The effects of humidity on gas transport properties of sulfonated copolyimides

Journal of Membrane Science 232 (2004) 115–122

The effects of humidity on gas transport properties of sulfonated copolyimides F. Piroux a,b , E. Espuche a,∗ , R. Mercier b a

Laboratoire des Matériaux Polymères et des Biomatériaux, Universite Claude Bernard, UMR 5627 Bˆat. ISTIL 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France b Laboratoire des Matériaux Organiques à Propriétés Spécifiques, BP 24, 69390 Vernaison, France Received 23 July 2003; accepted 4 December 2003

Abstract Hydrogen, oxygen and carbon dioxide transport properties are studied at various hydration rates for a large series of sulfonated copolyimides synthesized with a naphthalenic dianhydride, a sulfonated diamine and various non-sulfonated diamines. For all copolymers, the gas permeability coefficient decreases when the relative humidity increases in the range 0–70% and an increase of gas flux is observed at high hydration. The magnitude of the permeability coefficient variation depends on the gas polarity and on the copolymer composition. A detailed analysis of the gas transport properties at anhydrous state and of the water sorption mechanism as a function of the copolymers composition allows to explain all the gas permeability variations as a function of hydration. © 2003 Elsevier B.V. All rights reserved. Keywords: Sulfonated polyimides; Copolymers; Gas permeation; Hydration

1. Introduction Polyimides represent a very interesting materials family for gas transport applications and a lot of studies have focussed on the establishment of relationships between gas transport properties at anhydrous state and polyimide structures [1–5]. The influence of chain mobility, of substitution and polarity have been particularly studied and the effects of these parameters have been underlined on the membrane gas separation efficiency [6,7]. Nevertheless, in some applications and even for some gas separation process (e.g. O2 /N2 ), membranes are used in wet atmosphere. Some literature results have already shown that polyimide gas transport properties (H2 , CH4 , O2 , CO2 ) could be highly modified in these new environment conditions [8–10]. A general gas flow decrease was observed on the commercial Kapton films [9] but also on laboratory made polyimides [10]. From Pye et al. work [10] concerning the study of H2 and CH4 transport properties on polyimides based on 6-FDA and various diamines (metaphenylene diamine (MPD), ∗

Corresponding author. Tel.: +33-472448158; fax: +33-472431249. E-mail address: [email protected] (E. Espuche).

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.12.004

diaminobenzoic acid (3,5-DBA) and naphthalene diamine (1,5-ND)) it could be concluded that: (i) the decrease of the gas permeability coefficient depends on the gas nature. It was found slightly higher for H2 than for CH4 and (ii) the polymer structure is also an important parameter. The gas flows were divided by a factor equal to about 2 for the polyimide based on 3,5-BDA whereas, the decrease was found equal to about 16–20% for the other systems. The detailed analysis performed by Koros and coworkers [9] on Kapton film lead to the explanation of the decrease of the CO2 permeability coefficient by two effects: (i) due to hydration, Langmuir sites are partly occupied by water molecules and they are no more accessible to gas molecules: a decrease of solubility is thus observed and (ii) the diffusion of the gas molecules slows down due to a competition with the water molecules. Thus from these literature results, it can be concluded that a general reduction of the gas permeability coefficient in the presence of water molecules is observed on polyimides. The magnitude of the gas flow variation seems to depend on the nature of the gas and of the chemical and structural composition of the systems. Recently, sulfonated sequenced naphthalenic polyimides have been developed as alternative materials to

