Structure And Properties Of Graphene/polyimide Composite Films With Low Dieclectric Constant

In recent years,more and more interest has been focused on polyimide(PI)in the field of electronic packaging since its outstanding mechanical,thermal,and dielectric properties.However,it is required to further lower the dielectric constant of PI in order to minimize the crosstalk and to maximize the speed of signal propagation for the smaller and lighter microelectronic devices.There are three major strategies to lower the dielectric constant of PI.The one is to reduce the polarizability of PI,e.g.incorporation of fluorinated structure,crosslinking and/or alicylic structure.The another is to lower the density of PI,e.g.porous structure.The third is to employ nanoparticles,e.g.graphene derivatives and POSS.Based on the above researching background,it has been carried out in the dissertation to lower the dielectric constant of PI as well as improve its the mechanical properties.In the first part,functional graphene oxide(FGO)with photosensitivity was incorporated into the photosensitive polyimide(PSPI)matrix via solution blending,to prepare the photocrosslinkable nano-composite films.The crosslinked composite films were obtained through UV irradiation.Due to the confine of electronic and molecular polarization,the dielectric constant of the crosslinked FGO/PSPI films decreased effectively,which could be tuned by varying both the UV irradiation time and FGO loading.Among all the composite films,the film with 0.5 wt%FGO content exposed under UV light for 900 s possessed the lower dielectric constant(2.58).In order to further improve the mechanical properties of PI,fluorinated structure was introduced into graphene rather than PI molecular chains.Fluorine-containing graphene oxide(FCGO)was synthesized and employed to prepare FCGO/PI composite films.The dielectric constant of PI films with 0.6 wt%FCGO was decreased from 3.33 of pure PI to 2.34.Meanwhile,the tensile strength,tensile modulus and tensile energy to break of PI composite film were increased to 141 MPa,4.95 GPa and 31.6 MJ/m~3from 118 MPa,2.81 GPa and20.8MJ/m~3of pure PI,respectively.For the purpose of commercial production,a series of graphene fluoroxide/porous polyimide(GFO/pPI)nanocomposite films were fabricated.Polyethylene glycol(PEG)was introduced into the system to play the roles of an intercalator in exfoliation of graphite fluoroxide and a blowing agent in preparation of porous structure.Both the porous structure and the GFO nanosheets significantly decreased the dielectric constant of composite films,and the load transfer between GFO and PI matrix efficiently improved the mechanical properties of porou films.A very low dielectric constant of 2.29 as well as high tensile modulus of 4.43 GPa and tensile strength of 159 MPa were obtained of the composite film with the GFO loading at only 0.30 wt%.However,the porous structure of PI films formed via the method described above would collapse during the manufacture of flexible copper clad laminate.POSS was incorporated to fabricate the infrangible porous structure in PI.An alternating copolymer,synthesized from maleimide isobutyl polysilsesquioxane and 5-norbornene-2,3-dicarboxylic anhydride(poly(MIPOSS-alt-NA))via free radical copolymerization,was employed to prepare POSS/PI and GFO/POSS/PI composite films.Poly(MIPOSS-alt-NA)tended to form spherical nanoparticles in polar solvent during the preparation of PI composite films.The dielectric constant decreased to 2.09 of POSS/PI film containing 0.4 mol%poly(MIPOSS-alt-NA).At the same time,its tensile energy to break significantly increased to 23.5 MJ/m~3from 6.84MJ/m~3,althought the tensile strength and tensile modulus reduced to 96 MPa and 2.84 GPa from 107MPa and 3.25 GPa.Besides,the tensile strength and tensile modulus of GFO/POSS/PI composite film containing 0.4 mol%poly(MIPOSS-alt-NA)and 0.15 wt%GFO were re-improved to 123 MPa and 3.91 GPa.Meanwhile,the dielectric constant and tensile energy to break of GFO/POSS/PI and POSS/PI films were almost the same.Based on the above study,we prepared thermoplastic PI/PI/thermoplastic PI(TPI/PI/TPI)three-layer composite films.TPI was synthesized from 3,3',4,4'-biphenyl tetracarboxylic diandhydride(BPDA)and 4,4'-(4,4'-isopropylidenediphenyl-1,1'-diyldioxy)dianiline(BAPP),the middle-layer PI was synthesized from BPDA and para-phenylenediamine(PD).And the three-layer composite films were used to fabricate flexible copper clad laminate(FCCL).The mechanical propertis of three-layer film was similar to the middle-layer PI.The coefficient of thermal expansion(CTE)of three-layer film was 19.8 ppm/K,which is closed to that of copper foil.The peel strength of FCCL was up to 1.2 N/mm,which could meet the requirement of flexible printed circuit board.