【作者】 陈文帅；

【导师】 刘一星； 于海鹏；

【作者基本信息】 东北林业大学 ， 木材科学与技术， 2013， 博士

【摘要】 在可持续发展观已成为全球性共识的环境背景下,合理开发利用生物质材料日渐受到关注。生物质材料是由纳米纤维素所支撑的,将纳米纤维素从生物质材料中分离出来,利用其高比表面积、高强度和低热膨胀系数等特点,可开发出多种纳米纤维素基制品,这些材料有望在今后人们生产和生活的诸多方面获得应用。本论文集中阐述了生物质纳米纤维素的制备、结构与性能特点及其制品的开发与利用。通过改变生物质材料的种类及其开纤方法,制备出了多种生物质纳米纤维素,对其结构及性能进行了详细的表征和分析。随后以纳米纤维素气凝胶为例,研究在制备纳米纤维素基制品的过程中,纳米纤维素的自聚集机理及其制品的结构和性能特点。本论文主要开展了以下工作：(1)利用化学预处理结合高强度超声方法制备木质纳米纤维素,并系统研究纳米纤维素在制备过程中的结构及性能演变,以及高强度超声处理对纳米纤维素的结构及性能的影响。研究发现原料木粉为圆棒状结构,而在综纤维素中出现了源于导管的短而扁平的片状结构和源于木纤维的细长中空棒状结构。化学纯化纤维素也保持着细长的扁平片状结构,但厚度更薄,且彼此间相互分离。最终制备的纳米纤维素为细长的丝状结构。随着超声处理时间和超声波输出功率的提高,纯化纤维素的纳米开纤化程度随之提高。当超声波输出功率大于1000W,超声时间为30min时,制备了均匀分散的纳米纤维素水悬浊液。高强度超声处理对纳米纤维素的化学组分、聚集态结构及热稳定性等的影响不大。(2)对比研究了基于不同种生物质材料、不同种机械开纤方法制得的纳米纤维素的结构和性能差异。从木材等相对“低”纤维素含量的生物质材料中,可以制得形态尺寸均匀的纳米纤维素。但从麻纤维等相对“高”纤维素含量的生物质材料中,不易制得形态尺寸均匀的纳米纤维素。采用高速搅拌处理、高强度超声处理及高压匀质处理,均可实现生物质化学纯化纤维素的纳米开纤化,其中以高压匀质处理制得的纳米纤维素水悬浊液最为稳定。所制得的纳米纤维素多以簇状聚集体的形式存在。聚集体的长度大于13μm,宽度在30～250nm之间,其内部由平行排列的纳米纤维素所组成。(3)利用冷冻干燥方法处理离心后的纳米纤维素水悬浊液,制得了超长的纤维素纳米纤维,并对纳米纤维的结构与性能进行了表征。所制备的竹纤维素纳米纤维的直径为30-80nm,长度大于1mm,并保留了天然纤维素Ⅰ型结晶结构,结晶度值约为61.3%,热降解温度高于309℃。利用这一方法,也可从针叶树材、阔叶树材、麦秸及甘蔗渣等生物质材料中制备超长的纤维素纳米纤维。(4)通过对高强度超声纳米纤维素水悬浊液进行冷冻干燥处理,制备了轻质、柔韧的气凝胶,并对气凝胶的微观结构、密度和水分保持值等进行了表征。通过改变纳米纤维素水悬浊液的浓度,可以将气凝胶的三维多孔网状缠结结构转变为二维片状结构。在对高强度超声纳米纤维素水悬浊液进行离心处理后,冷冻干燥上清液,制得了密度为’0.2×10-3g/cm3的超低密度纳米纤维素气凝胶。(5)分别采用四种开纤方法制备出四种具有不同形态与表面性能的纳米纤维素,随后冷冻干燥其水悬浊液,制得了轻质、自支撑的纳米纤维素气凝胶。通过调整纳米纤维素的类型及其水悬浊液的浓度,可以控制气凝胶的微观结构。基于这一发现,我们提出了纳米纤维素在冷冻干燥过程中的自聚集形成机理。由于具有网状结构、高孔隙率、高表面活性等特点,所得纳米纤维素气凝胶显示出良好的力学柔韧性及耐压缩性,并具有非常高的水分承载值及染料吸附能力。纳米纤维素气凝胶还显示出良好的热绝缘与高频声吸附特征。更多还原

