Mechanism Research Of Cellulose Dissolution In LiOH/Urea Solution And Preparation & Properties Of Functional CelluloseMaterials

Cellulose is the most promising natural macromolecule on the earth because of its universality, reproducibility and environmental friendliness. LiOH/urea/H2O is a very efficient and green solvent system for cellulose. However, there is still no mature theory to explain the strong dissolution ability of cellulose and the roles of the Li+ions in the aqueous solution. In this work, the roles of Li+ in cellulose dissolution in LiOH/urea solution and the interaction of the components have been investigated. Moreover, the chain motility and the phase size of rigid phase of different cellulose materials were obtained by different NMR methods. Based on the alkali/urea system, a series of absorbable cellulose/PEI composite hydrogels and fluorescent carbon dots were prepared, and their structures and properties have been studied. A brief summary of results is as follow:(1) In this dissertation, the status of Li+ and the interactions of the components in LiOH/urea/cellulose system were investigated by employing fast-freeze-drying and solid-state NMR techniques. As many as four types of Li+ existing in the LiOH/urea/cellulose system were observed. This has never been observed before because of the rapid exchange in the aqueous solution system. Some direct evidences to prove the interactions from the H-X(13C,6Li) 2D NMR experiments were provided and a possible 3D interaction model was proposed to illustrate the roles of Li+ions in the dissolution of cellulose in LiOH/urea solution and the interactions of the components on the molecular level. A part of Li+ions tightly coordinate with the cellulose chains after OH- ions destroy the intermolecular hydrogen bonds between the cellulose chains. Some Li+ ions surround the Li+-cellulose chains in the form of Li+-OH--urea to prevent the self-aggregation of cellulose chains. Besides, a quite amount of Li+-OH- and free Li+ still exist in the LiOH/urea/cellulose system as substitution in the exchange process.(2) The crystallinities, chain motility and the phase size of rigid phase of different cellulose materials were obtained by several powerful NMR methods. After some processes such as hydrolysis, crosslinking or regeneration, the structures of these materials have changed a lot. In brief, the cellulose crystallite has the highest crystallinity and slowest chain motion. The cellulose hydrogel has the lowest crystallinity and fastest chain motion. The information of cellulose films is the middle between crystallite and hydrogel materials. By Dipolar Filter-Spin Diffusion method, the phase size of rigid phase has been obtained. Raw cellulose and cellulose crystallite have bigger phase size, cellulose hydrogel has smaller phase size, cellulose films’phase size is middle among these materials. It’s interesting that there are also some difference between the two kinds of films which were prepared from different system. The reason is probably due to the deference of the regeneration speed.(3) A series of Cellulose/poly-ethylene imine (PEI) composite hydrogels were prepared by grafting hyperbranched PEI onto cellulose chains in alkali/urea aqueous solvent system by "one step" method. The SEM showed that the cellulose/PEI hydrogel maintained porous structure. The adsorption abilities of Cellulose/PEI composite hydrogels toward Cu(Ⅱ) were studied through adsorption kinetics and adsorption thermodynamics. The adsorption kinetics curves agreed with pseudo-second-order model more. The adsorption isotherms could be described by both Langmuir and Freundlich isotherm models. The maximum adsorption amount was calculated to be 285.7 mg/g, much higher than the common adsorbents. The matrix stability provided by cellulose and strong chelation to Cu(II) provide by PEI made the hydrogels could be used in a wide range of pH and temperature. Besides, the Cu(H) loaded Cellulose/PEI hydrogel also could be easily regenerated by dilute sulfuric acid and still keep a major adsorption capacity. In addition, the cellulose/PEI hydrogel also can adsorb alizarin red efficiently in a wide pH range. It will be a kind of new potential environmentally friendly materials for sewage disposal.(4) By changing the components of the cellulose hydrogels and filtrating different size of the CDs, a series of fluorescent carbon dots were prepared. The factors which influence the structures and properties were investigated. The results showed that the fluorescence quantum yield can be improved a lot by adding NaOH or urea. Several methods (FT-IR, UV-vis, NMR) were used to study the structure difference of the carbon dots. All the CDs’ fluorescence are size dependent. The CDs prepared by NaOH/urea/cellulose hydrogel are very sensitive to pH values and Hg2+. Thus the CDs prepared by cellulose hydrogels are very potential in the fields of Hg2+ ion recognition. Besides, the CDs showed very good biocompatibility.