Preparation And Characterization Of Transparent Polyamide-imides And Their Composite Films

Polyimide(PI)is widely used in electronic devices due to its high temperature resistance,low temperature resistance,solvent resistance,flame retardancy,good mechanical properties,and excellent dielectric properties.However,the easy formation of CTC between diamine and dianhydride monomer molecules often results in a darker color and lower light transmittance of PI film.At the same time,the rigid structure of the molecule causes it to be difficult to process.With the development of the optoelectronic industry,there are higher requirements for polyimide,especially its heat resistance,light transmission and mechanical properties.Compared with polyimide,polyamide-imide(PAI)has both the excellent properties of polyamide and polyimide such as lower thermal expansion coefficient,excellent radiation resistance and corrosion resistance,as well as high light transmittance.Therefore,it becomes a preferred substrate for flexible electronic devices.However,there is a certain conflict between the heat resistance and light transmittance of PAI,which is difficult to meet the machining requirements.Therefore,its application in flexible electronic equipment is very limited.Based on this,a series of PAI films with high light transmittance,high heat resistance,and excellent mechanical properties are synthesized in this paper,and their performance has been detailed and analyzed.The main work content includes the following three aspects:(1)In the PAI molecule,an amide bond and an imide group are simultaneously containing.Compared to the PI,the molecule contains an amide bond,which can form a hydrogen bond inside the molecule,improve its mechanical properties and glass transition temperatures and overall performance.By the molecular structure design,the diamine monomer N,N'-bis(4-aminophenyl)phthalamide(APTA)was synthesized.Using APTA as diamine monomer,4,4'-(Hexafluoroisopropylidene)diphthalic anhydride(6FDA),3,3',4,4'-biphenyl tetracarboxylic dianhydride(BPDA)and 4,4'-oxygen bisphenylenedic anhydride(ODPA)as dianhydride monomer,three APTA series PAI films were synthesized.As a contrast,three kinds of PAI films of DABA series were prepared by using 4,4'-diamino-benzoylaniline(DABA)as diamine monomer and 6FDA,BPDA and ODPA as dianhydride monomer.The results showed that PAI films with 6FDA dianhydride monomer have better light transmittance,and the glass transition temperature of APTA series PAI films was significantly higher than that of DABA series PAI films.Among all PAI films,APTA-6FDA film has excellent comprehensive properties,the glass transition temperature reaches 354.1°C,the light transmittance of 550 nm is 81.41%,and the tensile strength is 142.51 MPa.(2)Both trifluoromethyl and alicyclic structures can damage the conjugate structure of the PAI molecular chain,reduce molecular density and thereby effectively inhibit the formation of CTC in molecular or molecular.It also can increase the free volume of PAI,and improve its light transmittance and solubility.A novel diamine monomer N,N'-(cyclohexane-1,4-diyl)bis(4-aminobenzanamide)(NCHABA)with an adipocyclic structure and an amide group was synthesized.Four NCHABA series copolymerized PAI films with different TFDB contents were prepared by mixing NCHABA with 2,2'-bis(trifluoromethyl)diaminobiphenyl(TFDB)in different proportions as diamine monomer and using 6FDA as dianhydride monomer.As a contrast,four kinds of APTA series copolymerized PAI films with different TFDB contents were prepared by mixing APTA and TFDB in different proportions as diamine monomer and using 6FDA as dianhydride monomer.All copolymerized PAI films have excellent solubility,and can dissolve in common organic solvents at room temperature.As the TFDB content increases,the light transmittance and thermal decomposition temperature of the film is increasing.The light transmittance of NCHABA series PAI films is higher than that of APTA series PAI films,and the highest light transmittance is 88.29%and 87.21%,respectively.The thermal decomposition temperature of the APTA series PAI film is higher than the NCHABA series PAI film,and the highest 5%thermal decomposition temperature is 516.3°C and 473.0°C,respectively.(3)Inorganic/organic polymer composites can concentrate the excellent properties of different materials,therefore,composite materials have good comprehensive performance.Si O₂ has low thermal expansion coefficient and good heat resistance.Adding nano-sized Si O₂ into PAI can improve the heat resistance and dimensional stability of PAI at the same time.Two series of PAI/Si O₂ composite films,APTA and NCHABA,were synthesized by sol-gel method by mixing APTA and NCHABA with TFDB at molar ratio of 1:4 as diamine monomer,6FDA as dianhydride monomer,KH550 as coupling agent and tetraethyl osilicate as main silicon source.The results show that as the Si O₂ content increases,the heat resistance of the composite film is significantly improved,and the thermal decomposition temperature and the glass transition temperature are increased.The highest 5%weight loss temperature of the APTA and NCHABA series composite films is 525.5°C and497.3°C,respectively,and the glass transition temperature was 336.7°C and 315.6°C.