Solubility studies on poly(ethylene oxide) and cellulose
This study has consisted of two related areas of research. The first area concerned the measurement of the proportions of gauche rotamer about C-C and C-O bonds ofpoly(ethylene oxide) (PEO) from NMR coupling constants. 3J HCCH and 3J HCOC couplings in 1,2-dimethoxyethane and bis-(2-methoxyethyl)ether, both model compounds for PEO, plus 3JHCCH in poly(ethylene oxide), have been obtained in five solvents by iterative fitting. The deduced proportions of gauche rotamers are higher than previous estimates, and higher still after allowance is made for the pentane effect. They fit well with gas phase electron diffraction data, with current gas phase theoretical calculations and with standard RIS parameters for the polymer. The influence of solvent arises more from its H-bond donor properties than in its dielectric. This study was undertaken partly in order to elucidate likely conformations of the glycosidic bond in the ensuing work on cellulose dissolution. The second area of research was concerned with the dissolution of cellulose in amine oxides. Courtaulds PLC have successfully used N-methylmorpholine-N-oxide (NMMO) to produce cellulosic spinning solutions. However, the interaction between cellulose and NMMO is not clearly understood. We have therefore sought to amplifY the understanding of this polymer-solvent interaction, first with optical microscopy experiments, and then with solution-state and solid-state NMR techniques. Optical microscopy experiments showed several different behaviours of ramie fibres (natural, highly crystalline cellulose I) as they dissolved in NMMO with added ~O. In 81.2 % NMMO solution, the fibres were observed to swell without dissolution, or burst into tiny fragments or else bend at certain regions creating a "zigzagging" pattern before dissolution. The rates of dissolution were also variable. Some amine oxides that are non-solvents for cellulose were also found to swell ramie, though in this case without subsequent dissolution. The changes in the proton NMR chemical shifts of NMMO and N-ethylmorpholine-N-oxide (NEMO), dissolved in dry DMSO-d6, on addition of water or methyl-P.o-glucopyranoside (a soluble model compound for cellulose) were used to obtain binding constants. The binding between methyl-P.oglucopyranoside and NMMO (a solvent for cellulose) is about twice as strong as the binding between the non-solvent NEMO and the same saccharide, under these conditions. In contrast, the binding between NEMO and water is stronger than that measured between NMMO and water. The stronger interaction of NMMO (relative to the interaction of NEMO) with the saccharide helps to explain its potency as a solvent for cellulose, but the reason for its occurrence, and the weakened interaction with water, is not fully understood. The carbon NMR shift differences of methyl-P.o-cellobioside on addition of various amine oxides (both solvents and non-solvents for cellulose) were found to be similar. We propose that the 1:1 interactions of all amine oxides with cellulose may be similar but that the competing interaction between amine oxide molecules decides if they become a solvent or non-solvent for cellulose. The variation of spin-lattice relaxation measurements on the cellobiose carbons upon addition of NMMO was also studied It indicates an increase of mobility of the C5-C6 bond in the presence ofNMMO. This implies the breaking ofH-bonds in this region. Solid-state NMR spectra (both CPMAS and HPDEC/MAS) were run on aliquots of cellulose samples in NMMO, taken out at various stages of the dissolution process. A faster disappearance of the C4 peak from the crystalline phase relative to the C4 peak from the amorphous phase suggests that the crystalline phase must first be penetrated by the solvent before dissolution. In contrast ramie treated with the non-solvent NEMO was found to have been converted from cellulose I to cellulose illr Theamorphous phase of ramie fibres treated with the non-solvent trimethylamine-N-oxide was found to increase at the expense of the crystalline phase, although in this case no cellulose ~ was detected. Evidently, some amine oxides that are non-solvents for cellulose can nevertheless penetrate between layers of cellulose that are held together by Vander Waals forces, even though they cannot pull apart adjacent chains of cellulose (necessary for dissolution to occur) that are held together by strong H-bonds.