Study On The Structural Characteristics Of Bamboo Cell Wall

Bamboo fiber has a special cell wall with a multilayer structure consist of primary andsecondary wall. Such a structure leads to strong rigidity, straight and stable mechanicalproperties of cell wall which is also the reason why the mechanical properties are so good.Further studying on the structure and properties of bamboo cell wall is not only important forknowing the growth mechanics of bamboo, but good to the utilization of bamboo resources,transformation of bioenergy and designing and making bionic.This research is studying on the structure of bamboo fiber cell wall with kinds of technicsand methods, and advanced instruments. The main contents includes three parts: studying themultilayer structure of bamboo cell wall with FE-SEM, TEM, Nanoindentation and AFM;studying the arrangement of cellulose microfibrils aggregates with AFM; and studying therelationship between hemicellulose of bamboo cell wall and nanostructure and mechanicalproperties with FT-IR, XRD and the technics for tensile testing of single fiber.The conclusions are as follows:（1）The multilayer structure of bamboo cell in treated by freezing and heating can beobserved in FE-SEM. The cell wall in the inner of bamboo splits more easily than that in theouter of bamboo after the freezing and heating treatment. The cell wall in the outside ofvascular bundle splits more easily. However, the FE-SEM method is only a qualitativeobservation. Comparing with the Nanoindentation, AFM has higher resolution, is effective formeasuring the modulus of cross section of bamboo cell wall. In comparison of TEM, observingbamboo cell wall structure with AFM is much simple which can not only observe the target cellwall,but can obtain more data. In addition, AFM is a effective method to study the cellulosemicrofibril and the distribution and diameter of microfibril aggregates in cross-section ofbamboo cell wall. While it need to notice the preparation of samples when using the AFMobserve the nanostructure of cell wall. （2）The multilayer structure and the thickness of each layer in different bamboo cell wallare different. The cellulose microfibril aggregates arrange randomly in the cross section of bothbamboo fiber cell wall and bamboo parenchyma cell wall.The distribution of microfibrilaggregates is different in different layers within on cell wall. There are more microfibrilaggregates in the place of the layer next to each other. Such a phenomenon is more obvious inparenchyma cell wall. The average diameter of microfibril aggregates in fiber cell wall is thesame, while that in parenchyma cell wall are different.（3）It is accessible to observe the nanoscale characteristics in bamboo fiber cell wall,especially the microfibrils, using AFM.The microfibril aggregates in primary cell wall ofbamboo fiber overlap, forming a randomly interwoven structure observed with AFM. And boththe ultrasonic treatment and the drying methods affected the arrangement and dimension ofmicrofibril aggregates. The structure of cell wall freeze-dried is near to the native structure.Ultrasonic treatment can increase roughness of bamboo fiber and exposure of microfibrilaggregates, which provides insight on enhancing wettability of bamboo fiber and observingmicrofibril aggregates.（4）Xylan and glucomannan degraded when bamboo fiber was treated by6%and8%NaOH solution. While the concentration increased to10%, xylan and glucomannan wereremoved. The pattern of cellulose microfibril aggregates treated by6%and8%NaOH solutiondid not change observing with AFM which changed sharply when the concentration of NaOHincrease to15%and25%. CelluloseⅠdid not change, while the CrI first increased, and thendecrease with the alkali treatments with6%,8%and10%concentration. However, when theconcentration increase to15%and25%, there some celluloseⅠtransform into celluloseⅡ.Meanwhile, the CrI increased sharply, but the average thickness of cellulose crystal increased.（5）The structure of bamboo cell wall change with the reduction of hemicellulose. Whenthe concentration of NaOH solution increased to10%, tinny pores began to appear betweencellulose microfibrils aggregates and the layer of cell wall split. With the extraction ofhemicellulose, both the tensile strength and tensile modulus of single fiber decreased. While when the concentration of NaOH increased to15%and25%, tensile modulus decreasedsharply, but tensile strength did not decrease. The elongation at break were similar when thefiber treated by6%and8%NaOH solution. When the concentration increased to10%, theelongation began to increase obviously which increased sharply when the concentration wentup to15%and25%. Besides, the form of fracture were different with different alkali treatment.The fracture form of fibers treated by6%and8%alkali solution liked the brush whichrepresented tenacity. When the concentration increased to10%, the fracture form were liketeeth, while the form were orderly when the concentration increased to15%and25%whichindicated the frangibility of fibers.