Gas transport properties of poly(ethylene oxide-co-epichlorohydrin) membranes

Journal of Membrane Science 230 (2004) 161–169

Gas transport properties of poly(ethylene oxide-coepichlorohydrin) membranes C. Charmette a , J. Sanchez a,∗ , Ph. Gramain a , A. Rudatsikira b a

Institut Européen des Membranes, UMR 5635, CNRS, ENSCM, UMII, cc 047, 2 Place Eugène Bataillon, 34095 Montpellier Cedex 5, France b Laboratoire d’Hétérochimie Moléculaire et Macromoléculaire, UMR 5076, CNRS, ENSCM, UMII, 8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France Received 8 April 2003; received in revised form 29 July 2003; accepted 27 October 2003

Abstract Gas permeation properties of crosslinked membranes prepared from a series of poly(ethylene oxide-co-epichlorohydrin) (P(EO/EP)) copolymers with different contents of ethylene oxide are determined by using the constant-volume and pressure-increase method. In addition to the chemical composition, the transport properties are related to the main characteristics of copolymers like the glass transition temperature, crystallinity and crosslinking ratio. Permeation measurements of He, H2 , N2 , O2 , CO2 and CH4 show that the permeabilities are nearly constant up to an EO content of about 75–80 mol%, then increase rapidly up to a maximum around 90 mol% of EO in the copolymers. The same behavior is observed for the diffusion coefficient and the CO2 sorption coefficient. The presence of an optimal EO composition is explained by the competition between crystalline and amorphous EO sequences. The copolymers present very high CO2 permeability and selectivity respect to other permanent gases even in gas mixtures and under high pressures. © 2003 Elsevier B.V. All rights reserved. Keywords: Poly(ethylene oxide-co-epichlorohydrin); Gas permeability; Diffusion coefficient; Sorption coefficient; CO2 permselectivity

1. Introduction The carbon dioxide separation by membrane process is placed among the most interesting gas separation applications, particularly for natural or landfill gas de-acidification [1,2] and in food packaging [3]. Many polymers have been studied for CO2 separation and particularly glassy polymers. Among the numerous factors affecting the permeability and selectivity results, the polymer polarity and thus the solubility factor are very important. For example, Bhide and Stern [4] showed that for films of polar polymers, the permeability tends to be lower and the selectivity higher respect to non-polar polymers. Silicone polymers are a typical example of elastomers reported as good polymers for CO2 separation but their CO2 selectivity is economically too low. Among the other suggested polar structures, polyethylene oxide (PEO) seems to be one of the most interesting due to its polarity and

∗

Corresponding author. Tel.: +33-467-149149; fax: +33-467-149149. E-mail address: [email protected] (J. Sanchez).

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.10.043

CO2 affinity. Membranes containing liquid low molecular weight PEO have been reported [5–7]. In order to obtain free-standing membranes, amorphous short PEO has been used as segments in copolymers leading to various interesting structures such as cellulose nitrate or acetate [8], polyurethanes [9,10], polyimides [10,11], polyamides [10,12], polymethacrylates [5]. In all cases, the PEO-based films present interesting performances in terms of CO2 permselectivity. The results have generally shown that CO2 permselectivity strongly depends on the PEO content. Yoshino et al. [10] studied the gas transport characteristics of PEO-polyurethanes, polyamides and polyimides segmented copolymers, with a maximum PEO content of 68.6%. They reported CO2 permeability up to 238 barrer with a CO2 /N2 ideal selectivity of 49. More recently, Yoshino et al. [13] studied the permeability properties of high molecular weight copolymers of EO, 2-(2-methoxyethoxy)ethylglycidyl ether and allylglycidyl ether (P(EO-EM-AGE)). Very good results for CO2 permeability (up to 773 barrer) and CO2 /N2 selectivity (up to 50) were reported. The difference on the CO2 permeability between the PEO-polyimide segmented copolymers and P(EO-EM-AGE) was attributed to the

162

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

lower Tg value observed for the last type of polymer. These authors concluded that the good permselectivity values observed are the consequence of the high CO2 solubility in the EO groups. Nevertheless, CO2 solubility measurements have not been reported. This paper is the continuity of previous publications on the permeation characteristics [14,15] and ionic conductivity [16] of self-standing films based on crosslinked ethylene oxide/epichlorohydrin (P(EO/EP)) copolymers. The membranes prepared from high molecular weight copolymers present different EO content (from 50 to 96 mol%) and different crosslinking ratios.

