Self-assembly of cellulose nanocrystals confined to square capillaries

Biological systems exploit restricted degrees of freedom to drive self-assembly of nano- and microarchitectures. Simplified systems, such as colloidal nanoparticles that behave as lyotropic liquid crystalline mesophases in confined geometric spaces, may be used to mimic biological structures. Cellulose nanocrystals (CNCs) are colloidally stable nanoparticles that self-assemble into chiral nematic (ChN) liquid crystalline mesophases. To date, the self-assembly of ChN mesophases of CNCs has been studied under confinement conditions within curved surfaces or under drying conditions that impose curvatures that can be exploited to control ChN ordering; however, their self-assembly has not been investigated in geometries with square cross-sections under static conditions. Here, we show that because of surface anchoring on perpendicular surfaces, the ChN CNC phase is unable to bend with the 90° angle of the square capillary under increasing confinement. Instead, the ChN phase forms radial layers in the shape of concentric squircle shells. With increasing layer distance from the capillary wall, the squircles transition into concentric cylinder shells. In larger capillaries, the radial shell layers appear as a continuous spiral pattern that engulfs fragmented ChN pseudolayers, a defect to accommodate the cylindrical confinement of the mesophase. These results are useful for understanding the fundamentals of self-assembling systems and development of new technologies.

Cathodic electrodeposition of organic nanocomposite coatings reinforced with cellulose nanocrystals

A systematic examination of the structure-property relations to create nanocomposite coatings via electrophoretic deposition (EPD) of a cathodic polymer - for instance, polyacrylate - reinforced with cellulose nanocrystals (CNCs) is discussed in this work. EPD, a scalable and green technique, has also been used in our approach to polymerize dopamine in the presence of CNCs and cathodic polymer to improve adhesion properties. This study investigated the interactions between CNCs, polydopamine (PDA) and the cathodic polyacrylic polymer to elucidate the dispersion state of the nanoparticles in the system by carefully examining ζ-potential, particle size, and electrophoretic mobility dependence in the colloidal suspension. We examined the morphology and structure of the electrodeposited nanocomposites using SEM and XPS, and the results indicate the formation of a compact, homogeneous structure. The incorporation of CNCs or PDA introduces different levels of roughness to the surface as revealed by AFM. Our results indicate improved adhesion, as determined by bond strength, using CNCs (>20%) or CNC-PDA (>30%) relative to the cathodic polymer, as well as significant improvement in hardness. The technique and materials used show promise to develop industrial organic coatings with low emissions for applications in automotive coatings and other applications.

Halloysite nanotubes enhance the mechanical properties and thermal stability of iridescent cellulose nanocrystal films

Films of cellulose nanocrystals (CNCs) with chiral nematic organization can show vivid iridescence that arises from their hierarchical structure. Unfortunately, the brittleness of the films limits their potential applications. In this paper, we investigate the incorporation of halloysite nanotubes (HNTs) into CNC films to prepare organic-inorganic composite films with enhanced mechanical properties, while preserving the chiral nematic structure and brilliant iridescence. The hybrid composite films containing 10 wt% HNTs are more elastic than pristine CNC films, with a 1.3-fold increase in tensile strength and a 1.6-fold increase in maximum strain. As well, the incorporation of HNTs slightly improves the thermal stability of the composite films. These materials mimic the hybrid composite structures of crab shells, leading to enhanced mechanical properties and thermal stability of CNC films while maintaining iridescence.

Mechanically Responsive Circularly Polarized Luminescence from Cellulose-Nanocrystal-Based Shape-Memory Polymers

Stimulus-responsive materials that display circularly polarized luminescence (CPL) have attracted great attention for application in chiral sensors and smart displays. However, due to difficulties in the regulation of chiral structures, fine control of CPL remains a challenge. Here, it is demonstrated that cellulose nanocrystal shape-memory polymers (CNC-SMPs) with luminescent components enable mechanically responsive CPL. The chiral nematic organization of CNCs in the material gives rise to a photonic bandgap. By manipulating the photonic bandgap or luminescence wavelengths of the luminescent CNC-SMPs, precise control of CPL emission with varied wavelengths and high dissymmetry factors (g_lum ) is achieved. Specifically, CPL emission can be switched reversibly by treating the luminescent CNC-SMPs with hot-pressing and recovery by heating. Pressure-responsive CPL with tunable g_lum values is ascribed to the pressure-responsive photonic bandgaps. Moreover, colorimetric and CPL-active patterns are created by imprinting desired forms into SMP samples. This study demonstrates a novel way to fabricate smart CPL systems using biomaterials.

