Journal of Membrane Science 214 (2003) 83–92

Effects of amidation on gas permeation propertiesof polyimide membranes

Ye Liu a, Mei Lin Chngb, Tai-Shung Chungb,∗, Rong Wangca Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore

b Department of Chemical and Environmental Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore 119260, Singapore

c Institute of Environmental Science& Engineering (IESE), 18 Nanyang Drive, Singapore 637723, Singapore

Received 4 February 2002; received in revised form 7 November 2002; accepted 11 November 2002

Abstract

The effects of amidation on gas separation properties of polyimide dense films were investigated. Using 6FDA-durene and6FDA-durene/mPDA (50:50) as the examples, the amidation was performed by immersing these polyimide dense films in a10% (w/v)N,N-dimethylaminoethyleneamine (DMEA) hexane solution for a certain period of time at ambient temperature.FTIR spectra indicate the intensities of the characteristic peaks of imide group decrease, while those of amide groups increasewith increasing the immersion time. Gas permeabilities of the modified polyimides to He, O2, N2, CO2 and CH4 weremeasured at 35◦C and the results suggest that the proposed amidation lowers the gas permeabilities of all gases, whileimproves the gas permselectivities of He/N2 and O2/N2. Experimental results also imply that the effects of amidation on gaspermselectivity strongly depend on the polyimide chemical structure. For the gas pairs of CO2/N2 and CO2/CH4, the DMEAmodified 6FDA-durene/mPDA (50:50) shows enhanced gas permselectivities, whereas the DMEA modified 6FDA-dureneexhibits worse gas permselectivities. Since the amidation by DMEA and the chemical cross-linking modification byp-xylenediamine display similar effects on gas separation properties of modified polyimides, one may conclude that the changes ofimide groups to amide groups in the cross-linking process has remarkable effects on gas separation properties. The changesin gas separation properties were discussed in terms of diffusion coefficients and solubility coefficients.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Gas separation membranes; Chemical modification; Amidation; Fluoropolyimide; 6FDA-durene polyimide

1. Introduction

Good physical and gas separation properties en-sure polyimides to be attractive membrane materialsfor gas separation. Extensive works have been car-ried out to molecularly design the chemical structureof polyimides in order to search for materials with

∗ Corresponding author. Tel.:+65-874-8373;fax: +65-779-1936.E-mail address:chencts@nus.edu.sg (T.-S. Chung).

better gas separation properties[1–6]. Meanwhile,cross-linking modification of polyimides inducedby UV-irradiation, thermal treatment or chemicalmethods was performed to impart polyimides withanti-plasiticisation, chemical resistant and better gasseparation properties. The combination of moleculardesign of polyimides and cross-linking modification isconsidered to be one of the most promising methods todevelop next generation polyimide gas separation sys-tems for the use in complex and harsh environments[1,7–16].

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(02)00537-9

84 Y. Liu et al. / Journal of Membrane Science 214 (2003) 83–92

However, most of the cross-linking methods pub-lished in the literature are not very applicable topolyimide hollow fibres because one may encounterprocessing difficulties, high operational temperatureand special requirements of functional groups. Re-cently we reported a chemical cross-linking methodfor polyimides, which was performed by immersingpolyimide membranes in ap-xylenediamine methanolsolution at ambient temperature[16]. This methodhas been successfully applied to fabricate cross-linkedpolyimide hollow fibres with remarkable improve-ments in gas separation properties, especially in anti-plasticisation and gas permselectivity for CO2/CH4[17]. However, the effects of the chemical cross-linking modification on gas separation properties de-pended on the chemical structures of polyimides. For6FDA-durene (poly(2,3,5,6-phenylene-2,2-bis(3,4-di-carboxyphenyl) hexafluoropropane) diimide) densefilms, the cross-linking modification led to decreasesin gas permeabilities for all gases studied[16]. How-ever, its effect on gas permselectivity is not straightfor-ward. It increased the gas permselectivities of He/N2and O2/N2, but decreased the permselectivities ofCO2/N2 and CO2/CH4. In contrast, gas permeabilitiesof the cross-linked copolyimide 6FDA-durene/mPDA(50:50) (copoly(2,3,5,6-phenylene/m-phenyl-2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane) diimide(50:50)) dense films were increased accompanied bylittle changes in gas permselectivities when the degreeof cross-linking was low. A higher degree of cross-linking reduced the gas permeabilities, increased thegas permselectivities of He/N2 and O2/N2, while keptthe gas permselectivities of CO2/N2 and CO2/CH4almost constant[18]. Therefore, further investigationis necessary to get a clear understanding on howthe chemical cross-linking modification modifies gasseparation properties of polyimides.

