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Gas transport properties of new aromatic cardo poly(aryl ether ketone)s
Journal of Membrane Science 283 (2006) 393–398
Gas transport properties of new aromatic cardo poly(aryl ether ketone)s C. Camacho-Zu˜niga a , F.A. Ruiz-Trevi˜no a,∗ , M.G. Zolotukhin b , L.F. del Castillo b , J. Guzman b , J. Chavez b , G. Torres b , N.G. Gileva c , E.A. Sedova c a
Universidad Iberoamericana, Departamento de Ingenier´ıas, Prol. Paseo de la Reforma No. 880, M´exico, D.F. 01210, Mexico b Instituto de Investigaciones en Materiales, Universidad Nacional Aut´ onoma de M´exico, Apartado Postal 70-360, CU, Coyoac´an, 04510 M´exico, D.F., M´exico c Institute of Organic Chemistry of Russian Academy of Sciences, UFA, Prospekt Oktuabrya, 71, Russia Received 6 March 2006; received in revised form 30 June 2006; accepted 8 July 2006 Available online 14 July 2006
Abstract New cardo poly(aryl ether ketone)s containing side phthalide groups and aryl ether ketones in different lengths have been synthesized and characterized in terms of their thermal, volumetric and gas transport properties to H2 , O2 , N2 , CH4 and CO2 . The polymers show high glass transition temperature (218–420 ◦ C), good solubility in chlorinated solvents and strong acids as well as excellent thermal stability (decomposition temperatures above 510 ◦ C). The most permeable membrane studied shows permeability coefficients of 11 to O2 and 72 to CO2 , with ideal selectivity factors of 4.6 for the pair O2 /N2 and 25 for CO2 /CH4 . The results, interpreted in terms of chain rigidity and chain packing ability, show that decreasing the length of the connector moieties between the cardo groups increases the fractional free volume, the glass transition temperature and the gas permeability coefficients. © 2006 Elsevier B.V. All rights reserved. Keywords: Cardo polymers; Transport properties; Gas separation; Membranes; Poly(aryl ether ketone)s
1. Introduction Gas separation membrane unit operations are recognized as clean technology, being the petrochemical and chemical industry one of their most promising applications. In such cases, they acquire greater interest especially when gas and oil prices increase as a consequence of the natural disasters caused by the severe global climate changes [1]. Since it can be a non-thermal process, it can reduce the amount of fuel burned around the world. However, to see its generalized application in the wild environments (aggressive chemical surroundings and high pressure and temperature) typically found in these industries, better polymer membranes should be achieved [2]. In this address, polymers that may possess high glass transition temperature, excellent mechanical toughness and thermo oxidative stability may be considered good candidates if their permeability–selectivity relationship is better or similar to the one shown by commercial membranes.
∗
Corresponding author. Tel.: +52 55 5950 4000; fax: +52 55 5950 4225. E-mail address: [email protected] (F.A. Ruiz-Trevi˜no).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.07.013
The term “cardo polymers”, introduced by the beginning of 1960 [3,4], refers to polymers containing bulky side groups. It has been shown for different polymer families that the presence of cardo groups inside the polymer structure disrupts crystallinity (if any), increases their glass transition temperature and improves their solubility. Generally, cardo groups in aromatic polymers twist the phenylene moieties of the main chain out of a planar conformation, thereby affecting the polymer free volume as well. All of these characteristics make aromatic cardo polymers suitable candidates for gas separation applications [5–20]. In this work, new cardo poly(aryl ether ketone)s (PEKs) polymers containing side phthalide groups were synthesized and characterized for their thermal, volumetric and gas transport properties. For comparison, another cardo polymer containing a biphenylene connector in its repeating unit was also studied (See Figs. 1 and 2). The systematic changes in the length of the chemical structure were expected to result in variations of their physical properties, e.g., packing density and segmental motion of the polymer chain. This work investigates these effects on the gas permeation behavior of their membranes.
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Fig. 1. Reaction scheme, chemical structure and nomenclature for cardo-PEKs synthesized in this work.
