Research On Modification Of Multiwalled Carbon Nanotube Surfaces And Composites Based On Thermoplastic Polymer

Carbon nanotubes (CNTs) are unique nanostructured materials with remarkablephysical and mechanical properties, such as high elastic modulus, as well asremarkable thermal and electrical conductivity, making them a very attractivecandidate in composite material formulations. For this reason a current flurry ofresearch is focused on the manufacturing of nanotube reinforced polymer matrixcomposites. However, dispersion of CNTs in solvents and polymer matrixes is anobstacle for their further applications due to the inert surfaces and the poor solubility.Some researches reported that a well dispersion of CNTs in solvents and polymermatrixes could be achieved through chemical modification of the CNT surfaces.Therefore, a proper purification was firstly carried out to achieve a large number ofactive groups (e.g.–OH and -COOH) on the CNT surfaces in this study. Based onthese active groups, CNT surfaces can be grafted and modified by organic chainsthrough reacting with organic molecules or polymer monomers. By using the modifiedCNTs, polymer-CNT composites were prepared via solution-mixing or melt-mixingapproaches. The effects of purification process and modification techniques on theCNTs were studied, and the properties of the polymer-CNT composites wereexamined and analyzed in this study. Some important rules about the polymer-CNTcomposites were found, which was instructive for the basic research and industrialmanufacture.Firstly, chlorine oxidation, composed of chlorine water treatment and ammoniawater treatment, was used to purify multiwalled carbon nanotubes (MWCNTs). Fromthe point view of practical application, this purification, comprising the merits ofgas-phase oxidation, liquid-phase oxidation and acid treatment, is easily carried outand can satisfy the need of purifying MWCNTs on a large scale in industry.Transmission electron microscope (TEM) observation showed that amorphous carbonon the outer-walls of pristine MWCNTs, carbon nanoparticles, and catalysts of metaloxides were eliminated via purification. Moreover, the closed tips of the pristineMWCNTs were opened. The contents of catalyst elements of Ni, Mo, and Mg in thepristine MWCNTs were 2.35, 3.58, and 0.91 wt%, respectively, which were estimatedby analysis of energy dispersive X-ray spectroscopy. However, there were no Mg andMo elements in the p-MWCNTs, and the Ni content was also dropped to 0.15 wt%. Importantly, the dispersion ability of MWCNTs in polar solvents such as water,ethanol, acetone, chloroform, and dimethylformamide, was greatly improved due to alarge number of carboxyl, hydroxyl groups, and C-Cl bonds, introduced on the CNTsurfaces after purification of chlorine oxidation. Therefore, p-MWCNTs could befurther modified via chemical reactions on the basis of these groups.Secondly, on the basis of the hydroxyl, p-MWCNT surfaces were modified withstyryl through reacting with 4-chloromethylstyrene. The modified MWCNTs withsurface styryl (s-MWCNTs) could participate in the polymerization of styrene, whichresulted in the covalent bonding of PS chains on the MWCNTs (g-MWCNTs). It wasfound that the PS layer was approximately 5 nm and 20 wt% of the g-MWCNTs. Aftermodified by PS, the MWCNTs were more compatible with poor polar solvent such astoluene and tetrahydrofuran. PS-p-MWCNT, PS-s-MWCNT, and PS-g-MWCNTcomposites were then prepared by solution-mixing approach. Scanning electronmicroscope (SEM) observation exhibited that because of the coating of PS layer, thecompatibility between the MWCNTs and PS matrix was greatly enhanced, resulting inthe homogeneous dispersion of g-MWCNTs. With the increase of MWCNT content,the fracture mechanism of the PS-g-MWCNT composite was transformed from brittlefracture to elastic one. Studying on the mechanical and thermal properties of thecomposits proved that with the increase of MWCNT content, the impact strength,tensile strength, and temperature of thermal stablility of PS-p-MWCNT,PS-s-MWCNT, and PS-g-MWCNT composites were all enhanced. With a betterinterfacial compatibility and homogeneous MWCNT dispersion, the