On the basis of a concept of the free volume, the mechanism of thermoplasticity for chemically-modified wood which was treated with trifluoroacetic acid anhydride and the fatty acid (TFAA method), is discussed. A proportional relationship between the volumetric swelling with the treatment and the free volume was derived from experimental results and the volume of various introduced side-chains. Moreover, systematic dependence of viscoelastic properties on the amount and the length of the introduced side-chains was due to creation of the free volume in wood substance. From both results and the assumption that TFAA method breaks down the lignin network, the mechanism of thermoplasticity is explained as follows. The introduction of side-chains into wood substance and breakdown of the lignin network during the reaction cause the volumetric swelling which is proportional to the sum of the volume of introduced side-chains and the free volume. Then, the interaction between molecules of wood components decreases because of the creation of the free volume and the modified wood substance has thermoplasticity, when large or many side-chains are introduced with chemical modification.
