Effect of Anisotropy of Cellulose Nanocrystal Suspensions on Stratification, Domain Structure Formation, and Structural Colors

Abstract

Outstanding optical and mechanical properties can be obtained from hierarchical assemblies of nanoparticles. Herein, the formation of helically ordered, chiral nematic films obtained from aqueous suspensions of cellulose nanocrystals (CNCs) were studied as a function of the initial suspension state. Specifically, nanoparticle organization and the structural colors displayed by the resultant dry films were investigated as a function of the anisotropic volume fraction (AVF), which depended on the initial CNC concentration and equilibration time. The development of structural color and the extent of macroscopic stratification were studied by optical and scanning electron microscopy as well as UV–vis spectroscopy. Overall, suspensions above the critical threshold required for formation of liquid crystals resulted in CNC films assembled with longer ranged order, more homogeneous pitches along the cross sections, and narrower specific absorption bands. This effect was more pronounced for the suspensions that were closer to equilibrium prior to drying. Thus, we show that high AVF and more extensive phase separation in CNC suspensions resulted in large, long-range ordered chiral nematic domains in dried films. Additionally, the average CNC aspect ratio and size distribution in the two separated phases were measured and correlated to the formation of structured domains in the dried assemblies.

Expanding the upper limits of robustness of cellulose nanocrystal aerogels: outstanding mechanical performance and associated pore compression response of chiral-nematic architectures

“…Further results from the characterization of the CNCs can be found in our previous study. 26 Milli-Q water (U.S.A. Millipore, Synergy UV) was used to dilute the CNC dispersion. Acetone was obtained from VWR Chemicals International S.A.S.…”

“…24 A particularly appealing aspect of using highly anisotropic cellulosic building blocks is associated with their long-range order in aqueous dispersion or in the solid state. 21,25,26 Specically, it has been demonstrated that nematic ordering of CNCs and CNFs can greatly enhance the properties of the formed aerogels, for instance, enabling a high directional toughness while maintaining high optical transparency and thermal insulation. 21,27 Compared to conventional aerogels with isotropic skeletal structures, the porous assemblies of nematic ordered aerogels can be further enhanced by uniaxial compression, whereby pore size harmonization in expense of macropores and in favor of mesopores occurs.…”

Asymmetrical coffee rings from cellulose nanocrystals and prospects in art and design

“…They were received as a suspension at 11.1% solids concentration. Details about the CNCs are listed in previous work (Klockars et al 2018). Ethanol (A, 94%) was obtained from Altia Fig.…”

“…The films in this study were all dried at 50% relative humidity to improve reproducibility, and to ensure a longer evaporation time and, therefore, to favor chiral nematic assembly as well as coffee ring formation via EISA (Klockars et al 2018;Tran et al 2018). Otherwise, at high enough drying rates, ''kinetic trapping or arrest'' would occur for particles, limiting the formation of the coffee ring (Li et al 2016).…”

“…Thus, CNC suspension drying near the corners is expected to be independent of the square size but rather depend on the film edge geometry. The CNC concentration (Klockars et al 2018), as well as the drying rate (Beck et al 2013;Natarajan et al 2017) may further affect coffee ring formation.…”