Defence of doctoral thesis: Li Zha – Surface Engineering of Cellulose Nanofibers for Advanced Biocomposites

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.

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-344942