Study On Weak Interface Characteristics And Gelatinous Fiber In Poplar Tension Wood

Poplar,as a typical tree species with tension wood,has rich gelatinous fibers(G-fiber)in its tension area,which provides a basis for the high growth stress generation of poplar,and also brings inconvenient in the processing and utilization of poplar.Therefore,from the perspective of wood basic properties,it is urgent to systematically and deeply study the characteristics of G-fibers as well as the cell wall,in order to enrich the basic theory of wood science.In this study,poplar tension wood with G-fiber appearing and then disappearing was selected.The microscopic characteristics and properties such as tension wood cell,gelatinous layer(G-layer),the microfibril arrangement,lignin micro-distribution,crosssectional characteristics of cells,morphological characteristics between cell layers,and the micromechanics of each cell layers were studied by means of the technology of paraffin/resin embedding,resin casting,multi-level directional separation of G-layer,ultra-thin sectioning,micro-infrared testing,multi-dimensional micro-surface imaging,depth sensitive indentation,and the observation instruments of optical microscope,atomic force microscope,nano indentation instrument.The main results of this research are as follows:(1)The tissue proportion of G-fiber in poplar tension area is between 9.18%and 41.38%,and the average value is 28.54%,which appears from the first growth ring,the richest in the fourth growth ring,and the least in the sixth growth ring.In cross section,G-fibers shows band-like and scattered distribution.The resin casting of normal fibers(N-fiber),G-fiber and fibers that remove the G-layers showed that the relatively complete textured pit pairs could be observed in both normal fibers and G-fibers.The interface between the G-layer and adjacent wall layers in G-fibers was precisely defined by microtome sectioning and resin casting techniques,and the corner middle lamella and cell corner middle lamella between fiber cells were weak interface bound.(2)The microfibril angle(MFA)of G-fibers,isolated G-layers and fibers that remove the G-layers in poplar tension area showed a decreasing trend from the direction of pith to bark,with the average MFA values of 12.89°,6.26° and 19.05°,respectively.The MFA of N-fibers also showed a decreasing trend from pith to outward,with an average value of 24.68°.The MFA of wood fibers with G-layers removed will be slightly lower than that of N-fibers.(3)The observation of the cell wall thickness of G-fiber and the density distribution of microfibril aggregates in each cell wall layer by AFM showed that the thickness of each cell wall layer of G-fiber varied greatly,and the G-layer was the thickest.The average thickness of secondary cell wall S1,S2,and G layer is 0.58 μm,2.26μm and 2.33 μm,respectively.The density and the microfibril aggregates in the S2 layer of G-fiber is large and closely arranged,indicating that the cell wall layer structure is compact,while the microfibril aggregates in the G-layer and the corner middle lamella(CML layer)are loosely arranged,especially at the interface between the G-layer and the adjacent wall layer,and the density distribution of microfibril aggregates varies widely.(4)Using fluorescence microscopy to analyze the lignin distribution of G-fiber,it can be seen that the cell corner middle lamella(CCML)is the highest,followed by the CML layer,and there is almost no fluorescence phenomenon in the G-layer,indicating that the lignin content in the G-layer is very low.The microscopic infrared analysis showed that the characteristic peak strength of G-fibers around 1739 cm-1 and 1504 cm-1 was lower than that of N-fibers,and the characteristic of peak-to-height ratio also indicated that the content of lignin and hemicellulose in G-fibers was relatively low.(5)According to nanoindentation measurement,the residual deformation of CCML and CML layer in both poplar G-fibers and normal fibers are about 50%and 70%respectively.The residual deformation of the S2 layer of N-fiber and the G-S2 interface layer of the Gfiber are both about 70%,indicating that the fiber cell wall was mainly plastic deformation,while the elastic deformation and plastic deformation of the cell corner are equal.There is a significant difference in the modulus of elasticity(MOE)of the interfaces between each cell wall layers of G-fibers,but the hardness value is not obvious.The mechanical properties of the N-fiber cell wall and intercellular interface show a similar conclusion to that of G-fiber.Compared with the MOE and hardness of CCML(13.07 GPa,0.52 GPa)and CML layer interface(15.90 GPa,0.52 GPa)of N-fiber,the MOE and hardness of CCML(7.28 GPa,0.19 GPa)and CML(10.52 GPa,0.17 GPa)of G-fiber are smaller,and the MOE and hardness of the G-S2 interface in G-fiber are 13.61 GPa and 0.27 GPa,respectively.(6)Based on the above analysis of the structure and mechanical properties of the cell wall layer of G-fibers,it can be seen that the structure arrangement of the cell wall interface of G-fibers and the G-S2 interface layer are loosely arranged,with low mechanical strength,and the interface is weakly bonded.The structural arrangement and mechanical properties of the composite intercellular layer and cell corners also exhibit weak interfacial phase.