Nanocellulose, originated from cellulose, the primary structural component of the cell walls of plants, has garnered significant attention for its excellent mechanical, optical, and barrier properties, as well as its renewable and sustainable nature. Various forms of nanocellulose, including cellulose nanocrystals and cellulose nanofibers (CNFs), are produced by breaking down lignocellulosic fibers into nanoscale dimensions, typically through mechanical or chemical processes. The large surface area and rich hydroxyl groups of CNFs are ideal for surface modifications, offering great versatility in the development of functional biocomposite materials. This thesis aims to design CNF-based composites with integrated multifunctionalities, including redispersibility, biocompatibility, mechanical robustness, wet integrity, as well as optical transparency, through surface engineering of cellulose nanofibers. The methodology involves strategically selecting CNFs, integrating CNFs with biopolymers, applying surface modifications, and implementing facile processing techniques. 

In Paper I, inspiration from plant cell wall was drawn to customize the interaction between water and CNFs. By Incorporating mixed-linkage beta-glucan from barley, superior rehydration, redispersion, and recycling of dried CNFs have been achieved. This advancement holds the potential to enhance the transportation and processability of CNF-based materials.

In Paper II, by leveraging the interaction between CNF and water, a facile material processing technique was introduced to fabricate CNF/regenerated silk fibroin (RSF) composites. This involved rehydration and swelling of TEMPO-oxidized CNF nanopaper structures with both random-oriented CNF and nematic-ordered CNF in the RSF solutions. Remarkably, the CNF/RSF composite films thus prepared exhibited exceptional mechanical properties in both dry conditions and in PBS, and demonstrated excellent biocompatibility when cultured with L929 fibroblast cell.

In Paper III, CNF/alginate double-network composites were prepared to investigate the impact of interfibrillar interactions and the G/M ratio (guluronic acid/mannuronic acid) of alginates on mechanical performance. The composite incorporating TEMPO-oxidized CNF and alginate with higher mannuronic acid content and molecular weight, exhibited high Young’s modulus of 20.3 GPa and high tensile strength of 331 MPa. The interfacial calcium ion crosslinking between CNF and alginate played a pivotal role in improving these properties. Furthermore, this composite was successfully demonstrated as a barrier spray coating for banana, significantly reducing weight loss when stored under ambient conditions, suggesting its potential for applications in food packaging.

In paper IV, carboxymethyl cellulose (CMC) was functionalized with quaternary ammonium salts, and subsequently used to modify the interface between holocellulose fibers network and acrylic resin. Strong and transparent composites were successfully fabricated, without the need for organic solvents or harsh chemicals that are often used during the covalent surface modification of cellulose. The hydrophobic functionalized CMCs facilitated homogeneous resin impregnation in cellulose fiber network, producing composites with enhanced interfacial adhesion strength, increased optical transparency and mechanical strength.

Nanocellulosa, som ursprungligen kommer från cellulosa, den primära strukturella komponenten i växters cellväggar, har fått betydande uppmärksamhet för sina unika egenskaper, inklusive utmärkta mekaniska, optiska och barriäregenskaper, samt dess förnybara och hållbara natur. Nanocellulosa, inklusive cellulosa nanokristaller och cellulosa nanofibrer (CNF), produceras genom att bryta ned lignocellulosa fibrer till nanoskala dimensioner, vanligtvis genom mekaniska eller kemiska processer. Den stora ytan och de rika hydroxylgrupperna hos CNF är idealiska för ytmodifieringar, vilket avsevärt ökar dess mångsidighet i utvecklingen av funktionella biokompositmaterial. Denna avhandling syftar till att designa CNF-baserade kompositer med integrerade multifunktionaliteter inklusive redispergerbarhet, biokompatibilitet, mekanisk robusthet, våtintegritet, samt optisk transparens genom ytmodifiering av cellulosananofibrer. Metodiken innefattar strategiskt val av CNF, integrering av CNF med biopolymerer, tillämpning av ytmodifieringar och implementering av enkla bearbetningstekniker.

I Paper I inspirerades man av växtcellväggen för att anpassa interaktionen mellan vatten och CNF. Genom att tillsätta betaglukan från korn, har bättre rehydrering, redispersion, och återvinning av torkade CNF uppnåtts. Denna framsteg möjliggör förbättring av transporten och bearbetningen av CNF-baserade material.

I Paper II, utnyttjades interaktionen mellan CNF och vatten, vilket introducerade en enkel materialbearbetningsteknik för att tillverka CNF/regenererad silkfibroin (RSF) kompositer. Detta involverade rehydrering och svullnad av TEMPO-oxiderade CNF nanopapperstrukturer med både slumpmässigt orienterade CNF och nematiskt ordnade CNF i RSF-lösningarna. De förberedda CNF/RSF kompositfilmerna uppvisade exceptionella mekaniska egenskaper både i torra förhållanden och i PBS, och visade utmärkt biokompatibilitet när de odlades med L929 fibroblastcell.

