Synthesis, Structures And Properties Of Polymers-Modified Multiwalled Carbon Nanotubes

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.