Holocellulose Nanofibers of High Molar Mass and Small Diameter for High-Strength Nanopaper

High-Lignin-Containing Cellulose Nanofibrils from Date Palm Waste Produced by Hydrothermal Treatment in the Presence of Maleic Acid

Lignin-containing cellulose nanofibrils (LCNFs) have attracted great attention because the presence of lignin brought additional merits to cellulose nanofibrils including hydrophobicity, ultraviolet (UV)-shielding capacity, and reduced water sensitivity. In the present work, LCNFs with lignin content up to 21 wt % were prepared with a high yield exceeding 70 wt %, from neat date palm waste, by a hydrothermal treatment (HTT) at 120-150 °C in the presence of 20-30 wt % maleic acid, followed by high-pressure homogenization. The chemical composition, degree of polymerization, morphology, and colloidal and rheological properties of the LCNFs were investigated to understand how the HTT in the presence of MA affected the properties of the resulting LCNFs. Nanopapers prepared from the LCNF suspensions exhibited mechanical properties lower than those from lignin-free CNF-based nanopapers, yet with decreased hydrophilicity. A mechanism explaining how the HTT in the presence of MA facilitated the disintegration of the biomass into nanoscale material was proposed. Overall, the present work demonstrated a feasible and scalable approach for the sustainable production of LCNF suspensions from neat agricultural residues, with a high yield and a high lignin content, without any need to perform a preliminary partial delignification.

The Colloidal Properties of Nanocellulose

Nanocelluloses are anisotropic nanoparticles of semicrystalline assemblies of glucan polymers. They have great potential as renewable building blocks in the materials platform of a more sustainable society. As a result, the research on nanocellulose has grown exponentially over the last decades. To fully utilize the properties of nanocelluloses, a fundamental understanding of their colloidal behavior is necessary. As elongated particles with dimensions in a critical nanosize range, their colloidal properties are complex, with several behaviors not covered by classical theories. In this comprehensive Review, we describe the most prominent colloidal behaviors of nanocellulose by combining experimental data and theoretical descriptions. We discuss the preparation and characterization of nanocellulose dispersions, how they form networks at low concentrations, how classical theories cannot describe their behavior, and how they interact with other colloids. We then show examples of how scientists can use this fundamental knowledge to control the assembly of nanocellulose into new materials with exceptional properties. We hope aspiring and established researchers will use this Review as a guide.

Understanding Nanocellulose-Water Interactions: Turning a Detriment into an Asset

Modern technology has enabled the isolation of nanocellulose from plant-based fibers, and the current trend focuses on utilizing nanocellulose in a broad range of sustainable materials applications. Water is generally seen as a detrimental component when in contact with nanocellulose-based materials, just like it is harmful for traditional cellulosic materials such as paper or cardboard. However, water is an integral component in plants, and many applications of nanocellulose already accept the presence of water or make use of it. This review gives a comprehensive account of nanocellulose-water interactions and their repercussions in all key areas of contemporary research: fundamental physical chemistry, chemical modification of nanocellulose, materials applications, and analytical methods to map the water interactions and the effect of water on a nanocellulose matrix.

Acidic and alkaline deep eutectic solvents pre-treatment to produce high aspect ratio microfibrillated cellulose

This study highlights the microfibrillation potential of three deep eutectic solvents (DES) composed of betaine hydrochloride-urea, choline chloride-urea and choline chloride-monoethanolamine. Cellulose fibres (eucalyptus and cotton) were first treated in DES (100 °C for 4 h) and then ground with an ultra-fine grinder to produce microfibrillated cellulose (MFC). DES pre-treatment especially betaine hydrochloride-urea system has significantly improved the microfibrillation process with reduced energy consumption comparable to that of enzymatic treatment (reference pre-treatment). Long and thin microfibril bundles (widths around 50 nm) and individualised microfibrils (widths between 5 and 10 nm) were obtained. MFC gels and nanopapers were characterised using several techniques. Nanopapers produced from DES treated MFC showed good mechanical properties with Young's modulus higher than 10 GPa. In addition, they exhibited higher quality index (between 73 and 76) than those produced from enzymatic hydrolysis (quality index around 68). DES pre-treatment is a promising way to produce MFC with high aspect ratio.

