Structural Design Of Low Dielectric Constant Polyimide And Fabrication Of The Fibers With High Performance

Aromatic Polyimide(PI)has been widely used in aerospace,microelectronics,industrial filtration etc,owing to its excellent integrative properties and designability.In recent years,with the rapid development of national defense,the wave-transmission composite material for radome has attracted increasing attention.Given severe service environment of radome,wave-transmission reinforcement fiber was required to to be of low dielectric,high specific strength and good thermal stability.However,the permittivity of currently commercialized polyimide fiber materials is between 3.2 and 3.8,which cannot well meet the requirements of wave-transmission materials.Therefore,it has become an urgent problem to further reduce the permittivity of polyimide while retaining its high performance.At present,there are many modification methods to reduce the dielectric constant of polyimide materials.Among them,the introduction of fluorine atom into PI chains can effectively bind the electron cloud,which can significantly reduce the dielectric constant of polyimide.PI blending with Nanoparticles can effectively limit the movement of molecular chains in the orientation polarization process,and increase the free volume between the molecular chains,so as to improve the dielectric properties of polyimide without significant performance degradation.Based on the discussion above,an amino-modified fluorine-containing hyperbranched polysiloxane(NH2-FHBPSi)was synthesized,and then introduced into the fully rigid(BPDA/p-PDA/BIA)polyimide by in-situ polymerization.NH2-FHBPSI/PI composite films and fibers with low dielectric constant and good comprehensive properties were prepared by "two-step" process.The structure-activity relationship between NH2-FHBPSi content and polyimide properties was studied.research results are as follows:1.Three silane coupling agents,?-aminopropyl methyl diethoxysilane(APDES),trifluoropropyl trimethoxy silane(TPTMS)and tetraethyl orthosilicate(TEOS),were using to prepare NH2-FHBPSi by hydrolysis-condensation method.The results showed that the amino group was successfully introduced and maintained in the preparation of hyperbranched polysiloxane with the content of 0.45?0.47 mmol/g.In addition,NH2-FHBPSi has obvious branched structure and only a small amount of mesopores and micropores inside.The average particle size of NH2-FHBPSi in ethanol is 45.6 nm,and can be dissolved in DMAc,NMP,DMSO and other polar aprotic solvents.Excellent solubility of NH2-FHBPSi will be conducive to in-situ modification with polyimide.2.NH2-FHBPSi nanoparticles were introduced into the PI chains by in-situ polymerization,and a series of PI composite films with different NH2-FHBPSi contents were obtained.The results show that the dielectric properties and moisture resistance of PI composite films have been improved obviously.When NH2-FHBPSi content is 10 wt%,the dielectric constant is reduced to2.61;the water absorption decreases by 34.3%;and the water contact Angle increases from 73.3° to92.8°.However,,when the NH2-FHBPSi content is more than 5 wt%,agglomeration appears due to the compatibility problem between fluorine-containing nanoparticles and PI matrix,which results in the decrease of mechanical properties,transparency and thermal properties of the films.3.PI composite fibers were prepared from NH2-FHBPSi/PAA spinning slurry with solid content of 13 wt% by wet forming process and thermal cyclization.In the process of wet fiber forming,when the ratio of H2 O to DMAc at coagulating bath is 4:6,cross profile of the nascent fibre is round,and the surface is smooth without defects.Using wide angle X-ray diffraction(WAXD)on fiber microstructure changes induced by thermal cyclization and the drawing of heat,the results show that diffraction peaks at meridian direction gradually strengthen with the increase of drawing ratio.And the appearance of two new diffraction peaks(2?= 6.7° and 31.3°)means that more orderly three-dimensional stacking structure is formed as well as the increase of molecular orientation along the fiber axial direction by thermal drafting process.When the drafting ratio reached 2.4,the tensile strength and modulus reached 2.83 GPa and 129.78 GPa,respectively.It was also found that with the increase of NH2-FHBPSi content,the shape and position of the peak in the equatorial direction did not change and the fiber was amorphous in general,while the 2? of the diffraction peak in the meridian direction decreased gradually.The corresponding d-spacing is increased by 0.06 (?).In terms of mechanical properties,when the NH2-FHBPSi content was 3 wt%,the tensile modulus and tensile strength of the fibers were increased by 27.1% and 1.7%,respectively.With the further increase of NH2-FHBPSi content,the modulus and strength of the fibers decreased significantly because of agglomeration.