Flexible Transparent Wood Composite Material with Shape Memory

Replacing plastics with composites that contain bio-based materials is interesting in the context of reducing the reliance on fossil resources, but could also lead to new materials with interesting properties. Functionalized wood-based materials, for example, are being developed—in particular, transparent wood materials have received research attention. They use the overall hierarchical structure of wood, which is then delignified and combined with a transparent polymer to give composites with possible applications, e.g., as building materials or in optoelectronics. However, such transparent wood materials are generally mechanically rigid and cannot be shaped or molded like some “pure” polymers.

Chengguo Liu, Nanjing Forestry University, China, Jianzhang Li, Beijing Forestry University, China, and colleagues have developed a flexible, transparent wood composite material whose shape can be changed and that even shows a shape memory effect. The team used a delignified wood skeleton. This skeleton’s microchannels and holes were then filled with a polymer that was prepared using 1,4-cyclohexanedimethanol divinylether (CDE) as a source of vinyl ether units together with styrene, 2-ethylhexyl methacrylate, and hydroxyethyl methacrylate as co-monomers, benzoyl peroxide and lauroyl peroxide as radical initiators, and octadecyl acrylate as a defoaming agent.

The resulting composite material is transparent and absorbs UV light, which could make it interesting for some applications. Flexible, chain-like segments in the co-polymer component provide mechanical flexibility at room temperature. The vinyl ether bonds and hydroxyl groups in the product can dynamically form and break connections via acetal formation. This provides the possibility of changing the shape of the material and is responsible for the shape memory effect. The team found that the material can be bent and shaped after heating to 40 °C and then fixed in its new shape by cooling. Upon reheating, the material straightened back out to almost its original state. Overall, the work could contribute to expanding the potential applications of wood-based composite materials.