Molecular Changes during Tensile Deformation of Single Wood Fibers Followed by Raman Microscopy

Revealing spatial distribution and accessibility of cell wall polymers in bamboo through chemical imaging and mild chemical treatments

Understanding the distribution and accessibility of polymers within plant cell walls is crucial for addressing biomass recalcitrance in lignocellulosic materials. In this work, Imaging Fourier Transform Infrared (FTIR) and Raman spectroscopy, coupled with targeted chemical treatments, were employed to investigate cell wall polymer distribution in two bamboo species at both tissue and cell wall levels. Tissue-level Imaging FTIR revealed significant disparities in the distribution and chemical activity of cell wall polymers between the fibrous sheath and fibrous strand. At the cell wall level, Imaging Raman spectroscopy delineated a distinct difference between the secondary wall and intercellular layer, with the latter containing higher levels of lignin, hydroxycinnamic acid (HCA), and xylan, and lower cellulose. Mild acidified sodium chlorite treatment led to partial removal of lignin, HCA, and xylan from the intercellular layer, albeit to a lesser extent than alkaline treatment, indicating susceptibility of these polymers to chemical treatment. In contrast, lignin in the secondary wall exhibited limited reactivity to acidified sodium chlorite but was slightly removed by alkaline treatment, suggesting stable chemical properties with slight alkaline intolerance. These findings provide valuable insights into the inherent design mechanism of plant cells and their efficient utilization.

Role of microfibril angle in molecular deformation of cellulose fibrils in Pinus massoniana compression wood and opposite wood studied by in-situ WAXS

Upon tensile stress, the spiral cellulose fibrils in wood cell walls rotate like springs with decreasing microfibril angle (MFA), and the cellulose molecules elongate in the chain direction. Compression wood with high MFA and opposite wood with low MFA were comparatively studied by in-situ tensile tests combined with synchrotron radiation WAXS in the present study. FTIR spectroscopy revealed that compression wood had a higher lignin content and fewer acetyl groups. For both types of wood, the lattice spacing d004 increased and the MFA decreased gradually with the increase of tensile stress. At stresses beyond the yield point, cellulose lattice strain depended linearly on macroscopic stress, while the MFA depended linearly on macroscopic strain. The deformation mechanisms of compression wood and opposite wood are not essentially different but differ in their deformation behavior. Specifically, the contribution ratio of lattice strain and cellulose fibril reorientation to macroscopic strain was 0.25 and 0.53 for compression wood, and 0.40 and 0.33 for opposite wood, respectively. Due to the geometric effects of MFA, a greater contribution of cellulose fibril reorientation to the macroscopic deformation was detected in compression wood than in opposite wood.

Raman developmental markers in root cell walls are associated with lodging tendency in tef

MAIN CONCLUSION

Using Raman micro-spectroscopy on tef roots, we could monitor cell wall maturation in lines with varied genetic lodging tendency. We describe the developing cell wall composition in root endodermis and cylinder tissue. Tef [Eragrostis tef (Zucc.) Trotter] is an important staple crop in Ethiopia and Eritrea, producing gluten-free and protein-rich grains. However, this crop is not adapted to modern farming practices due to high lodging susceptibility, which prevents the application of mechanical harvest. Lodging describes the displacement of roots (root lodging) or fracture of culms (stem lodging), forcing plants to bend or fall from their vertical position, causing significant yield losses. In this study, we aimed to understand the microstructural properties of crown roots, underlining tef tolerance/susceptibility to lodging. We analyzed plants at 5 and 10 weeks after emergence and compared trellised to lodged plants. Root cross sections from different tef genotypes were characterized by scanning electron microscopy, micro-computed tomography, and Raman micro-spectroscopy. Lodging susceptible genotypes exhibited early tissue maturation, including developed aerenchyma, intensive lignification, and lignin with high levels of crosslinks. A comparison between trellised and lodged plants suggested that lodging itself does not affect the histology of root tissue. Furthermore, cell wall composition along plant maturation was typical to each of the tested genotypes independently of trellising. Our results suggest that it is possible to select lines that exhibit slow maturation of crown roots. Such lines are predicted to show reduction in lodging and facilitate mechanical harvest.

A review on the hydration properties of dietary fibers derived from food waste and their interactions with other ingredients: opportunities and challenges for their application in the food industry

Dietary fiber (DF) significantly affects the quality attributes of food matrices. Depending on its chemical composition, molecular structure, and degree of hydration, the behavior of DF may differ. Numerous reports confirm that incorporating DF derived from food waste into food products has significant effects on textural, sensory, rheological, and antimicrobial properties. Additionally, the characteristics of DF, modification techniques (chemical, enzymatic, mechanical, thermal), and processing conditions (temperature, pH, ionic strength), as well as the presence of other components, can profoundly affect the functionalities of DF. This review aims to describe the interactions between DF and water, focusing on the effects of free water, freezing-bound water, and unfreezing-bound water on the hydration capacity of both soluble and insoluble DF. The review also explores how the structural, functional, and environmental properties of DF contribute to its hydration capacity. It becomes evident that the interactions between DF and water, and their effects on the rheological properties of food matrices, are complex and multifaceted subjects, offering both opportunities and challenges for further exploration. Utilizing DF extracted from food waste exhibits promise as a sustainable and viable strategy for the food industry to create nutritious and high-value-added products, while concurrently reducing reliance on primary virgin resources.