Mechanically strong and biodegradable holocellulose films prepared from Camellia oleifera shells

Bioplastic derived from renewable lignocellulosic biomass is an attractive alternative to petroleum-based plastics. Herein, Callmellia oleifera shells (COS), a unique byproduct from tea oil industry, were delignified and converted into high-performance bio-based films via a green citric acid treatment (15 %, 100 °C and 24 h), taking advantage of their high hemicellulose content. The structure-property relations of COS holocellulose (COSH) films were systematically analyzed considering different treatment conditions. The surface reactivity of COSH was improved via a partial hydrolysis route and strong hydrogen bonding formed between the holocellulose micro/nanofibrils. COSH films exhibited high mechanical strength, high optical transmittance, improved thermal stability, and biodegradability. A mechanical blending pretreatment of COSH, which disintegrated the COSH fibers before the citric acid reaction, further enhanced the tensile strength and Young's modulus of the films up to 123.48 and 5265.41 MPa, respectively. The films decomposed completely in soil, demonstrating an excellent balance between degradability and durability.

Nanocellulose for Sustainable Water Purification

Nanocelluloses (NC) are nature-based sustainable biomaterials, which not only possess cellulosic properties but also have the important hallmarks of nanomaterials, such as large surface area, versatile reactive sites or functionalities, and scaffolding stability to host inorganic nanoparticles. This class of nanomaterials offers new opportunities for a broad spectrum of applications for clean water production that were once thought impractical. This Review covers substantial discussions based on evaluative judgments of the recent literature and technical advancements in the fields of coagulation/flocculation, adsorption, photocatalysis, and membrane filtration for water decontamination through proper understanding of fundamental knowledge of NC, such as purity, crystallinity, surface chemistry and charge, suspension rheology, morphology, mechanical properties, and film stability. To supplement these, discussions on low-cost and scalable NC extraction, new characterizations including solution small-angle X-ray scattering evaluation, and structure-property relationships of NC are also reviewed. Identifying knowledge gaps and drawing perspectives could generate guidance to overcome uncertainties associated with the adaptation of NC-enabled water purification technologies. Furthermore, the topics of simultaneous removal of multipollutants disposal and proper handling of post/spent NC are discussed. We believe NC-enabled remediation nanomaterials can be integrated into a broad range of water treatments, greatly improving the cost-effectiveness and sustainability of water purification.

Plant-Based Structures as an Opportunity to Engineer Optical Functions in Next-Generation Light Management

This review addresses the reconstruction of structural plant components (cellulose, lignin, and hemicelluloses) into materials displaying advanced optical properties. The strategies to isolate the main building blocks are discussed, and the effects of fibrillation, fibril alignment, densification, self-assembly, surface-patterning, and compositing are presented considering their role in engineering optical performance. Then, key elements that enable lignocellulosic to be translated into materials that present optical functionality, such as transparency, haze, reflectance, UV-blocking, luminescence, and structural colors, are described. Mapping the optical landscape that is accessible from lignocellulosics is shown as an essential step toward their utilization in smart devices. Advanced materials built from sustainable resources, including those obtained from industrial or agricultural side streams, demonstrate enormous promise in optoelectronics due to their potentially lower cost, while meeting or even exceeding current demands in performance. The requirements are summarized for the production and application of plant-based optically functional materials in different smart material applications and the review is concluded with a perspective about this active field of knowledge.

Mechanical properties of cellulose nanofibril papers and their bionanocomposites: A review

Cellulose nanofibril (CNF) paper has various applications due to its unique advantages. Herein, we present the intrinsic mechanical properties of CNF papers, along with the preparation and properties of nanoparticle-reinforced CNF composite papers. The literature on CNF papers reveals a strong correlation between the intrafibrillar network structure and the resulting mechanical properties. This correlation is found to hold for all primary factors affecting mechanical properties, indicating that the performance of CNF materials depends directly on and can be tailored by controlling the intrafibrillar network of the system. The parameters that influence the mechanical properties of CNF papers were critically reviewed. Moreover, the effect on the mechanical properties by adding nanofillers to CNF papers to produce multifunctional composite products was discussed. We concluded this article with future perspectives and possible developments in CNFs and their bionanocomposite papers.

