Mesoporosity of Delignified Wood Investigated by Water Vapor Sorption

Biodegradable and Flexible Wood-Gelatin Composites for Soft Actuating Systems

Compliant materials are indispensable for many emerging soft robotics applications. Hence, concerns regarding sustainability and end-of-life options for these materials are growing, given that they are predominantly petroleum-based and non-recyclable. Despite efforts to explore alternative bio-derived soft materials like gelatin, they frequently fall short in delivering the mechanical performance required for soft actuating systems. To address this issue, we reinforced a compliant and transparent gelatin-glycerol matrix with structure-retained delignified wood, resulting in a flexible and entirely biobased composite (DW-flex). This DW-flex composite exhibits highly anisotropic mechanical behavior, possessing higher strength and stiffness in the fiber direction and high deformability perpendicular to it. Implementing a distinct anisotropy in otherwise isotropic soft materials unlocks new possibilities for more complex movement patterns. To demonstrate the capability and potential of DW-flex, we built and modeled a fin ray-inspired gripper finger, which deforms based on a twist-bending-coupled motion that is tailorable by adjusting the fiber direction. Moreover, we designed a demonstrator for a proof-of-concept suitable for gripping a soft object with a complex shape, i.e., a strawberry. We show that this composite is entirely biodegradable in soil, enabling more sustainable approaches for soft actuators in robotics applications.

Bone collagen tensile properties of the aging human proximal femur

Despite the dominant role of bone mass in osteoporotic fractures, aging bone tissue properties must be thoroughly understood to improve osteoporosis management. In this context, collagen content and integrity are considered important factors, although limited research has been conducted on the tensile behavior of demineralized compact bone in relation to its porosity and elastic properties in the native mineralized state. Therefore, this study aims (i) at examining the age-dependency of mineralized bone and collagen micromechanical properties; (ii) to test whether, and if so to which extent, collagen properties contribute to mineralized bone mechanical properties. Two cylindrical cortical bone samples from fresh frozen human anatomic donor material were extracted from 80 proximal diaphyseal sections from a cohort of 24 female and 19 male donors (57 to 96 years at death). One sample per section was tested in uniaxial tension under hydrated conditions. First, the native sample was tested elastically (0.25 % strain), and after demineralization, up to failure. Morphology and composition of the second specimen was assessed using micro-computed tomography, Raman spectroscopy, and gravimetric methods. Simple and multiple linear regression were employed to relate morphological, compositional, and mechanical variables with age and sex. Macro-tensile properties revealed that only elastic modulus of native samples was age dependent whereas apparent elastic modulus was sex dependent (p < 0.01). Compositional and morphological analysis detected a weak but significant age and sex dependency of relative mineral weight (r = -0.24, p < 0.05) and collagen disorder ratio (I∼1670/I∼1640, r = 0.25, p < 0.05) and a strong sex dependency of bone volume fraction while generally showing consistent results in mineral content assessment. Young's modulus of demineralized bone was significantly related to tissue mineral density and Young's modulus of native bone. The results indicate that mechanical properties of the organic phase, that include collagen and non-collagenous proteins, are independent of donor age. The observed reduction in relative mineral weight and corresponding overall stiffer response of the collagen network may be caused by a reduced number of mineral-collagen connections and a lack of extrafibrillar and intrafibrillar mineralization that induces a loss of waviness and a collagen fiber pre-stretch.

Releasing Free Radicals in Precursor Triggers the Formation of Closed Pores in Hard Carbon for Sodium-Ion Batteries

Increasing closed pore volume in hard carbon is considered to be the most effective way to enhance the electrochemical performance in sodium-ion batteries. However, there is a lack of systematic insights into the formation mechanisms of closed pores at molecular level. In this study, a regulation strategy of closed pores via adjustment of the content of free radicals is reported. Sufficient free radicals are exposed by part delignification of bamboo, which is related to the formation of well-developed carbon layers and rich closed pores. In addition, excessive free radicals from nearly total delignification lead to more reactive sites during pyrolysis, which competes for limited precursor debris to form smaller microcrystals and therefore compact the material. The optimal sample delivers a large closed pore volume of 0.203 cm3 g-1, which leads to a high reversible capacity of 350 mAh g-1 at 20 mA g-1 and enhanced Na+ transfer kinetics. This work provides insights into the formation mechanisms of closed pores at molecular level, enabling rational design of hard carbon pore structures.

