Molecular-level characterization of changes in the mechanical properties of wood in response to thermal treatment

Thermal treatment can improve the dimensional stability of wood, but it also decreases wood’s toughness, or increases its brittleness. In this paper, combining FTIR spectroscopy and mechanical analysis were used to monitor wood molecular straining and deepen the micromechanical understanding of thermally-treated wood. The degradation of hemicellulose increased as the thermal treatment temperature increase, and the 220 °C treatment also led to cellulose microfibrils reorientation. The results of static tension FTIR spectra of thermally-treated wood indicated that the absorption peak of cellulose glycosidic bond underwent a substantial bandshift to lower wave numbers as the tensile strain increase, but the characteristic peak position of the lignin was no obvious change during stretching. For the 160–200 °C treated samples, the bandshift ratios of cellulose C–O–C glycosidic bond increased upon increasing the temperature; Furthermore, the intensities of the two split peaks of cellulose at 1169 cm−1 and 1435 cm−1 in 0° polarization dynamic FTIR spectra decreased upon increasing the treatment temperature, indicating the elastic-like response of cellulose for thermally-treated wood decrease. For the 220 °C treated sample, the bandshift ratio of cellulose glycosidic bond decreased compared with other thermal treatment samples, but the intensities of the two split peaks of cellulose increased again. Those results indicated that the shear slipping between cellulose microfibrils decrease and microfibrils reorientation due to hemicellulose degradation after thermal treatment may cause the toughness of the thermally-treated wood decrease, or the brittleness increase.