Preparation and Characterization of Polybutylene Succinate Reinforced with Pure Cellulose Nanofibril and Lignocellulose Nanofibril Using Two-Step Process

This study reports the preparation of a polybutylene succinate (PBS) film reinforced with pure cellulose nanofibril (PCNF) and lignocellulose nanofibril (LCNF) by a two-step process that consists of solvent dispersion and twin-screw extrusion. Compared to the conventional one-step process, this method offered improved mechanical properties. The addition of 5% CNF increased the tensile properties up to 18.8%. Further, the effect of the lignin content was also studied by using LCNF as a reinforcement. The LCNF was prepared with and without a deep eutectic solvent (DES) pretreatment to gain LCNF with a lignin content that varied between 5, 19, and 30%. The mechanical properties results show that a 5% addition of LCNF to the PBS matrix increased its tensile strength and elastic modulus. Further, the morphological and thermal properties of the composites were also studied in detail.

Structural and Ecofriendly Holocellulose Materials from Wood: Microscale Fibers and Nanoscale Fibrils

Mildly delignified wood holocellulose fibers show well-preserved cellulose nanofibril (CNF) structure in the fiber cell wall. Fibers, paper, biocomposites, and compression-molded fiber materials demonstrate excellent mechanical properties. Here, wood holocellulose fibers and corresponding CNFs are discussed with respect to nanostructure, mechanical performance, and advanced materials potential. Functionalization routes are discussed, as well as materials selection, nanoscience of recycling, and the embodied energy in cellulosic candidates for multifunctional structural materials.

Mechanics Design in Cellulose-Enabled High-Performance Functional Materials

The abundance of cellulose found in natural resources such as wood, and the wide spectrum of structural diversity of cellulose nanomaterials in the form of micro-nano-sized particles and fibers, have sparked a tremendous interest to utilize cellulose's intriguing mechanical properties in designing high-performance functional materials, where cellulose's structure-mechanics relationships are pivotal. In this progress report, multiscale mechanics understanding of cellulose, including the key role of hydrogen bonding, the dependence of structural interfaces on the spatial hydrogen bond density, the effect of nanofiber size and orientation on the fracture toughness, are discussed along with recent development on enabling experimental design techniques such as structural alteration, manipulation of anisotropy, interface and topology engineering. Progress in these fronts renders cellulose a prospect of being effectuated in an array of emerging sustainable applications and being fabricated into high-performance structural materials that are both strong and tough.

Sustainable isolation of nanocellulose from cellulose and lignocellulosic feedstocks: Recent progress and perspectives

As a type of sustainable nanomaterials, nanocellulose has drawn increasing attention over the last two decades due to its great potential in diverse value-added applications such as electronics, sensors, energy storage, packaging, pharmaceuticals, biomedicine, and functional food. Sourcing nanocellulose from lignocellulose is commonly accomplished via the use of mineral acids, oxidizers, enzymes, and/or intensive mechanical energy. Yet, the economic and environmental concerns associated with these conventional isolation techniques pose major obstacles for commercialization. Considerable progress has been achieved in the last few years in developing sustainable nanocellulose isolation technologies involving organic acid/anhydride, Lewis acid, solid acid, ionic liquid, and deep eutectic solvent. This paper provides a comprehensive review of these alternatives with regard to general procedures and key advantages. Important knowledge gaps, including total biomass utilization, complete life cycle analysis, and health/safety, require urgently bridging in order to develop economically competitive and operationally feasible nanocellulose isolation technology for commercialization.

Eco-Friendly Bioinspired Interface Design for High-Performance Cellulose Nanofibril/Carbon Nanotube Nanocomposites

Inspired by a wood-like multicomponent structure, an interface-reinforced method was developed to fabricate high-performance cellulose nanofibril (CNF)/carbon nanotube (CNT) nanocomposites. Holocellulose nanofibrils (HCNFs) with core-shell structure were first obtained from bagasse via mild delignification and mechanical defibration process. The well-preserved native hemicellulose as the amphiphilic shell of HCNFs could act as a binding agent, sizing agent, and even dispersing agent between HCNFs and CNTs. Remarkably, both the tensile strength at high relative humidity (83% RH) and electrical conductivity of the HCNF/CNT nanocomposites were significantly improved up to 121 MPa and 321 S/m, respectively, demonstrating great superiority compared to normal CNF/CNT composite films. Furthermore, these HCNF/CNT composites with outstanding integrated performances exhibited great potential in the field of flexible liquid sensing.