Since the chemical cross-linking modification in ourprevious studies was formed through the amidationreaction between imide groups andp-xylenediamine[16–18], both the cross-linking and amidation led tothe changes in gas permeation properties. In order toseparate their combined effects and investigate howthe amidation alone affects gas separation propertiesof polyimides, we chose a diamino reagent which hasone of its two amino groups to be a tertiary aminogroup. As a result, the effects of cross-linking on poly-imides may be removed. In this paper, we will report

the effects of amidation on gas separation properties of6FDA-durene/mPDA (50:50) and 6FDA-durene mod-ified by N,N-dimethylaminoethyleneamine (DMEA).

2. Experimental

2.1. Materials

6FDA (2,2-bis(3,4-dicarboxyphenyl) hexafluoropr-opane dianhydride) and mPDA (m-phenyl diamine)were sublimated before use. Durene diamine (2,3,5,6-teramethyl-1,4-phenylene diamine) was recrystallizedfrom methanol, and NMP (N-methyl-pyrrolidone) wasdistilled at 42◦C/1 mbar after drying with molecularsieve before use. DMEA, hexane, dichloromethaneand methanol were used as received.

6FDA-durene and copolyimide 6FDA-durene/mP-DA (50:50) were synthesised and their chemicalstructures are shown inFig. 1. The synthesis of6FDA-durene was described previously[16]. Copoly-imide 6FDA-durene/mPDA (50:50) was prepared in asimilar way by using a mixture of equal molar durenediamine and mPDA instead of the pure durene di-amine. A stoichiometric 6FDA was added to a NMPsolution of durene diamine/mPDA with stirring underargon at ambient temperature. Twenty-four hours later,a mixture of acetic anhydride and triethylamine (witha 4:1 molar ratio of acetic anhydride/triethylamineto 6FDA) were slowly added to the solution to per-form imidization for 24 h. After being precipitatedin methanol, the polymers were filtered and dried at150◦C under vacuum for 24 h.

2.2. Membrane formation and modification

A 2% (w/v) dicholoromethane solution of 6FDA-durene or 6FDA-durene/mPDA was cast onto an opti-cal glass plate under ambient temperature. After mostof the solvent was evaporated slowly, the films weredried in a vacuum oven at 250◦C for 48 h to removethe residual solvent. Films with a thickness of around40�m were prepared for the gas permeation measure-ments and modifications.

The amidation was carried out by immersing thefilms in a 10% (w/v) hexane solution of DMEA fora certain period of time followed by taking out thefilms out of the solution, washing with fresh hexaneand drying at ambient temperature.

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Fig. 1. Chemical structures of polyimides.

2.3. Characterisation

ATR FTIR measurements were carried out using aPerkin-Elmer FTIR spectrometer. The method for themeasurement of gas permeation rates was the sameas previously reported[19].

3. Results and discussion

3.1. Amidation of polyimides by DMEA

Originally we planed to modify polyimides in bulk.DMEA with an equal molar ratio to imide group wasadded to a 5% (w/v) THF solution of polyimide, ei-ther 6FDA-durene or 6FDA-durene/mPDA (50:50).The reaction was performed at ambient temperaturefor 24 h. Since the DMEA modified polyimides weresoluble in methanol, their dry powder forms wereobtained by precipitation in water followed by dry-ing under vacuum. Dense films cast from methanolor THF solutions of the DMEA modified polyimideswere brittle and unsuitable for the test of gas perme-ation properties. Therefore, the surface modificationby immersing the unmodified polyimide dense filmsin a 10% (w/v) DMEA hexane solution for a certainperiod of time under ambient temperature was chosento study the effect of amidation on gas permeability.

