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Shear rheology of cellulose nanocrystal (CNC) aqueous suspensions Shafiei Sabet, Sadaf

Abstract

Scientific and commercial interests in renewable nanomaterials have been receiving increasing attention over the years. Cellulose nanocrystal (CNC) derived from entirely renewable resources promises wide applicability owing to its high strength, chirality, self-assembly and electromagnetic properties. In this thesis the rheology of CNC aqueous suspensions was studied and the rheological behaviour was correlated with their microstructure. It has been found that the CNC aqueous suspensions experience two microstructural transitions by increasing CNC concentration: a transition from isotropic to chiral nematic liquid crystal occurs above a first critical concentration, and by further increasing concentration, the suspensions go through another transition from chiral nematic liquid crystal to gel above a second critical concentration. The viscosity profile of anisotropic suspensions shows a three-region behaviour characteristic of liquid crystals, and after gel formation a single shear thinning is observed over the whole investigated range. CNC suspensions possessing a higher degree of sulfation have more tendency to form anisotropic chiral nematic structures, and form gels at relatively higher concentration compared to those with a lower degree of sulfation. Sonication up to 1000 J/g CNC, breaks all the aggregates in the system and significantly decreases the viscosity. Although the sonication-induced decrease in viscosity levels off through further sonication (>1000 J/g CNC), it still affects the viscosity of anisotropic suspensions at low shear rates by increasing the size of chiral nematic domains. The effects of adding NaCl to CNC aqueous suspensions have been evaluated in different concentration regimes: isotropic, anisotropic chiral nematic, and gel. For isotropic samples and gels, the viscosity decreases by the addition of NaCl up to 5 mM. For anisotropic samples, on the other hand, the viscosity at low shear rates increases by addition of NaCl up to 5 mM due to decrease in chiral nematic domain size. However, at high shear rates, where all the domains are broken, the viscosity decreases when adding NaCl. Further addition of NaCl (>5 mM) results in extensive aggregation in suspension, and thus the viscosity increases.

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