Cellulose degradation in pulp fibers studied as changes in molar mass distributions

In this thesis, size-exclusion chromatography (SEC) of wood polymers dissolved in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) has been used to characterize the molar mass distributions (MMD) of wood polymers in pulp fibers after chemical degradation. Characterization of birch kraft pulps subjected to ozone degradation and acid hydrolysis, respectively, rendered different changes in the MMD. Ozone degradation resulted in large redistributions of the original MMD, observed as the development of a distinct fraction of cellulose with intermediate molar mass. Acid hydrolysis resulted in minor changes of the original MMD compared to ozonation. Fibers subjected to acid hydrolysis were considerably weaker than ozonated fibers. These results indicated that there are differences in how the two chemicals degrade the fiber. The solubility of softwood kraft pulp fibers was enhanced by derivatization of the fiber polymers with ethyl-isocyanate during simultaneous dissolution in LiCl/DMAc. The derivatization made it possible to achieve reliable estimations of the MMD, and hence molar masses, of softwood kraft pulps. The derivatization procedure made it possible to dissolve 90 % of softwood kraft pulps with kappa numbers over 50. Severe alkaline degradation of birch and Norway spruce wood chips was studied both by varying the pulping time and by varying the initial alkali concentration. Differences were found in the MMD of the two fiber types, and the alkaline degradation was found to affect polymers in the entire MMD. Multi-angular laser light scattering (MALLS) was used as a detection technique with SEC on cellulosic samples. The MMD and average molar masses obtained through directstandard calibration with commercial standards were compared with MMD and molar masses as obtained by MALLS-detection. Large discrepancies were found, and two methods of correcting for these discrepancies were developed. Theoretical simulations of polymer degradation were performed. Random, or homogeneous degradation was used as a model for alkaline cellulose chain scission, and a resemblance with experimental data was observed. End-wise depolymerization of cellulose was also simulated and the results are discussed in the light of experimentally observed MMD.