Fabrications And Proformances Of Novel Polyimide Nanohybrid Films

Polyimide?PI?is a joint name of the same category organic polymer whose backbone contains acid imide loop“-CO-NH-CO-”.It has been extensively applied in aerospace,medical apparatus and instruments and microelectronic industry and fields,owing to unique physical and chemical properties,for example excellent mechanical strength,high dielectric properties,etc.However,with the rapid development of high technology,for example,electronic products become the thinner,lighter and more compatible and the lifetime of the high-voltage motor device also demands longer,lots of industries and relational fields require the materials with higher properties.As a result,the dielectric and mechanical properties of PI can no longer meet the extreme demands and the applications of PI are severely limited.Meanwhile,the conventional PIs with rigid backbone structure have indissolvable,refractory,machine shaping difficulty imperfections,et al.which limited also severely the applications of PIs.In recent years,many research groups devoted oneself to developing PI/inorganic hybrids and it has been shown that the unique properties of PI can be further improved by the careful selection of preparative approaches and the rational inclusion of inorganic nanoparticles.This is due to that the nano-sized particles differ from their bulk counterparts and possess unique advantages in enhancing the properties of the organic polymer matrix because of quantum size effect,surface effect,macro quantum tunnel effect and dielectric confinement effect.Therefore,PI/inorganic nano-hybrid materials have become an attractive topic.In order to improve processibility,mechanical,dielectric and thermo-stabilization properties,according to the relationship between molecular structure and performance,PI derived from pyromellitic dianhydride?PMDA?and a flexible diamine 4,4-bis?3-aminophenoxy?biphenyl?4,3-BAPOBP?that was comfirmed as the copolymerization monomer because its main chain possesses two ether bonds?-O-?and is more flexible than a conventional diamine,such as 4,4'-oxydianiline?4,4'-ODA?.Owing to well mechanical and dielectric properties and thermal stability,nano-sized alumina?Al₂O₃?,silica?SiO₂?,titania?TiO₂?and mica were selected and embedded into PI matrix,respectively.In the work,PI/inorganic hybrids with the improved mechanical,dielectric and thermal stability properties were prepared,dispersion of the inorganic particles in the matrix were investigated and the effect of nanoparticle contents on the properties of the matrix were also carefully studied and mainly reported.The main research works can be described as following:?1?Polyamic acid/nanoparticles?PAA/nanoparticles?is the precursor of PI nano-hybrid films whose process conditions is very important.For obtaining excellent PI nano-hybrid films,the nano Al₂O₃ and intrinsic viscosity were selected as the filler and the studying index,respectively.The optimal conditions for synthesizing PAA/Al₂O₃ were determined by the orthogonal experiments and the single factor experiments.The effects of the molar ratio of 4,3-BAPOBP and PMDA,the reaction temperature,the solid content and the reaction time on the intrinsic viscosity of PAA/Al₂O₃ were studied.The results showed that the optimal conditions were as follows:the molar ratio of 4,3-BAPOBP and PMDA was 1:1.01¹:1.02,the reaction temperature was 15²5?,the solid content was 18²0%,and the reaction time was about 5⁶ h.Based on the optimal conditions,the PI films were synthesized by using PMDA and three kinds of diamine monomers containing the flexible groups,respectively.The PI films'performances were tested and the relationship between the molecular structure and the properties was analyzed.Finally,PI?4,3-BAPOBP/PMDA?with excellent thermal stability and thermal processing was confirmed as the modified matrix by nanoparticles.?2?To improve the properties of PI,different mass fractions of Al₂O₃nanoparticles,unmodified or modified by KH550,were incorporated into PI matrix to form PI/Al₂O₃ hybrid films by in situ polymerization and thermal imidization method.The effects of Al₂O₃ additives on the structure,dielectric and mechanical properties,and glass transition temperatures?T_g?of the films were studied by scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,Fourier transform infra-red spectroscopy?FTIR?and other methods.Meanwhile,the dielectric and mechanical properties of the PI/modified Al₂O₃ hybrids were compared with the system in which unmodified Al₂O₃ nanoparticles were used in the PI matrix.The microstructures of the samples showed that the modified and unmodified Al₂O₃ inorganic particles were well dispersed in the matrix using in situ polymerization assisted by ultrasonic and mechanical methods.The results of performance testing revealed the dielectric and mechanical properties of PI films could be greatly enhanced with incorporating Al₂O₃ into the PI matrix.When modified Al₂O₃ content was 8 wt.%,the electrical breakdown strength increased from 254 to 311 MV/m and the tensile strength reached increased from 75.6 to 93.9MPa.?3?In preparing PI nano-hybrid films,the degree of dispersion of the nanoparticles is of great importance because it determines the expected properties of the composites so prepared.To overcome dispersion issues,coupling agents are usually added.But it has been proved that any residual coupling agents may reduce important properties of the composites.In order to obtain homogeneous nanoscale dispersibility and simplify the preparation technology of PI nano-hybrid films,hydrophobic aerosil was selected as the nano-SiO₂ precursor.A number of PI/SiO₂ hybrid films were prepared with 0,2,4,6,8 and 10 wt.%inorganic nanoparticle doping assisted by nanoparticles pre-dispersion technology,ultrasonic and mechanical methods.The results of FTIR and SEM showed that PI/SiO₂ nano-hybrid films could be successfully prepared by the experimental design scheme and SiO₂ distributed uniformly in the PI matrix.The test results of the mechanical properties showed that the tensile strength and Young's modulus of films could be markedly improved by loading the SiO₂ nano-particles into the PI matrix and reached their maximum values of 92.1 MPa and 1.82 GPa at a SiO₂content of 8 wt.%,respectively.According to the test results of the dielectric properties,we found that at a SiO₂ content of 8 wt.%,the breakdown strength increased by 28.3%,compared with the neat PI.?4?A series of PI/TiO₂ nano-hybrid films have been successfully prepared.The microstructure,dielectric and mechanical properties of the samples were characterized and measured by SEM,X-ray diffraction?XRD?,FTIR,ultraviolet–visible?UV–Vis?spectroscopy,LCR meter and the electronic tensile testing machine,respectively.The results showed that TiO₂ nanoparticles were dispersed evenly in the PI matrix when the TiO₂ content was lower,and large agglomerations formed as the TiO₂ content was higher than 9 wt.%.At the same TiO₂ mass fraction,the electrical breakdown strength and tensile strength of films reached a maximum value,respectively.Furthermore,the dielectric constants of films increased monotonously with the increasing of the TiO₂nanoparticles content.?5?Mica is composed of a natural layered silicate structure and is a perfect enhancer owning to good electrical,mechanical and thermal insulating properties.The mica was well dispersed in the PI matrix.The dependence of morphology,T_g,dielectric properties and mechanical properties at room temperature of the hybrid films on the content of mica was discussed.T_g,the breakdown strength and tensile strength of the hybrid films,were the highest when the mica content was 8 wt.%.Compared with neat PI,the maximums increased in the electrical breakdown strength?375 MV/m?and tensile strength?94.9 MPa?of the prepared films were 47%and25.5%,respectively.Meanwhile,the dielectric constant of PI/mica hybrid films increased with the increasing in the mica content;at the frequency of 100 Hz,the dielectric constant increased from 3.8 to about 5.3.