Effects of amidation on gas permeation properties of polyimide membranes

Journal of Membrane Science 214 (2003) 83–92

Effects of amidation on gas permeation properties of polyimide membranes Ye Liu a , Mei Lin Chng b , Tai-Shung Chung b,∗ , Rong Wang c a

c

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 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 and 6FDA-durene/mPDA (50:50) as the examples, the amidation was performed by immersing these polyimide dense films in a 10% (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 increase with increasing the immersion time. Gas permeabilities of the modified polyimides to He, O2 , N2 , CO2 and CH4 were measured at 35 ◦ C and the results suggest that the proposed amidation lowers the gas permeabilities of all gases, while improves the gas permselectivities of He/N2 and O2 /N2 . Experimental results also imply that the effects of amidation on gas permselectivity strongly depend on the polyimide chemical structure. For the gas pairs of CO2 /N2 and CO2 /CH4 , the DMEA modified 6FDA-durene/mPDA (50:50) shows enhanced gas permselectivities, whereas the DMEA modified 6FDA-durene exhibits worse gas permselectivities. Since the amidation by DMEA and the chemical cross-linking modification by p-xylene diamine display similar effects on gas separation properties of modified polyimides, one may conclude that the changes of imide groups to amide groups in the cross-linking process has remarkable effects on gas separation properties. The changes in 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 ensure polyimides to be attractive membrane materials for gas separation. Extensive works have been carried out to molecularly design the chemical structure of polyimides in order to search for materials with ∗ Corresponding author. Tel.: +65-874-8373; fax: +65-779-1936. E-mail address: [email protected] (T.-S. Chung).

better gas separation properties [1–6]. Meanwhile, cross-linking modification of polyimides induced by UV-irradiation, thermal treatment or chemical methods was performed to impart polyimides with anti-plasiticisation, chemical resistant and better gas separation properties. The combination of molecular design of polyimides and cross-linking modification is considered to be one of the most promising methods to develop next generation polyimide gas separation systems 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: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 5 3 7 - 9

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However, most of the cross-linking methods published in the literature are not very applicable to polyimide hollow fibres because one may encounter processing difficulties, high operational temperature and special requirements of functional groups. Recently we reported a chemical cross-linking method for polyimides, which was performed by immersing polyimide membranes in a p-xylenediamine methanol solution at ambient temperature [16]. This method has been successfully applied to fabricate cross-linked polyimide hollow fibres with remarkable improvements in gas separation properties, especially in antiplasticisation and gas permselectivity for CO2 /CH4 [17]. However, the effects of the chemical crosslinking modification on gas separation properties depended on the chemical structures of polyimides. For 6FDA-durene (poly(2,3,5,6-phenylene-2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane) diimide) dense films, the cross-linking modification led to decreases in gas permeabilities for all gases studied [16]. However, its effect on gas permselectivity is not straightforward. It increased the gas permselectivities of He/N2 and O2 /N2 , but decreased the permselectivities of CO2 /N2 and CO2 /CH4 . In contrast, gas permeabilities of 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 by little changes in gas permselectivities when the degree of cross-linking was low. A higher degree of crosslinking reduced the gas permeabilities, increased the gas permselectivities of He/N2 and O2 /N2 , while kept the gas permselectivities of CO2 /N2 and CO2 /CH4 almost constant [18]. Therefore, further investigation is necessary to get a clear understanding on how the chemical cross-linking modification modifies gas separation properties of polyimides. Since the chemical cross-linking modification in our previous studies was formed through the amidation reaction between imide groups and p-xylenediamine [16–18], both the cross-linking and amidation led to the changes in gas permeation properties. In order to separate their combined effects and investigate how the amidation alone affects gas separation properties of polyimides, we chose a diamino reagent which has one of its two amino groups to be a tertiary amino group. As a result, the effects of cross-linking on polyimides may be removed. In this paper, we will report

