Functionalized wood materials for smart filter technologies

Although being highly valued in the construction sector and in furniture industries, bulk wood is often seen as a low-value product. This is due to the fact that bulk wood is not associated with advanced modern materials and nanotechnological processes, because being pre-fabricated by nature it does not allow for assembling materials in a bottom-up approach (from monomer to material), in order to design functional materials for a given purpose. Thereby it is overlooked that wood possesses a highly sophisticated hierarchical structure which can be exploited to develop a vast variety of functional materials. Opposed to synthetic bottom-up materials this hierarchical structure provides already the intrinsic solution to up-scaling problems. However, wood is currently limited in its applications because there is a lack of post-functionalization techniques to exploit and further develop these advantageous features. The aim of this project is to develop and utilize facile and versatile protocols for the functionalization of wood resulting in wood-based materials with novel property profiles. In particular, our goal is to use the porous and anisotropic structure of wood as a scaffold for the design of functional materials. To achieve this, we plan to modify different wood species, by introducing polymer chains into the cell and cell wall structure. Synthetic polymer chains will be directly grown from the wood polymers, via radicals generated in-situ from pre-attached polymerization initiators. Through this method, we will ensure covalent attachment (for improved treatment stability) in the natural cell and cell wall scaffold. According to the choice of monomer, we anticipate that new wood properties can be implemented and/or new functions can be tailored and utilized. We have already developed the basic principle of this modular protocol and have used this approach to modify wood with polystyrene, in order to make it hydrophobic for an improved dimensional stability. The approach for bulk wood modification through the grafting of functional polymers would be the first of its kind and therefore offers an innovative and general strategy for obtaining wood-based materials with tunable properties. In order to control the location, distribution and function of the introduced polymers an in depth knowledge and comprehension of both the raw material as well as the modified material is essential. Adapted characterization techniques are therefore necessary to support the modification phase of the project. Chemical force microscopy (CFM) is considered as a key-technique being able to directly address the existing as well as the introduced function via the exposed functional groups available in wood and the introduced polymers. As CFM is a very versatile high resolution atomic force microscopy based technique, it is capable of accessing a wide range of functional groups in various liquid or gaseous conditions even at the smallest features of the cell scaffold. The targeted applications of the functionalized bulk wood bodies are membranes for waste water purification and oil-water separation, since in particular in these fields of utilization we spot main advantages of wood in comparison to other materials. Being optimized by nature for long-distance water transport and mechanical stability in the tree, wood stands for a highly porous material with excellent mechanical performance and robustness in the wet state which facilitates substantial throughput and large scale possibilities. Further, being a renewable and readily available resource, wood is a timely alternative material to petroleum based products.

Wood is considered as an aesthetic, natural and robust resource. In the past, it was shown that the property profile of this material can be extended and new characteristics can be added with chemical modification techniques, such as in transparent or magnetic wood. The goal of this international research project is to chemically functionalize the porous wood structure so that the obtained material can be applied for waste water purification or oil-water separation. The role of the bio-based functional wood is to provide mechanical stability, a water-conductive architecture and the efficient uptake of the contaminants. The novel modification strategies have to be fine-tuned and target certain positions within the wood matrix. Thus, new visualization and analysis methods are required to support the further development of the so-called functionalized wood. Therefore, in this part of the project, the high-resolution method chemical force microscopy, a variation of atomic force microscopy, was tested to localize and understand the chemical changes within the wood cell wall. The applied analyses aim at supporting the fundamental understanding of wood and the developed chemical modification methods so that the wood application possibilities can be increased. In detail, the experiments were applied on three substrates: chemically modified wood substrates, wood-imitating model surfaces and native wood. At the beginning of the project, model surfaces composed out of different lignocellulosic fibers were intensively analyzed to verify the chemical force microscopy method. It was shown that certain fiber compositions exhibit a gradient in the chemical surface properties within their cross-sections. This valuable data can hardly be achieved with any other microscopy method at this high spatial resolution. Furthermore, adhesion forces were collected on natural and chemically modified wood scaffolds in either temperature-varying environments or in pH-varying solutions. The analysis of these forces provided information about the local distribution of functional groups and could also confirm the added function within the wood cell wall. In addition, a new method which combines atomic force microscopy and infrared spectroscopy was applied to gather spectroscopic and mechanical signals with nanometer resolution simultaneously. Also with this approach, chemical surface properties of native and modified wood ultrastructures could be determined. The tested microscopy methods are characterized by the possibility to gain highly resolved chemical surface information and the ability to set the environmental conditions to test specific surface properties. The results show a high potential for the application of chemical force microscopy for the analysis of complex and heterogeneous materials such as wood.