Rapidly Responsive and Flexible Chiral Nematic Cellulose

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Rapidly Responsive and Flexible Chiral Nematic Cellulose Nanocrystal Composites as Multifunctional Rewritable Photonic Papers with Eco-Friendly Inks Hao Wan, Xiaofeng Li, Liang Zhang, Xiaopeng Li, Pengfei Liu, Zhiguo Jiang, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19375 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Materials & Interfaces

Rapidly Responsive and Flexible Chiral Nematic Cellulose Nanocrystal Composites as Multifunctional Rewritable Photonic Papers with Eco-Friendly Inks Hao Wan,† Xiaofeng Li,†* Liang Zhang,§* Xiaopeng Li,† Pengfei Liu,† Zhiguo Jiang† and Zhong-Zhen Yu†‖* †

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing

University of Chemical Technology, Beijing, 100029, China §

School of Chemistry and Biological Engineering, University of Science &

Technology Beijing, Beijing 100083, China ‖

State Key Laboratory of Organic-Inorganic Composites, College of Materials

Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected], [email protected], [email protected]

ABSTRACT: Rapidly responsive and flexible photonic papers are manufactured by co-assembly of cellulose nanocrystals (CNCs) and waterborne polyurethane (WPU) latex for fully taking advantage of the chiral nematic structure of CNCs and the flexibility of WPU elastomer. The resulting CNC/WPU composite papers exhibit not only tunable iridescent colors by adjusting the helical pitch size, but also instant optical responses to water and wet gas ascribed to the easy chain movement of the elastomeric WPU that does not restrict the fast water absorption induced swelling of CNCs. By choosing water, or NaCl aqueous solutions as inks, the colorful patterns on 1 ACS Paragon Plus Environment

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the CNC/WPU photonic paper can be temporary, durable, or even disguisable. In addition, the photonic paper is simultaneously rewritable for all these three-type patterns, and the disguisable patterns, which are invisible at normal times and show up under stimuli, exhibit a quick “reveal” conversion just by exhaling on the paper. The rewritability, rapid responsibility, easy fabrication, and the eco-friendly inks make the flexible photonic paper/ink combination highly promising in sensors, displays and photonic circuits. KEYWORDS: rewritable photonic paper, cellulose nanocrystals, stimuli response, disguisable pattern, chiral nematic composites

INTRODUCTION

Photonic paper, an important branch of photonic materials, has aroused increasing interests because of its potential applications in printing, decoration, security and anti-counterfeit systems.1-5 Colorful writing and printing on photonic papers could be accomplished

using

colorless

materials

without

the

use

of

chemical

chromophores/fluorophores, and their structural colors are more long-lasting as compared to traditional pigments and dyes.5 Generally, the matching of “paper” with “ink” is required: the photonic paper is usually made of elastomer containing periodic lattices of monodispersed and spherical colloids, while the ink is usually an organic solvent or monomer capable of swelling the elastomer matrix. When the ink molecules swell the elastomer in selected areas, the wavelength of Bragg-diffracted light would red-shift, creating a color variation that could be distinguished by naked 2 ACS Paragon Plus Environment

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eyes.3,

5

The color variation is exploited to write or display letters and patterns.

Although many efforts have been devoted to prepare photonic paper/ink system, several requirements are still challenging and must be met for practical usage: (1) easy fabrication process unbound with expensive equipment and tedious conditions, (2) little defects and cracks, (3) eco-friendly inks without the use of toxic components, and (4) rewritability. In addition, enhancements such as disguisable photonic pattern which would be invisible at normal times and show up under stimuli have added values for encryption and anti-counterfeiting purposes.6-13 Actually, nature always presents astonishing examples of biological systems with optimized structures to give us a better strategy. Besides the well-known examples such as feathers of birds,14 wings of butterflies,15 exoskeletons of beetles,16 and opals,17 which have inspired the long-lasting research on spherical colloids based photonic papers and also other photonic materials with applications in security labeling,18 cosmetics,19 textile industry,20

printing,21 etc,22-24 much attention has

recently been paid to the special helical organization of cellulosic material in plants.25 The elaborated helical nanostructures in fruits and leaves not only offer brilliant iridescence attracting pollinating species to increase seed dispersal, but also improve mechanical properties.26-27 Cellulose nanocrystals (CNCs), extracted from bulk cellulose by sulfuric acid hydrolysis,28-30 are able to assemble into chiral nematic liquid crystals in their aqueous suspension, which can be retained in the films of CNCs during slow drying under ambient conditions.31 The helical CNC films exhibit brilliant iridescence and structural colors by selectively reflecting left-handed 3 ACS Paragon Plus Environment