the effects of amidation on gas separation properties of 6FDA-durene/mPDA (50:50) and 6FDA-durene modified by N,N-dimethylaminoethyleneamine (DMEA). 2. Experimental 2.1. Materials 6FDA (2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride) and mPDA (m-phenyl diamine) were sublimated before use. Durene diamine (2,3,5,6teramethyl-1,4-phenylene diamine) was recrystallized from methanol, and NMP (N-methyl-pyrrolidone) was distilled at 42 ◦ C/1 mbar after drying with molecular sieve before use. DMEA, hexane, dichloromethane and methanol were used as received. 6FDA-durene and copolyimide 6FDA-durene/mPDA (50:50) were synthesised and their chemical structures are shown in Fig. 1. The synthesis of 6FDA-durene was described previously [16]. Copolyimide 6FDA-durene/mPDA (50:50) was prepared in a similar way by using a mixture of equal molar durene diamine and mPDA instead of the pure durene diamine. A stoichiometric 6FDA was added to a NMP solution of durene diamine/mPDA with stirring under argon at ambient temperature. Twenty-four hours later, a mixture of acetic anhydride and triethylamine (with a 4:1 molar ratio of acetic anhydride/triethylamine to 6FDA) were slowly added to the solution to perform imidization for 24 h. After being precipitated in methanol, the polymers were filtered and dried at 150 ◦ C under vacuum for 24 h. 2.2. Membrane formation and modification A 2% (w/v) dicholoromethane solution of 6FDAdurene or 6FDA-durene/mPDA was cast onto an optical glass plate under ambient temperature. After most of the solvent was evaporated slowly, the films were dried in a vacuum oven at 250 ◦ C for 48 h to remove the residual solvent. Films with a thickness of around 40 ␮m were prepared for the gas permeation measurements and modifications. The amidation was carried out by immersing the films in a 10% (w/v) hexane solution of DMEA for a certain period of time followed by taking out the films out of the solution, washing with fresh hexane and drying at ambient temperature.

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Fig. 1. Chemical structures of polyimides.

2.3. Characterisation ATR FTIR measurements were carried out using a Perkin-Elmer FTIR spectrometer. The method for the measurement of gas permeation rates was the same as 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 was added to a 5% (w/v) THF solution of polyimide, either 6FDA-durene or 6FDA-durene/mPDA (50:50). The reaction was performed at ambient temperature for 24 h. Since the DMEA modified polyimides were soluble in methanol, their dry powder forms were obtained by precipitation in water followed by drying under vacuum. Dense films cast from methanol or THF solutions of the DMEA modified polyimides were brittle and unsuitable for the test of gas permeation properties. Therefore, the surface modification by immersing the unmodified polyimide dense films in a 10% (w/v) DMEA hexane solution for a certain period of time under ambient temperature was chosen to study the effect of amidation on gas permeability. ATR FTIR was used to monitor the surface chemical structure changes of polyimide dense films during the modification. Fig. 2 shows the typical results of

6FDA-durene/mPDA (50:50) dense films. After immersing in the 10% (w/v) DMEA hexane solution for 24 h, the intensities of the characteristic peaks of imide groups at 1780 (asymmetric stretch of C=O in the imide group), 1728 (symmetric stretch of C=O in the imide group) and 1380 cm−1 (stretch of C–N in the imide group) decrease and the characteristic peaks of amide groups at 1639 (symmetric stretch of C=O in the amide group) and 1534 cm−1 (stretch of C–N and bend of N–H in the amide group) appear. When the immersion time is long enough (72 h), the characteristic peaks of imide groups almost disappear, indicating all the imide groups had been converted into amide groups. The amidation of polyimide is described in Scheme 1. 3.2. Effects of the amidation on gas separation properties of polyimide dense films Fig. 3 describes the dependence of gas permeabilities of the DMEA modified 6FDA-durene/mPDA (50:50) (obtained by an immersion of 24 h) on the upstream gas pressures in the range from 2 to 10 atm. For He, O2 , N2 , and CH4 , the gas permeabilities almost keep constant when the upstream gas pressure increases from 2 to 10 atm, but for CO2 , the gas permeability decreases from 45.7 to 36.0 barrer. This is due to the fact that gas transports through the membrane following the dual-mode sorption model and CO2 is the most condensable gas in these gases [20,21].

