Synthesis Of Cross-linkable Polyimide With Carboxyl Groups And Preparation Of Mixed Matrix Membrane With Mof For Gas Separation

Developing from 21th century,membrane separation is a promising and effective method for environment protection.Compared with the traditional separation process,membrane separation technology has the great advantage of high efficiency,low cost,low energy consumption,simple and flexible operation.The application is widely in the separation of natural gas,hydrogen,nitrogen,oxygen,carbon dioxide and other gases.In terms of the selection of membrane separation materials,polymer membrane materials have good processibility compared with others,especially polyimide.Because of excellent thermal stability,outstanding mechanical strength and good separation performance,polyimide has become preferred gas separation materials.However,as a result of high rigidity in the aromatic polyimides chains,high symmetry and strong intermolecular interaction,the low free volume and the gas permeability coefficient are not good for gas separation.Besides,polyimide membrane materials are prone to plasticization under high pressure conditions,which deteriorates the separation performance of the materials and limits the application range of the polyimide membrane materials.Crosslinking and mixing with fillers are emerging and effective methods for polymer to improve the properties of the gas separation membrane in recent years.Crosslinking is dedicated to form a three-dimensional network structure between polymer chains by heat treatment,addition of chemical cross-linker or ultraviolet irradiation,which influence the mobility of the molecular chain segment.As a result of crosslinking modification,the anti-plasticization performance of polyimide is expected to enhance,simultaneously.Another modified method is adding inorganic nanoparticles into the polymer matrix to prepare mixing matrix membranes.The addition of particles can restrict the movement of the polymer chain,provide additional transport channels,and improve the separation and plasticization resistance of the materials.In this paper,the effect of polyimide membrane on gas separation performance was investigated by two methods mentioned above respectively.Firstly,polyimide with trifluoromethyl group was successful synthesized according to our previous work.And two types of diamine were used as cross-linking reagents to investigate chemical cross-linking reaction and the influence of diamine type and crosslinking time on it.The results showed that the polyimide film dissolved in aliphatic diamines and didn't in aromatic diamines.Compared with the pristine polyimides,the crosslinked polyimide films showed thermal decomposition at more than 300?in TGA measurement,which was mainly due to the introduction of crosslinking agent.The results of XRD showed that the distance between molecular chains gradually increases with the extension of crosslinking time,and the gas permeability coefficient of membrane should increase in theory.However,the permeability coefficient decreases actually because of the increased crosslinking density.At the same time,the selectivity coefficient increases firstly and then decreases.Secondly,a series of co-polyimides containing carboxyl group were synthesized with different feed ratios by condensation reaction.In addition,a kind of organic metal framework,called Mg-MOF-74 was synthesized and characterized.The mixed matrix membrane?MMMs?containing carboxyl polyimide and Mg-MOF-74 were prepared and related the gas separation performance were investigated.The results showed that the glass transition temperature of carboxyl-containing polyimides increased with the carboxyl content,while the thermal decomposition temperature decreased due to the carboxyl groups which began to decompose at more than 200?.At the same time,with the increase of carboxyl group content,the gas permeability coefficient decreases and the selectivity increases.DBDA-30%with 30 mol%carboxyl content was chosen as a matrix to prepare MOF-containing MMMs.The content of Mg-MOF-74 in the MMMs was 5 wt%,10 wt%and 15 wt%,respectively.It was found by SEM that MOF can be well dispersed in the polymer to form a more uniform system.The glass transition temperature of MMMs exhibited increasing trend because the additions of Mg-MOF-74 limited the mobility of chain segment.Besides,MMMs showed favorable thermal stability.In addition,the permeability coefficients of N₂,CH₄,O₂ and CO₂ for MMMs reduced after mixing with MOF,which was suggested that the gas transportation path may be change.The polymer chain may expand into the pore channel of Mg-MOF-74,reducing the transfer route of gas molecules.At the same time,the addition of MOF limited the mobility of the polymer chain and reduced the free volume of the molecular chain.The synergistic effect of the two reduced the gas separation performance of MMMs.