Aqueous Suspensions Of Cellulose Nanocrystals Research Articles

Recent Progress on Photonic Cellulose Nanocrystal Films for Sensing Applications

Abstract: Cellulose nanocrystals (CNCs) have triggered considerable research interest in the last few years owing to their unique optical, biodegradation, and mechanical behavior. Herein, recent progress on the sensing application of photonic CNC films is summarized and discussed based on the analyses of the latest studies. We briefly introduce the three approaches for pre-paring CNCs: mechanical treatment, acid hydrolysis, and enzymatic hydrolysis, recapitulating their differences in preparation and properties. Then, when the aqueous suspension of cellu-lose nanocrystals (CNCs) reaches a specific concentration, it will self-assemble to form a left-handed nematic liquid crystal structure, and this structure can be maintained in films after water evaporation, which has strong photonic crystal properties. The periodic layered structure in the film interferes and diffracts with light, showing a rainbow color. Photonic CNC com-posites that combine CNCs and functional materials have good properties and broad prospects. Finally, we highlight the advanced applications of photonic CNC films, including mechanical sensing, thermal sensing, and humidity sensing. The prospects and ongoing challenges of pho-tonic CNC films were summarized.

Tannic Acid-Induced Gelation of Aqueous Suspensions of Cellulose Nanocrystals.

Nanocellulose hydrogels are a crucial category of soft biomaterials with versatile applications in tissue engineering, artificial extracellular matrices, and drug-delivery systems. In the present work, a simple and novel method, involving the self-assembly of cellulose nanocrystals (CNCs) induced by tannic acid (TA), was developed to construct a stable hydrogel (SH-CNC/TA) with oriented porous network structures. The gelation process is driven by the H-bonding interaction between the hydroxyl groups of CNCs and the catechol groups of TA, as substantiated by the atoms in molecules topology analysis and FTIR spectra. Interestingly, the assembled hydrogels exhibited a tunable hierarchical porous structure and mechanical moduli by varying the mass ratio of CNCs to TA. Furthermore, these hydrogels also demonstrate rapid self-healing ability due to the dynamic nature of the H-bond. Additionally, the structural stability of the SH-CNC/TA hydrogel could be further enhanced and adjusted by introducing coordination bonding between metal cations and TA. This H-bonding driven self-assembly method may promote the development of smart cellulose hydrogels with unique microstructures and properties for biomedical and other applications.

Uniaxial orientation of cellulose nanocrystals by zone-casting technique

Cellulose nanocrystals (CNCs) are biomass-based nanoparticles with attractive properties. Using a zone-casting technique, transparent films 2 cm wide and 4 cm long, and 2 µm thick, with uniaxially oriented CNCs were prepared from aqueous suspension of CNCs. The nanocrystals were aligned within the entire film perpendicular to the zone-casting direction. The orientation of the CNCs was confirmed by polarized light microscopy, X-ray diffraction and atomic force microscopy. The intensity of the light transmitted through the films depended on its polarization direction and was the strongest for the light polarized perpendicularly to the crystal orientation direction. The orientation of CNCs in the films resulting in optical anisotropy makes them promising materials for applications in optoelectronics.

Effect of Heating and Citric Acid on the Performance of Cellulose Nanocrystal Thin Films

Cellulose nanocrystals (CNCs) were extracted from bleached cotton by sulfuric acid hydrolysis. Thin films were prepared from the aqueous suspension of CNCs by casting and evaporation with 15% glycerol as a plasticizer. Our research aimed to create stable films resistant to water. The structure and the interactions of the films were modified by short (10 min) heating at different temperatures (100, 140, and 160 °C) and by adding different amounts of citric acid (0, 10, 20, and 30%). Various analytical methods were used to determine the structure, surface properties, and mechanical properties. The interaction of composite films with water and water vapor was also investigated. Heat treatment did not significantly affect the film properties. Citric acid, without heat treatment, acted as a plasticizer. It promoted the disintegration of films in water, increased water vapor sorption, and reduced tensile strength, resulting in flexible and easy-to-handle films. The combination of heat treatment and citric acid resulted in stable liquid-water-resistant films with excellent mechanical properties. A minimum heating temperature of 120 °C and a citric acid concentration of 20% were required to obtain a stable CNC film structure resistant to liquid water.