2.1. Materials Polyepichlorohydrin was supplied by Aldrich. The copolymers have been kindly provided by Zeon Chemical Company (Japan) and Daiso Chemical Company (Japan) (Table 1). These high molecular weight statistical copolymers are composed of ethylene oxide (–CH2 –CH2 O–) and epichlorohydrin (–CH–(CH2 Cl)–CH2 O–). 2,5-Dimercapto1,3,4-thiadiazol dipotassium salt (K-bismuthiol; Aldrich) was used as crosslinking agent. The weight of crosslinking agent by 100 g of copolymer (phr) characterizes the crosslinking degree. 2.1.1. Membranes preparation The free-standing crosslinked films were prepared by casting from a 4% (w/w) solution of copolymer into acetonitrile or THF. After the solubility process was completed, the solution was filtrated and concentrated under vacuum to a content of about 15 wt.% in copolymer, then an acetonitrile solution of crosslinking agent K-bismuthiol was added and the mixture was poured into a mold and slowly dried at room Table 1 Source and composition of studied copolymers

A0/100 Z50/50 D56/44 D61/39 D83/17 D86/14 D87/13 D88/12 D90/10 D93/7 D94/6 D96/4

Copolymer name

Aldrich Hydrin C2000 Epichlomer C Epichlomer D4 EO/EP 83/17 EO/EP 86/14 EO/EP 87/13 Daiso EP 12 Daiso EP 10 EO/EP 93/7 EO/EP 94/6 EO/EP 96/4

2.1.2. Membranes characteristics Membranes were characterized in terms of crosslinking ratio by measuring the swelling ratios in water and dichloromethane. The swelling ratio G was calculated by the following equation: G=

2. Experimental

Membranea

temperature. The quantity of K-bismuthiol was adjusted in order to replace between 1 and 50% of chloride atoms. After solvent evaporation, the films were oven-heated during 1.5 h at 363 K. The membranes were washed with distilled water, dried under vacuum for 2 days at 298 K and stored under silica gel. The film thickness was in the range of 2.3 × 10−4 to 6.0 × 10−4 m.

Copolymer composition (mol%) EO

EP

0 50.1 55.9 61 83 86 87 88 90 93 94 96

100 49.9 44.1 39 17 14 13 12 10 7 6 4

a Z, D and A for Zeon Chemical Company, Daiso Chemical Company and Aldrich, respectively.

m − m0 × 100 m0

(1)

where m0 and m are the corresponding weight of the dry and wet film. Most of the tested copolymers were characterized by differential scanning calorimetry (DSC) with a Pyris type Perkin-Elmer calorimeter. Complementary thermal properties for membranes Z50/50 and D88/12 were measured using a DSC form 2920MDSC TA Instruments. The samples were heated from 173 to 353 K at a heating rate of 10 K min−1 , followed by a rapid cooling. The glass transition temperatures (Tg ) were obtained at the thermogram inflexion point (midpoint) and the melting temperatures were estimated from the maximum of the transitions. The crystallinity ratios were calculated respect to the 100% crystalline PEO which presents a heat of fusion of 1.999 × 105 J kg−1 [17]. Since the crystalline structure of our samples is probably different of that of pure PEO, the obtained values are only relative. Scanning electron micrographs were realized with a HITACHI S-4500 I microscope. 2.2. Gas permeation and sorption experiments Permeation experiments were carried out by using the constant-volume and variable-pressure technique in a permeability apparatus at 298 K. The apparatus consists essentially of a two compartments permeation cell separated by the tested membrane. The permeability was obtained measuring the pressure increase in the downstream compartment (with a constant volume of 3.74 × 10−5 m3 ) and using different MKS Baratron pressure transducers (range from 0.0 to 1 × 105 Pa). The films and downstream cell walls where outgassed in situ during 48 h at high vacuum using a turbomolecular pump (Leybold, Turbovac 50, 50 l s−1 ). The permeability experiments were performed using 3.0 × 105 Pa of upstream pressure and recording the pressure increase in the downstream compartment. High pressure experiments (from 4.5 × 105 up to 3.55 × 106 Pa) with mixtures of 20% CO2 in H2 were realized at 308 K in an industrial laboratory. Gas chromatography was used to analyze the permeate composition. The permeability curves present successively the transitory and steady states. According to the Fick’s second law

