Preparation Of Crosslinked Porous Polyimide And The Properties Research

Polyimide(PI)is a kind of polymer material with excellent comprehensive properties.Polyimide can be divided into thermoplastic polyimide and thermosetting polyimide.Thermoplastic polyimide is particularly prominent in the preparation of aerogel;but the introduction of flexible chain will inevitably result in decrease of mechanical strength,heat resistance,and naturally limit its application in extreme conditions.While thermosetting polyimides can meet the needs of extreme environment owing to the characteristics of high strength and hardness.Thermosetting polyimides are usually prepared by crosslinking,which is a three-dimensional network of polyimides that can be synthesized in a variety of ways.In this study,threecross-linked porous polyimide Samples were prepared by different monomers and their properties were investigated.Sample 1 was prepared by a prolonged dianhydride(product from reaction of pyromellitic dianhydride and4,4'-diamino diphenyl ether with mole ratio of 2:1)and melamine which shows excellent adsorption performance.Polymethylene polyphenyl isocyanate(PM200)and PMDA were used to prepare sample 2 accompanied by blending of vacuum glass beads.This polyimide has lower shrinkage and thermal conductivity,better fire resistance and mechanical properties.Sample 3 was prepared by using aromatic dianhydride and triphenylmethane triisocyanate gum(JQ-1 gum).During the preparation process,the generated CO₂plays a foaming agent and hence porous PI is a one-step product actually.Sample 1 has good thermal stability and the temperature of mass loss 10%(T_d,10)begins at 435?in N₂atmosphere.It appears petal-like shape in FE-SEM view.The average pore diameter is 6.7 nm and the specific surface area of 110.38 m²/g from BET;the petal-like microstructure is favorable for adsorption.It has good adsorption performance and removal effect on acid orange 10(AO-10)and the average adsorption mass can reach 377 mg/g at 298 K;the adsorption capacity can reach 599 mg/g under acidic conditions when the initial concentration of AO-10 is 500 mg/Land the maximum desorption ratio can reach 38.9%under alkaline conditions.The second order kinetic model was better fitted by fitting the first order kinetic curve and the second order kinetic curve.The kinetic data fitted by Langmuir and Freundlich formula indicates the Langmuir model was more suitable for the adsorption experiment of AO-10.When the p H value reached 13,38.9%of the adsorbed AO-10 was released and 61.1%of the adsorption of AO-10 remained in pores.This shows that the polyimide can achieve adsorption and desorption,and realize the recycling of the material.For sample 2,introduction of vacuum glass beads brings the following impacts:apparent density decreases from 210.56 kg·m^-3to 143.76 kg?m^-3,shrinkage reduces from 20.97%to 6.78%,closed pore percentage increases from 54.9%to 79.3%,thermal conductivity decreases from 0.042 W/(m?k)to 0.032 W/(m?k).Sample 2 has excellent thermal stability and starts to decompose at 400?.With the addition of vacuum glass beads,the T_d,10of polyimide also increases correspondingly to 478?,and the LOI of polyimide increases from 35.88%to 41.74%,indicating that this polyimide has excellent high temperature resistance.It also has excellent mechanical properties.When the original sample has a deformation of 10%,the compressive strength is 0.84 MPa.With the addition of vacuum glass beads,the mechanical strength of the sample first increases and then decreases.The maximum value appears when the addition amount of vacuum glass beads is 5%;the pressure at 10%deformation was 1.02 MPa.A new polyimide was prepared by JQ-1 adhesive,with apparent density of 156kg·m^-3.Sample 3 has the pore diameter between 120 to 669?m in diameter,the closed pore percentage is 71.89%.It starts to decompose at about 420?and T_d,10is 479?,showing excellent high temperature resistance.The compressive strength is 0.75 MPa at10%deformation.When vertical combustion test is conducted on sample 3,a carbon layer will form on the surface of polyimide samples during combustion,and the carbon layer will form a protective layer on the surface of polyimide to prevent heat from diffusing inward.Sample 3 can self-extinguish from fire and the burned polyimide will basically maintain its original shape.The limit oxygen index of sample 3 is 35.35%,which can be classified as Class B₁flame retardant material.Only the sample surface is burned and the foam structure remains intact under the carbon layer,indicating high flame retardant performance.