Kinetic arrest during the drying of cellulose nanocrystal films from aqueous suspensions analogous to the freezing of thermal motions

A comprehensive understanding of controlling the iridescence of cellulose films by manipulating the alignment and helical pitch of cellulose nanocrystals (CNCs) is required to advance cellulose photonics and its optoelectronic applications. Aqueous suspensions of CNCs exhibit a cholesteric liquid crystal (LC) phase with structural color; however, attaining a uniformly colored film is extremely difficult. Presumably, because multiple interrelated factors influence the CNC molecular alignment and helical pitch, existing models are not necessarily conclusive and remain a subject of debate. To eventually achieve homogeneously colored films, we compare aqueous CNC suspensions as a lyotropic liquid LC with thermotropic ones, and we spectroscopically confirm that the coloration of CNC droplets originates from the periodic CNC structure. The suspension drying process significantly influences the quality of iridescence of CNC films. Rapidly drying a droplet of a CNC suspension forms a concentric rainbow film, with red edges and a blue center, typical of the coffee-ring effect observed in air-dried films. By contrast, slow drying under controlled humidity, which reduces capillary flow, provides higher uniformity and a large blue area. Orbitally shaking films while drying under high humidity further improves the uniformity. Therefore, the evaporation rate significantly influences the thermodynamically stabilized helical pitch of CNCs, which determines the structural color. We qualitatively model the kinetic arrest induced by the rapid evaporation of lyotropic LCs in a manner equivalent to that induced by the rate of temperature change in thermotropic LCs and other materials.

Shear Rate Imaging Using A Polarization Camera And A Birefringent Aqueous Cellulose Nanocrystal Suspension

Two aqueous cellulose nanocrystal suspensions are used to experimentally measure the shear and strain rates in a two-dimensional fluid flow. Cellulose nanocrystals are rod-like particles that align when subjected to shear, whereas at rest, they are randomly orientated by Brownian motion. The alignment causes birefringence, a phenomenon also known as flow-induced birefringence. The amount of birefringence is measured using a rotatable linear polarizer and a polarization camera. The linear polarizer is rotated to nine different positions. At each position, light from a light source becomes linear polarized before entering the birefringent fluid. Because of the birefringence the state of polarization is changed. This change is measured by the polarization camera. From the nine measurements the two-dimensional birefringence distribution is determined and from the amount of birefringence the strain rate is derived with the help of the data published by Lane et al. (2022a). We define Λ_0 as the angle between the (maximum) strain rate and the direction of flow. A shearing flow leads to Λ_0=45°, whereas Λ_0=0° and Λ_0=90° describe a flow down the centerline of a symmetrically converging and diverging channel, respectively. The measured strain rates are compared to simulations. The reference data in (Lane et al., 2022a) was taken at Λ_0=45°. For Λ_0≈45°, measurements and simulations correspond well to each other. If Λ_0≈0° is assumed, measured strain rates appear too high whereas they are too low for Λ_0≈90°. These results may be of interest for two reasons. First, shear rate imaging gave satisfying results in areas where Λ_0≈45°. Therefore, we propose the study of shear rates in a two-dimensional shearing flow by means of the presented method. Second, the results indicate that the flow state affects the degree of particle alignment and thus birefringence. This finding is in line with the statements made by Wayland (1960, 1964). The study of particle alignment and its influencing factors is of interest, and the experimental approach presented here is thought to be practicable for such studies.

Preparation of Cellulose Nanocrystals from Jujube Cores by Fractional Purification.

