Nanoscale Wood Surface Properties by Adhesion-Force AFM

Wood is the most important renewable resource and it is used beside an immense number of other applications in huge quantities for load bearing structures in building industry, furniture applications, pulp and papermaking. Recent research activities are also driven by the increased interest to use lignocellulosic biomass for biorefinery applications and for conversion into bioenergy. Wood is characterized by its excellent strength and stiffness to weight ratio which is based in cellular composition and hierarchical design. Hereby the cell wall composition, especially the distribution and interaction of the structural polymers of the cell wall at the nano- and sub- microscopic level is not fully understood. Typically wood surfaces are investigated which are challenging to study as their typical characteristics are described to be soft, porous, uneven, density varying, self-contamining, predominantly hydrophilic, hygroscopic, and anisotropic. Recently significant differences in the local (micron-scale) binding situation to different wood cell wall elements within adhesive bonds leaded us to own preliminary studies performed by atomic force microscopy (AFM) based adhesion-force mapping measurements. The gained results on longitudinal sections of wood showed significant higher adhesion-forces on the secondary cell wall S2 compared to native inner lumen surfaces. The observed differences were attributed to differences in polarity of the individual cell wall regions. Based on these preliminary findings, a systematic study on the observed effects is proposed using adhesion-force mapping, a state of the art AFM technique not yet established in wood material science. In a first step fundamental differences in wood surface properties of various structural elements of the wood cell wall will be addressed. Moisture is seen as main biasing factor to control the contrast induced by differences in polarity of the wood surface made visible by adhesion-force maps. Later on contrast originated by surface charge density will be used as driving force. With the help of this method the local variability of properties of the wood cell wall elements will be investigated and shall be attributed to the individual cell wall components which differ significantly in chemical composition. By looking at the wood surface from this new perspective, we hypothesize to contribute to a better understanding of the cell wall composition at the nanoscale level and additionally increase the understanding of different adhesion phenomena of the various interface types available in bonds containing wood as adherent (solid and particulate wood products, paper,). The proposed project is of a highly fundamental character. The proposed methodology is seen as challenging but realistic based on the existing experience of the proposers and their national and international collaboration partners.

Wood is the most important renewable raw material for material applications. Although the material has been used since ancient time, the ultrastructural organization and the associated properties have not yet been sufficiently elucidated at all length scales. In recent decades, high-resolution characterization methods based on atomic force microscopy for the determination of physical and chemical properties have become available. These methods were developed for perfectly flat and homogeneous surfaces and have not yet been used on wood. The aim of this project was to adapt these methods to the complex requirements of wood as a material, which is characterized, among other things, by high inhomogeneity, porosity, roughness, hygroscopicity and directional dependence of all properties, in order to gain new insights into the properties of the structural building blocks of wood. This information is particularly important in order to better understand the interaction of wood with other natural and synthetic polymers, such as those that occur, for example, in the bonding of wooden elements, the surface coating with paints and varnishes, or in paper production. Using atomic force microscopy, a surface is scanned with a tip with a radius of only a few nanometers, which allows structures to be imaged that are not accessible with conventional light microscopy. In addition to the mapping of the surface topography, this project also recorded the interaction of the measuring tip with the surface. The known physical properties of the measuring tip and the measurement of the adhesive force between the surface of the sample and the measuring tip allow conclusions to be drawn about the physical properties of the surface (e.g. local polarity). Since wood exchanges moisture with the environment, it was initially unclear how strongly the measurement conditions influence the measurement results and which measurement parameters would be of particular importance for this material. In the course of the project, the essential measurement parameters could be clarified, so that the adhesion-force atomic force microscopy method is now available for the area of material research of lignocellulosic material. In addition, essential knowledge of the surface properties of the wood polymers cellulose, hemicellulose and lignin and their aging behavior on the surface has already been determined. The influence of these surface properties on the adhesion effect of adhesives at the interface with the wood fibers has already been determined.