NMR Studies On The Mechanism Of Cellulose Dissolution In Alkali/Urea Solvent System

Cellulose is one of the most abundant naturally occurring polymeric materials in the world; it is almost inexhaustible, environmentally friendly and biocompatible. However, the application of cellulose is largely restrained by the difficulty of its dissolution in common solvents. It is widely accepted that the tight array of inter-and intrahydrogen bonds is the reason for this difficulty. In the last decades, intensive searches on new solvent system for cellulose dissolution, such as NaOH/additive system, NMMO, ionic liquids, have been carried out.NaOH system, cheap and environmentally friendly, was considered to have potential in commercial use. Zhang’s group did very intensive study on the dissolution of cellulose using a novel solvent system. They found that additives such as urea, thiourea and ZnO can improve the dissolution of cellulose in aqueous alkali solutions. The aqueous solution of7%NaOH/12%urea, pre-cooled to low temperatures, was found to be very adequate to the cellulose dissolution both in solubility and solution stability. It is suggested that NaOH cleaves the close chain packing of cellulose through formation of new hydrogen bonds between cellulose and the NaOH hydrates, while urea forms inclusion complex with them preventing the self-association of cellulose molecules.The aim of this dissertation is to investigate the dissolution mechanism of cellulose in alkali/urea aqueous solvent system by the means of nuclear magnetic resonance. By the investigation of chemical shift, longitudinal relaxation times and DOSY, the role of NaOH and urea in cellulose dissolution was investigated. The dissolution mechanism model of "hydrogen bond breaking-stabilization" was proposed.The main research contents and conclusion are divided into several parts. Chemical shifts and longitudinal relaxation times of cellulose NaOH/urea solutions were measured to elucidate the interaction between cellulose and the solvent molecules. It was found that NaOH plays an important role in cellulose dissolution, while there is no strong interaction between urea and cellulose. The hydrogen bonds of the solution were measured. The hydrogen bond network of NaOH solution is stronger than that of water, while urea can break the hydrogen bond network of water. The hydrogen bonds of NaOH solution went stronger with the increase of the concentration and the decrease of temperature. It is the breakage of hydrogen bonds by NaOH that leads to the dissolution of cellulose. While urea may play less important role in the breakage of hydrogen bonds of cellulose.As an alkali hydroxide, KOH can provide OH-just like LiOH and NaOH; but it is well known that LiOH and NaOH can dissolve cellulose, whereas KOH can only swell cellulose. The inability of KOH to dissolve cellulose was investigated and the mechanism of cellulose dissolving in alkali solutions was proposed. The dissolution behavior of cellulose and cellobiose in LiOH, NaOH and KOH were studied by means of1H and13C NMR as well as longitudinal relaxation times. The structure and properties of the three alkali solutions were compared. The results show that alkali share the same interaction mode with cellobiose and with the magnitude of LiOH>NaOH>KOH; the alkalis influence the structure of water also in the same order LiOH>NaOH>KOH. The different behavior of the three alkalis lies in the different structure of the cation hydration ions. Li+and Na+can form two hydration shells, while K+can only form loose first hydration shell. The key to the alkali solution can or cannot dissolve cellulose is whether the cation hydration ions can form stable complex with cellulose or not. K+cannot form stable complex with cellulose result in the KOH solution can only swell cellulose.Urea can improve the solubility and stability of cellulose in aqueous alkali solution, while its role has not come to a conclusion. To reveal the role of urea in solution, NMR was introduced to investigate the interaction between urea and the other components in solution. Results from chemical shifts and longitudinal relaxation times show that:(1) urea has no strong direct interaction with cellulose as well as NaOH;(2) urea does not have much influence on the structural dynamics of water. Urea may play its role through van der Waals force. It may accumulate on the cellulose hydrophobic region to prevent dissolved cellulose molecules from re-gathering. The driving force for the self-assembly of cellulose and urea molecules might be hydrophobic interaction. In the process of cellulose dissolution, OH" breaks the hydrogen bonds, Na+hydrations stabilize the hydrophilic hydroxyl groups and urea stabilizes the hydrophobic part of cellulose.The dissolution of methylcellulose in NaOH/urea solvent system was investigated. Dissolution of methylcellulose in water, urea solution, NaOH solution as well as NaOH/urea solution was investigated by the means of NMR. The dissolution of methylcellulose and cellulose in NaOH/urea solvent system was compared. The results conformed the conclusion of the previous studies. NaOH breaks the inter and intra hydrogen bonds of cellulose and stabilize the hydroxyl groups of cellulose, while urea stabilize the hydrophobic parts of cellulose, separating hydrophobic parts of cellulose to aggregate, stabilizing the solution.In this dissertation, the mechanism of cellulose dissolution in alkali/urea solvent system was studied systematically, the role of NaOH and urea in cellulose dissolution were proposed. The dissolution mechanism model proposed can apply to other solvent system of cellulose. It will help to get deeper understanding of the mechanism of cellulose, and will help to find and develop new and more efficient cellulose solvents. This research will enrich and improve the development and application of cellulose and other natural polymeric materials.