Cellulose Nanocrystal Materials Research Articles

Structural, Optical, and Mechanical Insights into Cellulose Nanocrystal Chiral Nematic Film Engineering by Two Assembly Techniques.

Iridescent cellulose nanocrystal (CNC) films with chiral nematic nanostructures exhibit great potential in optical devices, sensors, painting, and anticounterfeiting applications. CNCs can assemble into a chiral nematic liquid crystal structure by evaporation-assisted self-assembly (EISA) and vacuum-assisted self-assembly (VASA) techniques. However, there is a lack of comprehensive examinations of their structure-property correlations, which are essential for fabricating materials with unique properties. In this work, we gained insights into the optical, mechanical, and structural differences of CNC films engineered using the two techniques. In contrast to the random self-assembly at the liquid-air interface in EISA, the continuous external pressure in the VASA process forces CNCs to assemble at the filter-liquid interface. This results in fewer defects in the interfaces between tactoids and highly ordered cholesteric phases. Owing to the distinct CNC assembly behaviors, the films prepared by these two methods show great differences in the nanostructure, microstructure, and macroscopic morphology. Consequently, the highly ordered cholesteric structure gives VASA-CNC films a more uniform structural color and enhanced mechanical performance. These fundamental understandings of the relationship of structure-property nanoengineering through various assembly techniques are essential for designing and constructing high-performance chiral iridescent CNC materials.

Understanding the environmental impacts of large-scale cellulose nanocrystals production: Case studies in regions dependent on renewable and fossil fuel energy sources

Favorable functional properties of cellulose nanocrystals (CNCs) in several end-use applications as well as its ‘green’ credential as a bio-based material, stimulate significant interest in scaling up the manufacture of CNCs. As in any other process, there exist several design options for the overall process and decisions on adoption of a specific plant configuration should be based on the economic as well as the environmental data, preferably from the life cycle assessment (LCA) perspective. In this study we establish a benchmark LCA of a conceptual large-scale CNC manufacturing process based on sulfuric acid hydrolysis. We then employ the benchmark process model to explore several plant configurations under different acid recycling scenarios and targeting two different CNC materials, revealing key findings: (1) without acid recycling environmental impacts are dominated by input materials; (2) introducing acid recycling shifts environmental impacts from input materials to energy consumption: thus, complete acid recycling requires ∼ 14 times more energy; (3) based on the LCA study we recommend optimizing plant configuration through partial acid recycling for a balance between materials and energy consumption. Regions with abundant low-carbon energy, like Quebec and Sweden, favour high acid recycling, while fossil-dominated regions benefit from limited acid recycling. This study provides benchmark values of LCA impacts of CNC manufacture, which could be used for the assessment of carbon footprint and other environmental metrics of final products manufactured from CNCs.

Recent Advances in Flexible CNC-Based Chiral Nematic Film Materials.

Cellulose nanocrystal (CNC) is a renewable resource derived from lignocellulosic materials, known for its optical permeability, biocompatibility, and unique self-assembly properties. Recent years have seen great progresses in cellulose nanocrystal-based chiral photonic materials. However, due to its inherent brittleness, cellulose nanocrystal shows limitations in the fields of flexible materials, optical sensors and food freshness testing. In order to solve the above limitations, attempts have been made to improve the flexibility of cellulose nanocrystal materials without destroying their structural color. Despite these progresses, a systematic review on them is lacking. This review aims to fill this gap by providing an overview of the main strategies and the latest research findings on the flexibilization of cellulose nanocrystal-based chiral nematic film materials (FCNM). Specifically, typical substances and methods used for their preparation are summarized. Moreover, different kinds of cellulose nanocrystal-based composites are compared in terms of flexibility. Finally, potential applications and future challenges of flexible cellulose nanocrystal-based chiral nematic materials are discussed, inspiring further research in this field.

