| In the21st century, in order to solve the global warming and air pollution and other issues caused by the increasing depletion of petroleum resources and their consumption, science and technology have gradually turned to research development and utilization of renewable biomass resources. Cellulose is the most abundant natureal polymer on earth, which has a large number of hydroxyl groups to form strong hydrogen bonds. However, cellulose has not reached its potential applications because it cannot be dissolved in most common solvents and cannot be melted to fabricate into a desired form, attributing to the strong interstrong inter-and intra-molecular hydrogen bonding in cellulose. In our laboratory, novel solvents such as NaOH/thiourea, NaOH/urea and LiOH/urea aqueous solution that can dissolve cellulose rapidly after being precooled to low temperatures (-12~-5℃) have been developed. To create a new low temperature dissolved macromolecules theory and better guide the cellulose dissolution technology into the industrial production, a series of basic scientific problems such as the interaction between solvent small molecules and macromolecular, the formation of new hydrogen bonding and the system compound structure, should be further resolved. Recently, great progress has been made in computational chemistry. Especially in2013, the Nobel Prize in Chemistry was awarded to Martin Karplus, Michael Levitt, and Arieh Warshel for the "development of multiscal models for complex chemical systems", which would provide a pathway to model the complex construction of cellulose in aqueous solution.In this thesis, cellulose solution, the interaction between cellulose and solvents as well as the inclusion complex (IC) structure were studied by nuclear magnetic resolution (NMR), dynamic laser scattering (DLS), transmission electron microscope (TEM) and so on. Meanwhile, computational chemistry was used to infer the IC structure, which compared with the experimental results. The innovative points of this work are as follows.(1) a kind of water-soluble cellulose derivative, methylcellulose (MC), was used as solute in the NaOH/urea solvent system to prove that low temperature improved the formation and stability of hydrogen bonding between solvent and the hydroxyl groups on MC.(2) Variable temperature NMR and TEM were used to reveal the hydrogen bonding interaction between cellulose and NaOH/urea (or thiourea) cellulose aqueous solvent system and confirm the structure of the cellulose ICs.(3) The interaction between cellulose and solvent system as well as the structure of the cellulose ICs were estimated by DFT method.(4) A new technology about solubility characterization was established and the effect of the composition and proportion of the solvent system on the cellulose dissolution behavior was studied to find that, cellulose can be dissolved at room temperature when changing NaOH/thiourea ratio.The main contents and conclusions in this project are divided into the following parts. First, the interaction between NaOH/urea water solvent system and the hydroxyl group on cellulose at different temperatures was studied by means of a water-soluble macromolecules, methyl cellulose (MC). NMR and differential scanning calorimetry (DSC) results showed that-OH on MC interacted strongly with solvent molecules (NaOH, urea and water) to form hydrogen bonds. Dynamic light scattering results demonstrated that MC macromolecules existed mainly in the form of single chains at low temperature, whereas they were mainly in the form of aggregates when dissolving at room temperature. TEM and DLS results further confirmed that low temperature was favor for the formation of the MC-NaOH/urea complex in the aqueous solution, which is much more stable. Furthermore, MC presented in a random coil conformation in NaOH/urea solvent system, whereas cellulose chain without derivatization existed in a terms of rigid chain.TEM and a variety of NMR techniques were employed to study the hydrogen bonding interaction among the cellulose/NaOH/urea aqueous system and morphology of the complex.15N and23Na spectra shown that Na+ion in aqueous NaOH interacted directly with urea and the hydroxyl groups on cellulose, OH-ion interacted with the amino group of urea through hydrogen bonding but no direct interaction between cellulose and urea was existed. In solution, urea placed in the outermost layer of inclusion complexes associated with NaOH, urea, water and macromolecule cellulose to prevent aggregation of cellulose chains. Furthermore, the results of23Na relaxation time confirmed that adding urea in NaOH aqueous solution caused the cellulose ICs slow motion to increase the stability of the cellulose ICs and to promote cellulose dissolution. Low temperature mainly affected the interaction between Na+and H2O and enhanced the Na ion complex stability as well as the hydrogen bonding interaction between urea and OH". TEM results revealed that the cellulose ICs had an extended wormlike morphology with a diameter of3.6nm and a length about300nm, which is consistent with the average contour length. The above results proved a new explanation for cellulose dissolution in NaOH/urea solvent system at low temperature, confirmed the existence of the cellulose ICs as well as further clarified the ICs structure and its formation mechanism. Thus the work for the establishment of macromolecules dissolved in aqueous system theory at low temperature provides a sufficient basis.We first utilized the density functional theory (DFT) to study and analysis micro interaction between cellulose and NaOH/urea water aqueous solvent small molecules and possible space of complex structure. Based on the obtained experimental results, several kinds of NaOH/urea hydrate clusters were designed and optimized by M206X/6-31+G(d) to get the most stable NaOH(H2O)7’urea cluster structure. Detailed structural analysis shown that the Na+ions on the NaOH interacted with urea carbonyl O atoms, and OH" ion in NaOH directly