As a wise and profound teacher, nature provides numerous creatures with rich colors to us. To biomimic structural colors in nature as well as color changes responsive to environmental stimuli, there is a long way to go for the development of free-standing photonic films from natural polymers. Herein, a highly flexible, controllably iridescent, and multistimuli-responsive cellulose nanocrystal (CNC) film is prepared by simply introducing a small molecule as both plasticizer and hygroscopic agent. The presence of the additive does not block the self-assembly of CNC in aqueous solution but results in the enhancement of its mechanical toughness, making it possible to obtain free-standing iridescent CNC films with tunable structural colors. In response to environmental humidity and mechanical compression, such films can change structural colors smoothly by modulating their chiral nematic structures. Notably, the chromism is reversible by alternately changing relative humidity between 16 and 98%, mimicking the longhorn beetle Tmesisternus isabellae. This chromic effect enables various applications of the biofilms in colorimetric sensors, anticounterfeiting technology, and decorative coatings.
Due to spontaneous organization of cellulose nanocrystals (CNCs) into the chiral nematic structure that can selectively reflect circularly polarized light within a visible-light region, fabricating stretching deformation-responsive CNC materials is of great interest but is still a big challenge, despite such a function widely observed from existing creatures, like a chameleon, because of the inherent brittleness. Here, a flexible network structure is introduced in CNCs, exerting a bridge effect for the rigid nanomaterials. The as-prepared films display high flexibility with a fracture strain of up to 39%. Notably, stretching-induced structural color changes visible to the naked eye are realized, for the first time, for CNC materials. In addition, the soft materials show humidity-and compressionresponsive properties in terms of changing apparent structural colors. Colored marks left by ink-free writing can be shown or hidden by controlling the environmental humidities. This biobased photonic film, acting as a new "smart skin", is potentially used with multifunctions of chromogenic sensing, encryption, and anti-counterfeit.
The mesoporous structure and high exposure of the (001) facet are of great importance to the photocatalytic performance of TiO2. In this Letter, we report using cellulose nanocrystal (CNC) as a sacrificial template to develop mesoporous TiO2 with dominantly exposed (001) facets, for which CNC can provide confined space for the controlled crystal growth of TiO2 and create mesopores after being removed. Owing to the photoluminescence up-conversion, furthermore, carbon quantum dot (C-dot) is introduced to realize visible light catalytic property of TiO2. In particular, the TiO2/C-dot composite with an extremely low content of carbon dot exhibits high catalytic performance, for which the mechanism is discussed. These results indicate such biotemplating method offers the potential to develop more mesoporous nanomaterials with desirable structures.
Facing the limitation of weak compatibility on the use of cellulose nanofiber (CNF) as a reinforcement in polymer matrix nanocomposites, this work developed an alternative approach to fabricating a series of one-component CNF-reinforced nanocomposites by grafting polyacrylates from CNF via surface-initiated Cu(0)-mediated reversible deactivation radical polymerizations. An initiator group was initially introduced onto the surface of CNF through one-step esterification. Then, graft polymerization of methyl acrylate and nbutyl acrylate was carried out to obtain polyacrylate-grafted CNF. The graft polymerization exhibited several advantages, including simplicity, rapidity, and effectivity. A high graft ratio (up to 3000%) was obtained at room temperature after 3 h. The graft ratio was controlled by regulating the monomer concentration and reaction time. Characterizations confirmed that the chemical, morphological, and crystalline structure of CNF was maintained after surface modification and grafting. The uniform morphology and moderated transparency of solvent-casting polyacrylate-grafted CNF films proved the superior homogeneity of these nanocomposites. Tensile experiments revealed that poly(methyl acrylate)-grafted CNF nanocomposites had remarkable mechanical properties (tensile strength = 13.6−25.3 MPa; break elongation = 48.8−179.9%), contributing to an outstanding dispersity. By alternating the graft ratio, the mechanical behavior of the nanocomposite could transition from tough to ductile. This grafting approach has been proven to be a powerful tool for the design of CNF-reinforced nanocomposites for various purposes.
Bioinspired coloration of cellulose nanocrystals (CNCs) has advanced a new era of wide applications for decoration, sensing, and anti-counterfeiting. However, the solid structural color films suffer from strong angle-dependent iridescence and a limited color range due to the spatial arrangement caused by kinetic arrest during evaporation-induced self-assembly of CNCs. Here, a ternary co-assembly approach is demonstrated to prepare CNC films with weak/non-angular optical response and wide-range colors. Structural coloration is enabled by the long-range and short-range ordered photonic structure of CNCs. A cationic polymer, poly(dimethyl diallyl ammonium chloride), is introduced to influence the CNC self-assembly through electrostatic attraction, inducing the structural transition to the long-range disordered/short-range ordered structure as well as presenting the gradual change from iridescent structural colors to non-iridescent colors. Additionally, a hydrophilic small compound, glycerol, is used to adjust the helical pitch of nanostructured CNC arrays, offering the broad-range coloration. The CNC-based composite material is further used as an optical coating for anti-counterfeiting and realizing color patterning and angle-dependent selective color changes.
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