Cellulose Nanocrystal Elastomers with Reversible Visible Color

Abstract

Responsive photonic crystals have potential applications in mechanical sensors and soft displays; however, new materials are constantly desired to provide new innovations and improve on existing technologies. To address this, we report stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible color upon the application of mechanical stress. When stretched (or compressed) the colorless materials maintain their chiral nematic structure but the helical pitch is reduced into the visible region, resulting in coloration of the CNC‐elastomer composite. By increasing the percentage elongation of the material (ca. 50–300 %), the structural color can be tuned from red to blue. The color of the materials was characterized by reflectance optical microscopy and reflectance circular dichroism to confirm the wavelength and polarization of the reflected light. We also probed the mechanism of the structural color using 2D‐X‐ray diffraction. Finally, by either water‐patterning the starting CNC film, or by forming a CNC film with gradient color, through masked evaporation, we were able to prepare encoded stretchable chiral nematic CNC‐elastomers.

Infiltration of Proteins in Cholesteric Cellulose Structures

“…To address this issue, we evaluated a method to infiltrate cn-CNC films with macromolecules, which would otherwise promote a rapid and strong CNC aggregation in suspension ( Figure 1 ). We note that swelling and infiltration of cn-CNC films have been previously attempted using small molecules (ammonium hydroxide, 43 2,2-azobis(2-methylpropionitrile), 44 , 45 monomer solution cured to poly(dodecanediol- co -citrate) post infiltration 46 ). However, infiltration of swollen chiral nematic networks followed by dewatering (i.e., drying) is shown here to yield cn-CNC nanocomposite films with up to 50 wt % of an additive.…”

Geminate labels programmed by two-tone microdroplets combining structural and fluorescent color

“…One of the intriguing features of CLCs is to selectively reflect the circularly polarized light, according to Bragg's law, with the same handedness as the helix. This unique soft photonic crystal material can be easily prepared and constructed into multidimensional architectures from planar to spherical periodic structures [21][22][23][24][25][26][27][28][29][30][31][32][33] .…”

Mechanochromism of Structural‐Colored Materials

“…The mechanochromic LCs materials include cholesteric LCs, achiral nematic LCs and blue phase LCs. Various optical properties of the liquid crystals such as the reflectance wavelength, polarization and transparency can be modulated under external stimuli, [ 11,113–115 ] providing remarkable optical tunability based on mechanochromism.…”

“…With the extensive and sustainable natural source, the nanocrystals of cellulose and its derivatives have become promising building blocks for chiral mechanochromic materials. [ 11,113–119 ] They could be assembled into chiral helix structure with periodical refractive indexes, [ 11 ] whose stop band λ can be described by the following equation [ 112 ] where n eff is the effective refractive index of the liquid crystals, P is the helix pitch, and θ is the incident angle.…”

“…For example, MacLachlan's group has made considerable efforts to fabricate CNCs elastomers (CNC‐E) and investigate their mechanochromic behavior. [ 11,114 ] Specifically, the elastomer precursors (ethylacrylate: EA and 2‐hydroxyethyl acrylate: 2‐HEA) were infiltrated into the as‐assembled CNCs, followed by polymerization to obtain the CNC‐E (Figure 7b). The prepared CNC‐E was colorless and transparent because its Bragg reflective peak was located in the infrared region.…”