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Gas permeabilities of polymers with indan groups in the main chain.
Journal of Membrane Science 143 (1998) 115±123
Gas permeabilities of polymers with indan groups in the main chain. 2: Polyimides Gerhard Maiera,*, Martin Wolfa, Miroslav Blehab, Zbynek Pientkab a
Lehrstuhl fuÈr Makromolekulare Stoffe, Technische UniversitaÈt MuÈnchen, Lichtenbergstraûe 4, D-85747 Garching, Germany Institute of Macromolecular Chemistry, Department of Polymer Membranes, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, Prague 6 CZ-16206, Czech Republic
b
Received 16 July 1997; received in revised form 30 December 1997; accepted 30 December 1997
Abstract Permeability coef®cients and ideal selectivities for H2, CO2, O2, and N2 were determined for two series of polyimides with indan groups in the main chain. The bulkiness of the indan group was varied by replacing methyl groups at the indan ring system by bulky cyclohexyl groups. The effects of the linking group in the dianhydride part of the polymer repeating unit could be explained on the basis of bulkiness and ¯exibility. However, the introduction of two cyclohexyl substituents at the indan group to increase the bulkiness of this structural element had no uniform effect on permeability coef®cients and selectivities. Positive or negative effects were observed, depending on the linking group in the dianhydride moiety. This was tentatively attributed to the conformational ¯exibility of the cyclohexyl rings, which can result in different sterical requirements. # 1998 Elsevier Science B.V. Keywords: Gas separations; Structure±property relationships; Polyimides
1. Introduction Amorphous polymers with high glass transition temperatures such as polyimides or poly(arylene ether)s are of potential interest as gas separation membranes. Bulky groups in the main chain generally tend to increase free volume and hence permeability coef®cients [1±4]. High chain stiffness, as indicated by high glass transition temperatures, is expected to result in relatively high selectivities in gas separation applications [1±4]. However, exact structure±property relationships are not known in full detail yet. The *Corresponding author. Fax: +49 89 289 13562; e-mail: [email protected] 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00006-4
expectations described above are based on the idea that the permeation of gas particles through an amorphous, pore free polymer ®lm occurs by ``jumps'' of the particles between voids of free volume through channels which are opened and closed by thermally activated motions of certain chain segments of the polymer backbone. In this model, increasing free volume results in increasing permeability coef®cients, and increasing chain stiffness leads to increasing selectivity. In our view, to further elucidate exact structure± property relationships for a series of polymers, the chain segments which control the permeation need to be identi®ed. In a preceding paper [5] we described the results of permeability measurements of hydrogen,
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Scheme 1. Synthesis of the indan containing diamine monomers: 1 (Rmethyl) and 2 (Rcyclohexyl)
carbon dioxide, oxygen, and nitrogen for a series of amorphous indan containing poly(ether ketone)s. All polymers of this series exhibited only small variations of selectivity for the gas pairs H2/N2, CO2/N2, and O2/ N2 [5]. From these results it was concluded that the bulky indan groups in these polymers are rather immobile with regard to the timescale relevant for the permeation jumps, while the motions of the ¯exible phenylene ether segments between the indan groups are responsible for the permeation events. The present paper describes permeabilities and selectivities of a series of polyimides with structures similar to those of the poly(ether ketone)s discussed earlier. 1.1. Polymer structures The synthesis of the diamine monomers 1 and 2 as well as the polyimides 3 and 4 have been described
earlier [6±8], and are therefore outlined here only brie¯y in Schemes 1 and 2, and some properties of the polymers are summarized in Table 1. Except for polymer 3d all polyimides 3 and 4 are soluble in NMP above 508C, and except for 3d, 4c, and 4d they are also soluble in chloroform at ambient temperature [7,8]. In addition, 3a, 3b, 4a, and 4b are also soluble in cold NMP, DMAc, THF, and dichloromethane [7,8]. Glass transition temperatures are in the range between 2328C and 2748C [7,8], with those of the cyclohexyl substituted derivatives being approximately 308C higher than those of the methyl substituted polymers [8]. None of these polymers showed a melting endotherm in DSC measurements [7,8]. Molar masses were determined by GPC in chloroform with polystyrene calibration. Although this method does not yield absolute values, the results
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
117
Scheme 2. Structures of the polyimides 3 (Rmethyl) and 4 (Rcyclohexyl) (X±C(CF3)2±, ±SO2±, ±CO±, ±O±, Ð) Table 1 Properties of the polyimides 3 and 4 Polymer No.