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

260 Tg (K)

[18], the mass transfer coefficient Q in the permeation conditions can be simplified at infinite time:
 DC1 l2 Q = t− (2) l 6D t→∞

220

(3)

Sorption analysis was carried out in a Cahn 1100 microbalance at 3.0 × 105 Pa of gas pressure. The system was outgassed during 96 h under high vacuum using an oil diffusion pump (ALCATEL Crystal 63). The amount of sorbed gas was determined after subtraction of the buoyancy contribution using an adequate quantity of a non-absorbing material as reference. The sorption isotherms were performed by recording the weight uptake until attainment equilibrium conditions.

0

20 40 60 80 EO content (mol%) in copolymers

100

60 70 80 90 EO content (mol%) in copolymers

100

340 (b)

Tm (K)

l2 6θ

(a)

240

The diffusion coefficient D is then calculated from the extrapolation (θ) of the linear steady state on the time axis: D=

163

320

300

280 50

3. Results and discussion 3.1. Membranes characterization 60 wt% cristallinity

Table 1 lists the source and composition of copolymers used to prepare the membranes. Fig. 1a–c show, respectively, the evolution of the Tg , Tm and crystallinity of the uncrosslinked copolymers as a function of the EO content. It is observed (Fig. 1a) that Tg decreases with the EO content as expected considering the higher Tg of poly(epichlorohydrin). Tg values vary from 213 to 233 K, similar to those published by Kita and co-workers [10,11] for PEO-polyimide or PEO-polyurethanes segmented polymers but higher than for P(EO-EM-AGE) [13]. As a consequence of the statistical character of the EO/EP copolymerization process, crystallinity of the EO sequences appears above 75–80% EO and increases rapidly (Fig. 1c). It has to be remarked that the melting temperatures of formed PEO crystallites are lower than that of pure PEO and increase rapidly with the increase of EO content (Fig. 1b). This could be due to the presence of tiny little crystallites or defects in the crystal structure including some EP comonomer. The crosslinking reaction induces the formation of KCl crystals as shown in Fig. 2a for the copolymer Z50/50 crosslinked at 5 phr. After water washing process, KCl is eliminated and the films present unconnected little holes with diameters lower than 1 ␮m (Fig. 2b and c). Preliminary permeability measurements realized before and after washing show no noticeable variation. In order to estimate the crosslinking degree, the swelling ratio of films was measured in water (WSR) and dichloromethane (DSR). As example, Fig. 3a and b show the evolution of WSR and DSR with the EO content for membranes prepared at the same theoretical crosslinking

(c)

40

20

60

70 80 90 EO content (mol%) in copolymers

100

Fig. 1. Evolution of Tg (a), melting point Tm (b) and wt.% crystallinity (c) with the EO content of EO/EP copolymers.

ratio (5 phr). The swelling ratios increase with the EO content in both solvents. This demonstrates that the effective crosslinking decreases, a consequence of the decreasing content of CH2 Cl reactive sites. Fig. 4a and b show the WSR and DSR evolution with increasing phr for the membrane D88/12. The observed decreases of swelling with increasing phr confirm the crosslinking efficiency. Nevertheless, as shown in Table 2, the Tg and the relative crystallinity as determined by DSC are few affected for crosslinking ratios lower than 5 phr (that is to say about 0.7 mol of crosslinking agent for 100 mol of copolymer). This clearly indicates that within the phr range used to prepare the membranes, the effective crosslinking is low. However, further experiments are necessary to determine the effective crystallinity ratios.