Raman imaging: An indispensable technique to comprehend the functionalization of lignocellulosic material

With the increasing demands on sustainability in the material science and engineering landscape, the use of wood, a renewable and biodegradable material, for new material development has drawn increasing attentions in the materials science community. To promote the development of new wood-based materials, it is critical to understanding not only wood's hierarchical structure from molecule to macroscale level, but also the interactions of wood with other materials and chemicals upon modification and functionalization. In this review, we discuss the recent advances in the Raman imaging technique, a new approach that combines spectroscopy and microscopy, in wood characterization and structural evolution monitoring during functionalization. We introduce the principles of Raman spectroscopy and common Raman instrumentations. We survey the use of traditional Raman spectroscopy for lignocellulosic material characterizations including cellulose crystallinity determination, holocellulose discrimination, and lignin substructure evaluation. We briefly review the recent studies on wood property enhancement and functional wood-based material development through wood modification including thermal treatment, acetylation, furfurylation, methacrylation, delignification. Subsequently, we highlight the use of the Raman imaging for visualization, spatial and temporal distribution of wood cell wall structure, as well as the microstructure evolution upon functionalization. Finally, we discuss the future prospects of the field.

Paper-Based Substrate for a Surface-Enhanced Raman Spectroscopy Biosensing Platform-A Silver/Chitosan Nanocomposite Approach

Paper is a popular platform material in all areas of sensor research due to its porosity, large surface area, and biodegradability, to name but a few. Many paper-based nanocomposites have been reported in the last decade as novel substrates for surface-enhanced Raman spectroscopy (SERS). However, there are still limiting factors, like the low density of hot spots or loss of wettability. Herein, we designed a process to fabricate a silver-chitosan nanocomposite layer on paper celluloses by a layer-by-layer method and pH-triggered chitosan assembly. Under microscopic observation, the resulting material showed a nanoporous structure, and silver nanoparticles were anchored evenly over the nanocomposite layer. In SERS measurement, the detection limit of 4-aminothiophenol was 5.13 ppb. Furthermore, its mechanical property and a strategy toward further biosensing approaches were investigated.

Cell wall polymer distribution in bamboo visualized with in situ imaging FTIR

To better understand the high recalcitrance of bamboo during bioconversion, the fine spatial distribution of polymers in bamboo was studied with Imaging FTIR microscopy under both transmission and ATR modes, combined with PCA data processing. The results demonstrated that lignin, xylan and hydroxycinnamic acid (HCA) were more concentrated in the fibers near the xylem conduit, while cellulose was evenly distributed across the whole fiber sheath. PCA processing produced a clear separation between bamboo fibers and parenchyma cells, indicating that the parenchyma cells contains more pectin and HCA than fibers. It also demonstrated that cellulose, xylan and S-lignin were concentrated most heavily in bamboo fiber secondary cell walls, while G-lignin, pectin and HCA were found more in the compound middle lamella. The revealed information regarding polymer distribution is of great significance for better understanding of the inherent design mechanism of plant cell wall and its efficient utilization.

Zinc phthalocyanine conjugated cellulose nanocrystals for memory device applications

We present the electrical properties of zinc phthalocyanine covalently conjugated to cellulose nanocrystals (CNC@ZnPc). Thin films of CNC@ZnPc sandwiched between two gold electrodes showed pronounced hysteresis in their current-voltage characteristics. The layered metal-organic-metal sandwich devices exhibit distinct high and low conductive states when bias is applied, which can be used to store information. Density functional theory results confirmed wave function overlap between CNC and ZnPc in CNC@ZnPc, and helped visualize the lowest (lowest unoccupied molecular orbital) and highest molecular orbitals (highest occupied molecular orbital) in CNC@ZnPc. These results pave the way forward for all-organic electronic devices based on low cost, earth abundant CNCs and metallophthalocyanines.

Wood Derived Cellulose Scaffolds-Processing and Mechanics

Wood-derived cellulose materials obtained by structure-retaining delignification are attracting increasing attention due to their excellent mechanical properties and great potential to serve as renewable and CO₂ storing cellulose scaffolds for advanced hybrid materials with embedded functionality. Various delignification protocols and a multitude of further processing steps including polymer impregnation and densification are applied resulting in a large range of properties. However, treatment optimization requires a more comprehensive characterization of the developed materials in terms of structure, chemical composition, and mechanical properties for faster progress in the field. Herein, the current protocols for structure-retaining delignification are reviewed and the emphasis is placed on the mechanical characterization at different hierarchical levels of the cellulose scaffolds by experiments and modeling to reveal the underlying structure-property relationships.