Favorable combination of foldability and toughness of transparent cellulose nanofibril films by a PET fiber-reinforced strategy

Transparent cellulose nanofibril (CNF) films have been considered a promising sustainable alternative for nonrenewable and nonbiodegradable petroleum-based plastic films. However, their relatively low toughness and poor folding endurance are two remaining challenges for commercial application. In this work, inspired by fiber-reinforced polymer, a transparent CNF film with favorable combination of toughness and folding endurance is demonstrated by a facile and scalable polyethylene terephthalate (PET) fiber-reinforced strategy. The as-prepared PET fiber-reinforced CNF film not only exhibits a maximum average toughness of 13.7 ± 2.4 MJ/m³ that is nearly 4 times stronger than that of pure CNF film (3.7 ± 1.1 MJ/m³), but also presents superior bending flexibility with a folding endurance of over 10⁴ times that is an order of magnitude higher than that of pure CNF film (~4 × 10³ times). Moreover, its underlying principle for the enhanced toughness and foldability is explored. This work provides a facile strategy to prepare tough yet foldable CNF film and could promote its industrial uses in the fields of energy storage devices, packaging, and electronic devices.

Exploring Large Ductility in Cellulose Nanopaper Combining High Toughness and Strength

Cellulose nanopaper is a strong lightweight material made from renewable resources with a wide range of potential applications, from membranes to electronic displays. Most studies on nanopaper target high mechanical strength, which compromises ductility and toughness. Herein, we demonstrate the fabrication of highly ductile and tough cellulose nanopaper via mechanical fibrillation of hemicellulose-rich wood fibers and dispersion of the obtained cellulose nanofibrils (CNFs) in an ionic liquid (IL)-water mixture. This treatment allows hemicellulose swelling, which leads to dissociation of CNF bundles into highly disordered long flexible fibrils and the formation of a nanonetwork as supported by cryogenic transmission electron microscopy (cryo-TEM) imaging. Rheology of the suspensions shows a 300-fold increase in storage and loss moduli of CNF-IL-water suspensions, compared to their CNF-water counterparts. The nanopaper prepared by removing the IL-water shows a combination of large elongation (up to 35%), high strength (260 MPa), and toughness as high as 51 MJ/m³, because of efficient interfibrillar slippage and energy dissipation in the highly disordered isotropic structure. This work provides a nanostructure-engineered strategy of making ductile and tough cellulose nanopaper.

Strong Reinforcement Effects in 2D Cellulose Nanofibril-Graphene Oxide (CNF-GO) Nanocomposites due to GO-Induced CNF Ordering

Nanocomposites from native cellulose with low 2D nanoplatelet content are of interest as sustainable materials combining functional and structural performance. Cellulose nanofibril-graphene oxide (CNF-GO) nanocomposite films are prepared by a physical mixing-drying method, with focus on low GO content, the use of very large GO platelets (2-45μm) and nanostructural characterization using synchrotron x-ray source for WAXS and SAXS. These nanocomposites can be used as transparent coatings, strong films or membranes, as gas barriers or in laminated form. CNF nanofibrils with random in-plane orientation, form a continuous non-porous matrix with GO platelets oriented in-plane. GO reinforcement mechanisms in CNF are investigated, and relationships between nanostructure and suspension rheology, mechanical properties, optical transmittance and oxygen barrier properties are investigated as a function of GO content. A much higher modulus reinforcement efficency is observed than in previous polymer-GO studies. The absolute values for modulus and ultimate strength are as high as 17 GPa and 250 MPa at a GO content as small as 0.07 vol%. The remarkable reinforcement efficiency is due to improved organization of the CNF matrix; and this GO-induced mechanism is of general interest for nanostructural tailoring of CNF-2D nanoplatelet composites.