Cellulose Nanocrystal Gels with Tunable Mechanical Properties from Hybrid Thermal Strategies

Gels are useful materials for drug delivery, wound dressings, tissue engineering, and 3D printing. These various applications require gels with different mechanical properties that can be easily tuned, also preferably excluding the use of chemical additives, which can be toxic or harmful to the body or environment. Here, we report a novel strategy to synthesize cellulose nanocrystal (CNC) gels with tunable mechanical properties. Sequential freeze-thaw cycling and hydrothermal treatments were applied to CNC suspensions in different orders to give a series of pristine CNC hydrogels. Freeze-drying of the hydrogels also afforded a series of lightweight CNC aerogels. The mechanical properties of the hydrogels and aerogels were studied by rheological measurements and compression strength tests, respectively. Specifically, the complex modulus of CNC hydrogels ranged from 160 to 32,000 Pa among eight different hydrogels, while Young's modulus of CNC aerogels was tuned from 0.114 to 3.98 MPa across five different aerogels. The microstructures of aerogels were also investigated by scanning electron microscopy and X-ray microtomography, which revealed remarkable differences between the materials. Solvent sorption-desorption tests showed that the reinforced networks have excellent stability over the basic CNC aerogels in ethanol, demonstrating a material enhancement from the preparation strategies we developed. Thermal conductivity and thermal stability for these materials were also investigated, and it was found that the lowest thermal conductivity was 0.030 W/m K, and all of the aerogels are generally stable below 280 °C. These characteristics also expand the potential applications of this family of CNC gels to lightweight supporting materials and thermal insulators.

Coassembly of Cellulose Nanocrystals and Neutral Polymers in Iridescent Chiral Nematic Films

Photonic materials based on composite films of cellulose nanocrystals (CNCs) and polymers are promising as they can be renewable and show tunable optical and mechanical properties. However, the influence of polymers on CNC self-assembly is not always well understood, and conflicting results are present in the literature. In this study, we incorporate three neutral, water-soluble polymers-poly(ethylene glycol) (PEG), poly(vinyl pyrrolidone) (PVP), and poly(acrylic acid) (PAA)-with different molecular weights into CNC suspensions at various concentrations prior to obtaining iridescent composite thin films by solvent evaporation. Through spectroscopic, potentiometric, and rheological analyses, we find that PVP physically adsorbs to the surface of CNCs resulting in a bathochromic shift in film color with both increasing concentration and polymer molecular weight. In contrast, PEG induces depletion interactions that result in a decrease in the size of chiral nematic CNC domains, with a negligible change in film color. Finally, PAA hydrogen bonds to the hydroxyl groups of CNCs, resulting in a bathochromic color shift along with interesting rheological and liquid-state properties. This work demonstrates a deeper understanding of CNC-polymer interactions during coassembly and formation of iridescent chiral nematic films, allowing for greater control over optical properties of future CNC-based materials.

How hydrogen-bonding interactions and nanocrystal aspect ratios influence the morphology and mechanical performance of polymer nanocomposites reinforced with cellulose nanocrystals

The performance of polymer nanocomposites reinforced with cellulose nanocrystals (CNCs) is complicated by several factors, primarily CNC-polymer and polymer-polymer interactions. Our current work specifically seeks to address the effects of CNC geometry, CNC-polymer and polymer-polymer interactions on the structure and non-linear mechanical performance of nanocomposites prepared using two water-soluble polymers, polyethylene oxide (PEO) and polyvinyl alcohol (PVA), having different morphological and structural characteristics. PEO and PVA are chosen since they are compatible with CNCs, however, they interact quite differently with CNCs and result in different reinforcement mechanisms. PEO and PVA interact with CNCs via a nucleating effect and H-bonding, which influence the polymer structure in two opposite directions. The nucleating effect tends to lead the polymer chains to form more ordered structures, whereas H-bonding interactions restrict the mobility of the polymer chains. Since PEO has weaker interactions amongst molecular chains than PVA, the properties of PEO are more significantly influenced by CNCs.

