Fiber Morphology Responsible for Restricted Solubility of Softwood Kraft Pulp in LiCl/DMAc

Solid-state NMR spectroscopy and FTIR microspectroscopy were used to investigate the contribution of fiber ultrastructure to the limited solubility of softwood kraft pulp in LiCl/DMAc. The insoluble residue was enriched with glucomannan, and the NMR studies suggest that cellulose fibrils are covered with linear glucomannan, which together may form an insoluble complex. Damage to the fiber ultrastructure by mechanical treatment resulted in lower crystallinity and improved solubility. The interaction between cellulose and glucomannan was probably also destroyed in damaged fibers, improving the accessibility by the solvent. The insoluble material was found to contain highly swollen fibers with very low crystallinity, but also some intact fibers with nearly original pulp crystallinity.