The Impact of Fibre Oxidation on the Preparation of Cellulose Nanocrystals (CNC)

This study aimed to evaluate the effect of fibre oxidation on the extent of substituted sulfate on cellulose nanocrystals (CNC). In this investigation, fully bleached softwood (SW) and hardwood (HW) pulps from an Alberta pulp mill were oxidized under low (4%), medium (10%), and the higher-end of medium (14%) oxygen consistencies. The oxidized pulp samples were hydrolyzed with concentrated sulfuric acid under conventional procedures to produce CNC. The CNC materials were then characterized by different qualitative and quantitative techniques to evaluate the effect of oxidations on the number of substituted sulfates and the overall sulfate charge of isolated CNC to develop novel high-value applications. The experimental data show that fibre oxidation helped improve the overall CNC yield, with lower half-ester sulfate contents compared to the controls. The half-ester sulfate contents and the viscosity for SW CNC were found to be higher than their HW CNC counterparts. However, the thermal stability of CNC was found to be better for HW than SW pulps. The emerging data help to prepare and engineer CNC tailored to specific applications.

Responsive Chiral Photonic Cellulose Nanocrystal Materials

AbstractResponsive photonic crystals have attracted significant attention in fundamental scientific research and technological applications. Upon applying external stimuli, such as mechanical, electrical, magnetic, and optical triggers, the optical properties of photonic crystals can be actively tuned. Among a large number of photonic crystals, cellulose nanocrystals (CNCs) are considered one of the most promising materials due to their renewability, simplicity of preparation, and unprecedented chirality. A comprehensive overview of the fundamental design principles, methodologies, responsive mechanisms, and practical applications of responsive chiral photonic CNC materials is presented here. The changes in helix pitch, refractive index, orientation, and resulting structural and optical properties of chiral photonic CNCs in response to various stimuli are discussed. Thereafter, novel applications of responsive chiral photonic CNCs, such as colorimetric sensors, photonic actuators, rewritable photonic papers, and anti‐counterfeiting smart tags are described. We conclude by discussing future challenges and opportunities for developing high‐performance responsive chiral photonic CNC materials.

Theoretical study of cellulose II nanocrystals with different exposed facets

Derived from the most abundant natural polymer, cellulose nanocrystal materials have attracted attention in recent decades due to their chemical and mechanical properties. However, still unclear is the influence of different exposed facets of the cellulose nanocrystals on the physicochemical properties. Herein, we first designed cellulose II nanocrystals with different exposed facets, the hydroxymethyl conformations distribution, hydrogen bond (HB) analysis, as well as the relative structural stability of these models (including crystal facets {A, B, O} and Type-A models vary in size) are theoretically investigated. The results reveal that the HB network of terminal anhydroglucose depends on the adjacent chain’s contact sites in nanocrystals exposed with different facets. Compared to nanocrystals exposed with inclined facet, these exposed with flat facet tend to be the most stable. Therefore, the strategy of tuning exposed crystal facets will guide the design of novel cellulose nanocrystals with various physicochemical properties.

Ultrasonication pretreatment assisted rapid co-assembly of cellulose nanocrystal and metal ion for multifunctional application

Co-assembly of metal ion and cellulose nanocrystals (CNC) is a promising strategy to fabricate novel iridescent CNC materials with advanced applications. By combining ultrasonication pretreatment and vacuum-assisted self-assembly (VASA) technique, a facile and rapid strategy is proposed to prepare the Mn2+-doped carboxylated CNC (C-CNC) iridescent films with multifunctional application. The ultrasonication pretreatment temporarily disassembles the aggregates of C-CNC nanorods caused by the electrostatic interaction between negative charged C-CNC and Mn2+. The subsequent VASA process accelerates the self-assembly of chiral liquid crystals prior to the re-agglomeration of C-CNC by the bridge effect of Mn2+. Furthermore, the as-prepared Mn2+/CNC film exhibits a rapid and visible color change in ammonia atmosphere along with the formation of MnO2. The reversible change can be realized by the stimulation of reducing agent. The derived MnO2/C-CNC composite film displays efficient removal of methylene blue dye in aqueous solution by both of adsorption and degradation procedure.

