Abstract
Despite the importance of cell wall diffusion to nearly all aspects of wood utilization, diffusion mechanisms and the detailed effects of moisture remain poorly understood. In this perspective, we introduce and employ approaches established in polymer science to develop a phenomenological framework for understanding the effects of moisture on diffusion in unmodified wood cell walls. The premise for applying this polymer-science-based approach to wood is that wood polymers (cellulose, hemicelluloses, and lignin) behave like typical solid polymers. Therefore, the movement of chemicals through wood cell walls is a diffusion process through a solid polymer, which is in contrast to previous assertions that transport of some chemicals occurs via aqueous pathways in the cell wall layers. Diffusion in polymers depends on the interrelations between free volume in the polymer matrix, molecular motions of the polymer, diffusant dimensions, and solubility of the diffusant in the polymer matrix. Because diffusion strongly depends on whether a polymer is in a rigid glassy state or soft rubbery state, it is important to understand glass transitions in the amorphous wood polymers. Through a review and analysis of available literature, we conclude that in wood both lignin and the amorphous polysaccharides very likely have glass transitions. After developing and presenting this polymer-science-based perspective of diffusion through unmodified wood cell walls, suggested directions for future research are discussed. A key consideration is that a large difference between diffusion through wood polymers and typical polymers is the high swelling pressures that can develop in unmodified wood cell walls. This pressure likely arises from the hierarchical structure of wood and should be taken into consideration in the development of predictive models for diffusion in unmodified wood cell walls.
Highlights
Renewable wood resources are poised to play a major role in our future bioeconomy, both as a feedstock for biorefineries producing energy, chemicals, and fuels, as well as continuing to be the basis for wood-based construction materials [1,2,3]
We use the term cell wall porosity to refer to the volumes in wood cell walls that are not occupied by the native wood polymers or native chemicals deposited in the cell wall while the tree was living
Diffusion depends on complex interactions between the free volume of the polymer matrix, molecular motions, diffusant dimensions, and solubility of the diffusant in the polymer
Summary
Renewable wood resources are poised to play a major role in our future bioeconomy, both as a feedstock for biorefineries producing energy, chemicals, and fuels, as well as continuing to be the basis for wood-based construction materials [1,2,3]. Wood is a notoriously complex natural material composed of numerous types of polymers organized into hierarchical structures that span length scales from the molecular level (angstrom to a few nanometers) to the cellular level (micrometers to millimeters) and up to the tree level (centimeters to meters) This multiscale structure leads to difficulties associated with obtaining experimental data that de-couple intra-cell wall and inter-cell wall transport processes. This change in diffusion at the glass transition is linked to the increases in free volume and cooperative motion relaxations in the polymer as it changes from a glassy state to a rubbery one [36,37,38] In this perspective, the polymer science approaches are expanded to construct a polymer-science-based phenomenological framework to more broadly understand mechanisms for unmodified wood cell wall diffusion and the effects of changes in moisture.
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