Rapid synchronous synthesis of Ag nanoparticles and Ag nanoparticles/holocellulose nanofibrils: Hg(II) detection and dye discoloration

A novel microwave-assisted green method that synchronously synthesizes silver nanoparticles (AgNPs) and AgNPs decorated holocellulose nanofibrils (AgNPs/HCNF) within a minute and without using a reducing agent is reported. As obtained nanomaterials were well characterized using various analytical techniques. AgNPs applied as a colorimetric probe for the selective recognition of Hg(II) (linear range 10-200 μg L^-1, detection limit 1.16 μg L^-1). The probe was able to quantify Hg(II) in spiked tap, bore, and lake water samples and paper strips were developed to facilitate the onsite detection. Furthermore, freeze-drying of the AgNPs/HCNF nanocomposite produced aerogel that served as an excellent catalyst for the reduction of Congo red and methylene blue. The aerogel was easily recovered and reused without a decrease in activity or deterioration of its structure for five cycles. These results indicate the great potential of the AgNPs/HCNF aerogel for waste water treatment and catalytic applications.

Field-Assisted Alignment of Cellulose Nanofibrils in a Continuous Flow-Focusing System

The continuous production of macroscale filaments of 17 μm in diameter comprising aligned TEMPO-oxidized cellulose nanofibrils (CNFs) is conducted using a field-assisted flow-focusing process. The effect of an AC external field on the material's structure becomes significant at a certain voltage, beyond which augmentations of the CNF orientation factor up to 16% are obtained. Results indicate that the electric field significantly contributes to improve the CNF ordering in the bulk, while the CNF alignment on the filament surface is only slightly affected by the applied voltage. X-ray diffraction shows that CNFs are densely packed anisotropically in the plane parallel to the filament axis without any preferential out of plane orientation. The improved nanoscale ordering combined with the tight CNF packing yields impressive enhancements in mechanical properties, with stiffness up to 25 GPa and more than 63% (up to 260 MPa), 46% (up to 2.8%), and 120% (up to 4.7 kJ/m³) increase in tensile strength, strain-to-failure, and toughness, respectively. This study demonstrates for the first time the control over the structural ordering of anisotropic nanoparticles in a dynamic system using an electric field, which can have important implications for the development of sustainable alternatives to synthetic textiles.

Preparation and Characteristics of Wet-Spun Filament Made of Cellulose Nanofibrils with Different Chemical Compositions

In this study, wet-spun filaments were prepared using lignocellulose nanofibril (LCNF), with 6.0% and 13.0% of hemicellulose and lignin, respectively, holocellulose nanofibril (HCNF), with 37% hemicellulose, and nearly purified-cellulose nanofibril (NP-CNF) through wet-disk milling followed by high-pressure homogenization. The diameter was observed to increase in the order of NP-CNF ≤ HCNF < LCNF. The removal of lignin improved the defibrillation efficiency, thus increasing the specific surface area and filtration time. All samples showed the typical X-ray diffraction pattern of cellulose I. The orientation of CNFs in the wet-spun filaments was observed to increase at a low concentration of CNF suspensions and high spinning rate. The increase in the CNF orientation improved the tensile strength and elastic modulus of the wet-spun filaments. The tensile strength of the wet-spun filaments decreased in the order of HCNF > NP-CNF > LCNF.

Eco-Friendly Cellulose Nanofibrils Designed by Nature: Effects from Preserving Native State

Cellulose nanofibrils (CNFs) show high modulus and strength and are already used in industrial applications. Mechanical properties of neat CNF films or CNF-polymer matrix nanocomposites are usually much better than for polymer matrix composite films reinforced by clay, graphene, graphene oxide, or carbon nanotubes. In order to obtain small CNF diameter and colloidal stability, chemical modification has so far been necessary, but this increases cost and reduces eco-friendly attributes. In this study, an unmodified holocellulose CNF (Holo-CNF) with small diameter is obtained from mildly peracetic acid delignified wood fibers. CNF is readily defibrillated by low-energy kitchen blender processing. The hemicellulose coating on individual fibrils in the wood plant cell wall is largely preserved in Holo-CNF. This "native" CNF shows well-preserved native fibril structure in terms of length (∼2.1 μm), diameter (<5 nm), high crystallinity, high cellulose molar mass, electronegative charge, and limited mechanical processing damage. The hemicellulose coating contributes mechanical properties and high optical transmittance for CNF nanopaper, which can otherwise only be achieved with chemically modified CNFs. The CNF nanopaper shows superior mechanical properties with a Young's modulus of 21 GPa and an ultimate strength of 320 MPa. Moreover, hemicellulose imparts recyclability from the dried state. Altogether, this native CNF represents a class of colloidally stable, eco-friendly, low-cost CNF of small diameter for large-scale applications of nanopaper and nanomaterials.

