Study On Cell Wall Structure And Micromechanics Of Poplar Tension Wood

Poplar,as one of the commonly used fast-growing wood species in China,is susceptible to environmental and gravity influences during the growth process,resulting in tension wood.Compared with normal wood,there are great differences in the structure and performance of tension wood,and there are a series of problems in the processing and utilization,such as fluffing,clamping saw,cracking and so on.A systematic study of the cell wall structure,chemical composition and micromechanics of poplar tension wood is helpful for in-depth understanding of the structural characteristics of the cell wall of poplar tension wood and the differences between tension wood and normal wood,and provide theoretical basis for the efficient utilization and directional cultivation of poplar in plantations.Therefore,this paper takes poplar tension wood as the main research object,mainly used optical microscope(OM)and fiber segregation methods to observe the tissue and cell morphology of the poplar tension wood and the opposite wood;transmission electron microscope(TEM)and X-ray diffractometry(XRD)were used to observe the ultrastructure of the wood cell wall in poplar tension wood and opposite wood.The chemical composition and chemical composition of poplar tension wood were determined by NREL,FTIR and micro-Raman spectrometer;the micromechanics of the cell wall of poplar tension wood by used nano-indenter.The main research results of this paper are as follows:(1)The average tissue ratio of fiber,vessel and wood ray in tension wood is 69.50%,21.73% and 8.77%,and that in the opposite wood is 61.09%,30.35% and 8.56% respectively.The tissue ratio of wood fiber in the tension wood is higher than that in the opposite wood,the tissue ratio of vessel is low,and there is no significant difference in the wood rays.The ratio of gelatinous fiber in the wood in poplar tension wood is 48.66%.From the medulla to the bark,the seventh growth round wood fibers are all gelatinous fibers.The average length of the fibers in poplar tension wood and opposite wood are 1158.31μm and 1106.67μm,both of which are medium and long fibers.The average length-width ratio of the two is 55.24 and 44.18,respectively.The wood fiber in the tension wood is more suitable for pulping and papermaking.(2)The wall structure of the wood fiber cells in the tension wood is P + S1 + S2 + G structure,and the wall structure of the wood fiber cells in the opposite wood is typical P + S1 + S2 + S3 structure.At the same time,the average thickness of the S1 layer,S2 layer and G layer of the wood fiber cells in the tension wood is 0.61 μm,1.22 μm and 2.53 μm,respectively,and the G layer accounted for the highest proportion of fibers;the S1 layer of the wood fiber cells in the opposite wood,S2 layer and S3 layer have an average thickness of 0.33μm,2.28μm and 0.14μm,S2 layer is the thickest wall layer.(3)The average microfibril angle of S2 layer and G layer in poplar tension wood are 16.89 ° and 5.69 ° respectively,and the average microfibril angle of S2 layer in poplar opposite wood is 17.27 °.In the poplar tension wood,the microfibril angle of the S2 layer of the wood showes a decreasing trend along the direction of the pith to the bark.There is no significant difference in the microfibril angle of the G layer.The microfibril angle of the S2 layer of wood in the poplar opposite wood showes the same trend as the microfibril angle of the S2 layer of wood in the tension wood.(4)The average crystallinity of the wood in the poplar tension wood is 48.31%,and the average crystallinity in the opposite wood is 44.03%.The crystallinity of the wood in the tension wood is higher than that in theopposite wood.(5)The content of cellulose in the wood of poplar tension wood is 58.91%,the content of hemicellulose is 12.01%,and the content of lignin is 21.99%;the content of cellulose in the area of opposite wood is 41.53%.The absorption peak intensities of the wood in area of tension wood at 1730cm-1,1594cm-1 and 1509cm-1 are significantly lower than those in the area of opposite wood,and the results of the ratio of the characteristic peak intensity of the wood show that the chemical composition of tension wood is obviously different from the area of opposite wood.(6)The distribution of lignin in poplar tension wood and opposite wood is basically the same.A large amount of lignin is deposited at CC and CML.The lignin content in CC is the highest,and the lignin content in secondary wall is relatively low,and the relative content of lignin in the cell wall of the tension wood should be significantly lower than the relative content of lignin in the cell wall of the opposite wood.(7)The average longitudinal elastic modulus of G layer is the highest,followed by S2,and the cell wall of S1 is the lowest in tension wood.The wall layers of wood fibers in poplar opposite wood also have significant differences,the S2 layer is the highest.The highest of hardness in the tension wood is G layer,the opposite wood is S2 layer.In terms of wood cell wall stiffness,the wood fibers of the poplar tension wood and opposite wood produced the same results as the longitudinal elastic modulus.In addition,the average longitudinal elastic modulus,stiffness and hardness in the cell wall of the gelatinous fiber in the tension wood are significantly different from the cell wall in the opposite wood.The average longitudinal elastic modulus(12.64GPa)and hardness of the gelatinous fiber(0.38GPa)are higher than the wood fiber in the opposite wood(9.71GPa;0.26GPa).