±R
±X±
Tg a [8C]
n M
3a 3b 3c 3d 3e 4a 4b 4c 4d 4e
±methyl ±methyl ±methyl ±methyl ±methyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl
±C(CF3)2± ±SO2± ±CO± ±O± Ð ±C(CF3)2± ±SO2± ±CO± ±O± Ð
241 251 232 233 246 269 272 261 264 274
28 000 20 800 21 300
b
[g molÿ1]
d
31 500 30 700 8100 d d
25 800
Vf(H2) c
Vf(O2) c
0.163 0.144 0.124 0.134 0.131 0.179 0.160 0.138 0.147 0.145
0.161 0.139 0.124 0.119 0.132 0.169 0.144 0.127 0.122 0.135
a
DSC, 20 K/min. GPC in CHCl3, polystyrene calibration. c Density determined by floating film in aqueous CaCl2 solution. d Insoluble in CHCl3. b
can be compared within a series of similar polymers. As can be seen from Table 1, the values obtained for the polymers 3 and 4 are fairly uniform. In all cases the molar mass of the completely cyclized poly(imide)s were high enough to result in tough, ¯exible ®lms. 2. Experimental The synthesis and characterization of the polymers 3 and 4 has been described elsewhere. Permeabilities of hydrogen, carbon dioxide, oxygen, and nitrogen were measured for all polyimides 3 and 4 using circular samples of dense ®lms. Films were cast form poly(amic acid) solutions in DMAc. Imidization was achieved by heating to 508C (1 h), 1008C (1 h), 1508C (1 h), and ®nally 2608C (2 h). Completeness of imidization was checked by IR spectroscopy and thermogravimetry. Prior to measurements, the ®lms were stored under vacuum at 508C for several days. Three individual ®lms were cast from each polymer. The
data given for each polymer are the averages of the measurements of a sample of each of these three ®lms. The differences between the permeability coef®cients of the three samples is of the order of 2% for H2, 5% for CO2, 10% for O2 and N2, However, as the permeability coef®cients approach 0.1 barrer, the accuracy decreases to 50%. Selectivities are expressed as ideal selectivities (ratios of the permeability coef®cients). The preparation of the ®lms is critical for the quality of the measurements, especially in the case of polyimides, when ®lm preparation via the poly(amic acid) precursor is necessary. To further evaluate the reproducibility of our measurements, the permeabilities of two of our polymers (3a and 3d) for H2 and O2 were checked at Air Products and Chemicals. The deviations between these sets of results were far below 10%: 3a: P(O2)2.05; P(N2)0.33 (our results, Table 2: 2.05 and 0.32); 3d: P(O2)0.65; P(N2) 0.097 (our results, Table 2: 0.68 and 0.09). This clearly indicates the reliability of the measurements.
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Table 2 Gas permeability coefficients of the polyimides 3 and 4 [Barrers] Polymer No.
±R
±X±
P(H2)
P(CO2)
P(O2)
P(N2)
3a 3b 3c 3d 3e 4a 4b 4c 4d 4e
±methyl ±methyl ±methyl ±methyl ±methyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl
±C(CF3)2± ±SO2± ±CO± ±O± Ð ±C(CF3)2± ±SO2± ±CO± ±O± Ð
21.3 12.7 8.3 10.5 8.8 23.1 10.2 9.2 13.23 13.3
5.58 3.6 1.96 3.05 2.51 9.64 3.2 1.68 3.76 5.68
2.05 0.91 0.47 0.68 0.57 1.92 0.75 0.91 0.9 1.11
0.32 0.16 0.08 0.09 0.11 0.33 0.16 0.22 0.08 0.28
2.1. Permeability measurements Gas transport properties of the membranes were studied using a laboratory-made high-vacuum apparatus with a static permeation cell. The polymer membrane separates the injection side of the cell, containing the studied gas under constant pressure pi (1245 mbar), from the product side which is evacuated in the beginning. Permeability coef®cients P were determined from the pressure increase pp =t in the calibrated volume V of the product side of the cell. Only the linear part of the pressure increase was evaluated. P
pp V l 1 t A pi RT
Pressure pp at the product side was measured with a precision capacitance manometer CCM-10; the membrane area A was 1.24 cm2; l is the membrane thick-
ness; T is the temperature (22.58C); R is the gas constant. 3. Results of the permeability measurements The permeability coef®cients obtained by the procedure described above are summarized in Table 2, the selectivities in Table 3. The plots of selectivity vs. permeability coef®cient for the six gas pairs H2/N2, CO2/N2, O2/N2, H2/O2, H2/ CO2, and H2/CO2 are shown in the following Figs. 1± 6. The upper bounds and some data for polymers close to it [9,10] are added to the graphs for orientation. A comparison between the data for the methyl substituted polyimides 3 and their cyclohexyl substituted analogues 4 reveals that the additional bulkiness added to the indan groups by the two cyclohexyl rings has no uniform effect on the gas permeation properties
Table 3 Selectivities of the polyimides 3 and 4 Polymer No.
