Understanding Nanocellulose-Water Interactions: Turning a Detriment into an Asset.

Modern technology has enabled the isolation of nanocellulose from plant-based fibers, and the current trend focuses on utilizing nanocellulose in a broad range of sustainable materials applications. Water is generally seen as a detrimental component when in contact with nanocellulose-based materials, just like it is harmful for traditional cellulosic materials such as paper or cardboard. However, water is an integral component in plants, and many applications of nanocellulose already accept the presence of water or make use of it. This review gives a comprehensive account of nanocellulose-water interactions and their repercussions in all key areas of contemporary research: fundamental physical chemistry, chemical modification of nanocellulose, materials applications, and analytical methods to map the water interactions and the effect of water on a nanocellulose matrix.

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.

Tuning the Physicochemical Properties of Cellulose Nanocrystals through an In Situ Oligosaccharide Surface Modification Method.

The trend to replace petroleum-based products with sustainable alternatives has shifted research efforts toward plant-based materials such as cellulose nanocrystals (CNCs). CNCs show promise in numerous applications (e.g., composites and rheological modifiers); however, maximizing their performance often requires surface modifications with complex chemistries and purification steps. Presented here is a novel surface modification method with the potential to tune CNC properties through the in situ deposition of cellulose phosphate oligosaccharides during CNC production. This was achieved by leveraging the selective solubility of oligosaccharides, which are soluble at a low pH (during the CNC hydrolysis) yet become insoluble and precipitate onto CNC surfaces upon increasing pH during quenching. Oligosaccharide-coated CNCs demonstrated subtle changes including higher surface charge densities and lower water adsorption capacities and viscosities than their unmodified counterparts. CNC surface coverage was tuned by controlling the oligosaccharide degree of polymerization. Overall, this fundamental study introduces an easily scalable modification route that opens the door for expanded CNC functionality and applications.

Cellulose Nanocrystal (CNC)-Latex Nanocomposites: Effect of CNC Hydrophilicity and Charge on Rheological, Mechanical, and Adhesive Properties.

Cellulose nanocrystals (CNCs), a sustainable nanomaterial, are in situ incorporated into emulsion-based pressure-sensitive adhesives (PSAs). Commercially available CNCs with different surface hydrophilicity and surface charge (CNC101 and CNC103 from CelluForce) are used to explore their role in PSA property modification. Viscosity measurements and atomic force microscopy reveal differences in degree of association between the CNCs and the latex particles depending on the surface properties of the CNCs. The more hydrophilic and higher surface charge CNCs (CNC101) show less association with the latex particles. Dynamic strain sweep tests are used to analyze the strain-softening of the nanocomposites based on CNC type and loading. The CNC101 nanocomposites soften at lower strains than their CNC103 counterparts. This behavior is confirmed via dynamic frequency tests and modeling of the nanocomposites' storage moduli, which suggest the formation of CNC aggregates of, on average, 3.8 CNC101 and 1.3 CNC103 nanoparticles. Finally, PSA properties, i.e., tack, peel strength, and shear strength, simultaneously increase upon addition of both CNC types, although to different extents. The relationship between the PSA properties and CNC surface properties confirms that the less hydrophilic CNCs lead to improved CNC dispersion in the PSA films and therefore, enhance PSA properties.

How latex film formation and adhesion at the nanoscale correlate to performance of pressure sensitive adhesives with cellulose nanocrystals.

