Physico-Chemical Properties of Plant Polysaccharides Investigated with NMR Techniques
Abstract: In the lead of developing new biomaterials from plant sources it is of importance to be able to control and vary the properties, such as flexibility, strength, and hygroscopicity of the final material. These properties are highly directed by the interactions between constituents on a molecular level. To receive information about such interactions the work in this thesis is based on nuclear magnetic resonance (NMR) techniques, comprising studies of polysaccharides and their interactions with the surrounding in solution; in a gel; and in the state of fibers. It could be concluded that the dissolution mechanism for polysaccharides differs depending on whether the chain is substituted, as the hemicellulose xylan normally appears, or unsubstituted as native cellulose. The dissolution mechanism for cellulose was investigated in a solution of tetrabutylammonium fluoride in dimethyl sulfoxide (TBAF/DMSO), where it was revealed that the fluoride ion plays a dominant role as a hydrogen acceptor. Thereby the fluoride ion associates strongly to the hydroxyl groups of cellulose (-OHcellulose) thus replacing the hydrogen bonds between the cellulose chains, and electrostatically shields the chains. In this study it could further be elucidated that the ratio H2O : F -: -OHcellulose should be kept below 2:1:0.5, as gelling of cellulose in the solution is evident above these values. The proposed mechanism for such gel formation is that the interaction between water and fluoride is dominating compared to the association of fluoride to the hydroxyl groups of cellulose. This opens up for hydrogen bonds between cellulose chains due to less electrostatic shielding. The formation rate of this gel was further investigated by chemical shift imaging (CSI). As the gel is formed by the presence of water, the gel formation rate could be determined from the propagation of the detected water front to 5.5'10-11 m2/s, which is two orders of magnitude lower than the self-diffusion in water. NMR diffusometry was used to determine the size and molecular weight of xylans from various sources, by their self-diffusion coefficients in solution. The relation between the diffusion coefficient and the molecular weight was shown to strongly depend on the substitution pattern of the xylan backbone. The semi-soft structure of polysaccharides in the form of fibers was investigated by NMR cryoporometry. In this study the porosity and the pore size distribution as the effect of hornification in bleached wood fibers could be determined for pore radius up to 100 nm. Moreover, it could be shown that hornification of pulp could be prevented by adsorption of glucuronoxylan on the fiber surface prior to drying. The NMR methods used within this work; chemical shift imaging; diffusometry; cryoporometry; but also the study of line-broadening, are all shown to be suitable techniques to characterize physico-chemical properties of polysaccharides.
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