Transparent wood biocomposite (TW) is a sustainable optical material that combines high transmittance with mechanical strength but also exhibits pronounced haze. This haze limits applications where high optical transparency is required, and its physical origin remains insufficiently understood. In this study, photon transport in TW and related wood-based scaffolds at different stages of chemical modification, including delignified wood (DW), native wood (NW) and bleached wood (BW) templates is investigated. Time- and space-resolved transmission measurements are used to extract direction-dependent scattering and absorption coefficients. DW, BW, and NW samples exhibit anisotropic light propagation while TW both suppresses scattering and alters the scattering anisotropy, flipping 90° the dominant transport orientation relative to the fibers. Extracted optical parameters confirm low scattering coefficients, up to 2 orders of magnitude lower than the NW, BW, or DW. The main scattering mechanism for TW is identified as in-plane refraction, leading to predominant forward transmission or “snake-like” photon trajectories, marking a transition from diffusive to quasi-ballistic transport. These insights advance the fundamental understanding of light transport in hierarchical biocomposites and offer a framework for designing sustainable optical composites with broad haze control, increasing the functional potential of TW toward “wood glass”.

Translucent wood fiber composites offer new functions to stiff composites. Most "eco-friendly" thermoset resins are only partially biobased. Poly(limonene acrylate), PLIMA, can be fully biobased and is combined with hot-pressed softwood fibers (WF) by liquid resin impregnation and curing. Fibers are random-in-plane or strongly oriented and have different lignin characteristics. Microstructure-mechanical property relationships are compared for hot-pressed WF networks and WF/PLIMA biocomposites from the same fibers. Stress transfer in WF/PLIMA biocomposites is enhanced with a modulus of up to 16.7 GPa and a tensile strength of up to 139 MPa, compared to transparent plastics like poly(methyl methacrylate) (modulus similar to 3 GPa, tensile strength similar to 70 MPa). Optical transmittance is high, even at 35 vol % fiber content, suggesting translucent panels or lighting applications. Eco-indicators show that the PLIMA matrix accounts for similar to 80% of biocomposite cumulative energy demand (CED, cradle to gate) of 60 MJ/kg, compared to similar to 120 MJ/kg for glass fiber/thermoset composites.

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.

Abstract The sustainable development of functional energy-saving building materials is important for reducing thermal energy consumption and promoting natural indoor lighting. Phase-change materials embedded in wood-based materials are candidates for thermal energy storage. However, the renewable resource content is usually insufficient, the energy storage and mechanical properties are poor, and the sustainability aspect is unexplored. Here a novel fully bio-based transparent wood (TW) biocomposite for thermal energy storage, combining excellent heat storage properties, tunable optical transmittance, and mechanical performance is introduced. A bio-based matrix based on a synthesized limonene acrylate monomer and renewable 1-dodecanol is impregnated and in situ polymerized within mesoporous wood substrates. The TW demonstrates high latent heat (89 J g?1) exceeding commercial gypsum panels, combined with thermo-responsive optical transmittance (up to 86%) and mechanical strength up to 86 MPa. The life cycle assessment shows that the bio-based TW has a 39% lower environmental impact than transparent polycarbonate panels. The bio-based TW holds great potential as scalable and sustainable transparent heat storage solution.

Transparent wood (TW) shows interesting optical properties and offers a sustainable alternative to petroleum-based polymer glasses. The influence of the TW internal structure (e.g. fiber alignment, volume fraction of cellulose, lignin content, defects from preparation process) on the optical properties is poorly understood, which limits its use in various applications. It is also true for transparent cellulose biocomposites in general. In this thesis, eco-friendly TW biocomposites are investigated. The work focuses on experimental characterization, structure-optical property relationships and possibilities to quantify such relationships.  

                TWs made of delignified wood substrates with longitudinal direction of the tree parallel to the specimen surface are prepared. Relationships between anisotropic scattering and fiber alignment are studied by scattering angle measurement. Anisotropic photons distributions are compared between two fiber directions and various sample thicknesses. Next, attenuation coefficients (related to the anisotropic diffusion coefficients and absorption coefficient) for TWs are obtained by combining the photon diffusion equation with total transmittance measurements. The results indicate strong influence from the air gaps between wood substrate phase and polymer in the lumen pores on the scattering. Beside the airgaps between wood substrate and polymer, refractive index mismatch between polymer and wood substrate strongly influences the scattering. Thus, immersion liquid method (based on the total transmittance measurement) combined with a light transmission model (based on Fresnel reflection theory) is applied to estimate the refractive index of the delignified wood substrate. This facilitates TW design (i.e. the proper polymer selection for various applications) and modelling of the optical properties of delignified wood based transparent materials. Finally, extinction coefficients, Rayleigh scattering and absorption coefficients of TW are extracted from photon budget measurements combined with a light diffusion model developed. With higher volume fraction of cellulose, all these parameters are increased, although polymer-cellulose refractive index mismatch is the dominating factor controlling transmittance. The strong forward scattering in TW is analysed, and Rayleigh scattering has a strong effect on haze. The influence of lignin content on the absorption coefficient is also discussed.

Transparent trä (TW) har intressanta optiska egenskaper och erbjuder ett hållbart alternativ till petroleumbaserade polymerer. Förståelsen för inverkan av mikrostruktur hos TW (t.ex. fiberinriktning, volymandel av cellulosa, ligninhalt, defekter från beredningsprocessen) på de optiska egenskaperna är ofullständig, vilket begränsar dess användning i olika tillämpningar. Det gäller också generellt för transparenta cellulosabiokompositer. I denna avhandling studeras miljövänliga TW biokompositer, med fokus på experimentell karakterisering, samband mellan struktur och optiska egenskaper samt möjligheterna att kvantifiera sådana samband.

