A series of four cellulose nanocrystal (CNC) suspensions were prepared from bleached softwood kraft pulp using different conditions of sulfuric acid hydrolysis. The CNCs were identical in size (95 nm in length × 5 nm in width) but had different surface charges that increased with inceasing harshness of the hydrolysis conditions. Consequently, it was possible to isolate the effects of surface charge on the self-assembly and rheology of the CNC suspensions across a range of surface charges from 0.27 %S to 0.89 %S. The four suspensions (free of added electrolyte) all underwent liquid crystalline phase separation but the concentration onset for the emergence of the chiral nematic phase shifted to higher values with increasing surface charge. Similarly, suspension viscosity was also influenced by surface charge, with suspensions of lower surface charge CNCs more viscous and tending to gel at lower concentrations. The properties of the suspensions were interpreted in terms of the effective diameter of the particles, which decreased with increasing surface charge, but also by the formation of end-to-end aggregates at relatively low concentrations in the suspension of lowest surface charge CNCs. The results suggest a threshold surface charge density (~0.3 %S) above which effective volume considerations are dominant across the concentration range relevant to liquid crystalline phase formation. Above this threshold value, phase separation occurs at the same effective volume fraction of CNCs (~10 vol%), with a corresponding increase in critical concentration due to the decrease in effective diameter that occurs with increasing surface charge. Below this threshold value, the formation of end-to-end aggregates hastens gelation and interferes with ordered phase formation.

Phase separation and rheological properties of cellulose nanocrystals with different surface charge densities

255th National Meeting (2018)

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