bind to H atoms of urea amino to form new hydrogen bonding to induce hydrogen atom in amino transfer to OH-ion, which resulted in C-N with partial double bond character. At the same time, water of NaOH(H2O)7-urea replace the cellulose hydroxy and then get the stable similar NaOH(H2O)7urea cluster structure by M062X/6-31+G (d) optimization method. When NaOH/urea hydrate interacted with the primary hydroxyl function outside the ring of cellulose glucose, the C6-OH conformation of cellulose didn’t change. The cellulose ICs was calculated through M062X/6-31g (d) methods to reveal that the intramolecular hydrogen bond O3H...O5still existed in the cellulose ICs, and the basic structure of NaOH(H2O)7’urea cluster had no damage. The results supported that the cellulose should be rigid chain in aqueous solution. The binding energy of NaOH(H2O)6’urea cluster with glucose C6hydroxyl group is bigger than that of glucose and glucose as well as that of NaOH(H2O)6’urea cluster and water, suggesting that inclusion compound of the model is reasonable. According to the structural analysis of the diameter of the cellulose ICs through theoretical calculations, its diameter is2.8nm, which is in accordance with the results of TEM and cryo-TEM. Thus, the results from the theoretical calculation of simulation structure further confirmed that the cellulose evidence that the presence of cellulose ICs is reasonable.The intermolecular interaction in cellulose/NaOH/thiourea water system and its complex morphology were studied by1H,13C and23Na NMR spectrum and TEM. The results proved for the first time that OH-ion of NaOH interacted with the amino group of thiourea through hydrogen bonding but no direct interaction between Na+ion and C=S of thiourea was existed. The molecular interaction difference in both NaOH/thiourea system and NaOH/urea system attributed to difference in13C chemical shift of C=O in urea and C=S in thiourea. The association interaction between Na+and the O atom in urea carbonyl lead to small high shift of13C NMR spectra of C=O. However,13C NMR spectra of thiourea and thiourea/cellobiose shown that there are no hydrogen bonding interaction between thiourea and cellobiose, and further illustrated that thiourea didn’t interact directly with cellulose. Adding urea or thiourea in NaOH aqueous solution can promote the dissolution of cellulose, which is mainly due to the variance of the OH-hydrates structure, thus affecting the combination of it with hydroxyl groups of cellulose, leading to the improvement of the ability of dissolving cellulose. Therefore, OH-ion of NaOH is the key factor to dissolve cellulose. Reducing temperature,1H and13C spectra of cellulose both shifted high field, manifesting that the interaction between the atoms of cellulose chain with solvent molecules increased, thus forming stable cellulose ICs. In addition, decreasing temperature, the changes of13C chemical shift of cellulose at C-6atom was the biggest, investigating the association interaction between cellulose C-6hydroxyl group and solvent molecules was the strongest and it is more easily to destroy intermolecular hydrogen bond of cellulose. Therefore, low temperature is advantageous to form inclusion complex associated with cellulose and solvent molecules through hydrogen bonding, leading to cellulose dissolving in NaOH/thiourea aqueous solution. TEM results confirmed that the cellulose ICs had an extended wormlike morphology with a diameter of13-17nm and a length about300-500nm, which is consistent with the average contour length of cellulose chain. Thiourea prevented cellulose chains from aggregating and maintain the stability of cellulose solution.Based on hydrogen bonding between macromolecules and solvent molecules, cellulose solubility in NaOH/thiourea aqueous solution was studied by changing the thiourea content. The results revealed that cellulose can also interact with NaOH/thiourea aqueous solution through hydrogen bonding at room temperature, leading to changes in the resulting crystalline and structure of cellulose. X-ray diffraction (XRD) results confirmed that the conversion degree from cellulose I to cellulose II for cellulose treated with the alkali solvent system was connected with the cellulose dissolution. Thus, crystal structure transformation of the insoluble cellulose residue could preliminary determinate the ability of a solvent to dissolve cellulose. Furthermore, the transmittance of the cellulose solution could easily assess dissolving capacity of cellulose in NaOH/thiourea aqueous solvent. The experimental results shown that cellulose could be dissolved in10wt%NaOH/8wt%thiourea aqueous solution with cellulose concentration4%at10℃, and its solubility was more than80%, supporting that the cellulose solubility can be measure through transmittance. It revealed that when the thiourea content was enough to maintain a stable inclusion complex network of hydrogen bonds of cellulose, cellulose can be dissolved in the solvent at room temperature. Therefore, this work provides very important information to improve the cellulose new solvent system.This thesis mainly study cellulose and NaOH/urea (thiourea) aqueous solvent system combining the experimental data and theoretical calculations, especially the hydrogen bonding interaction and the structure of the formed inclusion complexes in the aqueous solvent system at low temperature. Experimental results under the guidance of the theory further optimize the composition of the aqueous solvent system at low temperature, and further improved the theory of the cellulose macromolecule dissolution at low temperature. The new theory of macromolecule dissolution at low temperature will effectively promote natural polymer research, development and utilization, and be in line with national sustainable development strategies and the Twelfth Five Year Plan. Therefore, this work is innovative, important academic value and application prospect. |
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