Preparation And Properties Of Silicon-Containing Polyimides And Their Composite Films

Aromatic polyimides possess excellent thermostability, mechanical property, electrical property and solvent resistance, and have been used in such applications as automobile and aircraft parts and packaging in printed electronic circuitry. Recently, syntheses of silicon-containing polyimides and their application are one of focuses in polyimides studies, since introduction of silicon-containing groups or polysiloxane segment into backbone of polyimides can make great improvement in their processability, solubility, impact resistance, anti-degradation in rich-oxygen environments and anti-ultraviolet. Moreover, these silicon-modified polyimides can also improve their adhesion with inorganic materials, including metal materials. By using their remarkable adhesive capacity, many sorts of polyimide nanocomposites can be prepared. In this thesis, a series of silicon-containing polyimide/barium titanate, or mesoporous silica nanocomposite films were prepared and characterized.Firstly, bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride (SIDA) monomer was synthesized through Wurtz coupling reaction, oxidization and cyclization reaction. The silicon-containing copoly(amic acid)s were synthesized from SIDA, pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (4,4′-ODA) in N,N-dimethylacetamide (DMAc). The copolyamic acid films were obtained by solution-cast method from DMAc solutions and thermally converted into transparent, flexible and tough copolyimide films. All copolyimides possessed amorphous character, and the regulation of those copolyimides decreased with the increase of the molar ratio of SIDA to PMDA. Introduction of silicon-containing group to polyimide backbone would make glass transition temperature shift to lower temperature. The decomposition temperature of the silicon-containing copolyimides decreased with the increase of the molar ratio of SIDA to PMDA. However, the optical transparency of the silicon-containing copolyimide thin films was superior to that of PMDA/4,4′-ODA polyimide thin films.Two silicon-containing polyimides were synthesized by solution polycondensation of SIDA with 3,4′-oxydianiline (3,4′-ODA) and 4,4′-ODA respectively. Physical properties of thin films of those polyimides were determined with the help of DSC, TGA, UV-visible spectroscopy and dynamic mechanical analysis. The polyimide from SIDA and 3,4′-ODA exhibited excellent energy-damping characteristic, mechanics properties and optical transparency, while that from SIDA and 4,4′-ODA possessed higher glass transition temperature and thermostability. Owing to the unsymmetric structure of the polyimide from SIDA and 3,4′-ODA, its increasing rate of linear coefficient of thermal expansion with temperature was quicker than that of the polyimide from SIDA and 4,4′-ODA.Secondly, SIDA, PMDA and 4,4′-diamino-3,3′-dimethyldiphenylmethane (MMDA) were used to prepare polyimides with good organo-solubility and light color. Intrinsic viscosities of polyimides in N,N-dimethylformamide (DMF) were ranged from 0.41 to 0.73 dL/g. These polyimides exhibited excellent solubility in the aprotonic polar organic solvents such as N,N-dimethylformamide (DMF), DMAc and low boiling point solvents like tetrahydrofuran. Introduction of dimethylsilylene group to the polyimide backbone would make the glass transition temperature shift to lower temperature, but optical transparency of the polyimides could be improved. The SIDA/MMDA polyimide had the highest thermal decomposition temperature in air. However, the copolyimide prepared from SIDA, PMDA and MMDA had the highest thermal decomposition temperature under nitrogen.Two polyimides containing silicon-ether-sulphone groups were synthesized by solution polycondensation of SIDA with bis[4-(3-aminophenoxy)-phenyl]sulfone (mBAPS) and bis[4-(4-aminophenoxy)-phenyl]sulfone (pBAPS), respectively. SIDA/pBAPS polyimide was semicrystalline polymer, whereas SIDA/mBAPS polyimide was amorphous polymer. Whether in air or in nitrogen, the two polyimides possessed higher thermal stability and antioxidation property. The variety of group position in polyimide molecular chains had little influence on the thermal stability and antioxidation property. The flexility, impact resistance at lower temperature, transparency in ultraviolet-visible area and solubility in organic solvents of polyimide prepared by SIDA and mBAPS were better than those of polyimide prepared by SIDA and pBAPS.Thirdly, SIDA?3,4′-ODA polyimide/BaTiO3, SIDA?4,4′-ODA polyimide/BaTiO3, SIDA?pBAPS polyimide/BaTiO3 and SIDA?mBAPS polyimide/coupling agent/BaTiO3 nanocomposite films were prepared by adding directly BaTiO3 nanoparticles or BaTiO3 nanoparticles treated with coupling agent into the poly(amic acid)s, followed by thermal imidization. The results showed BaTiO3 nanoparticles could not be dispersed uniformly in polyimides, the particles trended to form clusters. The clusters coalesced into more uniform structure at higher BaTiO3 filling than at lower BaTiO3 filling. That the coupling agent was added into the polyimide/BaTiO3 nanoparticles composites could not improve disperse of BaTiO3 nanoparticles in the polyimide. All polyimide/BaTiO3 nanocomposite films had higher thermal stability in air and glass transition temperature than those of the corresponding polyimides.The measurement of dielectric properties indicated that the relative dielectric constant of SIDA?3,4′-ODA polyimide/BaTiO3, SIDA?4,4′-ODA polyimide/BaTiO3, SIDA?pBAPS polyimide/BaTiO3 nanocomposite films decreased slowly with the increase of frequency. At 1MHz, the dielectric loss tangent value of these polyimide/BaTiO3 nanocomposite films was lower than the corresponding polyimides. And the relative dielectric constant of these polyimide/BaTiO3 nanocomposite films firstly descended then ascended with the increase of temperature.The measurement of infrared emission spectrum indicated, the addition of proper amounts of BaTiO3 nanoparticles into polyimides prepared by SIDA and 3,4′-ODA or SIDA and 4,4′-ODA could decrease the infrared emissivity of polyimide/BaTiO3 nanocomposite films, however the addition of BaTiO3 nanoparticles into polyimide prepared by SIDA and pBAPS made the infrared emissivity of polyimide/BaTiO3 nanocomposite film increase.Finally, Polyimide/mesoporous silica composite films were prepared by direct mixing of polyamic acid solution and silylated mesoporous silica particles, or by condensation polymerization of dianhydride and diamine with silylated mesoporous silica particles in DMAc, followed with thermal imidization. The results showed that the silylated mesoporous silica particles in the composites tended to form the aggregation with a strip shape due to phase separation. The composite films exhibited higher glass transition temperature as comparing with that of pure polyimide. It was found that the composite films presented lower infrared emissivity value than the pure polyimide and the magnitude of infrared emissivity value was related to the content of silylated mesoporous silica in the composite films. Inhibiting actions of silylated mesoporous silica on infrared emission of the composite films may be owing to presence of nanometer-scale pores in silylated mesoporous silica.In summary, silicon-containing polyimides and its nanocomposite films have outstandingly different properties from the other traditional polyimides. Hence, it has extensive application foreground in aviation, electric/electron, gas separation, preparation of wave-absorption materialsand other high-tech fields.