Cellulose nanocrystal films: effect of electrolyte and lignin addition on self-assembly, optical, and mechanical properties

Effect of electrolytes with various sizes (CsCl, NaCl), valencies (NaCl, CaCl 2 , AlCl 3 ), pH (NaCl, NaOH) on the self-assembly, optical, and mechanical properties of CNC film was studied. The cross polarized optical microscopy, UV–Vis transmission spectroscopy showed that the chiral nematic to isotropic transition concentration was dependent on the type of counterion and its interaction with the CNC rods. Notably for large size and higher valency of counterion, lower transition concentration was needed. Electrolyte concentration also influenced the CNC film transparency and pitch. At lower electrolyte concentrations CNC film retained iridescence while at higher concentrations hazy film was obtained due to CNC aggregation and electrolyte precipitation, the maximum transparency was obtained near transition concentration. At low electrolyte concentration, the CNC film pitch was higher which may be due to large effective diameter of negatively charged CNC rods, increase in electrolyte concentration reduced the pitch due to suppression of electrical double layer. Addition of electrolyte also showed improvement in the tensile strength (upto 60%) and % strain (upto 170%) of CNC film. This may be due to the presence of ionic interaction between the sulfate ester groups of CNC and positively charged counter ions of electrolyte thereby improving the stress transfer between the rods. Further, the CNC/kraft lignin (KL) based transparent, UV protection films were designed by incorporating hydrogen peroxide treated KL (PBKL) optimized for its transparency and UV blocking properties. CNC/PBKL composite films showed superior %Transparency at 550 nm and %UV blocking at 400 nm compared to CNC/4-amino benzoic acid (4-ABA), a commercial UV-absorbent and most lignin based transparent composite films reported in literature. Graphical abstract