Preparation, Thermal Conductivity And Electrical Resistivity Properties Of Polyurethane/Functionalized Multi-walled Carbon Nanotubes Composites

Carbon nanotubes (CNTs) could be considered to be formed by the rolling of graphiteare with one dimentional cylindrical nanostructure, which had outstanding menchanical properties, thermal and electrical conductivities. With the rapid development of semiconductor and highly integrated circuits technology, high-performance thermal interface materials were required for the heat dissipation. CNTs which were well-known for their excellent thermal conductivities had attracted much attention as idea filler. The thermal conduction mechanism of CNTs is ballistic phonon propagation. CNTs with large specific surface area, high aspect ratio and high surface energy have poor dispersibility and random distribution in polymer matrix as well as the weak compatibility and interface interaction between matrix and CNTs. Therefore, large interfacial thermal resistance between CNTs and polymers hindered the phonon propagation at the interface, lead to the reported experimental data were not in accord with theoretical results. Covalent functionalization of CNTs could improve the dispersibility and compatibility of CNTs in polymer matrix, interface interaction between matrix and CNTs, as well as the thermal conductivity of polymer/CNT composites.In the present dissertation, hydroxyl-terminated polybutadiene grafted multi-walled carbon nanotubes (HTPB-MWNTs) prepared through "graft to" method were incorporated in a polyurethane (PU) matrix based on poly(ethylene oxide-tetrahydrofuran) and aliphatic polyisocyanate resin as a curing agent. The mechanical properties, thermal conductivity and electrical resistivity of PU/HTPB-MWNT compostises were studied. The results showed that HTPB shell formed on MWNTs enhanced the dispersion of the MWNTs and their interfacial adhesion with the PU matrix and MWNTs, thereby improved the mechanical properties of the composites. Meanwhile, the HTPB shell retained the high electrical resistivity of these composites. Owing to above reasons, the interface thermal resistance between PU and MWNTs was decreased. There by increased the thermal conductivity of composites with 0.5 to 1 wt% MWNTs by 48.6% and 58.8% compared to neat PU. The apparent viscosity of the PEOT/HTPB-MWNT system was lower than PEOT/p-MWNT, result in better processing performance.Compared with the linear chain polymers, hyperbranched polymers have a dense spherical structure, the low bulk and solution viscosity, and rich -OH end groups. Therefore, we prepared hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotubes (HPU-MWNTs) through "graft from" method, and incorporated it in PU matrix. The effects of the HPU coating on PU/MWNT composite materials were studied, and compared with PU/HTPB-MWNT composites. MWNTs with 9-12 nm thick HPU shell had better dispersibility, compatibility and interfacial bonding strength with PU matix, and further improved thermal stability and mechanical properties of the PU/HPU-MWNT composites. And the composite had better electrical resistivity. The thermal conductivities of composites were increased with 0.5 to 1 wt% MWNTs by 58.2% and 67.8% compared to neat PU. Rheological tests showed that, apparent viscosity of PEOT/HPU-MWNT was lower than PEOT/HTPB-MWNT system. PEOT/HPU-MWNT system had better processing performance.The inorganic materials coated carbon nanotubes could endow CNTs with new new physical and chemical properties, resulting in the further development for their applications. Silica coated multi-walled carbon nanotubes (SiO2@MWNTs) with different coating thickness were synthesized by sol-gel method, and compounded with PU to prepare the PU/SiO2@MWNT composites. The effect of SiO2@MWNTs on the thermal conductivity and electrical insulation of PU were investigated. The results showed that the silica layer improved the interfacial interaction between MWNTs and PU, and enhanced the dispersion of MWNTs in PU matrix. Due to the electrical insulation of silica layer, PU/SiO2@MWNT composite had almost the same electrical insulation as neat PU. At the same time, the silica intermediate layer alleviated the modulus mismatch between the stiff MWNTs and the soft PU, decreased the probability of phonons scattering, and enhanced the thermal conductivity of the PU/SiO2@MWNT composites. The thermal conductivity of the PU/SiO2@MWNT composite was increased with the coating thickness.In order to enhancing the thermal stability of PU/MWNT composites, we synthesized a polyhedral oligomeric silsesquioxane grafted multi-walled carbon nanotubes (POSS@MWNTs), and incorporated it in PU matrix. POSS coating retained the electrical resistivity of these composites, improved the thermal conductivity of PU/MWNT composites, and enhanced the thermal stability of both of the MWNTs and PU/MWNT composite.