Mildly delignified softwood holocellulose fibers featuring native tracheid fiber cell wall structure and high hemicellulose content are prominent building blocks for wood derived fiber-based materials. However, preserving the natural alignment of long softwood fiber is challenging since top-down structure-retaining delignified softwood is unstable as extensive removal of lignin from intercellular space induces cracking and disintegration of wood structure. Here we report the use of chemical crosslinking pretreatment to improve the intercellular bonding between softwood fibers, therefore preserving the integrity of the naturally aligned softwood fibers after delignification. The crosslinked softwood veneer was delignified with peracetic acid and further densified into transparent and high-density film by thermal compression. The obtained transparent film of naturally aligned softwood holocellulose fibers showed high optical transmittance of 71 %, high haze of 85 %, strong optical anisotropy, as well as high tensile strength of 449 ± 58 MPa and high Young's modulus of 49.9 ± 5.6 GPa. This study provides a facile approach to preserve the natural alignment of softwood fibers for the fabrication of high performance holocellulose fibers-based materials.

Porous cellular foams, combining lightweight, high strength, and compressibility, hold great promise in a wide range of advanced applications. Here, the native structure of pine wood was modified by in-situ lignin sulfonation and unidirectional freezing, resulting in an alveolate structure inside the wood cell wall with arrays of sub-100 nm channels. The obtained wood foam exhibited highly enhanced permeability while retaining the native cellular arrangement and high lignin and hemicellulose content. Such engineered cellular foam contributed to superior mechanical performance with compressive strength of 9 MPa and Young's modulus of 344 MPa in the longitudinal direction. The high porosity allowed homogeneous infiltration of conductive polymer PEDOT:PSS inside the wood cell wall. The resulting composite exhibited high conductivity, sponge-like compressibility and the ability to modulate electrical resistance in a reversible manner in the radial direction. This rationally designed conductive wood demonstrated potential in durable and ultrasensitive pressure-responsive devices and strain sensors.

Rare earths (REs) incorporated in glasses, mostly in the form of RE3+ ions, have several applications such as lasers and optical amplifiers, spectral conversion layers for solar cells, light emitters and sensors. In this context, both the composition and the structural properties of the glass, as well as the dopant concentration play an important role in determining the optical properties and the efficiency of the system. Usually, the concentration of REs is small, below 1 at%, to avoid clustering and optical quenching. In this paper, we report the case of sol-gel Eu-doped silica-soda glass films. The addition of soda to silica can reduce RE clustering and precipitation, according to molecular dynamic simulations, but brings structural instabilities to the network. Here, sodium was varied from 10 to 30 at% and Eu from 0 to 8 at%. It was shown that Eu plays a significant role in the stabilization of the matrix, improving the transparency, the refractive index and the thickness of the films. The increase of Eu concentration provides a decrease of site symmetry and an increase of quantum efficiency (QY), reaching 71% for the highest 8 at% Eu doping, with remarkable absence of concentration quenching.

The nature of mass transport in plants has recently inspired the development of low-cost and sustainable wood-based electronics. Herein, we report a wood electrochemical transistor (WECT) where all three electrodes are fully made of conductive wood (CW). The CW is prepared using a two-step strategy of wood delignification followed by wood amalgamation with a mixed electron-ion conducting polymer, poly(3,4-ethylenedioxythiophene)–polystyrene sulfonate (PEDOT:PSS). The modified wood has an electrical conductivity of up to 69 Sm−1 induced by the formation of PEDOT:PSS microstructures inside the wood 3D scaffold. CW is then used to fabricate the WECT, which is capable of modulating an electrical current in a porous and thick transistor channel (1 mm) with an on/off ratio of 50. The device shows a good response to gate voltage modulation and exhibits dynamic switching properties similar to those of an organic electrochemical transistor. This wood-based device and the proposed working principle demonstrate the possibility to incorporate active electronic functionality into the wood, suggesting different types of bio-based electronic devices.

Through 270 million years of evolution, the finely tuned hierarchical structure of wood has been optimized for efficient nutrient transport and exceptional mechanical stability. Its distinctive orthotropic constitution can provide inspiration and design opportunities for the development of novel functional materials. In recent years, top-down modification approaches have adapted the wood structure for innovative applications, utilizing the hierarchical arrangement at different length scales. In doing so, preserving the structural integrity is of the essence.

This thesis explores new top-down modification techniques for the functionalization and structural control of wood-based materials. With the intent of better preserving and utilizing the natural wood organization and native components, two different modification routes were explored on softwood Scots pine: complete lignin removal and in-situ lignin modification. Complete delignification was achieved through preventive crosslinking of the polysaccharide matrix, enhancing intercellular adhesion between tracheids and preventing the disintegration of the cellular arrangement after lignin removal. The second approach focused on chemical modification of lignin by sulfonation as an alternative to complete lignin removal, resulting in wood templates of high negative charge up to 375 µmol g^-1 and with well-preserved residual lignin. 

Hot compression of the delignified wood veneers produced thin wood films with high optical transmittance of 71 % alongside exceptional tensile strength of 449 MPa and Young’s modulus of 50 GPa. Densification of lignin-retaining wood veneers yielded strong and transparent thin films with UV blocking ability. Additionally, these densified films could be easily recycled into discrete wood fibers. 

The integration of conductive polymers including poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polypyrrole in in-situ sulfonated wood resulted in bio-composites with high conductivity up to 203 S m^-1 and high pseudo-capacitance up to 38 mF cm^-2, indicating that tailoring the wood chemistry and activating the redox activity of lignin by sulfonation are important strategies for the fabrication of composites with potential for sustainable energy applications. 

