Abstract:

Objectve: This paper was proposed to compare the viscoelasticity of earlywood within different growth rings, and to investigate the relationships between absolutely dried density, rate of tracheid cell walls, microfibril angel and wood viscoelastic behavior, respectively, with an aim to reveal the deep mechanisms at cellular level. Method: The absolutely dried density, rate of tracheid cell walls, microfibril angel, modulus of elasticity, storage modulus, and loss modulus of earlywood within the third and sixth growth rings(heartwood)and the fourteenth and eighteenth growth rings(sapwood)were respectively measured by X-ray profile densimeter, ZEISS Imager A1 microscope, X-ray diffractometer, and dynamic mechanical analysis(DMA 2980). Result: 1) The absolutely dried density of sapwood(the fourteenth and eighteenth growth rings)was slightly higher than that of the heartwood(the third and sixth growth rings), and the rate of tracheid cell walls among the four growth rings exhibited no significant differences. With the increase of tree age, the microfibril angle decreased. 2) The elastic modulus, storage modulus and loss modulus increased with the increasing of tree age. The microfibril angle showed a significant negative correlation with elastic modulus, and the microfibril angle was a key factor affecting the stiffness and damping of earlywood. 3) In the temperature range(-120-120℃)of measurement, two mechanical relaxation processes were observed in all four growth rings earlywood. One was the α mechanical relaxation process at the temperature around 10℃, and there was no unified conclusion about its molecular movement; While the other β mechanical relaxation process at temperature around -40℃ was based on the reorientation of methylol groups in amorphous of wood cell wall. There was almost no difference in mechanical loss peak temperature among different growth rings. 4) With the testing frequency(1, 2, 5 and 10 Hz)increasing, the loss peak temperature of β mechanical relaxation process moved to a higher temperature range. But for the α mechanical relaxation process, the loss peak temperature almost did not change with the increase of testing frequency, that is to say, it had no frequency dependence. 5) Compared to the earlywood of sapwood(the fourteenth and eighteenth growth rings), the apparent activation energy of mechanical relaxation processes of the earlywood located in heartwood(the thirdth and sixth growth rings)were slightly higher, this could be because the heartwood had more extractives than sapwood for Chinese fir, and then the deposition of extractives limited the movement of molecular segments in cell walls. Conclusion: The MFA might be the key factor affecting the stiffness and damping of early wood in eralywood of different growth rings and the difference of apparent activation energy between heartwood and sapwood may be caused by the deposition of extracts.

Key words: Chinese fir, growth ring, earlywood, absolutely dried density, rate of tracheid cell wall, microfibril angle, viscoelasticity