Beyond What Meets the Eye: Imaging and Imagining Wood Mechanical-Structural Properties

Wood presents a hierarchical structure, containing features at all length scales: from the tracheids or vessels that make up its cellular structure, through to the microfibrils within the cell walls, down to the molecular architecture of the cellulose, lignin, and hemicelluloses that comprise its chemical makeup. This structure renders it with high mechanical (e.g., modulus and strength) and interesting physical (e.g., optical) properties. A better understanding of this structure, and how it plays a role in governing mechanical and other physical parameters, will help to better exploit this sustainable resource. Here, recent developments on the use of advanced imaging techniques for studying the structural properties of wood in relation to its mechanical properties are explored. The focus is on synchrotron nuclear magnetic resonance spectroscopy, X-ray diffraction, X-ray tomographical imaging, Raman and infrared spectroscopies, confocal microscopy, electron microscopy, and atomic force microscopy. Critical discussion on the role of imaging techniques and how fields are developing rapidly to incorporate both spatial and temporal ranges of analysis is presented.

Synthesis and Characterization of Zinc Phthalocyanine-Cellulose Nanocrystal (CNC) Conjugates: Toward Highly Functional CNCs

We report highly fluorescent cellulose nanocrystals (CNCs) formed by conjugating a carboxylated zinc phthalocyanine (ZnPc) to two different types of CNCs. The conjugated nanocrystals (henceforth called ZnPc@CNCs) were bright green in color and exhibited absorption and emission maxima at ∼690 and ∼715 nm, respectively. The esterification protocol employed to covalently bind carboxylated ZnPc to surface hydroxyl group rich CNCs was expected to result in a monolayer of ZnPc on the surface of the CNCs. However, dynamic light scattering (DLS) studies indicated a large increase in the hydrodynamic radius of CNCs following conjugation to ZnPc, which suggests the binding of multiple ZnPc molecular layers on the CNC surface. This binding could be through co-facial π-stacking of ZnPc, where ZnPc metallophthalocyanine rings are horizontal to the CNC surface. The other possible binding mode would give rise to conjugated systems where ZnPc metallophthalocyanine rings are oriented vertically on the CNC surface. Density functional theory based calculations showed stable geometry following the conjugation protocol that involved covalently attached ester bond formation. The conjugates demonstrated superior performance for potential sensing applications through higher photoluminescence quenching capabilities compared to pristine ZnPc.

FTIR Nanospectroscopy Shows Molecular Structures of Plant Biominerals and Cell Walls

Plant tissues are complex composite structures of organic and inorganic components whose function relies on molecular heterogeneity at the nanometer scale. Scattering-type near-field optical microscopy (s-SNOM) in the mid-infrared (IR) region is used here to collect IR nanospectra from both fixed and native plant samples. We compared structures of chemically extracted silica bodies (phytoliths) to silicified and nonsilicified cell walls prepared as a flat block of epoxy-embedded awns of wheat (Triticum turgidum), thin sections of native epidermis cells from sorghum (Sorghum bicolor) comprising silica phytoliths, and isolated cells from awns of oats (Avena sterilis). The correlation of the scanning-probe IR images and the mechanical phase image enables a combined probing of mechanical material properties together with the chemical composition and structure of both the cell walls and the phytolith structures. The data reveal a structural heterogeneity of the different silica bodies in situ, as well as different compositions and crystallinities of cell wall components. In conclusion, IR nanospectroscopy is suggested as an ideal tool for studies of native plant materials of varied origins and preparations and could be applied to other inorganic-organic hybrid materials.

Raman Spectroscopy Methods to Characterize the Mechanical Response of Soft Biomaterials

Raman spectroscopy has been used extensively to characterize the influence of mechanical deformation on microstructure changes in biomaterials. While traditional piezo-spectroscopy has been successful in assessing internal stresses of hard biomaterials by tracking prominent peak shifts, peak shifts due to applied loads are near or below the resolution limit of the spectrometer for soft biomaterials with moduli in the kilo- to mega-Pascal range. In this Review, in addition to peak shifts, other spectral features (e.g., polarized intensity and intensity ratio) that provide quantitative assessments of microstructural orientation and secondary structure in soft biomaterials and their strain dependence are discussed. We provide specific examples for each method and classify sensitive Raman characteristic bands common across natural (e.g., soft tissue) and synthetic (e.g., polymeric scaffolds) soft biomaterials upon mechanical deformation. This Review can provide guidance for researchers aiming to analyze micromechanics of soft tissues and engineered tissue constructs by Raman spectroscopy.

Field-Assisted Alignment of Cellulose Nanofibrils in a Continuous Flow-Focusing System

The continuous production of macroscale filaments of 17 μm in diameter comprising aligned TEMPO-oxidized cellulose nanofibrils (CNFs) is conducted using a field-assisted flow-focusing process. The effect of an AC external field on the material's structure becomes significant at a certain voltage, beyond which augmentations of the CNF orientation factor up to 16% are obtained. Results indicate that the electric field significantly contributes to improve the CNF ordering in the bulk, while the CNF alignment on the filament surface is only slightly affected by the applied voltage. X-ray diffraction shows that CNFs are densely packed anisotropically in the plane parallel to the filament axis without any preferential out of plane orientation. The improved nanoscale ordering combined with the tight CNF packing yields impressive enhancements in mechanical properties, with stiffness up to 25 GPa and more than 63% (up to 260 MPa), 46% (up to 2.8%), and 120% (up to 4.7 kJ/m³) increase in tensile strength, strain-to-failure, and toughness, respectively. This study demonstrates for the first time the control over the structural ordering of anisotropic nanoparticles in a dynamic system using an electric field, which can have important implications for the development of sustainable alternatives to synthetic textiles.