Emulsion polymerized latex-based pressure-sensitive adhesives (PSAs) are more environmentally benign because they are synthesized in water but often underperform compared to their solution polymerized counterparts. Studies have shown a simultaneous improvement in the tack, and peel and shear strength of various acrylic PSAs upon the addition of cellulose nanocrystals (CNCs). This work uses atomic force microscopy (AFM) to examine the role of CNCs in (i) the coalescence of hydrophobic 2-ethyl hexyl acrylate/n-butyl acrylate/methyl methacrylate (EHA/BA/MMA) latex films and (ii) as adhesion modifiers over multiple length scales. Thin films with varying solids content and CNC loading were prepared by spin coating. AFM revealed that CNCs lowered the solids content threshold for latex particle coalescence during film formation. This improved the cohesive strength of the films, which was directly reflected in the increased shear strength of the EHA/BA/MMA PSAs with increasing CNC loading. Colloidal probe AFM indicated that the nano-adhesion of thicker continuous latex films increased with CNC loading when measured over small contact areas where the effect of surface roughness was negligible. Conversely, the beneficial effects of the CNCs on macroscopic PSA tack and peel strength were outweighed by the effects of increased surface roughness with increasing CNC loading over larger surface areas. This highlights that CNCs can improve both cohesive and adhesive PSA properties; however, the effects are most pronounced when the CNCs interact favourably with the latex polymer and are uniformly dispersed throughout the adhesive film. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Mimicking the Humidity Response of the Plant Cell Wall by Using Two-Dimensional Systems: The Critical Role of Amorphous and Crystalline Polysaccharides.

Of the composite materials occurring in nature, the plant cell wall is among the most intricate, consisting of a complex arrangement of semicrystalline cellulose microfibrils in a dissipative matrix of lignin and hemicelluloses. Here, a biomimetic, two-dimensional cellulose system of the cell wall structure is introduced where cellulose nanocrystals compose the crystalline portion and regenerated amorphous cellulose composes the dissipative matrix. Spectroscopic ellipsometry and QCM-D are used to study the water vapor uptake of several two-layer systems. Quantitative analysis shows that the vapor-induced swelling of these ultrathin films can be controlled by varying ratios of the chemically identical ordered and unordered cellulose components. Intriguingly, increasing the share of crystalline cellulose appeared to increase the vapor uptake but only in cases for which the interfacial area between the crystalline and amorphous area was relatively large and the thickness of an amorphous overlayer was relatively small. The results show that a biomimetic approach may occasionally provide answers as to why certain native structures exist.

Water Vapor Uptake of Ultrathin Films of Biologically Derived Nanocrystals: Quantitative Assessment with Quartz Crystal Microbalance and Spectroscopic Ellipsometry.

Despite the relevance of water interactions, explicit analysis of vapor adsorption on biologically derived surfaces is often difficult. Here, a system was introduced to study the vapor uptake on a native polysaccharide surface; namely, cellulose nanocrystal (CNC) ultrathin films were examined with a quartz crystal microbalance with dissipation monitoring (QCM-D) and spectroscopic ellipsometry (SE). A significant mass uptake of water vapor by the CNC films was detected using the QCM-D upon increasing relative humidity. In addition, thickness changes proportional to changes in relative humidity were detected using SE. Quantitative analysis of the results attained indicated that in preference to being soaked by water at the point of hydration each individual CNC in the film became enveloped by a 1 nm thick layer of adsorbed water vapor, resulting in the detected thickness response.

2D dendritic fractal patterns from an amphiphilic polysaccharide.

Fractals are ubiquitous in nature and they are also useful in many modern applications because they are able to incorporate multiple length scales within the same material. However, the bottom-up construction of fractals has been difficult because the physico-chemical principles that underpin the fractal formation have remained elusive. Here, an experimental model setup was used to investigate the emergence of fractals in 2D: Langmuir-Schaefer deposition of an amphiphilic polysaccharide (namely, trimethylsilyl cellulose, TMSC) on substrates with systematically varying surface free energies. Continuous monolayers of TMSC on water appeared to turn into 2D fractal patterns when deposited horizontally on a solid substrate of low surface free energy such as hydrophobically modified (methylated) silica. The decreased surface energy results in higher entropy, which enables fractal formation, as fractals have the highest degree of uncertainty of all 2D patterns. Furthermore, morphological changes within the fractals during a chemical reaction and water immersion were investigated. The results provide a starting point for a better understanding of the bottom-up construction of 2D fractal patterns from soft materials.

Film thickness limits of a buckling-based method to determine mechanical properties of polymer coatings.

