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Digital color in cellulose nanocrystal films.

ABSTRACT: Cellulose nanocrystals (CNCs) form chiral nematic phases in aqueous suspensions that can be preserved upon evaporation of water. The resulting films show an intense directional coloration determined by their microstructure. Here, microreflection experiments correlated with analysis of the helicoidal nanostructure of the films reveal that the iridescent colors and the ordering of the individual nematic layers are strongly dependent on the polydispersity of the size distribution of the CNCs. We show how this affects the self-assembly process, and hence multidomain color formation in such bioinspired structural films.

SUBMITTER: Dumanli AG

PROVIDER: S-EPMC4251880 | biostudies-other | 2014 Aug

REPOSITORIES: biostudies-other

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Digital color in cellulose nanocrystal films.

Dumanli Ahu Gümrah AG van der Kooij Hanne M HM Kamita Gen G Reisner Erwin E Baumberg Jeremy J JJ Steiner Ullrich U Vignolini Silvia S

ACS applied materials & interfaces 20140728 15

Cellulose nanocrystals (CNCs) form chiral nematic phases in aqueous suspensions that can be preserved upon evaporation of water. The resulting films show an intense directional coloration determined by their microstructure. Here, microreflection experiments correlated with analysis of the helicoidal nanostructure of the films reveal that the iridescent colors and the ordering of the individual nematic layers are strongly dependent on the polydispersity of the size distribution of the CNCs. We sh ...[more]

PMID: 25007291

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Project description:A known deterrent to the large-scale development and use of cellulose nanocrystals (CNCs) in composite materials is their affinity for moisture, which has a profound effect on dispersion, wetting, interfacial adhesion, matrix crystallization, water uptake, and hydrothermal stability. To quantify and control the hydration and confinement of absorbed water in CNCs, we studied sulfated-CNCs neutralized with sodium cations and CNCs functionalized with less hydrophilic methyl(triphenyl)phosphonium cations. Films were cast from water suspensions at 20 °C under controlled humidity and drying rate, yielding CNC materials with distinguishably different dielectric properties and cholesteric structures. By controlling the evaporation rate, we obtained self-assembled chiral CNC films with extended uniformity, having helical modulation length (nominal pitch) tunable from 1300 to 600 nm. SEM imaging and UV-vis-NIR total reflectance spectra revealed tighter and more uniform CNC packing in films cast at slow evaporation rates or having lower surface energy when modified with phosphonium. The dielectric constant was measured by a noncontact microwave cavity perturbation method and fitted to a classical mixing model employing randomly oriented ellipsoidal water inclusions. The dielectric constant of absorbed water was found to be significantly smaller than that for free liquid indicating a limited mobility due to binding with the CNC "matrix". In the case of hydrophilic Na-modified CNCs, a decreasing pitch led to greater anisotropy in the shape of moisture inclusions (ellipsoidal to platelet-like) and greater confinement. In contrast, the structure of hydrophobic phosphonium-modified CNC films was found to have reduced pitch, yet the shape of confined water remained predominantly spherical. These results provide a useful perspective on the current state of understanding of CNC-water interactions as well as on CNC self-assembly mechanisms. More broadly, we believe that our results are beneficial for the realization of CNC-based functional materials and composites.

Dataset Information

Digital color in cellulose nanocrystal films.

Publications

Digital color in cellulose nanocrystal films.

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