±R
±X±
(O2/N2)
3a 3b 3c 3d 3e 4a 4b 4c 4d 4e
±methyl ±methyl ±methyl ±methyl ±methyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl
±C(CF3)2± ±SO2± ±CO± ±O± Ð ±C(CF3)2± ±SO2± ±CO± ±O± Ð
6.4 5.7 5.9 7.6 5.2 5.8 4.7 4.1 11.2 4.0
(H2/N2) 66.6 79.4 103.7 116.7 80 70 63.7 41.8 165.4 47.5
(CO2/N2)
(H2/CO2)
(CO2/O2)
(H2/O2)
17.4 22.5 24.5 33.9 22.8 29.2 20.6 7.6 47 20.3
3.8 3.5 4.2 3.4 3.5 2.4 3.1 5.5 3.5 2.3
2.7 4.0 4.2 4.5 4.4 5.0 4.4 1.8 4.2 5.1
10.4 14.0 17.7 15.4 15.4 12.0 13.6 10.1 14.7 12.0
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Fig. 1. Selectivity vs. permeability coefficient for the H2/N2 gas pair; (*) reference data from the literature [11]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
of the polymers. Although originally we expected increased permeability coef®cients by increasing the bulkiness of the indan group, in many cases the opposite effect is observed. Clearly, the effect of a single structural variation at one speci®c site of the repeating unit depends on the total structure of the repeating unit. 4. Discussion In the preceding paper of this series [5], gas permeabilities of poly(ether ketone)s 5 were studied. It was
119
observed that for the gas pairs H2/N2, CO2/N2, and O2/ N2 the selectivities of the poly(ether ketone)s were almost identical and did not vary much with the structure of the repeating unit. The polyimides 3 and 4 differ from the poly(ether ketone)s 5 by replacement of two phenyl rings by two phthalimide groups, indicated by bold lines in Scheme 3. In contrast to the observations in the case of the poly(ether ketone)s, the selectivities of the polyimides 3 and 4 vary strongly with the structure of the repeating units (Table 3, Figs. 1±6). Within both series of polyimides, the hexa¯uoroisopropyl group as linking group X results in the highest permeability coef®cients for the gases studied here. The carbonyl linking group results in the lowest values among the polymers 3 for all gases, and among the polymers 4 for hydrogen and carbon dioxide. The positive effect of the hexa¯uoroisopropyl group in the polymer backbone on permeability coef®cients is well known, and is generally explained by the bulkiness of this structural element [2,16,17]. For detailed discussion the knowledge of diffusion and solubility coef®cients is desirable. Due to limitations of our equipment we are presently not able to do sorption measurements. The determination of D from time-lag measurements has been shown to be unreliable for glassy polymers [18,19], when it is based on the assumption of Henry's law absorption and Fick's diffusion without detailed sorption measurements.
Fig. 2. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the CO2/N2 gas pair; (*) reference data from the literature [2,12]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Fig. 3. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the O2/N2 gas pair; (*) reference data from the literature [13,14]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
Fig. 4. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the H2/O2 gas pair; (*) reference data from the literature [3]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
However, for further interpretations, we can assume that the overall selectivities (X/Y) represent changes in diffusivity selectivities DX/DY, since it can be seen from the literature that the solubility selectivities of polyimides do not change much (usually less than 10%) with the chemical structure [2,20]. Also, end group effects could play a role. However, those polymers 3 and 4 which are soluble after imidization, exhibit comparable molar masses. The only exception n. The glass is polymer 4b, which has a lower M
transition of this polymer is in the range one can expect from the glass transition temperatures and chemical structures of the other polymers in this study. n at low molar masses, the Since Tg depends on M n for 4b is likely due to the apparently lower value of M fact that molar masses were determined by a relative method (GPC), which may not be suitable for all polymers 3 and 4 alike. If the molar mass of 4b were indeed much lower, one should also expect a much lower value of Tg.
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
121
Fig. 5. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the CO2/O2 gas pair; (*) reference data from the literature [3,15]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
Fig. 6. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the H2/CO2 gas pair; (*) reference data from the literature [12]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
In many cases, the hexa¯uoroisopropyl group has been found to increase (or at least not to decrease) the selectivity of membrane materials. However, from the data in Table 3 it is clear that in almost all cases studied here the polymers with X±O± exhibit higher (in some cases much higher) selectivities. The only exception is the gas pair CO2/O2. One can expect the ether linkage to be more ¯exible than the hexa¯uoroisopropyl group. Since increasing ¯exibility of the polymer backbone tends to result in decreased selectivity, this observation must be explained by hindered chain mobility due to more dense chain packing of the
polymers with the ether groups than those with the more bulky hexa¯uoroisopropyl groups. This view is supported by the fractional free volumes (Table 1), calculated from density measurements, using the increments developed recently by Park and Paul [21]. In Table 4 the changes of selectivity and permeability coef®cients caused by the replacement of two methyl substituents at the indan group by cyclohexyl groups. Although the cyclohexyl groups increases the bulkiness of the indan group (see fractional free volumes in Table 1), its effect on the permselectivities of the
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Scheme 3. Comparison of the structures of the polyimides 3 (Rmethyl) and 4 (Rcyclohexyl) with the poly(ether ketone)s 5 (X±C(CF3)2±, ±SO2±, ±CO±, ±O±, Ð). Table 4 Changes of selectivity and permeability coefficent for the faster gas Polymer
±X±
3a!4a
±C(CF3)2±
3b!4b
±SO2±
3c!4c
±CO±
3d!4d
±O±
3d!4d
Ð
P P P P P
O2/N2
H2/N2
ÿ0.6 ÿ0.13 ÿ1.0 ÿ0.16 ÿ1.8 0.44 3.6 0.22 ÿ1.2 0.54
3.4 1.8 ÿ15.7 ÿ2.5 ÿ61.9 0.9 48.7 2.73 ÿ32.5 4.5