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.

Fast Self-Assembly of Scalable Photonic Cellulose Nanocrystals and Hybrid Films via Electrophoresis

Nano-enabled, bio-based, functional materials are key for the transition to a sustainable society as they can be used, owing to both their performance and nontoxicity, to gradually replace existing nonrenewable engineering materials. Cellulose nanocrystals (CNCs), produced by acid hydrolysis of cellulosic biomass, have been shown to possess distinct self-assembly, optical, and electromechanical properties, and are anticipated to play an important role in the fabrication of photonic, optoelectronic, and functional hybrid materials. To facilitate CNCs' technological viability, a method suitable for industrial exploitation is developed to produce photonic films possessing long-range chirality on conductive, rigid, or flexible, substrates within a few minutes. The approach is based on electrophoretic deposition (EPD)-induced self-assembly of CNCs, where photonic films of any size can be produced by controlling CNC surface properties and EPD parameters. CNC film coloration can be determined by the CNC aqueous suspension characteristics, while their reflected intensity can be tuned by changing the duration and number of electrodeposition cycles. EPD-induced self-assembly of CNCs is compatible with in situ reduction of gold precursors without the need to use additional reducing agents (some of which are considered toxic), thereby allowing the preparation of hybrid photonic films with tunable plasmonic response in a one-pot process.

A rheological investigation of oil-in-water Pickering emulsions stabilized by cellulose nanocrystals

HYPOTHESIS

High and medium internal phase Pickering emulsions stabilized with cellulose nanocrystals (CNCs) exhibited very different performance compared to their peers stabilized with a surfactant. In this paper, we ascribed the difference to the formation of hydrogen bonding and van der Waals interactions between the CNC nanoparticles on adjacent oil droplets.

EXPERIMENTS

Rheological properties of CNC-stabilized oil-in-water medium internal phase emulsions (MIPEs, oil content = 65% v/v) and high internal phase emulsions (HIPEs, oil content = 80% v/v) were comprehensively characterized using both oscillatory and rotational tests.

FINDINGS

It was found that in the MIPEs, the van der Waals and hydrogen bonding interactions dominate the emulsion properties, whereas the compact structure of oil droplets plays a more important role in the HIPEs. CNC concentration in the aqueous phase also affects the emulsion properties, especially for the HIPEs, and the results can be correlated to the stabilization mechanisms we previously reported. The information from these tests provides a much-needed guidance for the practical application of CNC-stabilized emulsions.

Effect of thermal treatments on chiral nematic cellulose nanocrystal films

The ability to manipulate the optical appearance of materials is essential in virtually all products and areas of technology. Structurally coloured chiral nematic cellulose nanocrystal (CNC) films proved to be an excellent platform to design optical appearance, as their response can be moulded by organising them in hierarchical architectures. Here, we study how thermal treatments influence the optical appearance of structurally coloured CNC films. We demonstrate that the CNCs helicoidal architecture and the chiral optical response can be maintained up to 250 °C after base treatment and cross-linking with glutaraldehyde, while, alternatively, an exposure to vacuum allows for the helicoidal arrangement to be further preserved up to 900 °C, thus producing aromatic chiral carbon. The ability to retain the helicoidal arrangement, and thus the visual appearance, in CNC films up to 250 °C is highly desirable for high temperature colour-based industrial applications and for passive colorimetric heat sensors. Similarly, the production of chiral carbon provides a new type of conductive carbon for electrochemical applications.