Wood Deformation Leads to Rearrangement of Molecules at the Nanoscale

Wood, as the most abundant carbon dioxide storing bioresource, is currently driven beyond its traditional use through creative innovations and nanotechnology. For many properties the micro- and nanostructure plays a crucial role and one key challenge is control and detection of chemical and physical processes in the confined microstructure and nanopores of the wooden cell wall. In this study, correlative Raman and atomic force microscopy show high potential for tracking in situ molecular rearrangement of wood polymers during compression. More water molecules (interpreted as wider cellulose microfibril distances) and disentangling of hemicellulose chains are detected in the opened cell wall regions, whereas an increase of lignin is revealed in the compressed areas. These results support a new more "loose" cell wall model based on flexible lignin nanodomains and advance our knowledge of the molecular reorganization during deformation of wood for optimized processing and utilization.

Raman Imaging of Plant Cell Walls

Raman imaging is a microspectroscopic approach revealing the chemistry and structure of plant cell walls in situ on the micro- and nanoscale. The method is based on the Raman effect (inelastic scattering) that takes place when monochromatic laser light interacts with matter. The scattered light conveys a change in energy that is inherent of the involved molecule vibrations. The Raman spectra are thus characteristic for the chemical structure of the molecules and can be recorded spatially ordered with a lateral resolution of about 300 nm. Based on thousands of acquired Raman spectra, images can be assessed using univariate as well as multivariate data analysis approaches. One advantage compared to staining or labeling techniques is that not only one image is obtained as a result but different components and characteristics can be displayed in several images. Furthermore, as every pixel corresponds to a Raman spectrum, which is a kind of "molecular fingerprint," the imaging results should always be evaluated and further details revealed by analysis (e.g., band assignment) of extracted spectra. In this chapter, the basic theoretical background of the technique and instrumentation are described together with sample preparation requirements and tips for high-quality plant tissue sections and successful Raman measurements. Typical Raman spectra of the different plant cell wall components are shown as well as an exemplified analysis of Raman data acquired on the model plant Arabidopsis. Important preprocessing methods of the spectra are included as well as single component image generation (univariate) and spectral unmixing by means of multivariate approaches (e.g., vertex component analysis).

Optical imaging spectroscopy for plant research: more than a colorful picture

Optical imaging is a routine and indispensable tool in plant research. Here we review different emerging spectrally resolved optical imaging approaches and the wealth of information they can be used to obtain pertaining to the underlying chemistry, structure and mechanics of plants.

Measuring Molecular Strain in Rewetted and Never-Dried Eucalypt Wood with Raman Spectroscopy

To measure growth strain in wood using Raman spectroscopy, we investigated the Raman spectra of rewetted (water-saturated) Eucalyptus regnans and green Eucalyptus quadrangulata wood during tensile tests. Partial least squares models to predict the tensile strain were built from the Raman spectra. The best model could predict the tensile strain with a root mean square error of 427.5 με. Apart from the widely reported band shift at 1095 cm^-1 upon mechanical strain, spectral changes at 1420, 1120, 895, and 456 cm^-1 were identified. The assignments of these bands were discussed in relation to the molecular deformation of cellulose. The band shift rates during tensile tests were -3.06 and -2.15 cm^-1/% for rewetted E. regnans and green E. quadrangulata wood, respectively. We successfully detected the release of the molecular growth strain in green eucalyptus wood with Raman spectroscopy by observing band shifts of the 1095 cm^-1 signal. Further, there was a moderate correlation (r = 0.48) between the growth strain measured with strain gauges and the 1095 cm^-1 band position. The precision of the prediction of growth strain using Raman spectroscopy was negatively affected by variation attributed to the inhomogeneity of wood on the millimeter scale and instrumental instability.

Significant influence of lignin on axial elastic modulus of poplar wood at low microfibril angles under wet conditions

Wood is extensively used as a construction material. Despite increasing knowledge of its mechanical properties, the contribution of the cell-wall matrix polymers to wood mechanics is still not well understood. Previous studies have shown that axial stiffness correlates with lignin content only for cellulose microfibril angles larger than around 20°, while no influence is found for smaller angles. Here, by analysing the wood of poplar with reduced lignin content due to down-regulation of CAFFEOYL SHIKIMATE ESTERASE, we show that lignin content also influences axial stiffness at smaller angles. Micro-tensile tests of the xylem revealed that axial stiffness was strongly reduced in the low-lignin transgenic lines. Strikingly, microfibril angles were around 15° for both wild-type and transgenic poplars, suggesting that cellulose orientation is not responsible for the observed changes in mechanical behavior. Multiple linear regression analysis showed that the decrease in stiffness was almost completely related to the variation in both density and lignin content. We suggest that the influence of lignin content on axial stiffness may gradually increase as a function of the microfibril angle. Our results may help in building up comprehensive models of the cell wall that can unravel the individual roles of the matrix polymers.