ATR FTIR was used to monitor the surface chemi-cal structure changes of polyimide dense films duringthe modification.Fig. 2 shows the typical results of

6FDA-durene/mPDA (50:50) dense films. After im-mersing in the 10% (w/v) DMEA hexane solution for24 h, the intensities of the characteristic peaks of imidegroups at 1780 (asymmetric stretch of C=O in theimide group), 1728 (symmetric stretch of C=O in theimide group) and 1380 cm−1 (stretch of C–N in theimide group) decrease and the characteristic peaks ofamide groups at 1639 (symmetric stretch of C=O inthe amide group) and 1534 cm−1 (stretch of C–N andbend of N–H in the amide group) appear. When theimmersion time is long enough (72 h), the characteris-tic peaks of imide groups almost disappear, indicatingall the imide groups had been converted into amidegroups. The amidation of polyimide is described inScheme 1.

3.2. Effects of the amidation on gas separationproperties of polyimide dense films

Fig. 3 describes the dependence of gas permeabil-ities of the DMEA modified 6FDA-durene/mPDA(50:50) (obtained by an immersion of 24 h) on theupstream gas pressures in the range from 2 to 10 atm.For He, O2, N2, and CH4, the gas permeabilitiesalmost keep constant when the upstream gas pres-sure increases from 2 to 10 atm, but for CO2, thegas permeability decreases from 45.7 to 36.0 barrer.This is due to the fact that gas transports through themembrane following the dual-mode sorption modeland CO2 is the most condensable gas in these gases[20,21].

86 Y. Liu et al. / Journal of Membrane Science 214 (2003) 83–92

Fig. 2. A comparison of ATR FTIR spectra of 6FDA-durene/mPDA (50:50): (a) original samples; (b) and (c) DMEA modified samplesobtained by an immersion in 10% (w/v) DMEA hexane solution for 24 and 72 h at ambient temperature, respectively.

The gas permeabilities of DMEA modified poly-imide films are summarised inTable 1. The amidationof both 6FDA-durene/mPDA (50:50) and 6FDA-durene dense films lowers the gas permeabilities andimproves the permselectivities for the gas pairs ofHe/N2 and O2/N2. The effect of amidation on perms-electivities of CO2/N2 and CO2/CH4 depends on thepolyimide chemistry. The modified 6FDA-durene/mPDA (50:50) has higher values of CO2/N2 and CO2/CH4 permselectivities, whereas the modified 6FDA-durene has lower values. These phenomena are inagreement with those published data usingp-xylenediamine as the cross-linking agent with a long

Scheme 1. Amidation of polyimide by using DMEA.

immersion time[16,18]. Hence, it is reasonable toconclude that the amidation of polyimides contributesto the changes in gas separation properties, especiallythe gas permselectivities.

Gas diffusivity and solubility in polymers determinegas permeability. The apparent diffusion coefficientsand solubility coefficients of the modified polyimidesobtained by using the time lag method are tabulated inTable 2, except for those of He, which cannot be de-termined by the method for its fast permeation rates.For 6FDA-durene/mPDA (50:50), the effects of im-mersion time on the gas permeabilities, diffusion co-efficients, and solubility coefficients are depicted in

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Fig. 3. Dependence of gas permeabilities of the DMEA modified 6FDA-durene/mPDA (50:50) (obtained by an immersion in 10% (w/v)DMEA hexane solution for 24 h) on the upstream gas pressure.

