Study On Preparation And Creep Behavior Of Carbon Nanotubes-Reinforced Thermoplastic Nanocomposites

Carbon nanotubes-reinforced thermoplastic composites have become one of the most active topics in the academic research of material science and industrial application in recent years. Due to its advantages such as easy processing, the thermoplastic is extensively used as the composite matrix. However, the poor thermal-stability of thermoplastic polymer is an inherent defect caused by the creep deformation, thus it seriously limits the extensive engineering application of polymer. Meanwhile, creep recovery is also a key factor to determine the service life of the materials. Therefore, it is the urgent need to improve the creep and recovery performance of thermoplastic without sacrificing other important performances, such as mechanical and thermal performance. What’s more, the creep and recovery behavior of composites is more sensitive to the microstructural changes than the static mechanical behavior. The interaction mechanism between MWCNTs and polymer chains can be explored by studying the creep and recovery performance.In this thesis, multi-walled carbon nanotubes (MWCNTs) composites were prepared in order to improve the creep and recovery performances of two kinds of thermoplastic polymers, the polypropylene and polystyrene respectively. After the processing of the composites, various loading and mechanical states were employed as the experimental conditions to reveal their creep and recovery behaviors. The interaction mechanism of carbon nanotubes and polymer chains was discussed by both experimental analysis and modeling based on the plastic theories. The viscoelastic constitutive relationship of nanocomposites was explored. The details of the research are as following:The influence of the MWCNTs on the creep and recovery properties of polymer matrix was studied under different loading conditions. The composite materials with good dispersion of the MWCNTs were used as the samples. Different to the polymer matrix, both the tensile and shear creep results showed that the creep strain as well as unrecoverable stain of nanocomposites decreased, and the recovery ratio remarkablely increased. When the nanocomposites were subjected to the cyclic loading, the strain reduction and recovery improving effects of the MWCNTS were more critical to the behavior of the nanocomposites. The studies also indicated that with the increase of temperature, stress levels and the MWCNTs contents, the MWCNTs gave a better enhancement to the creep and recovery performances of the composites under all stress conditions.Secondly, creep behavior of nanocomposites in three different mechanical regions was investigated.In the glassy region, the influence of the MWCNTs on creep resistant of polystyrene was not apparent; in the glass-rubber transition region, carbon nanotubes could significantly decrease the creep strain and retard the transition process from the glassy state into viscoelastic and rubbery states; in the rubbery region, the addition of the carbon nanotubes could substantially prolong the creep life time. In the melt region, besides decreasing of creep deformation and improving recovery ratio, the MWCNTs also reduced the temperature dependence of the creep deformation of the polymer. All these results under various mechanical states could be the guidance for high-creep-resistance nano-material design.Then, based on the creep and recovery analysis, the interaction mechanism of carbon nanotubes and polymer chains were investigated. From shear creep results, it can be found that the mobility of polymer chains was closely related to the network-like structures formed by the MWCNTs and polymer chains, which would affect the creep and recovery properties of nanocomposites from the microscope view. Further research showed that a percolation threshold region (0.6-1.7vol%) existed. During this region the electrical, rheological, creep and recovery properties would have a sudden change. Carbon nanotubes-molecular chain network like structures could also strongly affect the cyclic creep behavior of nanocomposites.In addition to the experimental analysis, several viscoelastic models, such as Burger’s model, Findley power law, and Weibull distribution equation were applied to explore the viscoelastic constitutive relation of nanocomposites. Moreover, the prediction of the long-term creep property of nanocomposites was carried out based on time-temperature superposition principle. The predictable time scale extended up to tens of thousands of hours. To further understand the mechanisms, retardation spectrum was calculated to reveal that the existing of the MWCNTs inhibited the mobility of polymer chains.