Raman Spectroscopy in Nonwoody Plants

Confocal Raman spectroscopy (RS) enables obtaining molecular information from the nondestructive analysis of plant material in situ. It can thereby be a useful method to investigate spatial distribution and heterogeneity of cell-wall polymers. The authors' intention is to present some examples of RS application and its capabilities for investigations of nonwoody plants. In this context, we present protocols for qualitative analysis of main polymers of plant wall and application of RS in a semiquantitative study of the arrangement of selected polymers in the wall in its native state.

Surface Modification of Cellulose Nanocrystals with Succinic Anhydride

The surface modification of cellulose nanocrystals (CNC) is a key intermediate step in the development of new functionalities and the tailoring of nanomaterial properties for specific applications. In the area of polymeric nanocomposites, apart from good interfacial adhesion, the high thermal stability of cellulose nanomaterial is vitally required for the stable processing and improvement of material properties. In this respect, the heterogeneous esterification of CNC with succinic anhydride was investigated in this work in order to obtain CNC with optimised surface and thermal properties. The influence of reaction parameters, such as time, temperature, and molar ratio of reagents, on the structure, morphology and thermal properties, were systematically studied over a wide range of values by DLS, FTIR, XPS, WAXD, SEM and TGA methods. It was found that the degree of surface substitution of CNC increased with the molar ratio of succinic anhydride to cellulose hydroxyl groups (SA:OH), as well as the reaction time, whilst the temperature of reaction showed a moderate effect on the degree of esterification in the range of 70-110 °C. The studies on the thermal stability of modified nanoparticles indicated that there is a critical extent of surface esterification below which only a slight decrease of the initial temperature of degradation was observed in pyrolytic and oxidative atmospheres. A significant reduction of CNC thermal stability was observed only for the longest reaction time (240 min) and the highest molar ratio of SA:OH. This illustrates the possibility of manufacturing thermally stable, succinylated, CNC by controlling the reaction conditions and the degree of esterification.

Effects of mechanical stretching, desorption and isotope exchange on deuterated eucalypt wood studied by near infrared spectroscopy

Deuterium exchange combined with near infrared (NIR) spectroscopy was used to study the roles of accessible and inaccessible cellulose in the load transfer of eucalyptus wood. Monitoring the drying process helped to assign NIR bands of deuterated wood samples. Polarized NIR spectra of protonated and deuterated samples confirmed that inaccessible hydroxyl groups in eucalyptus wood were preferably oriented in the longitudinal direction. The spectral changes on NIR spectra caused by mechanical strain could be highlighted by averaging loading and unloading cycles to compensate for effects of desorption and isotope re-exchange due to environmental fluctuations. After deuteration, the bands affected by mechanical strain at around 6420, 6240 and 4670 cm^-1, which had been assigned to hydroxyl groups in cellulose, remained at these positions, suggesting the inaccessible cellulose fraction was the main load-bearing component in wood. A small band at around 4700 cm^-1 responding to mechanical strain, becoming visible in the deuterated spectra, indicated that accessible hydroxyls also contributed to the load transfer. Furthermore, the measurements confirmed previous reports of moisture adsorption of wood under tensile stress.

Molecular deformation of wood and cellulose studied by near infrared spectroscopy

Wood (Eucalyptus regnans and Pinus radiata) and paper samples were stretched to different strain levels using a purpose-built tensile test device fitted into a near infrared (NIR) spectrometer while collecting transmission spectra. Consistent spectral changes caused by mechanical strain, assigned to OH stretching bands, were observed for all three sample types. Bands at 6286 ± 5 cm^-1 and 6470 ± 10 cm^-1 were tentatively assigned to the OH groups connected with the 2OH⋯6O and 3OH⋯5O intramolecular hydrogen bonds of crystalline cellulose Iβ, respectively. Both bands shifted to higher wavenumbers indicating the elongation of the hydrogen bonds. A linear relationship was found between band shifts and mechanical strain. Band shift rates for the 3OH bond were more than twice that of the 2OH bond, consistent with bending of the glycosidic bond. Bending tests showed that the band at around 6286 cm^-1 shifted in opposite direction when under tension or compression.

Quantitative analysis of the distribution and mixing of cellulose nanocrystals in thermoplastic composites using Raman chemical imaging

Raman chemical imaging is presented to both quantify the dispersion and the degree of mixing in a cellulose nanocrystal composite.