polyimides is not uniform, even if we take into account only the most signi®cant changes (indicated by italic typeface in Table 4). For the polymers with X ±C(CF3)2±, both selectivity and permeability coef®cient are improved for the gas pairs H2/CO2 and CO2/O2. For the other gas pairs, the changes in and P are relatively small. In contrast, for the polymers with X±SO2±, both selectivity and permeability for H2/N2 are reduced by the introduction of the cyclohexyl rings, while for the other gas pairs small positive or negative changes of and P are observed. With X±O±, a tendency to increase both selectivity and permeability results. The remaining polymers (X ±CO± and XÐ) exhibit decreased selectivities and increased permeability coef®cients. Thus, selectivity and permeability coef®cients for the same gas
CO2/N2 11.8 4.06 ÿ1.9 ÿ0.4 ÿ16.9 ÿ0.28 13.1 0.71 ÿ2.5 3.17
H2/CO2
CO2/O2
H2/O2
ÿ1.4 1.8 ÿ0.4 ÿ2.5 1.3 0.9 0.1 2.73 ÿ1.2 4.5
2.3 4.06 0.4 ÿ0.4 ÿ2.4 ÿ0.28 ÿ0.3 0.71 0.7 3.17
1.6 1.8 ÿ0.4 ÿ2.5 ÿ7.6 0.9 ÿ0.7 2.73 ÿ3.4 4.5
pair can be increased or decreased by the introduction of the cyclohexyl substituents, depending on the linking group X in the dianhydride moiety of the repeating unit. The inconclusive effect of the cyclohexyl substituents can be explained by the inherent ¯exibility of this group due to its possible conformational changes. As a result of this built-in conformational ¯exibility, the effect of this substituents on packing density and hence main chain mobility can not be predicted easily and certainly depends on the structure of the rest of the repeating unit. Both polymer series exhibit low selectivities for hydrogen over carbon dioxide, although the size difference between these two gas molecules is larger than the difference between oxygen and nitrogen or oxygen and carbon dioxide. Yet, the O2/N2-
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
selectivities as well as the CO2/O2-selectivities are much higher (Table 3). This can not be explained by excessively high solubility of the CO2, because in that case, one should expect the CO2/O2- and CO2/N2selectivities to be unusually high. In addition, the measurements were performed at low pressures (1.2±1.3 bar), where CO2 does not exhibit unusual solubility behavior by swelling of the polymer. If low solubility of hydrogen in the polymers 3 and 4 were the reason, the H2/N2- and H2/O2-selectivities should also be signi®cantly decreased. Thus, at this point we can speculate that the selectivity behavior which we observe, is more likely the result of changes in diffusivity selectivity than changes in solubility selectivity. Future measurements of the sorption characteristics of these polymers will provide more insight into this problem. Also, introduction of a bulky substituent with less conformational freedom than the cyclohexyl group is planned for future studies. 5. Conclusions The incorporation of the bulky 1,1,3-trimethyl-3phenylindan group or the even more bulky 1,3-biscyclohexyl-1-methyl-3-phenylindan group into the diamine unit of polyimides did not result in the expected general increase in permeability coef®cients or selectivities. The cyclohexyl side groups at the indan group increase the global chain stiffness, as evidenced by a strong increase in glass transition temperatures. However, their in¯uence on permselectivity is not uniform and depends strongly on the structure of the rest of the repeating unit. Also, among the polymers studied here, those with the ether linkage in the dianhydride moiety exhibit higher selectivities than expected. This linkage is certainly quite ¯exible and should therefore result in relatively low selectivities. At this point, a detailed interpretation of structure-property relationships is dif®cult, and more work with similar polymers with different side groups at the indan ring is necessary to allow reliable conclusions. References [1] W.J. Koros, G.K. Fleming, Membrane-based gas separation, J. Membr. Sci. 83 (1993) 1±80.
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[2] S.A. Stern, Polymers for gas separations: the next decade, J. Membr. Sci. 94 (1994) 1±65. [3] D.R. Paul, Yu.P. Yampol'skii, Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. [4] R.E. Kesting, A.K. Fritzsche, Polymeric Gas Separation Membranes, Wiley, New York, 1993. [5] G. Maier, M. Wolf, M. Bleha, Z. Pientka, Gas permeabilties of polymers with indan groups in the main chain. 1: Poly(ether ketone)s, J. Membr. Sci. 143 (1998) 105±113. [6] G. Maier, D. Yang, M. Wol, O. Nuyken, Polyimides based on trimethylphenyl indan, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 35(1) (1994) 379±380. [7] G. Maier, D. Yang, M. Wolf, O. Nuyken, Soluble polyimides based on the phenylindan structure element, High Performance Polym. 6 (1994) 335±346. [8] G. Maier, M. Wolf, Polyimides with cyclohexyl substituted indan groups in the main chain, Macromol. Chem. Phys. 197 (1996) 781±796. [9] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165±185. [10] L.M. Robeson, W.F. Burgoyne, M. Langsam, A.C. Savoca, C.F. Tien, High performance polymers for membrane separation, Polymer 35 (1994) 4970±4978. [11] K.E. Min, D.R. Paul, Effect of tacticity on permeation properties of poly(methyl methacrylate), J. Polym. Sci., Part B: Polym. Phys. 26 (1988) 1021±1033. [12] J. Brandrup, H.E. Immergut, Polymer Handbook, 3rd ed., Wiley, New York, 1989, p. VI/444. [13] B. Gebben, M.H.V. Mulder, C.H. Smolders, Gas separation properties of a thermally stable and chemically resistant polytriazole membrane, J. Membr. Sci. 46 (1989) 29±41. [14] D.R.B. Walker, W.J. Koros, Transport characterization of a polypyrrolone for gas separations, J. Membr. Sci. 55 (1991) 99±117. [15] W.J. Koros, G.K. Fleming, S.M. Jordan, T.H. Kim, H.H. Hoehn, Polymeric membrane materials for solution±diffusion based permeation separations, Prog. Polym. Sci. 13 (1988) 339±401. [16] J.S. McHattie, W.J. Koros, D.R. Paul, Gas transport properties of polysulphones: 2. Effect of bisphenol connector group, Polymer 32 (1991) 2618±2625. [17] M.W. Hellums, W.J. Koros, G.R. Husk, D.R. Paul, Fluorinated polycarbonates for gas separation applications, J. Membr. Sci. 46 (1989) 93±112. [18] D.R. Paul, Effect of immobilizing adsorption on the diffusion time lag, J. Polym. Sci., Part A-2 (7) (1969) 1811±1818. [19] D.R. Paul, Gas sorption and transport in glassy polymers, Ber. Bunsenges. Phys. Chem. 83 (1979) 294±302. [20] 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. [21] J.Y. Park, D.R. Paul, Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method, J. Membr. Sci. 125 (1997) 23±39.
