Cellulosic nanomaterials are ideal reinforcers in hydrogel composites, but the current techniques that ensure defined nano-dimensions reduce sustainability. A different strategy for the synthesis of hydrogels from pulp fibers using green chemistry could offer a more sustainable solution. This work explores a mild, straightforward chemical modification with maleic anhydride that simultaneously decorates the fibers with carboxylate and alkene groups. Tuning the temperature of the reaction enables control over the surface charge ranging from 150 to 1,000 μmol/g. The fibers are used to construct a rubber-like, water-stable hydrogel composite prepared by in situ telechelic PEG polymerization followed by thermal or UV-induced free radical crosslinking. The initiation strategy, molecular weight of telechelic PEG, and degree of modification of the fibers enable control over the network formation within and around the fibers. The hydrogel composite is designed to be hydrolytically degradable under alkaline conditions, allowing separate recovery of both fibers and polymer precursors.

Nanocellulose has emerged as a widely utilized building block in nanostructured materials due to its availability, sustainability, large surface area, and high stiffness and aspect ratio. The wet or dry elastoplastic properties of these materials are determined by the fibrils' stiffness, chemical properties, hemicellulose content, and the number of fibril contacts. However, the specific contributions and relative importance of each factor remain unclear. Therefore, this work was devoted to systematically comparing the material properties of gels, aerogels, and wet and dry sheets prepared from CNFs with different aspect ratios, chemical functionality, and hemicellulose content. The fibrils were prepared by chemical and mechanical processing of different pulps. By preserving the native structure as much as possible, higher aspect ratio fibrils can be obtained, which allows for the development of more mechanically robust materials. The results demonstrate that higher aspect ratios lead to more interconnected networks at a lower solids concentration, resulting in a more evenly distributed stress and longer-range stress transfer, yielding stiffer and more ductile materials. The most important finding was that the aspect ratio influences the network formation, resulting in different network topologies. The results were also compared to earlier published data and integrated into a theoretical beam-bending model for a complete elastoplastic description of the network properties, including the influence of fibril aspect ratio and chemical functionality. This information improves our understanding and description of nanofibril networks for which general models have been missing. It can be used to optimize nanofibril preparation and, hence, the resulting eco-friendly materials.

The high structural anisotropy and colloidal stability of cellulose nanofibrils' enable the creation of self-standing fibrillar hydrogel networks at very low solid contents. Adding methacrylate moieties on the surface of TEMPO oxidized CNFs allows the formation of more robust covalently crosslinked networks by free radical polymerization of acrylic monomers, exploiting the mechanical properties of these networks more efficiently. This technique yields strong and elastic networks but with an undefined network structure. In this work, we use acrylate-capped telechelic polymers derived from the step-growth polymerization of PEG diacrylate and dithiothreitol to crosslink methacrylated TEMPO-oxidized cellulose nanofibrils (MATO CNF). This combination resulted in flexible and strong hydrogels, as observed through rheological studies, compression and tensile loading. The structure and mechanical properties of these hydrogel networks were found to depend on the dimensions of the CNFs and polymer crosslinkers. The structure of the networks and the role of individual components were evaluated with SAXS (Small-Angle X-ray Scattering) and photo-rheology. A thorough understanding of hybrid CNF/polymer networks and how to best exploit the capacity of these networks enable further advancement of cellulose-based materials for applications in packaging, soft robotics, and biomedical engineering.

