Ultrastructural Aspects of Pulp Fibers as Studied by Dynamic FT-IR Spectroscopy

Dynamic (or 2D) FT-IR spectroscopy in combination with polarized IR irradiation has been used in this work to study wood polymer orientation and interactions on the ultrastructural level in wood fibers in the native state as well as the effects of different pulping processes. The wood polymer interactions were studied under both dry and humid conditions. The matrix of lignin and hemicelluloses located between the well-ordered cellulose fibrils in the wood cell wall of spruce was here shown to be more highly ordered than has earlier been revealed. It was confirmed that glucomannan is oriented parallel to the cellulose fibrils and is highly coupled to it. The lignin was also shown to have a main orientation in the structure although this is probably not as strong as that of glucomannan. The orientation of the lignin may derive from the fact that the polysaccharides act as templates during the lignification of the cell wall. This organization implies that not only the cellulose but also the lignin and the hemicelluloses have different mechanical properties in the longitudinal and cross-fiber directions. The ability to gain molecular information on the stress transfer in polymers with dynamic FT-IR spectroscopy made it possible to verify experimentally earlier molecular calculations on the stress transfer within the cellulose chain. It was also possible to show, on the molecular level, the dominant importance of the cellulose fibrils for the stress transfer in the longitudinal direction of pulp fibers, including lignin-rich mechanical pulp fibers. The glucomannan of softwood fibers was also shown to participate in the stress transfer in the fiber direction indicating a close association with the cellulose, whereas the xylan showed no dynamic response. Already under dry conditions, the lignin was shown to have a more viscoelastic response than the polysaccharides during the loading of pulp fibers and it was thus able to move independently of the cellulose. The enhanced spectral resolution obtained with dynamic FT-IR spectroscopy made it possible to study the crystal structure/chain order of cellulose in pulp fibers. The possibility of following changes in the relative cellulose I allomorph composition of pulp fibers was demonstrated for some chemical pulps. Dynamic FT-IR experiments under humid conditions and of elevated temperatures made it possible to study the softening of the biopolymers in their native environment. This was also demonstrated for some different pulps, and this may be a promising tool for obtaining viscoelastic information on the molecular level in composite systems such as wood fibers.