Shear-Coated Linear Birefringent and Chiral Cellulose Nanocrystal Films Prepared from Non-Sonicated Suspensions with Different Storage Time

Nanocelluloses are very attractive materials for creating structured films with unique optical properties using different preparation techniques. Evaporation-induced self-assembly of cellulose nanocrystals (CNC) aqueous suspensions produces iridescent films with selective circular Bragg reflection. Blade coating of sonicated CNC suspensions leads to birefringent CNC films. In this work, fabrication of both birefringent and chiral films from non-sonicated CNC suspensions using a shear-coating method is studied. Polarization optical microscopy and steady-state viscosity profiles show that non-sonicated CNC suspensions (concentration of 6.5 wt%) evolve with storage time from a gel-like shear-thinning fluid to a mixture of isotropic and chiral nematic liquid crystalline phases. Shear-coated films prepared from non-sonicated fresh CNC suspensions are birefringent, whereas films prepared from suspensions stored several weeks show reflection of left-handed polarized light. Quantification of linear and circular birefringence as well circular dichroism in the films is achieved by using a Mueller matrix formalism.

Automated summary

Nanocelluloses are promising materials for creating structured films with unique optical properties through different preparation techniques. This study investigated the fabrication of birefringent and chiral films from non-sonicated CNC suspensions using a shear-coating method, showing the evolution of suspensions from a gel-like shear-thinning fluid to isotropic and chiral nematic liquid crystalline phases. The films prepared from non-sonicated fresh CNC suspensions exhibit birefringence, while those from stored suspensions reflect left-handed polarized light, with quantification achieved using a Mueller matrix formalism.