Abstract: [Objective]The changes of viscoelastic behavior of Chinese fir (Cunninghamia lanceolata) wood during moisture desorption processes and the influence of unstable state of cell wall on relaxation behaviors of wood polymers were investigated in this paper.[Method]Dynamic mechanical analysis (DMA Q800) was used at the given frequencies from 1 to 50 Hz to determine the changes of storage modulus E' and loss modulus E" of Chinese fir samples with a moisture content (MC) of 22.2%, which was corresponded to the environmental condition as 30℃ and 85% relative humidity (RH). The moisture desorption tests were performed by changing the RH levels into the DMA chamber. In this study, three RH conditions were selected as 0%RH, 30%RH and 60%RH. Under each RH condition, the moisture desorption process was divided into RH_ramp-down period and RH_isohume period. During the RH_ramp-down period, the RH decreased from 85%RH to 0%RH, 30%RH or 60%RH, respectively with a ramping rate of 2%RH·min^-1. During the RH_isohume period, RH kept constant at 0%RH, 30%RH or 60%RH for 120 min.[Result]Regardless of frequency, the storage modulus increased and loss modulus decreased with the increasing desorption time. Taken 1 Hz as an example, the time points that at the end of each RH_ramp-down period or RH_isohume period were selected to evaluate the differences of the changing rate of storage modulus (|ΔE'|) and loss modulus (|ΔE"|) and the changing rates per unit of MC (|ΔE'/ΔMC|and|ΔE"/ΔMC|) with the increasing desorption time. At the end of each RH_ramp-down period|ΔE'|was basically equal to or even higher than|ΔE"|, and|ΔE'/ΔMC|was basically equal to or even higher than|ΔE"/ΔMC|as well. Whereas, at the end of each RH_isohume period,|ΔE'| was less than |ΔE"|, and |ΔE'/ΔMC|was less than |ΔE"/ΔMC| as well. At both the end of RH_ramp-down period or RH_isohume period, the less the RH levels, the higher values of |ΔE'|,|ΔE"|,|ΔE'/ΔMC| and |ΔE"/ΔMC| were obtained. Furthermore, during the moisture desorption processes, higher value of storage modulus was tested at higher frequency. As frequency increased, the loss modulus decreased at first and then increased. The character frequency according to the minimum value of loss modulus occurred basically at 10-30 Hz, and shifted toward lower frequency with the increasing desorption time. The character frequency of wood during moisture desorption process was higher than that under an equilibrium state of moisture. During the moisture desorption processes, wood viscoelastic properties varied among frequencies. Taken the changes of loss modulus at 1 and 20 Hz (E"_1Hz/E"_20Hz) as an example, this value increased at first with the increasing desorption time, and then decreased. At each time points, 0% RH desorption condition had the most value of E" _1Hz/E"_20Hz, and 60% RH condition had the least one. Based on the changes of E"_1Hz/E"_20Hz, the residual ratio of unstable state of cell wall was 2.22%, 0.48% and 0.37%, corresponding to 0%, 30% and 60%RH_isohume conditions, respectively.[Conclusion]With the increasing desorption time, the wood stiffness increased and the damping decreased. The changes of stiffness was basically only related to the effect of reformed hydrogen bonds, and the changes of damping was related to the double-effects of reformed hydrogen bonds and free volume. With the increasing desorption time, the movements of water molecular and "water molecular-wood polymers" composed group became difficult, behaving as the transition of the α-relaxation process (attributed to the glass transition of hemicellulose) and the β-relaxation process (assigned to the reorientation of the methylol groups in amorphous wood cell walls and the reorientation of adsorbed water molecules) moved into lower frequency. Due to the existence of unstable state of wood cell wall, the movement difficulty of polymers weakened. With the increasing desorption time, the unstable state of wood cell wall diminished. The ratio of loss modulus at 1 and 20 Hz could predict the residual ratio of unstable state when wood approaching a new equilibrium moisture content.

Key words: Chinese fir, moisture desorption, viscoelastic behaviour, frequency dependency, unstable state of wood cell wall