A Study On The Relationship Between Viscoelastic Properties And Structural Factors In Earlywood And Latewood Of Chinese Fir

The macroscopic viscoelasticity of wood is the comprehensive response of its tissue structure,cell parameters and the molecular structure of cell wall.To better clarify the mechanism of viscoelastic properties of wood,this research studied the viscoelasticity of wood on growth ring scale and investigating the relationships between wood structure and viscoelastic properties from cell levelSamples were prepared from the the third(No.3)and sixth(No.6)growth rings(heartwood)and the fourteenth(No.14)and eighteenth(No.18)growth rings(sapwood)within a 25-year-old Chinese Fir(Cunninghamia lanceolata [Lamb.] Hook.).The absolutely dried density,percentage of tracheid cell walls,microfibril angle(MFA),modulus of elasticity(MOE),storage modulus(E′),and loss modulus(E′′)of EW and LW within the four growth rings were respectively measured by a X-ray profile densimeter,light microscope,X-ray diffractometer,and Dynamic Mechanical Analysis in this study.On the one hand,this study compared cell structure and viscoelastic properties between earlywood(EW)and latewood(LW)in the same growth ring.On the other hand,the study clarified their change regulations with tree age and revealed the relationships between wood structure and viscoelastic properties.The results showed that:(1)In the same growth ring,LW had a greater absolute dry density and larger percentage of tracheid cell walls than that of EW.LW density showed an increasing trend and EW density was similar with increasing tree age.For LW,the absolute dry density was directly proportional to the percentage of tracheid cell walls.(2)The MFA of EW was slightly larger than that of LW in No.14 and No.18 growth rings of sapwood within the same growth ring.However,the MFA of EW was smaller than that of LW in No.3 and No.6 growth rings of heartwood.The MFA in different growth rings decreased in both EW and LW with the increase of tree age.From the No.3 to No.18 growth ring,theMFA of EW decreased from 14.69° to 11.06°,and the MFA of LW decreased from 16.73° to10.64°.(3)In the same growth ring,LW had a greater MOE and larger E′ than that of EW,as caused by the higher density.(4)The MOE and E′ increased for both EW and LW with increasing tree age.From the No.3 to No.18 growth ring,the MOE of EW and LW increased from 970.9MPa to 1843.8MPa,2352.3 MPa to 8896.1 MPa,and the E′ of EW and LW increased from 1573 MPa to 3719 MPa,4268 MPa to 12938 MPa,respectively.On the one hand,there was no correlation between MOE and E′ of EW and density,while the MOE and E′ of LW were directly proportional to the density.On the other hand,for EW and LW in four growth rings,the MOE and E′ of cell walls were inversely proportional to the mean MFA values,Therefore,the MFA was the decessive factor in affecting the MOE and E′ of EW.Both density and the MFA affected the MOE and E′in LW.(5)E′ decreased with increasing temperature in both EW and LW within the four growth rings.The reduction degree was smaller in EW compared with LW.Furthermore,E′ increased slightly and E′′ decreased a little with the increase measurement frequency.(6)Temperature spectra of E′′ showed that the E′′ values for LW were higher than the EW values in individual growth rings,and E′′ values increased with increasing tree age in both EW and LW.In the temperature range(-120~120℃)of measurement,two mechanical relaxation processes were observed in earlywood and latewood of all four growth rings.One is the αmechanical relaxation process at the temperature around 12℃,showing no frequency dependence,and there is no unified conclusion about its molecular movement and conferring this was caused by the glass transition of low molecular weight hemicellulose;While the βmechanical relaxation process at temperature of-64~-38℃,shifting to a higher temperature range as testing frequency increased,was based on the reorientation of methylol groups in amorphous of wood cell wall.(7)The loss peak temperature of LW for the α relaxation process was slightly higher than that of EW within the same growth rings.The loss peak temperature of EW for the βmechanical relaxation process was obviously higher than that of LW.Almost no difference in αloss peak temperature was observed between the four EW growth rings,and the β loss peak temperatures were similar.The LW α loss peak temperature was almost the same in the four growth rings,but the β loss peak temperature of heartwood(No.3 and No.6 growth rings)was higher than that of sapwood(No.14 and No.18 growth rings).(8)The apparent activation energy of the β relaxation process of heartwood was greater than that of sapwood for both EW and LW,this was mainly caused by the MFA and extractives.On the one hand,compared to heartwood,the MFA of sapwood was lower.When wood cell walls are stretched in the longitudinal direction,the smaller MFA,the more barriers needed to overcome for the reorientation of methylol groups in the amorphous region of the cellulose molecules,that is to say,more energy was needed to motion of the molecular segments.On the other hand,the heartwood had more extractives than sapwood,and extractives can be linked to some related components in the cell wall by hydrogen bonding and copolymerize with pre-existing cell wall macromolecules,which could restrict the activity of molecular segments within the cell walls,so more dissipation energy was needed to overcome the barriers to motion of the molecular segments in the wood cell walls.Therefore,the values of apparent activation energy of the β relaxation process were determined by the net effect between MFA and extractive contents.In this study,the net effect was that high extractive contents dominated the apparent activation energy,making the heartwood require more apparent activation energy for the β relaxation process than sapwood.