Preparation And Properties Of Carbon Nanotubes/Polyimide Nanocomposites

In order to extensively improve the mechanical and thermal properties of polyimides,a type of high performance polymers,their modification has become one of the focuses in recent studies.Since the discovery of carbon nanotubes by Iijima in 1991,they have attracted considerable attentions because of their excellent mechanical,electrical,and other properties,and brought a widespread research for their unveiling the fundamental properties,fabrication techniques and novel applications. Therefore,as one of 1-D nanomaterials,carbon nanotubes are one of important candidates to modify polyimide materials.In this thesis,MWNTs were modified firstly using concentrated acid of H₂SO₄ and HNO₃,and then octadecyl isocyanate was grafted onto the surface of the MWNTs to improve the dispersion and the interaction between the filler and the matrix.Fourier transform infrared spectroscope (FTIR),Raman,transmission electronic microscope(TEM),elemental analysis(EA),and thermal gravimetric analysis(TGA)were employed to characterize the functionalized MWNTs.The result indicated that octadecyl isocyanate was successfully introduced onto the surface of the MWNTs,and the dispersion of nanomaterials was thus improved.Polyamic acid(PAA)solution were synthesized from pyromellitic dianhydride(PMDA)and 4,4'-diaminodiphenyl(ODA)using solution polycondensation with N,N'-dimethylacetamide(DMAc)as a solvent. The MWNT/PAA solution was prepared by mixing PAA solution and MWNTs in the solvent.The dynamic rheological properties of MWNT/PAA solutions were studied.As a result,the addition of the functionalized MWNTs in the matrix resulted in the increase of storage modulus(G')and loss modulus(G");whereas,the loss tangent(Tanδ) decreased as increasing MWNTs in the nanocomposites,indicating the obvious reinforced effect of the nanotubes.The increase of complex viscosity(η^*)with the addition of the MWNTs revealed that the nanocomposites containing nanotubes exhibited a very strong shear thinning effect.MWNT/PI films were obtained by casting MWNT/PAA solution, solvent evaporation and thermal imidization.FTIR,SEM,TGA,DMA and tensile instrument were employed to characterize morphologies and the properties of the nanocomposites.The appearance of the characteristic absorption peaks of PAA and PI at the FTIR spectra indicated that the transformation of PAA to PI.SEM images of the cross-section of the nanocomposites exhibited a good dispersion,which was helpful to improve the interracial interaction of the nanotubes and the matrix.The thermal stability and the decomposition kinetics of the nanocomposites were studied using TGA methods.As a result,the thermal decomposition activation energy of MWNT/PI nanocomposites were improved from 370KJ/mol of the pure polyimide to 449.2KJ/mol of 10%MWNT/PI nanocomposites,indicating the good thermal stability of the nanocomposites.Meanwhile,DMA results showed that glass transition temperature T_g increased,and T_g of the nanocomposite containing 5% MWNTs was up to 415℃from 400℃of the pure polyimide.Upon increasing MWNTs in the matrix,the mechanical property of MWNT/PI nanocomposites was greatly improved.The tensile strength of nanocomposite containing 7%MWNTs was up to the maximum 130 MPa, 46%higher than the pure polyimide.