Preparation and characterization of flexible lithium iron phosphate/graphene/cellulose electrode for lithium ion batteries

In this work, a free-standing flexible composite electrode was prepared by vacuum filtration method with LiFePO₄, graphene and nanofibrillated cellulose (NFC). Compared with the pure LiFePO₄ electrode, the resulting flexible composite (LiFePO₄/graphene/NFC) electrode showed excellent mechanical flexibility, and possessed an enhanced initial discharge capacity of 151 mA h/g (0.1 C) and a good capacity retention rate with only 5% loss after 60 cycles due to suitable electrolyte wettability at the interface. Furthermore, the NFC and graphene formed a three-dimensional conductive framework, which provided high-speed electron conduction in the composite and reduced electrode polarization during charging-discharging processes. Moreover, the composite electrode could endure bending tests up to 1000 times, highlighting preferable mechanical strength and durability. These results demonstrated that the as-fabricated electrodes could be applied as flexible electrodes with an embedded power supply.

New insights into plant cell walls by vibrational microspectroscopy

Vibrational spectroscopy provides non-destructively the molecular fingerprint of plant cells in the native state. In combination with microscopy, the chemical composition can be followed in context with the microstructure, and due to the non-destructive application, in-situ studies of changes during, e.g., degradation or mechanical load are possible. The two complementary vibrational microspectroscopic approaches, Fourier-Transform Infrared (FT-IR) Microspectroscopy and Confocal Raman spectroscopy, are based on different physical principles and the resulting different drawbacks and advantages in plant applications are reviewed. Examples for FT-IR and Raman microscopy applications on plant cell walls, including imaging as well as in-situ studies, are shown to have high potential to get a deeper understanding of structure-function relationships as well as biological processes and technical treatments. Both probe numerous different molecular vibrations of all components at once and thus result in spectra with many overlapping bands, a challenge for assignment and interpretation. With the help of multivariate unmixing methods (e.g., vertex components analysis), the most pure components can be revealed and their distribution mapped, even tiny layers and structures (250 nm). Instrumental as well as data analysis progresses make both microspectroscopic methods more and more promising tools in plant cell wall research.

Longitudinal Mechano-Sorptive Creep Behavior of Chinese Fir in Tension during Moisture Adsorption Processes

To provide comprehensive data on creep behaviors at relative humidity (RH) isohume conditions and find the basic characteristics of mechano-sorptive (MS) creep (MSC), the tensile creep behaviors, “viscoelastic creep (VEC)” at equilibrium moisture content and MSC during adsorption process, were performed on Chinese fir in the longitudinal direction under 20%, 40%, 60% and 80% RH (25 °C) and at 1, 1.3, and 1.6 MPa, respectively. The free swelling behavior was also measured, where the climate conditions corresponded with MSC tests. Based on the databases of free swelling, VEC, and MSC, the existence of MS effect was examined, and the application of the rheological model under the assumption of partitioned strain was investigated. The results revealed that both VEC and MSC increased with magnitude of applied stress, and the increasing RH level. Under all RH isohume conditions, the total strain of MSC was greater than that of VEC. The influence of RH level on VEC was attributed to the water plasticization effect, whereas that on MSC was presumed to be the effect of water plasticization and unstable state in the wood cell wall. In addition, the RH level promoted the relaxation behavior in MSC, while it slightly affected the relaxation behavior in VEC. In the future, the rheological model could consider the link between load configuration and the anatomic structural feature of wood.

Wood morphology and properties from molecular perspectives

• Background

It is with increasing interest that wood materials are now being considered as a green resource. For improving the product performance of wood derived materials new ways of separating them from wood are required. Thus, there is a great demand for a better understanding of the ultrastructure of wood and how the components are interaction on a molecular level in building up its properties.

• Material and method

By the use of microscopic and spectroscopic techniques combined with mechanical forces, new knowledge regarding especially the role of the matrix polymers, the hemicelluloses and lignin, has been gained. This relates specifically to molecular interaction and orientation.

• Results

It is here demonstrated that all of the wood polymers within the secondary cell wall exhibit a preferred orientation along the fibrils. The degree of orientation decreases in the order cellulose, hemicelluloses to the lignin which only shows a small degree of orientation, probably induced by structural constrains.

• Conclusion

This orientation distribution is probably what has to be considered to better predict transverse cell wall properties. Moisture accessible regions are also aligned in a parallel arrangement in the cellulose fibrils explaining its high moisture resistance. The lignin is surprisingly inactive in the stress transfer in the secondary wall. This could perhaps be related to the function of lignin providing compressive, hydrostatic resistance in the lenticular spaces between fibrils, when longitudinally straining the fibre. This knowledge of the ultrastructural properties of the wood polymers, here presented, provides for a better understanding of the cell wall properties.

Deformation micromechanics of all-cellulose nanocomposites: comparing matrix and reinforcing components

All-cellulose nanocomposites, comprising two different forms of cellulose nanowhiskers dispersed in two different matrix systems, are produced. Acid hydrolysis of both tunicate (T-CNWs) and cotton cellulose (CNWs) is carried out to produce the nanowhiskers. These nanowhiskers are then dispersed in a cellulose matrix material, produced using two dissolution methods; namely lithium chloride/N,N-dimethyl acetamide (LiCl/DMAc) and sodium hydroxide/urea (NaOH/urea). Crystallinity of both nanocomposite systems increases with the addition of nanowhiskers up to a volume fraction of 15 v/v%, after which a plateau is reached. Stress-transfer mechanisms, between the matrix and the nanowhiskers in both of these nanocomposites are reported. This is achieved by following both the mechanical deformation of the materials, and by following the molecular deformation of both the nanowhiskers and matrix phases using Raman spectroscopy. In order to carry out the latter of these analyses, two spectral peaks are used which correspond to different crystal allomorphs; cellulose-I for the nanowhiskers and cellulose-II for the matrix. It is shown that composites comprising a LiCl/DMAc based matrix perform better than NaOH/urea based systems, the T-CNWs provide better reinforcement than CNWs and that an optimum loading of nanowhiskers (at 15 v/v%) is required to obtain maximum tensile strength and modulus.

