Structure And Property Of The Blending-modified P84 Polyimide Gas Separation Membranes

The rapid development of society,accompanied by the burning of a large number of fossil fuels,has led to the excessive emission of greenhouse gases（CO₂,etc.）,which has seriously affected the environment and climate.Climate change has become an important issue in the world,and reducing greenhouse gas emissions to cope with climate warming has become a worldwide consensus.All major economies have put forward the goals of"carbon reduction"and"carbon neutrality".China aims to reach the peak of CO₂ emissions before 2030,and strive to achieve carbon neutrality before 2060.Developing energy-saving and environment-friendly CO₂separation technology is not only an important means to achieve"carbon peak"and"carbon neutrality",but also has great economic benefits.Gas separation membrane is mainly divided into inorganic membrane and polymer membrane,among which,inorganic membrane has high gas permeability,but it is brittle,difficult to process,difficult to achieve large quantities of industrial applications.The polymer membrane has the advantage of easy processing,and can be modified by blending the advantages of different materials to meet different industrial needs.Among many polymer membranes,polyimide membranes have attracted extensive attention because of their excellent heat resistance,mechanical properties and high gas separation performance.However,there is a trade-off limit between permeability and selectivity of polymer membranes,known as the Robeson upper limit,which greatly limits their application scenarios.Blending modification is one of the effective methods to adjust and improve the microstructure and gas permeability of polyimide membranes.Based on this,in order to solve the above problems,this paper adopts the method of multiphase blending.Through blending modification,high-throughput metal-organic frame materials MOF（UiO-66 and NH₂-UiO-66）and polyethylene glycol（PEG）are introduced into the P84 structure polyimide matrix,respectively.By adjusting MOF particle size,surface active groups,blends and other factors,the effects of the interface interaction of the composite system and the structure and performance of the separation membrane were investigated,and the comprehensive separation performance of the gas separation membrane was effectively improved.Specific research contents include:（1）Preparation and gas separation performance of UiO-66/P84 polyimide mixed matrix membranes:Porous metal-organic framework MOF material UiO-66 was mixed into P84polyimide.By controlling the size of UiO-66 nanoparticles and adjusting the two-phase interface structure of P84 polymer and UiO-66,UiO-66/P84 mixed matrix films（MMMs）with different mass fractions were prepared.The chemical structure and microstructure of UiO-66,P84 and MMMs were determined by FTIR,SEM and XRD.The thermal stability,mechanical properties and gas separation properties of MMMs were investigated by TGA,DSC,film tensile and gas separation tests.The results showed that the introduction of UiO-66 with high specific surface area provided an effective transport channel for gas molecules.When the UiO-66 load increased to 25 wt%,the permeability coefficient of CO2 increased by 179.7%and the ideal selectivity of CO₂/CH₄ increased by 59.6%.（2）Interfacial regulation and gas separation performance of PMOF/P84 polyimide mixed matrix membranes:Theoretically,the uniform and adjustable MOF pore structure can improve the gas permeability and separation performance of the polymer matrix.However,due to the poor interfacial compatibility between MOF particles and polymer,MOF particles are easy to aggregate in the composite membrane,which leads to the deterioration of the mechanical properties and mass transfer performance of the membrane.This greatly restricts the design and development of novel gas separation membranes with mixed matrix.In order to solve the above problems,amino-modified UiO-66（NH₂-UiO-66）was first synthesized.Based on the reactivity-NH₂ group on its surface,we further explored the amino-modified UiO-66,grafted P84 oligomer to form PMOF,and then the performance of the two-phase blending with P84 polyimide,and prepared a series of MMMs.The interaction between the two interfaces was studied by means of SEM and TEM.Compared with UiO-66,PMOF is coated with P84 short chain,which has better interfacial compatibility with P84polymer,which is conducive to increasing the gas permeability selectivity of MMMs.The results showed that when the PMOF dosage was 40 wt%,the CO₂ permeability coefficient reached 21.12Barrer and the ideal CO₂/CH₄ separation coefficient was 91.8,exceeding the upper bound of 2008.（3）Preparation and gas separation performance of P84 polyimide/PEG composite membrane:compared with MOF nanoparticles,it is easier to form good interfacial interaction between polymers/polymers with similar polarity.The flexible molecular structure of polyethylene glycol（PEG）and the dipole-quadrupole interaction between CO₂ molecules and PEG ether-oxygen unit give it a very high CO₂ permeability coefficient.In this paper,a series of composite membranes were prepared by blending PEG and P84 polyimide.The effect of the molecular weight and content of PEG on the blending membrane was studied systematically.The results showed that when the amount of PEG-20000 was 20 wt%,the blending composite membrane had the best gas separation performance,and the CO₂ gas permeability coefficient was 41.21 Barrer,about 7 times that of the pure polyimide membrane.