Adjustment of the Chiral Nematic Phase Properties of Cellulose Nanocrystals by Polymer Grafting.

Abstract

The self-organization properties of sulfated cellulose nanocrystals, TEMPO-oxidized cellulose nanocrystals and polymer-decorated cellulose nanocrystals suspensions in water were investigated and compared. Polarized light optical microscopy observations showed that these three systems phase separated to form a lower anisotropic chiral-nematic phase and an upper isotropic phase following a nucleation and growth mechanism, proving that surface-grafted polymer chains did not inhibit the self-organization properties of CNCs. The phase diagrams and pitch of the suspensions were shown to strongly depend on the surface chemistry of the nanoparticles and the nature of the interacting forces. Especially, the entropic repulsion contribution of the polymer chains to the overall interactions forces resulted in a decrease of the critical volume fractions due to an increase of the effective diameter of the rods. Additionally, above a cellulose volume fraction of 3.5% v/v, the pitch was significantly smaller for polymer-decorated CNC suspensions than for sulfated as-prepared CNC ones, revealing stronger chiral interactions with the surface-grafted chains. In all cases, the addition of small quantities of monovalent salt induced an increase of the critical concentrations, but values for polymer-decorated CNCs were always the smallest ones due to entropic repulsion forces. Overall, results show that polymer grafting provides more tunability to the chiral-nematic phase properties of CNCs, including an enhanced expression of the chirality.