Studies On Applications Of Ionic Liquids In The Dissolution And Graft Modification Of Natural Polysaccharides

Cellulose and chitin are two abundant renewable natural polysaccharides, and their development and utilization have great significance to alleviate resource constraints as well as energy crisis. The processing of cellulose and chitin are restricted by their limited solubility in common solvents and their inability to melt because of their numerous intermolecular and intramolecular hydrogen bonding. In recent years, a great breakthrough has been made by the utilization of ionic liquids as the solvents of natural polysaccharides. This has been providing a new and versatile platform for the wide utilization of biomass resources and creation of novel functional materials. In light of recent development of regarding applications of ionic liquids in natural polysaccharides, the work of this thesis is mainly concentrated on the following issues:(1) Dissolution of microcrystalline cellulose with ionic liquid 1-butyl-3- methyl imidazolium chloride and its rheological propertiesDissolution of Microcrystalline cellulose (MCC) in ionic liquid 1-butyl-3- methylimidazolium chloride (BmimCl) was studied. The structural differences between MCC and cellulose regenerated from the ionic liquid (BmimCl) have been investigated using X-ray diffraction. It was found that BmimCl was a good solvent for MCC and the regenerated cellulose belonged to celluloseⅡ. The rheological properties of MCC/BmimCl solutions have been investigated from steady shear and dynamic frequency analyses by means of ARES/RAF. In the steady shear measurements. The MCC/BmimCl solutions showed a shear thinning behavior at both high and low shear rates. The concentration of MCC /BmimCl solution and temperature had an important influence on the viscoelastic properties of MCC/BmimCl solution. Cross viscosity models was used to fit experimental data of complex viscosity. The activation energies for shear flow were determined by the Arrhenius equation.(2) Dissolution of chitin with ionic liquid 1-allyl-3-methylimidazolium bromide and its rheological propertiesDissolution of chitin (CT) in ionic liquid 1-allyl-3-methylimidazolium bromide (AmimBr) was studied. The experimental process was observed by the polar optical microscope, and the structural differences between native chitin and chitin regenerated from AmimBr have been investigated using X-ray diffraction. The results demonstrated that AmimBr was a good solvent for chitin and the native crystal structure ofα-chitin had a remarkable decrease in the crystallinity after being dissolved by the AmimBr solvent. The rheological properties of CT/AmimBr solutions have been investigated from steady shear and dynamic frequency analysis by means of ARES/RAF. In the steady shear measurements. All the CT/AmimBr solutions showed a shear thinning behavior that could be described by the power-law relationship in the range of measured shear rates. The dilute CT/AmimBr solutions show shear thickening region at low shear rates, which may reflect the characteristics of the AmimBr solvent. Shear viscosity showed exponential relationship with the concentration of chitin solution. A dynamic frequency sweep showed that higher concentration of chitins with IL behaved as weak gels.(3) Regenerating cellulose films from ionic liquid and their nanocompositesCellulose was dissolved in ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl), and then coagulated in water to obtain the cellulose hydrogel. Non-porous and micro-/nanoporous cellulose films could be obtained by directly drying and freeze- drying the cellulose hydrogel, respectively. Silver nanoparticles were formed on the regenerated cellulose films by reducing Ag+ in aqueous solution through in situ processing without using any other stabilizers. XRD, EDX, and SEM analyses were used to characterize the silver nanoparticles formed in the films. Particles of less than 100 nm were readily prepared using the described approach. The micro-/nanoporous structure and the high oxygen (ether and hydroxyl) density of the cellulose films constitute an effective nanoreactor for the in situ synthesis of silver nanoparticles. The nanopore is essential for the incorporation of silver ion and reductant into cellulose films as well as for the removal of unnecessary byproducts from films. The ether oxygen and the hydroxyl group not only anchor metal ions tightly in cellulose films via ion-dipole interactions, but they also stabilize metal nanoparticles by strong bonding interaction with their surface atoms. The preparative procedure is facile and versatile, and provides a simple route to the manufacturing of useful noble metal nanoparticles. The antibacterial experimental results showed that the nanosilver/cellulose composites had good antibacterial performances aginst Escherichia coli.(4) Cellulose/ε-caprolactone graft copolymers homogeneously synthesized in ionic liquids and their propertiesBy using stannous 2-ethyl hexanote [Sn(Oct)2] as an organic catalyst, cellulose/ε-caprolactone graft copolymers(cellulose-graft-PCL) with a molar substitution (MSPCL) of PCL in a range of 0.10-1.70 were successfully synthesized in ionic liquids . The molar ratios ofε-CL monomer to cellulose, reaction temperature, reaction time as well as catalyst amount affected the graft reaction .The structure and thermal properties of cellulose-g-PCL copolymers were characterized by FTIR , 1HNMR, 13CNMR, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and hot stage polarized light microscopy. The DSC results revealed that the copolymers (MS=1.70) exhibited a single glass transition temperature at 138℃, indicating the intermolecular and intramolecular hydrogen bonds in cellulose molecules had been effectively destroyed. DLS and SEM results indicated that the amphiphilic copolymers could self-assemble into spheric nanoparticles, ranging in size from 40 to 400 nm. The formation and size distribution of nanoparticles can be affected by molar substitution of graft chains as well as the preparation method.