Lightweight, Strong, and Transparent Wood Films Produced by Capillary Driven Self-Densification

Wood delignification and densification enable the production of high strength and/or transparent wood materials with exceptional properties. However, processing needs to be more sustainable and besides the chemical delignification treatments, energy intense hot-pressing calls for alternative approaches. Here, this study shows that additional softening of delignified wood via a mild swelling process using an ionic liquid-water mixture enables the densification of tube-line wood cells into layer-by-layer sheet structures without hot-pressing. The natural capillary force induces self-densification in a simple drying process resulting in a transparent wood film. The as-prepared films with ≈150 µm thickness possess an optical transmittance ≈70%, while maintaining optical haze >95%. Due to the densely packed sheet structure with a large interfacial area, the reassembled wood film is fivefold stronger and stiffer than the delignified wood in fiber direction. Owing to a low density, the specific tensile strength and elastic modulus are as high as 282 MPa cm3 g-1 and 31 GPa cm3 g-1. A facile and highly energy efficient wood nanotechnology approach are demonstrated toward more sustainable materials and processes by directly converting delignified wood into transparent wood omitting polymeric matrix infiltration or mechanical pressing.

Advanced Material Design and Engineering for Water-Based Evaporative Cooling

Water-based evaporative cooling is emerging as a promising technology to provide sustainable and low-cost cold to alleviate the rising global cooling demand. Given the significant and fast progress made in recent years, this review aims to provide a timely overview on the state-of-the-art material design and engineering in water-based evaporative cooling. The fundamental mechanisms and major components of three water-based evaporative cooling processes are introduced, including direct evaporative cooling, cyclic sorption-driven liquid water evaporative cooling (CSD-LWEC), and atmospheric water harvesting-based evaporative cooling (AWH-EC). The distinctive requirements on the sorbent materials in CSD-LWEC and AWH-EC are highlighted, which helps synthesize the literature information on the advanced material design and engineering for the purpose of improving cooling performance. The challenges and future outlooks on further improving the water-based evaporative cooling performance are also provided.

Shape and Stiffness Switchable Hydroplastic Wood with Programmability and Reproducibility

Stiffness switchable materials (e.g., supramolecular polymers, metals) that alter their shape and mechanical properties in response to specific stimuli are potentially utilized in the structural engineering field but still limited due to the use of petroleum-based synthetic monomers and large energy consumption. Herein, a sustainable and facile solvent casting strategy is proposed to fabricate the "hydroplastic wood" with shape and stiffness switchable properties via cell wall wetting, cell wall softening and subsequent moisture evaporation. Therein, a wetting agent with low surface tension and low viscosity is utilized for covering the rough surface of solid wood to form a liquid lubricating layer, thereby increasing the interfacial wettability and achieving uniform softening of the cell walls. This interface wetting treatment can easily break through the hydro-plasticization process for thick wood (Balsa wood, Ochroma lagopus Swartz, density: 0.25 g/cm3; Pinewood, Pinus armandii, density: 0.38 g/cm3). Additionally, the capillary force arising from moisture evaporation induces the self-densification of oriented cellulose nanofibrils and achieves moisture-mediated shape design capabilities through periodic saturation-dehydration. This work makes hydroplastic wood a promising candidate for engineering materials because of its combined advantages of strong durability, formability, and load-carrying capacity.