Table 1Gas separation properties of the DMEA modified polyimides

Polyimide Immersiontime (h)

P (barrer)a α(A/B)

He O2 N2 CO2 CH4 He/N2 O2/N2 CO2/N2 CO2/CH4

6FDA-durene/mPDA (50:50) 0 160 24.3 5.18 84.6 2.83 31.0 4.70 16.4 29.94 146 17.3 3.38 54.9 1.70 43.2 5.12 16.2 32.36 144 16.3 3.27 49.1 1.63 44.0 4.98 15.0 30.1

12 136 15.1 2.94 46.0 1.71 46.3 5.14 15.6 26.924 128 11.7 2.06 36.0 1.05 62.1 5.68 17.5 34.348 99.0 7.80 1.38 24.5 0.70 71.2 5.65 17.8 35.0

6FDA-durene 0 362 125 33.5 456 28.4 10.8 3.70 13.6 16.124 91.3 6.65 1.33 11.6 0.98 68.4 5.00 8.75 11.8

a 1 barrer= 1 × 10−10 cm3 (STP) cm/cm2 s cmHg.

Table 2Gas diffusion coefficients and solubility coefficients of the DMEA modified polyimides

Polyimide Immersiontime (h)

D (10−8 cm2/s) S (10−2 cm−3 (STP)/cm3 cmHg)

O2 N2 CO2 CH4 O2 N2 CO2 CH4

6FDA-durene/mPDA (50:50) 0 16.0 3.96 7.65 0.79 1.52 1.31 11.1 3.574 9.03 2.22 4.68 0.44 1.92 1.52 11.7 3.966 8.34 2.18 4.16 0.47 1.95 1.50 11.8 3.51

12 7.45 1.63 3.32 0.32 2.02 1.80 14.0 5.4124 6.28 1.34 3.18 0.28 1.91 1.54 11.3 3.7248 4.30 0.99 2.20 0.20 1.81 1.40 11.1 3.55

6FDA-durene 0 61.1 17.9 28.9 5.21 2.01 1.98 15.1 5.5124 2.85 0.55 1.16 0.26 2.34 2.28 10.1 3.79

88 Y. Liu et al. / Journal of Membrane Science 214 (2003) 83–92

Fig. 4. Effect of the immersion time on gas permeabilities of the DMEA modified 6FDA-durene/mPDA (50:50).

Fig. 5. Effect of the immersion time on the diffusion coefficients and solubility coefficients of the DMEA modified 6FDA-durene/mPDA(50:50).

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Figs. 4 and 5, respectively. As shown inFig. 4, thegas permeabilities decreased when the immersion timebecame longer. The degree of the decrease is in theorder of CH4 > N2 > CO2 > O2 > He and closeto each other for the all gases except for He. In termsof gas diffusion coefficient and solubility coefficient,Fig. 5 indicates that the amidation slightly improvesthe solubilities of O2, N2, CO2 and CH4 but reducestheir diffusion coefficients significantly. Therefore, thelowered gas permeabilities are mainly attributed to thedecreased diffusion coefficients. Both the increasedintersegmental interaction among the newly formedamide groups with the aid of hydrogen bonds and thereduced free volume due to the space filling effect ofthe DMEA groups contributed to the decreased diffu-sion coefficients. Meanwhile, the stronger interactionbetween the amide groups and gas molecules increasetheir solubility coefficients. As for the DMEA modi-fied 6FDA-durene, the decreases in gas permeabilitiesare also due to the significantly lowered diffusion co-efficients, as shown inTable 2. The solubility coeffi-cients of O2 and N2 are slightly improved; however,in contrast, those of CO2 and CH4 are somewhat de-creased. This phenomenon is attributed to the higherside group (CH3 + CF3) density in 6FDA-durene ascompared to 6FDA-durene/mPDA (50:50). The higherside group density possibly hinders the formation of

Fig. 6. Effect of the immersion time on permselectivities of the DMEA modified 6FDA-durene/mPDA (50:50).

charge transfer complex (CTC) between the imidegroups and leads to loose intersegmental packing dueto the steric hindrance and the chain rigidity, there-fore, facilitates the absorption of gas molecules be-ing highly condensable and of larger Lennard–Jonesdiameters, e.g. CO2 (0.40 nm) or CH4 (0.38 nm), ascompared with those of a smaller one, O2 (0.34 nm) orN2 (0.37 nm). After the amidization, the space fillingeffect of DMEA groups and the increased interseg-mental interaction due to the hydrogen bonding for-mation make the absorption of gas molecules of largerLennard–Jones diameters difficult and lead to reducedsolubilities.

