The Porous Structure And Its Influencing Factors Of Bamboo Cell Wall

Bamboo is a natural polymer composite material with a complex hierarchical structure,and it is also a complex porous material with micrometer to nanometer scale.The pore structure of the cell wall not only affects the accessibility of the modified chemical agent to the cell wall,but also has an important impact on the material’s thermal insulation,ion transmission,mass transfer and other properties.In this study,sympodial bamboo species-Neosinocalamus affinis（Rendle）Keng f.,Bambusa longispiculata Gamble,Bambusa pervariabilis Mc Clure,monopodial bamboo-Phyllostachys edulis（Carr.）H.de Lehaie,Phyllostachys nuda Mc Clure and Phyllostachys iridescens C.Y.Yao et S.Y.Chen as the research object,softwood（Cunning lanceolate）and hardwood（Populus tomentosa）are reference objects.The effects of drying methods on bamboo cell wall pores,the difference in bamboo wood cell wall pores,and the effect of directional removal of chemical components on bamboo cell wall pores were systematically studied using a variety of technologies and methods.The main conclusions of this study are as follows:（1）There are obvious differences in the cell wall pore structure between bamboo and wood,bamboo fiber and parenchyma cells.In terms of cell wall porosity,Chinese fir（5.06%）>poplar（3.98%）>bamboo（2.64%-3.75%）,parenchyma cells（5.07%）>bamboo fibers（2.91%）.There are micropores and mesopores in bamboo wood,and the shape of pores is slit-shape.Mesopore volume:bamboo（0.0015-0.0028 cm³/g）<poplar（0.0030 cm³/g）<Chinese fir（0.0042 cm³/g）,bamboo fibers（0.0023 cm³/g）<parenchyma cells（0.0034 cm³/g）.The mesopore size is mainly concentrated in the range of 2-10 nm.The pore volume of micropores:bamboo（0.0045-0.0074cm³/g）<poplar（0.0096 cm³/g）<Chinese fir（0.0102 cm³/g）,bamboo fibers（0.0091 cm³/g）<parenchyma cells（0.0116 cm³/g）.Generally speaking,bamboo cell walls are denser than wood cell walls,and bamboo fiber cell walls are denser than parenchymal cell walls.The micropore size is mainly concentrated in the range of 0.4-0.6 nm.The moisture absorption-desorption isotherms of bamboo and wood are both inverted"S"type curves.The differences in chemical composition and cell wall porosity of bamboo and wood together cause differences in moisture absorption and hysteresis between bamboo and wood.（2）The porosity of cell wall of Phyllostachys edulis after conventional drying（3.35%）was significantly lower than that of freeze-drying（4.89%）and CO₂ supercritical drying（5.35%）.Freeze-drying and CO₂ supercritical drying can better preserve the natural pore structure of cell wall of Phyllostachys edulis.The pores in the cell wall of Phyllostachys edulis are slit-shape,and the drying method will not change the pore shape.After drying,the pore size of the mesoporous area in the cell wall of Phyllostachys edulis is mainly between 2-10 nm,and the pore size of micropores is mainly between 0.4-0.6 nm.The effect of CO₂ supercritical drying on the retention of mesopores in cell wall of Phyllostachys edulis was significantly better than that of freeze drying,but the difference was not significant.In general,CO₂ supercritical drying has the best protection for the pore structure of the cell wall of the bamboo.The porosity of CO₂supercritical drying bamboo is significantly higher than that of conventional drying and freeze drying,so it can hold more capillary condensed water and has a strong hysteresis.However,the moisture absorption-desorption isotherms of the three are similar,and they are all reversed"S"curves,and the difference in fiber saturation point is not significant.（3）After benzene alcohol extraction,the cell wall porosity of Phyllostachys edulis,bamboo fibers and parenchyma cells increased by 29.62%,57.73%and 10.06%,the mesopore volume increased by 55.56%,30.43%and 11.76%,and the micropore volume increased by 36.49%,9.89%and 25.86%,respectively.Benzene alcohol extraction increased the pore volume in the range of2-10 nm,but had no significant effect on the pore volume in the range of 10-20 nm and 20-35nm.The extraction results in a significant increase of the dominant peak value in the pore size range of 0.4-0.6 nm,an increase in the number of pores,and an increase in the pore volume,especially in the pore size range of 0.6-0.8 nm and 0.8-1.0 nm.After delignification,the pore volume of Phyllostachys edulis,bamboo fiber and parenchyma cells did not increase significantly,which may be due to the collapse of some pores and the blockage of pore pathways.After delignification,the micropore volume decreased by 19.63%,58.04%and 26.71%respectively.There are still some pores in the pore size range of 0.4-0.6 nm,which may be caused by lignin residue.After hemicellulose removal,the removal of hemicellulose from the sample resulted in the opening of pore pathways,resulting in a large number of pores,and the mesopore volume increased to more than 2 times;The micropore volume was in the range of 0.0008-0.0009cm³/g.Phenyl alcohol extraction has no significant effect on the fiber saturation point of bamboo,bamboo fibers and parenchyma cells,but has a significant effect on moisture absorption and desorption hysteresis.At low relative humidity（RH<75%）,the hysteresis of moisture absorption and desorption is significantly reduced,and high relative humidity Under the conditions,it increases slightly,which is probably related to the increase of cell wall pores caused by extraction;after dehemicellulose and delignification,the fiber saturation point of moso bamboo,bamboo fiber and parenchyma cells is significantly increased,and the hygroscopic hysteresis is significant.Decrease,it is speculated that this is caused by the increase in the number of pores and free hydroxyl groups in the cell wall.