164

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

water swelling ratio (wt%)

400

(a) 300 200 100 0 40

50

60

70

80

90

100

90

100

CH2Cl2 swelling ratio (wt%)

EO content (mol%) in copolymers 700

(b) 600 500 400 300 200 40

50

60

70

80

EO content (mol%) in copolymers

Fig. 3. Evolution of the swelling ratio vs. the EO content for crosslinked EO/EP copolymers (5 phr) in: (a) water; (b) dichloromethane.

reported in Fig. 6. As indicated in Table 3, the data concern membranes crosslinked between 1.1 and 5 phr. It is generally accepted that crosslinking decreases the permeability due to a decrease of the free volume. However, results shown in

water swelling ratio (wt%)

500

(a)

400 300 200 100 0

2

4

6

8

10

8

10

12

14

phr

Fig. 2. SEM micrographs of an EO/EP membrane (Z50/50, 5 phr): (a) surface before washing process and showing KCl microcrystals; (b) surface after water washing; (c) cross-section after water washing.

3.2. Gas transport properties

CH2Cl2 swelling rate (wt%)

1000

(b) 800 600 400 200 0

Fig. 5 shows the evolution of the permeability coefficient as a function of the EO content in the different membranes for N2 , H2 and He, whereas the same evolution for CO2 is

2

4

6

12

14

phr

Fig. 4. Evolution of the swelling ratio with phr for the membrane D88/12 in: (a) water; (b) dichloromethane.

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169 Table 2 Example of Tg and wt.% crystallinity variations with the crosslinking ratio or phr for two membranes (DSC values) Membrane

phr

Tg (K)

Tm (K)

Z50/50

0 1 5

229.0 231.7 233.0

– – –

D88/12

0 2 10

215.6 218.6 221.1

297.5 290b 292b

Crystallinity (wt.%)a 0 0 0 17 18.9 8.6

phr = weight (g) of crosslinking agent for 100 g of copolymer. a Calculated from the all transition. b Very broad peak, T is determined in the middle of the transition. m

Fig. 7 for the membrane D88/12 confirm the preceding results [15], the measured permeabilities are few affected by the crosslinking ratio in the range used as observed for the Tg variation (Table 2). The Tg increases of few degrees with the crosslinking ratio whereas the swelling ratios are divided by a factor 2 (Fig. 4), indicating a little more compact polymer structure. Figs. 5 and 6 show that CO2 permeability is much higher (20–96 barrer) than the permeabilities of other gases (2–10 barrer). For all these gases, the permeability evolution with the EO content shows a similar behavior.

165

Almost constant from 0 to 61% EO and possibly up to 75% EO, the permeability suddenly increases up to a maximum around 90% EO and then decreases abruptly. As shown in Fig. 8, the variation of the CO2 diffusion coefficient follows the same dependency. In order to estimate the importance of the solubility parameter on the global CO2 permeability phenomena, the N2 , H2 and CO2 solubility were measured at different EO contents. Solubilities of N2 and H2 were not noticeable and many orders of magnitude lower than that of CO2 . Preliminary experiments realized with different crosslinking ratios between 1 and 5 phr demonstrated that CO2 solubility was not noticeably affected by this parameter. Fig. 9 shows the evolution of the CO2 sorption coefficient with the EO content. We observe that the solubility evolution is quite similar to that of the permeability and diffusion coefficient. This result indicates that EO/CO2 interactions are one of the main parameters controlling the CO2 transport phenomena through these films. Values of the ideal gas selectivities of different membranes (Table 3) show that CO2 selectivities respect to He, H2 and N2 increase with the EO content as expected considering the preceding results. As already discussed [15], it is also important to remark that for these copolymers, CO2 permeability and selectivity increase at the same time which is contrarily

Permeability (barrer)

12 10 8 6 4 2 0

0

50

60

70 EO content (mol%)

80

90

100

Fig. 5. H2 (䊏), He () and N2 (䊉) permeability vs. EO content in EO/EP membranes.

CO2 permeability (barrer)

100 80 60 40 20 0

0

50

60

70

80

90

EO content (mol%) Fig. 6. CO2 permeability vs. EO content in EO/EP membranes.

