Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying

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.

From Equilibrium Liquid Crystal Formation and Kinetic Arrest to Photonic Bandgap Films Using Suspensions of Cellulose Nanocrystals

“…Finally, and very importantly, the new helix axis m no longer describes a perfectly helicoidal modulation of the optic axis, in which the optic axis rotates by a constant amount dϕ/dm (where we define the variable m to indicate location along the new helix axis m ). Instead we find regions of decreased twist (dϕ/dm ) − alternating with regions of increased twist (dϕ/dm ) + > (dϕ/dm ) − (see [174] for details). This is significant because this deformed helix can no longer be described by a pure sinusoidal function.…”

“…This means that the path from the equilibrium CNC suspension to a selectively reflecting film covers a very extensive range of non-equilibrium behavior. Luckily, this range has recently been exquisitely analyzed by Frka-Petesic et al [174], allowing us to largely refer to their paper for the details of the process. In this final section of our review before it is time to conclude, we will, nevertheless, summarize the key components of this important step.…”

“…The answer must be sought in the non-equilibrium process that starts with kinetic arrest and ends with a dry colorful film, a stage of drying that was long left largely uninvestigated [11]. Frka-Petesic et al took on the task of filling this knowledge gap, considering the process first on a more qualitative level [14], then with extraordinary detail, combining theory, simulation and experiments, in an absolutely exquisite study [174]. We believe this will become a landmark paper, as it explains almost all mysteries of the optical characteristics of films prepared by drying CNC suspensions.…”

“…A value of α = 0.1 can readily be expected, the result seen nicely in Figure 4; the rods are stacked right on top of each other in the dry film, very differently from the equilibrium liquid crystal phase situation. The significance of this compression on the optical properties can be tremendous, because of the complex distortion of the helical structure that it entails, as illustrated in Figure 11, from [174].…”

“…With a non-zero angle β between m =ê 3 andẑ the compression will be smaller, as is qualitatively easy to see from Figure 11. Quantitatively, the helix pitch compression depends on the original helix tilt β as [174]:…”