Preparation And Properties Of Microfoamed Polyimide Film

In recent years,with the development of 5G communication technology,the demand for substrate materials is higher and higher.5G has the advantages of high speed,high frequency and reducing delay,which requires the substrate material to have low dielectric constant.Polyimide materials have attracted wide attention because of their outstanding thermal stability,mechanical properties and insulation.The dielectric constant of the packaging film used in 5G communication substrate material is required to be low in order to ensure the frequency of signal transmission,but the dielectric constant of traditional polyimide film is on the relatively high side.Generally speaking,the dielectric constant of foaming material is much lower than that of its bulk material.So the development of microfoamed polyimide film can broaden the application of polyimide materials in electronic industry,which has important theoretical and practical significance.In this paper,sixteen kinds of polyimide（PI）foamed films were prepared by adjusting the mass ratio of BTDA to PAPI and monomer concentration in the polymerization system by using BTDA and PAPI as polymerization monomers and DMAC as solvent.PAA solution was synthesized by BTDA and ODA,and BTDA and PAPI were added to PAA solution.By adjusting the mass of BTDA and PAPI,the microfoamed PI film with smaller diameter and better comprehensive properties was obtained,and the effect of post-treatment on the foam and other properties of the film was investigated by changing the vacuum time and the initial heating temperature.PMDA and ODPA were used instead of BTDA,to explore the effects of different anhydride monomers on the foaming effect and other properties of microfoamed PI films,hoping to improve their toughness.The thermal decomposition characteristics of BTDA-ODA-PAPI,PMDA-ODA-PAPI and ODPA-ODA-PAPI type microfoamed PI films were analyzed by thermal decomposition kinetics.The chemical structure,mechanical properties,thermal stability,cell size,contact angle and insulation of microfoamed PI films were studied by infrared spectrometer,universal tension tester,thermogravimetric analyzer,optical microscope,insulation resistance tester,contact angle tester and so on.The results indicate that the foaming effect of the film is the best when the mass ratio of PI and monomer is 1.4 and the concentration of polymeric monomer is 60 wt%,but the average cell diameter is larger,which is more than 100 um.When the content of BTDA and PAPI in PAA solution is 200%,bubbles in the film are small and uniform.The average diameter of the bubble is 2.52μm,the number of bubbles per unit area is 6.3×10⁵,the tensile strength is 97.42 MPa,T_5%and T_10%is 497.59℃and 528.37℃,respectively.When the vacuum time is 30 min and the initial heating temperature of the oven is 30℃,the bubble size of the film is smaller and the comprehensive properties are better.The average bubble diameter is 1.77μm,the number of cells per unit area（per square centimeter）is6.1×10⁵,the tensile strength is 108.02 MPa,T_5%and T_10%is 500.18℃and 530.76℃,respectively.BTDA-ODA-PAPI type microfoamed PI film has the largest bubble number and the smallest cell size.ODPA-ODA-PAPI type microfoamed PI film has the best toughness,elongation at break of 10.76%,average bubble diameter of 3.65μm,bubble number of 5.3×10⁵per unit area（per square centimeter）,and good performance in heat resistance and insulation properties.Therefore,the toughness of microfoamed PI films can be improved by appropriately adjusting the molecular chain flexibility of the polymer,and a better comprehensive performance can be achieved.Through the analysis of thermal decomposition kinetics of PI foamed films,it is found that the activation energies of PMDA-ODA-PAPI type,BTDA-ODA-PAPI type and ODPA-ODA-PAPI type microfoamed PI films decrease gradually in the same atmosphere.The thermal decomposition of three kinds of microfoamed PI films in N₂ambient is one step,and the thermal decomposition in atmosphere is divided into two steps.