The effect of moisture on the dielectric relaxations in wood

Abstract The dielectric constant ϵ′ and the loss factor ϵ″ of moistened wood were measured over a wide range of moisture content (MC), 0–40 wt% in the temperature range −196 to 0°C, and in the frequency range 30 Hz to 1 M Hz. Three relaxations were observed depending on the MC and frequency. The first ϵ″ peak at −103°C (30 Hz) in oven‐dried wood is known to be due to the methylol rotation in the amorphous regions. A new ϵ″ peak (second peak), which appeared for MC >0.6 wt% at −110 to −40°C (30Hz) depending on the MC, is assigned to the local mode motions of the complexes between moisture and polar groups in the wood system; it is enhanced as the overall loosening of the initial hydrogen bond network is promoted by the extended hydration due to further sorption of moisture. The peak temperatures and the activation energies for these relaxations changed complexly depending on the MC. At MC > 10 wt%, a novel ϵ″ peak (third peak) was disclosed around −40°C (30 Hz) as a shoulder of the ionic conduction. This novel peak may be attributed to the water frozen in wood on the basis of the peak temperature, relaxation strength vs. MC, the activation energy, and the fact that the peak is not observed for the sorption of various polar organic solvents. This suggests that some of the moisture sorbed by wood may probably exist in a frozen state at low temperatures, even at a MC well below the commonly accepted fiber saturation point.