116

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

perfluorosulfonated ionomer (Nafion® ) and fuel cell application [11,12]. As water is formed during the fuel cell working, it is of great interest to evaluate the gas transport properties of these materials at different relative humidities. Compared to classical polyimides, these copolymers are composed of two types of segments: the so-called hydrophilic sequences, e.g. the sulfonated ionic segments and the so-called hydrophobic sequences, e.g. the non-ionic part of the copolymer which structure can be chosen. The water sorption capacities determined on this materials family [13–15] are important (higher than 15% near saturation) compared to classical polyimides which water uptake is about 3–5% [16]. The water content depends on the system composition (e.g. the content of sulfonated segments) and on the nature of the components used in the non-sulfonated sequences. It seemed thus interesting to know if these peculiarities could lead to more complex behaviors as a function of hydration than classical polyimides. The aim of this work is to study the variation of the gas transport properties as a function of the relative humidity on sulfonated sequenced naphthalenic polyimides varying by their content of sulfonated segments and by the structure of the non-sulfonated sequences. Various gases differing by their kinetic diameter and their interaction capacity towards the polymers were chosen. To better understand the gas transport mechanism in presence of water a detailed analysis of the gas transport mechanism at anhydrous state and of the water sorption mechanism was performed.

O

O

N

N

O

O

2. Experimental 2.1. Materials Fig. 1 presents the chemical structure of the sulfonated copolyimides. The ionic sequences are composed by a naphthalenic dianhydride (1,4,5,8-tetracarboxylic acid (NTDA)) and an already sulfonated diamine (4,4 -diaminobiphenyl2,2 -disulfonic acid (BDSA)). The non-ionic sequences are composed by NTDA as dianhydride and a non-sulfonated diamine whose structure can be chosen. Segmented copolymers were obtained by a two-stage one pot method in m-cresol at 453.15 K (180 ◦ C) [11]. The first step involved the preparation of the sulfonated sequences: their average length x was adjusted by the molar ratio of the two monomers BDSA/NTDA. The presulfonated diamine was used under triethylammonium form in order to achieve a good solubility. In a second step, the polycondensation was pursued by adding: (i) the amount of the non-sulfonated diamine that determines the molar percentage of the sulfonated diamine in the total diamine content: [SD] and as a consequence the content of the sulfonated sequences and (ii) at last the appropriate amount of NTDA in order to respect the total stoichiometry. In this study, x was kept constant and equal to 5. Two types of copolymers were synthesized. For the first series, series A, the amount of sulfonated segments was varied. In

SO3H

HO3S

O

O

N

N

O

O y

x

Fig. 1. Structural composition of the sulfonated copolyimides. Table 1 Structure of the different non-sulfonated diamines used for the copolymer synthesis Abbreviation

Name

CARDO

9,9-Bis(4-aminophenyl)fluorene

BDAF

Bis[4-(4-aminophenoxy)phenyl hexafluoropropane

ODA

Oxydianiline

Diamine

Structure

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

2.2. Film preparation All polymer films were cast at 393.15 K (120 ◦ C) from m-cresol solvent at approximately 8 wt.% polymer onto a glass plate. The films were dried in a vacuum oven according to the following cycle: 2 h at 323.15 K (50 ◦ C), 28 h at 343.15 K (70 ◦ C) and 10 h at 363.15 K (90 ◦ C). They were unstuck from the glass plates support by immersion in methanol. Membranes with controlled thickness from 25 to 40 ␮m were obtained, they were acidified with 0.1 M HCl solution overnight and then rinsed with water. The complete triethylammonium/H+ ion-exchange was checked by both weight analysis and quantitative analysis. The films were finally dried for 4 h at 353.15 K (80 ◦ C) in a vacuum oven.

