Mechanical relaxation processes of wood in the low-temperature range

The dynamic viscoelastic properties of untreated and chemically modified wood specimens were determined in the temperature range 123 to 293 K and at constant frequencies. Absolutely dry wood specimens exhibited one relaxation process labeled gamma at around 180 K, being attributed to the motions of methylol groups in the amorphous region of the wood constituents. The changes in the gamma process due to the chemical modifications were explained by the reduction in the original gamma loss peak due to the decrease of methylol groups, and an additional relaxation induced by the other groups introduced. With moisture adsorption, an additional relaxation labeled beta was induced at 220-240 K. It appeared only when the wood adsorbed moisture irrespective of chemical modifications, and its characteristics were not affected by the formaldehyde and PEG treatments involving the remarkable changes in the mobility of amorphous molecules. These results suggested that the dominant mechanism of beta relaxation was not the segmental motions of the main chain, but the motion of the adsorbed water molecules. The positive activation entropy of the P relaxation was interpreted to reflect rearrangement of the adsorption sites required for the rotation of the adsorbed water molecules. (C) 2001 John Wiley & Sons, Inc.