Preparation And Properties Of Modified Cellulose Nanocrystals / Cellulose Acetate Nanofibers

To solve the increasingly serious environmental problems caused by the waste, recycling and degradation of materials, the application of green composite materials satisfy the requirements of global sustainable development strategy. Cellulose has a lot of characteristics, such as widespread sources, biocompatibility, and more importantly, can be expected as the main raw material for green composites in the future. Recently cellulose acetate electrospun nanofibers have superior properties, for example biodegradability, renewable and good chemical resistance, and also remarkable nanostructural features such as large specific surface area and high porosity. Thus cellulose acetate electrospun nanofibers show potentially broad application in the fields of biomedical materials and tissue engineering materials. However, their practical applications are restricted due to their poor mechanical properties. It is very important to take appropriate approach for improving their mechanical properties. Cellulose nanocrystals are ideal organic nanoreinforcements because of their nanoscale morphology, high strength and renewable, etc. Unfortunately, they are easily agglomerated duo to high surface hydroxyl content, which makes them difficult to disperse into the common solvents. Moreover, the compatibility with polymer matrix also needs to be improved. In this thesis, the hydroxyl groups on the surface of cellulose nanocrystals prepared through hydrochloric acid hydrolysis under hydrothermal conditions were converted to more lively carboxylic groups and then polyethylene glycol chain were grafted onto cellulose nanocrystals skeleton. The influences of three different types of cellulose nanocrystals as the nanoreinforcements on morphology structure and mechanical properties of electrospun composite nanofibers of cellulose acetate were discussed. Furthermore, the feasibility of electrospun composite nanofibers of cellulose acetate as a carrier in controlled drug delivery was evaluated. The main results are summarized as follows:1. Carboxyl cellulose nanocrystals were prepared through oxidation with ammonium persulfate by using cellulose nanocrystals produced by hydrochloric acid hydrolysis under hydrothermal conditions as raw materials. The influences of treatment methods, i.e. one-pot treatment, treated after ammonia neutralization, sodium hydroxide neutralization, and purification process on the extent of modification were studied. It is found that the greatest carboxyl degree of cellulose nanocrystals can be achieved via oxidation treatment after purification process. Furthermore, the effects of the reaction temperatures and concentration of ammonium persulfate on the morphology size, chemical structure, crystalline structure and thermal stability of cellulose nanocrystals were discussed. The greatest degree of oxidation was about 0.13. High dispersion stability of carboxyl cellulose nanocrystals in water, ethanol, dimethyl formamide, dimethyl acetamide, acetone, dimethyl sulfoxide, formic acid, and tetrahydrofuran can be achieved.2. The copolymers were prepared by grafting polyethylene glycol as flexible branched chains onto the surface of carboxyl cellulose nanocrystals as skeletons. Three catalysts including EDC/DMAP, EDC/HOBt and dibutyltin dilaurate on the graft yield were used to prepare the graft copolymers. The results show that the highest enthalpy for the copolymer with the graft yield of 32% can be obtained by using of dibutyltin dilaurate as catalyst. Compared with carboxyl cellulose nanocrystals, graft copolymers exhibited high thermal stability.3. Three kinds of cellulose acetate electrospun composite nanofibers were fabricated by using unmodified cellulose nanocrystals, carboxyl cellulose nanocrystals and cellulose nanocrystals with polyethylene glycol as branched chain as the nanoreinforcements. The influences of the nature and content of cellulose nanocrystals on the morphology, chemical structure and thermal stability of the composite nanofibers were investigated. It is demonstrated that the existence of hydrogen bonding interaction between cellulose nanocrystal and cellulose acetate appeared in the composite nanofibers. Moreover, high orientation of cellulose acetate along the direction of the nanofiber can be induced with carboxyl cellulose nanocrystals. More importantly, compared with that for pure nanofibers of cellulose acetate, nearly a 5000% improvement on the mechanical properties can be achieved for carboxyl cellulose nanocrystals, whereas significant toughening effect can be provided by cellulose nanocrystals with the branched chain of polyethylene glycol. Furthermore, hydrophilic properties and drug loadings of cellulose acetate nanofibers can be improved by introducing cellulose nanocrystals, especially when cellulose acetate nanofibers filled with cellulose nanocrystals grafted with polyethylene glycol chains.