Jujube cores are fiber-rich industrial waste. Dewaxing, alkali treatment, bleaching, and sulfuric acid hydrolysis were used to generate cellulose nanocrystals (CNCs) from the jujube cores in this study. The morphological, structural, crystallinity, and thermal properties of the fibers were investigated using FE-SEM, TEM, AFM, FT-IR, XRD, and TGA under various processes. CNCs’ zeta (ζ) potential and water contact angle (WAC) were also investigated. The findings demonstrate that non-fibrous components were effectively removed, and the fiber particles shrunk over time because of many activities. CNCs had a rod-like shape, with a length of 205.7 ± 52.4 nm and a 20.5 aspect ratio. The crystal structure of cellulose Iβ was preserved by the CNCs, and the crystallinity was 72.36%. The temperature of the fibers’ thermal degradation lowered during the operations, although CNCs still had outstanding thermal stability (>200 °C). Aside from the CNCs, the aqueous suspension of CNCs was slightly agglomerated; thus, the zeta (ζ) potential of the CNCs’ suspension was −23.72 ± 1.7 mV, and the powder had high hydrophilicity. This research will be valuable to individuals who want to explore the possibility for CNCs made of jujube cores.

Capillary Flow Characterizations of Chiral Nematic Cellulose Nanocrystal Suspensions.

Studying the flow-induced alignment of anisotropic liquid crystalline materials is of major importance in the 3D printing of advanced architectures. However, in situ characterization and quantitative measurements of local orientations during the 3D printing process are challenging. Here, we report a microfluidic strategy integrated with polarized optical microscopy (POM) to perform the in situ characterization of the alignment of cellulose nanocrystals (CNCs) under the shear-flow condition of the 3D printer's nozzle in the direct ink writing process. To quantify the alignment, we exploited birefringence measurements under white and monochromatic light. We show that the flow-induced birefringence patterns are significantly influenced by the initial structure of the aqueous CNC suspensions. Depending on the CNC concentration and sonication treatment, various structures can form in the CNC suspensions, such as isotropic, chiral nematic (cholesteric), and nematic (gel-like) structures. In the chiral nematic phase, in particular, the shear flow in the microfluidic capillary has a distinct effect on the alignment of the CNC particles. Our experimental results, complemented by hydrodynamic simulations, reveal that at high flow rates (Er ≈ 1000), individual CNC particles align with the flow exhibiting a weak chiral structure. In contrast, at lower flow rates (Er ≈ 241), they display the double-twisted cylinder structure. Understanding the flow effect on the alignment of the chiral liquid crystal can pave the way to designing 3D printed architectures with internal chirality for advanced mechanical and smart photonic applications.

Using rotation to organize cellulose nanocrystals inside a fiber.

We demonstrate for the first time that continuous rotation of a mixture of cellulose nanocrystals (CNCs) and monomer in a capillary tube results in well-organized structures. In the experiments, a capillary tube charged with an aqueous suspension of CNCs and hydroxyethyl acrylate was continuously rotated, then the structure was fixed in place by UV-initiated polymerization. The organization of the liquid crystalline structure that forms inside the tube depends on the rotation conditions and is captured in the polymer resin. The effects of rotation speed, rotation angle and CNC concentration were evaluated and are discussed based on fluid dynamic models. We demonstrate that it is possible to develop a core-shell fiber through this technique based on secondary Dean flow. The outer shell of the fiber shows well-ordered concentric rings with chiral nematic structure, while the inner core remains isotropic. Such fibers have potential applications in the field of optics. Overall, we demonstrate that rotation could be applied as a novel method to organize liquid crystals in a confined environment.

Water Dynamics in Composite Aqueous Suspensions of Cellulose Nanocrystals and a Clay Mineral Studied through Magnetic Resonance Relaxometry.

1H spin-lattice relaxation time (T1) measurements were performed to probe the dynamic behavior of water in aqueous suspensions of cellulose nanocrystals (CNCs) and a layered smectite clay mineral with different degrees of concentration. 1H-T1 experiments were carried out over a wide frequency domain, ranging from a few kilohertz to 500 MHz, with the aid of conventional and fast field cycling nuclear magnetic resonance (NMR) techniques. The experimental relaxometry data illustrate differences between the dynamic behavior of bulk water and that confined in the vicinity of CNC-clay surfaces. Clay alone in moderate concentration was found to enforce almost no effect on the water dynamics, whereas introducing CNCs to the system presented a significantly enhanced relaxivity. The modeling of the relaxation dispersions allowed the determination of dynamical processes and variables explaining the dynamic behavior of water in CNC-clay suspensions. It turned out that reorientations mediated by translational displacements are a leading NMR relaxation mechanism for water interacting with the surfaces of CNC-clay particles in the low-frequency domain. In the high-frequency regime, however, the inner-sphere paramagnetic relaxation mechanism dominates, which is caused by the interaction of water protons with dissolved Fe ions.

