Flexible and Responsive Chiral Nematic Cellulose Nanocrystal/Poly(ethylene glycol) Composite Films with Uniform and Tunable Structural Color

Abstract

The fabrication of responsive photonic structures from cellulose nanocrystals (CNCs) that can operate in the entire visible spectrum is challenging due to the requirements of precise periodic modulation of the pitch size of the self-assembled multilayer structures at the length scale within the wavelength of the visible light. The surface charge density of CNCs is an important factor in controlling the pitch size of the chiral nematic structure of the dried solid CNC films. The assembly of poly(ethylene glycol) (PEG) together with CNCs into smaller chiral nematic domains results in solid films with uniform helical structure upon slow drying. Large, flexible, and flat photonic composite films with uniform structure colors from blue to red are prepared by changing the composition of CNCs and PEG. The CNC/PEG(80/20) composite film demonstrates a reversible and smooth structural color change between green and transparent in response to an increase and decrease of relative humidity between 50% and 100% owing to the reversible swelling and dehydration of the chiral nematic structure. The composite also shows excellent mechanical and thermal properties, complementing the multifunctional property profile.

Flexible and Structural Coloured Composite Films from Cellulose Nanocrystals/Hydroxypropyl Cellulose Lyotropic Suspensions

“…Indeed, if the onset of the first degradation process is estimated, an increase of 10 • C, 16 • C and 21 • C on the composite system with 30 wt.% (T = 154 • C), 40 wt.% (T = 160 • C) and 50 wt.% (T = 165 • C) respectively, is observed when compared with the neat CNCs (T = 144 • C). Similar tendencies were observed in composite systems of CNCs/PEG [14]. In this work, the authors related this thermal stability to the formation of a uniform PEG layer onto the surface of the CNCs protecting them from degradation.…”

“…For instance, Yao et al were able to retain structural colouration after the addition of polyethylene glycol (up to 50wt.% CNC/PEG) to the CNCs suspension. The final product was humidity-responsive, since exposure to humidity increases the pitch value of the films in a reversible process, and became flexible as the authors increased the PEG content in the composite system [14]. Similar phenomena were reported when glycerol was added to the CNCs suspension by He et al [15] and by Xu et al [16].…”

“…Indeed, Mu et al attribute the observed increase of the pitch value of the films to the fact that D−(+)−glucose added to the CNCs suspension promotes gelation and therefore hindered further decreases in the pitch values as the solvent evaporates [18]. The increase in the wavelength reflected by the structure of the films for the composite mixtures of CNCs with glycerol [15,16] or polyethylene glycol [14] was also observed. In order to support this assumption measurements of the pitch value observed by POM images on the centre of each film were determined, as can be seen from Supplementary Figure S9.…”

Broadband Circular Polarizing Film Based on Chiral Nematic Liquid Crystals

“…Because of the structure, the films behave as 1D photonic crystals, reflecting circularly polarized light having the same sense as the cholesteric screw orientation . This property of CNCs is attractive for producing iridescent films with many potential applications …”

Cellulose Nanocrystal Elastomers with Reversible Visible Color

“…[22] Khan et al incorporated chiral nematic CNCs into thermoset polymers;these materials showed responsive color changes,b ut were not flexible. [23] Other strategies to make flexible CNC-containing materials that show structural color include the incorporation of surfactants, [24] polymers [25][26][27] and other additives [28] to produce flexible CNC films.H owever, these materials do not exhibit responsive color.…”