3. Transport properties analysis 3.1. Water sorption analysis The sorption isotherm was determined by a gravimetric method. The samples were introduced in a Setaram B92 microbalance. After desorption under vacuum (2 × 10−6 mbar) at a constant temperature (293.15 ± 1 K (20 ± 1 ◦ C)), a partial pressure of water was established within the apparatus by means of an evaporator placed at a temperature T. The water uptake was followed as a function of time until the equilibrium sorption was attained. The partial pressure was then incremented by successive steps by discrete changes of the temperature T. The water sorption isotherm was obtained from the equilibrium water uptakes at different partial pressures. 3.2. Gas permeation measurement The permeation cell consisted of two compartments separated by the studied membrane. The cell was thermostated at 293.15±1 K (20±1 ◦ C). A preliminary high vacuum desorption was realized to ensure that the static vacuum pressure changes in the downstream compartment were smaller than the pressure changes due to the gas or water vapor diffusion. (i) For gas permeation at anhydrous state, a 3.0 × 105 Pa gas pressure was introduced in the upstream. The pressure variations in the downstream compartment were measured with a datametrics pressure sensor. A steady-state line was obtained after a transitory state by plotting the measured pressure versus time. The

permeability coefficient P expressed in barrer units (1 barrer: 10−10 cm3STP cm cm−2 s−1 cm−1 (Hg) ) was calculated from the slope of the steady-state line. (ii) For gas permeation experiments on membranes at different relative humidities, the membrane was equilibrated at the desired water relative partial pressure by means of an evaporator before introducing in the upstream compartment the 3.0 × 105 Pa gas pressure. The permeability coefficient P was calculated from the slope of the straight line representative of the evolution of the pressure in the downstream compartment as a function of time. 4. Results 4.1. Gas transport properties at anhydrous state Fig. 2 presents the hydrogen permeability coefficients (PH2 ) for the different materials of series A. The analysis of the curve representative of the evolution of PH2 as a function of the sulfonated diamine content [SD] leads to three main conclusions: (i) The totally sulfonated polyimide has a very low permeability coefficient compared to the totally nonsulfonated CARDO/ODA polyimide. (ii) For the copolymers, the permeability coefficient decreases as the content of sulfonated diamine [SD] increases. The flux variation as a function of [SD] is not monotonous. Thus from a transport point of view, the copolymers cannot be considered as homogeneous systems. Their behaviors correspond to those of heterogeneous blends and three domains can be considered: • In [SD] range [0%; 30%], the evolution of the permeability coefficient can be described by Maxwell’s law (Eq. (1)) considering an impermeable dispersed

PH2 (barrer)

order to cover all the range of [SD] going from 0 to 100% and keeping the synthesis and casting parameters constant, a 50/50 molar mixture of CARDO and ODA diamines was chosen as the non-sulfonated diamine. For the second series, series B, the [SD] value was fixed to 30% and the nature of the non-sulfonated diamine varied (Table 1).

117

180 160 140 120 100 80 60 40 20 0 0

20

40

60

80

100

[SD](%)

Fig. 2. H2 permeability measured at anhydrous state for: (䉫) CARDO copolymer with [SD] = 30%; (䉬) CARDO/ODA based films with various [SD]; ( ) BDAF copolymer with [SD] = 30%; (䊐) ODA copolymer with [SD] = 30%. The dotted lines are relative to models applied to CARDO/ODA based films with various [SD]: in [SD] range 0–40% the dotted line is representative of Maxwell’s law considering a dispersed impermeable phase in a continuous permeable phase. The dotted line in [SD] range 60–100% is representative of a continuous low permeable phase in which a permeable phase is dispersed.

118

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

phase (the sulfonated phase) in a permeable continuous matrix (the non-sulfonated phase): P = 2Pc

1 − φd 2 + φd

(1)

where Pc is the permeability coefficient of the continuous phase (the non-sulfonated phase) and φd the volume fraction of the dispersed impermeable phase (e.g. the sulfonated phase). • A great decrease of permeability coefficient is observed in [SD] range 40–60% corresponding to a phase inversion. • In [SD] domain [70%; 100%], the transport can be described by the model considering a dispersed permeable phase in a continuous impermeable matrix (Eq. (2)):
 φd P = Pc 1 + 3 (2) φc with φc the volume fraction of the continuous impermeable phase. (iii) As a consequence of these two previous remarks, it can be concluded that in these copolymer series the gas transport takes place principally in the non-sulfonated phase. The analysis of the H2 permeability coefficients obtained for series B (Fig. 2) leads to the following observations. For a same molar content of sulfonated segments (e.g. [SD] = 30%), the gas permeability coefficient decreases going from CARDO diamine to CARDO/ODA mixture, BDAF and ODA diamine thus as the non-sulfonated diamine is less sterically hindered. The permeability coefficient of the copolymer based on ODA diamine is about twice the permeability coefficient of the totally sulfonated polyimide.

