Naturally derived cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) are emerging nanomaterials that display high strength, high surface area, and tunable surface chemistry, allowing for controlled interactions with polymers, nanoparticles, small molecules, and biological materials. Industrial production of nanocelluloses is increasing rapidly with several companies already producing on the tons-per-day scale, intensifying the quest for viable products across many sectors. While the hydrophilicity of the nanocellulose interface has posed a challenge to the use of CNCs and CNFs as reinforcing agents in conventional plastics, it is a significant benefit for creating reinforced or structured hydrogel composites (or, when dried, aerogels) exhibiting both mechanical reinforcement and a host of other desirable properties. In this context, this Review describes the quickly growing field of hydrogels and aerogels incorporating nanocelluloses; over 200 references are summarized in comprehensive tables covering the chemistry, preparation, properties, and applications of “nanocellulose-only” and “nanocellulose-containing” gels. Physical and chemical cross-linking strategies, postmodification steps, and routes to control gel structure are discussed, along with key developments and ongoing challenges in the field. Nanocellulose hydrogels and aerogels show great promise in a wide range of biomedical, energy storage, construction, separations, cosmetic, and food applications.

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.

Abstract:The renewability, biocompatibility and mechanical properties of cellulose nanocrystals (CNCs) have made them an attractive material for numerous composite, biomedical and rheological applications. However, for CNCs to shift from laboratory curiosity to commercial applications, researchers must transition from CNCs extracted at the bench scale to material produced at an industrial scale. There are a number of companies currently producing kilogram to ton per day quantities of sulfuric acid-hydrolyzed CNCs, as well as other nanocelluloses, as described herein.With the recent intensification of industrially produced CNCs, the variety of cellulose sources, hydrolysis methods and purification procedures, characterization of these materials becomes critical. This has further been justified by the past two decades of research which demonstrate that CNC stability and behaviour is highly dependent on surface chemistry, surface charge density and particle size. This work outlines key test methods that should be employed to characterize these properties to ensure a "known" starting material and consistent performance.Of the sulfuric acid-extracted CNCs examined, industrially produced material compared well with laboratory-made CNCs, exhibiting similar charge density, colloidal and thermal stability, crystallinity, morphology and self-assembly behaviour. In addition, it was observed that further purification of CNCs, using Soxhlet extraction in ethanol, had minimal impact on nanoparticle properties and is unlikely to be necessary for many applications. Overall the current standing of industrially produced CNCs is positive suggesting that the evolution to commercial scale applications will not be hindered by CNC production.

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An environmentally friendly procedure for the surface modification of cellulose nanocrystals (CNCs) in water is presented. Tannic acid (TA), a plant polyphenol, acts as the primer when mixed with CNCs in suspension, which are then reacted with decylamine (DA), the hydrophobe. Schiff base formation/Michael-type addition covalently attaches primary amines with long alkyl tails to CNC-TA, increasing the particle hydrophobicity (contact angle shift from 21° to 74°). After modification, the CNC-TA-DA particles in water phase separate, allowing for easy collection of modified material. The dried product is readily redispersible in toluene and other organic solvents, as demonstrated by turbidity measurements, dynamic light scattering, optical microscopy and liquid crystal self-assembly behavior. Electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, solid-state 13 C NMR, and X-ray diffraction support the successful surface modification and indicate that CNC particle morphology is retained. The modified CNCs have a slightly decreased onset of thermal degradation (ca. 10 °C lower) compared with unmodified CNCs. We believe that this surface modification strategy presents a scalable, simple and green approach to the production of hydrophobic bio-based nanoparticles which may lend themselves as reinforcing agents in nonpolar polymer composites, or stabilizers and rheological modifiers in non-aqueous liquid formulated products.