The differences of viscoelastic properties between the secondary wall S₂ layer and compound middle lamella of thermally treated wood

Thermal modification is a well-established method to improve the dimensional stability and the durability of wood for outdoor use. In this study, samples were thermally treated at 160–220 °C for 1 h. The differences of viscoelastic properties between the cell wall S₂ layer and compound middle lamella (CML) of thermally treated wood were investigated by creep compliance testing and dynamic modulus mapping of nanoindentation. The elastic parameters of the S₂ and CML, such as reduced modulus and storage modulus, decrease and then increase as the treatment temperature increases. By contrast, the loss moduli and loss factors of the two layers decrease steadily as the treatment temperature increases. In addition, the creep compliance results indicate that the S₂ layers of the untreated and treated samples exhibit greater elasticity than does the CML, but the rheological characteristics of the CML are more obvious. Finally, the loss modulus and loss factor of the S₂ layer are also larger than those of the CML. The changes of viscoelastic properties of cell walls of thermally treated wood are mainly related to hemicellulose degradation, the cross-linkage structure between cellulose and matrix breakage, and cellulose microfibrils arrangement.