【作者】 杨应奎；

【导师】 解孝林；

【作者基本信息】 华中科技大学 ， 高分子化学与物理， 2007， 博士

【摘要】 碳纳米管的独特结构,使其具有优异的光、电、磁、热、机械和化学性能,在功能材料、结构材料与纳米器件等方面具有许多特殊的应用。然而,碳纳米管自身的不溶不熔性、易缠结团聚以及难于加工操纵等问题,极大地限制了它的应用和研究。聚合物/碳纳米管复合材料,由于碳纳米管在基体中的难分散性、无序排列以及弱的相容性与界面粘接性能,削弱和降低了碳纳米管赋予聚合物的功能性与结构增强作用。因此,聚合物修饰碳纳米管的研究引起了广泛关注。一方面,聚合物修饰碳纳米管可以改变碳纳米管的表面性质,增加其溶解性,有利于在溶液状态下对碳纳米管进行结构表征,以及研究其溶液自组装与流变行为等;另一方面,利用功能性聚合物修饰碳纳米管,可赋予碳纳米管新的功能,进一步拓展其应用范围;再者,有利于碳纳米管在基体中均匀分散或取向排列,改善纳米管与基体之间的相容性和界面粘接性能,从而将碳纳米管的功能性与聚合物的加工性能结合起来,制备出高性能的功能材料与结构材料。本论文主要围绕聚合物共价修饰碳纳米管的合成,碳纳米管对聚合物结构与性能的影响进行研究。预先通过酰胺化反应在碳纳米管表面引入功能性有机小分子,再引发单体进行原位接枝聚合反应,制备了线性聚合物、超支化聚合物和液晶高分子接枝碳纳米管,并率先开展了聚合物接枝碳纳米管的溶液自组装、溶液流变学以及碳纳米管对偶氮苯光致异构化行为和液晶高分子液晶性影响的研究。预先在碳纳米管表面引入乙烯基,然后在引发剂作用下形成碳纳米管大分子自由基,引发苯乙烯单体进行原位接枝聚合,合成了线性聚苯乙烯接枝碳纳米管,这是一种自由基聚合共价修饰碳纳米管的新方法。结果表明,每100个纳米管碳中有1个碳被接枝上分子量为9800g/mol的聚苯乙烯链,形成核-壳纳米结构。碳纳米管在聚苯乙烯基体中均匀分散,明显地提高了聚合物的热稳定性和玻璃化温度。聚苯乙烯接枝碳纳米管在THF、DMF和CH2Cl2等溶剂中具有很好的溶解性,并随其溶液浓度增加,在玻璃基底上自组装形成了长度和直径逐渐增加的纳米针状结构。与线性聚合物相比,超支化聚合物具有更好的溶解性、更低的溶液与本体粘度以及多功能性末端基,且在制备方法上比树枝状聚合物的合成简单。本论文采用缩聚反应合成了超支化聚(脲-氨酯)接枝碳纳米管,并通过调节反应单体的物质量之比有效地控制了碳纳米管表面接枝聚合物的含量与厚度。重点研究了超支化聚(脲-氨酯)接枝碳纳米管的溶液流变性质,同时也比较了线性聚苯乙烯接枝碳纳米管的溶液流变行为。结果发现,依赖于接枝聚合物的含量与测试温度,超支化聚(脲-氨酯)接枝碳纳米管的溶液在低剪切速率下,表现出低温剪切增稠与高温剪切变稀的溶液行为;而高剪切速率下,均呈牛顿流体行为。不同温度下,聚苯乙烯接枝碳纳米管的溶液在低剪切速率下呈假塑性流体,高剪切速率下呈牛顿流体。这种新颖的溶液流变行为,与分子内和分子间氢键的形成、转换与破坏,以及剪切作用下聚合物链与碳纳米管的取向重排有关,并建立流变学模型讨论了溶液流变机理。此外,利用二异氰酸酯与双羟基偶氮苯液晶单体之间的缩聚反应,合成了具有柔性主链和主链较刚性的偶氮苯侧链液晶高分子接枝碳纳米管。结果发现,大尺度的刚性碳纳米管引入液晶高分子结构中,限制和阻碍了液晶高分子的取向排列,使分子链间的堆砌规整性降低,无序度增加,从而不再表现出液晶行为。研究液晶高分子接枝碳纳米管溶液的光化学行为,发现随紫外光照时间增加,偶氮苯部分逐渐从反式构型向顺式构型转化,并产生相应的光致变色现象。避光放置紫外光照射后的溶液,其颜色逐渐恢复到初始状态,并通过光照前后样品的1H NMR谱变化,证实了光致异构化反应的可逆性。碳纳米管的引入降低了偶氮苯基元的反-顺异构化速率,但增加接枝聚合物链的柔顺性,可提高其光响应速度。偶氮苯修饰碳纳米管结合了偶氮苯与碳纳米管的优异性能,在新型的功能材料与纳米器件方面具有潜在的应用。最后,将碳纳米管、4′-戊基-4-氰基联苯(5CB)小分子液晶和甲基丙烯酸(MAA)单体超声分散后,通过原位聚合反应制备了PMAA/5CB/MWNT复合材料。利用5CB在力作用下取向诱导了聚合物基体中碳纳米管的有序排列,并通过溶剂刻蚀的方法移除了体系中的5CB,从而制备了碳纳米管取向排列的聚合物薄膜,为研制新型的薄膜器件提供了一种简单的方法。更多还原