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Fig. 2. A comparison of ATR FTIR spectra of 6FDA-durene/mPDA (50:50): (a) original samples; (b) and (c) DMEA modified samples obtained 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 polyimide films are summarised in Table 1. The amidation of both 6FDA-durene/mPDA (50:50) and 6FDAdurene dense films lowers the gas permeabilities and improves the permselectivities for the gas pairs of He/N2 and O2 /N2 . The effect of amidation on permselectivities of CO2 /N2 and CO2 /CH4 depends on the polyimide chemistry. The modified 6FDA-durene/ mPDA (50:50) has higher values of CO2 /N2 and CO2 / CH4 permselectivities, whereas the modified 6FDAdurene has lower values. These phenomena are in agreement with those published data using p-xylene diamine as the cross-linking agent with a long

immersion time [16,18]. Hence, it is reasonable to conclude that the amidation of polyimides contributes to the changes in gas separation properties, especially the gas permselectivities. Gas diffusivity and solubility in polymers determine gas permeability. The apparent diffusion coefficients and solubility coefficients of the modified polyimides obtained by using the time lag method are tabulated in Table 2, except for those of He, which cannot be determined by the method for its fast permeation rates. For 6FDA-durene/mPDA (50:50), the effects of immersion time on the gas permeabilities, diffusion coefficients, and solubility coefficients are depicted in

Scheme 1. Amidation of polyimide by using DMEA.

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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 1 Gas separation properties of the DMEA modified polyimides Polyimide

Immersion time (h)

α(A/B)

P (barrer)a He

O2

N2

CO2

CH4

He/N2

O2 /N2

CO2 /N2

CO2 /CH4

6FDA-durene/mPDA (50:50)

0 4 6 12 24 48

160 146 144 136 128 99.0

24.3 17.3 16.3 15.1 11.7 7.80

5.18 3.38 3.27 2.94 2.06 1.38

84.6 54.9 49.1 46.0 36.0 24.5

2.83 1.70 1.63 1.71 1.05 0.70

31.0 43.2 44.0 46.3 62.1 71.2

4.70 5.12 4.98 5.14 5.68 5.65

16.4 16.2 15.0 15.6 17.5 17.8

29.9 32.3 30.1 26.9 34.3 35.0

6FDA-durene

0 24

362 91.3

125 6.65

33.5 1.33

456 11.6

28.4 0.98

10.8 68.4

3.70 5.00

13.6 8.75

16.1 11.8

a

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

Table 2 Gas diffusion coefficients and solubility coefficients of the DMEA modified polyimides Immersion time (h)

D (10−8 cm2 /s)

6FDA-durene/mPDA (50:50)

0 4 6 12 24 48

16.0 9.03 8.34 7.45 6.28 4.30

3.96 2.22 2.18 1.63 1.34 0.99

6FDA-durene

0 24

61.1 2.85

17.9 0.55

Polyimide

O2

N2

S (10−2 cm−3 (STP)/cm3 cmHg) CO2

CH4

O2

N2

CO2

CH4

7.65 4.68 4.16 3.32 3.18 2.20

0.79 0.44 0.47 0.32 0.28 0.20

1.52 1.92 1.95 2.02 1.91 1.81

1.31 1.52 1.50 1.80 1.54 1.40

11.1 11.7 11.8 14.0 11.3 11.1

3.57 3.96 3.51 5.41 3.72 3.55

28.9 1.16

5.21 0.26

2.01 2.34

1.98 2.28

15.1 10.1

5.51 3.79

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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).

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

Figs. 4 and 5, respectively. As shown in Fig. 4, the gas permeabilities decreased when the immersion time became longer. The degree of the decrease is in the order of CH4 > N2 > CO2 > O2 > He and close to each other for the all gases except for He. In terms of gas diffusion coefficient and solubility coefficient, Fig. 5 indicates that the amidation slightly improves the solubilities of O2 , N2 , CO2 and CH4 but reduces their diffusion coefficients significantly. Therefore, the lowered gas permeabilities are mainly attributed to the decreased diffusion coefficients. Both the increased intersegmental interaction among the newly formed amide groups with the aid of hydrogen bonds and the reduced free volume due to the space filling effect of the DMEA groups contributed to the decreased diffusion coefficients. Meanwhile, the stronger interaction between the amide groups and gas molecules increase their solubility coefficients. As for the DMEA modified 6FDA-durene, the decreases in gas permeabilities are also due to the significantly lowered diffusion coefficients, as shown in Table 2. The solubility coefficients of O2 and N2 are slightly improved; however, in contrast, those of CO2 and CH4 are somewhat decreased. This phenomenon is attributed to the higher side group (CH3 + CF3 ) density in 6FDA-durene as compared to 6FDA-durene/mPDA (50:50). The higher side group density possibly hinders the formation of