Achieving a sustainable foam production requires a complete substitution of synthetic components with natural and renewable alternatives, as well as development of an environment-friendly production process. This work demonstrates a synergetic combination of heat-treated beta-lactoglobulin proteins and cellulose nanofibrils (CNFs) to create fully bio-based and highly-stable wet foams. Furthermore, a gradual reduction in the pH, enabled oven-drying of the wet foams without any major structural collapse of the foam, resulting in the preparation of lightweight solid foams with the density of 10.2 kg.m(-3). First, the foaming behavior of heat-treated beta-lactoglobulin systems (HBSs) containing amyloid nanofibrils (ANFs) and non-converted peptides was investigated at different pHs. Subsequently, the HBS foams were stabilized using CNFs, followed by a gradual acidification of the system to a final pH of 4.5. To gain a deeper understanding of the stabilization mechanism of the foam, the interactions between the foam's components, their positioning in the foam structure, and the viscoelasticity of the fibrillar network were investigated using quartz crystal microgravimetry, confocal microscopy and rheology. The analysis of the obtained data suggests that the stability of the foams was associated with the accumulation of CNFs and ANFs at the air-water interface, and that the concomitant formation of an intertwined network surrounding the air bubbles. This together resulted in a significant decrease in drainage rate of the liquid in the foam lamellae, bubble coarsening and bubble coalescence within the foams. The results also show that the major surface-active component participating in the creation of the foam is the free peptide left in solution after the formation of the ANFs. A slow reduction in pH to 4.5 lead to further gelation of the fibrillar network and an improved storage modulus of the foam lamellae. This resulted in a strong coherent structure that could withstand oven-drying without collapse. The density, porosity, microstructure and compressive mechanical properties of such prepared dry foams were assessed. Overall, the results demonstrate the potential of HBSs to replace synthetic surfactants and outlines a sustainable preparation protocol for the preparation of light-weight porous composite structures of ANFs and CNFs.

The development of sustainable and renewable materials is paramount in today’s society. As the most abundant biopolymer on Earth, cellulose from cellulose-rich fibres is an excellent alternative for advanced and innovative material solutions. Nonetheless, competing with the impressive material properties and the low manufacturing costs of fossil-based plastics imposes great challenges. To increase the potential of cellulose fibres in a broader set of applications, the material properties of cellulose need to be tuned depending on the application. An in-depth study of the fibre structure and the application of different tailoring techniques is required to induce tailoring of the physical and chemical properties of the cellulose fibre materials. 

This thesis focuses on the structure-property relationship of fibrillar hydrogel networks as model structures for the delignified wet-fibre wall. First, a mathematical framework was developed to describe the characteristics of the swelling and mechanical behaviour of anisotropic fibrillar structures, considering the fibril aspect ratio, surface chemistry of the fibrils, and electrolyte concentration in the system. A chemical functionalisation was then introduced to the fibrillar structure, which provided the CNFs with colloidal stability and the ability to participate in free radical polymerisation with monomers and telechelic oligomers. As a result, fibrillar networks were crosslinked with flexible polymer links that provided the network with different mechanical and chemical properties. Additionally, by tailoring the molecular weight of the crosslinks, the ionic strength of the solution, and even the aspect ratio of the fibrils, the mechanical properties of the network were tuned to be either stiffer or more ductile. 

Finally, an innovative and more sustainable approach was developed to introduce charge and alkene functionality to the fibres. Following the lessons learned from the CNF model investigations, a polymerisation approach was developed in the presence of functionalised fibres. The polymers were grown from the fibre wall, followed by radical crosslinking to create strong Fibre reinforced hydrogel structures. Depending on the application, the method can be easily applied to introduce other types of molecules and functionalities to the fibres and tailor the properties of the fibres to suit a wide range of applications.

Utvecklingen av hållbara och förnyelsebara material är avgörande i dagens samhälle. Eftersom cellulosa ifrån växtfibrer är den mest förekommande biopolymeren på jorden är den ett utmärkt alternativ för användning i avancerade och innovativa materiallösningar. Det innebär dock en enorm utmaning att konkurrera med de imponerande materialegenskaperna och låga tillverkningskostnaderna hos fossilbaserade plaster. För att utnyttja den inneboende potentialen hos cellulosafibrerna och utveckla deras egenskapsrymd för användning i vidare tillämpningar är det helt nödvändigt att modifiera cellulosans materialegenskaper för att passa till specifika slutanvändningar. Det är därför nödvändigt att ingående studera hur olika modifieringstekniker kan användas för att skräddarsy fysikaliska och kemiska egenskaper hos cellulosan på olika strukturella nivåer i de delignifierade fibrerna.  