Gas permeabilities of polymers with indan groups in the main chain. 2: Polyimides Gerhard Maiera,*, Martin Wolfa, Miroslav Blehab, Zbynek Pientkab a
Lehrstuhl fuÈr Makromolekulare Stoffe, Technische UniversitaÈt MuÈnchen, Lichtenbergstraûe 4, D-85747 Garching, Germany Institute of Macromolecular Chemistry, Department of Polymer Membranes, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, Prague 6 CZ-16206, Czech Republic
b
Received 16 July 1997; received in revised form 30 December 1997; accepted 30 December 1997
Abstract Permeability coef®cients and ideal selectivities for H2, CO2, O2, and N2 were determined for two series of polyimides with indan groups in the main chain. The bulkiness of the indan group was varied by replacing methyl groups at the indan ring system by bulky cyclohexyl groups. The effects of the linking group in the dianhydride part of the polymer repeating unit could be explained on the basis of bulkiness and ¯exibility. However, the introduction of two cyclohexyl substituents at the indan group to increase the bulkiness of this structural element had no uniform effect on permeability coef®cients and selectivities. Positive or negative effects were observed, depending on the linking group in the dianhydride moiety. This was tentatively attributed to the conformational ¯exibility of the cyclohexyl rings, which can result in different sterical requirements. # 1998 Elsevier Science B.V. Keywords: Gas separations; Structure±property relationships; Polyimides
1. Introduction Amorphous polymers with high glass transition temperatures such as polyimides or poly(arylene ether)s are of potential interest as gas separation membranes. Bulky groups in the main chain generally tend to increase free volume and hence permeability coef®cients [1±4]. High chain stiffness, as indicated by high glass transition temperatures, is expected to result in relatively high selectivities in gas separation applications [1±4]. However, exact structure±property relationships are not known in full detail yet. The *Corresponding author. Fax: +49 89 289 13562; e-mail: [email protected] 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00006-4
expectations described above are based on the idea that the permeation of gas particles through an amorphous, pore free polymer ®lm occurs by ``jumps'' of the particles between voids of free volume through channels which are opened and closed by thermally activated motions of certain chain segments of the polymer backbone. In this model, increasing free volume results in increasing permeability coef®cients, and increasing chain stiffness leads to increasing selectivity. In our view, to further elucidate exact structure± property relationships for a series of polymers, the chain segments which control the permeation need to be identi®ed. In a preceding paper [5] we described the results of permeability measurements of hydrogen,
116
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Scheme 1. Synthesis of the indan containing diamine monomers: 1 (Rmethyl) and 2 (Rcyclohexyl)
carbon dioxide, oxygen, and nitrogen for a series of amorphous indan containing poly(ether ketone)s. All polymers of this series exhibited only small variations of selectivity for the gas pairs H2/N2, CO2/N2, and O2/ N2 [5]. From these results it was concluded that the bulky indan groups in these polymers are rather immobile with regard to the timescale relevant for the permeation jumps, while the motions of the ¯exible phenylene ether segments between the indan groups are responsible for the permeation events. The present paper describes permeabilities and selectivities of a series of polyimides with structures similar to those of the poly(ether ketone)s discussed earlier. 1.1. Polymer structures The synthesis of the diamine monomers 1 and 2 as well as the polyimides 3 and 4 have been described
earlier [6±8], and are therefore outlined here only brie¯y in Schemes 1 and 2, and some properties of the polymers are summarized in Table 1. Except for polymer 3d all polyimides 3 and 4 are soluble in NMP above 508C, and except for 3d, 4c, and 4d they are also soluble in chloroform at ambient temperature [7,8]. In addition, 3a, 3b, 4a, and 4b are also soluble in cold NMP, DMAc, THF, and dichloromethane [7,8]. Glass transition temperatures are in the range between 2328C and 2748C [7,8], with those of the cyclohexyl substituted derivatives being approximately 308C higher than those of the methyl substituted polymers [8]. None of these polymers showed a melting endotherm in DSC measurements [7,8]. Molar masses were determined by GPC in chloroform with polystyrene calibration. Although this method does not yield absolute values, the results
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
117
Scheme 2. Structures of the polyimides 3 (Rmethyl) and 4 (Rcyclohexyl) (X±C(CF3)2±, ±SO2±, ±CO±, ±O±, Ð) Table 1 Properties of the polyimides 3 and 4 Polymer No.
