Thermo-responsive liquid crystalline phases of cellulose nanocrystals with polymer brushes

Cellulose nanocrystals (CNCs) offer the potential for control over their contour length, aspect ratio, surface chemistry, and the resulting colloidal, thermal, and optical properties. They have garnered significant attention in the sustainable manufacturing of nanocomposites for applications in structural coloration, smart security tags, and colorimetric sensors. The underlying phase behavior including the complex interplay of surface-modified CNCs on the molecular interactions is still unclear and needs to be addressed prior the design of new functional materials.

We present self-assembled CNC liquid crystalline phase behavior mediated by Poly(N-isopropylacrylamide) (PNIPAM) brush grafted from the CNC surface and explore the impact of PNIPAM brush length and thermo-responsive behavior on the liquid crystalline phases. The grafted PNIPAM brushes allow to fine tune the CNC diameter and the resulting self-assembly.

CNCs bearing short polymer brushes form various liquid crystalline phases, with the critical points in the isotropic-cholesteric-nematic phase transition shifting to higher concentrations with longer brushes. This shift is further elucidated through the application of Onsager theory incorporating the changing electrostatic repulsion and steric hindrance. Excessively long polymer brushes inhibit the formation of cholesteric tactoids. The characteristic texture of the PNIPAM-CNC liquid crystalline phases observed under polarizing optical microscopes disappear and reappear upon heating and cooling, switched by the thermo-responsive PNIPAM brush aggregates on the CNC surface. These findings enrich our understanding of cholesteric liquid crystalline systems and extend their application in thermal stimuli-responsive soft nanotechnology.