Functional Materials from Cellulose‐Derived Liquid‐Crystal Templates

Cellulose nanocrystals (CNCs), known for more than 50 years, have attracted attention because of their unique properties such as high specific strength and modulus, high surface area, and fascinating optical properties. Just recently, however, their potential in supramolecular templating was identified by making use of their self‐assembly behavior in aqueous dispersions in the presence of compatible precursors. The combination of the mesoporosity, photonic properties, and chiral nematic order of the materials, which are available as freestanding films, has led to a significant number of interesting and promising discoveries towards new functional materials. This Review summarizes the use of cellulose derivatives, especially CNCs, as novel templates and gives an overview of the recent developments toward new functional materials.     

Flexible and iridescent chiral nematic mesoporous organosilica films

Nanocrystalline cellulose (NCC) has been used to template ethylene-bridged mesoporous organosilica films with long-range chirality and photonic properties. The structural color of the organosilica films results from their chiral nematic ordering, can be varied across the entire visible spectrum, and responds to the presence of chemicals within the mesopores. To synthesize these materials, acid hydrolysis was used to remove the NCC template without disrupting the organosilica framework. The resulting mesoporous organosilica films are much more flexible than brittle mesoporous silica films templated by NCC. These materials are the first of a novel family of chiral mesoporous organosilicas with photonic properties.     

Cellulose Nanocrystal Elastomers with Reversible Visible Color

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.     

Structural Transition in Liquid Crystal Bubbles Generated from Fluidic Nanocellulose Colloids

The structural transition in micrometer‐sized liquid crystal bubbles (LCBs) derived from rod‐like cellulose nanocrystals (CNCs) was studied. The CNC‐based LCBs were suspended in nematic or chiral nematic liquid‐crystalline CNCs, which generated topological defects and distinct birefringent textures around them. The ordering and structure of the LCBs shifted from a nematic to chiral nematic arrangement as water evaporation progressed. These packed LCBs exhibited a specific photonic cross‐communication property that is due to a combination of Bragg reflection and bubble curvature and size.     

Hard templating of nanocrystalline titanium dioxide with chiral nematic ordering

Anatase TiO(2) nanocrystals have been organized into high-surface-area (150-230 m(2) g(-1)) mesoporous films with long-range chiral nematic ordering. The chiral structure of the anatase films causes them to selectively reflect circularly polarized light and appear iridescent. These materials show replication of structural features found in the silica template on nanometer to millimeter length scales.     

Polymer and Mesoporous Silica Microspheres with Chiral Nematic Order from Cellulose Nanocrystals

Polymer microspheres with chiral nematic order were obtained from an emulsion polymerization technique using cellulose nanocrystals (CNCs) as the template. The growth of the liquid crystals from tiny tactoids to droplets with spherical symmetry was captured and investigated by both optical and electron microscopy for the first time. The size of the microspheres could be tuned between tens and hundreds of micrometers; to obtain single, integrated chiral nematic kernels, the size of water droplets in the emulsion should be similar to that of CNC tactoids. Through a double‐matrix templating method, novel silica microspheres with chiral nematic order were fabricated, which showed a high surface area and mesoporosity. The methods developed here may help to reveal the evolution of other self‐assembling systems, and these materials have potential applications in optical devices and chiral separations.     

Chiral nematic phase formation by aqueous suspensions of cellulose nanocrystals prepared by oxidation with ammonium persulfate

There is continuing interest in the growing family of nanocellulosic materials prepared from plant cell wall material. While most of the research on cellulose nanocrystals has focused on the product of sulfuric acid hydrolysis stabilized by surface sulfate half-esters, cellulose nanocrystals with surface carboxyl groups have also been prepared by oxidation of lignocellulosic materials with ammonium persulfate. The major difference is that the persulfate oxidation leads to nanocrystals stabilized by surface carboxyl groups. Some properties of cellulose nanocrystals from cotton and wood, prepared by persulfate oxidation, are compared with those observed for nanocrystals prepared by sulfuric acid hydrolysis. Evidence from polarized light microscopy showed that the nanocrystal suspensions prepared by persulfate oxidation also form chiral nematic ordered phases in water.     

Photonic Patterns Printed in Chiral Nematic Mesoporous Resins

Chiral nematic mesoporous phenol‐formaldehyde resins, which were prepared using cellulose nanocrystals as a template, can be used as a substrate to produce latent photonic images. These resins undergo swelling, which changes their reflected color. By writing on the films with chemical inks, the density of methylol groups in the resin changes, subsequently affecting their degree of swelling and, consequently, their color. Writing on the films gives latent images that are revealed only upon swelling of the films. Using inkjet printing, it is possible to make higher resolution photonic patterns both as text and images that can be visualized by swelling and erased by drying. This novel approach to printing photonic patterns in resin films may be applied to anti‐counterfeit tags, signage, and decorative applications.     

Mesoporous Silica and Organosilica Films Templated by Nanocrystalline Chitin

Liquid crystalline phases can be used to impart order into inorganic solids, creating materials that mimic natural architectures. Herein, mesoporous silica and organosilica films with layered structures and high surface areas have been templated by nanocrystalline chitin. Aqueous suspensions of spindle‐shaped chitin nanocrystals were prepared by sequential deacetylation and hydrolysis of chitin fibrils isolated from king crab shells. The nanocrystalline chitin self‐assembles into a nematic liquid‐crystalline phase that has been used to template silica and organosilica composites. Removal of the chitin template by either calcination or sulfuric‐acid‐catalyzed hydrolysis gave mesoporous silica and ethylene‐bridged organosilica films. The large, crack‐free mesoporous films have layered structures with features that originate from the nematic organization of the nanocrystalline chitin.     

纤维素纳米晶体手性复合材料:结构色的调控与应用

结构色在自然界中扮演了重要的角色,在昆虫外骨骼、鸟类羽毛以及植物果实中广泛分布.纤维素纳米晶体(CNCs)的水悬浮液达到一定浓度时会自组装形成左旋的手性向列液晶结构,这种手性向列结构在水分挥发后仍能保持并形成光子晶体虹彩薄膜,具有极强的手性和光子晶体的双重性质.膜内的周期性层状结构与光线产生干涉、衍射作用,表现出复杂的...