【Abstract】 With the increasing attention on sustainable development, utilization of bio-based materials became more and more important. The bio-based materials are supported with nanocellulose. Because nanocellulose has prospective properties such as high Young’s modulus and very low coefficient of thermal expansion, they are expected to be utilized as a filler of next generation nanocomposites. The present dissertation will focus on the preparation, characterization, and application of nanocellulose and their aerogels. First, a series of nanocellulose were produced from various bio-based materials with different nanofibrillation methods. Second, the structures and properties of the nanocellulose were characterized and compared. Third, aerogels were fabricated by freeze-drying the nanocellulose suspensions. The self-aggregation mechanism of nanocellulose and the structures and properties of their aerogels were further presented. The main results can be summarized as follows:(1) Nanocellulose was successfully fabricated from poplar wood using high-intensity ultrasonication combined with chemical pretreatments. In this work, the structural and property evolution during the preparation of nanocellulose were investigated. The effect of ultrasonication treatment on the structures and properties of nanocellulose were also studied. With the removing of lignin and most of hemicelluloses, the size of the cellulose fibers decreased apparently. The diameter distributions of the nanocellulose were dependent on the treating time and output power of the ultrasonication. Uniform nanocellulose can be produced when the treating time reached30min, and the output power was greater than1000W. The ultrasonication treatment has little effect on the chemical composition, crystal structure and thermal stability of the nanocellulose.(2) The structures and properties of the nanocellulose isolated from various bio-based materials with different nanofibrillation methods were investigated. We can isolate long nanocellulose with10～40nm in width from the resources with "low" cellulose content such as wood, bamboo, and wheat straw. However, it is very hard for us to extract long nanocellulose from the resources with "high" cellulose content such as cotton and flax. Nanocellulose can be individualized by nanofibrillation the cellulose fibers using a high-speed blender, an ultrasonicator, and a high-pressure homogenizer. The suspensions after treated with a high-pressure homogenizer exhibited the best stable behavior. Large amounts of bundles were existed in the suspensions. The bundles were>13μm long and30～250nm wide. The bundles were organized with parallel aligned nanocellulose with3～5nm in width.(3) Ultralong and highly uniform cellulose nanofibers were successfully prepared, by using chemical pretreatment combined with high intensity ultrasonication to exact nanocellulose, and then self-aggregation of them into ultralong nanofibers by freeze-drying. The as-prepared bamboo cellulose nanofibers displayed fine structures with lengths>1mm, diameters of30-80nm, and aspect ratios>10,000. Similar findings were also observed from the micrographs of nanofibers fabricated from softwood, hardwood, wheat straw and cane bagasse. With the removal of the matrix materials, the cellulose Ⅰ crystal structure was maintained, whereas the crystallinity and thermal stability of the fibers increased. The crystallinity and thermal degradation temperature of the bamboo cellulose nanofibers reached61.3%and over309℃, respectively.(4) Ultralight and highly flexible nanocellulose aerogels were fabricated by freeze-drying the nanocellulose suspensions which were produced by nanofibrillation the cellulose fibers using an ultrasonicator. The structure, density, and water uptake capability of the aerogels were characterized. The microstructures of the aerogels can be transformed from open3D porous nanofibrillar network to2D sheet-like skeletons by adjusting the concentration of the nanocellulose suspension. When the transparent supernatant fraction, which has-0.018wt%solid content obtained via centrifugation of the nanocellulose suspensions, was subjected to freeze-drying, ultra-low density aerogels (0.2×10-3g/cm3) were successfully produced due to the self-aggregated of the nanocellulose and their bundles along the longitudinal direction.(5) By carefully modulating the nanofibrillation process, four types of nanocellulose with different morphologies and surface properties were readily fabricated. Then, free-standing lightweight aerogels were obtained from the corresponding aqueous nanocellulose suspensions via freeze-drying. The structures of the aerogels could be controlled by manipulating the type of nanocellulose as well as the concentrations of their suspensions. A possible mechanism for the self-aggregation of nanocellulose into aerogel nanostructures was further proposed. Owing to web-like structure, high porosity and high surface reactivity, the nanocellulose aerogels exhibited high mechanical flexibility and ductility, and excellent properties for water uptake, removal of dye pollutants, and the use as thermal insulation materials. The aerogels also displayed high sound adsorption capability at high frequencies.更多还原