Preparation And Characterization Of Polyimide Gas Separation Membranes Containing Bulky Pendant Groups

With the rapid development of social civilization,in modern industrial life,people's environmental protection concepts continue to deepen,the recycling of resources has become the primary task and goal of sustainable economic and social development.A large amount of mixed gas existed or be produced in the world every day,and it can be recovered only after purification,separation and enrichment.Polymer membranes have huge advantages when be used to separate gas,such as light weight,economy,high efficiency,saving energy,and saving space.Among them,polyimide contains many excellent properties,showing great potential in the field of gas separation.It has been gradually recognized and widely used.In 1991,Robeson aggregated and published Robeson's upper bound theory,pointing out that there is a trade-off relationship between gas permeability and selectivity for polymer membranes.Therefore,the development of some new materials that can approach or exceed this upper bound is the goal and the driving force for the continuous development of gas separation membranes.This article starts from the viewpoint of structural modification,according to the relationship between the molecular structure of polyimide and gas separation performance,two diamine molecules containing large side groups were designed and synthesized:3'-(trifluoromethyl)-[1,1'-biphenyl]-2,5-diamine,3',5'-bis(trifluoromethyl)-[1,1'-biphenyl]-2,5-diamine,at the same time,fluorine-containing structures are also introduced into each molecule,in addition,and use p-phenylenediamine(PDA)without pendant groups as a comparative experiment.Combine the above diamines with 4,4'-(hexafluoroisopropylene)diphthalic anhydride(6FDA)and biphenyl dianhydride 3,3',4,4'-biphenyltetracarboxylic dianhydride(BPDA)by"Two-step method"polymerization,a total of six kinds of transparent PI films.Using FT-IR,~1H-NMR and other methods to complete the structural analysis and discussion of small molecules,using UV-Vis,TGA,DSC,TMA,universal testing machine,etc.to analyze and discuss solubility,heat resistance,mechanical properties,etc ofpolymer films.at the same time,the gas separation performance of the material in different atmospheres was tested.The study found that this series of films have good comprehensive performance,and the UV cut-off wavelength range of all films is 322~397 nm;no weight lossphenomenon occurs for all films below 400?,Explain that the Polyamic acid is fully imidized,the initial thermal decomposition temperature range of each film is between 430~527?,the glass transition temperature is above273?;The elongation at break of all samples is 4.7~7.4%,the tensile strength is 62~129 MPa,the Young's modulus is 1.56~2.22 GPa;As the molecular chain free volume increases,the permeability coefficient of each gas molecule in the film increases,and the improvement of the permeability coefficient is accomplished by increasing the diffusion capacity.