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Molecular rheology of coniferous wood tissues Chow, Sue-Zone

Abstract

The time dependent molecular motions of wood components at straining parallel to fibre direction were observed by infrared polarization technique. A two-stage molecular motion involving three wood components, cellulose, hemicellulose and lignin is suggested as the course of wood molecular relaxation. The first stage begins at equilibrium, when the specimen is not stressed, and extends immediately to a minimum dichroic ratio (formula omitted) of carbo-hydrate components represented by 1160 cm⁻¹ and 1730 cm⁻¹ bands, and the maximum dichroism of lignin (1500 cm⁻¹). The second stage starts at the end of the first stage and extends to equilibrium recovery. Regardless of the form of external excitation (creep or stress relaxation), and the time of excitation (ramp- or step-loading), the basic pattern of the two-stage molecular motion was followed, while damping of the molecules accompanied the whole rheological process. Thus, the wood macromolecular structure maintains an "internal state" of equilibrium on receiving external excitation. This equilibrium state is achieved by moving the carbohydrate and lignin components in opposite directions. The described pattern of molecular motion for a component in wood is a compensatory result from the interference of other components. Removing one or more components from wood changes the motion patterns of the remaining components. The conformation of cellulose in the specimen without the presence of lignin and hemicellulose is comparable to that of other synthetic linear polymers. Energy transfer system of wood was postulated as being due to the directional movement of molecular components which results in association and high steric interference between carbohydrates and lignin, similar to cross-linked chains of lignin and carbohydrates. This energy transfer system of wood is further facilitated by the existence of a systematic structure of wood microfibrils which permits a zone of gradual transition from high crystallinity to a diffuse state. The lignin network of the system may do more than transfer energy, it may act also as an "energy sink" and thereby function to maintain the memory of the excitation.

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