Functional Wood Materials Characterization

The group focuses on revealing the chemistry, structure and mechanics of native and functionalized plant material on a cell wall level by using high resolution analysis techniques such as Raman spectroscopy imaging and atomic force microscopy (AFM). In addition, the group aims at using the wood inherent cellulose scaffold for the development of high performance novel functional composite materials. 
Densified Cellulose Composites
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Schematic illustration of the newly developed process for the development of densified cellulose composites based on the delignification followed by densification of wood
In this project novel high performance natural fiber composites based on a delignification and densification procedure of wood are developed. The hierarchical structure and directionality of wood are preserved during the process and thus the obtained composite material is of high homogenity, combining highly desired materials properties as high strength and toughness. For an optimized mechanical performance of the composite materials different delignification and densification procedures are tested and furthermore various matrix systems (e.g. thermoplasts, duroplasts) will be evaluated. In addition, to the determination of the macroscopic mechanical properties of the composites a special focus will be laid on a detailed characterization of the structural and mechanical characteristics on the cell wall level via atomic force microscopy (AFM) in order to gain new insights into the relevant structure-function relationships at the micro- and nanoscale.
Ultrastructural and mechanical characterization of native and functionalized plant cell walls via atomic force microscopy
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Working principle of the QI-mode: A force-distance curve is recorded in every pixel of the mapping. From the maximum difference between the baseline and the retraction curve, the pull-off forces, the adhesion image is build which can be overlayed with the topography image

Wood represents a unique hierarchical biological material with excellent macroscopic properties originating from the specific structural arrangement of the constituents on the micro- and nanoscale. For a detailed analysis of the corresponding structure-property relationships high resolution analytical techniques, beyond conventionallyused SEM, TEM and nanoindentation, are needed. In this regard atomic force microscopy (AFM) can push the limit of resolution down to the nanoscale in terms of imaging the topography and mechanical properties. So we have adapted the so-called quantitative imaging mode (JPK instruments) based on recording a force distance curve in every pixel for the nano-scale structural respectively mechanical characterization of wood cell walls without the need of embedding. Thereby, it was possible to differentiate for the first time between the different cell wall layers, (CML, S1, S2, S3) on a spruce cross section on the basis of their mechanical properties. Furthermore, we have visualized the change in micro fibril angle in the transition zone from the S1 to the S2 layer characterized by a stiffness gradient, and proven the model of a concentric lamellar arrangement of the constituents within the S2 layer. 

Quantitative imaging is not limited to the structural/mechanical analysis of natural unmodified plant/wood cell walls. Moreover, it additionally represents a powerful analytical tool for the detailed characterization of modified/functionalized wood. QI enabled for example the visualization of a homogeneous polyelectrolyte layer by layer buildup on wood surfaces already within the first nm thick layers based on differences of the adhesion values of the differently charged polyelectrolyte layers. Thus, it was possible to gain new insights into the stability of polyelectrolyte coatings on a complex biomaterial surface.

Characterization of wood adhesive and primer systems on a cell wall level

Within

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Illustration of the analysis of primer treated respectively adhesive bonded wood samples via atomic force microscopy and confocal raman spectroscopy imaging
the last years the appliction of 1C-PUR adhesive systems for loadbearing timber constructions becomes more and more important due to its advantages including: representing a one component system; fast curing at ambient temperatures and formaldehyde free. However, in order to fulfill certain standards with reference to the comparatively low performance in delamination tests in wet state need to be overcome. In this regard specific primer systems are applied on the wood surface resulting in a significant improvement of the delamination behaviour. So far the exact mode of action of the applied primer system remains unclear and therefore we are using raman spectroscopy imaging and atomic force microscopy for a detailed characterization of the influence of the primer on the interface region between wood and adhesive. Just recently we have shown that via AFM it is possible to proof an entering of the primer into the surface near wood cell walls by studying individual force-distance curves which reveal a distinct enhancement of the work of adhesion of the primer treated wood surfaces. 

The gained knowledge in this project will unravel fundamental information concerning wood-glue-primer interactions and potentially help to improve the formulation of adhesives in the future.

This project is performed in close collaboration with Henkel, engineered wood. 

Relevant publications
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[1] Casdorff, K., Keplinger, T., Bellanger, H., Michen, B., Schön, S., Burgert, I.: High-resolution adhesion mapping of the odd-even effect on a layer-by-layer coated biomaterial by Atomic-Force-Microscopy, ACS Applied Materials & Interfaces; 2017

 

[2] Casdorff K., Keplinger T., Burgert I.: Nano-mechanical characterization of the wood cell wall by AFM studies – comparison between AC- and QITM mode; Plant Methods; 2017

 

[1] Casdorff K., Kläusler O., Gabriel J., Amen C., Lehringer C., Burgert I., Keplinger T.: About the influence of a water based priming system on the interaction between wood and one-component polyurethane adhesive studied by Atomic Force Microscopy and Confocal Raman Spectroscopy Imaging; International Journal of Adhesion and Adhesives; 2017