Fabriation Of Surface-Silvered Polyimide Films And The Mechanism Investigation

Surface-silvered polyimide(PI)film,due to the combination of the excellent thermal and mechanical properties of the polyimide matrix and the unmatched optical reflectivity and electrical conductivity of the surface silver(Ag)layer as well as its many other outstanding performances such as flexibility,light weight,and high strength,has been found widely attractive in aerospace and microelectronic industry and therefore has been extensively studied in recent years.In this dissertation,we report our efforts on the preparation of such silver-metallized polyimide films via two different techniques,in-situ single-stage self-metallization and direct ion-exchange self-metallization.The surface reflectivity and conductivity,thermal and mechanical properties of the composite film were characterized.Factors influencing the film metallization process and related mechanism were investigated.With the in-situ single-stage self-metallization protocol,surface-silvered polyimide film has been prepared using pyromellitic dianhydride/ 4,4'-oxydianiline(PMDA/ODA)-based polyimide as the matrix and (1,1,1-trifluoro-2,4-pentadionato)silver(AgTFA)as the silver precursor. Surface reflectance of the metallized film increases with increasing silver concentration.However,the reflectivity for the films prepared from this system is relatively low,with a value in the range of 20～40%.Electrical conductivity was obtained only under rapid thermal treatment cycle and the films were prepared with surface resisitivity of ca.5Ωsq^-1.However,because of the rigid macromolecule structure,the thermal stability of the metallized film was greatly reduced and serious decomposition would occur on the film after 4 h at 300℃.Block and random pyromellitic dianhydride - 4,4'-oxydiphthalic anhydride /4,4'-oxydianiline(PMDA-ODPA/ODA)-based copolyimides were synthesized in our work and used as the matrix to investigate the influence of copolymerization ratio and sequence on the properties of the metallized films. For the silvered random copolyimide,only 40%reflectivity was achieved and no conductivity was observed.However,both reflectivity and conductivity were obtained on the block matrix with a maximum reflectivity over 55%and surface resistivity of ca.5Ωsq^-1,which is suggested to be due to the synergistic effect of the PMDA/ODA block chain and the ODPA/ODA block chain existing in the block copolyimide.The final metallized films retain the essential thermal and mechanical properties of the pristine polyimide film.The step-wise variation of reflectivity during the thermal curing cycle was fully clarified through scanning electron microscopy(SEM)and X-ray photoelectron spectroscopic(XPS)analysis.The relationship between surface properties and surface morphology has been established.Silver reduction, aggregation and the formation of surface silver layer has been traced in the work using transmission electron microscopy(TEM)and X-ray diffraction (XRD).And the mechanism pertaining to the silver layer formation has been discussed.A direct ion-exchange self-metallization approach was developed in our work and double-surface-silvered polyimide film has been fabricated through the ion exchange of damp-dry poly(amic acid)(PAA)films in aqueous silver ion solution and subsequent thermal treatment.In this dissertation,three different polyimides including PMDA/ODA,ODPA/ODA and 3,3',4,4'-benzophenonetetracarboxylic dianhydride - 4,4'-oxydianiline (BTDA/ODA)were used as the matrices and three different simple silver salts including silver nitrate,silver fluoride and silver ammonia complex cation were employed as the silver origins for the film preparation.The results suggest that both polyimide structure and silver structure have significant influence on the surface reflectivity and conductivity of the final metallized film.Films prepared using BTDA/ODA-based polyimide as the matrix achieved very high reflectivity and conductivity.The optimum one was the film prepared from the BTDA/ODA-AgF system,with maximum reflectivity over 80%/ 100%and surface resisitivity of 0.6 / 0.2Ωsq^-1on the upside and underside,respectively.Silver ammonia complex cation has the highest efficiency for polyimide silver metallization.Films with maximum reflectivity of 81.8%/ 93.1%and surface resistance of 0.6 / 0.6Ωsq^-1on the upside/underside could be fabricated by employing very dilute silver ion solution(0.01 M)and very short ion exchange time(5 min).Surface differences were observed on the metallized film,that is,the reflectivity and conductivity of the underside are always superior to that of the upside.And surface metallization usually(?)curs earlier on the underside.It is suggested that slight imidizaiton occurring on the upside of the poly(amic acid)film should be responsible for all these differences.SEM,XRD,XPS,TEM and ultraviolet/visible(UV/Vis)analysis were carried out on the films cured at different thermal stage to investigate the film metallization process.We have clarified the silver reduction and aggregation process and established the relationships of surface properties with the surface morphology and the surface composition of the hybrid films.The migration of small silver particles from the near-surface bulk to the top surface of the film was observed by TEM in our work.However,we figure that this effect doesn't play very important role in the formation of well-established silver layer.And we have demonstrated through differential scanning calorimetry(DSC), thermogravimetry(TGA),XPS and SEM analysis that the formation of highly reflective and conductive silver layers on polyimide film is a result of the silver-catalyzed and oxygen-assisted decomposition of the polymer overlayer and the self-accelerated aggregation of silver clusters on the film surface.The surface silver layers were well-boned to the underlying polyimide.XPS analysis indicates that there are strong chemical bonging interactions between the silver and the polymer groups.However,silver present on the surface mainly exists as native metal in the form of face-centered-cubic(FCC)crystal.The ion exchange process between poly(amic acid)and silver ions was extensively studied in our work.Detailed chemical interactions occurring between the macromolecule and silver ions have been disclosed through Fourier transform infrared spectroscopy(FTIR)and XPS analysis.A metal-ion-induced cross-linking behavior of the matrix was observed during the loading of silver into poly(amic acid)via ion exchange.We have developed a method and measured the mass loss of poly(amic acid)during the ion exchange process.The results indicate that water molecules have very weak damaging effect on the precursor,while the silver ions have strong catalytic and accelerated effect on the hydrolysis of poly(amic acid) molecules.Compared to the pristine film,the thermal stability of the hybrid films in air were ca.130～160℃degraded after the incorporation of silver.However,the final metallized film maintains the basic mechanical properties of the polyimide matrix.Recently,the application of the direct ion-exchange self-metallization approach was extended in our work.Monodispersed silver nanocubes have been fabricated via this method with silver adding amount no more than 1 mol%by employing water-soluble PMDA/ODA-based poly(amic acid)as the intermediate and silver nitrate as the silver origin.The size of silver particles could be controlled in the range of 90～150 nm by adjusting the thermal treatment time.