Preparation And Properties Of Colorless And Transparent Semi-aromatic Polyimide Film

With the popularity of portable mobile terminals,flexible display technology becomes more and more important.The flexible display which can be bent and folded becomes the focus of research.Polymer materials have excellent flexibility,but generally the thermal properties are poor.Polyimide（PI）is a kind of polymer material with excellent mechanical properties,heat resistance,low dielectric coefficient and radiation resistance.Especially,its heat resistance property is unique among polymer materials.It is widely used in aerospace,military production,microelectronics and other fields.Although the rigid imide ring gives PI material excellent comprehensive properties,the presence of electron donor and acceptor in PI molecule will produce charge transfer complex,resulting in decreased transmittance and yellow color of PI material.In order to be able to play a role in the field of flexible display,it is urgent to develop a colorless transparent polyimide（CPI）which need excellent heat resistance,high optical transparency and low thermal expansion coefficient.However,optical transparency,heat resistance and dimensional stability are often mutually restricted relations,so how to achieve the balance them is the key to achieve CPI using at flexible display.This topic aims to prepare a variety of semi-aromatic colorless transparent polyimide films through molecular structure design,with rigid lipid ring carboxylic anhydride as the main body,and the homomeric reaction of trifluoromethyl containing electron-drawing groups,ether bonds regulating the rigid flexibility of molecular chain segments and amide units with hydrogen bonds.The influences of polymer molecular structure,polymerization reaction process,cyclic reaction and molecular stacking on polymer transparency,heat resistance,dimensional stability and physical and mechanical properties were studied,and the correlation between polymer structure and film properties was established.The main contents are as follows:（1）Semi-aromatic colorless transparent polyimide films were prepared with Cyclopentanone bis-spironorbornane tetracarboxylic dianhydride（Cp ODA）as the main frame by thermal cyclation reaction with different commercial diamine monomers.The effects of molecular flexibility,strong electron-absorbing trifluoromethyl group and polyhydrogen bonded amide unit as the main chain on the properties of polyimide films were analyzed,and a CPI film with rigid structure unit and trifluoromethyl TFMB/Cp ODA（PI-3）was obtained.The optical transmittance at 450 nm(T_{450 nm})reaches 85%,the thermal resistance glass transition temperature（T_g）reaches 419℃,and the linear coefficient of thermal expansion(CTE_50-250℃)is only 23.2 ppm/K,which provides the basis for the development of colorless transparent polyimide films for flexible display.In addition,it has been found that polyimide films can perform better in dimensional stability by proper introduction of an amide unit,and the CTE_50²50℃of semi-aromatic polyimide films is only 7.4 ppm/K.（2）In order to improve the transparency of the film and avoid adverse effects on the heat resistance and linear expansion property as much as possible,rigid diamine BIA with the structure of kinking benzimidazole was introduced to ternary copolymerization with semi-aromatic polyimide AB-TFMB/Cp ODA（PI-5）which has a low linear thermal expansion coefficient.It is found that introduction of BIA can effectively reduce the fog value of the film（19.4%to 3.1%）and improve the optical transparency.The larger the BIA copolymerization ratio,the better the dimensional stability of the film.When polyimide film is imidization at the highest temperature of 300℃,the optical transmission range of 450 nm is 57～80%,the linear expansion coefficient between 50～300℃is-1.12～8.67 ppm/K,the T_grange is 408～426℃.（3）A rigid diamine monomer（2,3,5,6-tetrafluoro-4-ethylene-phenoxy）triphenyl methane（DFPTM）with latent coexisting interaction was designed and synthesized based on the theoretical basis that the cross-linking structure can effectively restrict the movement of molecular chain segments.Crosslinked polyimide films were prepared by thermal cyclization with 2,2’-bis（trifluoromethyl）-4,4’-diaminophenyl ether（6FODA）and rigid lipid cyclic dianhydride Cp ODA.The effects of cross-linking degree on the transmissibility,linear expansion coefficient and glass transition temperature of polyimide films were investigated.It is found that with the increase of crosslinking degree,the thermal expansion coefficient is significantly improved.At high crosslinking degree,T_g can be effectively improved,while the high temperature stability is lost,which also brings adverse effects on the optical properties.The linear thermal expansion coefficients of the prepared series of crosslinked copolymerized films are 37.44～64.40 ppm/K and 37.49～58.38 ppm/K in the range of 50～250℃and 50～300℃,respectively.The dimensional stability of crosslinked films at high temperature is better than that at low temperature(CTE_50～250℃>CTE_50～350℃).