【作者】 柯刚；

【导师】 官文超；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2007， 博士

【摘要】 1991年,日本科学家S. Iijima发现了碳纳米管(Carbon nanotubes, CNTs)。经过十几年的发展,碳纳米管已经成为纳米科技重要的研究前沿,其重大研究成果层出不穷,在21世纪科技发展中占有举足轻重的地位。碳纳米管奇特的准一维中空管结构使其在吸附、电学、磁学、场致发射、力学、电化学等许多方面具有优异的性能。然而,极差的溶解性严重限制了碳纳米管的研究与应用。通过化学修饰不仅能有效改善碳纳米管的溶解性,而且能赋予其更多功能,进一步拓宽其应用领域。近几年来,利用高分子对碳纳米管进行共价化学修饰尤其得到了广泛重视,它已成为制备具有某些特定功能的碳纳米管及其复合材料的有力手段,对研发相关纳米器件和新型材料有很大的理论和现实意义。目前,国内外利用高分子共价修饰碳纳米管的很多研究主要集中于合成高分子,还没有利用天然高分子共价修饰碳纳米管的研究报道。然而,合成高分子主要来源于日益枯竭的化石资源,而且用来修饰的一些高分子还含有毒性苯环等芳香烃。与合成高分子相比,壳聚糖、纤维素和环糊精等天然高分子及其衍生物是无毒、生物相容、生物可降解、环境友好、用之不竭的可再生资源,作为解决人类面临的能源、资源、环境三大问题的重要材料,在众多领域中应用广泛。因此,天然高分子及其衍生物可作为共价修饰碳纳米管的理想高分子。本研究首次明确提出选择天然高分子及其衍生物对碳纳米管进行化学修饰,通过共价键将两者相结合,合成出结构新颖的天然高分子/碳纳米管共价衍生物,在改善碳纳米管溶解性的同时,可望为最终研发出兼具甚至优于两者性能的,在分离、吸附、催化、环保、电极和微摩擦等领域有应用前景的,绿色环保的新型纳米复合材料奠定较好的基础。这将开拓碳纳米管化学与天然高分子化学,以及超分子化学相交叉的新的研究分支,属于开创性的前沿基础研究,具有重要的理论意义和潜在的应用价值。与单壁碳纳米管(SWNTs)相比,短切的多壁碳纳米管(MWNTs)在吸附、分离、催化、摩擦等方面更具研究和应用价值。因此,本研究在对原始MWNTs进行球磨短切、纯化、酸化和酰氯化处理的基础上,首先采用易溶于水、反应活性较强的低分子量壳聚糖(LMCS,Mw≈8770 g·mol-1)对MWNTs进行共价修饰,首次合成出结构新颖的低分子量壳聚糖/多壁碳纳米管共价衍生物(MWNT-LMCS),并利用FTIR、13C NMR、XPS、Raman和TEM对MWNT-LMCS的结构进行了综合表征,结果说明,LMCS分子的部分氨基及伯羟基与MWNTs的酰氯基之间发生亲核取代反应,形成了新的酰胺键和酯键,从而将LMCS的分子链接枝在MWNTs表面。MWNT-LMCS由81.6% C、11% O和7.4% N组成。同时,MWNT-LMCS中LMCS的含量约为58 wt%,并且MWNTs的每1000个碳原子上接枝的LMCS分子链数大约为4。十分有趣的是,XRD测试表明,LMCS在接枝到MWNTs表面后,其非晶态结构发生很大改变,LMCS的结晶性增强。MWNTs与LMCS间的共价和非共价作用可能共同导致LMCS的结晶性增强。经计算确定MWNT-LMCS的晶胞属单斜晶系,其结晶度为26%。MWNT-LMCS在醋酸水溶液,以及DMF、DMAc、DMSO等有机溶剂中具有良好的溶解性。尽管采用LMCS对MWNTs成功进行了修饰,然而实验证明,目前在碳纳米管酰氯化的基础上直接进行化学修饰的普遍方法对高分子量壳聚糖等天然高分子并不可行。这一方面可能是由于MWNTs属碳素无机物,具有管状的石墨片层结构,其化学稳定性较高,空间位阻大,且不能在溶剂中溶解,因而MWNTs的反应活性差。另一方面,由于分子量大幅度增加,空间位阻增大,且分子链具有一定的刚性,导致高分子量壳聚糖等的反应活性比LMCS差。为此,本研究另辟蹊径,在MWNTs短切、酸化和酰氯化的基础上,首先采用链延长的方法,利用活性强、结构简单的1,3-丙二胺对MWNTs进行修饰,合成出1,3-丙二胺/多壁碳纳米管共价衍生物(MWNT-NH2),使反应基团从MWNTs的外层管壁“伸长”出来,以减小空间位阻对反应的影响。在链延长的基础上,采用拥有能与天然高分子的羟基或氨基反应的活泼氯原子的三氯均三嗪,通过与MWNT-NH2的低温均相反应,首次合成出结构新颖的2, 4, 6-三氯- 1, 3, 5-三嗪/多壁碳纳米管共价衍生物(MWNT-triazine)。同时,利用FTIR、13C NMR、XPS和UV等对MWNT-NH2和MWNT-triazine的结构进行了表征。MWNT-triazine中三嗪环的含量约为0.8 mmol/g。MWNT-triazine能溶于水、酸溶液和醇,在DMF、DMAc、DMSO等有机溶剂中也具有很好的溶解性。MWNT-triazine的合成达到了在MWNTs表面引入活泼基团、增加活性基团数量的同时,减小反应的空间位阻,并改善MWNTs溶解性的目的,为天然高分子对MWNTs的化学修饰提供了有利条件。由于直接采用高分子量壳聚糖修饰MWNTs导致产物的分离与纯化很困难,因此在碱性条件下,首先利用氯乙酸对脱乙酰度为84.7%、粘均分子量为1.99×105的壳聚糖进行化学改性,制备了取代度约为16.9%的水溶性N,O-CMC。在此基础上,采用N,O-CMC对MWNT-triazine进行化学修饰,首次合成出结构新颖的N,O-羧甲基壳聚糖/多壁碳纳米管共价衍生物(MWNT-CMC)。同时,尝试采用简便的UV测试手段对纯化效果进行分析。利用FTIR、13C NMR、XPS、UV和TEM对MWNT-CMC进行表征,结果说明,MWNT-triazine的氯原子被N,O-CMC分子链中的羟基和氨基亲核取代,从而将N,O-CMC的氨基葡萄糖单元接枝在MWNTs表面。MWNT-CMC由78.1% C、15.6% O和6.3% N组成,且MWNTs表面接枝的N,O-CMC的含量约为31.9 wt%。与结晶性差的反应物N,O-CMC(结晶度为14%)和非晶态的MWNT-triazine相比,MWNT-CMC的结晶性增强,其结晶度为21%。MWNT-CMC能部分溶于盐酸和醋酸水溶液,并在DMF、DMAc、DMSO及NMP等溶剂中具有良好的溶解性。进一步采用β-环糊精(β-CD)、醋酸纤维素(CA)、羟乙基纤维素(HEC)和甲基纤维素(MC)对MWNT-triazine分别进行化学修饰,首次合成出β-环糊精/多壁碳纳米管共价衍生物(MWNT-β-CD)、醋酸纤维素/多壁碳纳米管共价衍生物(MWNT-CA)、羟乙基纤维素/多壁碳纳米管共价衍生物(MWNT-HEC)和甲基纤维素/多壁碳纳米管共价衍生物(MWNT-MC)。由于这四种天然高分子/多壁碳纳米管共价衍生物都是通过均相反应合成得到,因此是对碳纳米管的众多非均相修饰的一个重要突破。同时,本研究还尝试利用简便的UV测试手段对这四种共价衍生物的纯化效果分别进行了分析,结果表明,衍生物中不存在UV可检测出的游离的天然高分子,与TEM的测试结果一致。利用FTIR、13C NMR、XPS、UV和TEM等多种分析手段对这四种衍生物的结构分别进行表征,结果表明,MWNT-triazine的氯原子分别被β-CD、CA、HEC或MC分子链中的羟基亲核取代,从而将β-CD、CA、HEC或MC的吡喃葡萄糖环结构接枝在MWNTs表面。MWNT-β-CD由79.7% C、14.3% O和6.0% N组成,且MWNTs表面接枝的β-CD的含量约为26.6 wt%。MWNT-CA由76.3% C、18.4% O和5.3% N组成,且MWNTs表面接枝的CA的含量约为42.8 wt%。MWNT-HEC由76.2% C、19.5% O和4.3% N组成,且MWNTs表面接枝的HEC的含量约为32.1 wt%。MWNT-MC由77.1% C、18.9% O和4.0% N组成,且MWNTs表面接枝的MC的含量约为38.3 wt%。结晶性研究表明,与MWNT-triazine的反应分别破坏了β-CD、CA、HEC及MC原有的晶形结构。