【作者】 胡伟立；

【导师】 王华平；

【作者基本信息】 东华大学 ， 材料学， 2013， 博士

【摘要】 细菌纤维素(BC)是国际公认的一种优异的天然纳米生物材料,具有独特的精细三维网状结构,因其“纳米效应”而具有高比表面积、高吸水性及保水性、高气液透过率、高湿态强度等优异特性。目前BC膜材料已经实现规模化生产,主要应用于食品及医用领域,解决了静电纺纳米纤维产量低,难以工业化生产的难题,具有十分广泛的应用前景。但目前BC材料的研究主要集中在生物发酵优化以及低成本化方向,对于具有差异化结构性能的BC改性材料及其相关功能纳米材料的研究较少,而已报道的国内外关于BC功能材料的研究主要集中于增强及医用领域,其应用领域也有待进一步拓展。在BC衍生化及功能化过程中,其表面特性决定了BC的表面修饰方法选取与功能化复合体系的建立。而目前BC的性能研究大多集中在基础理化特性的表征,而对其表面特性的研究报道较少。因此,本篇论文通过系统化研究BC表面特性,包括BC表面可及羟基数量,不同羟基的可及性及氢键有序程度,BC纤维表面电动力学行为等,从而更好地指导差异化BC改性材料及BC基功能化材料的设计及制备,理解BC纳米纤维在其中的模板作用以及纳米反应器效应。根据性能及应用需求,探索BC改性材料的结构设计和制备技术,通过对BC纳米材料进行表面修饰,实现BC表面活性基团的可控设计；并进一步基于BC及其衍生化BC纳米纤维活性位点控制的原位制备机理,采用不同方法原位可控构筑新型BC基功能化纳米复合材料体系,为提高BC附加值,拓展BC在光电信息等新材料领域应用提供更为有效的方法和新的合成思路。BC表面性能、表面修饰及功能化研究具体内容如下：1.研究了BC表面羟基可及度及其形态结构与性能的关系,为BC表面修饰及功能化体系的拓展提供了理论依据。结果表明BC表面羟基可及数目为1.28(最大值为3),BC中O(2)H是三种羟基中最可及的,O(3)H最不可及,其与高度有序的O(3)H..O(5’)分子内氢键一致。BC的等电势点pH为3.7,Splateau为-7.5mV。在中性及碱性条件下的BC模板效应最显著,其表面羟基基团的大量解离可使其作为有效的反应活性位点来控制纳米颗粒及纳米线材料的生长与分布。BC膜具有高比表面积(55.37m2/g),且其比表面积及其微观形态可以通过不同的干燥方式及后处理进行调控。2.将BC优异表面特性与实际应用相结合,利用BC表面大量的可及性羟基基团以及比表面积的可调控性,将BC膜材料固定在石英晶体微天平(QCM)表面,制备了一种新颖的具有高稳定性和灵敏度的低成本湿度传感器。结果表明传感器展示了优异的传感特性,频移对数对相对湿度显示了线性关系,且基于BC膜的传感器在97%相对湿度时灵敏度比相应的纤维素膜增加了4倍多,所得传感器具有优异的可逆性及长期稳定性。3.通过采用乙酰化及偕胺肟化学改性方法,以乙酰基团及胺肟基团部分取代BC纳米纤维表面大量的羟基官能团,拓展BC在疏水增强基体材料及金属离子吸附领域的应用前景。使用碘作为催化剂,通过绿色高效无溶剂方法对BC纳米纤维表面官能团进行乙酰化改性,制备的乙酰化BC膜具有良好的表面疏水性能及优异的机械性能(杨氏模量13.4GPa,拉伸强度225.8MPa),有利于作为疏水的非极性聚合物基体的增强材料。在保留BC聚集态结构和一定物理机械性能的同时制备了偕胺肟改性BC,有效地提高了金属离子的吸附容量,扩展并丰富了BC纳米纤维的模板效应。4.利用BC纳米纤维表面大量的可及性羟基基团与所引入组分的相互作用,可以通过简单的表面修饰方法制备新颖的BC功能膜材料,有效拓展BC膜的应用领域。通过在BC表面引入NO2SP组分成功制备了一种新型的BC-NO2SP光致变色纳米纤维膜,该膜颜色可随着BC-NO2SP膜结构中吡喃组分的异构化发生可逆变化；通过在BC表面引入聚乙烯亚胺(PEI)组分进行表面修饰,成功制得了一种新颖、简单且可重复使用的PEI-BC纳米纤维膜基的QCM气体传感器,该传感器在室温下甲醛浓度1-100ppm范围内具有良好的线性,表现出高灵敏度,良好的可重复性和选择性,开创BC纳米纤维膜新的应用领域。5.利用BC及偕胺肟BC(Am-BC)的活性位点控制的原位合成机理以及纳米反应器效应,成功制备了具有光催化特性的ZnO/BC, ZnO/Am-BC及光致发光特性的CdSe/BC纳米复合膜材料。结果表明所得复合膜结构性能受反应液浓度及反应时间的影响,在优化条件下,直径为20-50nm的ZnO及CdSe纳米粒子均匀分布在BC纳米纤维的表面。Am-BC中胺肟基团的引入为ZnO成核及生长提供了更多的有效活性位点,有效提高了ZnO纳米颗粒负载量。相同条件下(120min),ZnO/BC/及ZnO/Am-BC复合膜对甲基橙溶液的光催化降解效率分别可达70%及91%,可应用于有机污水处理,且易于回收,可循环利用。所得CdSe/BC柔性复合膜在紫外光激发下显示出均匀的绿色荧光,可应用于证券纸,传感器及柔性荧光膜材料等领域。6.采用BC作为模板材料,利用BC表面大量羟基与苯胺中的胺基相互作用,过硫酸铵作为氧化剂,通过原位氧化聚合苯胺制备了新颖的PANI/BC柔性导电纳米复合膜。研究了反应时间,掺杂酸对纳米复合膜性能的影响。优化条件下,PANI颗粒均匀沉积在BC纳米纤维表面,沿着BC模板形成连续的直径为200nm的纳米鞘结构,该复合膜电导率可达5.0×10-2S/m,并具有优异的柔性及良好的机械性能(杨氏模量5.6GPa,拉伸强度95.7MPa),且对应力具有敏感性。该材料可应用在传感器,柔性电极,柔性显示材料及其它柔性导电膜等领域,同时本工作也为BC应用领域的拓展提供了新的方向。更多还原

【Abstract】 Bacterial cellulose (BC) is internationally recognized as a type of natural bio-nanomaterial with a fine three-dimensional network. Due to its "nano effect", it has high water absorbing and holding capacity, large specific surface area, high gas-liquid permeability, and excellent mechanical properties. So far, BC has been successfully applied in the food and medical fields with a large-scale production. However, the current research on BC materials mainly focuses on the optimization of biosynthesis process and the low-cost preparation. The modified BC with differentiated structure and properties and the related functional nanomaterials are rarely studied. So far, the reported BC-based functional nanomaterials concentrate on the reinforcing and medical fields, the application field of BC needs to be further expanded. For the derivatization and functionalization of BC, the surface characteristic plays a crucial role which determines the selection of modification methods and the construction strategies of functional nanocomposite system. However, the surface properties and the relationship between the structure and properties of BC have not been intensively investigated.Therefore, in this thesis, we conducted a systematic study on the surface properties of BC, including the amount of accessible hydroxyl groups, the availability