Concentric chiral nematic polymeric fibers from cellulose nanocrystals

Hierarchical biological materials, such as osteons and plant cell walls, are complex structures that are difficult to mimic. Here, we combine liquid crystal systems and polymerization techniques within confined systems to develop complex structures. A single-domain concentric chiral nematic polymeric fiber was obtained by confining cellulose nanocrystals (CNCs) and hydroxyethyl acrylate inside a capillary tube followed by UV-initiated polymerization. The concentric chiral nematic structure continues uniformly throughout the length of the fiber. The pitch of the chiral nematic structure could be controlled by changing the CNC concentration. We tracked the formation of the concentric structure over time and under different conditions with variation of the tube orientation, CNC concentration, CNC type, and capillary tube size. We show that the inner radius of the capillary tube is important and a single-domain structure was only obtained inside small-diameter tubes. At low CNC concentration, the concentric chiral nematic structure did not completely cover the cross-section of the fiber. The highly ordered structure was studied using imaging techniques and X-ray diffraction, and the mechanical properties and structure of the chiral nematic fiber were compared to a pseudo-nematic fiber. CNC polymeric fibers could become a platform for many applications from photonics to complex hierarchical materials.

Thermal Degradation of Cellulose Filaments and Nanocrystals

Cellulose-derived materials, such as microcellulose and nanocellulose, are sustainable materials with a wide range of applications. Here, through a multi-analytical approach, we investigate the thermal degradation of microfibrillar cellulose filaments (CFs); acidic cellulose nanocrystals (CNC-H), containing sulfate half-ester groups on the surface; and neutralized cellulose nanocrystals (CNC-Na), where the protons are replaced by sodium ions. CFs have a simple degradation mechanism, associated with extensive dehydration, decarboxylation, and decarbonylation, and the highest thermal stability of the three (∼325 °C) despite the abundance of amorphous regions and inhomogeneous fibrous mass that make them structurally and morphologically less homogeneous than high-crystallinity CNCs. CNC-H decompose in a complex way below 200 °C, with large char fractions and evaporation of sulfur compounds at high temperatures, while sodium counterions in CNC-Na can improve the thermal stability up to 300 °C, where the pyrolysis leads to partial rehydration and formation of sodium hydroxide on the surface.

Thermal Degradation of Cellulose Filaments and Nanocrystals

Cellulose-derived materials, such as microcellulose and nanocellulose, are sustainable materials with a wide range of applications. Here, through a multi-analytical approach, we investigate the thermal degradation of microfibrillar cellulose filaments (CFs); acidic cellulose nanocrystals (CNC-H), containing sulfate half-ester groups on the surface; and neutralized cellulose nanocrystals (CNC-Na), where the protons are replaced by sodium ions. CFs have a simple degradation mechanism, associated with extensive dehydration, decarboxylation, and decarbonylation, and the highest thermal stability of the three (∼325 °C) despite the abundance of amorphous regions and inhomogeneous fibrous mass that make them structurally and morphologically less homogeneous than high-crystallinity CNCs. CNC-H decompose in a complex way below 200 °C, with large char fractions and evaporation of sulfur compounds at high temperatures, while sodium counterions in CNC-Na can improve the thermal stability up to 300 °C, where the pyrolysis leads to partial rehydration and formation of sodium hydroxide on the surface.

Properties and stabilization mechanism of oil-in-water Pickering emulsions stabilized by cellulose filaments

Cellulose filaments (CFs) are produced from wood-pulp fibres through solely mechanical treatment. They are inhomogeneous materials comprising both relatively large fibre fragments and fine fibrils. CFs can stabilize oil-in-water medium- and high-internal phase Pickering emulsions, where the presence of fibre fragments plays a critical role in emulsion stabilization by preventing CF fibrils from forming highly entangled structures. The emulsion properties are shown to be influenced by both the refining energy and CF concentration. CFs are distinct from cellulose nanofibrils (CNF) or microfibrillated cellulose (MFC), which flocculate and collapse owing to the entanglement of the homogeneous fibrous mass. Typically, other additives - surfactants or cellulose nanocrystals - are necessary to stabilize such emulsions. Further, lignin-containing CFs, produced from unbleached pulps, are particularly beneficial for emulsion stabilization at higher pH when the functional groups on lignin molecules are ionized. Our findings present CFs as potentially inexpensive, sustainable, and efficient emulsifiers for a wide variety of systems, from food to personal care products or engineering materials.