与非晶态的MWNT-triazine相比,MWNT-β-CD和MWNT-HEC的结晶性增强,而MWNT-MC及MWNT-CA的结晶性则有所增强。经计算,MWNT-β-CD、MWNT-CA、MWNT-HEC和MWNT-MC的结晶度分别为19%、15%、18%和17%。溶解性测试表明,衍生物MWNT-β-CD、MWNT-CA、MWNT-HEC和MWNT-MC在DMF、DMAc、DMSO及NMP等有机溶剂中具有良好的溶解性。以上研究说明,对碳纳米管表面先采用扩链和引入活泼基团的手段进行修饰,合成出衍生物MWNT-triazine,以改善MWNTs的溶解性和化学反应活性,再通过亲核取代反应,将天然高分子及其衍生物接枝到MWNTs表面,是一种新的十分有效修饰方法。综上所述,本研究在国家自然科学基金(50374039)、中国科学院纤维素化学重点实验室开放基金(LCLC-2005-159)和华中科技大学优秀博士学位论文基金(2004-044)的资助下,分别采用低分子量壳聚糖、N,O-羧甲基壳聚糖、β-环糊精、醋酸纤维素、羟乙基纤维素和甲基纤维素对多壁碳纳米管进行化学修饰,首次合成出六种结构新颖的天然高分子/多壁碳纳米管共价衍生物,并综合利用多种分析手段对其结构进行充分表征。在此基础上,研究了含有刚性碳纳米管的衍生物的结晶性质和溶解性。本研究采用独特的化学修饰方法以及活性衍生物MWNT-triazine,不仅成功合成得到五种天然高分子/碳纳米管共价衍生物,而且实现了醋酸纤维素等四种天然高分子对碳纳米管的均相化学修饰,这是对碳纳米管的众多非均相修饰的一个重要突破,将为实现包括天然高分子在内的众多高分子化合物对MWNTs的共价化学修饰提供新的有效方法。本研究在改善碳纳米管溶解性的同时,为研发出在众多领域有应用前景、绿色环保的新型纳米复合材料提供了可选择的基础材料。这将开拓碳纳米管化学与天然高分子化学,以及超分子化学相交叉的新的研究分支,具有重要的理论意义和潜在的应用价值。更多还原

【Abstract】 In 1991, Japanese scientist S. Iijima discovered carbon nanotubes (CNTs). Through the development of more than ten years, the CNTs has already become the important research frontier of nanoscience and nanotechnology, with vital fruits piling up one after another. Thus the CNTs occupies the prominent position in the development of science and technology in the 21st centuries,Due to the unique one-dimension like hollow tube structure, the CNTs has many attractive properties in the fields of adsorb, electricity, magnetism, field emission, mechanics and electrochemistry, etc. However, solubility of the CNTs is very poor, therefore limited their research and application seriously. Chemical modifications not only improve the solubility of the CNTs effectively, but also endow the CNTs with more functions, thus further broaden the application fields of the carbon nanotubes.Covalent modifications of the CNTs with polymers aroused peculiar attentions in the last few years. The strategy of polymer modifications not only has become of an effective means to prepare CNTs and their composites with special functions, but also has great theoretical and realistic significance to the development of nano-apparatus and novel materials. Thus far, most of the studies focused on synthetic polymers, and there is little report relating to natural polymers. However, these synthetic polymers are generally derived from nonrenewable and increasingly finite fossil resources, and some bear toxic aromatic rings. Compared with the synthetic polymers, the natural polymers, such as chitosan, cellulose and cyclodextrin, etc, are renewable resources with properties of nontoxicity, biocompatibility, biodegradability, abundance and environmental friend. As the important materials to resolve the problems of energy sources, resources and environment, natural polymers have broad applications in many fields. Thus, it is ideal to functionalize the CNTs with natural polymers and their derivatives.For the first time, this study presents definitely the strategy that utilizes natural polymers and their derivatives to modify the CNTs, and combines the CNTs and natural polymers through covalent bonds, thus obtains covalent derivatives of the CNTs and natural polymers with novel structures. The strategy will not only improve the solubility of the CNTs effectively, but also appear as a desirable way to develop ultimately environment friendly nanocomposites provided with properties that are inherent in both components. And these novel nanocomposites will probably be applied in