Cellulose nanocrystals (CNCs) are bio-sourced chiral nanorods that can form aqueous colloidal suspensions able to spontaneously self-assemble above a critical concentration into a cholesteric liquid crystal. This order is characterized by a helicoidal alignment of the CNCs, locally described within each domain by the helical axis direction and its periodicity, or pitch. When these suspensions are dried on a substrate, the resulting solid-state films display a photonic response characterized by a strong reflection at visible wavelengths.[1] In this talk, we describe the optical response of CNC films in terms of specular reflection and scattering properties, and we show that the reflected wavelength is always red-shifted at off-specular angles with respect to specular conditions. This observation indicates that the pitch is not uniform across the sample but varies with the orientation of the different cholesteric domains. We propose a model accounting for this effect, which describes the vertical compression of the cholesteric suspension upon drying, starting at the onset of the kinetic arrest and ending when solvent evaporation is complete.[2,3] Our model is in excellent agreement with observations and allows for retrieving the history of the self-assembly process. This allows us to access the precise composition and the pitch of the suspension when it transited into a kinetically arrested phase directly from the optical analysis of the film. We demonstrate its use to investigate the kinetic arrest transition in an elegant way, avoiding the usual complications from common rheological approaches.

Optical analysis of cellulose nanocrystal films enables access to their self-assembly history

2019 Spring National Meeting

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