Cellulose Nanocrystals' Assembly under Ionic Strength Variation: From High Orientation Ordering to a Random Orientation

Tuning the phase separation of cellulose nanocrystals with hydrolysis times: influence of effective dimensions

This study attempts to quantify a relatively unexplored and very important subject: cellulose nanocrystal (CNC) bundles and their effective dimensions on phase separation and subsequent chiral resolution in CNC suspensions. Currently, there is little data discussing how effective bundle dimensions affect the onset of chiral nematic phase formation despite the fact that theory and experimental data indicate they are important factors. The effect of the extent of hydrolysis on the phase behavior of CNC suspensions was analyzed by correlating it with the critical weight concentration (w 0), which is the CNC weight corresponding to the onset of the chiral nematic phase. From Onsager theory and its extension, w 0 is primarily a function of CNC size while surface charge exerts a non-negligible effect. CNCs were produced from never-dried bleached softwood pulp under varying acid hydrolysis times to systematically alter sizes and surface charges. Concentration-dependent phase diagrams were mapped to ascertain the w 0 of the produced suspensions. The data revealed a clear decrease in w 0 when the hydrolysis time increased from 25 to 90 minutes, despite similar individual CNC size and increasing surface charges. This latter discovery following shape and size distribution indicated an increased area-equivalent (AE) diameter from extended hydrolysis, suggesting particle aggregation/bundling. This result was corroborated by elevated particle surface charges from enhanced lateral adherence between CNCs. In contrast to our findings that higher surface charge reduces the effective diameter, the observed decrease in w 0 suggests that an earlier onset of the anisotropic phase is driven by CNC bundles, which were more prevalent in samples with elevated surface charge. These observations indicate that CNC bundles play a significant role in promoting the anisotropic phase, counteracting the effect of surface charge on w 0. This work therefore provides invaluable insights into the complex interplay of CNC surface charge, shape, and size by shedding light on the importance of hydrolysis time on particle aggregation and phase behavior in CNC suspensions.

Stretchable Laminates with Tunable Structural Colors from Layered Stacks of Elastomeric, Ionic, and Natural Polymers

Natural polymers such as plant-derived cellulose nanocrystals (CNCs) are renowned for color iridescence due to their internal helical organization, but they show modest stretchability and bending abilities, because of the brittle nature of highly crystalline needlelike nanocrystals. Herein, we report the highly stretchable composite materials built from these nanocrystals and branched ionic polymers (bIPs) with terminal amine-terminated poly(N-isopropylacrylamide) (PNIPAM) stacked between elastomeric layers. These layered elastomeric composites preserve the high mechanical stretchability of polyurethane outer layers up to 150%. Furthermore, the toughness increased manyfold, due to the sequential initiation and arresting of concurrent transversal cracks within the reinforcing central nanocomposite layer. Moreover, vivid structural colors of CNC helical organization preserved within these laminated composites show the ability to respond to humidity and temperature. We suggest that these elastomeric composite laminates with preserved structural colors of helical nanocellulose organization can be considered to be promising candidates for demanding applications such as robust wearable sensors, flexible optical labels, and photonic devices.

Magneto-Responsive Chiral Optical Materials: Flow-Induced Twisting of Cellulose Nanocrystals in Patterned Magnetic Fields

Magnetic fields have been used to uniformly align the lyotropic chiral nematic (cholesteric) liquid crystalline (LC) phase of biopolymers to a global orientation and optical appearance. Here, we demonstrate that, in contrast, weak and patterned magnetic field gradients can create a complex optical appearance with the variable spatial local organization of needle-like magnetically decorated cellulose nanocrystals. The formation of optically patterned thin films with left- and right-handed chiral and achiral regions is observed and related to local magnetic gradient-driven vortices during LC suspension flow. We trace the localized flow directions of the magnetically decorated nanocrystals during evaporation-induced assembly, demonstrating how competing evaporation and field-induced localized flow affect the twisted organization within magnetically induced vortices. The simulations suggested that localized twisting inversion originates from the interplay between the direction and strength of the local-depth-related magnetic gradients and the receding front through peripheral magnetic gaps. We propose that this finding will lead to magnetically patterned photonic films.

