Cellulose nanocrystals: Extraction from wood waste and their thermo-responsive composites with block copolymer vesicles

An essential goal for achieving a circular economy is the reuse of different waste types. Wood waste is a growing issue since it comprises a significant portion of total solid waste. The valorization of wood has many benefits over current waste management methods, including incineration and landfilling, and it has the potential to achieve a more sustainable environment. Cellulose nanocrystal (CNC) is a high-value and promising sustainable material characterized by its high surface area, aspect ratio, biocompatibility, abundance, and outstanding mechanical properties. These nanoparticles have been used in a wide range of applications, from rheology modifiers to biosensors, hydrogels, energy storage, flocculants, and drug delivery systems. However, CNC extraction starting from bulk wood chips and the resulting hardwood and softwood CNC properties have not been investigated in detail. Also, since CNCs lack stimuli-responsiveness, turning CNCs into smart systems remains challenging. In the first part of this thesis, CNCs were isolated from pine (softwood) and oak (hardwood) wood chips obtained from a waste stream of a panel production facility. The waste wood chips were alkali-treated and bleached to remove non-cellulosic content. The resulting nanocrystals were compared in terms of morphology and physicochemical properties. Although they underwent the same chemical process, CNCs from hardwood and softwood wastes displayed different properties, especially in size, zeta potential, and crystallinity. The results showed that hardwood CNC has higher crystallinity compared to softwood CNC. Moreover, hardwood CNCs have smaller hydrodynamic diameters with greater zeta potential than softwood CNCs. In the second part, we aimed to create a thermo-responsive nanocomposite system constructed with wood-derived CNCs and temperature-responsive reversible polymersome-forming PEO-PPO-PEO block copolymers, specifically Pluronic L121. The morphology, phase behavior, and mechanical properties of the composite gels were investigated in detail. Two different CNC concentrations (4 wt. % and 5 %) were studied by varying the L121 concentration from 1% to 20% to understand the effect of unimers and polymersomes on the CNC network. The results showed that dilute and high concentrations of Pluronic L121 have different effects on the CNC gelation behavior. Adding Pluronic L121 up to 5% softened the composite below the transition temperature. The composite became stronger with L121 addition from 10 to 20% and a gel network was obtained above the transition temperature. Interestingly, the CNC hydrogel network became more deformable and resistant to microstructural breakdown at large strains due to the inclusion of large vesicles. The results demonstrate that CNC-Pluronic L121 hydrogels showed thermo-reversible rheological behavior, making them potential candidates for developing stimuli-responsive functional materials for biomedical applications.