For this system and contrary to the copolymers based on CARDO or CARDO/ODA mixtures the contribution of the sulfonated phase to the gas transport properties cannot be considered as negligible. In conclusion, the gas transport mechanism in anhydrous conditions results from the contribution of two phases: the sulfonated and the non-sulfonated phases. The participation of the sulfonated phase to the transport can be considered as negligible when the non-sulfonated diamine structure leads to a high free volume in the non-sulfonated phase. The same conclusions have already been drawn for CO2 and O2 in a previous study [12]. 4.2. Water sorption Fig. 3 presents the water sorption isotherms relative to the materials of series A. For the totally sulfonated polyimide the isotherm is of BET type 2 and the water uptakes are particularly important in all the range of activity (about 30% at a water activity equal to 0.6 and near 60% at a water activity equal to 0.9). A detailed analysis of the water transport mechanism has already shown [14,15] that the sorbed water molecules at low activity contribute to the formation of the ion hydration sphere but that they are also located in the holes formed by the triethylammonium/H+ exchange. The high increase of water uptake noticed at activity higher than 0.7 has been attributed to a swelling phenomena. In this domain, water clusters are formed around the first sorbed water molecules. Compared to the very hydrophilic character of the totally sulfonated polyimide, the non-sulfonated CARDO–ODA polyimide behaves as a more hydrophobic material. Nevertheless, even if lower amount of water is sorbed by this polyimide, the sorption capacity cannot be considered as negligible. Furthermore, the sorption mechanism is different

80% [SD]=100%

70%

[SD]=85%

60%

[SD]=40% [SD]=30%

M (%)

50%

[SD]=0%

40% 30% 20% 10% 0% 0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

P/P0 Fig. 3. Water sorption isotherms of CARDO/ODA based copolymer with various [SD].

1

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

119

30 25 M (%)

20 15 10 5 0 0

0,2

0,4

0,6

0,8

1

P/Po Fig. 4. Water sorption isotherms of: (䉫) CARDO copolymer; (䉬) CARDO/ODA copolymer; ( ) BDAF copolymer. [SD] = 30% for all the copolymers.

from that of the totally sulfonated material. The linear shape of the isotherm is representative of a Henry type sorption implying a random distribution of water molecules in the polymer. Swelling phenomenon is thus excluded in this system at high activity. The sorption isotherms relative to the copolymers are of BET type 2. For these films and whatever the composition, the water uptakes result from the contribution of both sulfonated and non-sulfonated phases. Indeed, the experimental isotherms can be described by an additivity law represented by the dotted lines in Fig. 3. Fig. 4 describes the water sorption isotherms obtained for the polymers of series B. It allows to compare the influence of the structure of the non-sulfonated diamine on the water uptake of copolymers at a constant [SD] value, 30%. The water uptakes decrease going from CARDO to CARDO–ODA and to BDAF diamine. For this last system and despite the free volume brought by CF3 groups, the water sorption is quite low probably due to the hydrophobic character of these groups. It can be concluded from this study that the water sorption is dominant in the sulfonated phase. Nevertheless, water molecules are also located in the non-sulfonated phase. The water content depends then on the free volume brought by the non-sulfonated diamine and also on the hydrophilic or hydrophobic character of the non-sulfonated diamine. 4.3. Gas transport at different relative humidities All polymers are equilibrated at the chosen relative humidity before the gas experiment is performed. In a first part we will focus on the evolution of H2 transport properties as a function of hydration and we will discuss the influence of the materials composition. The second part will show the influence of the gas nature. 4.3.1. H2 transport properties as a function of hydration: influence of materials composition Fig. 5 presents the evolution of H2 permeability coefficient as a function of the relative humidity (RH) for the materials of series A.

