Characterizing lignin-containing microfibrillated cellulose based on water interactions, fibril properties, and imaging.

Morphological characterization of microfibrillated cellulose (MFC) is critically important to process control in production and product specification for trade and product development yet is extremely difficult. This study evaluated several indirect methods for relative comparison of the morphology of lignin-free and lignin-containing ((L)MFCs). The (L)MFCs studied were produced using a commercial grinder through different passes from a dry lap bleached kraft eucalyptus pulp, a virgin mixed (maple and birch) unbleached kraft hardwood pulp, and two virgin-unbleached kraft softwood (loblolly pine) pulps with one bleachable grade (low lignin content) and one liner grade (high lignin content). The (L)MFCs were indirectly characterized using techniques based on water interactions, i.e., water retention value (WRV) and fibril suspension stability, as well as fibril properties, i.e., cellulose crystallinity and fine content. Optical microscopy and scanning electron microscopy were also applied to directly visualize the (L)MFCs to provide some objective measure of the morphology of the (L)MFCs. The results indicate that most measures such as WRV, cellulose crystallinity, fine content cannot be used to compare (L)MFCs from different pulp fibers. Measures based on water interactions such as (L)MFC WRV and suspension stability appeared can provide some degree of indirect assessment. This study provided the utilities and limits of these indirect methods for relative comparison of the morphologies of (L)MFCs.

Towards sustainable production and utilization of plant-biomass-based nanomaterials: a review and analysis of recent developments.

Plant-biomass-based nanomaterials have attracted great interest recently for their potential to replace petroleum-sourced polymeric materials for sustained economic development. However, challenges associated with sustainable production of lignocellulosic nanoscale polymeric materials (NPMs) need to be addressed. Producing materials from lignocellulosic biomass is a value-added proposition compared with fuel-centric approach. This report focuses on recent progress made in understanding NPMs-specifically lignin nanoparticles (LNPs) and cellulosic nanomaterials (CNMs)-and their sustainable production. Special attention is focused on understanding key issues in nano-level deconstruction of cell walls and utilization of key properties of the resultant NPMs to allow flexibility in production to promote sustainability. Specifically, suitable processes for producing LNPs and their potential for scaled-up production, along with the resultant LNP properties and prospective applications, are discussed. In the case of CNMs, terminologies such as cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) used in the literature are examined. The term cellulose nano-whiskers (CNWs) is used here to describe a class of CNMs that has a morphology similar to CNCs but without specifying its crystallinity, because most applications of CNCs do not need its crystalline characteristic. Additionally, progress in enzymatic processing and drying of NPMs is also summarized. Finally, the report provides some perspective of future research that is likely to result in commercialization of plant-based NPMs.

Contributions of Crystalline and Noncrystalline Cellulose Can Occur in the Same Spectral Regions: Evidence Based on Raman and IR and Its Implication for Crystallinity Measurements.

Aggregated states of celluloses remain poorly understood, and therefore, the topic requires careful investigation. In this study, Raman, IR, and X-ray diffraction (XRDs) were used to study cotton microcrystalline cellulose (MCC) and MCC that has been ball-milled to various degrees. Raman and IR spectroscopy methods indicated that when these ball-milled samples were wet with water, most underwent conformational changes at the molecular level. Although formation of cellulose II was observed in longer duration ball-milled samples, the changes primarily gave rise to increased contributions in spectral and diffraction regions typically associated with the contributions of crystalline cellulose I. Moreover, when the wet samples were air-dried at 25 °C, the newly formed cellulose I-like structures partly reverted to the previous form present in the initial dry state. These findings explained for the previously reported XRD and NMR observations, where the addition of water resulted in increased crystallinities of cellulose samples. The implications of these findings to cellulose crystallinity measurements and other situations are discussed.

Towards the scalable isolation of cellulose nanocrystals from tunicates.

In order for sustainable nanomaterials such as cellulose nanocrystals (CNCs) to be utilized in industrial applications, a large-scale production capacity for CNCs must exist. Currently the only CNCs available commercially in kilogram scale are obtained from wood pulp (W-CNCs). Scaling the production capacity of W-CNCs isolation has led to their use in broader applications and captured the interest of researchers, industries and governments alike. Another source of CNCs with potential for commercial scale production are tunicates, a species of marine animal. Tunicate derived CNCs (T-CNCs) are a high aspect ratio CNC, which can complement commercially available W-CNCs in the growing global CNC market. Herein we report the isolation and characterization of T-CNCs from the tunicate Styela clava, an invasive species currently causing significant harm to local aquaculture communities. The reported procedure utilizes scalable CNC processing techniques and is based on our experiences from laboratory scale T-CNC isolation and pilot scale W-CNC isolation. To our best knowledge, this study represents the largest scale where T-CNCs have been isolated from any tunicate species, under any reaction conditions. Demonstrating a significant step towards commercial scale isolation of T-CNCs, and offering a potential solution to the numerous challenges which invasive tunicates pose to global aquaculture communities.

