Investigation of the molecular dynamics of restricted water in wood by broadband dielectric measurements

Dielectric measurements are one of the most reliable techniques for investigating the molecular dynamics of water in moist materials. However, dielectric measurements of moist wood have not yet been carried out in a wide frequency range that can be used to evaluate the molecular dynamics of water in wood. We performed dielectric measurements of a deciduous tree, Zelkova serrata, along the fiber direction in the frequency range of 40 Hz to 10 GHz at room temperature around the fiber saturation point of wood to investigate the molecular dynamics of water in wood. Cole–Cole-type relaxation process reflecting the molecular dynamics of the water is observed in the GHz region. The water content dependences of the relaxation time and strength of this process are similar to those of the relaxation process of free water observed in polymer–water mixtures. However, the τ − β CC diagram of this process markedly deviates from that of the relaxation process of free water in polymer–water mixtures. The molecular mechanism of this characteristic relaxation process is interpreted as the formation of the local structure of water restricted in the void spaces of wood. The water molecules adsorbed on the inner walls of the void spaces form a local structure, and the local structure grows in the length direction along the walls of the void spaces with increasing water content of wood. The molecular dynamics of these water molecules is strongly restricted between the inner walls of the void spaces and air spaces, and the strongly restricted molecular dynamics of the water leads to the characteristic relaxation process observed in the GHz region. We give molecular descriptions of the strongly restricted water adsorbed on the inner walls of the void spaces of wood around the fiber saturation point.