Orienting Cellulose Nanocrystal Functionalities Tunes the Wettability of Water-Cast Films.

Cellulose nanocrystal (CNC)-based materials display apparently erratic wetting behaviors with contact angle (CA) variations as large as 30° from sample to sample. This work hypothesizes that it is the orientation of CNC amphiphilic functionalities at the interface with air that causes the variability in CA. By exploiting relationships with the Hansen solubility parameter theory, a set of surface tension parameters is proposed for both the polar and the non-polar surfaces of cellulose Iβ nanocrystals. These coefficients elucidate the wettability of CNC materials by establishing a correlation between the wetting properties of the air/sample interface and its chemical composition in terms of non-polar moieties. Advancing/receding CA experiments suggest that, while spin-coating CNC suspensions yield purely polar films, oven-casting them produces amphiphilic surfaces. We proposed a mechanism where the state of dispersion (individual or agglomerated) in which CNCs reach the air/water interface during casting is the determining factor: while individual nanocrystals find it more stable to orient their non-polar surfaces toward the interface, the aspect ratio of CNC agglomerates favors an orientation of their polar surfaces. This represents the first compelling evidence of CNC orientation at an interface and can be applied to Pickering emulsions and nanocomposites and to the production of CNC materials with tuned wettability.

Chameleon-Inspired Variable Coloration Enabled by a Highly Flexible Photonic Cellulose Film.

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 compression-responsive 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.

Thermostable and Redispersible Cellulose Nanocrystals with Thixotropic Gelation Behavior by a Facile Desulfation Process

As a ecofriendly and promising nanomaterial, cellulose nanocrystals (CNCs) have attracted significant interest. However, the intrinsic sulfate-modified surface and self-aggregation behavior of the most common acid-hydrolyzed CNCs with sulfuric acid lead to the low thermostability and poor redispersion of solid-state CNCs materials, which limit their practical applications. In this work, we propose a simple and effective strategy to not only remove the sulfates in CNCs but also resuscitate their dispersibility by a facile post-treatment process, including an acid-triggering gelation, solvent exchange with ethanol, and supercritical CO2 drying. The resultant CNC powder exhibits a high thermostability, much higher than that of the common acid-hydrolyzed CNCs and similar to that of the pristine cellulose. Moreover, the CNC powder can be readily redispersed in various solvents by a short ultrasonic treatment. Subsequently, as the CNC concentration increases, CNC gels spontaneously form and exhibit thixotropic behaviors. Taking advantage of these characteristics of the above desulfated CNC powder, we fabricated a CNC/Ag nanoparticle gel, magnetic CNC/Fe3O4 nanoparticle gel, pressure-sensitive polymer/CNC composite gel, and colorless and transparent poly(lactic acid)/CNC thermoplastic films. Therefore, the highly thermostable, redispersible, and easy-to-gel CNC powder has a huge potential as a building block for various stable, well-dispersed, and functional nanocomposite materials.

Effect of Purification Methods on Commercially Available Cellulose Nanocrystal Properties and TEMPO Oxidation

Cellulose nanocrystals (CNCs) are attractive for use in polymer composites, biomedical applications, and barrier packaging. In all of these applications they are mixed with other components and compatibility is a major design consideration, as CNCs naturally have a high density of surface hydroxyl groups and primarily disperse well in polar media. Numerous surface modification approaches have been used to address these issues, but challenges remain due to the variability in the commercially available CNC materials. CNCs can be produced from biomass using several extraction methods, most notably acid hydrolysis and biomass extraction, also known as the American Value Added Pulping process. The production method of the CNC material has an impact on both physical and surface properties of CNCs, including size, shape, crystal structure, and zeta potential. In addition, post-treatments can be used to purify the CNC material and further alter these properties. This work studies the properties of CNCs from three different commercial suppliers and after conducting three different post-treatments: dialysis, Soxhlet extraction, and acetone washing to understand the effect of the commercial source and purification on CNC surface properties and modification via 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) mediated oxidation. We show that there is significant variation in CNC physical and surface properties between different commercial suppliers before and after purification. Importantly, we show that for CNCs produced through acid hydrolysis, acetone washing or Soxhlet extraction in ethanol decreases the achievable degree of TEMPO modification, but makes it more consistent between the different commercial suppliers. This has important implications for improving reproducibility in CNC research as well as aiding the expanding fields of applications.