Fig. 5. Evolution of PH2 as a function of relative humidity for: ( ) CARDO/ODA non-sulfonated polymer; (䊐) CARDO/ODA copolymer with [SD] = 30%; (䊏) CARDO/ODA copolymer with [SD] = 40%; (䉫) totally sulfonated polyimide.

As generally mentioned in the literature [8–10], a decrease of the gas permeability coefficient is observed for the non-sulfonated CARDO/ODA polyimide. This decrease is very important in the range of relative humidity 0–60% where PH2 goes from 144 to 21 barrer. For higher RH range, the permeability variation are lower (from 21 to 5.6 barrer). The effects of small partial pressures of water have already been underlined on gas flux through a series of stiff-chain polyimides. Flux depressions ranging from 21 to 59% were reported for H2 and CH4 due to the presence of water vapor (about 50% RH) in the feed [10]. This gas flux decrease was explained by a competition of mixed penetrants for sorption sites. Our experimental conditions differ from those described in the literature as our membranes are at first equilibrated at a chosen RH before gas diffusion is performed. Thus depending on the chosen relative humidity a certain amount of free holes are partly or totally filled with water before gas experiments. Gas molecules can thus only diffuse through the pathways formed by the unoccupied holes of sufficient sizes. This can explain in part the greater evolution of the gas flux we have observed compared to the literature results and also the non-monotonous decrease of the gas permeability coefficient whereas the water uptake in the CARDO/ODA polyimide increases quite linearly as a function of the relative humidity (Fig. 3). The evolution of the gas flux for the totally sulfonated polyimide can also be divided in two domains but in this case a decrease of the gas flux from 7 to 1 barrer is observed for RH ranging from 0 to 70% followed by an increase of the gas flux at higher RH (PH2 is equal to 4.5 barrer at 100% RH). The first evolution can be explained, as for the totally non-sulfonated CARDO/ODA polyimide, by the decrease of the number and size of the holes available for the gas diffusion due to the presence of the first sorbed water molecules. The increase of the gas flux at high RH is the consequence of the water swelling effect described in the

120

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

water sorption study. This last behavior is often observed for hydrophilic materials and it is related to a plasticization phenomenon making easier the gas transport [17,18]. For the copolymers, a decrease of gas permeability coefficient is also observed in RH range 0–70% and for higher RH, the gas permeability coefficient increases. We can remark that if PH2 values are very different at anhydrous state as a function of the copolymers composition, the gas permeability coefficients measured at 100% RH are very similar (Table 2). As a consequence, the global variation of permeability due to the effect of water between 0 and 100% RH is higher when the initial permeability coefficient is important. This can be explained owing to the detailed gas and water transport analysis we have performed. Indeed, we have shown that in these systems the non-sulfonated phase had the major role on the gas transport properties at anhydrous state. Thus the more the non-sulfonated phase fraction is important in the copolymer the more the initial permeability is high. When the hydration increases in the range 0–70%, water molecules are sorbed in the sulfonated and non-sulfonated phases without swelling effect: the water progressively fills the free volumes contained in these both phases leading to a global decrease of the gas permeability coefficient. The decrease of gas flux is very important in the non-sulfonated phase (Fig. 5) and we can consider that near 70% RH, the gas transport properties of both sulfonated and non-sulfonated phases become very similar. Thus as a swelling phenomenon occurs in the sulfonated phase at high relative humidity, the

Fig. 6. Evolution of PH2 as a function of relative humidity for sulfonated copolymers with [SD] = 30% based on different non-sulfonated diamines: (䉫) CARDO; (䉬) CARDO/ODA; ( ) BDAF; (䊐) ODA.