                TW baserade på delignifierade träsubstrat har i denna studie trädets fiberriktning parallell med provytan. Samband mellan anisotrop ljusspridning och fiberorientering studeras genom mätning av spridningsvinkel. Anisotropa fotonfördelningar jämförs mellan två fiberriktningar och olika provtjocklekar. Därefter erhålls koefficienter för ”attenuering” (försvagning), som är relaterade till de anisotropa diffusionskoefficienterna och absorptionskoefficienten för TW. De bestäms genom att kombinera en modell för fotondiffusion med mätningar av total optisk transmittans. Resultaten indikerar en stark påverkan på ljusspridningen av luftspalter mellan träsubstrat och polymer i lumen, som är en form av debondsprickor. Utöver debondsprickor mellan träsubstrat och polymer, så påverkas ljusspridningen även av skillnaden i brytningsindex mellan polymer och träsubstrat. Av det skälet utvecklas en metod för att mäta brytningsindex hos träsubstratet. Det porösa substratet sänks ned i en vätska med känt brytningsindex och optisk transmittans mäts och kombineras med en modell för ljustransmission baserad på fresnelreflektion. Goda data för träsubstratets brytningsindex underlättar vid formgivning av TW biokompositer (dvs. rätt polymerval för olika applikationer) och är också viktigt för modellering av de optiska egenskaperna hos transparenta material från delignifierat trä. I avhandlingens sista del kombineras en modell för ljusdiffusion med systematiska mätningar av ljusspridning, reflektion och transmittans hos olika materialprover. Data för ”extinktionskoefficienter”, Rayleigh-spridning och absorptionskoefficienter kan bestämmas, liksom hela fotonbudgeten för materialet. Med högre volymfraktion av cellulosa ökar värdena för alla dessa parametrar, även om skillnaden i brytningsindex mellan polymer och cellulosa är den dominerande faktorn som styr transmittansen. Den starka ljusspridningen framåt (”haze”) i TW analyseras, och även Rayleighspridningen har en stor effekt på ljusspridning. Ligninhaltens inverkan på absorptionskoefficienten diskuteras också.

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.

Composite polymer hydrogels are of significant interests for high optical transparency and mechanical performance. In this work, a strong and transparent composite hydrogel is developed from a highly mesoporous cellulose network prepared from wood via top-down delignification followed by TEMPO-mediated oxidation and in situ polymerization of PNIPAM. Individualization of cellulose microfibrils inside the wood cell wall is critical for the fabrication of free-standing composite hydrogel with high water content of 94.9 wt% and high optical transmittance of 85.8% with anisotropic light scattering behavior. The composite hydrogel also showed anisotropic mechanical properties with a tensile strength, Young's modulus and toughness of 317 kPa, 5.4 MPa, and 39.2 kJ m- 3 in axial direction, and 152 kPa, 0.31 MPa and 57.1 kJ m- 3 in the transverse direction, respectively. It also showed thermochromic behavior, i.e., reversibly changing between transparent and brightly white by a temperature change between 25 and 40 degrees C, demonstrating great potential for optical applications.

Transparent wood (TW)-based composites are of significant interest for smart window applications. In this research, we demonstrate a facile dual-stimuli-responsive chromic TW where optical properties are reversibly controlled in response to changes in temperature and UV-radiation. For this functionality, bleached wood was impregnated with solvent-free thiol and ene monomers containing chromic components, consisting of a mixture of thermo- and photoresponsive chromophores, and was then UV-polymerized. Independent optical properties of individual chromic components were retained in the compositional mixture. This allowed to enhance the absolute optical transmission to 4 times above the phase change temperature. At the same time, the transmission at 550 nm could be reduced 11−77%, on exposure to UV by changing the concentration of chromic components. Chromic components were localized inside the lumen of the wood structure, and durable reversible optical properties were demonstrated by multiple cycling testing. In addition, the chromic TW composites showed reversible energy absorption capabilities for heat storage applications and demonstrated an enhancement of 64% in the tensile modulus as compared to a native thiol−ene polymer. This study elucidates the polymerization process and effect of chromic components distribution and composition on the material’s performance and perspectives toward the development of smart photoresponsive windows with energy storage capabilities.

Refractive index (RI) determination for delignified wood templates is vital for transparent wood composite fabrication. Reported RIs in the literature are based on either single plant fibers or wood powder, measured by the immersion liquid method (ILM) combined with mathematical fitting. However, wood structure complexity and the physical background of the fitting were not considered. In this work, RIs of delignified wood templates were measured by the ILM combined with a light transmission model developed from the Fresnel reflection/refraction theory for composite materials. The RIs of delignified balsa wood are 1.536 ± 0.006 and 1.525 ± 0.008 at the wavelength of 589 nm for light propagating perpendicular and parallel to the wood fiber direction, respectively. For delignified birch wood, corresponding values are 1.537 ± 0.005 and 1.529 ± 0.006, respectively. The RI data for delignified wood scaffolds are important for tailoring optical properties of transparent wood biocomposites, and also vital in optical properties investigations by theoretical modelling of complex light propagation in transparent wood and related composites. The developed light transmission model in combination with the immersion liquid method can be used to determine the RI of complex porous or layered solid materials and composites.

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.