of different hydroxyl groups, hydrogen bonding system on the accessible surfaces andζ-potentials, in order to understand the soft template and nanoreactor effects of the BC nanofibers and realize the controllable preparation of modified BC materials and functionalized BC-based nanocomposites.Based on the application requirements, we have explored the structural design and preparation techniques to obtain novel modified BC nanomaterials, in order to achieve the controlled design of surface functional groups of BC nanofibers with adjustable characteristics. In addition, based on the BC and modified BC nanomaterials and the controllable reactive sites mechanism in the in-situ synthesis process, we used different in-situ preparation methods to establish the novel BC-based functionalized nanocomposite system. These studies have further provided a more efficient synthetic method and a new way for the controllable preparation of large-scale and low-cost BC-based nanocomposites applied in sensor, photocatalytic, and photoelectric fields.The detailed studies on surface characteristic, modification and functionalization of BC are as follows.1. The accessibility of surface hydroxyl groups and the relationship between structural features and properties of BC have been studied, in order to provide a theoretical basis for the surface modification of BC and the preparation of functional BC-based nanocomposites. The amount of accessible surface hydroxyl group has been found to be1.28(maximum of3) using dynamic vapor sorption method. The chemical microstructure analysis has shown the O(2)H has the maximum availability in the three kinds of hydroxyl groups, which has implied it has no or minimum contribution to the hydrogen bonding system on accessible surface. The O(3)H has lowest availability which is due to the highly ordered intra-molecular hydrogen bonding with O(5’).BC has a low isoelectric point at pH3.7and a ζplateau of-7.5mV. The template effect of BC can be exerted sufficiently in neutral and alkaline conditions due to the dissociation of its surface hydroxyl groups, which can serve as effective reactive sites to control the distribution and growth of the nanoparticles and nanowires in the BC matrix. The BC film has a high specific surface area of55.37m2/g, and the specific surface area and morphology can be adjusted by different drying and post-processing methods.2. With a combination of unique surface characteristics and practical application of BC material, a novel highly stable and sensitive humidity sensor based on beating-treated BC coated quartz crystal microbalance (QCM) has been successfully fabricated. The results showed that the sensors possessed good sensing characteristics by increasing more than two orders of magnitude with increasing relative humidity (RH) from5to97%, and the Log (Δf) showed good linearity. The sensitivity of sensors coated with BC membranes was four times higher than that of the corresponding cellulose membranes at97%RH. In addition, the sensors exhibited a good reversible behavior and good long term stability.3. The acetylated BC and amidoximated BC (Am-BC) films with hydroxyl groups of BC partially substituted by acetyl groups and amidoxime groups have been prepared by using chemical modification methods. These modifications can expand the application prospects of BC in hydrophobic enhancing materials and metal ion adsorption fields. BC preserving the microfibrillar morphology was partially acetylated by the solvent-free acetylation method using iodine as a catalyst. The obtained acetylated BC membrane shows more hydrophobic surface and good mechanical properties (Young’s modulus is13.4GPa and tensile strength is225.8MPa), which is in favor of