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A multifunctional soft material with high ionic and electrical conductivity, combined with high mechanical properties and the ability to change shape can enable bioinspired responsive devices and systems. The incorporation of all these characteristics in a single material is very challenging, as the improvement of one property tends to reduce other properties. Here, a nanocomposite film based on charged, high-aspect-ratio 1D flexible nanocellulose fibrils, and 2D Ti3C2Tx MXene is presented. The self-assembly process results in a stratified structure with the nanoparticles aligned in-plane, providing high ionotronic conductivity and mechanical strength, as well as large water uptake. In hydrogel form with 20 wt% liquid, the electrical conductivity is over 200 S cm−1 and the in-plane tensile strength is close to 100 MPa. This multifunctional performance results from the uniquely layered composite structure at nano- and mesoscales. A new type of electrical soft actuator is assembled where voltage as low as ±1 V resulted in osmotic effects and giant reversible out-of-plane swelling, reaching 85% strain., QC 20240624

To address the rising need of sustainable solutions in electronic devices, the development of electronically conductive composites based on lightweight but mechanically strong wood structures is highly desirable. Here, a facile approach for the fabrication of highly conductive wood/polypyrrole composites through top-down modification of native lignin followed by polymerization of pyrrole in wood cell wall. By sodium sulfite treatment under neutral condition, sulfonated wood veneers with increased porosity but well-preserved cell wall structure containing native lignin and lignosulfonates are obtained. The wood structure has a content of sulfonic groups up to 343 mu mol g(-1) owing to in situ sulfonated lignin which facilitates subsequent oxidative polymerization of pyrrole, achieving a weight gain of polypyrrole as high as 35 wt%. The lignosulfonates in the wood structure act as dopant and stabilizer for the synthesized polypyrrole. The composite reaches a high conductivity of 186 S m(-1) and a specific pseudocapacitance of 1.71 F cm(-2) at the current density of 8.0 mA cm(-2). These results indicate that tailoring the wood/polymer interface in the cell wall and activating the redox activity of native lignin by sulfonation are important strategies for the fabrication of porous and lightweight wood/conductive polymer composites with potential for sustainable energy applications., Funding Agencies|Wallenberg Wood Science Center - Knut and Alice Wallenberg Foundation

Nature has evolved elegant ways to alter the wood cell wall structure through carbohydrate-active enzymes, offering environmentally friendly solutions to tailor the microstructure of wood for high-performance materials. In this work, the cell wall structure of delignified wood is modified under mild reaction conditions using an oxidative enzyme, lytic polysaccharide monooxygenase (LPMO). LPMO oxidation results in nanofibrillation of cellulose microfibril bundles inside the wood cell wall, allowing densification of delignified wood under ambient conditions and low pressure into transparent anisotropic films. The enzymatic nanofibrillation facilitates microfibril fusion and enhances the adhesion between the adjacent wood fiber cells during densification process, thereby significantly improving the mechanical performance of the films in both longitudinal and transverse directions. These results improve the understanding of LPMO-induced microstructural changes in wood and offer an environmentally friendly alternative for harsh chemical treatments and energy-intensive densification processes thus representing a significant advance in sustainable production of high-performance wood-derived materials., QC 20230627

The preparation of wood substrates modified by charged inorganic nanoparticles (NPs) diffusing into the internal cell wall structure is investigated for generating functional properties. The flammability problem of wood biocomposites is addressed. NPs applied from colloidal sols carry charge to stabilize them against aggregation. The influence of charge on particle diffusion and adsorption should play a role for their spatial distribution and localization in the wood substrate biocomposite. It is hypothesized that improved dispersion, infiltration, and stability of NPs into the wood structure can be achieved by charge control diffusion, also restricting NP agglomeration and limiting distribution to the wood cell wall. Cationic and anionic silica NPs of ≈30 nm are therefore allowed to diffuse into bleached wood. The influence of charge on distribution in wood is investigated as a function of initial sol concentration. Transparent wood is fabricated by in situ polymerization of a thiol­ene in the wood pore space. These biocomposites demonstrate excellent flame retardancy with self­extinguishing characteristics. The approach has potential for commercial fabrication of flame retardant transparent composites for glazing and other building applications., QC 20220523

The optical response of hierarchical materials is convoluted, which hinders their direct study and property control. Transparent wood (TW) is an emerging biocomposite in this category, which adds optical function to the structural properties of wood. Nano- and microscale inhomogeneities in composition, structure and at interfaces strongly affect light transmission and haze. While interface manipulation can tailor TW properties, the realization of optically clear wood requires detailed understanding of light-TW interaction mechanisms. Here we show how material scattering and absorption coefficients can be extracted from a combination of experimental spectroscopic measurements and a photon diffusion model. Contributions from different length scales can thus be deciphered and quantified. It is shown that forward scattering dominates haze in TW, primarily caused by refractive index mismatch between the wood substrate and the polymer phase. Rayleigh scattering from the wood cell wall and absorption from residual lignin have minor effects on transmittance, but the former affects haze. Results provide guidance for material design of transparent hierarchical composites towards desired optical functionality; we demonstrate experimentally how transmittance and haze of TW can be controlled over a broad range., QC 20220125