Ectopic callose deposition into woody biomass modulates the nano-architecture of macrofibrils

Plant biomass plays an increasingly important role in the circular bioeconomy, replacing non-renewable fossil resources. Genetic engineering of this lignocellulosic biomass could benefit biorefinery transformation chains by lowering economic and technological barriers to industrial processing. However, previous efforts have mostly targeted the major constituents of woody biomass: cellulose, hemicellulose and lignin. Here we report the engineering of wood structure through the introduction of callose, a polysaccharide novel to most secondary cell walls. Our multiscale analysis of genetically engineered poplar trees shows that callose deposition modulates cell wall porosity, water and lignin contents and increases the lignin-cellulose distance, ultimately resulting in substantially decreased biomass recalcitrance. We provide a model of the wood cell wall nano-architecture engineered to accommodate the hydrated callose inclusions. Ectopic polymer introduction into biomass manifests in new physico-chemical properties and offers new avenues when considering lignocellulose engineering.

Transparent Wood Biocomposite of Well-Dispersed Dye Content for Fluorescence and Lasing Applications

Aggregation-induced quenching often restricts emissive performance of optically active solid materials with embedded fluorescent dyes. Delignified and nanoporous wood readily adsorbs organic dyes and is investigated as a host material for rhodamine 6G (R6G). High concentration of R6G (>35 mM) is achieved in delignified wood without any ground-state dye aggregation. To evaluate emissive performance, a solid-state random dye laser is prepared using the dye-doped wood substrates. The performance in terms of lasing threshold and efficiency was improved with increased dye content due to the ability of delignified wood to disperse R6G.

Wood xerogel for fabrication of high-performance transparent wood

Optically transparent wood has been fabricated by structure-retaining delignification of wood and subsequent infiltration of thermo- or photocurable polymer resins but still limited by the intrinsic low mesopore volume of the delignified wood. Here we report a facile approach to fabricate strong transparent wood composites using the wood xerogel which allows solvent-free infiltration of resin monomers into the wood cell wall under ambient conditions. The wood xerogel with high specific surface area (260 m² g^-1) and high mesopore volume (0.37 cm³ g^-1) is prepared by evaporative drying of delignified wood comprising fibrillated cell walls at ambient pressure. The mesoporous wood xerogel is compressible in the transverse direction and provides precise control of the microstructure, wood volume fraction, and mechanical properties for the transparent wood composites without compromising the optical transmittance. Transparent wood composites of large size and high wood volume fraction (50%) are successfully prepared, demonstrating potential scalability of the method.

Sustainability in Wood Products: A New Perspective for Handling Natural Diversity

Wood is a renewable resource with excellent qualities and the potential to become a key element of a future bioeconomy. The increasing environmental awareness and drive to achieve sustainability is leading to a resurgence of research on wood materials. Nevertheless, the global climate changes and associated consequences will soon challenge the wood-value chains in several regions (e.g., central Europe). To cope with these challenges, it is necessary to rethink the current practice of wood sourcing and transformation. The goal of this review is to address the intrinsic natural diversity of wood, from its origin to its technological consequences for the present and future manufacturing of wood products. So far, industrial processes have been optimized to repress the variability of wood properties, enabling more efficient processing and production of reliable products. However, the need to preserve biodiversity and the impact of climate change on forests call for new wood processing techniques and green chemistry protocols for wood modification as enabling factors necessary for managing a more diverse wood provision in the future. This article discusses the past developments that have resulted in the current wood value chains and provides a perspective about how natural variability could be turned into an asset for making truly sustainable wood products. After briefly introducing the chemical and structural complexity of wood, the methods conventionally adopted for industrial homogenization and modification of wood are discussed in relation to their evolution toward increased sustainability. Finally, a perspective is given on technological potentials of machine learning techniques and of novel functional wood materials. Here the main message is that through a combination of sustainable forestry, adherence to green chemistry principles and adapted processes based on machine learning, the wood industry could not only overcome current challenges but also thrive in the near future despite the awaiting challenges.