Arbetet i denna avhandling har fokuserats på att klarlägga struktur-egenskapsförhållandena för cellulosa-rika fibrilstrukturer. Initialt användes fibrillära hydrogelnätverk som modell för den delignifierade våtfibreväggen. Till att börja med utvecklades ett matematiskt ramverk för att beskriva det typiska svällningsbeteendet och de mekaniska egenskaperna hos de anisotropa fibrillstrukturerna med avseende på fibrillernas längs/tvärs förhållande, ytkemi och elektrolytkoncentration i systemet.  Efter detta modifierades fibrillerna på ett sådant sätt att de erhöll en god kolloidal stabilitet samtidigt som de försågs med en vinyl-funktionalitet som innebar att de kunde användas i friradikalpolymerisation med olika typer av monomerer och telecheliska oligomerer. Via denna typ av radikalpolymerisation var det möjligt att skapa fibrillnätverk med flexibla polymerkopplingar som resulterade i skräddarsydda mekaniska och kemiska egenskaper. Genom att kontrollera tvärbindningarnas molekylvikt, lösningens jonstyrka och fibrillernas längs/tvärs förhållande kunde nätverkets mekaniska egenskaper kontrolleras så att de antingen blev mer töjbara eller styva. 

Med hjälp av en innovativ och mer hållbar modifieringsteknik visade det sig vidare möjligt att samtidigt skapa hög laddning och att introducera en vinylfunktionalitet hos cellulosa-rika fibrer. Genom att använda de tidigare erfarenheterna ifrån de modifierade fibrillnätverken visade det sig möjligt att utveckla en polymerisationsmetod i närvaro av de funktionaliserade fibrer där polymerisation initierades både inuti och omkring de modifierade fibrerna. Polymererna ympades ifrån den modifierade fibreväggen både inuti och omkring fibrerna, följt av radikaltvärbindning för att skapa helt nya typer av starka, fibrebaserade hydrogelstrukturer. Beroende på den slutliga tillämpningen, av denna nya typ av fiberförstärkta hydrogeler, är det enkelt att använda metoden för att inkludera andra molekyler och funktionaliteter till fibrerna för att skräddarsy gelegenskaperna.

Converting omnipresent environmental energy through the assistance of spontaneous water evaporation is an emerging technology for sustainable energy systems. Developing bio-based hydrovoltaic materials further pushes the sustainability, where wood is a prospect due to its native hydrophilic and anisotropic structure. However, current wood-based water evaporation-assisted power generators are facing the challenge of low power density. Here, an efficient hydrovoltaic wood power generator is reported based on wood cell wall nanoengineering. A highly porous wood with cellulosic network filling the lumen is fabricated through a green, one-step treatment using sodium hydroxide to maximize the wood surface area, introduce chemical functionality, and enhance the cell wall permeability of water. An open-circuit potential of ≈140 mV in deionized water is realized, over ten times higher than native wood. Further tuning the pH difference between wood and water, due to an ion concentration gradient, a potential up to 1 V and a remarkable power output of 1.35 µW cm−2 is achieved. The findings in this study provide a new strategy for efficient wood power generators.

Fibrillar hydrogels are remarkably stiff, low-density networks that can hold vast amounts of water. These hydrogels can easily be made anisotropic by orienting the fibrils using different methods. Unlike the detailed and established descriptions of polymer gels, there is no coherent theoretical framework describing the elastoplastic behavior of fibrillar gels, especially concerning anisotropy. In this work, the swelling pressures of anisotropic fibrillar hydrogels made from cellulose nanofibrils were measured in the direction perpendicular to the fibril alignment. This experimental data was used to develop a model comprising three mechanical elements representing the network and the osmotic pressure due to non-ionic and ionic surface groups on the fibrils. At low solidity, the stiffness of the hydrogels was dominated by the ionic swelling pressure governed by the osmotic ingress of water. Fibrils with different functionality show the influence of aspect ratio, chemical functionality, and the remaining amount of hemicelluloses. This general model describes physically crosslinked hydrogels comprising fibrils with high flexural rigidity - that is, with a persistence length larger than the mesh size. The experimental technique is a framework to study and understand the importance of fibrillar networks for the evolution of multicellular organisms, like plants, and the influence of different components in plant cell walls.