100

166

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

Table 3 Permselectivity of different EO/EP membranes Membrane

phr

A0/100 Z50/50 Z50/50 D56/44 D61/39 D83/17 D86/14 D87/13 D88/12 D88/12 D88/12 D88/12 D90/10 D93/7 D96/4

0 5 1.1 3 3 5 1.5 5 1.8 3.6 6.8 11.7 1.8 5 5

CO2 /He

CO2 /H2

CO2 /N2

4.5 4.4 6.0 8.1 15.3 15.4 16.1

3.4 3.8 5.1 4.9 8.9 6.8 9.0 7.8 6.1 9.5 31 6.8 11.0 10.6

1.3 31.6 6.6 10.9 45.4 37.1 50.6 48.5 14 9.3 11.1 7.0 17.1 62.9 61.8

8.4 13.9 18.0

CO2 /O2

CO2 /CH4

O2 /N2

15.4

15.4

2.1

16.4

0.7 2.8 1.9 2.7 5.0

15.7 16.3 19.2 18.5 9.7

12.74 7.7 8.3

22.9 20.9

16.9 18.0

2.8 3.0

Permeability (barrer)

50 40 30 20 10 0 0

4

2

10

8

6

12

14

phr Fig. 7. Evolution of H2 (䊏), He () and N2 (䊊) permeability vs. phr for membranes D88/12.

to the tendency currently reported. The best permeability value observed for CO2 is 95.6 barrer with selectivities of 62.9 for CO2 /N2 , 16.9 for CO2 /CH4 and 11 for CO2 /H2 . These values with those quite recently reported by Yoshino et al. [13] are among the highest published up to now in the literature and are over the limits reported by Robeson [19].

All these data suggest that the main parameter governing the permeability properties of these membranes is the copolymer composition. Permeability (P), diffusion coefficient (D) and CO2 solubility (SCO2 ) demonstrate the same dependence with the EO content. Two domains may be considered. From 0 to about 75–80% EO, P, D and SCO2 are

12

2 -1

8

-7

D CO2 (10 . cm .s )

10

6 4 2 0

40

50

60

70

80

90

100

EO content (mol%) in copolymers Fig. 8. Evolution of CO2 diffusion coefficient (experimental) vs. EO content in EO/EP membranes.

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

167

Sorption coefficient (10-3.cm3(STP).cm-3.cmHg-1)

16 14 12 10 8 50

60

70

80

90

100

EO content (mol%) in copolymers Fig. 9. Sorption coefficient of CO2 (experimental) for different EO/EP membranes at 298 K.

nearly constant and then very few dependent on the EO content. Such behavior is in contradiction with the important Tg decrease observed in this domain for the copolymers (Fig. 1). Such a decrease corresponds generally to an increase of free volume and therefore of permeability that is not observed here. From 75–80 up to 100% EO, P, D and SCO2 increase up to a maximum for the same EO content (around 90%). Interpretation of these behaviors may be understood by considering the structure and texture of the membranes. The observed independence of the permeability properties (P and D) with the EO content up to 75–80%, domain where the copolymers are amorphous, clearly indicates that the EO content and then the properties of the whole films are not the controlling factor. This suggests that the film interfaces could be the controlling parameter and that within this EO range, permeability measurements describe the interfacial permeability. According to this hypothesis, the composition and texture of the film interface must be different of that of the internal films. It has to be recalled that EP comonomer is hydrophobic contrarily to EO comonomer. As a consequence, in the presence of air, it is quite likely that up to 20–25% EP, the interfaces are mainly composed of chloromethyl groups. As demonstrated by the low permeability of the homopolymer poly(epichlorohydrin) (Table 3), the interface permeability stays low up to an EP content too low to cover the all surfaces. This hypothesis is suggested by two experimental behaviors. First, up to 75–80% EO, the surfaces of films are not sticky contrarily to the sticky behavior of samples with higher EO content. Secondly, the particularity of the interfaces was observed in a preceding study concerning the use of such membranes as separator in all solid batteries in the presence of conducting salt soluble in the EO phase [16]. It was shown that, in this range of EP content, the interfacial electrical resistance was very high compared to that of the internal resistance (unpublished results). Further experiments are in progress to analyze the composition of the membrane interfaces. However, the presence of such interfaces cannot explain the lack of variation of CO2 sorption coefficient since the data are obtained at the equilibrium. The chemical