Pilot-Scale Production of Cellulosic Nanowhiskers With Similar Morphology to Cellulose Nanocrystals.

This study describes a class of cellulosic nanomaterials, cellulosic nanowhiskers (CNWs), and demonstrates scaled-up production with acid recovery using less expensive equipment made of common stainless steel rather than glass-lined steel. CNWs produced using concentrated maleic acid (MA) hydrolysis followed by mechanical fibrillation have morphology similar to MA-produced cellulose nanocrystals (CNCs) and sulfuric-acid-produced CNCs (S-CNCs) but differ in crystallinity. Applications of CNWs as a substitute for CNCs for which morphology and surface charge, rather than crystallinity, are the pertinent characteristics are presented. The tested CNW suspensions have a wider viscosity range of 0.001 to 1000 Pa.s over a variety of shear rates of 0.01 to 1000 1/s compared to S-CNCs of 0.001 to 0.1 Pa.s and are better suited for applications such as rheology modification and 3D printing. This study proposes CNWs as a less expensive and sustainable replacement for CNCs in applications that do not require crystalline properties.

Analysis of Cellulose and Lignocellulose Materials by Raman Spectroscopy: A Review of the Current Status.

This review is a summary of the Raman spectroscopy applications made over the last 10 years in the field of cellulose and lignocellulose materials. This paper functions as a status report on the kinds of information that can be generated by applying Raman spectroscopy. The information in the review is taken from the published papers and author's own research-most of which is in print. Although, at the molecular level, focus of the investigations has been on cellulose and lignin, hemicelluloses have also received some attention. The progress over the last decade in applying Raman spectroscopy is a direct consequence of the technical advances in the field of Raman spectroscopy, in particular, the application of new Raman techniques (e.g., Raman imaging and coherent anti-Stokes Raman or CARS), novel ways of spectral analysis, and quantum chemical calculations. On the basis of this analysis, it is clear that Raman spectroscopy continues to play an important role in the field of cellulose and lignocellulose research across a wide range of areas and applications, and thereby provides useful information at the molecular level.

Estimation of Syringyl Units in Wood Lignins by FT-Raman Spectroscopy.

Syringyl (S) lignin content and the syringyl-to-guaiacyl (S/G) lignin ratio are important characteristics of wood and lignocellulosic biomass. Although numerous methods are available for estimating S lignin units and the S/G ratio, in this work, a new method based on Raman spectroscopy that uses the 370 cm-1 Raman band-area intensity (370-area) was developed. The reliability of the Raman approach for determining S content was first tested by the quantitative analysis of three syringyl lignin models by sampling them, separately, in dioxane and in Avicel. Good linear correlations between the 370 cm-1 intensity and model concentrations were obtained. Next, the percent syringyl (%S) lignin units in various woods were measured by correlating the 370 cm-1 Raman intensity data with values of S units in lignin determined by three regularly used methods, namely, thioacidolysis, DFRC, and 2D-HSQC NMR. The former two methods take into account only the monomers cleaved from ß-O-4-linked lignin units, whereas the NMR method reports S content on the whole cell wall lignin. When the 370-area intensities and %S values from the regularly used methods were correlated, good linear correlations were obtained ( R2 = 0.767, 0.731, and 0.804, respectively, for the three methods). The correlation with the highest R2, i.e., with the 2D NMR method, is proposed for estimating S units in wood lignins by Raman spectroscopy as, in principle, both represent the whole cell wall lignin and not just the portion of lignin that gets cleaved to release monomers. The Raman analysis method is quick, uses minimal harmful chemicals, is carried out nondestructively, and is insensitive to the wet or dry state of the sample. The only limitations are that the sample of wood contains at least 30% S and not be significantly fluorescent, although the latter can be mitigated in some cases.

New cellulose crystallinity estimation method that differentiates between organized and crystalline phases.