enhancing the hydrophobic non-polar polymeric matrix. The Am-BC hydrogel can be obtained under moderate conditions by the successive polymer analogous reactions using acrylonitrile in an alkaline solution medium. The results revealed that the amidoximated modification could enhance the interactions between guest metal ions and host BC nanofiber while preserving the microfibrillar morphology, which can enrich the template effect of BC nanofibers.4. By introducing different functional components to interact with the large number of hydroxyl groups of BC, we have used simple surface modification methods to effectively expand the application fields of BC. The photochromic BC nanofibrous membranes were successfully prepared by surface modification of BC nanofibers with spiropyran photochromes. UV/vis spectrometry of the resulting BC-NO2SP revealed that the membranes show reversible photochromic property by changing their color from colorless to pink forming a merocyanine structure upon UV irradiation, and returning back again to colorless spiropyran structure by visible light. A novel formaldehyde sensor based on nanofibrous polyethyleneimine (PEI)-modified BC membranes coated QCM has been successfully fabricated. The sensor showed high sensitivity with good linearity and exhibited a good reversibility and selectivity towards formaldehyde in the concentration range of1-100ppm at room temperature, which make possibility for the application of BC in gas sensor fields.5. Using the soft template and nanoreactor effect of BC and Am-BC, we have successfully in-situ prepared photocatalytic ZnO/Am-BC and ZnO/Am-BC nanocomposites and flexible luminescent CdSe/BC nanocomposite membranes. The results have shown the structure and properties of the nanocomposites were affected by the preparation method and the concentration of reaction solution. Under optimized conditions, the spherical ZnO and CdSe nanoparticles with a size of20-50nm were homogeneously dispersed on the BC nanofibers. The introduced amidoxime group of Am-BC has provided more effective active sites for the nucleation and growth of ZnO nanoparticels, so the ZnO/Am-BC composite film with higher loading of ZnO nanoparticles exhibited improved photocatalytic efficiency (91%at120min) for the degradation of methyl orange solution compared to the ZnO/BC nanocomposite film. The resultant composite film can be applied to the treatment of organic wastewater, and can be easily reused. The CdSe/BC nanocomposite membranes emitted green fluorescence upon UV excitation, which are promising for applications in the fields of security papers, sensors and flexible luminescent membranes.The novel conductive flexible polyaniline (PANI)/BC nanocomposite membranes have been synthesized in situ by oxidative polymerization of aniline with ammonium persulfate as an oxidant and BC as a template. It was found that the PANI nanoparticles deposited on the surface of BC connected to form a continuous nanosheath by taking along the BC template, which greatly increases the thermal stability of BC. In addition, the acids remarkably improve the accessibility and reactivity of the hydroxyl groups of BC. The results indicate that the composites exhibit excellent electrical conductivity (the highest value was5.0×10-2S/cm) and good mechanical properties (Young’s modulus was5.6GPa and tensile strength was95.7MPa). Moreover, the electrical conductivity of the membrane is sensitive to the strain. This work provides a straightforward method to prepare flexible films with high conductivity and good mechanical properties, which could be applied in sensors, flexible electrodes, and flexible displays. It also opens a new field of potential applications of BC materials.更多还原