Characterizing the mechanical properties of polymer coatings typically requires access to specialty equipment, the analysis through which can be tedious despite instrumental precision. An alternative method reported in the literature,strain-induced elastic buckling instability for mechanical measurements (SIEBIMM), is a high throughput, facile yet accurate method, used to characterize the Young's modulus of supported films and coatings. SIEBIMM can easily be implemented in both academic and industrial settings. HYPOTHESIS: We hypothesize that the SIEBIMM method has an upper coating thickness limit beyond which the assumptions and practicality of the method are no longer valid. EXPERIMENTS: The Young's moduli of model polyvinyl alcohol coatings (on polydimethylsiloxanesubstrates) with thicknesses ranging from 67 nm to 40 µm were determined using the SIEBIMM method and the data were subjected to a rigorous statistical analysis. FINDINGS: SIEBIMM could accurately characterize coatings up to 35 µm thick. The Young's modulus of all coatings ≤ 35 µm was 1.6 ± 0.1 GPa at 50% RH, which agreed with free-standing polyvinyl alcohol films measured by traditional tensile testing. For the method to be used on thicker coatings, it is essential to consistently measure coating thickness and buckling wavelength at the same location to minimize potential error.

Validation of a diffusion-based single droplet drying model for encapsulation of a viral-vectored vaccine using an acoustic levitator.

Development of thermally stable spray dried viral-vectored vaccine powders is dependent on the selection of a proper excipient or excipient blend for encapsulation, which can be a time and resource intensive process. In this work, a diffusion-based droplet drying model was developed to compute droplet drying time, size, and component distribution. The model predictions were validated using an acoustic levitator to dry droplets containing protein-coated or fluorescently labelled silica nanoparticles (as adenoviral vector analogues) and a range of excipient blends. Surface morphology of the dried particles was characterized by atomic force microscopy and the distribution of silica nanoparticles was quantified by confocal microscopy. The modelled distributions of adenovirus agreed with the microscopy results for three mannitol/dextran excipient blends with varying molecular weight dextrans, verifying the equations and assumptions of the model. Viral vector activity data for adenovirus in a range of (poly)saccharide/sugar alcohol formulations were also compared to the model outputs, suggesting that viral activity decreases when the model predicts increasing adenovirus concentrations near the air-solid interface. Using a validated model with excipient property inputs that are readily available in the literature can facilitate the development of viral-vectored vaccines by identifying promising excipients without the need for experimentation.

Bottom-up assembly of nanocellulose structures.

Nanocelluloses, both cellulose nanofibrils and cellulose nanocrystals, are gaining research traction due to their viability as key components in commercial applications and industrial processes. Significant efforts have been made to understand both the potential of assembling nanocelluloses, and the limits and prospectives of the resulting structures. This Review focuses on bottom-up techniques used to prepare nanocellulose-only structures, and details the intermolecular and surface forces driving their assembly. Additionally, the interactions that contribute to their structural integrity are discussed along with alternate pathways and suggestions for improved properties. Six categories of nanocellulose structures are presented: (1) powders, beads, and droplets; (2) capsules; (3) continuous fibres; (4) films; (5) hydrogels; and (6) aerogels and dried foams. Although research on nanocellulose assembly often focuses on fundamental science, this Review also provides insight on the potential utilization of such structures in a wide array of applications.

Incorporating Cellulose Nanocrystals into the Core of Polymer Latex Particles via Polymer Grafting.

Surface-initiated atom transfer radical polymerization was used to graft hydrophobic poly(butyl acrylate) from cellulose nanocrystals (CNCs) resulting in compatibilized CNCs that were successfully incorporated inside the core of polymer latex particles. CNCs are anisotropic nanoparticles derived from renewable resources and have potential as reinforcing agents in nanocomposites. However, challenges due to the incompatibility between cellulose and hydrophobic polymers and processing difficulties, such as aggregation, have limited the performance of CNC nanocomposites produced to date. Here, CNCs were incorporated into the miniemulsion polymerization of methyl methacrylate by adding polymer-grafted CNCs to the monomer phase. A poly(methyl methacrylate)-CNC nanocomposite latex was subsequently produced in situ, whereby polymer-grafted CNCs (with optimized graft length) were located inside the latex particles, as shown by transmission electron microscopy. This work provides a method for controlling the location of CNCs in latex-based nanocomposites and may extend the use of CNCs in commercial adhesives and coatings.