many fields, such as separation, absorbance, catalysis, environment protection, electrode and micro friction, etc. As an innovative frontier and basis research, this research will develop a new research branch that crosses the CNTs chemistry, polymer chemistry and supermolecule chemistry, thus has great theoretical significance and potential utility value.Compared with siglewalled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs) are of better research and utility value in fields of adsorb, separation, catalysis and tribology, etc. Therefore in the study, the MWNTs were shortened, purified, oxidized and acylated with acyl chlorides, and further modified with a natural low molecular weight chitosan (LMCS, Mw≈8770 g·mol-1), which was water-soluble and relatively active. The obtained novel derivative of the MWNTs covalently modified with the LMCS (MWNT-LMCS) was characterized by FTIR, 13C NMR, XPS, Raman and TEM. Results indicate that nucleophilic substitution reaction happened between acyl chlorides of the MWNTs and a part of amino and primary hydroxyl groups of the LMCS. Consequently, amide and ester bonds formed between the LMCS and the MWNTs, and molecular chains of the LMCS were attached to surfaces of the MWNTs. The MWNT-LMCS consists of 81.6% C、11% O and 7.4% N. Moreover, the LMCS content in the MWNT-LMCS is about 58 wt %, and about four molecular chains of the LMCS are attached to 1000 carbon atoms of the nanotube sidewalls. Most interestingly, the amorphous packing structure of the LMCS changed dramatically when it attached to the surfaces of the MWNTs. The crystallinity of the attached LMCS was much improved. Covalent and noncovalent interactions between the MWNTs and the attached LMCS might induce the crystalline character of the LMCS. By complex calculations, the unit cell of the MWNT-LMCS was determined to be monoclinic, and degree of crystallinity of the MWNT-LMCS was 26%. The MWNT-LMCS was soluble in DMF, DMAc, DMSO and acetic acid aqueous solution.Although the MWNTs was successfully modified with the LMCS, however, further experiments indicated that the common modification approach that based on the acylated MWNTs was not feasible to natural polymers such as high molecular weight chitosan and etc. On the one hand, as inorganic carbon chemicals, the carbon nanotubes have tube like structures of graphitic sheet, which conduce to a higher chemical stability, considerable steric hindrance, and very poor solubility. So the reactivity of the MWNTs is poor. On the other hand, the great increase in molecular weight may cause considerable steric hindrance. Moreover, the molecular chains are usually stiff. Thus the reactivity of natural polymers such as high molecular weight chitosan is much lower than the LMCS.As the reaction between the MWNT-COCl and high molecular weight chitosan was not efficient, chain extension tactics was first adopted in the study. Based on the shortened, oxidized and acylated MWNTs, 1,3-propanediamine, which is very active with simple structure, was chosen to modify the MWNTs. Consequently, novel derivative of the MWNTs covalently modified with the 1,3-propanediamine (MWNT-NH2) was obtained. The active groups were extending out from the surfaces of the MWNTs in order to decrease the steric hindrance in reactions. On the basis of chain extension, 2,4,6-trichloro-1,3,5- triazine, which has active chlorine atoms, was chosen to homogeneously react