Plant micro- and nanomechanics: experimental techniques for plant cell-wall analysis

In the last few decades, micro- and nanomechanical methods have become increasingly important analytical techniques to gain deeper insight into the nanostructure and mechanical design of plant cell walls. The objective of this article is to review the most common micro- and nanomechanical approaches that are utilized to study primary and secondary cell walls from a biomechanics perspective. In light of their quite disparate functions, the common and opposing structural features of primary and secondary cell walls are reviewed briefly. A significant part of the article is devoted to an overview of the methodological aspects of the mechanical characterization techniques with a particular focus on new developments and advancements in the field of nanomechanics. This is followed and complemented by a review of numerous studies on the mechanical role of cellulose fibrils and the various matrix components as well as the polymer interactions in the context of primary and secondary cell-wall function.

The hierarchical structure and mechanics of plant materials

The cell walls in plants are made up of just four basic building blocks: cellulose (the main structural fibre of the plant kingdom) hemicellulose, lignin and pectin. Although the microstructure of plant cell walls varies in different types of plants, broadly speaking, cellulose fibres reinforce a matrix of hemicellulose and either pectin or lignin. The cellular structure of plants varies too, from the largely honeycomb-like cells of wood to the closed-cell, liquid-filled foam-like parenchyma cells of apples and potatoes and to composites of these two cellular structures, as in arborescent palm stems. The arrangement of the four basic building blocks in plant cell walls and the variations in cellular structure give rise to a remarkably wide range of mechanical properties: Young's modulus varies from 0.3 MPa in parenchyma to 30 GPa in the densest palm, while the compressive strength varies from 0.3 MPa in parenchyma to over 300 MPa in dense palm. The moduli and compressive strength of plant materials span this entire range. This study reviews the composition and microstructure of the cell wall as well as the cellular structure in three plant materials (wood, parenchyma and arborescent palm stems) to explain the wide range in mechanical properties in plants as well as their remarkable mechanical efficiency.

Crystalline and amorphous cellulose in the secondary walls of Arabidopsis

In the cell walls of higher plants, cellulose chains are present in crystalline microfibril, with an amorphous part at the surface, or present as amorphous material. To assess the distribution and relative occurrence of the two forms of cellulose in the inflorescence stem of Arabidopsis, we used two carbohydrate-binding modules, CBM3a and CBM28, specific for crystalline and amorphous cellulose, respectively, with immunogold detection in TEM. The binding of the two CBMs displayed specific patterns suggesting that the synthesis of cellulose leads to variable nanodomains of cellulose structures according to cell type. In developing cell walls, only CBM3a bound significantly to the incipient primary walls, indicating that at the onset of its deposition cellulose is in a crystalline structure. As the secondary wall develops, the labeling with both CBMs becomes more intense. The variation of the labeling pattern by CBM3a between transverse and longitudinal sections appeared related to microfibril orientation and differed between fibers and vessels. Although the two CBMs do not allow the description of the complete status of cellulose microstructures, they revealed the dynamics of the deposition of crystalline and amorphous forms of cellulose during wall formation and between cell types adapting cellulose microstructures to the cell function.

Influence of magnetic field alignment of cellulose whiskers on the mechanics of all-cellulose nanocomposites

Orientation of cellulose nanowhiskers (CNWs) derived from tunicates, in an all-cellulose nanocomposite, is achieved through the application of a magnetic field. CNWs are incorporated into a dissolved cellulose matrix system and during solvent casting of the nanocomposite a magnetic field is applied to induce their alignment. Unoriented CNW samples, without the presence of a magnetic field, are also produced. The CNWs are found to orient under the action of the magnetic field, leading to enhanced stiffness and strength of the composites, but not to the level that is theoretically predicted for a fully aligned system. Lowering the volume fraction of the CNWs is shown to allow them to orient more readily in the magnetic field, leading to larger relative increases in the mechanical properties. It is shown, using polarized light microscopy, that the all-cellulose composites have a domain structure, with some domains showing pronounced orientation of CNWs and others where no preferred orientation occurs. Raman spectroscopy is used to both follow the position of bands located at ~1095 and ~895 cm(-1) with deformation and also their intensity as a function rotation angle of the specimens. It is shown that these approaches give valuable independent information on the respective molecular deformation and orientation of the CNWs, and the molecules in the matrix phase, in oriented and nonoriented domains of all-cellulose composites. These data are then related to an increase in the level of molecular deformation in the axial direction, as revealed by the Raman technique. Little orientation of the matrix phase is observed under the action of the magnetic field indicating the dominance of the stiff CNWs in governing mechanical properties.