Polysaccharide Nanocrystals-Based Chiral Nematic Structures: From Self-Assembly Mechanisms, Regulation, to Applications

Chiral architectures, one of the key structural features of natural systems ranging from the nanoscale to macroscale, are an infinite source of inspiration for functional materials. Researchers have been, and still are, strongly pursuing the goal of constructing such structures with renewable and sustainable building blocks via simple and efficient strategies. With the merits of high sustainability, renewability, and the ability to self-assemble into chiral nematic structures in aqueous suspensions that can be preserved in the solid state, polysaccharide nanocrystals (PNs) including cellulose nanocrystals (CNCs) and chitin nanocrystals (ChNCs) offer opportunities to reach the target. We herein provide a comprehensive review that focuses on the development of CNCs and ChNCs for the use in advanced functional materials. First, the introduction of CNCs and ChNCs, and cellulose- and chitin-formed chiral nematic organizations in the natural world, are given. Then, the self-assembly process of such PNs and the factors influencing this process are comprehensively discussed. After that, we showcased the emerging applications of the self-assembled chiral nematic structures of CNCs and ChNCs. Finally, this review concludes with perspectives on the challenges and opportunities in this field.

Modulation of Tannic Acid on the Cholesteric Structure of Cellulose Nanocrystals

The chiral nematic phase structure, formed by the self-assembly of cellulose nanocrystals (CNCs) in an aqueous suspension and maintained in a solid film, shows great potential for optical applications. To achieve complex structures in optical devices, it is crucial to subject CNCs to specific shearing processes, such as spinning and printing. Understanding the structural and property changes of the CNC liquid crystal phase in these processes is of utmost importance. In this study, we investigated the effect of adding tannic acid (TA) on the rheological properties and cholesteric phase structures of CNCs/TA mixed suspensions. By calculating the surface site interaction points, we observed that TA can adsorb onto the surface of CNC rods in suspensions through hydrogen bonding. Through characterization techniques, such as polarized optical microscopy, rheology, and synchrotron SAXS, we examined the effects of TA addition on the microstructure and rheological properties of the CNC liquid crystal phase and clarified the change relating to the system composition. Under the same CNC concentration, the volume fraction of the anisotropic phase, the pitch, and the rod spacing of the cholesteric phase were not significantly affected by the addition of TA. However, the system viscosity was significantly reduced with the appropriate amount of TA (2 wt %), in a wide range of CNC concentrations (up to 15 wt % CNCs). The flow indexes (n) in Region I and Region III of steady-state shear curves of CNCs/TA systems (11-15 wt % CNCs) were compared. Moreover, we introduced the well-established theoretical models for liquid crystal polymers to tentatively interpret Region I of the CNCs/TA cholesteric phase and realized that increased numbers of smaller cholesteric-phase domains in the CNCs/TA system and interfacial modification by TA may contribute to the fluidity change. The feature of the domain texture of CNCs/TA systems is verified by polarized optical microscopy observations.

Structural Color from Cellulose Nanocrystals or Chitin Nanocrystals: Self-Assembly, Optics, and Applications

Widespread concerns over the impact of human activity on the environment have resulted in a desire to replace artificial functional materials with naturally derived alternatives. As such, polysaccharides are drawing increasing attention due to offering a renewable, biodegradable, and biocompatible feedstock for functional nanomaterials. In particular, nanocrystals of cellulose and chitin have emerged as versatile and sustainable building blocks for diverse applications, ranging from mechanical reinforcement to structural coloration. Much of this interest arises from the tendency of these colloidally stable nanoparticles to self-organize in water into a lyotropic cholesteric liquid crystal, which can be readily manipulated in terms of its periodicity, structure, and geometry. Importantly, this helicoidal ordering can be retained into the solid-state, offering an accessible route to complex nanostructured films, coatings, and particles. In this review, the process of forming iridescent, structurally colored films from suspensions of cellulose nanocrystals (CNCs) is summarized and the mechanisms underlying the chemical and physical phenomena at each stage in the process explored. Analogy is then drawn with chitin nanocrystals (ChNCs), allowing for key differences to be critically assessed and strategies toward structural coloration to be presented. Importantly, the progress toward translating this technology from academia to industry is summarized, with unresolved scientific and technical questions put forward as challenges to the community.

Modulating the chiral nematic structure of cellulose nanocrystal suspensions with electrolytes

HYPOTHESIS

The iridescent optical properties of films made of cellulose nanocrystals (CNC) are controlled by the pitch and range of the chiral nematic structures. These are further tuned with the addition of electrolyte.

EXPERIMENTS

Electrolyte type, valency and concentration were varied. The bulk CNC suspension properties were investigated by combining rheology, polarised optical photography and microscopy, while the spacing between crystals was determined using SAXS.

