Recent Advances in Chiral Nematic Structure and Iridescent Color of Cellulose Nanocrystal Films

Abstract

One unique property of cellulose nanocrystals (CNC) is their property of forming suspensions with chiral nematic order. This order can be preserved in films cast from the suspensions, raising the possibility of applications as photonic materials and templates. However, it has proved difficult to generate uniform, well-ordered chiral nematic materials from CNC. Recently, the importance of kinetic arrest due to gel formation in the later stages of evaporation has been recognized as a key step in film formation. In this brief review, recent developments regarding the structure of chiral nematic suspensions and films as monitored by polarized light microscopy are outlined, and attention is drawn to the importance of shear forces on the self-organization process.

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets

“…We interpret this pattern as the result of mechanical buckling of the cholesteric structure upon unidirectional vertical compression perpendicularly to its helix axis m. The vertical compression is expected to play an important role in this drying geometry (i.e., the solvent evaporates from the top while the surface area is kept constant) from the moment the drying suspension reaches a threshold concentration where the kinetic arrest occurs and the mutual orientation of the rods cannot relax anymore. [61][62][63][64] These undulations are fundamentally different from the Helfrich-Hurault or related instabilities [65][66][67][68] observed in confined cholesterics under the influence of external fields coupling parallel to the local director n. So far we observed simi lar buckling waves only in a spherical microdroplet geometry, [62] and is consistent with a lower compressibility of the cholesteric structure normal to m, which is expected in this system. [63] While we clearly demonstrate that casting CNC suspensions on NdFeB magnets does successfully induce an alignment of the cholesteric structure in the final film, it would be misleading to conclude that the final alignment in the dry film corresponds to the local direction of the magnetic field applied during casting.…”

Unexpected microphase transitions in flow towards nematic order of cellulose nanocrystals

“…The resulting cellulose nanocrystals (CNC) have lengths in hundreds of nanometers and widths in the 3-5 nm range (Rånby 1951;Marchessault et al 1959;Habibi et al 2010). The molecular and particular chirality has been identified as a source for selfassembly of the CNC in suspensions in liquid crystal phases that induce color in visible range but also via birefringence (Gray 2016).…”

Optical Properties of Self‐Assembled Cellulose Nanocrystals Films Suspended at Planar–Symmetrical Interfaces

“…For optical properties of materials formed from CNC, a nematic order translates into layers or structures with a reduced opacity. On the other hand, chiral nematic order allows control over the reflectance and transmittance of the material, imbuing it with tunable colors . The nematic order is observed when a film forms from a rapidly drying CNC suspension or under shear; in contrast, chiral nematic order requires a longer equilibration time.…”