Multiple mechanical relaxations of wood under nonequilibrium molecular states during the moisture sorption process

Abstract

Changes in the viscoelastic properties of wood in the intermediate-humidity range during moisture sorption have been attributed to hemicellulose softening, primarily based on the softening behavior of isolated hemicellulose. This study examined whether the loss modulus ( E ″) peak observed in this humidity range corresponds to frequency-dependent mechanical relaxation of cell-wall polymers and aimed to identify the components involved. The effects of drying history, conditioning period, measurement frequency, wood species, and component-removal treatments on E ″ during the moisture sorption process were investigated. The E ″ peak in the intermediate-humidity range was shown to be a superposition of two relaxations: (1) frequency-dependent relaxation occurring at higher humidity (approximately 60 %RH at 0.1 Hz), in which the molecular state altered by drying history persists as equilibrium is approached; (2) relaxation occurring at lower humidity (approximately 40 %RH) that disappears with time after drying. The latter relaxation remained after hemicellulose removal but was altered by delignification, suggesting the involvement of lignin.