polarized light like those structures in the biological photonic systems. Compared to photonic systems based on spherical colloids, CNC-based photonic materials are of great interests because of their wide availability, relatively low cost, simple self-assembly process and specular chiral nematic structure. Due to the brittleness and the tendency to collapse in water, CNCs are preferred to combine with a polymer, in which the CNC component retains its unique optical properties while the organic polymer provides mechanical flexibility and anti-solvent performance. The main criterion to the successful fabrication of photonic CNC composites is to find suitable conditions of solidification to retain the chiral nematic structure that is very sensitive to pH value and ionic strength. In most cases, low contents of CNCs are introduced into polymers to fabricate composites with enhanced mechanical properties but a sacrifice of photonic properties.32-37 With the exploration of new fabrication methodology and suitable polymers, CNC/polymer composites with both iridescent coloration and satisfactory mechanical performances have been successfully prepared.38-47 However, they have rarely been used as photonic papers for display applications, because the polymers used are usually cross-linked resins with compromised stimuli-responsive properties to inks, or soluble polymers with low resistance to inks. Till now, it is challenging to use CNC/polymer composites directly as the photonic papers for colorful writing. To improve the stimuli-responsive properties, CNCs have been carefully removed from the CNC/polymer composites to get mesoporous materials by supercritical CO2 drying, and the cross-linked mesoporous polymer films showed improved 4 ACS Paragon Plus Environment

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stimuli-responsive performance.42,48 In addition, one of these porous films could be directly written with disguisable patterns by using hydrochloric acid solution as the ink, which is much more convenient to conventional photolithography method.6-8 However, it was hard to erase the patterns and write again due to the change of the interior chemical structure.48 To the best of our knowledge, there is no report on the rewritability of photonic papers for disguisable patterns so far. Herein, we demonstrate a highly efficient and simple approach to fabricate chiral nematic structured CNC-based photonic papers with rewritable performance, rapid stimuli-responsive ability, and high flexibility by drying aqueous suspensions of CNC nanorods and waterborne polyurethane (WPU) latex. On the rewritable CNC/WPU photonic paper, the temporary, durable, and even disguisable patterns can be easily written or erased by introducing or removing water, or different content of sodium chloride on the paper surface. In addition, because of different water absorption ability of CNC and the ink, along with the easy chain movement of elastomeric WPU, the disguisable patterns exhibit a quick “reveal” conversion just by exhaling on the paper. EXPERIMENTAL SECTION Materials: Commercial microcrystalline cellulose particles with a mean size of 25 µm were purchased from Aladdin Industrial Co. (China). Sodium hydroxide (NaOH) and sulfuric acid (H2SO4, 95-98%) were supplied by Beijing Chemical Factory (China). WPU suspension with a solid content of 35 wt% (Leasys® 5530) was provided by Wanhua Chemical Group Co. Ltd. (China). Cross-linking agent of 5 ACS Paragon Plus Environment


waterborne polyurethane (hydrophilic blocked aliphatic polyisocyanate, FB-15, Siwo Chem) was bought from Shanghai Sisheng Polymer Materials Co., Ltd. (China). All chemicals were used without further purification. Preparation of Cellulose Nanocrystals: Microcrystalline cellulose was acid-hydrolyzed with 64 wt% of sulfuric acid at 45 oC under vigorous stirring at 600 rpm for 45 min. The resultant yellowish-white cellulose suspension was then diluted with cold de-ionized water of ~10 times the volume of the acid solution to stop the hydrolysis. The resulted suspension was allowed to settle overnight. The clear top phase of the settled suspension was decanted while the remaining cloudy bottom phase was centrifuged at 15000 rpm for 10 min (15 oC). After decanting the supernatant, the resulting thick white suspension was washed with deionized water and centrifuged again. To remove almost of the soluble cellulosic materials, the centrifugation-washing procedure ended until the supernatant became turbid. Then, the suspension was transferred into the dialysis membrane tubes (8,000~14,000 molecular weight cut-off) and dialyzed against deionized water for 3-4 days until the pH value of the water outside the tube was close to neutral. Afterwards, the suspension was stocked with mixed bed resin of LUBAO (MB-12, Shanghai Resin Factory, China) for 24 h to eliminate residual electrolytes. After separation with mixed bed resin by Whatman 541 filter paper, the suspension with light blue color was ultrasonicated for 2 min (Scientz JY92-IIDN, 900W) at 30% output. The aqueous suspension was approximately 0.2 wt% and then concentrated by evaporation of water

