and start-up of steady shear that were performed for all the nanocrystals suspensions prepared at different concentrations. Differences observed were related to the crystals surface modifications or addition of the dispersing agent. A persistent periodic oscillation of the viscosity was measured in the start-up of steady shear tests and the periods of the oscillations were determined and related to the different types of crystals considered.