A new method is proposed for estimation of cellulose crystallinity (CrI) based on 93 cm-1 Raman band in spectra of cellulose I materials. In this method (93-Raman), CrI was determined based on regression that was developed using the ratios of peak-heights of the 93 and 1096 cm-1 Raman bands (I93/I1096). For calibration purposes, a set of eight samples, all derived from cotton microcrystalline cellulose Whatman CC31 were selected. When the peak intensity ratios (I93/I1096) were plotted against the calculated CrIs of the calibration set samples, the plot showed an excellent linear correlation (R2 = 0.9888). The 93-Raman method was used to estimate crystallinities of a number of cellulose materials including poplar wood samples that were hydrothermally treated at various temperatures. The wood 93-Raman CrI data showed that the method is able to differentiate between organized and crystalline phases of cellulose, a capability lacking in many other CrI estimation methods.

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.

Chemical modification of nanocellulose with canola oil fatty acid methyl ester.

Cellulose nanocrystals (CNCs), produced from dissolving wood pulp, were chemically functionalized by transesterification with canola oil fatty acid methyl ester (CME). CME performs as both the reaction reagent and solvent. Transesterified CNC (CNCFE) was characterized for their chemical structure, morphology, crystalline structure, thermal stability, and hydrophobicity. Analysis by Fourier transform infrared (FTIR) and FT-Raman spectroscopies showed that the long chain hydrocarbon structure was successfully grafted onto CNC surfaces. After transesterification the crystal size and crystallinity of nanocrystals were not changed as determined by Raman spectroscopy and wide angle X-ray diffraction (XRD). CNCFE showed higher thermal stability and smaller particle size than unmodified CNCs. Water contact angle measurement indicated the CNCFE surface has significantly higher hydrophobicity than unmodified CNCs. The transesterified CNCs could be potentially used as hydrophobic coatings and reinforcing agents to hydrophobic polymer for nanocomposites.

1064 nm FT-Raman spectroscopy for investigations of plant cell walls and other biomass materials.

Raman spectroscopy with its various special techniques and methods has been applied to study plant biomass for about 30 years. Such investigations have been performed at both macro- and micro-levels. However, with the availability of the Near Infrared (NIR) (1064 nm) Fourier Transform (FT)-Raman instruments where, in most materials, successful fluorescence suppression can be achieved, the utility of the Raman investigations has increased significantly. Moreover, the development of several new capabilities such as estimation of cellulose-crystallinity, ability to analyze changes in cellulose conformation at the local and molecular level, and examination of water-cellulose interactions have made this technique essential for research in the field of plant science. The FT-Raman method has also been applied to research studies in the arenas of biofuels and nanocelluloses. Moreover, the ability to investigate plant lignins has been further refined with the availability of near-IR Raman. In this paper, we present 1064-nm FT-Raman spectroscopy methodology to investigate various compositional and structural properties of plant material. It is hoped that the described studies will motivate the research community in the plant biomass field to adapt this technique to investigate their specific research needs.

A comparative study of cellulose nanofibrils disintegrated via multiple processing approaches.

Various cellulose nanofibrils (CNFs) created by refining and microfluidization, in combination with enzymatic or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized pretreatment were compared. The morphological properties, degree of polymerization, and crystallinity for the obtained nanofibrils, as well as physical and mechanical properties of the corresponding films were evaluated. Compared to refining, intense microfluidization contributed greater separation of nanofibril bundles, which led to an enhancement of mechanical strength and transparency for the resultant film. The selected enzymatic pre-treatments produced shortened fibers due to preferential hydrolysis of amorphous cellulose and, in combination with mechanical treatments, resulted in short and stiff cellulose nanocrystal (CNC)-like materials. Despite films from these CNC-like fibrils having inferior tensile strength, their tensile modulus and transparency were significantly improved compared to CNFs prepared without pre-treatment. The unique fiber morphology and high crystallinity potentially offer a green and ecologically friendly alternative for the preparation of CNCs and CNFs as part of an integrated biorefinery approach.

Estimation of cellulose crystallinity of lignocelluloses using near-IR FT-Raman spectroscopy and comparison of the Raman and Segal-WAXS methods.