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at ambient conditions. The final cellulose nanocrystals suspension was stored in a refrigerator for further use. Preparation of CNC/WPU Composite Films: WPU suspension was diluted to 5 wt% with deionized water and crosslinkers was added to make a WPU/cross-linker content ratio of 10:1 by weight. The mixture was stirred for 10 min and ultrasonicated for 10 min to be homogeneous. Sodium hydroxide solution (0.1 M) was added into the CNC suspension to adjust the pH to be higher than 4, and the concentration of the suspension was adjusted to ~3.5 wt% using deionized water. The suspension of CNCs and WPU were mixed together and stirred for 30 min, followed by ultrasonication for 40 min. A certain amount of the final mixture was poured into a petri dish (6 cm in diameter, 3 ml mixture) or spread onto glass slide (2 ml mixture) and dried at ambient conditions (20~25 oC, RH 20~50%) to obtain CNC/WPU composite films (20-40 µm), which were finally heated at 105 oC for 2 h in an oven to complete the cross-linking reactions. Characterization: Atomic force microscope (AFM) measurements were carried out on a Bruker Dimension Fastscan instrument in tapping mode uisng the specimens prepared by drying a droplet of dilute CNC suspension (ca. 0.005 wt %) on a freshly cleaved mica. The measurement of length and width of CNC was performed using Nanoscope Analysis software and calculated by taking the average of at least 15 CNC rods. The electrophoretic mobility of CNCs was measured by a Brookhaven Instruments NanoBrook90PlusPALS zeta potential analyzer. A Brookhaven Instruments NanoBrook90PlusPALS dynamic light scattering (DLS) was used to 7 ACS Paragon Plus Environment


measure the sizes of the nanoparticles in suspensions. The reflective wavelength of the composite was characterized by a Shimadzu UV-1800 UV-vis spectrophotometer in transmission mode using the integrating sphere accessories. The composite film was stuck onto a quartz plate and mounted to be perpendicular to the beam path. To measure the reflected wavelength in a series of polar solvents, the composite films was cut into small rectangular shape and put into the cuvette filled with different portions of ethanol/water mixture. The film was pressed against the wall of the cuvette by a small patch of rubber, so as to be perpendicular to the beam path. Morphology of the composite films was observed with a Hitachi S4700 scanning electron microscope (SEM) at an accelerating voltage of 5 kV. The specimens were frozen in liquid nitrogen, fractured and coated with gold before SEM characterization. Mechanical properties of the films were measured by an Instron1185 universal testing machine with a nominal strain rate of 0.5 mm min-1. The specimen sizes used were 50 mm × 5 mm with a gap width of 30 mm. The results were an average of eight replicates of each specimen. RESULTS AND DISCUSSION For the fabrication of CNC/WPU photonic paper, CNCs are prepared from sulfuric acid-catalyzed hydrolysis of microcrystalline cellulose, and have an average length of 220±48 nm and average width of 6±1 nm determined by statistical analysis of AFM image (Figure S1). WPU, a water-insoluble polymer, was dispersed in water as latex with an average diameter about 174 nm (Table S1). Usually the CNC/polymer composites were prepared by drying the suspension of CNCs with soluble polymer or 8 ACS Paragon Plus Environment

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monomer. When an aqueous CNCs suspension was used, the mixed systems were limited to water-soluble polymer or precursors.43,

47

When compounded with

water-insoluble polymers, CNCs have to be modified and dispersed in organic solvent.40 Herein, when dispersed as latex, water-insoluble polymers, such as WPU, could be used in aqueous mixed system to prepare CNC/polymer composites, avoiding the amount use of organic solvent. The CNCs nanorods and WPU latex can be dispersed homogeneously in water due to their negative surface charges.49-50 Their mixed suspension maintains a stable colloidal system, which is proved by the highly negative zeta potential (-37.63 mV, Table S1).51 Intense birefringence patterns are observed (Figure S2), indicating the chiral nematic state of CNCs in the CNC/WPU suspension. The suspensions with different mass ratios of CNC/WPU components are slowly dried in ambient environment to get flexible films. During the drying process, WPU latex loses the spherical shape and is homogeneously dispersed between the chiral nematic structure of CNCs due to the low glass transition temperature (Tg< -50 o