Fracture Properties Evaluation of Cellulose Nanocrystals Cement Paste.

Due to the need for high-performance and sustainable building materials, the investigation of the determination of fracture toughness of cement paste using new and sustainable materials, such as cellulose nanocrystals (CNCs) is worthwhile. Contrary to other well-known nano-reinforcement particles, such as carbon nanotubes, CNCs are less toxic; therefore, they have less safety and environmental risks. Fracture behavior of cement paste has been studied intensively for a long time. However, the incorporation of new materials in the cement paste, such as cellulose nanocrystal materials (CNCs), has not been fully investigated. In this paper, the fracture behavior, compressive strength, and hydration properties of cement paste reinforced with cellulose nanocrystal particles were studied. At the age of 3, 7, and 28 days, a three-point bending moment test, and a calorimetry and thermogravimetric analysis, scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX) analysis were performed on the water-to-binder-weight ratio of 0.35 cement paste, containing 0.0%, 0.2%, and 1.0% volume cellulose nanocrystals. Results indicated that the fracture properties and compressive strength were improved for the sample containing 0.2% CNCs. Preliminary results indicate that CNCs can improve the fracture behavior of cementitious materials and can be considered as a renewable and sustainable material in construction.

Recent Progress in Cellulose Nanocrystal Alignment and Its Applications.

Cellulose nanocrystal (CNC) research has addressed increased attention in the bioprocessing community, especially the utility of these materials in different fields, including biomedical and materials applications. In this review, the different preparation methods and properties of CNC materials will be initially briefly discussed. The latter focuses special attention on the alignment of CNCs and its applications. Different methods to induce CNC alignment, including self-assembly, shear-based alignment, magnetic and electric field alignment, spin-coating, dip-coating, etc., are reviewed in detail, followed by the different characterization techniques to quantify the alignments of CNCs. The current and potential applications of aligned CNCs, such as fillers to enhance the mechanical and thermal properties of biocomposites, photonic materials to deal with structural colors and enhancement in optical properties of iridescent films, and reduced wettability for high-resolution printing, are further discussed to provide more insights into this promising research direction. We hope that this review will shine light on CNC alignment and also open the door to different types of applications.

Degradation of pesticides using amine-functionalized cellulose nanocrystals

A series of amine-functionalized cellulose nanocrystal materials were successfully synthesized, characterized, and evaluated for the remediation of pesticide contaminants from organic and aqueous media. Their ability to degrade malathion in organic systems has been examined, resulting in up to 100% degradation of the compound into detectable lower molecular weight by-products. A poly(ethylenimine) cellulose nanocrystal (CNC-PEI) material was also capable of degrading aqueous solutions of malathion, deltamethrin, and permethrin with 100%, 95%, and 78% degradation, respectively. Thus, these materials can potentially serve as a new and viable remediation technique based on their ability to effectively degrade various pesticides. The reusability of the CNC-PEI was also explored. The CNC-PEI material maintained its ability to degrade malathion throughout two wash and re-use cycles.

