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Summary In order to define the role of lignin for stress transfer in the fiber wall, its orientation and mechanical interaction with other cell-wall components were investigated. Polarized infrared spectra of oriented thermomechanical pulp fiber sheets were used to determine the molecular orientation, and dynamic infrared spectroscopy was used to study the rheology of lignin in the cell wall. The results indicate that there is a preferred orientation of the phenyl-propane units of lignin along the fiber axis in spruce tracheids. This implies there is an ordered structure of lignin in the secondary wall of tracheids analogous to that of the cell-wall polysaccharides. The dynamic IR-spectra also indicated that lignin exhibits a much more viscoelastic behavior than do the carbohydrates, a knowledge of importance in the interpretation of transverse fiber properties.

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The properties of cellulose materials are dependent on interactions between and within the cellulose chains. To investigate the deformation behavior of cellulose and its relation to molecular straining, sheets with fibers oriented preferably in one direction were studied by dynamic FT-IR spectroscopy. Celluloses with different origins (spruce pulp, Cladophora cellulose, cotton linters) were used. The sheets were stretched sinusoidally at low strains and small amplitudes while being irradiated with polarized infrared radiation. The cellulose fibers showed mainly an elastic response. The cellulose fibers showed mainly an elastic response. The glucose rings and the C-O-C bridges connecting adjacent rings, as well as the O(3)H.O(5) intramolecular hydrogen bonds are the components mainly deformed under stress, whereas the O(2)H.O(6) intramolecular hydrogen bonds play a minor role. The load distribution was also found to be different in the different allomorphic forms of cellulose I, namely, I(alpha) and I(beta).