C ) (Figure 1a,b).52 CNC/WPU composite films with homogeneous chiral nematic

structure are fabricated when the WPU contents are lower than 40 wt%. Further increasing the WPU content causes the coexistence of chiral nematic CNCs-rich region and WPU-rich region. The circular dichroism spectra (Figure S3) of the composite films exhibits strong positive signal, confirming the left-handed helical structure of CNCs in the composite films. This homogeneous arrangement of WPU latex is quite different from the CNC/polyethyl methacrylate latex system, in which the latex nanoparticles keep their spherical shape due to the high Tg of polyethyl 9 ACS Paragon Plus Environment


ethacrylate and the composite consists of planar disordered layers of the latex nanoparticles alternating with chiral nematic CNCs-rich regions.53

Figure 1. (a) Schematic illustrating the self-assembly of CNCs in the presence of WPU latex. (b) SEM images of the cross-sections of CNC/WPU composite films with different WPU contents. (c) Stress-strain curves of CNC/WPU composite films with different WPU contents. Inset is a photograph of a flexible CNC/WPU composite film.


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Although neat chiral nematic CNCs films are of great interest, they are usually brittle and hard to be handled, limiting their practical applications. Incorporation of WPU not only effectively preserves the chiral nematic structure but also improves the flexibility of the films. As presented in Figure 1c and Table S2, the tensile strength of the CNC/WPU composite films increases with the WPU content up to 50 wt%. Further increase of WPU content causes the reduction in tensile strength due to the low strength of WPU component itself and phase separation. Because of the super-elasticity of WPU, the elongation-at-break of the composite films continuously increases, although their Young’s modulus decreases as compared to that of neat CNCs film. These results indicate that the incorporation of WPU into the chiral nematic CNCs leads to the formation of flexible and iridescent films. The CNC/WPU composite films exhibit iridescent colors, which can be readily tuned by varying the mass ratios of CNCs and WPU components (Figure 2a, b). Generally, the peak wavelength (λmax) reflected by chiral nematic structures depends on several parameters including the helical pitch (P), the angle of incident light (θ), and the average refractive index (navg) of the material, which obey the following Eq. (1):54 λmax = navg P sin(θ)

(1)

λmax can be modulated by manipulating the helical pitch or the refractive index. The difference of navg among the composite films with different CNC/WPU ratios is almost negligible, because CNCs and WPU have very similar refractive indices of 1.5455 and 1.50,56-57 respectively. Therefore, the λmax is mainly modulated by varying 11 ACS Paragon Plus Environment


the helical pitch. It is seen that the helical pitch of the chiral nematic structure enlarges continuously with increasing the WPU content, and λmax shifts smoothly from ~332 nm for neat CNCs film to ~625 nm for the CNC/WPU (40/60) composite film (Figure 2c). Further increasing the WPU content to 70 wt% causes the loss of color, which is ascribed to the serious loss of the chiral nematic structure in the composite film. In addition, as predicted by Eq. (1), the incident light angle influences the reflected color of the composite films (Figure 2d).

Figure 2. (a) Photographs and (b) UV-vis spectra of the CNC/WPU composite films with different WPU contents. (c) Plots of helical pitch measured from SEM images and λmax from UV-vis spectra of chiral nematic CNC/WPU composites as a function of WPU content. (d) Photographs of CNC/WPU composite films with 30 wt% of WPU taken from viewing angles of 90o (top) and 50o (bottom). The structural color of the CNC/WPU composite films can be readily modulated by the solvents with different polarities. Upon immersion in the solvents of water and 12 ACS Paragon Plus Environment

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ethanol with different ratios, the composite films swell rapidly, leading to red-shift of the reflection peak wavelength (Figure 3). As ethanol and water have similar refractive indices, the change in color is mainly attributed to the variation in the helical pitch of the CNCs by swelling. The CNC/WPU composite films exhibit a red shift of their reflection peak from 441 nm in neat ethanol to ~600 nm in the ethanol/water (40/60) mixture. Further increase of the water content could not tune the reflection peak effectively due to the restriction of the cross-linked polymer network. In other words, the cross-linked polymer network protects the chiral nematic structure from unlimited swelling and demolishing. The swelling response is reversible and rapid, occurring within seconds (Movie S1). In contrast, the uncross-linked composite film is vulnerable to water (Figure S4). Most of reported CNC/polymer composite films do not have the equivalent rapid and reversible solvent-responsive feature because of the confinement of CNCs by rigid polymer chains or their low resistance to solvents.42, 46 Poly (ethylene glycol)/CNC composite shows rapid water response because both poly (ethylene glycol) and CNCs have water absorption ability, along with their uncross-linked sturcutre.47 However, the crosslinking of the polymer to increase the water resistant ability should decrease the responsive speed.41 Herein the cross-linked WPU is water resistant and also hydrophobic. The water contact angle of pure WPU film is about 96o, and the water absorption of pure WPU film is only 0.88 wt% when dipped in water for 5 min. However, the fast water response of the CNC/WPU composite paper is comparable to that of poly (ethylene glycol)/CNC composite and also neat CNC film.47, 58 The main 13 ACS Paragon Plus Environment