FINDINGS

The addition of electrolyte to a CNC suspension containing chiral nematic structures first causes the nematic pitch to increase indicating the suspension has a weaker structure. Further increases in electrolyte concentration cause aggregation and complete breakdown of the chiral nematic structures. The univalent species cause larger changes to the chiral nematic structure with the onset and magnitude of structure breakdown occurring at lower ionic strengths compared with the divalent species. Cation size influences the chiral nematic structure with the order of influence being K+ > Na+ ≈ Ca2+ > Mg2+, which corresponds from the largest to smallest cation. This work demonstrates that both ion valency, concentration and species play a significant role in controlling the chiral nematic structures of CNC suspensions and will be a vital step in the development of CNC liquid crystals, optical materials and sensors.

Accelerating Cellulose Nanocrystal Assembly into Chiral Nanostructures

Cellulose nanocrystal (CNC) suspensions self-assembled into chiral nematic liquid crystals. This property has enabled the development of versatile optical materials with fascinating properties. Nevertheless, the scale-up production and commercial success of chiral nematic CNC superstructures face significant challenges. Fabrication of chiral nematic CNC nanostructures suffers from a ubiquitous pernicious trade-off between uniform chiral nematic structure and rapid self-assembly. Specifically, the chiral nematic assembly of CNCs is a time-consuming, spontaneous process that involves the organization of particles into ordered nanostructures as the solvent evaporates. This review is driven by the interest in accelerating chiral nematic CNC assembly and promoting a long-range oriented chiral nematic CNC superstructure. To start this review, the chirality origins of CNC and CNC aggregates are analyzed. This is followed by a summary of the recent advances in stimuli-accelerated chiral nematic CNC self-assembly procedures, including evaporation-induced self-assembly, continuous coating, vacuum-assisted self-assembly, and shear-induced CNC assembly under confinement. In particular, stimuli-induced unwinding, alignment, and relaxation of chiral nematic structures were highlighted, offering a significant link between the accelerated assembly approaches and uniform chiral nematic nanostructures. Ultimately, future opportunities and challenges for rapid chiral nematic CNC assembly are discussed for more innovative and exciting applications.

pH and Salt-Assisted Macroscopic Chirality Inversion of Gadolinium Coordination Polymer

The precise adjustment of handedness of helical architectures is important to regulate their functions. Macroscopic chirality inversion has been achieved in organic supramolecular systems by pH, metal ions, solvents, chiral and non-chiral additives, temperature, and light, but rarely in coordination polymers (CPs). In particular, salt-assisted macroscopic chirality inversion has not been reported. In this work, we carried out a systematic investigation on the role of pH and salt in regulating the morphology of CPs based on Gd(NO₃)₃ and R-(1-phenylethylamino)methylphosphonic acid (R-pempH₂). Without extra NO₃^-, the chirality inversion from the left-handed superhelix R-M to the right-handed superhelix R-P can be achieved by pH modulation from 3.2 to 3.8. The addition of NaNO₃ (2.0 eq) at pH 3.8 results in an inversion of chiral sense from R-P to R-M as a pure phase. To our knowledge, this is the first example of salt-assisted macroscopic helical inversion in artificial systems.

Uniformly aligned flexible magnetic films from bacterial nanocelluloses for fast actuating optical materials

Naturally derived biopolymers have attracted great interest to construct photonic materials with multi-scale ordering, adaptive birefringence, chiral organization, actuation and robustness. Nevertheless, traditional processing commonly results in non-uniform organization across large-scale areas. Here, we report magnetically steerable uniform biophotonic organization of cellulose nanocrystals decorated with superparamagnetic nanoparticles with strong magnetic susceptibility, enabling transformation from helicoidal cholesteric (chiral nematic) to uniaxial nematic phase with near-perfect orientation order parameter of 0.98 across large areas. We demonstrate that magnetically triggered high shearing rate of circular flow exceeds those for conventional evaporation-based assembly by two orders of magnitude. This high rate shearing facilitates unconventional unidirectional orientation of nanocrystals along gradient magnetic field and untwisting helical organization. These translucent magnetic films are flexible, robust, and possess anisotropic birefringence and light scattering combined with relatively high optical transparency reaching 75%. Enhanced mechanical robustness and uniform organization facilitate fast, multimodal, and repeatable actuation in response to magnetic field, humidity variation, and light illumination.

Naturally derived biopolymers attracted great interest to construct photonic materials but traditional processing commonly results in non-uniform organization across largescale areas. Here, the authors report a uniform biophotonic organization of cellulose nanocrystals decorated with superparamagnetic nanoparticles enabling transformation from helicoidal cholesteric to uniaxial nematic phase with near-perfect orientation.