Viscoelastic properties of woody hemp core

The aim of this study was to determine the effect of removing extractives from the woody core of hemp (chenevotte) on the chain mobility of hemicelluloses and lignins, which can react during technological transformation such as de-fibering and/or composite materials production. Extractives are molecules with low molecular weight, which are present in the cell wall matrix and can be readily removed by solvents. In the present paper, the nature and amounts of extractives, removed under different conditions and with solvents of different polarities, were determined. The mobility and structural relaxations of lignins and hemicelluloses were studied in situ by dynamic mechanical analysis and dielectric analysis under controlled moisture content. Extractions at low temperature led to rigidification of lignins and plasticizing of hemicelluloses, probably due to local changes by the selective removal of molecules interacting with the polymers. Probably, the accessibility of hemicelluloses to plasticizing water was increased at controlled humidity. In contrast, hot extractions including water induced rigidification of the hemicelluloses and plasticizing of lignins. This could be related to a combination of molecule extractions and chemical modifications of both polymers. This interpretation is supported by the variation of activation energy for relaxation of hemicelluloses. It can be concluded that each type of extraction has a clear specific effect on the relaxation properties of the amorphous cell wall polymers.