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

Derek Gray

doi:10.3390/nano6110213

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.

Highlights

Cellulose nanocrystals (CNC) and other forms of nanocellulose are gaining recognition as an important new family of sustainable materials with a wide range of potential applications [1,2]
A surprising property of cellulose nanocrystal suspensions was that the rod-like nanocrystals formed a chiral nematic phase above a critical concentration [3], and that this suspension dried to form an iridescent film with the optical properties of a chiral nematic liquid crystal [4]
The wavelength of the reflected band changes with viewing angle. These properties were explained by de Vries [5] on the basis of a helicoidal arrangement of birefringent layers. He predicted that the wavelength of reflected circularly polarized light, λ, would depend on the pitch, P, of the chiral nematic structure, λ = nP where n is the mean refractive index of the chiral nematic phase

Summary

Introduction

Cellulose nanocrystals (CNC) and other forms of nanocellulose are gaining recognition as an important new family of sustainable materials with a wide range of potential applications [1,2]. The wavelength of the reflected band changes with viewing angle These properties were explained by de Vries [5] on the basis of a helicoidal arrangement of birefringent layers. He predicted that the wavelength of reflected circularly polarized light, λ, would depend on the pitch, P, of the chiral nematic structure, λ = nP where n is the mean refractive index of the chiral nematic phase. The handedness of the reflected light depends on the handedness of the chiral nematic structure This relation between wavelength and pitch holds for samples viewed along the chiral nematic axis (i.e., normal to the sample surface). The many interesting organic and inorganic chiral nematic materials prepared from CNC and CNC templates are not considered here

Overview of Drying Process

Carboxyl-Stabilized CNC Suspensions and Films

Liquid Crystalline Textures Observed by Polarized Light Microscopy

Polarized Light Microscopy of Solid CNC Films