Intercellular Matrix Infiltration Improves the Wet Strength of Delignified Wood Composites

Delignified wood (DW) represents a promising bio-based fibrous material as a reinforcing component in high-performance composites. These cellulose composites possess excellent strength and stiffness in the dry state, which are significantly higher than for natural wood. However, in the wet state, a penetrating water layer enters the intercellular regions and disrupts the stress transfer mechanisms between cell fibers in fully DW. This water layer initially facilitates complex shaping of the material but imparts DW composites with very low wet stiffness and strength. Therefore, a sufficient stress transfer in the wet state necessitates a resin impregnation of these intercellular regions, establishing bonding mechanisms between adjacent fibers. Here, we utilize a water-based dimethyloldihydroxyethylene urea thermosetting matrix (DMDHEU) and compare it with a non-water-based epoxy matrix. We infiltrate these resins into DW and investigate their spatial distribution by scanning electron microscopy, atomic force microscopy, and confocal Raman spectroscopy. The water-based resin impregnates the intercellular areas and generates an artificial compound middle lamella, while the epoxy infiltrates only the cell lumina of the dry DW. Tensile tests in the dry and wet states show that the DMDHEU matrix infiltration of the intercellular areas and the cell wall results in a higher tensile strength and stiffness compared to the epoxy resin. Here, the artificial compound middle lamella made of DMDHEU bonds adjacent fibers together and substantially increases the composites' wet strength. This study elucidates the importance of the interaction and spatial distribution of the resin system within the DW structure to improve mechanical properties, particularly in the wet state.

Effect of Moisture on Polymer Deconstruction in HCl Gas Hydrolysis of Wood

The HCl gas system previously used to produce cellulose nanocrystals was applied on Scots pine wood, aiming at a controlled deconstruction of its macrostructure while understanding the effect on its microstructure. The HCl gas treatments resulted in a well-preserved cellular structure of the wood. Differences in wood initial moisture content (iMC) prior to HCl gas treatment played a key role in hydrolysis rather than the studied range of exposure time to the acidic gas. Higher iMCs were correlated with a higher degradation of hemicellulose, while crystalline cellulose microfibrils were not largely affected by the treatments. Remarkably, the hydrogen-deuterium exchange technique showed an increase in accessible OH group concentration at higher iMCs, despite the additional loss in hemicelluloses. Unrelated to changes in the accessible OH group concentration, the HCl gas treatment reduced the concentration of absorbed D₂O molecules.

Delignification Effects on Indonesian Momala (Homalium foetidum) and Korean Red Toon (Toona sinensis) Hardwood Pore Structure and Sound Absorption Capabilities

Among the various methods used to improve the sound absorption capability of wood, we focused on delignification in Indonesian momala (Homalium foetidum) and Korean red toon (Toona sinensis). We performed gas permeability, pore size, and porosity analyses and evaluated how the change in the pore structure affects the sound absorption capabilities. Results show that delignification increased the through-pore porosity and improved sound absorption capability in both species. In addition, the air gap in the rear space maximized the sound absorption of momala and the red toon. The noise reduction coefficient (NRC) of delignified momala (90 min) with a 3 cm air gap was 0.359 ± 0.023. This is approximately 154.6% higher than that of untreated momala without an air gap. The NRC of delignificated red toon (90 min) with a 3 cm air gap was 0.324 ± 0.040, an increase of 604.3% over untreated red toon without an air gap.

