(Nanowerk Spotlight) Cellulose, the most abundant biopolymer on Earth, has long been recognized for its potential as a sustainable alternative to petroleum-based materials. As concerns about environmental pollution and resource depletion grow, researchers have been exploring ways to harness cellulose’s unique properties for various applications.
Two forms of nanocellulose – cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs) – have emerged as particularly promising. CNCs, rod-shaped nanoparticles derived from cellulose, possess the remarkable ability to self-assemble into chiral nematic structures, resulting in iridescent films with vibrant structural colors. This property has attracted significant interest for applications in fields ranging from food packaging to security labels.
However, the development of practical cellulose-based materials has faced persistent challenges. While CNC films exhibit striking optical properties, they are inherently brittle, limiting their use in applications requiring flexibility. Previous attempts to enhance the mechanical properties of CNC films often involved introducing non-cellulose additives, compromising the material’s biodegradability and biocompatibility.
The integration of CNFs, known for their reinforcing capabilities, into CNC systems has shown promise in improving mechanical properties. Yet, these efforts have typically resulted in the loss of the desirable structural colors, as the addition of CNFs disrupts the self-assembly process of CNCs.
The interplay between the surface charges of CNCs and CNFs, along with the complex dynamics of their interactions during the drying process, has presented a formidable obstacle to creating flexible, colorful, all-cellulose films. Researchers have grappled with balancing the enhancement of mechanical properties while preserving the unique optical characteristics of CNCs. This challenge has underscored the need for innovative approaches to cellulose nanoparticle engineering and composite design.
Recent advancements in cellulose modification techniques and a deeper understanding of nanoparticle self-assembly have paved the way for new strategies to overcome these limitations. By carefully manipulating the surface properties of cellulose nanoparticles and controlling their interactions, researchers are now exploring novel methods to create cellulose-based materials that combine flexibility, strength, and structural color.
In a significant step forward, researchers in China have developed a method to produce flexible, structurally colored films composed entirely of cellulose. This innovative approach, detailed in a recent study in Advanced Functional Materials (“All-Cellulose-Based Flexible Photonic Films”), addresses the long-standing challenge of combining the optical properties of CNCs with the mechanical strength of CNFs while maintaining an all-cellulose composition.
Photographs of a pure CNCs film: a) before and b) after bending. Photographs of a CNCs-MLCNFs film: c) before and d,e) after bending at a certain angle. The mass ratio of CNCs to MLCNFs in this CNCs-MLCNFs film was 1:0.3. The CNCs suspension was sonicated for 10 mins before use. The scale bar is 5 cm. (Image: reproduced from DOI:10.1002/adfm.202408464, CC BY)
The key to this breakthrough lies in a multi-step process that modifies CNFs to minimize their interference with CNC self-assembly. The researchers first reduced the surface charge of CNFs through a chemical treatment involving 4-(4,6-Dimethoxy-1,3,5-triazin-2-yl)−4-methylmorpholin-4-ium chloride (DMTMM) activation followed by sodium borohydride reduction. This step decreased the carboxyl group content on the CNFs from 0.76 to 0.26 mmol/g, significantly lowering their zeta potential and reducing their potential to disrupt CNC organization.
The modified CNFs, termed LCNFs (low-charge CNFs), were then functionalized with methacrylic anhydride to introduce carbon-carbon double bonds. This modification allowed for the subsequent cross-linking of the LCNFs under ultraviolet light, creating a network structure. The resulting modified LCNFs, dubbed MLCNFs, formed the basis of a hydrogel network into which CNC suspensions could be introduced.
By carefully controlling the incorporation of CNCs into the MLCNF network and managing the evaporation process, the researchers successfully produced films that retained the structural colors characteristic of CNC self-assembly while exhibiting significantly enhanced flexibility. A key innovation in this process was the ability to tune the structural colors of the films by adjusting the sonication energy applied to the initial CNC suspensions. By varying the sonication time from 10 to 35 minutes, the researchers were able to produce films with vibrant red, green, and blue colors. This simple yet effective method of color control adds an important dimension of customizability to the material.
Crucially, the mechanical properties of these all-cellulose films showed marked improvements compared to pure CNC films. While pure CNC films exhibited a tensile strength of 60 ± 6 MPa with a maximum strain of only 0.40 ± 0.07%, the composite films with the highest MLCNF content (30% by mass) demonstrated a fivefold increase in strain to 2.28 ± 0.11% and a 20% increase in tensile strength to 72 ± 7 MPa. These quantitative improvements highlight the significant enhancement in flexibility and strength achieved through the incorporation of MLCNFs. Importantly, these improvements were achieved without significantly compromising the films’ high Young’s modulus, which only decreased from 6.5 ± 0.21 to 4.7 ± 0.17 GPa with the addition of MLCNFs.
The success of this approach hinges on two critical factors: the reduction of surface charges on the CNFs and their immobilization within a cross-linked network. By minimizing the electrostatic interactions between CNFs and CNCs and restricting the mobility of the CNFs, the researchers were able to preserve the self-assembly process of CNCs that gives rise to their structural colors. At the same time, the MLCNF network provides a flexible supporting structure that enhances the overall mechanical properties of the composite.
X-ray diffraction analysis revealed that the incorporation of MLCNFs had only a modest impact on the crystallinity of the composite films. The crystallinity index decreased from 89% for pure CNC films to 82% for films with 30% MLCNF content, indicating that the fundamental crystalline structure of the CNCs was largely preserved. This retention of crystallinity is significant, as it suggests that the optical and mechanical properties of the CNCs are maintained even with the addition of the MLCNF network.
The development of these flexible, colorful, all-cellulose films represents a significant advance in sustainable materials science. By overcoming the previously conflicting requirements of structural color and mechanical flexibility, this research opens up new possibilities for cellulose-based materials in a wide range of applications.
The potential applications for these materials are diverse and promising. The combination of flexibility, strength, and tunable structural color makes them attractive candidates for use in flexible electronics, such as foldable displays or sensors. Their all-cellulose composition ensures biodegradability and biocompatibility, addressing growing concerns about electronic waste and the environmental impact of conventional materials.
In the field of anti-counterfeiting, these films could provide a sustainable alternative to current technologies, offering unique optical properties that are difficult to replicate without access to the specific fabrication process. The ability to tune the color through simple sonication of the CNC suspension adds an additional layer of security and customization.
Moreover, the principles developed in this research may have broader implications for the design of other composite materials. The strategy of modifying and immobilizing one component to preserve the self-assembly properties of another could be applied to other systems where maintaining nanostructure is crucial.
While this research represents a significant step forward, further work will be necessary to optimize the production process and explore the full range of potential applications. Scaling up the fabrication of these composite films, improving their durability under various environmental conditions, and investigating their long-term stability will be important areas for future research.
The development of flexible, structurally colored, all-cellulose films demonstrates the continuing potential of cellulose as a versatile and sustainable material. By cleverly manipulating the properties of cellulose nanoparticles, researchers have created a material that combines the best properties of CNCs and CNFs, opening new avenues for the application of bio-based materials in advanced technologies.
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