2. Experimental 2.1. Materials Dichloroethane and anhydrous aluminum chloride were obtained from Aldrich and purified by distillation and sublimation, respectively. Aromatic hydrocarbons reported on Fig. 1: 4,4 -diphenoxybenzophenone (for PEK-2), 1,4bis (4-phenoxybenzoyl)benzene (for PEK-3) and 1,3-bis(4phenoxybenzoyl)benzene (for PEK-4) were prepared according to the published method [21]. Dichloride of 3,3bis(4,4 -carboxyphenyl)phthalide was synthesized as previously reported [22]. 2.2. Polymer synthesis Three different cardo poly(aryl ether ketone)s (PEK2-4) were prepared via electrophilic polycondensation according to the scheme outlined in Fig. 1. Every synthesis was carried out by the reaction of aromatic electrophilic substitution of Friedel-Crafts of the acid dichloride with the corresponding aromatic hydrocarbon in presence of aluminum chloride. Other conditions were similar to those described elsewhere [21].
Fig. 2. Cardo polymer repeating unit based on phthalide groups and biphenylene.
A standard procedure for the preparation of cardo poly(aryl ether ketone)s is described below. For PEK-2, a 300 ml threeneck flask equipped with magnetic stirrer, nitrogen inlet, thermometer, solids addition funnel and gas outlet, was purged with dry nitrogen and charged with 3.66 g (0.01 mol) of 4,4 -diphenoxybenzophenone, 4.11 g (0.01 mol) of dichloride of 3,3-bis(4,4 -carboxyphenyl)phthalide and 125 ml of 1,2dichloroethane. The addition funnel was charged with 5.05 g (0.038 mol) of high purity A1C13 . The transparent colorless solution was cooled to −15 ◦ C, aluminum chloride was added and then the reaction mixture was stirred for 30 min. Thereafter, the temperature was raised to 20 ◦ C over 2 h and the reaction was allowed to continue at this temperature for 20 h. The suspension obtained was filtered. The precipitate was washed with methanol, extracted with boiling methanol for 20 h and allowed to dry in air. The air-dried product was heated at 100 ◦ C overnight under vacuum to give 7.04 g of the polymer (95% yield). Similar preparations were carried out to obtain the polymers PEK-3 and PEK-4. Cardo polymer CP-1 was prepared according to the previously reported method [23]. 2.3. Polymer characterization The inherent viscosities of 0.2% polymer solutions in symmtetrachloroethtane were measured at 25 ◦ C using an Ubbelohde viscometer. The 1 H and 13 C NMR spectra were recorded using a Bruker Avance 400 spectrometer, operating at 400.13 and 100 MHz for 1 H and 13 C, respectively. Thermogravimetric analyses (TGA) were carried out in air and under nitrogen at a heating rate of 10 ◦ C min−1 on a TGA 2950 Thermogravimetric Analyzer, TA Instruments. The glass transition temperature for every polymer was evaluated by differential scanning calorimetry (DSC) measurements at 20 ◦ C min−1 on a DSC 2910 TA Instruments. Finally, the polymer densities were determined from a dry film sample in a density gradient column containing aqueous zinc chloride solution at 30 ◦ C.
