Structure And Extraction Of Cellulose Of Plant Cell Wall And Cellulose Based Functional Materials

Cellulose is the major component of plant cell walls.Plants produce hundreds of millions of tons of cellulose through photosynthesis every year.They are inexhaustible and renewable resources in nature.And the materials synthesized from cellulose are biodegradable and environmentally friendly.They can be degraded into water,carbon dioxide and other small molecules in natural environment.In the background of energy crisis and increasingly serious environmental pollution,as a kind of natural,renewable and biodegradable raw materials,cellulose has great application potential in many fields.There are several hot issues existing in the application of cellulose:the extraction of cellulose from plant cell wall,the green solvents of cellulose,and clean and efficient preparation of various new cellulose based functional materials.This thesis is working around these issues.The big difficulty encountered in using natural cellulose is how to extract cellulose from plant cell walls efficiently.The structure of the cell wall of plants is very complex.The outer layer of walls are covered by the protective wax and inorganic salts,while the internal layer are composed of cellulose,hemicellulose lignin and protein etc..Cellulose is closely linked with other chemical components.The crystalline and non crystalline regions of cellulose stack together in the cellulose microfibrils,while different crystalline forms coexist in the crystalling regions.All these facts impede the application of natural cellulose.In recent years,biotechnology has been vigorously pushed to solve the problem,but only with full understanding of the spatial relationship among each component and the existing forms of cellulose crystals,can the enzyme or bacterial separating cellulose out feasible.So the research on the inner structure of plant cell wall is vital important to the development of biological technology and chemical methods for extracting cellulose.In Chapter 2,a new pretreatment method of cell wall samples was presented to study cellulose structure.The condition of treatment was mild so that the natural structure of each component was preserved.The relationship between each component was explored by the atomic force microscope(AFM)on the molecular scale,and the crystalline structure of natural cellulose was analyzed by AFM in fluid.By comparing the images obtained with the unit cell model,the different crystalline forms were identified and the form of stacking in the crystalline region was disclosed.This part of the work provides visual images of the aggregation and crystalline layout of cellulose,which will promote the development of biological and chemical methods to turn straw waste into valuable substance.Expanding the application of cellulose to various fields is the goal of researchers.There are two ways to achieve this goal:one is to improve the traditional technologies,reduce the cost,energy consumption and pollution caused by the production process,making the processes more consistent with the requirements of green chemistry.The other is following the development of science and technology,to design new cellulose-based functional materials so as to replace petrochemical products.The second half of chapter2,Chapter 3 and 4 are working about the preparation of green cellulose functional materials.Cellulose was extracted from seaweed and straw,and the extractive as well as microcrystalline cellulose were used as raw materials.The polyelectrolyte ultrathin membrane was prepared by self assembly technology;cellulose was dissolved and cross-linked in the LiCl/urea solvent,then regenerated to prepare the transparent film and azo dye adsorption resin.The hydrophobic membrane and the pH sensitive membrane were prepared by modifying the regenerated cellulose membrane.The complicated hydrogen bonds and crystalline structures of cellulose molecules make cellulose very difficult to be dissolved in ordinary solvents.The dissolution issue has restricted the application of cellulose seriously.Searching for efficient solvents and homogeneous reaction environments for cellulose are important topic in cellulose application.In Chapter 5,the molten ZnCl2 hydrate was tried to dissolve cellulose.The factors that might affect the dissolution were discussed,including suitable temperature and time,the content range of cellulose,and the molar ratio of ZnCl2 to H2O.The regeneration of cellulose and homogeneous synthesis of cellulose acetate were conducted in this system.The separation of products as well as the recovery and re-utilization of inorganic salts in the system were also studied.The researches in this paper provide rich experimental data and fruitful conclusions,which will help to realize wider development and application and of cellulose science.