Sustainable and superhydrophobic cellulose nanocrystal-based aerogel derived from waste tissue paper as a sorbent for efficient oil/water separation

Extraction of cellulose nanocrystals (CNCs) from wastes is a valuable strategy from the environmental and economical points of view. Herein, a novel and cost-effective method for the preparation of superhydrophobic, environmentally friendly, and recyclable bio-aerogel by freeze-drying the aqueous suspension of CNCs extracted from waste tissue paper and polyvinyl alcohol (PVA) in the presence of hydrolyzed tetraethyl orthosilicate (TEOS) sol has been developed for the first time. The highly porous (98.42 %), ultralow density (0.017 g/cm3), and superhydrophobic (WCA of 154.93°±4.14) CNCs/PVA/TEOS aerogel can selectively remove oily contaminants from water (BET surface area of 76 m2/g). The aerogel showed a high sorption capacity in the range of 69−168 g/g for 6 oils and 8 organic solvents. The reusability experiments showed the aerogel could maintain more than 92 % of its sorption capacity even after 20 cycles of sorption-squeezing. The cyclic compressive stress-strain tests confirmed the good mechanical properties of the aerogel with 89 % of shape recovery after 50 cycles. The CNCs/PVA/TEOS aerogel can be used as a recyclable sorbent for removing oils/organic solvents from water.

Controlling Ultrafiltration Membrane Rejection via Shear-Aligned Deposition of Cellulose Nanocrystals from Aqueous Suspensions

Cellulose nanocrystals (CNCs) of 180 nm length and 8 nm diameter were deposited on porous supports by tangential flow filtration followed by salt permeation to form ultrafiltration membranes. At a high enough shear rate on the support surface, CNCs aligned in the direction of flow, showing a nematic order. The shear rates for transition to the nematic phase determined from rheology analysis, polarized optical microscopy, and membrane performance were consistent with one another, at ca. 10 s-1. Permeating an AlCl3 solution through the shear-aligned CNC deposit stabilized the CNC layer by screening repulsive electrostatic interactions, and the stable CNC layer was obtained. On changing the surface shear rate from 10 to 50 s-1, the order parameter of CNCs increased from 0.17 to 0.7 and the rejection for Blue Dextran (5 kDa) increased from 80.4 to 92.7% and that for β-lactoglobulin (18 kDa) increased from 89.6 to 95.4%. Hence, a simple and scalable method for controlling rejection properties of ultrafiltration membranes is developed, which uses aqueous CNC suspensions to form the selective layer.

Benchmarking Cellulose Nanocrystals Part II: New Industrially Produced Materials.

The demand for industrially produced cellulose nanocrystals (CNCs) has been growing since 2012, when CelluForce Inc. opened its inaugural demonstration plant with a production capacity of 1 tonne per day. Currently, there are 10 industrial CNC producers worldwide, each producing a unique material. Thus, academic researchers and commercial users alike must consider the properties of all available CNCs and carefully select the material which will optimize the performance of their desired application. To support these efforts, this article presents a thorough characterization of four new industrially produced CNCs including sulfated CNCs from NORAM Engineering and Constructors Ltd. (in cooperation with InnoTech Alberta and Alberta-Pacific Forest Industries Inc.) and Melodea Ltd., as well as carboxylated CNCs from Anomera Inc. and Blue Goose Biorefineries Inc. These materials were benchmarked against typical lab-made, sulfated CNCs. While all CNCs were similar in size, shape, crystallinity, and suspension quality, the sulfated CNCs had a higher surface charge density than their carboxylated counterparts, leading to higher colloidal stability. Additionally, significant differences in the rheological profiles of aqueous CNC suspensions, as well as CNC thermal stability and self-assembly behavior, were observed. As such, this article highlights both the subtle and significant differences between five CNC types and acts as a guide for end-users looking to optimize the performance of CNC-based materials.

