A Study On Preparation And Gas Separation Performance Of New Polyimide Membranes

Compared with traditional gas separation technologies such as low temperature fractionation and pressure swing adsorption（PSA）,membrane separation has found increasing industrial applications because it offers many advantages such as low energy consumption,easy operation,simple process,low capital investment cost,and small land occupation.However,there is a trade-off relationship between gas permeability coefficient and selectivity,i.e.,the membrane having higher gas permeability coefficient tends to have lower selectivity,and vice versa.To develop polymer membranes with both high permeability coefficient and high selectivity,two approaches have been explored in this thesis.Firstly,large polar pendant groups were introduced into a five-membered ring polyimide（PI）via chemical modification to increase the gas permeability coefficients,while high selectivities were maintained.Secondly,a series of six-membered ring polyimides with different flexible/rigid backbone structures were synthesized,and the influences of flexible/rigid backbone structures and contents of polar sulfonyl group were studied.By opmimizing the polyimide structure,the membranes with high gas permeability coefficients and selectivities have been developed.In Chapter 2,a five-membered ring polyimide（BPDA-Tr MPD）with excellent thermal resistance was synthesized via the conventional two-step method from an aromatic dianhydride,3,3’,4,4’-biphenyltetracarboxylic dianhydride（BPDA）,and an aromatic diamine,2,4,6-trimethyl-1,3-phenylenediamine（Tr MPD）.Three brominated polyimides were prepared by the radical substitution reaction using a brominating reagent,N-bromosuccinimide（NBS）,an initiator,peroxybenzoyl（BPO）,and the polyimide.After bromination,the polyimides show increased permeability coefficients but decreased selectivities.One of the brominated polyimide,PI-Br0.7（here,‘0.7’refers to the feed ratio of NBS to the parent polyimide）was employed to undergo next step reaction with three compounds,potassium phthalimide,pyridine and N-methylimidazole yielding three graft polyimides.The graft polyimides exhibit obviously increased selectivities compared to brominated polyimides.Among them,the polymide membrane grafted with potassium phthalimide（PI-Br0.7-1）shows two times higher gas permeability coefficients and slightly higher selectivities（O₂/N₂ and CO₂/CH₄）to that of the pristine polyimide membrane（BPDA-Tr MPD）.This indicates that the graft-modification method of this study is favorable for improving gas separation performance of the polyimide membrane.In Chapter 3,a series of six-membered ring polyimides were synthesized by random copolymerization of 1,4,5,8-naphthalenetetracarboxylic dianhydride（NTDA）and a series of aromatic diamines such as 2,2’-bis（trifluoromethyl）benzidine（BTFBz）,2,4,6-trimethyl-1,3-phenylenediamine（Tr MPD）,9,9-bis（4-aminophenyl）fluorene（BAPF）,3,7-diamino-2,8-dimethyldibenzothiazole sulfone（DDBT）,and 4,4’-bis（3-aminophenoxy）diphenyl sulfone（m BAPS）.The resulting polyimides exhibit excellent thermal resistance.Compared with the polyimide homopolymers,the copolyimides exhibit significantly improved solubility because the structural regulation of polymer backbones is destroyed due to random copolymerization.Tough membranes with good mechanical properties were obtained by solution cast method.Among the copolyimide membranes,NTDA-DDBT/BTFBz（1/1）shows the highest tensile stress of 244 MPa and an elongation at break of 23.1%.XRD patterns suggest that they are amorphous and have large segmental distances（0.50–0.75 nm）.The gas permeation properties of polyimides were investigated,and the results showed that gas permeability coefficients and selectivities were strongly related to the rigidity and flexibility of the main chain structure.The membranes with more rigid backbone tend to have larger gas permeability coefficients but lower selectivities.Gas permeability coefficients of rigid polyimides prepared by DDBT,Tr MPD,BAPF and BTFBz are about two orders of magnitude higher than that of flexible polyimides prepared by m BAPS,but the differences in selectivities are not so significant.Among them,copolyimide membranes consisting DDBT exhibit excellent gas separation performances.For example,the gas permeability coefficient of NTDA-BAPF/DDBT（1/1）reaches 227 Barrer for oxygen and 1200 Barrer for CO₂(1 Barrer=10^-10cm³（STP）·cm^-1s^-1cm Hg^-1),while the selectivity is 4.46 for O₂/N₂ and 24.1 for CO₂/CH₄,respectively.The separation performance of this copolyimide membrane just reaches the latest upper bound reported by Robeson in 2008.