Synthesis,Properties And Applications Of Polyimides Modified By Rigid Semi-Alicyclic Structure

With the application of flexible OLED screens for smart phones represented by iPhone X,the development of the flexible display field has been greatly promoted.The flexible OLED market is now dominated by Samsung Korean,and domestic manufacturers:the BOE,Tianma and CSOT have also increased the investment of flexible OLED.High-performance,flexible substrates are one of the essential raw materials for the preparation of flexible OLEDs.The temperature of low temperature polysilicon（LTPS）process is up to 400°C.However,the maximum using temperature of conventional flexible polymer substrates such as:polymethyl methacrylate（PMMA）,polyether sulfone（PS）,polyethylene naphthalate（PEN）and polyethylene terephthalate（PET）et.al.are lower than 150°C.And it will cause distortion during high temperature processing,which will seriously affect the performance of the device.Highly thermal resistant polyimides（PIs）have the decomposition temperature of approximately 600°C and good solvent resistance,mechanical properties and dimensional stability,and making them are the ideal materials for flexible substrates.The traditional aromatic polyimides（ArPIs）exhibited excellent thermal stability and deep yellow or brown color due to the strong charge transfer complex effects（CTC）and the inter-and intra-molecular interactions.These ArPIs substrates will reduce the transmittance of the flexible electrode,affect the light absorption and transmission and decrease the device properties.Therefore,how to balance the relationship between good thermal,dimensional stability and good optical performance are the focus of this article.Considering that PIs based on commonly used dianhydride monomers 4,4’-（hexafluoroisopropylidene）diphthalic anhydride（6FDA）and1,2,4,5-cyclohexanetetracarboxylic dianhydride（HPMDA）exhibited good optical properties,but poor dimensional stability.To overcome these drawbacks,we have designed and synthesized a series of isomerized rigid semi-alicyclic diamine monomers and a rigid semi-alicyclic modified fluorine-containing dianhydride monomer.And two series of the PIs based on diamine and dianhydride were prepared,respectively.Two kinds of the PIs with excellent performance were selected to prepare flexible transparent electrodes,and further to fabricate QLEDs,OPVs and Sb₂Se₃ thin film solar cells devices.The following works have been carried out around these issues mentioned above:（1）Preparation and properties of PIs based on rigid alicyclic diamine monomers.A rigid semi-alicyclic functionalized isomeric diamine 5（6）-amino-1-（4-aminophenyl）-1,3,3-trimethylindane（5（6）-DAPI）was synthesized and separated by column to obtain5-amino-1-（4-aminophenyl）-1,3,3-trimethylindane（5-DAPI）,6-amino-1-（4-aminophenyl）-1,3,3-trimethylindane（6-DAPI）.A series of isomeric polyimides（CPI-x and TPI-x）were prepared by chemical imidation and thermal imidization.The effects of incorporation of isomeric rigid alicyclic structure into polymer backbones have been systematically investigated in terms of optical,thermal,mechanical,dielectric and surface properties,respectively.Single crystal X-ray diffraction（SCXRD）found that 5-DAPI has a larger dihedral,more tortuous/non-planar steric structure than 6-DAPI,it reduced the inter-and intra-molecular interactions and the degree of chain’s packing,increased the average chain distance.Thus,the solubility,optical properties and dielectric performances of5-DAPI-based films were improved.The bigger steric hindrance of 6-DAPI possessed stronger molecular interactions,and 6-DAPI series PIs exhibited better thermal stability.Meanwhile,due to the mild polymerization conditions of chemical imidization,the glass transition temperature（T_g）of the CPI-x series is in the range of 329-429°C,which was higher than the T_g of TPI-x（321.9-370.7°C）.（2）The structural design,monomer synthesis and geometric configuration transition of rigid semi-alicyclic fluorine-containing dianhydride monomer.Considering PIs based on 6FDA and HPMDA exhibited excellent optical properties and big CTE value.A rigid semi-alicyclic modified and fluorine-containing dianhydride:9,10-difluoro-9,10-bis（trifluoromethyl）-9,10-dihydroanthracene-2,3,6,7-tetracarboxylic acid dianhydride（8FDA）was designed and synthesized.8FDA monomer and the reaction intermediates were well characterized.The mechanism study of the reaction revealed that the geometric configuration transition happened from trans-to cis-when the hydroxyl group was substituted by fluoride atom originated from diethylaminosulfur trifluoride（DAST）.Meanwhile,the SCXRD of 8FDA found three different crystal forms which indicated that8FDA was a kind of polymorphism.（3）Preparation and properties of PIs based on 8FDA.A series of PIs were prepared by the polymerization of 8FDA,6FDA and HPMDA with two fluorine-containing diamines 2,2’-bis（trifluoromethyl）-4,4’-diaminobiphenyl（TFDB）and 4,4’-oxybis（3-（trifluoromethyl）aniline）（TFODA）.The comparative research found that 8FDA-based PIs possessed higher glass transition temperature,thermal decomposition temperature（T_g>365°C）and better dimensional stability(coefficient of thermal expansion lower than20 ppm K^-1)than the contrast PIs.At the same time,8FDA-based PIs were colorless or light yellow,showed good optical transparency.These results indicated that the8FDA-based PIs can be promising candidates for high-performance flexible substrates used in flexible displays,wearable devices,and other applications.（4）Preparation and properties of the flexible devices based on CPI-1 and PI-1.This chapter provided two methods of the preparation flexible devices based on transparent PIs.According to the difference performance of these two PIs,an ultra-thin metal electrode was prepared by the thermal evaporation on the surface of CPI-1 with 3 nm MoO₃,2 nm Au and 4 nm Ag.Then the flexible QLED and OPV device was fabricated using this flexible electrode.The flexible QLED device achieved high brightness of 5230 cd/m² and EQE of 5.2%,meanwhile,OPV device obtained 4.36%energy conversion efficiency.The indium tin oxide（ITO）was sputtered on the surface of PI-1 by magnetron sputtering to prepare an ITO flexible electrode and fabricated the flexible antimony selenide thin film solar cell device,exhibited the device efficiency of 2.89%.