composition of films in this domain is quite probably the reason of this behavior. As well demonstrated, the exceptional complexation/solubilization properties of PEO are in part due to its chain flexibility and concern only sequences of EO long enough to interact cooperatively. Since the used copolymers have a statistical composition, it is worth to suppose that up to a content of 75–80% EO (EP/EO = 1/3), most of the EO sequences are too short to interact with CO2 . The second domain of EO content showing increasing permeabilities and the observation of a maximum are easily understood by considering the crystallization influence of EO sequences. The EO content increase promotes the formation of longer EO sequences and then CO2 solubilization. At the same time, the possibility of crystallization increases. Since crystallites are impermeable, competition between amorphous EO sequences and crystallites takes place. At very high EO contents, crystallization becomes as important as in pure poly(ethylene oxide) and the permeability decreases rapidly. In order to check this interpretation, Figs. 10 and 11 show the CO2 permeability and sorption coefficient as a function of the amorphous EO content in copolymers. The calculation of amorphous EO content was based on DSC measurements with reference to pure PEO and for uncrosslinked copolymers. It is quite probable that the obtained values are minimized. Nevertheless, the proposed explanation is confirmed. After a slow increase, PCO2 and SCO2 increase rapidly with the content of amorphous EO sequences and no more maximum is observed. The data presented up to now concern the permeability of pure gas. With mixtures of gas, it was often observed changes of properties due to plastifying effects originated by the interactions of some gas with the membranes. Also the gas pressure used for the measurements may be an influent factor. In order to appreciate the separation performances of these membranes with gas mixtures and under different pressures, separation experiments with two different membranes (D86/14 and D93/7) were realized. Gas mixtures (20% CO2 in H2 ) at 308 K and increasing pressures from 4.5 × 105 up to 3.55 × 106 Pa were used. Results reported

168

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

CO2 permeability (barrer)

100 80 60 40 20 0 0

20

40

60

Amorphous EO content (wt%) in copolymers Fig. 10. CO2 permeability vs. amorphous EO content (wt.%) in copolymers.

Sorption coefficient (10-3cm3.cm-3.cm-1)

16

14

12

10

8 30

40

50

60

70

Amorphous EO content (wt%) in copolymers Fig. 11. Sorption coefficient of CO2 vs. amorphous EO content (wt.%) in copolymers.

Table 4 CO2 and H2 permeability and selectivity at different pressures for two compositions of membrane. Measurements perform at 308 K with mixtures of 20% CO2 in H2 Pressure (×105 Pa)

4.5 7.9 7.9 14.8 14.8 35.5 35.5

Membrane D86/14 (1.5 phr)

Membrane D93/7 (3 phr)

Permeability (barrer)

Selectivity

Permeability (barrer)

Selectivity

CO2

H2

CO2 /H2

CO2

H2

CO2 /H2

334 314

40 41

8.4 7.6

314 314 337

42 42 85

7.5 7.4 4.0

266 268 256 260 260 256 248

28 29 29 29 29 34 31

9.4 9.1 8.9 8.9 8.9 7.6 8.1

C. Charmette et al. / Journal of Membrane Science 230 (2004) 161–169

in Table 4 show that the obtained CO2 /H2 selectivity values are of the same order than those reported in Table 3 for experiments realized at 298 K and 3 × 105 Pa. Only an increase of H2 permeability is observed for the membrane D93/7 at very high pressures (3.55 × 106 Pa). Observation of these results allows us to conclude that, in the studied range, the pressure does not influence appreciably the CO2 and H2 permeability. It is believed that this behavior comes from the presence of the crystallites reinforcing the mechanical properties of membranes. As far as the CO2 permeability remains almost constant with the pressure, we can also conclude that the maximum capacity of solubility is already attained at low pressures and that no plasticization effect occurs.

4. Conclusions In this work, the interest of polymeric membranes based on EO for CO2 separation is confirmed. The use of EO/EP membranes prepared from high molecular weight copolymers seems particularly advantageous. The high permeability and selectivity observed for CO2 is a consequence of the elastomer character of these copolymers together with the high content in amorphous EO sequences. It is worth to remark that these new membranes can be prepared by reactive extrusion leading to very thin films (some micrometers) and then high fluxes. These unique properties make such membranes commercially attractive for CO2 separation. Further studies are in progress to precise the structure and texture of these membranes and confirm the permeability mechanism proposed.

