Gelation Kinetics and Network Structure of Cellulose Nanocrystals in Aqueous Solution

Cellulose nanocrystals (CNC) are rod-like biosourced nanoparticles that are widely used in a range of applications. Charged CNC was obtained by acid extraction from cotton and dispersed in aqueous solution using ultrasound and characterized by light scattering. Aggregation and gelation of CNC induced by addition of NaCl was investigated by light scattering as a function of the NaCl concentration (30-70 mM), the CNC concentration (0.5-5 g/L), and the temperature (10-60 °C). Formation of fractal aggregates was observed that grow with time until they percolate and form a weak system spanning network. The aggregation rate and gel time were found to decrease very steeply with increasing NaCl concentration and more weakly with increasing CNC concentration. A decrease of the gel time was also observed with increasing temperature for T > 20 °C. The structure of the CNC networks was studied using confocal laser scanning microscopy and light scattering. The local structure of the networks was fractal and reflected that of the constituting aggregates. The gels were homogeneous on length scales larger than the correlation length, which decreased with increasing CNC concentration. The CNC gels flowed when tilted for C < 12 g/L and sedimentation was observed macroscopically for C < 4 g/L due to the collapse of the CNC network under gravity. The speed and extent of sedimentation was investigated as a function of the ionic strength and the CNC concentration. Gelled CNC could be completely redispersed by applying ultrasound.