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We describe an approach to prepare polymer composites with chiral nematic photonic structures through the self-assembly of cellulose nanocrystal (CNC) dispersions in organic solvents. Contrary to previous reports, we demonstrate that dispersions of neutralized sulfated CNCs in polar organic media readily form lyotropic chiral nematic liquid crystalline phases. We have investigated the effect of the neutralizing base on the CNC self-assembly, observing chiral nematic ordering for all counterions studied. The self-assembly of the organic CNC dispersions can be exploited to prepare iridescent polymeric composites simply by casting the CNC dispersion with a suitable polymer soluble in the organic solvent. Photonic properties of the composite films can be easily controlled by either varying the ratio of CNCs to polymer or adding salts.

Stimuli-responsive photonic materials are useful in sensors, optical filters, inks, displays, and other technologies. [1, 2] In particular, photonic hydrogels can show large color changes in response to variations in osmotic pressure, and have a wide range of tunable functionality through selection of suitable hydrogel monomer(s). [3][4][5] Chiral nematic liquid crystals (LCs) can have unique photonic properties that could be valuable in the form of a hydrogel.[6] Although there are some reports of photonic hydrogels with chiral nematic structures prepared mostly through polymerization of functionalized molecular LCs, [7] a general approach to achieve the broad range of responsive functionalities found in conventional hydrogels (for example, temperature, pH, solvent) is still lacking. Herein we report novel nanocomposite hydrogels prepared by self-assembly of nanocrystalline cellulose (NCC) with various hydrogel monomers. These new chiral nematic hydrogels have tunable colors, respond to various stimuli, and have interesting mechanical and swelling behavior. We demonstrate that the hydrogels can be prepared as large, freestanding films and can be photopatterned. Furthermore, the embedded NCC offers new potential for modification to produce functional materials. Our findings will be important for developing sensors and other chiral optoelectronic devices.Materials with chiral nematic order show selective reflection of light with circular polarization. They currently have applications such as distributed feedback lasing and polarized photoluminescence.[6] To construct a photonic hydrogel with chiral nematic order, we turned to cellulose, a widely-available, inexpensive, and biocompatible material. Cellulose-based hydrogels are used commercially in wound dressings, superabsorbent materials, and other applications. [8] The surface of cellulose and its derivatives can be tailored to produce hydrogels that respond to external stimuli such as temperature or pH. [9] Nanocrystalline cellulose (NCC), prepared by acid hydrolysis of cellulose, [10][11][12] has been used to reinforce hydrogels to improve their typically poor mechanical properties. [13][14][15][16] NCC forms a chiral nematic LC phase in water, where the NCC rods are organized in a left-handed helical assembly. These lyotropic phases exhibit photonic color when the helical pitch of the assembly is on the order of the wavelength of visible light, selectively reflecting left-handed circularly polarized light. Tatsumi et al. recently created a hydrogel with liquid crystalline order using NCC, but it did not show any photonic properties. [15] A main criterion to successfully prepare photonic hydrogels using NCC was to find suitable conditions for polymerization of hydrogel precursors while retaining the formation of the chiral nematic phase of NCC, which is very sensitive to changes in pH and ionic strength. [17] In our experiments, we used sulfuric acid hydrolysis to prepare acidic dispersions of NCC. Mixing NCC (3 wt %, pH 2.4) with nonionic hydrogel precursors ...

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