Micromechanics of TEMPO-oxidized fibrillated cellulose composites

Composites of poly(lactic) acid (PLA) reinforced with TEMPO-oxidized fibrillated cellulose (TOFC) were prepared to 15, 20, 25, and 30% fiber weight fractions. To aid dispersion and to improve stress transfer, we acetylated the TOFC prior to the fabrication of TOFC-PLA composite films. Raman spectroscopy was employed to study the deformation micromechanics in these systems. Microtensile specimens were prepared from the films and deformed in tension with Raman spectra being collected simultaneously during deformation. A shift in a Raman peak initially located at ~1095 cm(-1), assigned to C-O-C stretching of the cellulose backbone, was observed upon deformation, indicating stress transfer from the matrix to the TOFC reinforcement. The highest band shift rate, with respect to strain, was observed in composites having a 30% weight fraction of TOFC. These composites also displayed a significantly higher strain to failure compared to pure acetylated TOFC film, and to the composites having lower weight fractions of TOFC. The stress-transfer processes that occur in microfibrillated cellulose composites are discussed with reference to the micromechanical data presented. It is shown that these TOFC-based composite materials are progressively dominated by the mechanics of the networks, and a shear-lag type stress transfer between fibers.

Observations of multiscale, stress-induced changes of collagen orientation in tendon by polarized Raman spectroscopy

Collagen is a versatile structural molecule in nature and is used as a building block in many highly organized tissues, such as bone, skin, and cornea. The functionality and performance of these tissues are controlled by their hierarchical organization ranging from the molecular up to macroscopic length scales. In the present study, polarized Raman microspectroscopic and imaging analyses were used to elucidate collagen fibril orientation at various levels of structure in native rat tail tendon under mechanical load. In situ humidity-controlled uniaxial tensile tests have been performed concurrently with Raman confocal microscopy to evaluate strain-induced chemical and structural changes of collagen in tendon. The methodology is based on the sensitivity of specific Raman scattering bands (associated with distinct molecular vibrations, such as the amide I) to the orientation and the polarization direction of the incident laser light. Our results, based on the changing intensity of Raman lines as a function of orientation and polarization, support a model where the crimp and gap regions of collagen hierarchical structure are straightened at the tissue and molecular level, respectively. However, the lack of measurable changes in Raman peak positions throughout the whole range of strains investigated indicates that no significant changes of the collagen backbone occurs with tensing and suggests that deformation is rather redistributed through other levels of the hierarchical structure.

Interfacial energy dissipation in a cellulose nanowhisker composite

Cyclic tensile and compressive deformation is applied to cellulose nanowhisker-epoxy resin based model nanocomposites. The molecular deformation of the cellulose nanowhiskers within the epoxy resin matrix is followed using a Raman spectroscopy technique, whereby shifts in the position of a band located at ∼ 1095 cm(-1) are shown to correlate directly with a breakdown in the interfaces between the resin and the nanowhiskers and between nanowhiskers themselves. A theoretical model is used to determine the dissipation of energy at the interfaces between whiskers and at the whisker-matrix interface. This approach is shown to be useful for interpreting the local micromechanics of these materials by providing a quantitative measure of the quality of the interface.

Ultra-structural organisation of cell wall polymers in normal and tension wood of aspen revealed by polarisation FTIR microspectroscopy

Polarisation Fourier transform infra-red (FTIR) microspectroscopy was used to characterize the organisation and orientation of wood polymers in normal wood and tension wood from hybrid aspen (Populus tremula × Populus tremuloides). It is shown that both xylan and lignin in normal wood are highly oriented in the fibre wall. Their orientation is parallel with the cellulose microfibrils and hence in the direction of the fibre axis. In tension wood a similar orientation of lignin was found. However, in tension wood absorption peaks normally assigned to xylan exhibited a 90° change in the orientation dependence of the vibrations as compared with normal wood. The molecular origin of these vibrations are not known, but they are abundant enough to mask the orientation dependence of the xylan signal from the S₂ layer in tension wood and could possibly come from other pentose sugars present in, or associated with, the gelatinous layer of tension wood fibres.

Cross-linked bacterial cellulose networks using glyoxalization

In this study, we demonstrate that bacterial cellulose (BC) networks can be cross-linked via glyoxalization. The fracture surfaces of samples show that, in the dry state, less delamination occurs for glyoxalized BC networks compared to unmodified BC networks, suggesting that covalent bond coupling between BC layers occurs during the glyoxalization process. Young's moduli of dry unmodified BC networks do not change significantly after glyoxalization. The stress and strain at failure are, however, reduced after glyoxalization. However, the wet mechanical properties of the BC networks are improved by glyoxalization. Raman spectroscopy is used to demonstrate that the stress-transfer efficiency of deformed dry and wet glyoxalized BC networks is significantly increased compared to unmodified material. This enhanced stress-transfer within the networks is shown to be a consequence of the covalent coupling induced during glyoxalization and offers a facile route for enhancing the mechanical properties of BC networks for a variety of applications.