Synthesis Of Polyimide Containing Crosslinkable Pendant Groups For Gas Separation Properties

How to effectively separated CO₂ from mixed gas is an important separation process in the field of chemical industry and environment,etc.compared to traditional separation process,the CO₂ absorption process based on membrane has high efficiency,low cost,low energy,high flexibility,etc.Polyimide not only has excellent comprehensive performance,but also has strong gas selectivity.However,as a result of rigid larger aromatic ring structure contained with polyimide main chain and the strong molecular inter-atomic forces between chains,there is small free volume.And its easy plasticizing faults under high pressure make the gas eparation performance decrease at the same time the operating costs increase,which greatly limits the polyimide membrane materials to be used widely in the field of gas separation membrane.Crosslinking is an emerging method of polymer modified to improve the performance of membrane gas separation in recent years.Because of the formation of the crosslinking network,ability of the molecular chain segment can be effectively limited,which can decrease permeability and increase gas selectivity.As a result of crosslinking modification,plasticizing performance of polyimide will be increased to form polyimide with better performance will be in high-pressure condensate gas or harsh operating environments separation.Mixing matrix membranes with inorganic particle is a hot spot in the modification of polymer materials in recent years.Nanoscale diameter and smooth channel structure of MWCNTs for gas molecular diffusion through good channel with higer coefficient of permeability make itself be ideal inorganic fillers.In this article,ally groups and trifluoromethyl groups will be introduced to polyimide to study polymer characteristics,and mixing matrix membranes will be producted with CNTs.First of all a novel diamine monomer 4,4'-??3,3'-diallyl-[1,1'-biphenyl]-4,4'-diyl?bis?oxy??bis?3-?trifluoromethyl?aniline??DBDA?was designed and synthesized.Based on this diamine monomer,a series of linear co-polyimides containing different content of cross-linkable pendant groups were prepared.It is indicated that we synthesized the target product successfully from the result of FIR,H1-NMR and MS.The synthetic PIs showed excellent mechanical properties,thermal stability and high Tg with increasing trend as the content of allyl decreased.It is seemed that the crosslinking reactions between allyl groups of polymer side chain take place to form three-dimensional networks with the increase of temperature,which results to theappearance of second loss tangent peak.UV irradiation method is used to study the crosslinking behavior of polymer.FTIR results showed that 1678 cm-1 attributed to C=C absorption peaks disappeared after crosslinking,which proved that there was a crosslinking reaction between allyl groups.It is observed from DMA tests results that the values of loss factor tangent for PIs increase 4-9? after crosslinking,and the second loss tangent peak disappeared for the crosslinking samples which can further demonstrate that crosslinking reaction happened.Thermal stability and mechanical properties of crosslinked polymer improved.50%PI was chosen to prepare CNT-containing MMMs from a series of polymers we synthesized before.DMA and TGA test results showed that,Tg and thermal stability increased because the addition of carbon nanotubes reduce the mobility of chain segment.It was indicated that the values of Tg and Td5% for PIs increased after adding CNTs.It was suggested the three-dimensional network structure formed by crosslinking reaction limited polymer molecular chain movement,thus reduced the free volume,which resulted to the increment of polymers' Tg and thermal stability.Tg and Td5% of CNT50%PI was increased after crosslinking,It was showed that the cross-section of the sample without CNTs was dispersed in MMMs homogenously from the results of SEM.The permeability coefficients of CO₂ and N₂ for 50%PI reduce after crosslinking which is suggested to form a three-dimensional network structure.At the same time,CO₂/N₂ selectivity coefficient was found to increase.MMMs CNT50%PI prepared from 50%PI and 1wt% MWCNTs showed obviously improved gas permeation properties,but selective properties keeped the same level compared with 50%PI,which was attributed to the nanoscale diameter and smooth channel structure of MWCNTs for gas molecular diffusion through good channel with higer coefficient of permeability.The coefficient of permeability of CO₂ and N₂ increased 42% and 53% repectively,meanwhile selectivity coefficient of CO₂/N₂ is almost the same compared to the polymer,which break the trade-off rule between permeability and selectivity improved the efficiency of industrial operation.