Microscale compressive behavior of hydrated lamellar bone at high strain rates

The increased risk of fracture in the elderly associated with metabolic conditions like osteoporosis poses a significant strain on health care systems worldwide. Due to bone's hierarchical nature, it is necessary to study its mechanical properties and failure mechanisms at several length scales. We conducted micropillar compression experiments on ovine cortical bone to assess the anisotropic mechanical response at the lamellar scale over a wide range of strain rates (10^-4 to 8·10² s^-1). At the microscale, lamellar bone exhibits a strain rate sensitivity similar to what is reported at the macroscale suggesting that it is an intrinsic property of the extracellular matrix. Significant shear band thickening was observed at high strain rates by HRSEM and STEM imaging. This is likely caused by the material's inability to accommodate the imposed deformation by propagation of thin kink bands and shear cracks at high strain rates, leading to shear band thickening and nucleation. The post-yield behavior is strain rate and direction dependent: hardening was observed for transverse oriented micropillars and hardening modulus increases with strain rate by a factor of almost 2, while axially oriented micropillars showed strain softening and an increase of the softening peak width and work to ultimate stress as a function of strain rate. This suggests that for compression at the micrometer scale, energy absorption in bone increases with strain rate. This study highlights the importance of investigating bone strength and post-yield behavior at lower length scales, under hydrated conditions and at clinically relevant strain rates. STATEMENT OF SIGNIFICANCE: We performed micropillar compression experiments of ovine cortical bone at two different orientations and over seven orders of magnitude of strain rate. Experiments were performed under humid condition to mimic the natural conditions of bone in a human body using a newly developed micro-indenter setup. The strain rate sensitivity was found to be of a similar magnitude to what has been reported for higher length scales, suggesting that the strain rate sensitivity is an intrinsic property of the bone extracellular matrix. In addition, localized shear deformation in thick bands was observed for the first time at high strain rates, highlighting the importance of investigating bone under conditions representative of an accident or fall at several length scales.

The Study of Bound Water Status and Pore Size Distribution of Chinese Fir and Poplar Cell Wall by Low-Field NMR

With the increasing shortage of timber resources and the advancement of environmental protection projects, many artificial fast-growing forests are planted and used as raw materials in China. There are significant differences in the properties of natural forest wood and artificial fast-growing forest wood, and the properties of wood mainly depend on the change in the status of bound water in the cell wall. In this study, the fiber saturation point (FSP) and pore size distributions within the cell wall of six species of fast-growing forest wood were studied by low-temperature nuclear magnetic resonance (NMR) technology. The effects of species, growth rings, and extractives on the FSP and pore structure were analyzed. The water vapor sorption experiments were performed, and the adsorption isotherms of the samples were fitted through the Guggenheim-Anderson-de Boer (GAB) equation. According to the least-square regression of the adsorption isotherms and combined with the low-temperature NMR experiments, the content and proportion of the different types of bound water were analyzed. The results showed that the average FSP of each Chinese fir was about 40% and that of each poplar was about 35%. There is about a 10% difference between the FSP measured by NMR technology and the adsorption bound water content obtained by adsorption isothermal. The pore size distribution results show that in all samples, the proportion of pores larger than 10.5 nm is very low, about 10%; the proportion of 1.92-10.5 nm pores is about 30%; and the proportion of pores smaller than 1.92nm is more than 50%. This work will be helpful to the study of the wood moisture status and provide reference data for wood modification.

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.

Plant Nanomaterials and Inspiration from Nature: Water Interactions and Hierarchically Structured Hydrogels

Recent developments in the area of plant-based hydrogels are introduced, especially those derived from wood as a widely available, multiscale, and hierarchical source of nanomaterials, as well as other cell wall elements. With water being fundamental in a hydrogel, water interactions, hydration, and swelling, all critically important in designing, processing, and achieving the desired properties of sustainable and functional hydrogels, are highlighted. A plant, by itself, is a form of a hydrogel, at least at given states of development, and for this reason phenomena such as fluid transport, diffusion, capillarity, and ionic effects are examined. These aspects are highly relevant not only to plants, especially lignified tissues, but also to the porous structures produced after removal of water (foams, sponges, cryogels, xerogels, and aerogels). Thus, a useful source of critical and comprehensive information is provided regarding the synthesis of hydrogels from plant materials (and especially wood nanostructures), and about the role of water, not only for processing but for developing hydrogel properties and uses.