role of this phase becomes dominant for the gas transport at high RH and an increase of the gas flux is then observed. The conclusions we have drawn for series A copolymers are reinforced by the analysis of the results obtained for the copolymers based on different non-sulfonated diamines (series B). Fig. 6 shows that the same trends are observed for all copolymers, e.g. a first decrease of H2 permeability coefficient as a function of RH and then an increase of PH2 at high relative humidity. The decrease magnitude observed in the first domain depends on the non-sulfonated diamine. It is higher when the initial gas permeability is important thus

Table 2 Gas permeability, P (barrer) of the different films at 0 and 100% RH Non-sulfonated diamine

[SD]

fa

RH (%) 0

40

50

55

60

100

– – –

– – –

13 1.6 62

5.6 1.4 55

25.7 17.1 2.7

0

PH2 PO 2 PCO2

144 24 147

30 5.2 89

30

PH 2 PO 2 PCO2

89 11 70

– – –

2 – 6.5

– – –

– – –

4.3 1.4 42

20.7 7.8 1.7

40

PH 2 PCO2

76 56

– –

– –

– –

– –

3.7 43

20.5 1.3

50

PH 2 PO 2 PCO2

42 3.3 24

– – –

– – –

– – –

– – –

4.3 1.7 50

9.8 2 0.5

100

PH 2

7.1

–

–

–

–

4.5

1.6

CARDO

30

PH 2 PO 2 PCO2

– – –

6.9 – 32

– – –

– – –

3.7 1.6 51

45 15 3.2

ODA

30

PH 2 PO 2 PCO2

14 0.8 5.2

– – –

– – –

– – –

– – –

2.5 0.8 12

5.6 1 0.4

BDAF

30

PH 2 PCO2

43 28

– –

– –

6 7.6

– –

5 17

8.6 1.6

CARDO/ODA

–

a

166 24 164

The ratio of the permeability at anhydrous state to the permeability at 100% RH.

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

121

observe that whatever the system considered, the following order is verified: fH2 > fO2 > fCO2

Fig. 7. Evolution of PCO2 as a function of relative humidity for: ( ) CARDO/ODA non-sulfonated polyimide; (䊏) CARDO/ODA based copolymer with [SD] = 40%; (䉫) CARDO based copolymer with [SD] = 30%; ( ) BDAF based copolymer with [SD] = 30%; (䊐) ODA based copolymer with [SD] = 30%.

when the free volume in the non-sulfonated phase increases. In the second domain, e.g. for RH higher than 70%, all the curves are very similar confirming here the major role of the lonely common part of all these copolymers: the sulfonated phase. 4.3.2. Gas transport properties as a function of hydration: influence of the gas nature Fig. 7 presents the evolution of the CO2 permeability coefficient as a function of hydration for some materials of series A and B. As for H2 , a two-step decrease of CO2 permeability is observed for the non-sulfonated CARDO–ODA based polyimide. For the copolymers and whatever the nature of the non-sulfonated diamine a decrease of PCO2 is observed in RH range 0–70% followed by an increase of the gas flux at higher relative humidity. Thus the same trends as those described for H2 are observed whatever the gas nature. Nevertheless it can be observed that for some systems, the CO2 permeability coefficients are similar at 100 and 0% RH or even higher at 100% RH than at 0% RH. The next part of the study has been then particularly focussed on the evolution of the gas permeability coefficients between 0 and 100% RH for the following gases H2 , O2 and CO2 . Table 2 presents the different permeability coefficient values measured and the factor f corresponding to the ratio of the permeability of a considered gas at anhydrous state to the permeability of the same gas at 100% RH. The analysis of Table 2 data leads to some remarks. Concerning H2 , f values are higher than 1 for all the studied films indicating a decrease of the permeability coefficient going from 0 to 100% RH. For O2 , the same trend is observed except for ODA based copolymer. For this system, the oxygen permeability values are equal at 0 and 100% RH. For CO2 , f values higher than 1 are observed except for ODA based copolymer and CARDO/ODA based copolymer with [SD] equal to 50%. For these two systems, f values are lower than 1: an increase of the CO2 permeability coefficient is thus measured between dry and wet states. At last we can