Polymer Grafting Inside Wood Cellulose Fibers by Improved Hydroxyl Accessibility from Fiber Swelling

Chemical modification of wood cellulose fibers is important for tailored wood-polymer interfaces, reduced moisture sorption, and novel grades of chemical wood pulp. The present study shows how the reaction solvent system influences hydroxyl accessibility during chemical fiber modification. Surface initiated ring-opening polymerization of ε-caprolactone from wood cellulose fibers was investigated in a wide range of solvent systems. The hydrogen bond donor strength of the solvent increased graft density and the amount of grafted polycaprolactone (PCL) on the fiber surface, and on nanoscale fibrils inside the fiber. Specifically, the reaction system with acetic acid as a new, green solvent for cellulose grafting increased graft density 24 times compared to bulk polymerization conditions. The results show relationships between solvent properties, hydroxyl accessibility, and grafting results in cellulosic plant fibers. The study clarifies the opportunities provided by controlling the interior of the cellulosic plant fiber cell wall during chemical modification so that the fiber becomes a swollen cellulose nanofibril gel.

High-Density Molded Cellulose Fibers and Transparent Biocomposites Based on Oriented Holocellulose

Ecofriendly materials based on well-preserved and nanostructured wood cellulose fibers are investigated for the purpose of load-bearing applications, where optical transmittance may be advantageous. Wood fibers are subjected to mild delignification, flow orientation, and hot-pressing to form an oriented material of low porosity. The biopolymer composition of the fibers is determined. Their morphology is studied by scanning electron microscopy, cellulose orientation is quantified by X-ray diffraction, and the effect of beating is investigated. Hot-pressed networks are impregnated by a methyl methacrylate monomer and polymerized to form thermoplastic wood fiber/poly(methyl methacrylate) biocomposites. Tensile tests are performed, as well as optical transmittance measurements. Structure-property relationships are discussed. High-density molded fibers from holocellulose have mechanical properties comparable with nanocellulose materials and are recyclable. The thermoplastic matrix biocomposites showed superior mechanical properties (Young's modulus of 20 GPa and ultimate strength of 310 MPa) at a fiber volume fraction of 52%, with high optical transmittance of 90%. The study presents a scalable approach for strong, stiff, and transparent molded fibers/biocomposites.

Synthesis, characterization and enzymatic surface roughing of cellulose/xylan composite films

Upgrading renewable cellulose biopolymer to various high-value material/chemical is of great importance in building a sustainable bio-economy. This work assessed the technical feasibility of fabricating transparent cellulose/xylan composite films using facile solution-casting method. More importantly, this work also initially assessed the technical potential of xylanase treatment to selectively modify the surface of the obtained composite films with the goal of extending their applications. When bleached Kraft xylan addition was lower than 20 wt%, the composite films could still retain their original mechanical and structural advantages. Xylanase treatment specifically removed 26.0% and 32.3% xylan of the composite films with an enzyme loading of 2 and 5 mg g^-1 cellulose, respectively. It was shown that xylan component was heterogeneously located in the surface of the composite films during film-casting process, which allowed the subsequent surface etching/roughing at nanoscale using facile xylanase treatment without compromising their structural advantages.