Sustainable Wood Nanotechnologies for Wood Composites Processed by In-Situ Polymerization

The development of large, multifunctional structures from sustainable wood nanomaterials is challenging. The need to improve mechanical performance, reduce moisture sensitivity, and add new functionalities, provides motivation for nanostructural tailoring. Although existing wood composites are commercially successful, materials development has not targeted nano-structural control of the wood cell wall, which could extend the property range. For sustainable development, non-toxic reactants, green chemistry and processing, lowered cumulative energy requirements, and lowered CO2-emissions are important targets. Here, modified wood substrates in the form of veneer are suggested as nanomaterial components for large, load-bearing structures. Examples include polymerization of bio-based monomers inside the cell wall, green chemistry wood modification, and addition of functional inorganic nanoparticles inside the cell wall. The perspective aims to describe bio-based polymers and green processing concepts for this purpose, along with wood nanoscience challenges.

Wood Modification as a Tool to Understand Moisture in Wood

Moisture plays a central role in the performance of wood products because it affects important material properties such as the resistance to decomposition, the mechanical properties, and the dimensions. To improve wood performance, a wide range of wood modification techniques that alter the wood chemistry in various ways have been described in the literature. Typically, these modifications aim to improve resistance to decomposition, dimensional stability, or, to introduce novel functionalities in the wood. However, wood modification techniques can also be an important tool to improve our understanding of the interactions between wood and moisture. In this review, we describe current knowledge gaps in our understanding of moisture in wood and how modification has been and could be used to clarify some of these gaps. This review shows that introducing specific chemical changes, and even controlling the distribution of these, in combination with the variety of experimental methods available for characterization of moisture in wood, could give novel insights into the interaction between moisture and wood. Such insights could further contribute to applications in several related fields of research such as how to enhance the resistance to decomposition, how to improve the performance of moisture-induced wooden actuators, or how to improve the utilization of wood biomass with challenging swelling anisotropy.

Bundling of cellulose microfibrils in native and polyethylene glycol-containing wood cell walls revealed by small-angle neutron scattering

Wood and other plant-based resources provide abundant, renewable raw materials for a variety of applications. Nevertheless, their utilization would greatly benefit from more efficient and accurate methods to characterize the detailed nanoscale architecture of plant cell walls. Non-invasive techniques such as neutron and X-ray scattering hold a promise for elucidating the hierarchical cell wall structure and any changes in its morphology, but their use is hindered by challenges in interpreting the experimental data. We used small-angle neutron scattering in combination with contrast variation by poly(ethylene glycol) (PEG) to identify the scattering contribution from cellulose microfibril bundles in native wood cell walls. Using this method, mean diameters for the microfibril bundles from 12 to 19 nm were determined, without the necessity of cutting, drying or freezing the cell wall. The packing distance of the individual microfibrils inside the bundles can be obtained from the same data. This finding opens up possibilities for further utilization of small-angle scattering in characterizing the plant cell wall nanostructure and its response to chemical, physical and biological modifications or even in situ treatments. Moreover, our results give new insights into the interaction between PEG and the wood nanostructure, which may be helpful for preservation of archaeological woods.

Determination of mesopores in the wood cell wall at dry and wet state

Wood porosity is of great interest for basic research and applications. One aspect is the cell wall porosity at total dry state. When water is absorbed by wood, the uptake of water within the cell wall leads to a dimension change of the material. A hypothesis for possible structures that hold the water is induced cell wall porosity. Nitrogen and krypton physisorption as well as high pressure hydrogen sorption and thermoporosimetry were applied to softwood and hardwood (pine and beech) in dry and wet state for determining surface area and porosity. Physisorption is not able to detect pores or surface area within the cell wall. Krypton physisorption shows surface area up 5 times lower than nitrogen with higher accuracy. With high pressure sorption no inaccessible pore volumes were seen at higher pressures. Thermoporosimetry was not able to detect mesopores within the hygroscopic water sorption region. Physisorption has to be handled carefully regarding the differences between adsorptives. The absence of water-induced mesopores within the hygroscopic region raise doubts on existing water sorption theories that assume these pore dimensions. When using the term “cell wall porosity”, it is important to distinguish between pores on the cell wall surface and pores that exist because of biological structure, as there are no water-induced mesopores present. The finding offers the possibility to renew wood-water-sorption theories because based on the presented results transport of water in the cell wall must be realized by structures lower than two 2 nm. Nanoporous structures in wood at wet state should be investigated more intensively in future.