Tunable Diffraction Gratings from Biosourced Lyotropic Liquid Crystals

Diffraction gratings are important for modern optical components, such as optical multiplexers and signal processors. Although liquid crystal (LC) gratings based on thermotropic LCs have been extensively explored, they often require expensive molecules and complicated manufacturing processes. Lyotropic LCs, which can be broadly obtained from both synthetic and natural sources, have not yet been applied in optical gratings. Herein, a facile grating fabrication method using a biosourced lyotropic LC formed by cellulose nanocrystals (CNCs), a material extracted from plants, is reported. Hydrogel sheets with vertically aligned uniform periodic structures are obtained by fixing the highly oriented chiral nematic LC of CNCs in polymer networks under the cooperative effects of gravity on phase separation and a magnetic field on LC orientation. The hydrogel generates up to sixth-order diffraction spots and shows linear polarization selectivity, with tunable grating periodicity controlled through LC concentration regulation. This synthesis strategy can be broadly applied to various grating materials and opens up a new area of optical materials from lyotropic LCs.

Improving Thin-Film Properties of Poly(vinyl alcohol) by the Addition of Low-Weight Percentages of Cellulose Nanocrystals

The increased demand for electronic devices, combined with a desire to minimize the environmental impact, necessitates the development of new eco-friendly materials. One promising approach is the incorporation of renewable and green materials that possess the desired mechanical and electrical properties while allowing for more ecologically friendly disposal of these devices. The addition of low-weight percentages (0.25-0.75 wt %) of cellulose nanocrystals (CNCs) was investigated as an environmentally friendly additive in aqueous dispersions of poly(vinyl alcohol) (PVA). It was found that these low CNC loadings were sufficient to induce a favorable increase in viscosity, which in turn dramatically enhanced the film quality of the PVA blends through an improvement in the critical radius of the spun film, overall film thickness, and homogeneity of the thin film. This corresponded to an increase in the number of functioning organic electronic devices that could be fabricated by spin coating, including metal-insulator-metal (MIM) capacitors and organic thin-film transistors (OTFTs). Most importantly, the incorporation of CNCs into PVA did not significantly alter the native dielectric properties of the polymer thin films when incorporated into both MIM capacitors and OTFTs.

Iridescent Cellulose Nanocrystal Films Modified with Hydroxypropyl Cellulose

The introduction of polymers into a chiral nematic cellulose nanocrystal (CNC) matrix allows for the tuning of optical and mechanical properties, enabling the development of responsive photonic materials. In this study, we explored the incorporation of hydroxypropyl cellulose (HPC) into a CNC film prepared by slow evaporation. In the composite CNC/HPC thin films, the CNCs adopt a chiral nematic structure, which can selectively reflect certain wavelengths of light to yield a colored film. The color could be tuned across the visible spectrum by changing the concentration or molecular weight of the HPC. Importantly, the composite films were more flexible than pure CNC films with up to a ten-fold increase in elasticity and a decrease in stiffness and tensile strength of up to six times and four times, respectively. Surface modification of the films with methacrylate groups increased the hydrophobicity of the films, and therefore, the water stability of these materials was also improved.

Host-Guest Chemistry Within Cellulose Nanocrystal Gel Receptors

Cellulose nanocrystals (CNCs) spontaneously assemble into gels when mixed with a polyionic organic or inorganic salt. Here, we have used this ion-induced gelation strategy to create functional CNC gels with a rigid tetracationic macrocycle, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ ). Addition of [CBPQT]Cl₄ to CNCs causes gelation and embeds an active host inside the material. The fabricated CNC gels can reversibly absorb guest molecules from solution then undergo molecular recognition processes that create colorful host-guest complexes. These materials have been implemented in gel chromatography (for guest exchange and separation), and as elements to encode 2- and 3-dimensional patterns. We anticipate that this concept might be extended to design a set of responsive and selective gel-like materials functioning as, for instance, water-pollutant scavengers, substrates for chiral separations, or molecular flasks.

