Etienne Cabane

A versatile strategy for grafting polymers to wood cell walls.

The hierarchical structure of wood is composed of a cellulose skeleton of high structural order at various length scales. At the nanoscale and microscale the specific structural features of the cells and cell walls result in a lightweight structure with an anisotropic material profile of excellent mechanical performance. By being able to specifically functionalize wood at the level of cell and cell walls one can insert new properties and inevitably upscale them along the intrinsic hierarchical structure, to a level of large-scale engineering materials applications. For this purpose, however, precise control of the spatial distribution of the modifying substances in the complex wood structure is needed. Here we demonstrate a method to insert methacryl groups into wood cell walls using two different chemistry routes. By using these methacryl groups as the anchor points for grafting, various polymers can be inserted into the wood structure. Strikingly, depending on the methacryl precursor, the spatial distribution of the polymer differs strongly. As a proof of concept we grafted polystyrene as a model compound in the second modification step. In the case of methacryloyl chloride the polymer was located mainly at the interface between the cell lumina and the cell wall covering the inner surface of the cells and being traceable up to 2-3 μm in the cell wall, whereas in the case of methacrylic anhydride the polymer was located inside the whole cell wall. Scanning electron microscopy, Fourier transform infrared spectroscopy and especially Raman spectroscopy were used for an in-depth analysis of the modified wood at the cell wall level.

Bio-inspired functional wood-based materials – hybrids and replicates

Wood is a CO2 storing material from renewable resoures with excellent mechanical properties and a sophisticated hierarchical structure from the nanoscale of cell wall polymers up to the macroscale of tree size. In recent years, one can observe an increasing research interest in modifying and functionalising wood cell and cell wall assemblies as well as cell wall components. In this context, three fundamental approaches aiming at developing novel and advanced bio-inspired and bio-based functional materials can be identified. At the level of bulk wood, this review will cover two research directions of wood functionalisation which have in common the utilisation of the hierarchical structure at different length scales, but differ in terms of the preservation of the organic scaffold of the wood cell wall. In those protocols that modify and functionalise wood cell walls with the emphasis on retaining their structural integrity, hybrid materials leading to polymer-or mineral-related wood products are developed. In the second wood functionalisation approach, the hierarchical structure of wood is used as a template to engineer wood-derived inorganic non-metallic materials. The third approach is at the level of cell wall components, where bio-based materials from wood cellulose nanofibres (CNF) are discussed. The use of CNF allows for designing structures with different porosities and new combinations of anisotropic properties beyond the predetermined hierarchical structure of wood. The review will cover fundamental aspects and various protocols for the three to date surprisingly separately treated approaches with a focus on synthesis procedures and characterisation of the modified materials with respect to the targeted functionalisation as well as potential fields of application.

Flavonoid insertion into cell walls improves wood properties

Wood has an excellent mechanical performance, but wider utilization of this renewable resource as an engineering material is limited by unfavorable properties such as low dimensional stability upon moisture changes and a low durability. However, some wood species are known to produce a wood of higher quality by inserting mainly phenolic substances in the already formed cell walls--a process so-called heartwood formation. In the present study, we used the heartwood formation in black locust (Robinia pseudoacacia) as a source of bioinspiration and transferred principles of the modification in order to improve spruce wood properties (Picea abies) by a chemical treatment with commercially available flavonoids. We were able to effectively insert hydrophobic flavonoids in the cell wall after a tosylation treatment for activation. The chemical treatment reduced the water uptake of the wood cell walls and increased the dimensional stability of the bulk spruce wood. Further analysis of the chemical interaction of the flavonoid with the structural cell wall components revealed the basic principle of this bioinspired modification. Contrary to established modification treatments, which mainly address the hydroxyl groups of the carbohydrates with hydrophilic substances, the hydrophobic flavonoids are effective by a physical bulking in the cell wall most probably stabilized by π-π interactions. A biomimetic transfer of the underlying principle may lead to alternative cell wall modification procedures and improve the performance of wood as an engineering material.

Fully biodegradable modification of wood for improvement of dimensional stability and water absorption properties by poly(ε-caprolactone) grafting into the cell walls

Materials derived from renewable resources are highly desirable in view of more sustainable manufacturing. Among the available natural materials, wood is one of the key candidates, because of its excellent mechanical properties. However, wood and wood-based materials in engineering applications suffer from various restraints, such as dimensional instability upon humidity changes. Several wood modification treatments increase water repellence, but the insertion of hydrophobic polymers can result in a composite material which cannot be considered as renewable anymore. In this study, we report on the grafting of the fully biodegradable poly(e-caprolactone) (PCL) inside the wood cell walls by Sn(Oct)2 catalysed ring-opening polymerization (ROP). The presence of polyester chains within the wood cell wall structure is monitored by confocal Raman imaging and spectroscopy as well as scanning electron microscopy. Physical tests reveal that the modified wood is more hydrophobic due to the bulking of the cell wall structure with the polyester chains, which results in a novel fully biodegradable wood material with improved dimensional stability.