±R
±X±
Tg a [8C]
n M
3a 3b 3c 3d 3e 4a 4b 4c 4d 4e
±methyl ±methyl ±methyl ±methyl ±methyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl
±C(CF3)2± ±SO2± ±CO± ±O± Ð ±C(CF3)2± ±SO2± ±CO± ±O± Ð
241 251 232 233 246 269 272 261 264 274
28 000 20 800 21 300
b
[g molÿ1]
d
31 500 30 700 8100 d d
25 800
Vf(H2) c
Vf(O2) c
0.163 0.144 0.124 0.134 0.131 0.179 0.160 0.138 0.147 0.145
0.161 0.139 0.124 0.119 0.132 0.169 0.144 0.127 0.122 0.135
a
DSC, 20 K/min. GPC in CHCl3, polystyrene calibration. c Density determined by floating film in aqueous CaCl2 solution. d Insoluble in CHCl3. b
can be compared within a series of similar polymers. As can be seen from Table 1, the values obtained for the polymers 3 and 4 are fairly uniform. In all cases the molar mass of the completely cyclized poly(imide)s were high enough to result in tough, ¯exible ®lms. 2. Experimental The synthesis and characterization of the polymers 3 and 4 has been described elsewhere. Permeabilities of hydrogen, carbon dioxide, oxygen, and nitrogen were measured for all polyimides 3 and 4 using circular samples of dense ®lms. Films were cast form poly(amic acid) solutions in DMAc. Imidization was achieved by heating to 508C (1 h), 1008C (1 h), 1508C (1 h), and ®nally 2608C (2 h). Completeness of imidization was checked by IR spectroscopy and thermogravimetry. Prior to measurements, the ®lms were stored under vacuum at 508C for several days. Three individual ®lms were cast from each polymer. The
data given for each polymer are the averages of the measurements of a sample of each of these three ®lms. The differences between the permeability coef®cients of the three samples is of the order of 2% for H2, 5% for CO2, 10% for O2 and N2, However, as the permeability coef®cients approach 0.1 barrer, the accuracy decreases to 50%. Selectivities are expressed as ideal selectivities (ratios of the permeability coef®cients). The preparation of the ®lms is critical for the quality of the measurements, especially in the case of polyimides, when ®lm preparation via the poly(amic acid) precursor is necessary. To further evaluate the reproducibility of our measurements, the permeabilities of two of our polymers (3a and 3d) for H2 and O2 were checked at Air Products and Chemicals. The deviations between these sets of results were far below 10%: 3a: P(O2)2.05; P(N2)0.33 (our results, Table 2: 2.05 and 0.32); 3d: P(O2)0.65; P(N2) 0.097 (our results, Table 2: 0.68 and 0.09). This clearly indicates the reliability of the measurements.
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Table 2 Gas permeability coefficients of the polyimides 3 and 4 [Barrers] Polymer No.
±R
±X±
P(H2)
P(CO2)
P(O2)
P(N2)
3a 3b 3c 3d 3e 4a 4b 4c 4d 4e
±methyl ±methyl ±methyl ±methyl ±methyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl
±C(CF3)2± ±SO2± ±CO± ±O± Ð ±C(CF3)2± ±SO2± ±CO± ±O± Ð
21.3 12.7 8.3 10.5 8.8 23.1 10.2 9.2 13.23 13.3
5.58 3.6 1.96 3.05 2.51 9.64 3.2 1.68 3.76 5.68
2.05 0.91 0.47 0.68 0.57 1.92 0.75 0.91 0.9 1.11
0.32 0.16 0.08 0.09 0.11 0.33 0.16 0.22 0.08 0.28
2.1. Permeability measurements Gas transport properties of the membranes were studied using a laboratory-made high-vacuum apparatus with a static permeation cell. The polymer membrane separates the injection side of the cell, containing the studied gas under constant pressure pi (1245 mbar), from the product side which is evacuated in the beginning. Permeability coef®cients P were determined from the pressure increase pp =t in the calibrated volume V of the product side of the cell. Only the linear part of the pressure increase was evaluated. P
pp V l 1 t A pi RT
Pressure pp at the product side was measured with a precision capacitance manometer CCM-10; the membrane area A was 1.24 cm2; l is the membrane thick-
ness; T is the temperature (22.58C); R is the gas constant. 3. Results of the permeability measurements The permeability coef®cients obtained by the procedure described above are summarized in Table 2, the selectivities in Table 3. The plots of selectivity vs. permeability coef®cient for the six gas pairs H2/N2, CO2/N2, O2/N2, H2/O2, H2/ CO2, and H2/CO2 are shown in the following Figs. 1± 6. The upper bounds and some data for polymers close to it [9,10] are added to the graphs for orientation. A comparison between the data for the methyl substituted polyimides 3 and their cyclohexyl substituted analogues 4 reveals that the additional bulkiness added to the indan groups by the two cyclohexyl rings has no uniform effect on the gas permeation properties
Table 3 Selectivities of the polyimides 3 and 4 Polymer No.
