| Nowadays, global interests in renewable energy are increasing because oflack of coal and oil. As the most abundant natural polymers on the earth, cellulosehas attracted great attention by its unique superiority, such as environmentallyfriendly property, biocompatible and so on. Moreover, many functional cellulosematerials were successfully prepared from the doped cellulose. However, theinter-and intra-molecular hydrogen bonds have made the dissolution of cellulosea difficult process in common solvents, and it limits its potential applications. Inour laboratory, novel solvents such as NaOH/thiourea, NaOH/urea and LiOH/ureaaqueous solution that can dissolve cellulose rapidly after being precooled to lowtemperatures (–12~–5°C) have been developed. We have proved that thecellulose dissolution at–12°C can arise as a result of a fast dynamicself-assembly process among solvent small molecules (NaOH, urea, and water)and cellulose macromolecules. NaOH―hydrates‖can be more easily attracted tocellulose chains through the formation of new hydrogen-bonded networks at lowtemperature, while the urea hydrates can possibly be self-assembled at the surfaceof the NaOH hydrogen-bonded cellulose to form an inclusion complex (IC),leading to dissolution of cellulose. However, cellulose ICs are unstable and can bedestroyed probably, leading to the cellulose IC aggregates. Therefore, the singleICs co-exist with IC aggregates in this system. As a result, the cellulose solution isrelatively unstable and sensitive to temperature, polymer concentration, andstorage time. The aim of this dissertation is to investigate the aggregate behaviorof cellulose single ICs and IC aggregates in NaOH/urea system to optimize thedissolution procedure, as well as the chemical cross-linking rheological behaviorto supply useful information for the industrialization of the green cellulosesolvent.The innovation of this dissertation are as follows:(1) for the first time, theeffect of the proportion of NaOH and urea on the aggregation behavior ofcellulose was investigated, and the mechanism of the solvent small molecules andtemperature on the cellulose solution stability was established.(2) The effect ofstirring conditions on dissolution of cellulose in NaOH/urea system was studiedsystematically, and a schematic diagram of the cellulose solubility was plotted.(3)The chemical cross-linking process of cellulose/epichlorohydrin (ECH) was monitored by rheological, and the characteritics of cellulose/ECH hydrogels wasrevealed.(4) A conducting hybrid composite of cellulose/polypyrrole (PPy) wassuccessfully prepared by in situ synthesized by using the micropores in cellulosefilms as micro-reactors.The main research contents and conclusions are divided into several parts.For the7wt%NaOH/12wt%urea solvent, a small increase of NaOH couldincrease the amount of singe cellulose ICs and weaken the aggregationphenomenan, as a result of the reinforcement of hydrogen bond between celluloseand NaOH hydrates. Increase of urea decreased the IC aggregates percentageslightly because urea could form more complete shell of the ICs structure.Furthermore, the portion of NaOH/urea solvent has been optimized to be9/13, inwhich the proportion of cellulose single ICs was0.96. Temperature also exhibitedgreat influence on the aggregation phenomenon, and cellulose solution wasrelatively stable at low temperature.Influences of the stirring area, stirring time, stirring rates of different stirringblade and environment temperature on the cellulose solubility in NaOH/urea wereinvestigated comprehensively. Lager stirring area, longer stirring time and higherstirring rate could enhance the saturation solubility (cmax) value, and after the cmaxvalue reached a maximum, further increase of stirring time and rates made littleinfluence on the dissolution, indicating a completed dissolution has been achieved.Moreover, a schematic diagram of the cellulose solubility was plotted, as aninstruction of cellulose dissolution in NaOH/urea. The results from DLSexperiments manifested that increase of rates could improve dispersion anddissolution of cellulose. On the other hand, the cmaxvalues could be significantlycontrolled by varying the stirring environment temperature, and the favorableenvironment temperature range was from–5to–10°C.Rheological properties of gelation process and mechanical properties ofbiomass hydrogels prepared from cellulose in NaOH/urea aqueous solutions byusing ECH as cross-linker were studied. Oscillatory frequency, time andtemperature sweeps were performed on an ARES-RFS III rheometer to monitorthe effect of convent of cross-linker, cellulose concentration and temperature onsol-gel rheology. The results indicated that the concentration of cross-linker andthe polymer could influence the crosslinking process. The higher concentration ofECH and cellulose could accelerate the gelatin, because of the entanglement among the polymer. Compare with ECH, the rheological behavior was moresensitive to the cellulose concentration. Temperature also played an important role,and higher temperature could result in a lower gelatin time. The mechanicalproperties of the cellulose/ECH hydrogels was adjusted to the concentration ofECH and cellulose. ECH/cellulose hydrogels exhibited excellent mechanicalproperties. Equilibrium swelling degrees of the ECH/cellulose hydrogensenhanced markedly with the cellulose concenstration.Cellulose/PPy conducting hybrid films were fabricated successfully by in situsynthesis of pyrrole with FeCl3as the oxidant and dopant in the micropores ofcellulose films as micro-reactors. The morphology of the resultant PPy wasspherical, and the size and distribution could be controlled by the pyrroleconcentration. The results from the IR spectroscopy indicated the hydrogen bondexited between PPy and cellulose. The cellulose films were planted conductivityafter combination of PPy, and the conductivities were affected by the size,distribution of PPy and the dopant FeCl3. The dielectric constant of the hybridfilms wwere high, which resulted in the strong absorption of the electromagneticwaves. This work solved the problem in molding PPy, and provided a simple andenvironmentally friendly method to prepare biodegradable and conductivepolymer composite material.In this dissertation, the dissolution process and stability of cellulose inNaOH/urea aqueous solution were studied systematically. The dissolvingprocedure of cellulose were optimized to promote the development of―green‖cellulose solvent and it provids important scientific proofs for cellulose materialfabrication in industry. The mechanism of chemical and physical cross-linking incellulose/ECH was explored, and a conductive composite film was prepared by insitu synthesis method. This research will enrich and improve the development andapplication of other natural polymers and functional materials, promoting thebasic research and application of biomass materials in our country. Thus, weprovided important scientific proofs to realize the low-cost, nontoxic and rapidprocess for fabrication cellulose materials on industry. Therefore, there were greatscientific significance and prospects of applications and it well accords with thetarget of our country and has the great importance for a sunstainable development. |
PDF Full Text Request