Study On Synthesis And Properties Of Derivatives Of Carbon Nanotubes Covalently Modified With Natural Polymers

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.