Preparation, Structure And Properties Of Polymer Nanocomposites Based On Carbon Nanotubes And Layered Double Hydroxides

Polymer based organic/inorganic nanocomposites combine the advantages of the inorganic materials (rigidity, high thermal stability and unique optical, electronic and magnetic properties) and the organic polymers (flexibility, electric, ductility and processability), which is different from the single materials and conventional microscopic composites. Polymer/inorganic nanocomposites have potential applications in mechanical, thermal, electronical, optical, and nonlinear optical fields. Therefore, the preparation/fabrication and utilization of high performance and multifunctional polymer/inorganic nanocomposites have great academic and practical significance.In this thesis, inorganic materials including carbon nanotubes (CNT) and layered double hydroxides (LDH) and several kinds of polymers (polyvinyl alcohol, nylon, polyvinylidene difluoride) were chosen as nanofillers and polymeric matrices, respectively. A series of polymer/inorganic nanocomposites are prepared using a variety of methods (solution blending, in situ polymerization, melt compounding, electrospinning, and layer by layer assembly) and their structure and properties are extensively studied. The main contents and conclusions of the thesis are as follows:1. Grafting of nylon 6 chains onto the multi-walled carbon nanotubes (MWNT) has been achieved under microwave irradiation. The novel approach is very convenient and highly efficient for the functionalization of carbon nanotubes. The resulting carbon nanotube samples were characterized using FTIR, Raman, TEM, and TGA techniques. The results indicated the covalent functionalization of MWNT with nylon 6. According to TGA results, the weight of grafted nylon 6 is about 47% in the functionalized MWNT.2. Nylon 11 nanocomposites with different loadings of MWNT were prepared by melt compounding. Scanning electron microscopy images on the fracture surfaces of the composites showed a uniform dispersion of MWNT throughout the matrix. The presence of the MWNT significantly improved the thermal stability and enhanced the storage modulus (G) of the polymer matrix. Melt rheology studies showed that, compared with neat PA11, the incorporation of MWNT into the matrix resulted in higher complex viscosities (|?*|), storage modulus (G), loss modulus (G), and lower loss factor (tan?). PA11 and its nanocomposites containing less than 1 wt% MWNT showed similar frequency dependencies and reached a Newtonian plateau at low frequencies. For the nanocomposite with 2 wt% MWNT, the regional network was destroyed and the orientation of the MWNT during shearing exhibited a very strong shear thinning effect.3. Polyvinylidene difluoride (PVDF) solutions containing a very low concentration of single-walled carbon nanotubes (SWNT) and multiwalled carbon nanotubes (MWNT) of similar surface chemistry, respectively, were electrospun, and the nanofibers formed were collected using a modified rotating disk collector. The polymorphic behavior and crystal orientation of the nanofibers were studied using wide-angle X-ray diffraction and infrared spectroscopy, while the nanotube alignment and interfacial interactions in the nanofibers were probed by transmission electron microscopy and Raman spectroscopy. It is shown that the interfacial interaction between the SWNT and PVDF and the extensional force experienced by the nanofibers in the electrospinning and collection processes can work synergistically to induce highly oriented?-form crystallites extensively. In contrast, the MWNT could not be well aligned along the nanofiber axis, which leads to a lower degree of crystal orientation.4. Nano-sized Mg-Al layered double hydroxide (LDH) was synthesized by a fast nucleation and slow aging method. Poly(vinyl alcohol) (PVA) nanocomposites with different LDH loadings were prepared by water solution casting method. TEM observations showed that the LDH nanoplatelets were uniformly dispersed in the PVA matrix. Tensile tests indicated that the elastic modulus and the tensile strength of PVA were enhanced by incorporating with LDH. In addition, the presence of LDH decreases the onset decomposition temperature but improves the amount of residues of PVA nanocomposites. Meanwhile, the transparency of PVA/LDH nanocomposite films is excellent and comparable with that of neat PVA. 5. Nano-sized Mg-A1 LDH which can be suspended in water was mixed with water solution of PVA, and then PVA/LDH nanocomposite hydrogels were prepared by using a cyclic freezing/thawing method. TEM results showed that the LDHs were well dispersed in PVA matrix, and increased the physical crosslinking density. DSC measurements reveal that the addition of LDH promotes the crystallization of PVA on the LDH surfaces. The Young's modulus and tensile strength of PVA nanocomposite hydrogels were greatly improved even at very low loading level of LDH. The water swelling ability of PVA/LDH nanocomposite hydrogels decreased due to high crosslinking density caused by non-swollen LDH.6. Highly crystalline and monodisperse Co-Al-CO3 LDH samples were prepared by precipitation through urea hydrolysis under hydrothermal condition which were then delaminated to single layer after decarbonation and anion exchange to Co-Al-NO3 LDH. After vigorous stirring in water, negatively charged montmorillonite (MMT) nanosheets were obtained. PVA as intermediate linkers can make sure the formation of hydrogen bonds between MMT and LDH nanosheets. This hydrogen bonding is essential in order to obtain uniform growth of (PVA/MMT/PVA/LDH)m multilayer films by layer by layer (LBL) technique. Taking the homogenous ultrathin films with LDH or MMT as comparisons, the heterogeneous hybrid films are expected to possess a tunable function due to their structural diversity and combined functionality.7. The exfoliated LDH nanosheets with positive charges could adsorb negative CNTs on their platelet surfaces with assistance of mechanical shaking. Co-Al LDH and CNTs have been assembled to form exfoliated LDH nanosheet/carbon nanotube hybrids via electrostatic force. The LDH/CNT hybrids with unique 3D nanostructure are novel nanofillers which combine 2D LDH platelets and 1D carbon nanotubes together. Furthermore, the unique three-dimensional (3D) hybrids thus prepared were used as reinforcing nanofillers to enhance the performance of polyamide 6 (PA6). It was found that the synergic effect of the CNT and LDH nanoplatelets resulted in homogeneous dispersion of the hybrid nanofillers throughout the PA6 matrix and strong combination with the matrix, thus providing an efficient mechanical improvement for the PA6 nanocomposites.