Preparation And Properties Of Polymer/Carbon Nanotube Composites

Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, many efforts have shown that CNTs have unique structural, mechanical, electrical and chemical properties. Theoretical and experimental results showed extremely high elastic modulus and tensile strength as high as 1 TPa and 200 GPa, respectively. These superior properties enable the use of CNTs as nano-reinforcements or additives for advanced structures to improve their mechanical properties. In addition, the CNTs also possess extraordinary conducting properties together with the high aspect ratio, by appropriately adding certain amounts of CNTs into polymer; it is possible to improve the electrostatic discharge and electromagnetic-radio frequency interference protection. So, the interests of researchers are more and more focus on the CNTs/polymer composites. Moreover, owing to the recent rapid development of technology, CNTs are available in kilogram quantities with a price that is acceptable compared to that at the initial stage of their discovery. This makes the large-scale production of CNTs composites possible and practical. Studies of CNTs/polymer composites have achieved great success, but some problems in their preparation are still needed to solve, and their practical applications are to be further developed. The remarkable properties of CNTs and its size make them suitable candidates for applications in polymer composite functional fiber. So, in this work, the fiber forming polymer polypropylene (PP), nylon-6 as the matrix materials, the preparation, structure and properties of the CNTs nanocomposites are studied, and the PP/CNT, nylon-6/CNT composite fiber with improved properties are prepared. The main results obtained are as follows: 1. PP/multi-wall carbon nanotube (MWNT) composites with MWNT content 0.5 wt %～5 wt% were fabricated by solution coagulation method. The observation by scanning electron microscope (SEM) demonstrated that the MWNT were well dispersed in the composites at the level of nanometer. The effects of MWNT content on the non-isothermal crystallization behavior of the composites were investigated by differential scanning calorimetry (DSC), and the crystallization kinetics of the composites were analyzed by the Jeziorny and the Mo Zhishen methods. It was found that MWNT acts as nucleating agents, the crystallization peak temperature (T_p) and onset temperature (T_onset) of the composites shifts to high direction, though their variations indicate no significant dependence on fluctuations of the MWNT content from 0.5 to 5wt%. The analyses by super cooling degree (T_p—T_onset) showed that the overall crystallization rate of composites are higher than that of control PP and decreased with the increase of MWNT contents (from 0.5 wt% to 5 wt%), suggesting a impediment action of MWNT on the movement of PP segments. The crystallization kinetics parameters showed that the composite exhibits a smaller half crystallization time, a larger crystallization rate constant and a smaller Avrami exponent compared with pure PP, and are independent of the MWNT content. In order to achieve the identical crystallinity, the PP/MWNT composites need a characteristic cooling rate F(T) that are lower and the values of a are higher than that for pure PP. MWNT can act as nucleating agents of PP. The MWNT has no influence on the crystallinity and melting peak temperature of PP in the composite. Results of the wide-angle X-ray diffraction (WAXD) showed that the introducing of MWNT does not change the crystal form of PP in the composites. At the MWNT content of 0.5wt%, a decrease of spherulite size was observed.2. Chemical modification of CNTs can help it to disperse homogeneously throughout the matrix. Polypropylene grafted MWNT (PP-g-MWNT) were prepared through radical reactions in the PP/MWNT suspension irradiated with ultraviolet (UV) for 24h in the presence of initiator. The existence of PP-g-MWNT structure was confirmed and characterized by means of transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), Wide-angle X-ray diffraction (WAXD), Thermo gravimetric analysis (TGA). The results showed that PP chains wrapped onto the MWNT of shorter length. The absorptions and diffractions of PP characteristics were found on the FTIR spectrum and WAXD profiles of the PP-g-MWNT, which were indicative of the occurrence of graft reaction. WAXD results also suggested that the grafted PP has different crystal structure from that of the PP control. TGA results showed that the degree of grafting PP onto MWNT was about 10.4wt%. No significant changes were found in the D line and