Multi-axis alignment of Rod-like cellulose nanocrystals in drying droplets

HypothesisRadial capillary flow in evaporating droplets carry suspended nanoparticles to its periphery where they are deposited and form a coffee-ring. Rod-like nanoparticles seeking to minimize their capillary energy will align with their long-axis parallel to the contact line. Particles exhibiting electrostatic repulsion, such as cellulose nanocrystals (CNCs), establish a competition between capillary flow-induced impingement against a growing coffee-ring and entropic minimization leading to enhanced particle mobility. Therefore, balancing these effects by manipulating the local particle concentration in drying droplets should result in deposition with a controlled orientation of CNCs. ExperimentsThe dynamic local order in aqueous suspensions of CNCs in evaporating sessile droplets was investigated through time-resolved polarized light microscopy. The spatial distribution of alignment in deposited CNCs was explored as a function of nanoparticle concentration, droplet volume, initial degree of anisotropy, and substrate hydrophobicity. Computational analysis of the rotational Péclet number during evaporation was also investigated to evaluate any effects of shear-induced alignment. FindingsMultiple modes of orientation were identified suggesting local control over CNC orientation and subsequent properties can be attained via droplet-based patterning methods. Specifically, high local particle concentrations led to tangential alignment and lower local particle concentrations resulted in new evidence for radial alignment near the center of dried droplets.

Polymer Induced Gelation of Aqueous Suspensions of Cellulose Nanocrystals.

We investigated the gelation of cellulose nanocrystals (CNCs) in polyelectrolyte and neutral polymer solutions. Cellulose nanocrystals (CNCs) with half-ester sulfate groups produced by acid hydrolysis of wood pulp were used in this study. The microstructure of CNCs/polymer suspensions was investigated in semidilute concentration regimes by selecting carboxymethyl cellulose (CMC700) as an anionic polymer and poly(ethylene oxide) (PEO600) as a neutral polymer solution. Together with quartz crystal microbalance with dissipation monitoring (QCM-D), rheology, scanning electron microscopy (SEM), and cryo-transmission electron microscopy (cryo-TEM), we characterized CNCs-polymer interactions, the suspension microstructure, and the macroscopic gel flow. Significant viscosity increases at low shear rates coupled with high shear-thinning behaviors were observed in CNC colloid-CMC700 polymer mixtures, but not those CNCs in PEO600 solutions. The apparent differences between CNCs-CMC700 and CNCs-PEO600 mixtures were due to their chain confirmations. On the basis of the evaluations from STEM, cryo-TEM, and polarized optical microscopy, we proposed that the excess CMC700 molecules in solutions result in the depletion of CNCs and the formation of anisotropic domains.

Thermodynamics and kinetics of CO2 hydrate formation in the presence of cellulose nanocrystals with statistical treatment of data

Carbon dioxide hydrate was formed in aqueous suspensions of cellulose nanocrystals (CNC) and the induction time and rate of hydrate crystal growth was compared to data obtained when CO2 hydrate was formed in pure water (blank). CNC concentrations of 0.13, 0.5 and 0.75 wt% were employed to determine whether CNC could reduce the induction time and enhance hydrate growth rate as it has been reported for carbon nanotubes and other nanoparticle suspensions. Hydrate formation experiments were carried out in two different crystallizers. The first set-up was in semi-batch operation mode while the second set-up was in batch operation mode. The results show that the CNC did not have a statistically significant impact on CO2 hydrate formation. This study also raises questions about the validity of conclusions and claims made in the literature about the effect of carbon nanotubes and other nanoparticles on gas hydrate formation. Those claims were based on very small sample sizes and without statistical data analysis.

