Structure And Properties Of Multi-walled Carbon Nanotubes/Polyamide 6 Composites

1,6-hexametbylene diamine (HMD) were grafted onto multi-walled carbon nanotubes (MWNTs) via acid-thionyl chloride route. The characterization of treated MWNTs was carried out by Fourier transform infrared spectrum (FTIR), Raman spectrum, transmission electron microscopy (TEM) and scanning electron microscopy (SEM), etc. The results show that HMD is covalently bonded on the MWNTs after activating by thionyl chloride. A microstructure transformation of MWNTs is found during the treatment process. Acidification makes the MWNTs compact and grafting HMD promotes the compact structure loose again. The chemical processing of MWNTs modification was analyzed theoretically in detail and the mechanism of microstructure transformation was speculated. It is proposed that HMD molecules inserted into different carbon nanotubes and break the hydrogen band structure formed in acidification process. Therefore the dense structure derived from acidification is modified, which lead to the transformation of MWNTs.The MWNTs after different treatment were used to fabricate MWNTs/PA6 composites through melt blending. The dispersion of different MWNTs in PA6 was observed by a combination of SEM, TEM and optical microscopy (OM). The results show that the amino-functionalized MWNTs (f-MWNTs) are dispersed more homogeneously in PA6 than the pristine MWNTs (p-MWNTs) and the poorest dispersion is achieved for acid treated MWNTs (a-MWNTs) because of their compact structure. It is indicated that the loose structure and functionalized surface of MWNTs benefit the dispersion of MWNTs in PA6. In addition, the amino-functionalization of MWNTs improves the compatibility between the MWNTs and PA6, resulting in stronger interfacial adhesion.MWNTs play an important role in the crystallization behavior of PA6. Firstly, the effect of MWNTs on the isothermal crystallization and melting behavior of PA6 was investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The Avrami and Lauritzen-Hoffmann equations were used to describe the isothermal crystallization kinetics. It is indicated that the MWNTs act as effective nucleating agents, the crystallization rate of PA 6 in the composites is thus increased. However the stiff MWNTs inhinder the motion of PA6 chains, thus reduce the spherulite radius of PA6. This reduction is more significant for p-MWNTs. The more the MWNTs are, the more obvious the effect is. All the isothermally crystallizedsamples exhibit triple melting endotherms at lower crystallization temperature and double melting endotherms at higher crystallization temperature. The multiple melting endotherms are mainly caused by the recrystallization of PA6 spherulites with different crystal sizes, and perfection during heating. The MWNTs have little effect on the crystalline structure of PA6 under isothermal condition, however the presence of MWNTs increases the lamellar thickness of PA6 crystal. The apparent equilibrium melting temperature of the composites is lower than the apparent equilibrium melting temperature of neat PA6.The non-isothermal crystallization and melting behavior of PA6 and its composites were investigated by DSC. The Jeziorny, Ozawa and Mo equations are used to describe the non-isothermal crystallization kinetics. The results show that Ozawa and Jeziorny equations can not describe the nonisothermal crystallization process, while the Mo equations exhibits great advantages in treating the nonisothermal crystallization kinetics. The addition of MWNTs into PA6 has an effect on the mechanism of nucleation and the growth of PA6 crystallites. The MWNTs act as effective nucleating agents but hinder the motion of PA6 chains which resulted in the decrease of crystallization rate. The result is different with the isothermal process, indicating the complexity of non-isothermal crystallization. The polymorphism is found for the nonisothermal crystallized samples, which is dependent on the cooling rates. The lower temperature peak corresponds to the y crystalline form and the higher temperature peak is related to the a crystalline form. The addition of MWNTs favors the formation of a crystalline form, and promotes the y crystalline form transform into a crystalline form at higher cooling rates.A combination of DSC, XRD and polarized microscopy (PLM) was used to investigate the effect of MWNTs on the crystallization structure of PA6. The results indicate the presence of polymorphism in PA6 and its composites, which is dependent on the MWNTs concentration and the cooling rate. More MWNTs and slow cooling from the melt favors the formation of a crystalline form. It is proposed that the plane structure of a crystalline form prior to be formed on the smooth surface of MWNTs, thus causing the oc-favoring phenomenon. With the cooling rates increasing, the crystallinity of neat PA6 decreases, and that of the composites decreases firstly but increases afterward. The heterogeneous nucleation induced by MWNTs and the restricted mobility of polymer chains are considered as the main factors. Theinfluence of thermal treatment on the crystalline structure of MWNTs/PA6 composites is also discussed. The MWNTs promote the y-a phase transition of composites took place at a lower temperature than that of neat PA6. The annealing peaks of the composites annealed at 160°C are higher than that of neat PA6, and the highest annealing peak is obtained for f-MWNTs/PA6 composites. This phenomenon is closely related to the different nucleation and recrystallization behavior produced by various MWNTs in confined space. The compatibility between f-MWNTs and PA6 is improved by grafting HMD, thus weakens the effect of MWNTs on the crystallization behavior of PA6.The tensile properties of MWNTs/PA6 composites were investigated. It is found that p-MWNTs cause the tensile strength of composites decreased, but low content of f-MWNTs make the tensile strength and modulus increased. The elongation at break of the composites is decreased. The better properties of the composites result from the more homogeneous dispersion of f-MWNTs in PA6 and stronger interfacial adhesion between f-MWNTs and PA6. The storage modulus of the composites is similar to neat PA6 owing to the crystallization and stiff effect of MWNTs, however the glass transition temperature (Tg) characterized by tan8 peak for all composites is lower than that of neat PA6. Amino-functionalization of MWNTs improves the interaction between MWNTs and PA6, leading to a higher Tg than p-MWNTs/PA6. The apparent viscosity of the composites is lower than PA6 at lower MWNTs content, but the opposite is true at higher MWNTs concentration. This is attributed to the broken hydrogen band junction in PA6 chains by MWNTs and the stiff effect of MWNTs. Amino-functionalization of MWNTs weakens their stiff effect, resulting in lower apparent viscosity for the f-MWNTs/PA6 composites. The thermal degradation behavior of MWNTs/PA6 composites was studied using thermogravimetric analysis (TGA). The presence of MWNTs improves the thermal stability of PA6 under air obviously, but has little effect on the thermal degradation behavior of PA6 under nitrogen. This improvement is attributed to the excellent thermal stability of MWNTs.