G line of the Raman spectra of the PP-g-MWNT, suggesting that MWNT structure remained unchanged after the grafting reactions. PP/MWNT and PP/(PP-g-MWNT) composites with MWNT content 1 wt % were fabricated by solution coagulation method. Dynamic mechanical analysis indicated the storage modulus of composites is increased by the stiffening effect of the (PP-g-MWNT), which is particularly significant at higher temperature. The storage modulus of PP/(PP-g-MWNT) composites is increased by 122% and 159% at 25℃and 75℃, respectively. The magnitude of the tanδpeak decreased significantly, and the loss modulus peak broadened towards higher temperature.3. A new antistatic master batch with high efficacy and durability was obtained by dispersing MWNT into the antistatic agent PR-86. One merit of the antistatic master batch was that good antistatic PP fiber with "fibril in matrix" structure can be prepared with just 0.5wt% (MWNT content: 0.05wt %) of it. The friction electrostatic voltage of antistatic PP fiber is only 160V. The antistatic mechanism of the fiber is the polarization of the "island" in the fiber under the action of electric field and discharging electric charge within the fiber. By using the MWNT, the degree of the polarization of "island" is enhanced, which further strengthens the effect of the master batch's antistatic property. The fiber has a durable antistatic characteristic and is free from the influence of relative humidity. And the antistatic master batch has no influence on other properties of the fiber.4. To successfully achieve the outstanding mechanical and electrical performances of new advanced CNTs nanocomposite materials depends strongly on the ability to disperse CNTs homogeneously throughout the matrix. Furthermore, good interfacial bonding is required to achieve load transfer across the CNT-matrix interface. In-situ polymerization can help to achieve these desired goals. In this work, the nylon-6/MWNT composites were prepared by acyl chloride MWNT in-situ polymerized with amino-capped nylon-6, and comparison with the composite prepared by solution coagulation method. The observation by optical microscope and SEM demonstrated that the MWNT were well dispersed in the composites at the level of nanometer. The results of FTIR, UV-Vis absorption spectrum and the intrinsic viscosity of the nylon-6 and nylon-6/MWNT composite indicated that there are chemical bond linkage between MWNT and nylon-6 in the composite. The crystallization behavior of the composites was studied by DSC. The heating scans from glass state of DSC measurement results showed that the MWNT additions led to an increase in the glass transition temperature and crystallization peak temperature, a decrease in the melting temperature in comparison with the control nylon-6, which suggest that the MWNT hinders the motion of the molecular chain of nylon-6 in the crystallization process. The Jeziorny and Mo's methods were applied to analysis the kinetics of nonisothermal crystallization by DSC cooling scans of the composite, the results of crystallization peak temperatures, half crystallization time, crystallization rate constant and the characteristic cooling rate F(T) showed that MWNT has heterogeneous nucleating effect on nylon-6. The overall crystallization rate of composites are higher than that of control nylon-6, when the MWNT content is 1 wt%, But the chemical bond interaction between nylon-6 and MWNT restricts the motion of nylon-6 macromolecular segments greatly, which thus leading to the overall crystallization rate decrease of the nylon-6/MWNT (2wt%) composites in comparison with control nylon-6. Results of the SEM and WAXD showed that the introducing of MWNT does not change the spherulite structure and crystal form of nylon-6 in the composites.5. The nylon-6/MWNT composite fiber with different MWNT concentration were fabricated by melt-blend-spining with nylon-6 by using the master batch nylon-6/MWNT (5wt%) composite prepared from in-situ polymerization, and comparison with the composite fiber prepared by using the master batch obtained from solution mixing method. The mechanical properties of the composite fibers were tested by Instron 1122, Results showed that the addition of only 0.5 wt % carbon nanotubes in the nylon-6/MWNT composite fiber prepared by using the master batch obtained from in-situ polymerization increases the tensile strength and initial modulus by 60% and 86% respectively, and not affect the breaking elongation. Microstructure characterization, performed by SEM, showed that the MWNT disperse well and interact with nylon-6 matrix, and there are bridging effect on composite cracks.