The effects of immersion time on the gas permse-lectivity, diffusion selectivity and solubility selectivityof modified 6FDA-durene/mPDA (50:50) polyimidesare illuminated inFigs. 6–8, respectively. The in-creased permselectivity of He/N2, as shown inFig. 6,should result from the denser packing of intermolec-ular chains because of the formation of hydrogenbonds among the formed amide groups and the spacefilling effect of the DMEA groups, which makes thediffusion of larger nitrogen molecules more difficult.Fig. 7 demonstrates that the diffusion selectivitiesof all gas pairs increase with increasing the immer-sion time. However, the solubility selectivity, illus-trated inFig. 8, increases slightly for O2/N2, keeps

90 Y. Liu et al. / Journal of Membrane Science 214 (2003) 83–92

Fig. 7. Effect of the immersion time on diffusion selectivities of the DMEA modified 6FDA-durene/mPDA (50:50).

almost constant for CO2/CH4, but decreases slightlyfor CO2/N2. All these factors contribute to the in-creased gas permselectivities of O2/N2, CO2/N2 andCO2/CH4 after the amidation as shown inFig. 6.

Fig. 8. Effects of the immersion time on solubility selectivities of the DMEA modified 6FDA-durene/mPDA (50:50).

As listed inTable 3, the increased diffusion selec-tivity of O2/N2 is the main factor to yield improvedgas permselectivities of O2/N2 for the DMEA mod-ified 6FDA-durene. The significantly decreased CO2

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Table 3Diffusion selectivities and solubility selectivities of the DMEA modified 6FDA-durene

Polyimide Immersion time (h) Diffusion selectivity Solubility selectivity

O2/N2 CO2/N2 CO2/CH4 O2/N2 CO2/N2 CO2/CH4

6FDA-durene 0 3.39 1.61 5.58 1.00 7.50 2.7324 4.83 2.00 4.00 1.00 4.35 2.63

solubility leads to a lowered solubility selectivity ofCO2/N2, therefore, a decreased gas permselectivity ofCO2/N2. The reduction in both the diffusion selectiv-ity and solubility selectivity of CO2/CH4 contributedto a lower permselectivity due to the amidation.

4. Conclusions

The amidation of polyimides, 6FDA-durene/mPDA(50:50) and 6FDA-durene, were realised by immers-ing the dense films in a DMEA hexane solution fora certain period of time at ambient temperature. Forboth polyimides, the amidation significantly loweredgas diffusion coefficients and gas permeabilities forall the gases studied. This phenomenon was probablyresulted from a stronger intersegmental interactionamong the newly formed amide groups with the aidof hydrogen bonds and the space filling effects ofDMEA. The gas permselectivities of the DMEA mod-ified polyimides were improved for the separation ofHe/N2 and O2/N2. However, the effects of the amida-tion on gas permselectivities of CO2/N2 and CO2/CH4depend on the polyimide chemical structure. Forthe DMEA modified 6FDA-durene/mPDA (50:50),the gas permselectivities of CO2/N2 and CO2/CH4increase because of the increase in diffusion selectivi-ties after amidation, whereas for the DMEA modified6FDA-durene, CO2/N2 and CO2/CH4 permselectiv-ities decrease because of the drop in diffusion se-lectivity of CO2/CH4, accompanying the reductionsin solubility selectivities of CO2/N2 and CO2/CH4for a significantly decreased CO2 solubility after theamidization.

This study indicates that the amidation of poly-imides has similar impact on gas separation propertiesof polyimides as that of chemical cross-linking mod-ification usingp-xylene diamine. Therefore, one may

conclude that the modification of imide groups toamide groups has remarkable effects on gas separationproperties of polyimides in the cross-linking process.

Acknowledgements

The authors would like to thank the British Gasgrant and the NUS grant (R-279-000-108-112) forfunding this research.

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