Microsuspension Polymerization of Styrene Using Cellulose Nanocrystals as Pickering Emulsifiers: On the Evolution of Latex Particles

We report a mechanistic study of the microsuspension polymerization of styrene stabilized by cellulose nanocrystals (CNCs) in its native form as well as graft-modified with copolymers of styrene and N-3-(dimethylamino)propyl methacrylamide (DMAPMAm) or N,N-(diethylamino)ethyl methacrylate (DEAEMA). Native CNCs and graft-modified CNCs were shown to form stable styrene emulsions with an average droplet diameter of 18-20 and 5-9 μm, respectively. Initiators of widely varying water solubilities [2,2'-azobisisobutyronitrile (AIBN), 2-2'-azobis(2,4-dimethylvaleronitrile) (Vazo-52), and lauroyl peroxide (LPO)] were employed for the polymerizations. The type of initiator and the type of CNC were shown to directly affect the microsuspension polymerization kinetics, particle size, and molecular weight distribution. Using AIBN and Vazo-52, submicron latex particles were observed in the final latex in addition to the desired 3-20 μm CNC-armored microsuspension particles. The resulting latex and microsuspension polystyrene particles were studied for their CNC coverage and surface charge. We found that the presence of CNCs in the aqueous phase did not lead to Pickering emulsion polymerization by heterogeneous nucleation.

Cellulose Nanocrystal Elastomers with Reversible Visible Color

Responsive photonic crystals have potential applications in mechanical sensors and soft displays; however, new materials are constantly desired to provide new innovations and improve on existing technologies. To address this, we report stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible color upon the application of mechanical stress. When stretched (or compressed) the colorless materials maintain their chiral nematic structure but the helical pitch is reduced into the visible region, resulting in coloration of the CNC-elastomer composite. By increasing the percentage elongation of the material (ca. 50-300 %), the structural color can be tuned from red to blue. The color of the materials was characterized by reflectance optical microscopy and reflectance circular dichroism to confirm the wavelength and polarization of the reflected light. We also probed the mechanism of the structural color using 2D-X-ray diffraction. Finally, by either water-patterning the starting CNC film, or by forming a CNC film with gradient color, through masked evaporation, we were able to prepare encoded stretchable chiral nematic CNC-elastomers.

Freeze-Thaw Gelation of Cellulose Nanocrystals

Gels are attractive for applications in drug delivery, tissue engineering, and 3D printing. Here, physical colloidal gels were prepared by freeze-thaw (FT) cycling of cellulose nanocrystal (CNC) suspensions. The aggregation of CNCs was driven by the physical confinement of CNCs between growing ice crystal domains. FT cycling was employed to form larger aggregates of CNCs without changing the surface chemistry or ionic strength of the suspensions. Gelation of CNC suspensions by FT cycling was demonstrated in water and other polar solvents. The mechanical and structural properties of the gels were investigated using rheometry, electron microscopy, X-ray diffraction, and dynamic light scattering. We found that the rheology could be tuned by varying the freezing time, the number of FT cycles, and concentration of CNCs in suspension.

Pressure-Responsive Hierarchical Chiral Photonic Aerogels

Pressure-responsive chiral photonic aerogels are fabricated by combining liquid crystal self-assembly and ice-templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure-induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent-sensitive ink that shows quick color response to different solvents is designed using the pressure-responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.

Solid-state 23Na NMR spectroscopy studies of ordered and disordered cellulose nanocrystal films

Cellulose nanocrystal films with either disordered or chiral nematic structures of varying helical pitch were investigated using ²³Na solid-state nuclear magnetic resonance (NMR) spectroscopy. Spin lattice relaxation of ¹H correlated with ²³Na analyzed by indirect observation using polarization transfer from ¹H nuclei to ²³Na nuclei showed that the Na⁺ cations are well hydrated in the cellulose nanocrystal films. Linewidth analysis in solid-state ²³Na NMR showed that the Na⁺ cations move in confined spaces, and that the Na⁺ cations in the film having disordered structure are more dynamic than in the films having ordered structure. From lineshape analysis of the ²³Na 2D nutation NMR spectra, we can distinguish the Na⁺ environments within the ordered and disordered films, and find trends in anisotropic interaction parameters between ordered samples with different pitches. These are the first detailed ²³Na NMR spectroscopic studies of CNC-Na⁺ films, and they show that this technique may be a powerful probe for characterizing the extent of order in nanocellulose samples.