±R
±X±
(O2/N2)
3a 3b 3c 3d 3e 4a 4b 4c 4d 4e
±methyl ±methyl ±methyl ±methyl ±methyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl ±cyclohexyl
±C(CF3)2± ±SO2± ±CO± ±O± Ð ±C(CF3)2± ±SO2± ±CO± ±O± Ð
6.4 5.7 5.9 7.6 5.2 5.8 4.7 4.1 11.2 4.0
(H2/N2) 66.6 79.4 103.7 116.7 80 70 63.7 41.8 165.4 47.5
(CO2/N2)
(H2/CO2)
(CO2/O2)
(H2/O2)
17.4 22.5 24.5 33.9 22.8 29.2 20.6 7.6 47 20.3
3.8 3.5 4.2 3.4 3.5 2.4 3.1 5.5 3.5 2.3
2.7 4.0 4.2 4.5 4.4 5.0 4.4 1.8 4.2 5.1
10.4 14.0 17.7 15.4 15.4 12.0 13.6 10.1 14.7 12.0
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Fig. 1. Selectivity vs. permeability coefficient for the H2/N2 gas pair; (*) reference data from the literature [11]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
of the polymers. Although originally we expected increased permeability coef®cients by increasing the bulkiness of the indan group, in many cases the opposite effect is observed. Clearly, the effect of a single structural variation at one speci®c site of the repeating unit depends on the total structure of the repeating unit. 4. Discussion In the preceding paper of this series [5], gas permeabilities of poly(ether ketone)s 5 were studied. It was
119
observed that for the gas pairs H2/N2, CO2/N2, and O2/ N2 the selectivities of the poly(ether ketone)s were almost identical and did not vary much with the structure of the repeating unit. The polyimides 3 and 4 differ from the poly(ether ketone)s 5 by replacement of two phenyl rings by two phthalimide groups, indicated by bold lines in Scheme 3. In contrast to the observations in the case of the poly(ether ketone)s, the selectivities of the polyimides 3 and 4 vary strongly with the structure of the repeating units (Table 3, Figs. 1±6). Within both series of polyimides, the hexa¯uoroisopropyl group as linking group X results in the highest permeability coef®cients for the gases studied here. The carbonyl linking group results in the lowest values among the polymers 3 for all gases, and among the polymers 4 for hydrogen and carbon dioxide. The positive effect of the hexa¯uoroisopropyl group in the polymer backbone on permeability coef®cients is well known, and is generally explained by the bulkiness of this structural element [2,16,17]. For detailed discussion the knowledge of diffusion and solubility coef®cients is desirable. Due to limitations of our equipment we are presently not able to do sorption measurements. The determination of D from time-lag measurements has been shown to be unreliable for glassy polymers [18,19], when it is based on the assumption of Henry's law absorption and Fick's diffusion without detailed sorption measurements.
Fig. 2. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the CO2/N2 gas pair; (*) reference data from the literature [2,12]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Fig. 3. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the O2/N2 gas pair; (*) reference data from the literature [13,14]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
Fig. 4. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the H2/O2 gas pair; (*) reference data from the literature [3]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
However, for further interpretations, we can assume that the overall selectivities (X/Y) represent changes in diffusivity selectivities DX/DY, since it can be seen from the literature that the solubility selectivities of polyimides do not change much (usually less than 10%) with the chemical structure [2,20]. Also, end group effects could play a role. However, those polymers 3 and 4 which are soluble after imidization, exhibit comparable molar masses. The only exception n. The glass is polymer 4b, which has a lower M
transition of this polymer is in the range one can expect from the glass transition temperatures and chemical structures of the other polymers in this study. n at low molar masses, the Since Tg depends on M n for 4b is likely due to the apparently lower value of M fact that molar masses were determined by a relative method (GPC), which may not be suitable for all polymers 3 and 4 alike. If the molar mass of 4b were indeed much lower, one should also expect a much lower value of Tg.
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
121
Fig. 5. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the CO2/O2 gas pair; (*) reference data from the literature [3,15]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
Fig. 6. Selectivity vs. permeability coefficient of the indan-poly(imide)s for the H2/CO2 gas pair; (*) reference data from the literature [12]; (~) methyl substituted indan groups; (&) cyclohexyl substituted indan groups.
