Abstract
Cellulose nanocrystals (CNCs) are bio-sourced chiral nanorods that can form stable colloidal suspensions able to spontaneously assemble above a critical concentration into a cholesteric liquid crystal, with a cholesteric pitch usually in the micron range. When these suspensions are dried on a substrate, solid films with a pitch of the order of few hundreds of nanometers can be produced, leading to intense reflection in the visible range. However, the resulting cholesteric nanostructure is usually not homogeneous within a sample and comports important variations of the cholesteric domain orientation and pitch, which affect the photonic properties. In this work, we first propose a model accounting for the formation of the photonic structure from the vertical compression of the cholesteric suspension upon solvent evaporation, starting at the onset of the kinetic arrest of the drying suspension and ending when solvent evaporation is complete. From that assumption, various structural features of the films can be derived, such as the variation of the cholesteric pitch with the domain tilt, the orientation distribution density of the final cholesteric domains and the distortion of the helix from the unperturbed cholesteric case. The angular-resolved optical response of such films is then derived, including the iridescence and the generation of higher order reflection bands, and a simulation of the angular optical response is provided, including its tailoring under external magnetic fields. Second, we conducted an experimental investigation of CNC films covering a structural and optical analysis of the films. The macroscopic appearance of the films is discussed and complemented with angular-resolved optical spectroscopy, optical and electron microscopy, and our quantitative analysis shows an excellent agreement with the proposed model. 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. This work highlights the key role that the anisotropic compression of the kinetically arrested state plays in the formation of CNC films and is relevant to the broader case of structure formation in cast dispersions and colloidal self-assembly upon solvent evaporation.
Highlights
We describe the formation of Cellulose Nanocrystals (CNCs) films as a set of cholesteric domains of given initial orientation that are subject to a linear deformation process
Our model is based on a simple assumption that, once the kinetic arrest has taken place, the arrangement in the liquid crystalline phase in suspension is locked and upon further solvent evaporation the structure is subject to a vertical compression that can be described in the limit of affine deformation
We find it essential to emphasize this interpretation of the stripes, as they are often ascribed in the literature of CNC films to the common fingerprint pattern of cholesterics
Summary
Colloidal self-assembly is a powerful pathway to produce a variety of nanostructures that can display selective optical properties analogous to some biological tissues found in nature.[1,2,3] Understanding and controlling these selfassembly routes remains challenging, as many competing phenomena operate simultaneously at different length scales to produce the final structure in the material, and their optical response. Our model is based on a simple assumption that, once the kinetic arrest has taken place, the arrangement in the liquid crystalline phase in suspension is locked and upon further solvent evaporation the structure is subject to a vertical compression that can be described in the limit of affine deformation This model allows predicting several aspects of the structure of CNC films, namely: the angular dependence of the cholesteric pitch, the reorientation of the domains, the angular distribution of the domains provided the initial one in the arrested suspension is known, the distortion of the helical order and its visualization in film cross-section. We discuss the implications of our model and its limitations and summarize our main findings in key bullet points
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