I Paper III, förbereddes CNF/alginate dubbelnätverkskompositer för att undersöka effekten av inter-fibrillära interaktioner och G/M-förhållandet (guluronsyra/mannuronsyra) av alginater på mekanisk prestanda. Kompositen som innehöll TEMPO-oxiderad CNF och alginate med högre innehåll av mannuronsyra och högre molekylvikt, uppvisade hög Youngs modul på 20,3 GPa och hög draghållfasthet på 331 MPa. Interfacial kalciumjonkorslänkning mellan CNF och alginate spelade en central roll i att uppnå dessa egenskaper. Dessutom demonstrerades denna komposit framgångsrikt som en spraybeläggning för banan, vilket betydligt fördröjde viktförlusten när den förvarades i omgivningsförhållanden, vilket antyder dess möjliga tillämpningar inom livsmedelsförpackning.

I Papper IV funktionaliserades karboximetylcellulosa (CMC) med kvartära ammoniumsalter och användes sedan för att modifiera gränssnittet mellan holocellulosafibernätverk och en akrylresin. Starka och transparenta kompositer tillverkades framgångsrikt utan behov av organiska lösningsmedel eller de hårda kemikalier som ofta används vid kovalent ytmodifiering av cellulosa. Den hydrofoba, funktionaliserade CMC:n underlättade en homogen resinimpregnering i cellulosafibernätverket, vilket resulterade i en komposit med förbättrad gränsytfästningsstyrka, ökad optisk transparens och mekanisk styrka.

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.

Improving the redispersion and recycling of dried cellulose nanofibrils (CNFs) without compromising their nanoscopic dimensions and inherent mechanical properties are essential for their large-scale applications. Herein, mixed-linkage (1,3;1,4)-beta-D-glucan (MLG) was studied as a rehydration medium for the redispersion and recycling of dried CNFs, benefiting from the intrinsic affinity of MLG to both cellulose and water molecules as inspired from plant cell wall. MLG from barley with a lower molar ratio of cellotriosyl to cellotetraosyl units was found homogeneously coated on CNFs, facilitating rehydration of the network of individualized CNFs. The addition of barley MLG did not impair the mechanical properties of the CNF/MLG composites as compared to neat CNFs nanopaper. With the addition of 10 wt% barley MLG, dry CNF/MLG composite film was successfully redispersed in water and recycled with well-maintained mechanical properties, while lichenan from Icelandic moss, cationic starch, and xyloglucan could not help the redispersion of dried CNFs.

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.

Integrating synthetic low-dimensional nanomaterials such as metal–organic framework (MOF) nanosheets with a sustainable biopolymer is a promising strategy to endow composites with attractive structural and functional properties for expanded applications. Herein, aggregation-induced-emission luminogen (AIEgen)-based MOF bulk crystals are successfully exfoliated into ultrathin 2D nanosheets. Seaweed cellulose nanofibrils (CNFs) are assembled with low amounts (0.3 to 4.0 wt%) of the 2D nanosheets to generate luminescent composites. The 2D nanosheets are adsorbed onto the CNFs in dilute water suspensions owing to the flexibility of the MOF nanosheets and the high aspect ratio of the CNFs. Transparent films are prepared by solution casting from a water suspension of the CNF-MOF assembly. The fluorescence emission of the composite films is enhanced because of the favored affinity between MOF nanosheets and CNFs. Remarkably, these films demonstrate excellent UV-shielding capacity and high optical transmittance at the visible wavelength range. The composite films also show reversible changes in fluorescence emission intensity in response to ambient humidity. The tensile strength and modulus of the composite films are also enhanced owing to the increased adhesion between CNFs through the adsorbed MOF nanosheets. This work provides a novel pathway to fabricate luminescent CNFs-based composites with tunable optical properties for functional materials. 

Lytic polysaccharide monooxygenase (LPMO)-catalysed oxidation of cellulose has emerged as a green alternative to chemical modifications in the production of cellulose nanofibrils (CNFs) from wood pulp fibres. The effect of the hemicellulose content of the starting pulp fibres on the oxidation capabilities of cellulose-active LPMO is important and has not been investigated previously. In this study, the production of LPMO-oxidised CNFs was evaluated on two commercial softwood pulp fibres with different hemicellulose contents. Thin and colloidally stable CNFs were readily obtained from kraft pulp with a hemicellulose content of 16%. The preserved hemicellulose fraction in the kraft pulp enhanced the access of LPMO into the fibre cell wall, enabling the production of homogeneous CNFs with a thin width of 3.7 ± 1.7 nm. By contrast, the LPMO-oxidised dissolving pulp with a lower hemicellulose content of 4% could only be partially disintegrated into thin CNFs, leaving a large amount of cellulose microfibril aggregates with widths of around 50 to 100 nm. CNFs disintegrated from the LPMO-oxidised kraft pulp could be processed into nanopapers with excellent properties including an optical transmittance of 86%, tensile strength of 260 MPa, and Young's modulus of 16.9 GPa. Such CNFs also showed acid-triggered nanofibril gelation owing to the introduced carboxyl groups on cellulose microfibril surfaces. These results indicate that the inherent hemicelluloses present in the wood cell wall are essential for LPMO-mediated CNF production from wood pulp fibres.