reason is that WPU is waterproof but breathable, so it does not hinder the penetration of water in the composite film. Moreover, the elastomeric WPU does not restrict the water absorption induced fast swelling of CNCs because of the easy chain movement. The water-resistant ability along with the rapidly responsive behavior indicating a significant improved performance of the composite with elastomer which have rarely reported. Meanwhile, the swelling-induced color change in different polar solvents made the CNC/WPU composite films promising as sensors and the co-assembly of CNCs with aqueous latex of WPU can be expanded to other aqueous rubber latex system.

Figure 3. (a) Photographs and (b) UV-vis spectra of CNC/WPU composite film with 30 wt% of WPU when immersed in the water/ethanol solvents with different volume ratios.


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In view of their rapid stimuli-responsive feature and excellent anti-solvent performance, the flexible CNC/WPU composite film is employed as a photonic paper. One piece of the CNC/WPU film is handwritten with a brush absorbed with different kinds of “inks” (Figure 4). Three types of aqueous “inks” are used because of the hydrophilic nature of CNCs. When water is used as an ink, the color of the wetted area changes from blue to orange within seconds, due to the increased helical pitch derived from the swelling and the red shift of the reflection peak. The boundary of the characters is quite incisive due to the hydrophobicity of the WPU matrix, which is an advantage for precise writing and printing. These characters can be called as “temporary” or “evaporative” patterns because the wetted region would recover to the original blue color after the water evaporation. The writing of “temporary” pattern could be repeated and the λmax of the patterns fluctuates slightly around 585 nm, indicating a good rewritability of the photonic paper for “temporary pattern” (Figure S5). In addition to the rapid responsive behavior to liquid water, the photonic paper also shows a response to wet gas. When exhaling on the paper, the paper quickly changes its color from blue to yellowish green. The response is also reversible: the film quickly recovers to its original color after the stop of exhalation. This unique property for the CNC based chiral nematic polymer composite is also ascribed to the water breathable capacity along with the easy chain movement of elastomeric WPU.



Figure 4. (a) One piece of the CNC/WPU film with 20 wt% of WPU was used as the photonic paper, demonstrating multiple functions with selected “ink” at different conditions. Temporary characters written by water (top images), durable characters written by high concentration NaCl solution of 0.1 M (middle images), and disguisable characters written by low concentration NaCl solution of 0.025 M (bottom images). The rewritability of the CNC/WPU photonic paper is well demonstrated by the conversion of the λmax of (b) repeated writing of durable patterns and


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washing/drying processes, and (c) repeated writing of disguisable patterns and their appearance by exhaling to the paper. Besides the evaporative characters, durable patterns can also be realized by using NaCl aqueous solution as the ink. Upon writing with 0.1 M NaCl solution, the pattern turns to orange immediately. After the water evaporation, the color of the pattern fades to an extent but is still visible by naked eyes. This durable pattern should result from the penetration of the salt into the helix structure, inducing a durable change of the helical pitch. Because CNCs and NaCl have contrasting water absorption abilities, simply exhaling on the surface of the paper can result in a much distinct pattern with color change. It is noteworthy that the CNC/WPU photonic paper could be regenerated for repeat writing, because the NaCl ink can be erased by washing with excessive water. As shown in Figure 4b, the writing and erasing operations are repeated for 5 cycles using the 0.1 M NaCl solution as the ink. The reflection peaks are recorded to show the good rewritability of the durable pattern. The reflection peak of the durable pattern fluctuates slightly around 380 nm and the relative deviations are acceptable. After rinsing with excessive water and drying, the reflection peak of the blank photonic paper presents at ~352 nm, indicating that the water rinsing is efficient in removing the residual salt from the paper. More interestingly, this CNC/WPU photonic paper could also be written with disguisable patterns. Disguisable photonic pattern is more complex than most responsive photonic materials, because it requires both recordability and invisibility. Additionally, the disguisable patterns must have reversibility and the ability of quick 17 ACS Paragon Plus Environment