Unwinding a spiral of cellulose nanocrystals for stimuli-responsive stretchable optics

Cellulose nanocrystals (CNCs) derived from biomass spontaneously organize into a helical arrangement, termed a chiral nematic structure. This structure mimics the organization of chitin found in the exoskeletons of arthropods, where it contributes to their remarkable mechanical strength. Here, we demonstrate a photonic sensory mechanism based on the reversible unwinding of chiral nematic CNCs embedded in an elastomer, leading the materials to display stimuli-responsive stretchable optics. Vivid interference colors appear as the film is stretched and disappear when the elastomer returns to its original shape. This reversible optical effect is caused by a mechanically-induced transition of the CNCs between a chiral nematic and pseudo-nematic arrangement.

Boundary Geometry Effects on the Coalescence of Liquid Crystalline Tactoids and Formation of Topological Defects

In many lyotropic liquid crystals, the evolution of macroscopic anisotropic phases is mediated by tactoids, which are discrete ordered microdroplets existing in continuous disordered phases. Here we report the effects of boundary conditions on the movement and transformation of liquid crystalline tactoids of cellulose nanocrystals (CNCs) in nonspherical droplets. Using an in situ photopolymerization method, we obtained three-dimensional views of the initial emergence and expansion of macroscopic ordered phases. These processes, as well as the evolution of topological defects, were significantly influenced by the boundary geometry (or Marangoni flows and pinning effects) of the droplets. This phenomenon helps explain the influence of the substrate on the photonic properties of chiral nematic films of CNCs and may also provide new insights into the self-assembly process in lyotropic liquid crystals.

Current characterization methods for cellulose nanomaterials

A new family of materials comprised of cellulose, cellulose nanomaterials (CNMs), having properties and functionalities distinct from molecular cellulose and wood pulp, is being developed for applications that were once thought impossible for cellulosic materials. Commercialization, paralleled by research in this field, is fueled by the unique combination of characteristics, such as high on-axis stiffness, sustainability, scalability, and mechanical reinforcement of a wide variety of materials, leading to their utility across a broad spectrum of high-performance material applications. However, with this exponential growth in interest/activity, the development of measurement protocols necessary for consistent, reliable and accurate materials characterization has been outpaced. These protocols, developed in the broader research community, are critical for the advancement in understanding, process optimization, and utilization of CNMs in materials development. This review establishes detailed best practices, methods and techniques for characterizing CNM particle morphology, surface chemistry, surface charge, purity, crystallinity, rheological properties, mechanical properties, and toxicity for two distinct forms of CNMs: cellulose nanocrystals and cellulose nanofibrils.

Aerogel materials with periodic structures imprinted with cellulose nanocrystals

Novel aerogel materials with periodic structures derived from chiral nematic liquid crystalline cellulose nanocrystals (CNCs) are reported. The liquid crystalline structure of phase-separated CNCs is locked by a simple solvent exchange method or silica condensation. Both cellulose and silica/cellulose aerogel materials were obtained after critical point drying, and subsequent calcination of the silica/cellulose composite afforded a silica aerogel with periodic order. Gas adsorption and electron microscopy studies revealed that these materials have high surface areas and a unique chiral nematic structure imparted from the helicoidal CNC template. This is a new, scalable approach to aerogel materials with highly anisotropic structures. The high porosity and periodic, chiral features of these new materials may make them suitable for applications that require anisotropic properties or as hard templates for the construction of other ordered aerogels.