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2.4. Membrane preparation
S(i) =
Dense polymer films were prepared by casting CHCl3 solutions containing 10% (w/v) polymer onto glass plates and dried for 24 h at 40 ◦ C in an oven. They were removed from the glass plates and dried for another 24 h at 100 ◦ C under vacuum in order to evaporate residual solvent. All membranes were treated according to the same procedure and their thickness, measured with a Mitutoyo caliper, varied from 20 to 30 m. 2.5. Gas permeability measurements A standard permeation cell was used to determine the pure gas permeability coefficients at 35 ◦ C and 2 atm. Before measurements were performed, vacuum was pull overnight in order to remove any residual gas dissolved in the membrane. During the permeation experiment, the increase in pressure in the downstream side of the membrane was followed by an absolute pressure transducer, MKS Baratron type 627B, and kept below 1 mmHg while the upstream pressure was maintained at 2 atm. Ultrahigh purity gases H2 , O2 , N2 , CH4 and CO2 were used in that order to avoid the plasticization effect of CO2 , the most condensable gas. Gas permeability coefficients for each gas, P(i), were determined from the slope of the downstream pressure versus time plot once steady state had been achieved. Permeability coefficients were evaluated for at least two different membranes of the same polymer and their difference was smaller than 3%. This fact confirms reproducibility of the reported values.The apparent diffusion coefficients, D(i), and apparent solubility coefficients, S(i), for each gas were calculated from the time lag θ(i) using the equations: D(i) =
L2 6θ(i)
(1)
P(i) D(i)
395
(2)
In Eq. (1) L is the film thickness. The ideal separation factor, α(i/j) = P(i)/P(j), was calculated from the ratio of the pure gas permeability coefficients. 3. Results and discussion The cardo polymers synthesized are not soluble in alcohols, aromatic hydrocarbons or ethers but fully soluble in chlorinated solvents (such as chloroform and tetrachloroethane) and strong acids. Their good solubility would allow the formation of an ultra thin active layer as asymmetric or composite membrane. Such arrangements can yield a high gas flux and therefore these materials fulfill the requirement of processability for their application in gas separation membrane technology. It is well known that the type of reactions used in these syntheses is not very selective and, frequently, leads to isomer mixtures. For that reason, it is necessary to analyze the obtained chemical structures. Their solubility allowed performing reliable 1 H and 13 C NMR spectral studies to define their structures. As an example, the 1 H NMR spectrum of polymer PEK-2 is given in Fig. 3. It is well resolved: the aromatic resonances anticipated for the diphenoxybenzophenone (doublets at 7.15, 7.50, 7.78, and 7.00 ppm) and biphenyl fragments (two doublets near 7.85 ppm) are all evident and no other resonances can be observed. Similarly, its 13 C NMR spectrum, not included for simplicity purposes, showed a highly resolved pattern with no evidence of ortho- or meta-substitution. Similar to that of PEK-2, spectra of PEK-3 and PEK-4 are also well resolved and confirm the corresponding substitution in the aromatic fragments of the main chain. Table 1 summarizes the cardo polymers properties in terms of their inherent viscosity of their solutions, the glass transition temperature, the thermal degradation temperature at a 10 wt.%
Fig. 3. 1 H NMR spectrum determined for polymer PEK-2 (solution in CDCl3 ).
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Table 1 Inherent viscosity, glass transition temperature, thermal degradation temperature, specific and fractional free volume measured for the cardo polymers synthesized in this work Polymer
ηinh , dl/g (TCE)
Tg (◦ C)
Tdecom (◦ C, onset)
V (cm3 /g)
FFVa
CP-1 PEK-2 PEK-3 PEK-4
0.86 1.24 1.16 1.35
420 238 227 218
510 517 520 517
0.8309 0.7927 0.7921 0.7857
0.182 0.152 0.149 0.142
◦ 0 FFV = V (30)−V V (30) , where V(30) is the specific volume at 30 C, V0 = 1.3Vw and Vw is the van der Waals volume of the repeating unit estimated by the method outlined by Bondi. a