release with convenient means. Disguisable patterns were usually prepared by photolithography with complex procedure,6-8 and photonic papers for directly writing disguisable photonic patterns were not easy to realize,48 especially for those with rewritability. In the present work, when a low concentration NaCl solution (~0.025 M) is used as the ink, the pattern turns invisible after the water is evaporated. It is calculated that there is ~7 nm difference in the average reflection peak wavelength between the blank photonic paper (~352 nm, Figure 4b) and disguisable pattern (~359 nm, Figure 4c). This difference is caused by the change of the helical pitch induced by the incorporated salt. As this tiny difference could not be distinguished by naked eyes, the pattern is thus invisible. Just by exhaling on the paper, the pattern reveals rapidly due to the contrasting expansion of the helical pitch between the pattern and the blank paper caused by the different water absorption ability of NaCl and CNCs. At a dry condition, like room condition, the pattern conceals quickly. As expected, the disguisable pattern with reversible and quick “conceal-reveal” conversion is realized by the very convenient means. The conversion is mainly based on the highly sensitive and rapid wet gas response performance of the CNC/WPU composite paper, which can be ascribed to the elastomeric WPU with easy chain movement. Similar to the durable pattern, the disguisable pattern can be erased with water and written again. The reflection peak wavelengths of the disguised and their revealed patterns are stably around 359 and 377 nm after repeated washing and writing (Figure 4c). This excellent rewritability results from the reversible physical change between the pristine state and


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the salt-incorporated state. In contrast, previously reported invisible patterns were not rewritable due to the unavoidable irreversible chemical change.6-8,48 CONCLUSIONS In summary, chiral nematic structured CNC/WPU composite films with iridescent coloration are prepared by self-assembly of CNCs in the presence of aqueous WPU latex. The variation in the mass ratio of WPU and CNCs components alters the helical nanostructure of CNCs, thus tunes the color of the composite films. In addition, the composite films exhibit dynamic photonic properties, responding rapidly to external stimuli including solvent polarity and humidity. The CNC/WPU composite films are tested as rewritable photonic papers for temporary, durable, and disguisable patterns, by using water, high-concentration, and low-concentration NaCl solutions as inks, respectively. The rewritable and tunable photonic properties, fast responsive ability, eco-friendly inks, high mechanical performances, and the straightforward fabrication methodology make the flexible CNC/WPU photonic papers highly promising for applications in sensors, photonic circuits and display systems. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. AFM Image of CNCs; digital images of CNC film and suspension; circular dichroism spectra; particle sizes and zeta potentials of suspension; mechanical properties of CNC/WPU composite films. Movie of the reversible swelling response of CNC/WPU composite film. Notes 19 ACS Paragon Plus Environment


The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51403016, 51773008, 51773019, 51533001, 51521062), and the National Key Research and Development Program of China (2016YFC0801302) is gratefully acknowledged.

REFERENCES

(1) Ge, J.; Goebl, J.; He, L.; Lu, Z.; Yin, Y. Rewritable Photonic Paper with Hygroscopic Salt Solution as Ink. Adv. Mater. 2009, 21, 4259-4264. (2) Fudouzi, H.; Xia, Y. Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers. Langmuir 2003, 19, 9653-9660. (3) Wang, Z.; Zhang, J.; Xie, J.; Wang, Z.; Yin, Y.; Li, J.; Li, Y.; Liang, S.; Zhang, L.; Cui, L. Polymer Bragg Stack as Color Tunable Photonic Paper. J. Mater. Chem. 2012, 22, 7887-7893. (4) Aguirre, C. I.; Reguera, E.; Stein, A. Tunable Colors in Opals and Inverse Opal Photonic Crystals. Adv. Funct. Mater. 2010, 20, 2565-2578. (5) Fudouzi, H.; Xia, Y. Photonic Papers and Inks: Color Writing with Colorless Materials. Adv. Mater. 2003, 15, 892-896. (6) Xuan, R.; Ge, J. Invisible Photonic Prints Shown by Water. J. Mater. Chem. 2012, 22, 367-372. (7) Ye, S.; Fu, Q.; Ge, J. Invisible Photonic Prints Shown by Deformation. Adv. Funct. Mater. 2014, 24, 6430-6438. 20 ACS Paragon Plus Environment

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Rapidly Responsive and Flexible Chiral Nematic Cellulose

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