with the MWNT-NH2 under low temperatures. The obtained novel derivative of the MWNTs covalently modified with the 2,4,6-trichloro-1,3,5-triazine (MWNT-triazine) was characterized by XPS, FTIR, 13C NMR and UV spectra. The triazine ring content in the MWNT-triazine sample is 0.8 mmol/g. The MWNT-triazine can dissolve in distilled water, acid aqueous solution, alcohols, DMF, DMAc and DMSO, etc. The synthesis of the MWNT-triazine attained purposes such as introducing reactive groups to the surfaces of the MWNTs, increasing amount of the active groups, decreasing the steric hindrance in reactions and improving the solubility of the MWNTs. Therefore the MWNT-triazine will be of advantage to the modification of the MWNTs with natural polymers.As it was very difficult to separate and purify the product of the MWNTs modified directly with high molecular weight chitosan, a chitosan sample, which has a degree of deacetylation of 84.7% and viscometric average molecular weight of 1.99×105, was first modified with chloroacetic acid under an alkaline condition. And a water soluble N,O-carboxymethyl chitosan (N,O-CMC), which had a overall degree of substitution of about 16.9%, was obtained. Then the MWNT-triazine was further modified with the N,O-CMC, and UV spectrum was adopted to attempt to analyze the purification effect of the product. The obtained novel derivative of the MWNTs covalently modified with the N,O-CMC (MWNT-CMC) was characterized by FTIR, 13C NMR, XPS, UV and TEM. Results indicate that chlorine atoms of the MWNT-triazine were nucleophilic substituted by a part of amino and hydroxyl groups of the N,O-CMC. Consequently, chitosamine units of the N,O-CMC were attached to surfaces of the MWNTs. The MWNT-CMC consists of 78.1% C, 15.6% O and 6.3% N. Moreover, the N,O-CMC content in the MWNT-CMC is about 31.9 wt%. Compared with reactant N,O-CMC, which had a low crystallinity and degree of crystallization of 14%, and the amorphous packing structure of the MWNT-triazine, the crystallinity of the MWNT-CMC was improved. The degree of crystallization of the MWNT-CMC was about 21%. The MWNT-CMC dissolved in DMF, DMAc, DMSO and NMP, and was partly soluble in hydrochloric acid and acetic acid aqueous solutions.The MWNT-triazine was further modified withβ-cyclodextrin (β-CD), cellulose acetate (CA), hydroxyl ethyl cellulose (HEC) and methyl cellulose (MC), respectively. Consequently, four novel derivatives of the MWNTs, namely MWNT-β-CD, MWNT-CA, MWNT-HEC and MWNT-MC were obtained respectively. As the four novel derivatives were synthesized in homogeneous systems, the modification strategy in the study is a significant breakthrough, compared with the many covalent modifications of the CNTs that carried out in non-homogeneous systems. Moreover, UV spectrum was adopted to attempt to analyze the purification effect of the four products, results verified that there was not any individual natural polymers that physical mixed in the products and could be detected with UV spectrum. This was consistent with the results of TEM imaging. The obtained four novel derivative of the MWNTs were characterized by FTIR, 13C NMR, XPS, UV and TEM, respectively. Results indicate that chlorine atoms of the MWNT-triazine were nucleophilic substituted by a part of hydroxyl groups in the molecular chains of theβ-CD, CA, HEC or MC, respectively. Consequently, glucopyranose rings of theβ-CD, CA, HEC or MC were attached