Acknowledgements We gratefully acknowledge the Daiso Chemical Company for providing most of the EO/EP copolymers.

References [1] W.J. Schell, Commercial applications for gas permeation membrane systems, J. Membr. Sci. 22 (1985) 217.

169

[2] R. Rautenbach, K. Welsch, Treatment of landfill gas by gas permeation—pilot plant results and comparison to alternatives, Desalination 90 (1993) 193. [3] M.L. Rooney, Active Food Packaging, 1st ed., Chapman & Hall, 1995, p. 111. [4] B.D. Bhide, S.A. Stern, A new evaluation of membrane processes for the oxygen-enrichment of air. I. Identification of optimum operating conditions and process configuration, J. Membr. Sci. 62 (1991) 13. [5] M. Kawakami, H. Iwanaga, Y. Yamashita, M. Yamasaki, M. Iwamoto, S. Kagawa, Enhancement of carbon dioxide permselectivity of immobilized liquid polyethylene glycol membrane by addition of metal salts, Nihon Kagaku Kaishi 6 (1983) 847. [6] J.H. Kim, S.Y. Ha, S.Y. Nam, J.W. Rim, K.H. Paek, Y.M. Lee, Selective permeation of CO2 through pore-filled polyacrylonitrile membrane with poly(ethylene glycol), J. Membr. Sci. 186 (2001) 94. [7] M. Kawakami, H. Iwanaga, Y. Hara, M. Iwamoto, S. Kagawa, Gas permeabilities of cellulose nitrate/poly(ethylene glycol) blend membranes, J. Appl. Polym. Sci. 27 (1982) 2387. [8] J. Li, S. Wang, K. Nakai, T. Nakagawa, A. Mau, Effect of polyethylene glycol (PEG) on gas permeabilities and permselectivities in its cellulose acetate (CA) membranes, J. Membr. Sci. 138 (1998) 143. [9] G. Qipeng, X. Hechang, M. Dezhu, Effect of temperature on gas permeation of polymer blends. I. Poly(ethylene oxide)/copolymer polyurethane, J. Appl. Polym. Sci. 39 (1990) 2321. [10] M. Yoshino, K. Ito, H. Kita, K. Okamoto, Effects of hardsegment polymers on CO2 /N2 gas-separation properties of poly(ethyleneoxide)-segmented copolymers, J. Polym. Sci. B: Polym. Phys. 38 (2000) 1707. [11] K. Okamoto, M. Fuji, S. Okamyo, H. Suzuki, K. Tanaka, H. Kita, Gas permeation properties of poly(ether imide) segmented copolymers, Macromolecules 28 (1995) 6950. [12] J.H. Kim, S.Y. Ha, Y.M. Lee, Gas permeation of poly(amide-6-bethylene oxide) copolymer, J. Membr. Sci. 190 (2001) 179. [13] M. Yoshino, H. Kita, K. Okamoto, M. Tabuchi, T. Sakai, CO2 /N2 gas separation properties of poly(ethylene oxide) containing polymer membranes, Trans Mater. Res. Soc. Jpn. 27 (2002) 419. [14] Ph. Gramain, J. Sanchez, Membranes pour la séparation sélective de gaz, French Patent FR 0011811 (2000), to CNRS; International Patent Application PCT/FR01/02833 (2001). [15] J. Sanchez, C. Charmette, Ph. Gramain, Poly(ethylene oxide-coepichlorohydrin) membranes for carbon dioxide separation, J. Membr. Sci. 205 (2002) 259. [16] S. Lascaud, P. Baudry, Ph. Gramain, Nouvel électrolyte polymère solide et ensemble électrochimique multicouche comprenant un tel électrolyte polymère solide, French Patent FR 9712952 (1997), to CNRS. [17] J.R. Craven, R.H. Moobs, J.R.M. Giles, Synthesis of oxymethylenelinked poly(oxyethylene) elastomers, Makromol. Chem. Rapid Commun. 7 (1986) 81. [18] J. Crank, The Mathematics of Diffusion, 2nd ed., Clarendon Press, Oxford, 1975. [19] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165.