【Abstract】 Carbon nanotubes (CNTs) have been proposed for many potential applications in functional materials, structural materials and nanodevices due to their unique optical, electronic, thermal, mechanical and chemical properties. However, the lack of solubility and fusibility and the entanglements as well as the difficult manipulation have imposed great limitations to their applications and development of CNTs. Despite the extraordinary promise of polymer/CNT nanocomposites, their realistic applications have been hindered by their poor dispersibility and random distribution in polymer matrix as well as the weak compatibility and interface interaction between matrix and CNTs. Therefore, functionalization of CNTs with various polymers has generated great attention. On the one hand, polymer-modified CNT changes the surface properties of CNTs, which not only improves their solubility, but also has benefits to investigate their structures, self-assembly and rheology behavior in solutions. On the other hand, modification of CNTs using functional polymers can endow CNTs with new properties, resulting in the further development for their applications. Moreover, the functionalization improves the dispersibility or alignment of CNTs in polymer matrix, and enhances the compatibility and interface adhesion between polymer matrix and CNTs, which promises the high performance materials with good functionality, mechanical properties and excellent processability.In the present dissertation, the synthesis of polymer covalently modified CNTs, and the effect of CNTs on the structures and properties of polymers were mainly studied. The organic small molecules with functional groups were preliminarily grafted onto the CNT surfaces via amidation to generate CNT-supported macroinitiators. Then linear polymers, hyperbranched polymers and liquid crystalline polymers (LCPs) were covalently grafted onto CNTs by grafting-from approaches based on the in-situ polymerization of monomers in the presence of CNT-macroinitiators. The self-assembly and rheology of the polymer-grafted CNTs solutions were investigated, and the influence of CNTs on the photoisomerization of azobenzene moieties and liquid crystallinity of LCPs was also discussed.Linear polystyrene-grafted CNTs (NT-LPS) with a core-shell nanostructure have been synthesized by a new in-situ free radical polymerization of styrene in the presence of CNTs terminated with vinyl groups. The results showed that 1 of 100 every 100 carbon atoms in CNTs is functionalized at the tips and side walls of CNTs and grafted by PS with a molecular weight of 9800 g/mol. CNTs are dispersed uniformly in polymer matrix, and the thermal stability and glass-transition temperature (Tg) of PS in NT-LPS are clearly enhanced. NT-LPS is soluble in THF, DMF and CH2Cl2. The self-assembly of NT-LPS that accompanies the evaporation of their THF solutions on glass substrates forms nanopins whose lengths and diameters increase with an increase in the solution concentration.Compared with linear polymers, hyperbranched polymers show better solubility, lower solution and melt viscosities and multifunctionality at end groups, and their synthetical methods are relatively ease. In the present dissertation, hyperbranched poly(urea-urethane)s (HPUs) have been grafted onto the surfaces of multiwalled carbon nanotubes (MWNTs) using one-pot polycondensation of diethanolamine (DEOA) and tolylene 2, 4-diisocyanate (TDI) in the presence of MWNT terminated with multi-hydroxyl groups (MWNT-OH). The grafted-HPU content and thickness on MWNTs were well-controlled by adjusting the feed ratio of TDI to DEOA. The solution rheology of the HPU-functionalized MWNTs was investigated for the first time by steady shear measurements. For comparison, the solution rheology of NT-LPS with comparable grafted-polymer content was also discussed. Depending on the grafted-HPU content and the temperature measured, the HPU-grafted MWNT solutions at low shear rates exhibit typical shear-thickening at low temperature and shear-thinning at high temperature, and show Newtonian fluids under high shear flow with a decreased viscosity. However, the NT-LPS solutions always represent shear-thinning at low shear rates and Newtonian behavior at high shear rates when they are measured at different temperatures. These novel rheological behaviors are related to the formation, conversion and disassociations of hydrogen-bonds as well as the orientation of polymer chains and MWNTs under shear. The rheology model was proposed to account for their rheological behaviors of polymer-grafted MWNT solutions.LCP-grafted MWNTs (MWNT-LCPs) have been synthesized by an in-situ polycondensation of azobenzene monomer containing bishydroxyl (AzoM) and diisocyanate in the presence of MWNT-OH. Two kinds of diisocyanates, an aliphatic diisocyanate (HDI) and an aromatic diisocyanate (TDI), were employed to react with AzoM, generating soft and semi-rigid LCPs with azobenzene side-chains onto the MWNT surfaces. Incorporating MWNTs into the molecular structures of LCPs increases the conformational and configurational disorder, and the ordered structures of liquid crystalline phase are destroyed, resulting in the disappearance of LCPs’liquid crystallinity due to the large scale and rigidity of MWNTs. The trans-isomers of azobenzene moieties in MWNT-LCP are gradually transformed to cis-isomers upon UV-irradiation, and accompanied by photochromism. When the UV-irradiated MWNT-LCP solution is kept in the dark for several days, the original color of solution is gradually recovered and shown the same 1H NMR spectrum, indicating the reversible photoisomerization. As compared with the parent polymer, the photoisomerization rate constant of MWNT-LCP decreases due to the heat sinks, and steric effects of MWNTs. However, the responsive rate of MWNT-LCP can be effectively controlled by adjusting the main-chain flexibility of the grafted-LCP on MWNTs, which might be the basis for developing novel high-performance optic and photonic devices.Finally, PMAA/5CB/MWNT composites have been synthesized by an in-situ free radical polymerization of methyl acrylic acid (MAA) in the presence of MWNTs and low molar mass liquid crystal, 4′-n-pentyl-4-cyanobiphenyl (5CB). The orientation of 5CB under shear induces the alignment of MWNTs in PMAA matrix, and 5CB is fully dialysed from PMAA matrix using a good solvent for 5CB and nonsolvency for PMMA. The oriented-MWNT polymeric films might have many potential applications in the thin-film devices.更多还原