Nonlocal density functional theory of water taking into account many-body dipole correlations: binodal and surface tension of 'liquid-vapour' interface

In this paper we formulate a nonlocal density functional theory of inhomogeneous water. We model a water molecule as a couple of oppositely charged sites. The negatively charged sites interact with each other through the Lennard-Jones potential (steric and dispersion interactions), square-well potential (short-range specific interactions due to electron charge transfer), and Coulomb potential, whereas the positively charged sites interact with all types of sites by applying the Coulomb potential only. Taking into account the nonlocal packing effects via the fundamental measure theory, dispersion and specific interactions in the mean-field approximation, and electrostatic interactions at the many-body level through the random phase approximation, we describe the liquid-vapour interface. We demonstrate that our model without explicit account of the association of water molecules due to hydrogen bonding and with explicit account of the electrostatic interactions at the many-body level is able to describe the liquid-vapour coexistence curve and the surface tension at the ambient pressures and temperatures. We obtain very good agreement with available in the literature MD simulation results for density profile of liquid-vapour interface at ambient state parameters. The formulated theory can be used as a theoretical background for describing of the capillary phenomena, occurring in micro- and mesoporous materials.

Atomic force microscopy imaging of delignified secondary cell walls in liquid conditions facilitates interpretation of wood ultrastructure

Deep understanding of the physicochemical and structural characteristics of wood at the nanoscale is essential for improving wood usage in biorefining and advancing new high performance materials design. Herein, we use in situ atomic force microscopy and a simple delignification treatment to elucidate the nanoscale architecture of individual secondary cell wall layers. Advantages of this approach are: (i) minimal sample preparation that reduces the introduction of potential artifacts; (ii) prevention of structural rearrangements due to dehydration; (iii) increased accessibility to structural details masked by the lignin matrix; and (iv) possibility to complement results with other analytical techniques without sample manipulation. The methodology permits the visualization of parallel and helicoidally arranged microfibril aggregates in the S1 layer and the determination of lignin contribution to microfibril aggregates forming S2 layers. Cellulose and hemicelluloses constitute the core of the aggregates with a mean diameter of approximately 19 nm, and lignin encloses the core forming single structural entities of about 30 nm diameter. Furthermore, we highlight the implications of sample preparation and imaging parameters on the characterization of microfibril aggregates by AFM.

Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy

Fresh soot particles are generally hydrophobic, however, particle hydrophilicity can be increased through atmospheric aging processes. At present little is known on how particle chemical composition and hydrophilicity change upon atmospheric aging and associated uncertainties governing the ice cloud formation potential of soot. Here we sampled two propane flame soots referred to as brown and black soot, characterized as organic carbon rich and poor, respectively. We investigated how the ice nucleation activity of these particles changed through aging in water and aqueous acidic solutions, using a continuous flow diffusion chamber operated at cirrus cloud temperatures (T ≤ 233 K). Single aggregates of both unaged and aged soot were chemically characterized by scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM/NEXAFS) measurements. Particle wettability was determined through water sorption measurements. Unaged black and brown soot particles exhibited significantly different ice nucleation activities. Our experiments revealed significantly enhanced ice nucleation activity of the aged soot particles compared to the fresh samples, lowering the required relative humidities at which ice formation can take place at T = 218 K by up to 15% with respect to water (ΔRHi ≈ 25%). We observed an enhanced water uptake capacity for the aged compared to the unaged samples, which was more pronounced for the black soot. From these measurements we concluded that there is a change in ice nucleation mechanism when aging brown soot. Comparison of the NEXAFS spectra of unaged soot samples revealed a unique spectral feature around 287.5 eV in the case of black soot that was absent for the brown soot, indicative of carbon with hydroxyl functionalities. Comparison of the NEXAFS spectra of unaged and aged soot particles indicates changes in organic functional groups, and the aged spectra were found to be largely similar across soot types, with the exception of the water aged brown soot. Overall, we conclude that atmospheric aging is important to representatively assess the ice cloud formation activity of soot particles.