By tailoring both wood chemistry and morphology, a wood foam with unique microstructure, enhanced permeability, along with high ultimate strength of 9 MPa and Young’s modulus of 364 MPa was obtained. When combined with the conductive polymer PEDOT:PSS, the composite demonstrated uniform conductivity of 215 S m^-1 and mechanoresponsive electrical resistance, showing promise in sensing and mechanoresponsive devices.

Therefore, in-situ engineering of lignin proved to be a versatile toolkit to obtain wood templates of improved permeability and porosity, greater compliance to densification, and enhanced compatibility with conductive polymers.

Träets noggranna hierarkiska struktur har genom 270 miljoner års evolution optimerats för effektiv transport av näringsämnen och enastående mekanisk stabilitet. Dess distinkta ortotropa struktur kan inspirera samt ge utvecklingsmöjligheter för nya funktionella material. Via top-down metoder har man under de senaste åren lyckats nyttja träets hierarkiska struktur i olika storleksordningar för innovativa applikationer. I detta sammanhang är bevarandet av den strukturella integriteten av största vikt.

Denna avhandling undersöker nya top-down metoder för att funktionalisera och kontrollera trämaterialens struktur. Med avsikt att bättre bevara och nyttja träets naturliga struktur och komponenter har två olika modifieringsmetoder undersökts för tall (pinus sylverstris): fullständig avlägsning av lignin samt in-situ modifiering av lignin. För att motverka cellstrukturens sönderfall efter fullständig delignifiering krävs det att polysackarider tvärbinds för att stärka vidhäftningen mellan trakeiderna. Modifiering av lignin (sulfonering) undersöktes som ett alternativ till fullständig delignifiering och resulterade i trämaterial med hög negativ laddning, upp till 375 µmol g^-1, och med välbevarat lignin. 

Varmpressning av delignifierade träfanér resulterade i tunna träfilmer med hög optisk transmittans på 71 %, enastående draghållfasthet på 449 MPa och Youngs modul på 50 GPa. Förtätning av fanér med bevarat lignin gav starka och transparenta filmer med UV-blockerande förmåga vars massafibrer även kunde återvinnas.

Integrering med elektriskt ledande polymerer, som poly(3,4-etylendioxytiopen) polystyrensulfonat (PEDOT:PSS) och polypyrrol, i in-situ-sulfonerat trä resulterade i biokompositer med hög konduktivitet, upp till 203 S m^-1, och hög pseudo-kapacitans upp till 38 mF cm^-2. Detta visar att kontroll av träkemin och redox-aktivitet hos lignin genom sulfonering är viktiga strategier för att tillverka kompositer med potential för hållbara energitillämpningar.

Ett poröst trämaterial med unik mirkostruktur framställdes genom att justera träets kemi och morfologi. Materialet har förbättrad permeabilitet, en hög tryckhållfasthet på 9 MPa och en Young's modul på 364 MPa. Genom att tillsätta PEDOT:PSS till materialet så uppnåddes en enastående ledningsförmåga på 215 S m^-1 och ett mekanoresponsivt elektriskt motstånd. Materialet därmed kan potentiellt användas för sensorer och mekanoresponsiva enheter.

In-situ modifiering av lignin är en mångsidig metod för att framställa trämaterial med förhöjd permeabilitet och porositet vilket förbättrar kompatibiliteten med elektriskt ledande polymerer och möjligheten för förtätning.

Wood is a high-strength lightweight material owing to its orthotropic cellular structure and composite-like constitution. In conventional fabrication of wood-derived functional materials, the removal of the potentially beneficial components, such as lignin and hemicellulose, often leads to the disruption of the native hierarchical wood structure. Herein, we developed a facile method of in situ wood sulfonation followed by hot pressing for pine veneers to prepare high-density transparent thin films with preserved wood components and the natural fiber alignment. An optimum lignin content of the hot-pressed films was found to be 20.6% where both mechanical and optical properties were significantly enhanced with a more dense and compact structure. The hot-pressed transparent wood films also showed UV-blocking capability and could be recycled into discrete wood fibers owing to the sulfonate groups endowed by the in situ sulfonation step. The unique combination of properties achieved for thin wood films marks an important step in engineering functional wood-based materials that utilize both the structure of aligned fibers and the complex components of natural wood.

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 µmol 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. 

Nanostructured wood veneer with added electroactive functionality combines structural and functional properties into eco-friendly, low-cost nanocomposites for electronics and energy technologies. Here, we report novel conducting polymer-impregnated wood veneer electrodes where the native lignin is preserved, but functionalized for redox activity and used as an active component. The resulting electrodes display a well-preserved structure, redox activity, and high conductivity. Wood samples were sodium sulfite-treated under neutral conditions at 165 °C, followed by the tailored distribution of PEDOT:PSS, not previously used for this purpose. The mild sulfite process introduces sulfonic acid groups inside the nanostructured cell wall, facilitating electrostatic interaction on a molecular level between the residual lignin and PEDOT. The electrodes exhibit a conductivity of up to 203 S m−1 and a specific pseudo-capacitance of up to 38 mF cm−2, with a capacitive contribution from PEDOT:PSS and a faradaic component originating from lignin. We also demonstrate an asymmetric wood pseudo-capacitor reaching a specific capacitance of 22.9 mF cm−2 at 1.2 mA cm−2 current density. This new wood composite design and preparation scheme will support the development of wood-based materials for use in electronics and energy storage.