Isotropic Paper Directly from Anisotropic Wood: Top-Down Green Transparent Substrate Toward Biodegradable Electronics

Flexible electronics have found useful applications in both the scientific and industrial communities. However, substrates traditionally used for flexible electronics, such as plastic, cause many environmental issues. Therefore, a transparent substrate made from natural materials provides a promising alternative because it can be degraded in nature. The traditional bottom-up fabrication method for transparent paper is expensive, environmentally unfriendly, and time-consuming. In this work, for the first time, we developed a top-down method to fabricate isotropic, transparent paper directly from anisotropic wood. The top-down method includes two steps: a delignification process to bleach the wood by lignin removal and a pressing process for removing light-reflecting and -scattering sources. The resulting isotropic, transparent paper has high transmittance of about 90% and high haze over 80% and is demonstrated as a nature-disposable substrate for electronic/optical devices. Adjusting the pressing ratio used changes the density of the resulting paper, which tunes the microstructure-related properties of the isotropic, transparent paper. This top-down method is simple, fast, environmentally friendly, and cost-effective, which can greatly promote the development of paper-based green optical and electronic devices.

Advanced Materials through Assembly of Nanocelluloses

There is an emerging quest for lightweight materials with excellent mechanical properties and economic production, while still being sustainable and functionalizable. They could form the basis of the future bioeconomy for energy and material efficiency. Cellulose has long been recognized as an abundant polymer. Modified celluloses were, in fact, among the first polymers used in technical applications; however, they were later replaced by petroleum-based synthetic polymers. Currently, there is a resurgence of interest to utilize renewable resources, where cellulose is foreseen to make again a major impact, this time in the development of advanced materials. This is because of its availability and properties, as well as economic and sustainable production. Among cellulose-based structures, cellulose nanofibrils and nanocrystals display nanoscale lateral dimensions and lengths ranging from nanometers to micrometers. Their excellent mechanical properties are, in part, due to their crystalline assembly via hydrogen bonds. Owing to their abundant surface hydroxyl groups, they can be easily modified with nanoparticles, (bio)polymers, inorganics, or nanocarbons to form functional fibers, films, bulk matter, and porous aerogels and foams. Here, some of the recent progress in the development of advanced materials within this rapidly growing field is reviewed.

Preserving Cellulose Structure: Delignified Wood Fibers for Paper Structures of High Strength and Transparency

To expand the use of renewable materials, paper products with superior mechanical and optical properties are needed. Although beating, bleaching, and additives are known to improve industrially produced Kraft pulp papers, properties are limited by the quality of the fibers. While the use of nanocellulose has been shown to significantly increase paper properties, the current cost associated with their production has limited their industrial relevance. Here, using a simple mild peracetic acid (PAA) delignification process on spruce, we produce hemicellulose-rich holocellulose fibers (28.8 wt %) with high intrinsic strength (1200 MPa for fibers with microfibrillar angle smaller than 10°). We show that PAA treatment causes less cellulose/hemicellulose degradation and better preserves cellulose nanostructure in comparison to conventional Kraft pulping. High-density holocellulose papers with superior mechanical properties (Young's modulus of 18 GPa and ultimate strength of 195 MPa) are manufactured using a water-based hot-pressing process, without the use of beating or additives. We propose that the preserved hemicelluloses act as "glue" in the interfiber region, improving both mechanical and optical properties of papers. Holocellulose fibers may be affordable and applicable candidates for making special paper/composites where high mechanical performance and/or optical transmittance are of interest.

Anisotropic flexible transparent films from remaining wood microstructures for screen protection and AgNW conductive substrate

Flexible transparent conductive films or substrates prepared from plastics or cellulose are widely used in optoelectronic devices. However, all of these films or substrates are fabricated by complex and expensive methods, which consume much energy and time. In this work, we report for the first time a remarkably facile and effective approach for fabricating flexible transparent films directly from wood. The resulting films exhibit an array of exceptional optical and mechanical properties. The well-aligned cell structures in natural wood are maintained during delignification, leading to anisotropic films with high transparency (≈90% transmittance). These anisotropic films with well-aligned cell structures show mechanical tensile strengths higher than those of the original wood, and can be used as screen protection films for cellphones. Furthermore, ultrathin, highly transparent, and outstandingly conductive films have been prepared from such films and silver nanowires (AgNWs) using the Meyer technique. A conductive film with an optimal area density (341 mg m^-2) of AgNWs showed outstanding synergistic properties, with a transmittance of 80% and a sheet resistance of 11 Ω sq^-1, equal to the conductivity of ITO. Of importance here is that the low-cost anisotropic transparent wood film shows promising potential for electronics applications in solar cells, flexible displays, and other products.