In the context of three-dimensional (3D) cell culture and tissue engineering, 3D printing is a powerful tool for customizing in vitro 3D cell culture models that are critical for understanding the cell-matrix and cell-cell interactions. Cellulose nanofibril (CNF) hydrogels are emerging in constructing scaffolds able to imitate tissue in a microenvironment. A direct modification of the methacryloyl (MA) group onto CNF is an appealing approach to synthesize photocross-linkable building blocks in formulating CNF-based bioinks for light-assisted 3D printing; however, it faces the challenge of the low efficiency of heterogenous surface modification. Here, a multistep approach yields CNF methacrylate (CNF-MA) with a decent degree of substitution while maintaining a highly dispersible CNF hydrogel, and CNF-MA is further formulated and copolymerized with monomeric acrylamide (AA) to form a super transparent hydrogel with tuneable mechanical strength (compression modulus, approximately 5-15 kPa). The resulting photocurable hydrogel shows good printability in direct ink writing and good cytocompatibility with HeLa and human dermal fibroblast cell lines. Moreover, the hydrogel reswells in water and expands to all directions to restore its original dimension after being air-dried, with further enhanced mechanical properties, for example, Young’s modulus of a 1.1% CNF-MA/1% PAA hydrogel after reswelling in water increases to 10.3 kPa from 5.5 kPa.

Development of strong cellulose nanofibril (CNF) networks for advanced applications, such as in the biomedical field, is of high importance owing to the biocompatible nature and plant-based origin of cellulose nanofibrils. Nevertheless, lack of mechanical strength and complex synthesis methods hinder the application of these ma-terials in areas where both toughness and manufacturing simplicity are required. In this work, we introduce a facile method for the synthesis of a low solid content (< 2 wt%), covalently crosslinked CNF hydrogel where Poly (N-isopropylacrylamide) (NIPAM) chains are utilized as crosslinks between the nanofibrils. The resulting net-works have the capability to fully recover the shape in which they were formed after various drying and rewetting cycles. Characterization of the hydrogel and its constitutive components was performed using X-ray scattering, rheological investigations and uniaxial testing in compression. Influence of covalent crosslinks was compared with networks crosslinked by the addition of CaCl2. Among other things the results show that the mechanical properties of the hydrogels can be tuned by controlling the ionic strength of the surrounding me-dium. Finally, a mathematical model was developed based on the experimental results, which describes and predicts to a decent degree the large-deformation, elastoplastic behavior, and fracture of these networks.

Polymer shape-memory aerogels (PSMAs) are prospects in various fields of application ranging from aerospace to biomedicine, as advanced thermal insulators, actuators, or sensors. However, the fabrication of PSMAs with good mechanical performance is challenging and is currently dominated by fossil-based polymers. In this work, strong, shape-memory bio-aerogels with high specific surface areas (up to 220 m2/g) and low radial thermal conductivity (0.042 W/mK) were prepared through a one-step treatment of native wood using an ionic liquid mixture of [MTBD]+[MMP]−/DMSO. The aerogel showed similar chemical composition similar to native wood. Nanoscale spatial rearrangement of wood biopolymers in the cell wall and lumen was achieved, resulting in flexible hydrogels, offering design freedom for subsequent aerogels with intricate geometries. Shape-memory function under stimuli of water was reported. The chemical composition and distribution, morphology, and mechanical performance of the aerogel were carefully studied using confocal Raman spectroscopy, AFM, SAXS/WAXS, NMR, digital image correlation, etc. With its simplicity, sustainability, and the broad range of applicability, the methodology developed for nanoscale reassembly of wood is an advancement for the design of biobased shape-memory aerogels.