Investigation of the formation mechanisms in high internal phase Pickering emulsions stabilized by cellulose nanocrystals

Medium and high internal phase Pickering emulsions stabilized by cellulose nanocrystals (CNCs) have been prepared and the effects of CNC concentration and type of oil phase on the properties of emulsions were studied. The maximum oil phase volume that can be stabilized by CNCs is 87% when the CNC concentration is 0.6 wt.%; this slightly decreases to 83% when the CNC concentration is increased to 1.2 wt.% or higher. In addition, the oil droplets stabilized with 0.6 wt.% CNC suspensions have a larger size than those stabilized with higher concentration CNC suspensions. As evidenced by the change in oil droplet morphology and size, two different emulsion formation mechanisms are proposed. For a CNC concentration of 0.6 wt.%, the extra oil added into the emulsion is accommodated by the expansion of oil droplet size, whereas for CNC concentrations of 1.2 wt.% and higher, the oil is stabilized mainly by the formation of new oil droplets.

Tactoid Annealing Improves Order in Self-Assembled Cellulose Nanocrystal Films with Chiral Nematic Structures

The self-assembly process in cellulose nanocrystal (CNC) film formation was studied as a function of evaporation time. It is known that the total evaporation time of CNC dispersions affects the structure of the film obtained, but the extension of different phases of the evaporation has not been explored. By extending the evaporation time of CNC suspensions after the onset of liquid crystallinity, the homogeneity of the resulting films could be improved as observed by polarized optical microscopy and scanning electron microscopy. Here, we show that an intermediate stage of self-assembly, between phase separation and gel vitrification, called tactoid annealing, helps explain the discrepancies in order for chiral nematic CNC films dried at varying evaporation times. This intermediate stage of self-assembly may be useful for designing highly ordered and homogenous CNC-based materials.

Transparent Depolarizing Organic and Inorganic Films for Optics and Sensors

Fabrication of novel organic and inorganic depolarizing films derived from quasinematic cellulose nanocrystal (CNC) organization is demonstrated. These films convert linearly polarized and circularly polarized light into unpolarized light over the entire visible region. Patterning of the quasinematic CNCs on top of a chiral nematic film gives latent images that are revealed only upon observation through the circularly polarizing filters.

Flexible Photonic Cellulose Nanocrystal Films

The fabrication of self-assembled cellulose nanocrystal (CNC) films of tunable photonic and mechanical properties using a facile, green approach is demonstrated. The combination of tunable flexibility and iridescence can dramatically expand CNC coating and film barrier capabilities for paints and coating applications, sustainable consumer packaging products, as well as effective templates for photonic and optoelectronic materials and structures.

Shape Memory Cellulose-Based Photonic Reflectors

Biopolymer-based composites enable to combine different functionalities using renewable materials and cost-effective routes. Here we fabricate novel thermoresponsive photonic films combining cellulose nanocrystals (CNCs) with a polydiolcitrate elastomer exhibiting shape memory properties, known as hydroxyl-dominant poly(dodecanediol-co-citrate) (PDDC-HD). Iridescent films of CNCs are first made by evaporation-induced self-assembly, then embedded in the PDDC-HD prepolymer, and finally cured to obtain a cross-linked composite with shape memory properties. The fabricated samples are characterized by polarized optical microscopy, scanning electron microscopy, and thermomechanical cycling. The obtained hybrid material combines both intense structural coloration and shape memory effect. The association of stiff cellulose nanocrystals and soft polydiolcitrate elastomer enhances the overall mechanical properties (increased modulus and reduced brittleness). This hybrid nanocomposite takes advantage of two promising materials and expands their possibilities to cover a wide range of potential applications as multiresponsive devices and sensors. As they perform from room to body temperatures, they could be also good candidates for biomedical applications.

Magnetic Mesoporous Photonic Cellulose Films

Novel hybrid materials of cellulose and magnetic nanoparticles (NPs) were synthesized and characterized. The materials combine the chiral nematic structural features of mesoporous photonic cellulose (MPC) with the magnetic properties of cobalt ferrite (CoFe2O4). The photonic, magnetic, and dielectric properties of the hybrid materials were investigated during the dynamic swelling and deswelling of the MPC films. It was observed that the dielectric properties of the generated MPC films increased tremendously following swelling in water, endorsing efficient swelling ability of the generated mesoporous films. The high magnetic permeability of the developed MPC films in conjunction with their superior dielectric properties, predominantly in the swollen state, makes them interesting for electromagnetic interference shielding applications.