Liquid crystal ordering transition and colloidal interactions of cellulose nanocrystals (CNCs)

CNCs are materials of increasing interest because of their size, aspect ratio, stiffness, renewability, and liquid crystal (LC) ordering. CNCs self-assemble into LC phases above a critical concentration, which is dictated by particle size and surface properties. In this work, we study this transition by observing the interactions of CNCs across a range of concentrations using isothermal titration calorimetry (ITC). This technique allows us to take highly precise heat measurements from the interactions of very small sample volumes. Addition of electrolytes also has a pronounced effect on the suspension behavior: at low ionic strengths (0-10 mM), the electrostatic double layer is compressed and the effective particle size is reduced. With increasing ionic strength (>20 mM), the electrolytes destabilize the suspension by neutralizing the electrostatic repulsive forces and inducing aggregation. The nature of this transition is examined for different electrolytes and CNC concentrations, with ITC results for both liquid crystal transition and ionic strength effects being corroborated using rheology and polarized optical microscopy (POM). These techniques provide relevant microstructural information in the form of rheology, and the presence and extent of anisotropic domains.