Preparation And Properties Of Polyimide Films With Low Coefficient Of Thermal Expansion

In recent years,with the rapid development of flexible circuit and flexible display,it is an urgent need to enhance the high temperature resistance,low dielectric loss,excellent mechanical properties and low coefficient of thermal expansion(CTE)properties of polymer materials which are used in electronic products.Polyimide(PI)material is famous for its excellent comprehensive properties,and it is widely used in microelectronics industry.However,the CTE value of conventional polyimide is relatively high,and this high CTE limits its application in flexible electronic devices.Therefore,it is a major trend to produce polyimide films with low CTE as well as good over-all properties.In this paper,the blending modification of inorganic filler,high temperature heat treating and multicomponent polymerization are employed to prepare the polyimide film with low CTE and excellent over-all properties.Firstly,the polyimide of PMDA-ODA type is synthesized as a matrix,and two kinds of silicates as well as three kinds of silica are used as particles to prepare hybrid films.Then,the heat treating of different temperatures is employed to the PI/SiO2 hybrid film,so as to decrease the CTE value of PI film,and meanwhile find the suitable heat treating temperature.Finally,PMDA-ODA-PDA?PMDA-TPEQ-PDA?PMDA-ODPA-PDA and PMDA-BPDA-PDA type polyimide copolymers are synthesized through copolymerization.The influence of different types and contents of rigid monomers on the CTE value of polyimide films is studied,and the relationship between the rigidity of polymer chains and the CTE value of polyimide is researched.The fourier transform infrared spectroscopy(FTIR),X-radial diffractometer(XRD),scanning electron microscopy(SEM),polarizing microscopy,thermo mechanical analysis(TMA),thermo gravimetric(TGA),precision impedance analysis,electronic universal tensile testing machine and insulation resistance tester are employed to characterize the chemical structures,aggregation structures,morphology,dimensional stabilities,thermal stabilities,dielectric properties,mechanical properties,insulation resistance and water absorption of PI films.The results indicate that:the CTE values of polyimide films decrease with the addition of mica or meerschaum,but the CTE value drops a little as well as the mechanical and thermal properties of hybrid film decrease sharply.All of the three SiO2 can decrease the CTE value of PI and the mechanical as well as thermal properties of PI are good,when the mass fraction of KH570 modified SiO2is 15%,the mechanical and thermal properties as well as the dielectric property of hybrid are good,and the CTE is just 23.38x10-6·k-1,which can basically meet the demand of flexible circuit board.The PI/SiO2 hybrid with optimal comprehensive performance is selected to carry out heat treating,as a result the heat treating can improve the dimensional stability,mechanical and thermal properties of PI/SiO2 hybrid,when the heat treating temperature is 380?,the PI film get the best over-all performance,with 117.68 MPa of tensile strength,556.3 ? of T0.05,good electrical property,and its CTE value is 18.25×10-6·k-1,which is the same as the CTE of copper foil,and can be well applied in the flexible circuit board industry.With the increasing content of PDA,all of the CTE values of the PMDA-ODA-PDA and PMDA-TPEQ-PDA type PI films sharply decline,and when the molar ratio of n(TPEQ):n(PDA)=2:8,the PMDA-TPEQ-PDA type PI films get the top comprehensive properties,with 124.49 MPa of tensile strength,506.3 ? of T0.05,the combination property is excellent and its CTE value decreases to 12.63×10-6·k-1,which is relatively low,however it is still higher than the CTE of silicon.With the increasing content of PMDA,the CTE values of both the PMDA-ODPA-PDA and PMDA-BPDA-PDA type PI films decrease distinctly,and the mechanical as well as thermal properties of PI make better,when the molar ratio of n(BPDA):n(PMDA)=4:6,the PI films get the optimal overall performance,its tensile strength is 159.70 MPa,the T0.05 is 582.3 ?,the dielectric constant is 3.07,the water absorption is 0.73%,and the CTE value is 5.43×10-6·k-1,which is very close to the CTE value of silicon,the dimensional stability is excellent,and it has great potential in the application of high performance flexible display parts such as AMOLED.