Heat-induced changes in cellulose nanocrystal/amino-aldehyde biocomposite systems

Cellulose nanocrystals (CNCs) were extracted from natural cellulosic fibres such as bleached cotton and flax with a controlled multi-step sulphuric acid hydrolysis. From the aqueous suspensions of CNCs, the biocomposite films were prepared by casting and evaporation, with an amino-aldehyde (AA) compound in a wide concentration range from 0 to 30%. The AA compound (dimethylol dihydroxy ethylene urea) was considered both as a cross-linker of the CNC and as a matrix polymer for the CNC-reinforced composite system. Two series of films were prepared using different polyols such as sorbitol and glycerol as plasticizers to improve tractability. Heat treatment of the films was performed at elevated temperatures ranging from 140 to 200 °C for 10 min. Results clearly proved that besides temperature, the factors affecting the response of CNC-based nanocomposites to heat treatment were the source of cellulose, the type of plasticizer and the amount of cross-linking agent. Films based on flax–CNC and plasticized with glycerol showed a higher increase in yellowness and a more significant decrease in haze than those derived from cotton–CNC and plasticized with sorbitol, respectively. The cross-linking agent (AA) had a moderating effect on the heat-induced changes of properties. Furthermore, thermal gravimetric analysis (TG) of films revealed that thermal stability of the CNC films improved considerably when AA was added and cross-linking occurred. The increase in Tmax was more significant for the flax–CNC films (from about 230 to 290 °C) than for the cotton–CNC ones (from about 250 to 280 °C).

Quantitative Calorimetric Studies of the Chiral Nematic Mesophase in Aqueous Cellulose Nanocrystal Suspensions

Aqueous suspensions of cellulose nanocrystals (CNC) can spontaneously form a chiral nematic mesophase at a critical concentration (c*). Unfortunately, no current analytical technique permits rapid detection of c*. Herein, we introduce a facile and accurate approach to assess c* rapidly (< 2 h) from a small sample volume and compare our results to those obtained by conventional methods. Our strategy employs isothermal titration calorimetry (ITC) to measure the heat of interaction, which can identify the onset of mesophase formation since it is sensitive to suspension viscosity and thus capable of detecting small changes in the suspension environment. We measure c* for CNC samples differing in surface charge and aspect ratio and find that lower aspect ratios and higher surface charges both increase c*. Our ITC results reveal the role of CNC interactions prior to visual observation of mesophase formation and elucidate effects related to nanocrystals and their suspensions.

Nonlinear viscoelastic characterization of charged cellulose nanocrystal network structure in the presence of salt in aqueous media

The change in ionic strength of cellulose nanocrystal (CNC) suspensions is shown to contribute to a respective change in colloidal behavior, such as stiffness and fractal gelation. In this study, dynamic colloidal behavior and stability of aqueous CNC suspensions and their correlation with nonlinear viscoelastic properties of the CNC gel structures in the presence of different concentrations of sodium chloride (NaCl) salt were investigated. The microstructure of CNC/salt suspensions/gels were investigated with a wide range of characterization technique. To obtain further insight into the network structure of CNC/salt systems, for the first time, nonlinear rheology of the suspensions/gels was analyzed to correlate macro-mechanical viscoelastic response of the CNC/salt aqueous systems to structural changes as a response to strain. The intra-cycle viscoelasticity, explained utilizing qualitative Lissajous–Bowditch plots and quantitative nonlinear parameters, demonstrates a strong dependence of the nonlinear response of the samples to salt concentration, CNC concentration, and frequency of deformation. Higher intra-cycle nonlinearity was observed upon increasing the salt loading.

Retrieving the Coassembly Pathway of Composite Cellulose Nanocrystal Photonic Films from their Angular Optical Response.

Aqueous suspensions of cellulose nanocrystals (CNCs) are known to self-assemble into a chiral nematic liquid crystalline phase, leading to solid-state nanostructured colored films upon solvent evaporation, even in the presence of templating agents. The angular optical response of these structures, and therefore their visual appearance, are completely determined by the spatial arrangement of the CNCs when the drying suspension undergoes a transition from a flowing and liquid crystalline state to a kinetically arrested state. Here, it is demonstrated how the angular response of the final film allows for retrieval of key physical properties and the chemical composition of the suspension at the onset of the kinetic arrest, thus capturing a snapshot of the past. To illustrate this methodology, a dynamically evolving sol-gel coassembly process is investigated by adding various amounts of organosilica precursor, namely, 1,2-bis(trimethoxysilyl)ethane. The influence of organosilica condensation on the kinetic arrest can be tracked and thus explains the angular response of the resulting films. The a posteriori and in situ approach is general; it can be applied to a variety of additives in CNC-based films and it allows access to key rheological information of the suspension without using any dedicated rheological technique.