Hygroscopicity of Waterlogged Archaeological Wood from Xiaobaijiao No.1 Shipwreck Related to Its Deterioration State

Waterlogged archaeological wood (WAW) artifacts, made of natural biodegradable polymers, are important parts of many precious cultural heritages. It is of great importance to understand the hygroscopic behavior of WAW in different deterioration states for the development of optimal drying processes and choices of safe storage in varying conditions. This was investigated in a case-study using two Hopea (Giam) and two Tectona (Teak) WAW samples collected from the Xiaobaijiao No.1 shipwreck. The deterioration state of WAW was evaluated by the maximum water content (MWC) method and by the cell morphological structure. Both Hopea and Tectona WAW could be classified into moderately and less decayed WAW. The hygroscopic behavior of moderately and less decayed WAW was then comparatively investigated using Dynamic Vapor Sorption (DVS) measurements alongside two sorption fitting models. Compositional analysis and hydroxyl accessibility measurements of WAW cell walls were shown to correlate with the hygroscopicity of WAW in different deterioration states. It was concluded that moderately decayed WAW possessed higher hygroscopicity and hysteresis than less decayed WAW because of the lower relative content of polysaccharides and the higher relative content of lignin, including the slow hydrolysis of O-acetyl groups of xylan and the partial breakage of β-O-4 interlinks, accompanied by an increased hydroxyl accessibility. This work helps in deciding on which consolidation measures are advised for shipwreck restauration, i.e., pretreatments with specific consolidates during wood drying, particularly for wooden artifacts displayed in museums.

Delignified Wood-Polymer Interpenetrating Composites Exceeding the Rule of Mixtures

Wood is increasingly considered in sustainable structural materials development due to its hierarchical structure, including an oriented reinforcing cellulose phase combined with carbon capturing and renewability. Top-down manufacturing techniques can provide direct access to this hierarchical cellulose scaffold for use in new functional materials. For high-performance load-bearing wood-based materials, the volume content of the reinforcing phase needs to be increased to much higher fiber volume contents (FVCs). This has been achieved by structure-retaining delignification followed by densification. The obtained matrix-free materials possess high tensile stiffness due to preservation of hierarchical fiber alignment; however, they demonstrate low mechanical properties in bending and cannot be used in moist conditions due to their propensity for water absorption. In order to address these two challenges, an interpenetrating wood polymer phase composite is developed using a delignified wood scaffold as a continuous reinforcing phase and epoxy resin as the interconnected matrix phase. We utilize the continuous flow channels in delignified wood for vacuum-assisted matrix infiltration in a condition of open continuous porosity in the wood scaffold. Prior to matrix curing, the material is densified in order to increase the FVC, decrease porosity, and reduce density variations in the wood scaffold. Due to the compressibility of delignified cellulose fibers, interpenetrating phase composites (IPCs) with very high FVCs of up to 80% could be produced, leading to exceptionally high tensile stiffness and strength of up to 70 GPa and 600 MPa. The obtained stiffness values far exceed the upper limit of the rule of mixtures due to an enhanced stress transfer through mechanically interlocked fiber-fiber interfaces combined with the stiffness providing matrix phase that further aids stress transfer between neighboring wood cells via their pits. This new approach paves the way for an efficient production of high-performance sustainable materials that can be used as alternative for glass fiber reinforced composites or natural fiber composites.