QC 20220404

Lightweight polymer foams from synthetic polymers are commonly used in a wide-spread spectrum of application fields. Their intrinsic flammability coupled with restrictions on flame retardant chemicals poses a severe threat to safety. Here, fire resistant foams comprising biobased components capable of replacing petroleum-based foams are investigated. Cellulose nanofibers are combined with 2D montmorillonite nanoplatelets and a native xyloglucan hemicellulose binder, using a water-based freeze casting approach. Due to the silicate nanoplatelets, these lightweight foams self-extinguish the flame during flammability tests. The limiting oxygen index is as high as 31.5% and in the same range as the best fire-retardant synthetic foams available. In cone calorimetry, the foams display extremely low combustion rates. Smoke release is near the detection limit of the instrument. In addition, the foams are withstanding the penetration of a flame torch focused on one side of the specimen (T on surface 800 °C) and structural integrity is maintained. At the same time, the unexposed side is insulated, as demonstrated by a through-thickness temperature drop of 680 °C cm−1. The results represent a tremendous opportunity for the development of fire-safe foams combining excellent sustainability with multifunctional performance., QC 20220810

Transparent wood (TW), a biocomposite material demonstrating optical transparency in the visible range, has attracted much interest in recent years due to great potential for ecofriendly applications, for instance, in construction industry and functionalized organic materials. Optical properties of TW, including transparency and haze, depend on a particular structure of cellulose-based backbone compound, (mis-)matching of the refractive indices (RIs) between TW compounds, and the polymer matrix. Although there are data of cellulose RIs for various forms of cellulose (fibers, powder, hot-pressed films, etc.), these values might differ from an effective RI of the TW substrate. Herein, a numerical model of light propagation in the TW, based on the real cellular structure in wood, is presented and applied to estimate an effective RI of the delignified wood reinforcement in the experimentally investigated TW material. Ray-tracing and rigorous electromagnetic approaches are compared for modeling light propagation in the TW. Ray tracing demonstrates considerably simplified yet accurate and efficient solutions. The work brings substantial progress toward realistic and practical wood modeling for the purpose of applications, materials design, and fundamental studies., Funding agencies: Knut and Alice Wallenberg foundation through the Wallenberg Wood Science Center at KTH Royal Institute of Technology and Linköping University European Research Council Advanced Grant Wood NanoTech. Grant Number: 742733

In the native wood cell wall, cellulose microfibrils are highly aligned and organized in the secondary cell wall. A new preparation strategy is developed to achieve individualization of cellulose microfibrils within the wood cell wall structure without introducing mechanical disintegration. The resulting mesoporous wood structure has a high specific surface area of 197 m2 g−1 when prepared by freeze‐drying using liquid nitrogen, and 249 m2 g−1 by supercritical drying. These values are 5 to 7 times higher than conventional delignified wood (36 m2 g−1) dried by supercritical drying. Such highly mesoporous structure with individualized cellulose microfibrils maintaining their natural alignment and organization can be processed into aerogels with high porosity and high compressive strength. In addition, a strong film with a tensile strength of 449.1 ± 21.8 MPa and a Young's modulus of 51.1 ± 5.2 GPa along the fiber direction is obtained simply by air drying owing to the self‐densification of cellulose microfibrils driven by the elastocapillary forces upon water evaporation. The self‐densified film also shows high optical transmittance (80%) and high optical haze (70%) with interesting biaxial light scattering behavior owing to the natural alignment of cellulose microfibrils., QC 20201221

Polyurethane (PU) nanocomposites utilizing cellulose nanocrystals (CNCs) as nanofiller and amorphous PU matrix were synthesized in a novel solvent-free bulk process. A green nanofiller, CNCs, was studied as reinforcement and was further modified by grafting poly(ethylene glycol) (PEG) on the CNC surface (CNC-PEG). Transmission electron microscopy revealed an excellent dispersion of the PEGylated CNC nanoparticles in the PU matrix, whereas as-received CNCs formed agglomerates. The results indicated strong improvements in tensile properties with Young's modulus increasing up to 50% and strength up to 25% for both, PU/CNC and PU/CNC-PEG nanocomposites. The enhanced tensile modulus was attributed to stiff particle reinforcement together with an increase in glass transition temperature., QC 20220215