| Silicone materials are widely used in the fields of electronic appliances,erospace,construction,chemical industry and transportation because of their good resistance to high and low temperature,radiation resistance,oxidation resistance and weather resistance.However,conventional silicone still has poor thermal conductivity and mechanical properties.The unique structure and excellent electrical,thermal and mechanical properties of carbon nanotubes make them ideal fillers for high-performance polymer-based composites.However,carbon nanotubes tend to agglomerate and have poor compatibility with polymers.Therefore,the first problem to be solved for carbon nanotubes as filler is how to prevent the carbon nanotubes from agglomerating and to improve the compatibility between the polymer and carbon nanotubes.This dissertation mainly discusses the preparation of liquid crystal-carbon nanotubes/silicone nanocomposites by using three different liquid crystal modified carbon nanotubes as the filler of organic silicon.In addition,the influence of the liquid crystal induced carbon nanotube orientation on the organosilicon composite material after application of an electric field was also investigated.Specific content includes:(1)In this thesis,three types of liquid crystal molecules: 4-methoxybenzylidene-4'-n-butylaniline(MBBA),4'-(Octyloxy)-4-biphenylcarbonitrie(8OCB)and 4-[4-(4-ethenylcyclohexyl)cyclohexyl]-1,2-difluorobenzene(ECFB)were used to modify carbon nanotubes.We characterized the liquid crystal modified carbon nanotube composites by fluorescence spectrometer,UV-Vis spectrophotometer,infrared spectroscopy,XRD and TGA.Research results showed that carbon nanotubes were combined with three kinds of liquid crystal molecules by non-covalent bonding(?-? stacking).(2)Silicone nanocomposites were prepared by using three kinds of liquid crystal modified carbon nanotubes as fillers,respectively.The morphology of the composites was observed by SEM and TEM.The results showed that after the modification of the liquid crystal,the dispersibility of MWCNTs in the organic silicon was improved obviously and the compatibility with the silicone was also correspondingly enhanced.The mechanical properties and thermal properties of the silicone were improved obviously as well.When the content of carbon nanotube composites was 1 wt%,the tensile strength of the three liquid crystal-carbon nanotube/silicone composites reached the maximum.The tensile strengths of MBBA-MWCNTs/organosilicon composites,ECFB-MWCNTs/organosilicon composites and 8OCB-MWCNTs/organosilicon composites were 6.78 MPa,8.85 MPa and 11.83 MPa respectively,which were 36%,78%,138% higher than that of compared with the same content of MWCNTs/silicone composites.When the mass fraction of MWCNTs was 14wt%,the thermal conductivity of MBBA-MWCNTs/silicone composites reached 0.5615 W/(m · K),which was about four times higher than that of pure silicone and 54% higher than that of MWCNTs/silicone with the same mass fraction.The thermal conductivity of ECFB-MWCNTs/silicone composites reached 0.8101 W/(m·K),which was about 5.3 times that of pure organic silicon materials,and was 122% higher than that of the same mass fraction of MWCNTs/silicone materials.The thermal conductivity of the 8OCB-MWCNTs/silicone composites was 0.9543 W/(m·K),which was about 6.2 times that of the pure silicone material and 162% higher than that of the same mass fraction of the MWCNTs/silicone material.(3)Under the effect of an applied electric field,polarized light microscopes and computer-measured polarization experimental detectors revealed that the two liquid crystals,ECFB and 8OCB,would deflect along the direction of the electric field as the voltage changes,indicating that the two liquid crystals had a photoelectric effect.ECFB-MWCNTs and 8OCB-MWCNTs composites were used as the fillers of organic silicon matrix respectively.During the curing process of composites,carbon nanotubes(CNTs)were induced by electric field to prepare highly oriented carbon nanotube thermal conductive composites.The thermal conductivity of ECFB-MWCNTs/silicone and 8OCB-MWCNTs/silicone composites was significantly improved after the application of electric field.When the mass fraction of carbon nanotubes was 11 wt%,the thermal conductivity of ECFB-MWCNTs/silicone composites after applying an electric field was 1.5112 W/(m·K),which was 4 times that of MWCNTs/silicone composites after applying an electric field,which was 883% higher than that of the pure silicone substrate,about 93% higher than that of the same sample without the electric field and about 326% more than the same mass fraction of MWCNTs/silicone composites.When the mass fraction of carbon nanotubes was 11 wt%,the thermal conductivity of 8OCB-MWCNTs/silicone composites after applying an electric field was 2.3574 W/(m·K),which was 6.5 times that of MWCNTs/silicone composites after applying an electric field,which was about 1433% higher than that of the pure silicone matrix,about 160% higher than the same sample without the electric field and about 565% more than the same mass fraction of MWCNTs/silicone composites.(4)A series of studies had shown that liquid crystals effectively improve the dispersion of carbon nanotubes in the silicone matrix and improve the interface between the carbon nanotubes and the matrix.An electric field was applied to the liquid crystal functionalized carbon nanotube organic silicon composite material,and through the study of the relationship between the structure and the properties,the field induction effect of the liquid crystal functionalized carbon nanotube composite material and the mechanism of the performance enhancement were revealed.It was found that under the action of an electric field,liquid crystals can induce the alignment and distribution of carbon nanotubes along the direction of the electric field.The uniform direction of carbon nanotubes can reduce the collision and scattering of phonons,which was more conducive to forming more thermal conduction paths in organic silicon composite materials and improve the thermal conductivity of composite materials. |
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