Abstract
Cellulose nanocrystals (CNCs) self-assemble in water suspensions into liquid crystalline assemblies. Here, we elucidate the microstructural changes associated with nonlinear deformations in (2–9 wt%) CNC suspensions through nonlinear rheological analysis, that was performed in parallel with coupled rheology—polarized light imaging. We show that nonlinear material parameters from Fourier-transform rheology and stress decomposition are sensitive to all CNC phases investigated, i.e. isotropic, biphasic and liquid crystalline. This is in contrast to steady shear and linear viscoelastic dynamic moduli where the three-region behavior and weak strain overshoot cannot distinguish between biphasic and liquid crystalline phases. Thus, the inter-cycle and intra-cycle nonlinear parameters investigated are a more sensitive approach to relate rheological measurements to CNC phase behavior.
Highlights
Cellulose nanocrystals (CNCs) show enormous potential for many applications, either as a dominant material component or part of multicomponent systems
The linear and nonlinear viscoelasticity of isotropic (2 wt% CNC), biphasic (3-5 wt% CNC) and liquid crystalline (≥ 6 wt%) phases in CNC water suspensions were investigated in this study
The phases were determined based on a combined analysis of polarized optical microscopy, linear viscoelastic oscillatory shear, steady shear viscosity functions and combined rheo-polarized light imaging (PLI) experiments
Summary
Cellulose nanocrystals (CNCs) show enormous potential for many applications, either as a dominant material component or part of multicomponent systems. Our hypothesis is that nonlinear oscillatory shear analysis through increased measurement sensitivity and additional nonlinear rheological parameters is a more sensitive framework to elucidate CNC phases from rheology. To our knowledge, this is the first study to address this. Space-time diagrams were constructed from the video recordings by extracting one line of pixels at a fixed position out of each still frame and appended to a new image having the x-axis corresponding to the experimental time and the y-axis corresponding to the length L (Kádár et al 2020a), see Fig. S1 Both oscillatory and steady shear measurements were performed on the setup. Where S > 0 indicates intra-cycle strain-stiffening, S < 0 intra-cycle strain-softening, T > 0 intra-cycle shear-thickening and T < 0 intra-cycle shear-thinning
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