Functional lignocellulosic materials prepared by ATRP from a wood scaffold

Wood, a natural and abundant source of organic polymers, has been used as a scaffold to develop novel wood-polymer hybrid materials. Through a two-step surface-initiated Atom Transfer Radical Polymerization (ATRP), the porous wood structure can be effectively modified with polymer chains of various nature. In the present study, polystyrene and poly(N-isopropylacrylamide) were used. As shown with various characterization techniques including confocal Raman microscopy, FTIR, and SEM/EDX, the native wood ultrastructure and features are retained and the polymer chains can be introduced deep within the wood, i.e. inside the wood cell walls. The physical properties of the new materials have been studied, and results indicate that the insertion of polymer chains inside the wood cell wall alters the intrinsic properties of wood to yield a hybrid composite material with new functionalities. This approach to the functionalization of wood could lead to the fabrication of a new class of interesting functional materials and promote innovative utilizations of the renewable resource wood.

UV-protection of wood surfaces by controlled morphology fine-tuning of ZnO nanostructures

Abstract One of the most significant limitations for a wider utilisation of the renewable and CO2-storing resource wood is its low ultraviolet (UV) light stability. The protection of the wood surface without altering its aesthetic appeal requires an optically transparent but UV protective coating which should be strongly attached to the rough and inhomogeneous substrate. For this purpose, ZnO nanostructures were deposited onto the wood surface via a chemical bath deposition process. The morphology of crystalline ZnO was controlled by aluminium nitrate or ammonium citrate in the growth step resulting in nanorod arrays or platelet structures, respectively. Detailed structural, chemical and mechanical characterisations as well as accelerated weathering exposure revealed the effective performance of the platelet structure, which formed a dense and thin ZnO coating on spruce. The total colour change (ΔE in the CIE system) was calculated to be 20.5 for unmodified wood, while it was about three for the modified samples after 4 weeks accelerated weathering test. Moreover, the ZnO coating also suppressed crack initiation and propagation indicating a substantial increase in durability.

Biomaterial Wood: Wood-Based and Bioinspired Materials

In view of the aimed transition toward sustainable societies, wood has a great future potential in being a renewable and carbon-storing resource, as well as due to its peculiar structural multiscale features and excellent properties. A useful approach for developing wood or wood-based materials with improved properties or new functionalities is to combine bioinspired materials science and wood materials research. For this, recent developments in polymer chemistry and nanotechnology can be used to transfer and apply the basic principles and mechanisms from nature for the fabrication of hybrid wood materials. In this chapter, we review the recent developments in the field and provide examples of our own work on wood materials synthesis and characterization, which illustrate the general concept and its future potential.

A straightforward thiol–ene click reaction to modify lignocellulosic scaffolds in water

We report on a sustainable and straightfoward approach to develop a hydrophobic layer on the surface of wood veneers via the Michael addition click reaction of alkanethiols. After treatment, the naturally hygroscopic wood scaffold shows superhydrophobic properties with contact angles above 150°.

Functional lignocellulosic material for the remediation of copper(II) ions from water: Towards the design of a wood filter

In this study, the chemical modification of bulk beech wood is described along with its utilization as biosorbent for the remediation of copper from water. The material was prepared by esterification using anhydrides, and reaction conditions were optimized to propose a greener process, in particular by reducing the amount of solvent. This modification yields a lignocellulosic material whose native structure is preserved, with an increased amount of carboxylic groups (up to 3 mmol/g). We demonstrate that the material can remove up to 95% of copper from low concentration solutions (100- 500 ppm). The adsorption efficiency decreases with concentrated copper solutions, and we show that a limited number of -COOH groups participate in copper binding (ca. 0.1 Cu/-COOH). This result suggests a limited accessibility of -COOH groups in the wood scaffold. This was demonstrated by the characterization of -COOH and copper distributions inside wood. Raman and EDX imaging confirmed that most -COOH groups are located inside the wood cell walls, thereby limiting interactions with copper. According to this study, critical limitations of bulk wood as a biosorbent were identified, and the results will be used to improve the material and design an efficient wood filter for heavy metal remediation.

Enhancing the performance of beech-timber concrete hybrids by a wood surface pre-treatment using sol-gel chemistry

Timber-concrete composites require reliable connections between both components, which are usually obtained by metal fasteners or slots in the wood. In this study, an alternative approach is presented based on a fully glued connection in combination with a primer treated wood surface, to enhance the compatibility and the adhesion properties at the interface between beech wood and concrete. Prior to the gluing and the concrete application in a wet-on-wet process, the wood surface was functionalised with a xerogel obtained by means of a sol-gel process, consisting of two layers of silane nanofilms, with different functional groups, which are capable of undergoing further chemical crosslinking reactions with the adhesive. The coating with its functionalities allows for reducing the penetration of the epoxy adhesives into the wood structure and an additional chemical connection to the adhesive can be established. The main objective of this study was to analyse the effect of the surface treatment on the mechanical properties of such composites in 3-point and 4-point bending tests as well as push-out-tests. The results showed that the pre-treatment can improve the load bearing capacity of the timber-concrete composites, but that a ductile behaviour cannot be achieved with the tested adhesives.