Of the recently developed univariate and multivariate near-IR FT-Raman methods for estimating cellulose crystallinity, the former method was applied to a variety of lignocelluloses: softwoods, hardwoods, wood pulps, and agricultural residues/fibers. The effect of autofluorescence on the crystallinity estimation was minimized by solvent extraction or chemical treatment or both. Additionally, when the roles of lignin and hemicellulose in the Raman crystallinity assessment were investigated, it was found that syringyl lignin containing lignocelluloses generated somewhat higher crystallinity, whereas the presence of hemicellulose reduced the crystallinity. Overall, when autofluorescence was minimized and corrections made for hemicellulose and syringyl lignin contributions, the univariate Raman method performed well and estimated cellulose crystallinity accurately. Moreover, when the Raman and Segal-WAXS methods were compared, we observed that in the absence of significant fluorescence, the Raman method was influenced mostly by hemicellulose and syringyl lignin, whereas the Segal-WAXS was affected by various types of lignin and hemicellulose. It was concluded that the near-IR FT-Raman method with corrections for influences of syringyl lignin and hemicellulose can be used to correctly estimate cellulose crystallinity.

Spatially resolved characterization of cellulose nanocrystal-polypropylene composite by confocal Raman microscopy.

Raman spectroscopy was used to analyze cellulose nanocrystal (CNC) -polypropylene (PP) composites and to investigate the spatial distribution of CNCs in extruded composite filaments. Three composites were made from two forms of nanocellulose (CNCs from wood pulp and the nano-scale fraction of microcrystalline cellulose) and two of the three composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cm(-1), respectively, obtained at 1 µm spatial resolution showed that the CNCs were aggregated to various degrees in the PP matrix. Of the three composites analyzed, two showed clear existence of phase-separated regions: Raman images with strong PP and absent/weak cellulose or vice versa. For the third composite, the situation was slightly improved but a clear transition interface between the PP-abundant and CNC-abundant regions was observed, indicating that the CNC remained poorly dispersed. The spectroscopic approach to investigating spatial distribution of the composite components was helpful in evaluating CNC dispersion in the composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs.

Restructuring the crystalline cellulose hydrogen bond network enhances its depolymerization rate.

Conversion of lignocellulose to biofuels is partly inefficient due to the deleterious impact of cellulose crystallinity on enzymatic saccharification. We demonstrate how the synergistic activity of cellulases was enhanced by altering the hydrogen bond network within crystalline cellulose fibrils. We provide a molecular-scale explanation of these phenomena through molecular dynamics (MD) simulations and enzymatic assays. Ammonia transformed the naturally occurring crystalline allomorph I(ß) to III(I), which led to a decrease in the number of cellulose intrasheet hydrogen bonds and an increase in the number of intersheet hydrogen bonds. This rearrangement of the hydrogen bond network within cellulose III(I), which increased the number of solvent-exposed glucan chain hydrogen bonds with water by ~50%, was accompanied by enhanced saccharification rates by up to 5-fold (closest to amorphous cellulose) and 60-70% lower maximum surface-bound cellulase capacity. The enhancement in apparent cellulase activity was attributed to the "amorphous-like" nature of the cellulose III(I) fibril surface that facilitated easier glucan chain extraction. Unrestricted substrate accessibility to active-site clefts of certain endocellulase families further accelerated deconstruction of cellulose III(I). Structural and dynamical features of cellulose III(I), revealed by MD simulations, gave additional insights into the role of cellulose crystal structure on fibril surface hydration that influences interfacial enzyme binding. Subtle alterations within the cellulose hydrogen bond network provide an attractive way to enhance its deconstruction and offer unique insight into the nature of cellulose recalcitrance. This approach can lead to unconventional pathways for development of novel pretreatments and engineered cellulases for cost-effective biofuels production.

Raman imaging to investigate ultrastructure and composition of plant cell walls: distribution of lignin and cellulose in black spruce wood (Picea mariana).

A detailed understanding of the structural organization of the cell wall of vascular plants is important from both the perspectives of plant biology and chemistry and of commercial utilization. A state-of-the-art 633-nm laser-based confocal Raman microscope was used to determine the distribution of cell wall components in the cross section of black spruce wood in situ. Chemical information from morphologically distinct cell wall regions was obtained and Raman images of lignin and cellulose spatial distribution were generated. While cell corner (CC) lignin concentration was the highest on average, lignin concentration in compound middle lamella (CmL) was not significantly different from that in secondary wall (S2 and S2-S3). Images generated using the 1,650 cm(-1) band showed that coniferaldehyde and coniferyl alcohol distribution followed that of lignin and no particular cell wall layer/region was therefore enriched in the ethylenic residue. In contrast, cellulose distribution showed the opposite pattern-low concentration in CC and CmL and high in S2 regions. Nevertheless, cellulose concentration varied significantly in some areas, and concentrations of both lignin and cellulose were high in other areas. Though intensity maps of lignin and cellulose distributions are currently interpreted solely in terms of concentration differences, the effect of orientation needs to be carefully considered to reveal the organization of the wood cell wall.