Carbon Nanotubes Functionalized With Polymers And Their Preliminary Applications In High-Performance Polymer Nanocomposites

Covalent functionalization of carbon nanotubes (CNTs) is currently developing as one of the most powerful tools enabling their processing, manipulation, and assembly from solution and polymer matrices. There are still some problems necessary to overcome in this area, for example, in the existing "grafting to" approaches one needs to balance the molecular weight and grafting density of polymers, and harsh conditions (high temperature and long reaction time) and fussy procedures in polymer functionalizations. The purpose of our investigations is to find some possible routes to overcome the above difficulties. Meanwhile, we hope to explore possible applications of these functionalized CNTs in high-performance polymer nanocomposites. The primary contents of this dissertation are outlined as follows:Single-walled carbon nanotubes (SWNTs) with high covalent bonding density of polymer layers were prepared by "grafting to" approach, where the benzyl chloride groups of styrene copolymers (M_n = 47 600) reacted with the phenyl alkyne groups on SWNTs under relatively mild conditions. This resulted in a grafting efficiency as high as 81 wt % in TGA. Microscopic observations displayed the uniform, thick polymer layers on the SWNT surface. The high density of covalent bonding between polymer and nanotubes was confirmed by Raman, ~1H NMR and FTIR, which makes them well dissolved in organic solvents and homogeneously dispersed in the polymer matrix. The in situ UV-vis observations during the dissolution indicated that for such a multifunctional system almost no cross-links occurred between SWNTs due to the physical absorption and steric hindrance of polymer chains during the functionalization of SWNTs. The current method suggests a possibility to control the covalent bonding density at the interface and to tune the thickness of polymer layers by altering the molecular weights of functional blocks and whole copolymer molecules. It should be helpful for systematically studying the effect of interfacial bonding density on macroscopic properties and further optimizing the properties of SWNT-based nanocmposites.Few-walled carbon nanotubes (FWNTs) have attracted much attention due to their unique structure that may be potential in solving some existing problems associated with SWNTs and multi-walled carbon nanotubes (MWNTs). We here extended the method applied in the first section to the FWNT-polymer interface, in which the pre-synthesized multi-functional diblock copolymers, and then uniform thick polymer layers were coated on the FWNT surface. From the recent molecular simulations and experimental observations, it was recognized that the interfacial shear strength between SWNTs and amorphous/crystalline polymers could be increased with increasing interfacial chemical cross-links. This opinion was further confirmed in our work It was shown that the polymer grafted CNTs with higher bonding density exhibited a pronounced reinforcement effect on the mechanical properties of polymer nanocomposites. The addition of only 0.06 wt % SWNTs resulted in 82% and 78% of increases in tensile strength and elastic modulus of the composites, respectively. This reflects an efficient interfacial stress transfer between SWNTs and polymer. The same increase appeared in thermal property of nanocompostes.The facility and efficiency is important in considering of polymer funcionalization of CNTs. We here report an easy method to functionalize CNTs with polystyrene (PS). We first prepared PS in which 2, 2 -Azobis (isobutyronitrile) (AIBN) was used as an initiator and 2,2,6,6-Tetramethyl-1-piperidineoxy (TEMPO) as a modulator. After the decomposition of AIBN to initiate the polymerization, cyanic groups are left at the PS chain ends. Subsequently, CNTs were functionalized with p-aminophenylalkyne (APA). With the assistance of trifluoromethane sulphonic acid, the coupling reaction between cyanic and phenyl alkyne groups was completed at room temperature, resulting in a 32 wt % grafting efficiency in TGA. The details of PS-grafted CNTs were further characterized by TEM, AFM, Raman and UV-vis. To the best of our knowledge, it is for the first time exhibited that the PS functionalization of CNTs was able to be completed at room temperature and moreover, the grafting polymer (PS) did not require additional incorporation of reactive groups (only the residual cyanic groups at the PS ends were utilized). This is in contrast with the strategies adopted in other "grafting to" methods, and thus simplifies the functionalization procedure of CNTs.Styrene can copolymerize with many vinyl monomers, and adapt to any polymerization initiated methods. We here propose a facile method to introduce lots of styrene monomers to the SWNT surface so that the polymer modification can be realized by means of in-situ polymerization of those styrene monomers covalently linked on SWNTs. To achieve this, phenyl alkyne groups were first grafted to the SWNT surface, followed by the coupling reaction between phenyl alkyne and benzyl chloride of p-chloromethylstyrene. The polymer layers were then formed on the SWNT surface via in-situ polymerization initiated by AIBN and modulated by TEMPO. The results from HR-TEM and SEM proved the micro-configuration of the polymer-grafted SWNTs. Because of introduction of lots of monomers onto the SWNT surface, the solubility of SWNTs was improved and can be dispersed in various organic solvents. The method presented not only affords a chance to copolymerize SWNTs with extensive vinyl monomers, but also can prepare directly nanocomposites by melt-mixing with polymers.