Preparation And Properties Of Transparent Polyimide Films

In recent years,the widespread use of high-temperature resistant colorless polymers as a plastic substrate for image display devices,liquid crystal alignment layers,optical films,flexible printed circuit boards,touch panels,and other optoelectronic devices has attracted a lot of attention.If the current fragile inorganic glass substrates（400-700μm）are replaced with plastic substrates（<50μm）,display devices can be made lighter,stronger,and more flexible.For transparent plastic substrates,a high glass transition temperature（Tg>300°C）is required to meet the high temperature processing.It has good optical transparency,high heat resistance and dimensional stability,and film-forming process compatibility in the UV-visible wavelength range.In this study,a series of CPI films containing amide structure were prepared and synthesized from the design of molecular backbone of transparent polyimide（CPI）films.The optical,thermal,and mechanical properties of the synthesized films are also investigated and discussed in depth.（1）Starting from trans-1,4-cyclohexanedicarboxylic acid,a dianhydride monomer（TCDA）containing an alicyclic and amide structure was synthesized by an acylation reaction,which was combined with a variety of different diamines(2,2′-bis（trifluoromethyl）-4,4′-diaminobiphenyl（TFMB）,trans-1,4-cyclohexanediamine（t-CHDA）,1,4-bis（4-amino-2-trifluoromethylphenoxy）benzene(The properties of CPI films can be systematically regulated by changing the linearity of the molecular chains,and by forming hydrogen bonds between molecular chains to promote tight stacking of molecular chains.Among them,PI-1（TCDA/TFMB）and PI-2（TCDA/t-CHDA）have the best overall performance.The absence ofπ-electrons in the aliphatic ring structure can effectively inhibit the electron flow in the polyimide chains and reduce the CTC effect,as well as the introduction of-CF₃,which has a strong electron-absorbing effect,into the molecular backbone,increasing the spatial site resistance,limiting the interactions between polymer backbones and chain stacking,inhibiting the electron flow in the molecular backbone,and greatly weakening the formation of charge-transfer complexes.This resulted in CPI films with transmittance（T）>89%at 550 nm,yellowness index（YI）is 3.23,and haze（Haze）is 3.23.The thermal glass transition temperature Tg>320°C,the coefficient of thermal expansion CTE<16.2 ppm/K,and T_d5%>434°C are due to the rigid and linear backbone structure and the strong hydrogen bonding between the molecular chains due to the introduction of amide bonds.（2）Starting from 4-aminophthalic acid,four new aromatic dianhydride monomers containing biphenyl structure,benzene structure and amide and ether bonds,namely s-ABDA,i-ABDA,EADA,and p-DAMDA,were synthesized by acylation reactions.s-ABDA,i-ABDA,EADA,and p-DAMDA were combined with 2,2’-bis（trifluoromethyl）-4,4’-diaminobiphenyl（TFMB）,trans-1,4-cyclohexanedi amine（t-CHDA）diamine by a two-step process to prepare a series of transparent polyimide films.CPI films have excellent optical properties,ultra-low coefficient of thermal expansion,high glass transition temperature and adequate mechanical properties.The experimental results show that the introduction of biphenyl,benzene,ether bonds and amide structure in the dianhydride monomer makes these new transparent polyimide films exhibit excellent optical properties（T550 nm>88%）,good thermal stability（CTE<4.4 ppm/K;Tg>314 ℃;Td5%>478 ℃）and mechanical strength（σ>208 MPa）.In particular,the PI-1（s-ABDA/TFMB）films exhibit an ultralow coefficient of thermal expansion（CTE = 4.4 ppm/K）,which in combination with this maximum refractive index（Δn=0.1832）suggests that the role of hydrogen bonding at high temperatures is crucial for the regulation of thermal expansion.In addition,the tensile strength（σm）of CPI films ranged from 45.8 to 208.3 Mpa,the tensile modulus（Et）was in the range of 1.6 to 30.3 Gpa,and the elongation at break（εb）ranged from 3.2 to 13.4%.（3）In this chapter,A new dianhydride monomer（ATDA）containing adamantane and amide structures was synthesized and polymerized with three diamines TFMB,6FAPB and ODA to prepare a series of transparent polyimide films.Test results show that the CPI films have excellent optical properties,including PI-1（ATDA/TFMB）with a transmittance of over 89% at 550 nm,and high heat resistance（Tg up to 352 ℃）.A series of CPI films with high transparency,high heat resistance and excellent tensile properties were designed by introducing both adamantane and amide groups of alicyclic caged alkane structure into the molecular skeleton.（4）In this study,HPMDA was reacted with 4-aminophthalic acid to synthesize a new dianhydride monomer（IADA）containing an imine ring,which was polymerized with the diamines TFMB,6FAPB,and ODA.The new dianhydride monomer synthesized by the reaction with 4-aminophthalic acid greatly improved the reactivity of HPMDA polymerization with diamines,and the difficulty of HPMDA polymerization with diamines was solved.And the light transmission at 550 nm can exceed 87%.However,due to the large number of flexible groups and alicyclic structure in the polymer backbone,the thermal expansion coefficient of the polymer at high temperature is significantly reduced,among which the thermal expansion coefficient of PI-3（IADA/6FAPB）is 71.49 ppm/K.