Lignin-containing cellulose nanocrystals/sodium alginate beads as highly effective adsorbents for cationic organic dyes

Cellulose nanocrystals (CNCs) is an exciting class of sustainable and carbohydrate material, which has great potential applications in molecular adsorption. However, the complex preparation process and limited adsorption capacity of CNCs hinder its commercial application. In this study, we design a novel functional cellulose nanocrystals-based adsorbent by an ingenious mixing of lignin-containing cellulose nanocrystals (LCNCs), sodium alginate (SA), and calcium chloride solution. Benefiting from the sulfonate groups of lignin, carboxyl groups of SA, the maximum adsorptive capability of LCNCs/SA beads for methylene blue was found to be 1181 mg g−1, which was significantly higher than previously reported biomass-based adsorbents. More importantly, LCNCs/SA beads can be reused several times. This strategy can not only improve the adsorption performance of CNCs-based materials, but also simplify the production technology of CNCs, which greatly promote the commercial application of CNCs materials.

Research needs for nanocellulose commercialization and applications

This short review deals with some applications and research needs for nanocellulosic (NC) materials; primarily cellulose nanocrystals (CNC), cellulose nanofibers (CNF), and bacterial cellulose (BC). Whereas CNC and BC materials are fairly homogenous, CNF materials represent a wide sector of different materials, often with a high heterogeneity. This is due to different pretreatment methods (mechanical, chemical, enzymatic), woodbased or agricultural-based materials, delignification and bleaching procedures, etc. The purpose of this comprehensive review is not to discuss the various production methods, for which the reader may consult with a selected number of reviews [1-6]; thus, the focus is on practical applications. Practical applications and potential markets were also discussed some years ago by other investigators [7-8]. Upscaling and choice of pretreatment methods, as well as economic considerations and different business models, have also been discussed, along with: • Toxicity and environmental issues [9-10] • The complex characterization of cellulose nanomaterials [4] The reader should also be aware of new contenders to the three classic groups of cellulosic nanomaterials, which are already in a commercial phase. These include cellulose filaments [11-12] and materials from mechanical grinding processes [13], and these materials may be nanostructures or not, depending on our classification. Finally, as indicated by the editorial on p. 275, scientists are currently taking a deep dive into the fundamental features of nanocellulosic materials [14-15].

Amine functionalization of cellulose nanocrystals for acid–base organocatalysis: surface chemistry, cross-linking, and solvent effects

Sulfuric acid-derived cellulose nanocrystals (CNCs) are investigated as an alternative support for bifunctional heterogeneous organocatalyst materials. CNC catalysts are developed through direct surface functionalization by TEMPO-mediated oxidation and subsequent EDC amide coupling with alkyl diamines, used to impart both acid and base character to CNC surfaces. Sulfate ester content is controlled by partial acid hydrolysis prior to chemical functionalization, optionally reducing the initial sulfur content by approximately 50%. The resultant acidic and basic catalytic moieties are analyzed qualitatively by FT-IR spectroscopy and quantitatively by conductometric titration and elemental analysis. Relatively high degrees of substitution with amine functionality are achieved with diamine loadings around 0.9 mmol/g. This synthesis partially cross-links the CNCs, the extent of which is analyzed by dynamic light scattering of materials functionalized with 2–6 carbon spacer alkyl diamines. These CNC materials are demonstrated to be effective acid–base catalysts for an aldol condensation test reaction in batch liquid phase kinetic experiments. Relatively slow reaction rates were achieved in water:acetone reaction mixtures, in which decreased sulfate ester content is shown to decrease catalytic activity. Significant rate improvements are achieved by elimination of rate limiting transport and miscibility issues by solvent exchange to organic-only acetonitrile:acetone co-solvent reaction mixtures. Cross-linking and limited acid–base cooperativity are shown to decrease activity in CNCs functionalized with too long and too short diamines, respectively. This study demonstrates CNCs as alternative supports for bifunctional heterogeneous organocatalysis with site-specific activity on the order of that of well-studied aminosilica organocatalysts.