89

charge transfer complex (CTC) between the imide groups and leads to loose intersegmental packing due to the steric hindrance and the chain rigidity, therefore, facilitates the absorption of gas molecules being highly condensable and of larger Lennard–Jones diameters, e.g. CO2 (0.40 nm) or CH4 (0.38 nm), as compared with those of a smaller one, O2 (0.34 nm) or N2 (0.37 nm). After the amidization, the space filling effect of DMEA groups and the increased intersegmental interaction due to the hydrogen bonding formation make the absorption of gas molecules of larger Lennard–Jones diameters difficult and lead to reduced solubilities. The effects of immersion time on the gas permselectivity, diffusion selectivity and solubility selectivity of modified 6FDA-durene/mPDA (50:50) polyimides are illuminated in Figs. 6–8, respectively. The increased permselectivity of He/N2 , as shown in Fig. 6, should result from the denser packing of intermolecular chains because of the formation of hydrogen bonds among the formed amide groups and the space filling effect of the DMEA groups, which makes the diffusion of larger nitrogen molecules more difficult. Fig. 7 demonstrates that the diffusion selectivities of all gas pairs increase with increasing the immersion time. However, the solubility selectivity, illustrated in Fig. 8, increases slightly for O2 /N2 , keeps

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

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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 slightly for CO2 /N2 . All these factors contribute to the increased gas permselectivities of O2 /N2 , CO2 /N2 and CO2 /CH4 after the amidation as shown in Fig. 6.

As listed in Table 3, the increased diffusion selectivity of O2 /N2 is the main factor to yield improved gas permselectivities of O2 /N2 for the DMEA modified 6FDA-durene. The significantly decreased CO2

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

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

6FDA-durene

Immersion time (h)

0 24

Diffusion selectivity

Solubility selectivity

O2 /N2

CO2 /N2

CO2 /CH4

O2 /N2

CO2 /N2

CO2 /CH4

3.39 4.83

1.61 2.00

5.58 4.00

1.00 1.00

7.50 4.35

2.73 2.63

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

conclude that the modification of imide groups to amide groups has remarkable effects on gas separation properties of polyimides in the cross-linking process.

Acknowledgements 4. Conclusions The amidation of polyimides, 6FDA-durene/mPDA (50:50) and 6FDA-durene, were realised by immersing the dense films in a DMEA hexane solution for a certain period of time at ambient temperature. For both polyimides, the amidation significantly lowered gas diffusion coefficients and gas permeabilities for all the gases studied. This phenomenon was probably resulted from a stronger intersegmental interaction among the newly formed amide groups with the aid of hydrogen bonds and the space filling effects of DMEA. The gas permselectivities of the DMEA modified polyimides were improved for the separation of He/N2 and O2 /N2 . However, the effects of the amidation on gas permselectivities of CO2 /N2 and CO2 /CH4 depend on the polyimide chemical structure. For the DMEA modified 6FDA-durene/mPDA (50:50), the gas permselectivities of CO2 /N2 and CO2 /CH4 increase because of the increase in diffusion selectivities after amidation, whereas for the DMEA modified 6FDA-durene, CO2 /N2 and CO2 /CH4 permselectivities decrease because of the drop in diffusion selectivity of CO2 /CH4 , accompanying the reductions in solubility selectivities of CO2 /N2 and CO2 /CH4 for a significantly decreased CO2 solubility after the amidization. This study indicates that the amidation of polyimides has similar impact on gas separation properties of polyimides as that of chemical cross-linking modification using p-xylene diamine. Therefore, one may

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

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