Preparation，Structure And Properties Of Polyimide/layered Nanopartical Composites With High Barrier

In recent years,polyimides(PIs)exhibit broad market prospects in the packaging field of flexible Organic Light-Emitting Devices(FOLED)due to its outstanding heat resistance,dimensional stability and comprehensive performance.Despite these advantages,the poor resistance of the polyimides film to water vapor and oxygen limits their application in the field of FOLED encapsulation.However,the strong designability of the polyimides molecular structure offers huge potential to improve its barrier performance.In addition,the rapid development of nanomaterials and related preparation technologies provides a favorable route for the development of high-barrier polyimides.Therefore,the application of PI nanocomposites has become one of the most important and popular techniques to improve the barrier performance of polyimides.The relationships between the molecular chain structure,aggregation structure,film making technology and barrier performance were explored in this paper.Combing with the molecular dynamics simulation,the dianhydride/diamine monomer containing rigid plane structure and polar groups results in close molecular chain packing,high crystallinity,small free volume,as well as high barrier intrinsic polyimide.On this basis,according to the Nieslen model,the inorganic nanoparticles lamellar structure were uniformly dispersed in the polyimide substrate,as a result,the “double plane” polyimides nanocomposite with high exfoliation,high dispersion and high orientation were prepared to further improve barrier properties.Thermal gravimetic analysis(TGA),differential scanning calorimetry(DSC),dynamic thermodynamics(DMA),thermomechanical analysis(TMA),transmission electron microscopy(TEM),polarized light microscopy(POM),X-ray diffraction(XRD),positron annihilation lifetime spectrometer(PALS),moisture vapor permeability instrument and oxygen permeability instrument were used to characterize and test the structure and properties of polyimide films.The specific contents and results are as follows:(1)Polyimide films with rigid planar structure and polar groups were prepared by two-step method using DABA and PMDA.The results show that the DABPI film is a semi-crystalline polymer with a crystallinity of 39.3%.According to the 2D-XRD testing result,DABPI crystals show high orientation,and the orientation direction is almost parallel to the surface of the film,which greatly prolongs the diffusion path of the permeation molecules.The water vapor permeability coefficient and oxygen permeability coefficient of DABPI were 3.57 g-mil/[m2-day] and 5.61 ccmil/[m2-day],respectively.The water vapor permeability coefficient and oxygen permeability coefficient of Kapton films obtained by the same preparation process in this paper were 148.67 g-mil/[m2-day] and 465.83 cc-mil/[m2-day],respectively.In comparison,permeability of DABPI film decreased by nearly two orders of magnitude.Besides,DABPI also have excellent thermal stability,with 5% weight loss temperatures of 542 °C and thermal expansion coefficient(50~300 °C)of-3.23 ppm/K.(2)The effects of different preparation technology(solid content,curing temperature and thickness)on the aggregation structure and barrier properties of polyimide films were also investigated in detail.The structure and properties of the polyimide films showed that the high solid content(≥12%)polyamide acid(PAA)with rigid planar structure and polar group has “liquefied crystallization” phenomenon.The mobility of polyamide acid molecular chains in solvent was restricted by the formation of the ordered structure.During the curing process,the low heating rate contributes to the evaporation of residual solvents and the escape of moisture generated by thermal imidization.The film contraction in the thickness direction,leads to the orientation of the molecular chains in the plane.It can be found that the thinner thickness of the polyimide films and the more obvious chain orientation result in the smaller the coefficient of thermal expansion and better barrier properties.When the solid content of PAA is 9%,the curing temperature higher than the glass transition temperature and the film thickness is 10 um,the oxygen permeability coefficient of the DABPI is 3.41 cc-mil/m2-day,and the water vapor permeability coefficient is 1.16 g-mil/m2-day.(3)Based on the previous research,the layered nanoparticles montmorillonite(MMT)and graphene(G)were selected to prepare polyimide nanocomposites.The structure and properties of polyimide composite films show that the two-dimensional layered nanoparticles are uniformly dispersed in the DABPI substrate with obvious orientation.Due to the orientation and nucleation of the nanoparticles,the crystallinity of the nanocomposite films is significantly improved as well as the barrier properties and thermal stability.When the addition amounts of MMT and G were 5 wt% and 0.5 wt%,the barrier properties of polyimide nanocomposites reached maximum,respectively.Compared with the intrinsic DABPI film,the oxygen permeability coefficient and the water vapor permeability coefficient of DABPI/MMT films were 1.82 ccmil/[m2-day] and 0.8 g-mil/[m2-day],decreased by 79% and 73% respectively;and the oxygen permeability coefficient and the water vapor permeability coefficient of DABPI/G were 0.69 cc-mil/[m2-day] and 0.44 g-mil/[m2-day],decreased by 92% and 85% respectively.