The permeability coefficient decrease is higher for the gas which has the smallest kinetic diameter. The same observations have been reported by Pye et al. [10] considering H2 and CH4 transport properties in different hydrated polyimides (RH = 50%). The decrease they observed for H2 was slightly higher than for CH4 in spite of the very different size of these two molecules. Thus all these variations cannot be explained considering simply and only the evolution of the number and size of the free available holes as a function of hydration. Indeed, it has been already demonstrated that for example, thermomechanical treatments (e.g. orientation) leading to a reduction of the sizes and number of free volume lead generally to higher variation in the transport of bigger gases [19]. Furthermore, assuming that only morphological factors are at the origin of the gas permeability variation, should imply that free volume is mainly constituted in our systems by small size free holes (e.g. holes small enough to allow H2 and not O2 diffusion at anhydrous state and holes large enough to allow water sorption impeding highly H2 diffusion at wet state and in a much less extend O2 and CO2 diffusion). This assumption is then not consistent with the high values of the CO2 permeability coefficient measured at anhydrous state on some copolymers and with the increase of PCO2 observed for some other systems between dry and wet state. Thus complex mechanisms operate in our systems due in a great part to the modification of the medium polarity under hydration. Indeed, we can remark that the decrease of the permeability coefficient is less important for the more polar gas molecule. We can then think that a higher affinity is developed between this type of molecule and the medium as the hydration increases. Thus compared to H2 behavior, the CO2 permeability coefficient decreases are attenuated at moderate hydration (Table 2) and the permeability coefficient increases noticed at high hydration are emphasized leading even for some systems to a higher flux at wet state than at dry state. This last behavior is observed when the role of the sulfonated phase cannot be neglected on the gas transport properties at anhydrous state. Two types of copolymers are concerned: the copolymers with a dense non-sulfonated phase, e.g. ODA based systems and the copolymers for which the continuous phase is the sulfonated phase, e.g. CARDO/ODA based copolymers with [SD] values higher than 40%. The swelling capacity of the sulfonated phase at high relative humidity associated to the higher interaction developed between the polar gas molecules and the water contained polymer can explain the important CO2 gas flux measured for these systems at hydrated state. As a conclusion, the composition of the copolymer but also the nature of the permeate play an important role on the evolution of the gas flux at different relative humidities.

122

F. Piroux et al. / Journal of Membrane Science 232 (2004) 115–122

5. Conclusion The present study underlines the specific behavior as a function of hydration of glassy copolymers composed by a highly hydrophilic phase and a more hydrophobic phase. Contrary to the decrease of the gas permeability coefficient observed on classical polyimides, the variation of the gas flux is not monotonous in these systems and a drastic change of the gas transport mechanism is observed between dry and wet state. The non-sulfonated “hydrophobic” phase has the major role in dry state whereas the sulfonated “hydrophilic” phase essentially governs the transport properties in wet state. A decrease of the gas permeability coefficients occurs for water activity lower than 0.7 and an increase of the gas flux is observed at higher activity. The magnitude of the gas flux variation depends on the polarity of the considered gas. Indeed for H2 , the gas evolution is essentially related to the number and size of the holes available for the diffusion. The decrease of the permeability coefficient observed between dry and wet state is more important when the initial permeability is high and the values measured at wet state are similar whatever the composition of the copolymers. For more polar gases such as CO2 , interaction effects between the gas molecules and the hydrated medium have also to be considered. In this case, the composition of the copolymer becomes more important and for some systems (systems with dense non-sulfonated continuous phase or with sulfonated continuous phase) higher CO2 permeability coefficients are measured in wet state than in dry state. Acknowledgements The authors acknowledge financial support of ADEME and GIE-PSA-Renault and fruitful discussions with M. Pinéri (C.E.A., 17 Rue des Martyrs, 38054 Grenoble Cedex 9). References [1] M.R. Coleman, W.J. Koros, Isomeric polyimides based on fluorinated dianhydrides and diamines for gas separation applications, J. Membr. Sci. 50 (1990) 285–297.