to surfaces of the MWNTs, respectively. The MWNT-β-CD consists of 79.7% C, 14.3% O and 6.0% N, and theβ-CD content in the MWNT-β-CD is about 26.6 wt%. The MWNT-CA consists of 76.3% C, 18.4% O and 5.3% N, and the CA content in the MWNT-CA is about 42.8 wt%. The MWNT-HEC consists of 76.2% C, 19.5% O and 4.3% N, and the HEC content in the MWNT-HEC is about 32.1 wt%. The MWNT-MC consists of 77.1% C, 18.9% O and 4.0% N, and the HEC content in the MWNT-HEC is about 38.3 wt%.Crystallinity studies indicated that reactions with the MWNT-triazine destroyed original crystal forms of theβ-CD, CA, HEC or MC, respectively. Compared with the amorphous packing structure of the reactant MWNT-triazine, the crystallinity of the MWNT-β-CD and MWNT-HEC was improved, and the crystallinity of the MWNT-MC and MWNT-CA was somewhat improved. The degree of crystallization of the MWNT-β-CD, MWNT-HEC, MWNT-MC and MWNT-CA was about 19%、15%、18% and 17%, respectively. The four novel derivatives of the MWNTs were easily soluble in DMF, DMAc, DMSO and NMP. Above studies illuminate the strategy, which adopts chain extension and introducing active groups to obtain novel active derivative MWNT-triazine first, in order to improve the reactivity and solubility of the MWNTs, and then attach the molecular chains of the natural polymers and their derivatives to the surfaces of the MWNTs through the nucleophilic substitution reactions, is novel and very effective to the covalent modifications of the MWNTs.In summary, based on the supporting by the National Natural Science Foundation of China (No. 50374039), the opening foundation of key laboratory of cellulose and lignocellulosics chemistry of the Chinese Academy of Sciences (LCLC-2005-159), and excellent doctorial dissertation foundation of the Huazhong University of Science and Technology (2004-044), this study adopted natural polymers and their derivatives, such as low molecular weight chitosan, N,O-carboxymethyl chitosan,β-cyclodextrin (β-CD), cellulose acetate (CA), hydroxyl ethyl cellulose (HEC) and methyl cellulose (MC) to covalently modify MWNTs, respectively. Consequently, six novel derivatives of the MWNTs were synthesized for the first time, and further characterized by several modern analysis instruments, respectively. Moreover, crystallinity and solubility of the derivatives that contain rigid carbon nanotubes were studied.Based on the novel strategy and the active derivative MWNT-triazine, the study not only synthesized successfully five novel derivatives of the MWNTs modified with natural polymers, but also realized the syntheses of four novel derivatives in homogeneous systems. This is a significant breakthrough compared with the many covalent modifications of the CNTs that carried out in non-homogeneous systems, and will provide a novel and very effective method to the covalent modifications of the MWNTs with many polymers, including natural polymers and their derivatives. The study not only improves the solubility of the CNTs effectively, but also provides several basic materials, which might be useful in the development of novel environment friendly nanocomposites that will be applied in many fields. As an innovative frontier and basis research, this research will develop a new research branch that crosses the CNTs chemistry, polymer chemistry and supermolecule chemistry, thus has great theoretical significance and potential utility value.更多还原