Biomimetic Inks Based on Cellulose Nanofibrils and Cross-Linkable Xylans for 3D Printing

This paper presents a sustainable all-wood-based ink which can be used for 3D printing of constructs for a large variety of applications such as clothes, furniture, electronics, and health care products with a customized design and versatile gel properties. The 3D printing technologies where the material is dispensed in the form of liquids, so called inks, have proven suitable for 3D printing dispersions of cellulose nanofibrils (CNFs) because of their unique shear thinning properties. In this study, novel inks were developed with a biomimetic approach where the structural properties of cellulose and the cross-linking function of hemicelluloses that are found in the plant cell wall were utilized. The CNF was mixed with xylan, a hemicellulose extracted from spruce, to introduce cross-linking properties which are essential for the final stability of the printed ink. For xylan to be cross-linkable, it was functionalized with tyramine at different degrees. Evaluation of different ink compositions by rheology measurements and 3D printing tests showed that the degree of tyramine substitution and the ratio of CNFs to xylan-tyramine in the prepared inks influenced the printability and cross-linking density. Both two-layered gridded structures and more complex 3D constructs were printed. Similarly to conventional composites, the interactions between the components and their miscibility are important for the stability of the printed and cross-linked ink. Thus, the influence of tyramine on the adsorption of xylan to cellulose was studied with a quartz crystal microbalance to verify that the functionalization had little influence on xylan's adsorption to cellulose. Utilizing xylan-tyramine in the CNF dispersions resulted in all-wood-based inks which after 3D printing can be cross-linked to form freestanding gels while at the same time, the excellent printing properties of CNFs remain intact.

Aligning cellulose nanofibril dispersions for tougher fibers

Nanocomposite materials made from cellulose show a great potential as future high-performance and sustainable materials. We show how high aspect ratio cellulose nanofibrils can be efficiently aligned in extrusion to fibers, leading to increased modulus of toughness (area under the stress-strain curve), Young’s modulus, and yield strength by increasing the extrusion capillary length, decreasing its diameter, and increasing the flow rate. The materials showed significant property combinations, manifesting as high modulus of toughness (~28–31 MJ/m³) vs. high stiffness (~19–20 GPa), and vs. high yield strength (~130–150 MPa). Wide angle X-ray scattering confirmed that the enhanced mechanical properties directly correlated with increased alignment. The achieved moduli of toughness are approximately double or more when compared to values reported in the literature for corresponding strength and stiffness. Our results highlight a possibly general pathway that can be integrated to gel-spinning process, suggesting the hypothesis that that high stiffness, strength and toughness can be achieved simultaneously, if the alignment is induced while the CNF are in the free-flowing state during the extrusion step by shear at relatively low concentration and in pure water, after which they can be coagulated.

Highly Carboxylated Cellulose Nanofibers via Succinic Anhydride Esterification of Wheat Fibers and Facile Mechanical Disintegration

We report herein the preparation of 4-6 nm wide carboxyl-functionalized cellulose nanofibers (CNF) via the esterification of wheat fibers with cyclic anhydrides (maleic, phtalic, and succinic) followed by an energy-efficient mechanical disintegration process. Remarkable results were achieved via succinic anhydride esterification that enabled CNF isolation by a single pass through the microfluidizer yielding a transparent and thick gel. These CNF carry the highest content of carboxyl groups ever reported for native cellulose nanofibers (3.8 mmol g^-1). Compared to conventional carboxylated cellulose nanofibers prepared via Tempo-mediated oxidation of wheat fibers, the present esterified CNF display a higher molar-mass and a better thermal stability. Moreover, highly carboxylated CNF from succinic anhydride esterification were effectively integrated into paper filters for the removal of lead from aqueous solution and are potentially of interest as carrier of active molecules or as transparent films for packaging, biomedical or electronic applications.

Highly Anisotropic, Highly Transparent Wood Composites

For the first time, two types of highly anisotropic, highly transparent wood composites are demonstrated by taking advantage of the macro-structures in original wood. These wood composites are highly transparent with a total transmittance up to 90% but exhibit dramatically different optical and mechanical properties.