[2] M. Langsam, in: M.K. Gosh, K.L. Mittal (Eds.), Polyimides: Fundamentals and Applications, Marcel Dekker, New York, 1996, Chapter 22, p. 697. [3] M. Langsam, W.F. Burgoyne, Effects of diamine monomer structure on the gas permeability of polyimides. I. Bridged diamines, J. Polym. Sci. 31 (1993) 909–921. [4] S.A. Stern, Polymers for gas separations: the next decade, J. Membr. Sci. 94 (1994) 1–65. [5] K. Tanaka, Y. Osada, H. Kita, K.I. Okamoto, Gas permeability and permselectivity of polyimides with large aromatic rings, J. Polym. Sci. Polym. Phys. 33 (1995) 1907–1915. [6] M. Al-Masri, D. Fritsch, H.R. Kricheldorf, New polyimides for gas separation. 2. Polyimides derived from substituted catechol bis(etherphthalic anhydrides)’s, Macromolecules 33 (19) (2000) 7127–7135. [7] H. Jianhua Fang, H. Kita, K.I. Okamoto, Hyperbranched polyimides for gas separation applications. 1. Synthesis and characterization, Macromolecules 33 (13) (2000) 4639–4646. [8] J.Y. Dolveck, Thesis, Université Claude Bernard, Lyon, February 12, 1993. [9] R.T. Chern, W.J. Koros, E.S. Sanders, R. Yui, Second component effects in sorption and permeation of gases in glassy polymers, J. Membr. Sci. 15 (1983) 157–169. [10] D.G. Pye, H.H. Hoehn, M. Panar, J. Appl. Polym. Sci. 20 (1976) 287. [11] C. Genies, R. Mercier, B. Sillion, N. Cornet, G. Gebel, M. Pineri, Soluble sulfonated naphthalenic polyimides as materials for proton exchange membranes, Polymer 42 (2001) 359– 373. [12] F. Piroux, E. Espuche, R. Mercier, M. Pinéri, G. Gebel, Gas transport mechanism in sulfonated polyimides. Consequences on gas selectivity, J. Membr. Sci. 209 (2002) 241–253. [13] N. Cornet, G. Beaudoing, G. Gebel, Influence of the structure separation and purification technology 22–23 (2001) 681. [14] V. Detallante, D. Langevin, C. Chappey, M. Métayer, R. Mercier, M. Pinéri, Water vapor sorption in naphthalenic sulfonated polyimide membranes, J. Membr. Sci. 190 (2001) 227–241. [15] F. Piroux, E. Espuche, R. Mercier, M. Pinéri, Water vapour transport mechanism in naphthalenic sulfonated polyimides, J. Membr. Sci. 223 (2003) 127–139. [16] W.J. Koros, D.R. Paul, J. Polym. Sci.: Polym. Phys. Ed. 16 (1978) 1947. [17] S. Zhou, S.A. Stern, The effect of plasticization on the transport of gases in and through glassy polymers, J. Polym. Sci. 27 (1989) 205–222. [18] S. Despond, E. Espuche, A. Domard, Water sorption and permeation in chitosan films: relation between gas permeability and relative humidity, J. Polym. Sci. Polym. Phys. 39 (2001) 3114– 3127. [19] J.A. Webb, D.I. Bower, I.M. Ward, P.T. Cardew, J. Polym. Sci. Polym. Phys. 31 (1993) 743.