Preparation,Properties And Molecular Simulation Of High Barrier Polyimide Containing Amide Structure

Polyimide（PI）,with its excellent comprehensive performance,has become a key material for flexible displays.However,the high water vapor and oxygen permeability of traditional polyimide film cannot guarantee the service life of flexible organic light-emitting diodes（FOLED）.Therefore,improving its barrier performance has become a research focus at present.The introduction of amide structures into the main chain of the polyimide molecular chain has been preliminarily proven to be an effective method of improving the barrier properties.At the same time,the monomer containing the amide structures has not been studied in depth.In this paper,to increase the number of amide structures in polyimide repeat units as the main line of research,a series of novel diamine/dianhydride monomers containing amide structures were designed and synthesized from the perspective of spatial heterogeneity and different monomers,and a series of intrinsic high-barrier polyimide films were prepared accordingly.At the same time,in order to improve the theoretical basis of the introduction of amide structures into polyimide to improve the barrier performance,Material Studio software was used to systematically simulate and calculate the related parameters of the gas permeability of the new polyimide.The specific research contents and results are as follows:（1）A novel diamine monomer containing a single amide group,3-amino-N-（3-aminophenyl）benzamide（m-DABA）with an amino substituent in the intermediate position was synthesized by considering spatial isomerism,and the polyimide m-DABPI film was prepared via the polymerization of m-DABA with the commercial phthalic anhydride（PMDA）.Compared to the reported para-linked polyimide p-DABPI,the interposition-linked polyimide m-DABPI showed better barrier performance,with water vapor transmittance（WVTR）and oxygen transmittance（OTR）as low as 2.8 g·m^-2·day^-1and 3.3 cm³·m^-2·day^-1,respectively,which were 54.9%and 41.8%of that of p-DABPI.Molecular simulation and test characterization showed that m-DABPI has a positive effect on the improvement of barrier performance due to its interpositional substitution,which is more conducive to the stacking of molecular chains,reducing free volume and chain mobility.（2）By increasing the number of amide groups and studying the effects of spatial isomerism on the barrier performance of polyimide further,4-amino-N’-（4-aminobenzoyl）benzohydrazide（p-AAPDA）and3-amino-N’-（3-aminobenzoyl）benzohydrazide（m-AAPDA）diamine monomers containing two amide groups were synthesized,and the corresponding polyimide p-AAPPI and m-AAPPI were prepared via polymerization with PMDA.Both polyimides had excellent thermal stability and mechanical properties.In particular,p-AAPPI has a glass transition temperature of 423°C,a thermal expansion coefficient of only2.58 ppm·K^-1,and a tensile strength of 143 MPa.Moreover,it also has excellent barrier performance,with its WVTR and OTR being 1.0g·m^-2·day^-1and 1.7 cm³·m^-2·day^-1,respectively.Found in the repeat unit contains two amide groups,barrier performance is much better than the repeat unit of polyimide film contains only one amide groups,and counterpoint between heterogeneous relative to the isomerism is more conducive to the formation of hydrogen bonds,molecular inter-atomic forces,molecular chain packing density increases,and reduced free volume.Therefore,p-AAPPI with para-position connections has better barrier performance than m-AAPPI with interposition connections.（3）Anewtypeofdianhydridemonomer N-（1,3-diox-1,3-dihydroisobenzofuran-5-yl）-1,3-dioxo-1,3-dihydroisobe nzofuran-5-carboxamide（DAPA）containing an amide structure was designed and synthesized,and polyimide film DAPPI was prepared in two steps with commercial p-phenylenediamine PDA.Through molecular simulation and test characterization studies,it was found that the molecular chain spacing of DAPPI is smaller than that of p-DABPI prepared based on the amide structure located in diamine monomer,the molecular chain is more tightly packed and the free volume is smaller,making the diffusion and penetration of small molecules more difficult.However,the molecular chain mobility of DAPPI and p-DABPI is similar,so the molecular chain mobility is not the key factor in determining the barrier performance of the two polyimides.DAPPI has better barrier performance than p-DABPI because of its smaller free volume.（4）A novel polyimide AD-PAI containing three amide groups was prepared via the polymerization of two amide groups’diamine p-AAPDA with anhydride DAPA containing one amide groups.With the increase in the number of amide groups,the ability to form hydrogen bonds in the matrix is greatly improved,and 211 hydrogen bonds can be formed in the simulation unit,which is much higher than that of other systems.A large number of hydrogen bonds make it have stronger intermolecular forces,and the cohesive energy density is as high as 618J/cm3,which makes the glass transition temperature and tensile strength as high as 441°C and 155 MPa,respectively.More importantly,the strong intermolecular force makes the molecular chain stack closer,further reducing the free volume and local chain mobility of the polyimide.Therefore,the barrier performance of AD-PAI is far better than that of other systems,with the WVTR and OTR as low as 0.37g·m^-2·day^-1and 0.30 cm³·m^-2·day^-1,respectively.