PS-g-MWCNTcomposite had the better mechanical properties and thermal properties, compared toPS-p-MWCNT and PS-s-MWCNT composites. When the MWCNT content was 1.5wt%, the impact strength and tensile strength of PS-g-MWCNT composite wereincreased to 138.8% and 133.6% of neat PS, respectively. And the temperature ofthermal stability of PS-g-MWCNT composite was enhanced by 25 oC, compared toneat PS.Thirdly, p-MWCNTs were modified by PS via the polymerization of styrene undermicrowave irradiation. It was found that the PS layer (called PS coat layer in thefollowing) on the modified MWCNTs (m-MWCNTs) was approximately 3 nm and 10wt% of the m-MWCNTs. The mechanism of the modification was that the groups (e.g.,-H, -CH2OH, -CH2Cl and -CH3) on the p-MWCNTs were actived by microwaveirradiation and participated the polymerization of styrene, which resulted in theconvalent bond between PS and MWCNT surfaces. The PS coat layer, covalently bonding on the MWCNT surfaces, could not be washed off by solvents, proved bydispersion tests. Whereas, in the case of a control experiment conducted throughconventional heating, the PS was noncovalently bonding on MWCNT surfaces andcould be washed off by solvents. Then, the PS-p-MWCNT and PS-m-MWCNTmasterbatches with MWCNT content of 20 wt% were prerared by solution-mixing.And PS-p-MWCNT and PS-m-MWCNT composites were manufactured viamelt-mixing of masterbatches with neat PS on industrial extruder and injectionmoulding machine. In PS-m-MWCNT composite, the MWCNTs were enwrapped bytwo PS layers, which proved by SEM. One was the PS coat layer achieved duringmodification. The other was a PS middle layer lying between the PS coat layer and PSmatrix, whose thickness was approximately 80 nm. On the contrary, there was not PSmiddle layer in PS-p-MWCNT composite. TEM observations of the PS-m-MWCNTcomposite showed that the MWCNTs were individually dispersed in PS matrix andoriented along the injection direction of PS melt during moulding. In PS-p-MWCNTcomposite, the MWCNTs were also individually dispersed, but they were disordered.The mechanical and thermal properties of PS-m-MWCNT composite were better thanthat of PS-p-MWCNT composite, due to the influence of PS coat layer and PS middlelayer, improving the compatibility and interfacial adhersion between MWCNTs andPS matrix. For instance, when the MWCNT content was 0.32 wt%, thePS-m-MWCNT composite had a 250% increase of impact strength as compared toneat PS, but the PS-p-MWCNT composite had only a 150% increase.Fourthly, to confirm the fact that modification of MWCNT surfaces is universal forother polymer matrix, the preparation and properties of polycarbonate (PC)-MWCNTcomposite were also studied. In order to covalently modify MWCNT surfaces with PCchains, based on the reaction activity of C=C bonds and anhydride groups in maleicanhydride (MAH), and improve the compatibility between MWCNTs and PC matrix, aPC masterbatch was prepared via solution-mixing and reflux reaction of PC,MWCNTs, and MAH. Using industrial extruder and injection moulding, kilogramscale of PC-MWCNT composites were prepared via melt-mixing. SEM observationdisplayed that the claval aggregations of approximately 500 nm diameter, composedof several MWCNTs, were homogeneously dispersed in matrix and oriented along theinjection direction of PC melt during moulding. Because of the inducing effect ofMWCNTs, PC chains were regularly arrayed near the MWCNT surfaces and a largeamount of microcrystalline domains were formed, improving the thermal stability ofPC-MWCNT composite. Therefore, when the MWCNT content was 0.32 wt%, the impact strength of PC-MWCNT composite were increased to 160% of neat PS, thetemperature of thermal stability was enhanced with 30 oC.In conclusion, the above researches prove that it is feasible to chemically modifyMWCNT on the basis of the active groups (e.g., -H, -CH2OH, -CH2Cl and -CH3) onp-MWCNT surfaces. After modification, the dispersion ability of MWCNTs insolvents and polymer matrices can be improved, and interfacial adhersion betweenMWCNTs and polymer matrices can be enhanced, resulting in the improvement ofcomprehensive properties of polymer-MWCNT composites.