Cellulose nanocrystals with different morphologies and chiral properties

The study reports the morphology and optical properties of a variety of cellulose nanocrystals (CNC) obtained from different natural sources with different dimensions and composition. A library of CNCs components with different dimensions, sulfate contents, and crystallite sizes was prepared under identical hydrolysis conditions from five distinct sources representing traditional choices ranging from soft and hard wood pulps to microcrystalline cellulose. High-resolution atomic force microscopy, scanning electron microscopy, scanning transmission electron microscopy, and X-ray diffraction confirmed that all CNCs have a well-defined needle-like morphology with different aspect ratios and axi-asymmetric cross-sections. Varying the cellulose sources resulted in wide variability of the CNC dimensions, including length (120–210 nm), aspect ratio (30–70), height (2.9–3.6 nm), and width (6–11 nm). Specifically, the CNCs from microcrystalline sources have large cross-sectional dimensions and produce straight CNC bundles, but CNCs from wood pulps have small cross-sections and form twisted bundles of a few individual nanocrystals. Chemical composition and surface potentials were found to be less critical to the resulting chiral characteristics and structural colors while the CNCs with high aspect ratios form chiral films with large pitch values and thus longer wavelengths of selective reflection. Such flexible chiral CNC materials with controlled optical signature can be further considered for development of advanced materials for colorimetric sensors, tunable and active photonic materials, optical coatings, chiral inks and 3D printed photonic structures.

New Orleans hosts ACS national meeting

Chemists from around the world gathered in the Crescent City for the 255th ACS National Meeting & Exposition on March 18–22. With more than 16,600 attendees, this meeting was the largest that New Orleans has ever seen as a host city for ACS. The meeting’s theme, “Nexus of Food, Energy & Water,” focused on chemistry’s role in achieving global food, energy, and water security. Roughly 8,200 papers and 5,000 posters were presented in programming by 29 technical divisions and seven committees. Among the meeting’s highlights, Emily Cranston of McMaster University presented The Kavli Foundation Emerging Leader in Chemistry Lecture on her research on developing hybrid cellulose nanocrystal materials for emulsions, injectable hydrogels, and cross-linked aerogels. Angela Belcher of Massachusetts Institute of Technology delivered The Fred Kavli Innovations in Chemistry Lecture on using nature and biomaterials as inspiration to design biological hybrid multidimensional materials. Other notable symposia and special e...

Dielectric Characterization of Confined Water in Chiral Cellulose Nanocrystal Films.

A known deterrent to the large-scale development and use of cellulose nanocrystals (CNCs) in composite materials is their affinity for moisture, which has a profound effect on dispersion, wetting, interfacial adhesion, matrix crystallization, water uptake, and hydrothermal stability. To quantify and control the hydration and confinement of absorbed water in CNCs, we studied sulfated-CNCs neutralized with sodium cations and CNCs functionalized with less hydrophilic methyl(triphenyl)phosphonium cations. Films were cast from water suspensions at 20 °C under controlled humidity and drying rate, yielding CNC materials with distinguishably different dielectric properties and cholesteric structures. By controlling the evaporation rate, we obtained self-assembled chiral CNC films with extended uniformity, having helical modulation length (nominal pitch) tunable from 1300 to 600 nm. SEM imaging and UV-vis-NIR total reflectance spectra revealed tighter and more uniform CNC packing in films cast at slow evaporation rates or having lower surface energy when modified with phosphonium. The dielectric constant was measured by a noncontact microwave cavity perturbation method and fitted to a classical mixing model employing randomly oriented ellipsoidal water inclusions. The dielectric constant of absorbed water was found to be significantly smaller than that for free liquid indicating a limited mobility due to binding with the CNC "matrix". In the case of hydrophilic Na-modified CNCs, a decreasing pitch led to greater anisotropy in the shape of moisture inclusions (ellipsoidal to platelet-like) and greater confinement. In contrast, the structure of hydrophobic phosphonium-modified CNC films was found to have reduced pitch, yet the shape of confined water remained predominantly spherical. These results provide a useful perspective on the current state of understanding of CNC-water interactions as well as on CNC self-assembly mechanisms. More broadly, we believe that our results are beneficial for the realization of CNC-based functional materials and composites.