In many cases, the hexa¯uoroisopropyl group has been found to increase (or at least not to decrease) the selectivity of membrane materials. However, from the data in Table 3 it is clear that in almost all cases studied here the polymers with X±O± exhibit higher (in some cases much higher) selectivities. The only exception is the gas pair CO2/O2. One can expect the ether linkage to be more ¯exible than the hexa¯uoroisopropyl group. Since increasing ¯exibility of the polymer backbone tends to result in decreased selectivity, this observation must be explained by hindered chain mobility due to more dense chain packing of the
polymers with the ether groups than those with the more bulky hexa¯uoroisopropyl groups. This view is supported by the fractional free volumes (Table 1), calculated from density measurements, using the increments developed recently by Park and Paul [21]. In Table 4 the changes of selectivity and permeability coef®cients caused by the replacement of two methyl substituents at the indan group by cyclohexyl groups. Although the cyclohexyl groups increases the bulkiness of the indan group (see fractional free volumes in Table 1), its effect on the permselectivities of the
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G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
Scheme 3. Comparison of the structures of the polyimides 3 (Rmethyl) and 4 (Rcyclohexyl) with the poly(ether ketone)s 5 (X±C(CF3)2±, ±SO2±, ±CO±, ±O±, Ð). Table 4 Changes of selectivity and permeability coefficent for the faster gas Polymer
±X±
3a!4a
±C(CF3)2±
3b!4b
±SO2±
3c!4c
±CO±
3d!4d
±O±
3d!4d
Ð
P P P P P
O2/N2
H2/N2
ÿ0.6 ÿ0.13 ÿ1.0 ÿ0.16 ÿ1.8 0.44 3.6 0.22 ÿ1.2 0.54
3.4 1.8 ÿ15.7 ÿ2.5 ÿ61.9 0.9 48.7 2.73 ÿ32.5 4.5
polyimides is not uniform, even if we take into account only the most signi®cant changes (indicated by italic typeface in Table 4). For the polymers with X ±C(CF3)2±, both selectivity and permeability coef®cient are improved for the gas pairs H2/CO2 and CO2/O2. For the other gas pairs, the changes in and P are relatively small. In contrast, for the polymers with X±SO2±, both selectivity and permeability for H2/N2 are reduced by the introduction of the cyclohexyl rings, while for the other gas pairs small positive or negative changes of and P are observed. With X±O±, a tendency to increase both selectivity and permeability results. The remaining polymers (X ±CO± and XÐ) exhibit decreased selectivities and increased permeability coef®cients. Thus, selectivity and permeability coef®cients for the same gas
CO2/N2 11.8 4.06 ÿ1.9 ÿ0.4 ÿ16.9 ÿ0.28 13.1 0.71 ÿ2.5 3.17
H2/CO2
CO2/O2
H2/O2
ÿ1.4 1.8 ÿ0.4 ÿ2.5 1.3 0.9 0.1 2.73 ÿ1.2 4.5
2.3 4.06 0.4 ÿ0.4 ÿ2.4 ÿ0.28 ÿ0.3 0.71 0.7 3.17
1.6 1.8 ÿ0.4 ÿ2.5 ÿ7.6 0.9 ÿ0.7 2.73 ÿ3.4 4.5
pair can be increased or decreased by the introduction of the cyclohexyl substituents, depending on the linking group X in the dianhydride moiety of the repeating unit. The inconclusive effect of the cyclohexyl substituents can be explained by the inherent ¯exibility of this group due to its possible conformational changes. As a result of this built-in conformational ¯exibility, the effect of this substituents on packing density and hence main chain mobility can not be predicted easily and certainly depends on the structure of the rest of the repeating unit. Both polymer series exhibit low selectivities for hydrogen over carbon dioxide, although the size difference between these two gas molecules is larger than the difference between oxygen and nitrogen or oxygen and carbon dioxide. Yet, the O2/N2-
G. Maier et al. / Journal of Membrane Science 143 (1998) 115±123
selectivities as well as the CO2/O2-selectivities are much higher (Table 3). This can not be explained by excessively high solubility of the CO2, because in that case, one should expect the CO2/O2- and CO2/N2selectivities to be unusually high. In addition, the measurements were performed at low pressures (1.2±1.3 bar), where CO2 does not exhibit unusual solubility behavior by swelling of the polymer. If low solubility of hydrogen in the polymers 3 and 4 were the reason, the H2/N2- and H2/O2-selectivities should also be signi®cantly decreased. Thus, at this point we can speculate that the selectivity behavior which we observe, is more likely the result of changes in diffusivity selectivity than changes in solubility selectivity. Future measurements of the sorption characteristics of these polymers will provide more insight into this problem. Also, introduction of a bulky substituent with less conformational freedom than the cyclohexyl group is planned for future studies. 5. Conclusions The incorporation of the bulky 1,1,3-trimethyl-3phenylindan group or the even more bulky 1,3-biscyclohexyl-1-methyl-3-phenylindan group into the diamine unit of polyimides did not result in the expected general increase in permeability coef®cients or selectivities. The cyclohexyl side groups at the indan group increase the global chain stiffness, as evidenced by a strong increase in glass transition temperatures. However, their in¯uence on permselectivity is not uniform and depends strongly on the structure of the rest of the repeating unit. Also, among the polymers studied here, those with the ether linkage in the dianhydride moiety exhibit higher selectivities than expected. This linkage is certainly quite ¯exible and should therefore result in relatively low selectivities. At this point, a detailed interpretation of structure-property relationships is dif®cult, and more work with similar polymers with different side groups at the indan ring is necessary to allow reliable conclusions. References [1] W.J. Koros, G.K. Fleming, Membrane-based gas separation, J. Membr. Sci. 83 (1993) 1±80.
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