Molecular Design,Synthesis And Properties Of Polyimide With High Transparency And High Temperature Resistant

The rapid development of flexible display technology promotes the innovation of flexible substrate materials.As an important part of flexible display devices,flexible substrate materials are put forward high requirements on their performances such as high temperature resistant,optical properties and mechanical properties.Polyimide has been widely used in the field of microelectronics by virtue of its admirable comprehensive performances and structural diversity as well as designability,of which highly transparent polyimides with high temperature resistant are considered as one of the most promising transparent substrate materials for flexible display.However,the traditional aromatic polyimide is generally prepared by polymerization of diamine with strong electron-donating ability and dianhydride with strong electron-accepting ability.The polymer molecular chains are tightly packed with strong interactions between aromatic rings,which could generate charge transfer complex(CTC)effects among intra/interchains of polyimide,resulting in a low transmittance in the visible light range and limiting their applications sin bottom-emission flexible display devices.In order to improve the optical transparency of polyimide film in visible region,the formation of CTC effect could be inhibited by reducing the rigidity of polymer chains or the intra/interchains interactions,while the heat resistance and mechanical properties of polyimide will be sacrificed at the same time.Therefore,how to balance the optical transparency and high temperature resistant of polyimide is particularly important.For this purpose,this thesis starts from the molecular design to achieve the high optical transparency and high temperature resistance of polyimide by synergistically regulating the intra/interchain interactions of polyimide.A kind of diamine containing trifluoromethyl and meta-benzene structure,two diamines with bulky pendent groups,two diacids by the introduction of amide groups and a kind of dianhydride by embedding amide group were designed and synthesized.Four series of highly transparent polyimides were then fabricated by utilizing them to react with commercial monomers containing trifluoromethyl,alicyclic unit or benzimidazole,respectively,and the structure-activity relationship between different chemical structures and properties of polyimides were studied.The main contents are as follows:1.The introduction of fluorine-containing substituents and meta-substituted structure into polyimide molecular chain could reduce the charge density of benzene ring and destroy the tightly packing of molecular chain,which is beneficial to improve the optical transparency of polyimide.Therefore,a rigid diamine 3,5-diaminotrifluorotoluene(m-TFPDA)containing trifluoromethyl and m-benzene structure was designed and synthesized,and then reacted with commercial hexafluorohydride(6FDA)to synthesize a novel highly transparent polyimide of m-TFPDA/6FDA through a two-step method.The prepared films possess good solubility in common polar solvents with high boiling point or non-polar solvents with low boiling point.The transmittance exceeds 85%at wavelength of 450 nm.The glass transition temperature(T_g)is around 300 ^oC.The linear thermal expansion coefficient(CTE)is 48 parts per million/^oC(ppm/^oC).The dielectric constant and dielectric loss are 2.27 and 1.3×10^-3at 10KHz,respectively,and the low dielectric properties could remain stable up to 280°C.By comparing with p-phenylenediamine/hexafluoroanhydride(p-PDA/6FDA)and m-phenylenediamine/hexafluoroanhydride(m-PDA/6FDA)polyimide films,it is found that the introduction of trifluoromethyl and m-benzene structures make the molecular chains of polyimide distort and bend,inhibit the formation and CTC and increase the free volume of polyimide.Thus,the optical transparency and dielectric constant of polyimide films can be effectively improved.However,the introduction of trifluoromethyl and m-benzene structures also lead to poor regularity of molecular chains and weaken interchain interactions,resulting in a large CTE of the film and need to be further improved.2.In order to further reduce the CTE of m-TFPDA/6FDA film,two rigid diacids 2,2'-(5-(trifluoromethyl)-1,3-phenylene)bis(1,3-dioxoisoindoline-5-carboxylicacid)(DAc-1)and2,2'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(1,3-dioxoisoindoline-5-carboxylic acid)(DAc-2)containing imide ring were designed and synthesized,and utilizing them with diamine m-TFPDA to synthesize two novel highly transparent polyimides containing amide units m-TFPDA/DAc-1 and m-TFPDA/DAc-2,respectively.The prepared films possess good solubility in common polar solvents with high boiling point or non-polar solvents with low boiling point.The CTEs are successfully reduced below 20 ppm/^oC.There T_gs are improved to 320 ^oC,the transmittance still exceeds 80%at wavelength of 450 nm.By comparing with m-TFPDA/6FDA film,it is found that the introduction of amide units in transparent polyimide chain can improve the rigidity of molecular chain and the interchain interactions,which could make the molecular chain in-plane orientation,thus effectively reducing the CTE,improving the T_gand maintaining a high optical transmittance of polyimide films.3.The design of bulky pendent groups into polyimide molecular chain could destroy the tightly packing of the molecular chain and inhibit the CTC effect,beneficially improving the optical transparency of polyimide.Meanwhile,the bulky pendent groups could also restrict the movement of molecular chain segment,favorably improving the T_gof polyimide.Therefore,two diamines with bulky pendant groups based on 4,4'-diamine triphenylmethane structure TMDT and 3FTMDT were designed and synthesized,and then reacted with 6FDA to synthesize two novel highly transparent polyimides TMDT/6FDA and 3FTMDT/6FDA through a one-step method,respectively.The prepared films possess good solubility in common polar solvents with high boiling point or non-polar solvents with low boiling point.The transmittance exceed 85%at wavelength of 450 nm.The T_gs are about 340 ^oC and the CTEs are around 60 ppm/^oC.In order to further improve the dimensional stability of3FTMDT/6FDAfilm,adianhydridecontainingamideunitof N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)-bis(1,3-dioxo-1,3-dihydroisobenzofu ran-5-carboxamide)(TA-TFMB)was designed and synthesized,and then reacted with3FTMDT to synthesize a novel highly transparent polyimide 3FTMDT/TA-TFMB.The CTE could reduce to 45 ppm/^oC and the T_gis improved to 360 ^oC.The transmittance still exceeds80%at wavelength of 450 nm.It is found that introducing substituted benzene structure into the side chain of polyimide chain could obtain polyimide with high optical transparency and high T_g,but the CTEs are large.Further introduction of amide units into polyimide main chains could improve the rigidity of molecular chains and the interchain interactions,effectively reducing the CTE,improving the T_gand maintaining a high optical transmittance of polyimide films.4.Introducing aliphatic ring structure into polyimide molecular chain could destroy the conjugated structure within the molecule and inhibit the CTC effect,beneficially improving the optical transparency of polyimide.While the presence of abundant single bonds in the aliphatic ring leads to poor thermal stability of the material.The semi-aromatic and semi-aliphatic polyimide has good optical transparency and heat resistant.Therefore,1,2,4,5-cyclohexane tetracarboxylic anhydride(HPMDA)containing aliphatic ring structure was copolymerized with two aromatic diamines 2-(4-aminophenyl)-5-aminobenzimidazole(BIA)containing benzimidazole and 2,2'-bis(trifluoromethyl)benzidine(TFMB)containing trifluoromethyl to synthesize highly transparent copolyimides with different ratios through a one-step method.It is found that the rigidity and interaction of the copolyimide chains gradually increased with the increase of BIA content,improving the T_gand reducing the CTE.The T_gs are improved from 400 ^oC to over 450 ^oC and the CTEs are reduced from 58 to 13ppm/?.While the optical transparency of the film would be decrease with decreased transmittance from 88%to 25%at wavelength of 450 nm.Therefore,the optical transparency and high temperature resistant of copolyimide could be tuned by controlling the ratio of two diamines of TFMB and BIA.