loss in air and the specific and fractional free volume. Their inherent viscosities with an average of 1.15 dl/g indicate, at least from a qualitative point of view, that the polymers have a high molecular weight. This fact allowed the formation of flexible membranes with enough mechanical strength for gas permeation measurements. Moreover, their flexibility suggests an amorphous nature, which can also be inferred from the optical transparency of their films and the typical metallic sound when they are waved in air [25]. Two more attributes of the polymers studied here are their high Tg (above 218 ◦ C and up to 420 ◦ C) and high thermal decomposition temperatures (above 510 ◦ C). This high thermal oxidative stability makes them good candidates for applications in the chemical industries, besides offering a wide range of temperatures to process them. Their Tg and FFV (above 0.142) rank in the following order: PEK-4 < PEK-3 < PEK-2 < CP-1. Such trend for the Tg can be understood in terms of chain rigidity. The Tg of CP-1 is 420 ◦ C, a lot higher than PEKs’. Its repeating unit contains a cardo group connected by a byphenylene moiety. It is an extremely rigid structure with severe rotational hindrance due to the phenylene groups connected to a quaternary carbon. Analyzing PEKs’ chemical structures it can be seen that the rigidity of CP-1 is effectively lowered by increasing the length of the repeating unit with aryl ether ketones. In this case, the chain rigidity of the polymer chains is also related to the behavior of the FFV. CP-1 has the highest FFV. The associated rigidity of CP-1’s repeating unit, its non planar conformation and the high frequency of appearance of the cardo group seriously affect the packing ability of the chains. Including the phenylene, carbonyl and ether connectors in PEK’s repeating unit not only softens the chains but also reduces the frequency of appearance of the cardo group resulting in a better packing of the chains. This effect can also be appreciated in the specific volume that follows the same behavior as FFV and Tg and varies between 0.7857 and 0.8309 cm3 /g. The gas permeability coefficients to the more permeable gases, as well as the corresponding selectivity for the gas pairs H2 /N2 , H2 /CH4 , O2 /N2 and CO2 /CH4 , of the membranes synthesized here are summarized in Table 2. The poly(aryl ether ketone) membranes result with relatively high permeability coefficients for all the gases measured since they are practically in the same order of magnitude as the coefficients reported for other families such as polysulfones (see Fig. 5), polycarbonates
and some polyarylates [24]. Similar chemical structures to the ones studied in this paper have been published before [14] showing permeability values in the same order of magnitude as those reported in Table 2. The gas permeability coefficients of the PEKs show the following order: PEK-4 < PEK-3 < PEK-2 < CP-1, the same trend followed by Tg , V and FFV. These results can be understood by thinking of the highly restricted segmental motion of an opened polymer matrix due to the presence of the cardo group. CP-1 has very rigid polymer chains with the highest free volume; consequently it is the most permeable material (11 Barrer to O2 ). In contrast, PEK-4, with less rigid polymer chains and the lowest free volume, has a permeability coefficient smaller than CP-1 by two orders of magnitude (0.27 Barrer to O2 ). One can conclude that in a structure consisting mainly of cardo groups, like that of CP-1, the inclusion of the phenylene, carbonyl and ether groups, in similar lengths to the ones studied, softens the chain and improves chain packing diminishing the permeability of the materials. Concerning PEKs’ chemical structure, the only difference between PEK-3 and PEK-4 is the position of the two carbonyl groups, the first one being in para and the second one in meta. The cardo polymer based on the para-position is more permeable than the one based on the meta-position; similar behaviors have been reported with polyarylates and polyamides. The difference between PEK-2 and PEK-3 is the length of the branch (aryl ether ketone). PEK-3 is larger in one phenylene-carbonyl group than PEK-2 and the former is less permeable than the latter. The gas permeability coefficients increase with smaller (aryl ether ketone) branches and this effect can be explained by the associated increment in FFV as it was mentioned. Besides the simultaneous increase in FFV and permeability characteristic of amorphous polymers, the permeability coefficients correlate very well with the FFV according to the well known empirical equation [25]: P = Ae−B/FFV
(3)
where A and B are characteristic parameters for each gas that may also depend to some degree on the polymer family. Fig. 4 shows these relationships and reports trend lines equations for each gas. Ideal selectivity factors (see Table 2) rank in the following order: CP-1 < PEK-2 < PEK-3 < PEK-4 for the four pairs of gases studied. In general, the poly(aryl ether ketone)s membranes are more selective than PSF. For example, PEK-4, the most selective material, has a factor of 171 for H2 /N2, 189 for H2 /CH4 , 8.7 for O2 /N2 and 33 for CO2 /CH4 . Fig. 5 shows O2 /N2 selectivity versus O2 permeability of the polymers analyzed here. The experimental “upper bound” limit observed by Robeson [26] as well as the properties of PSF – whose P(O2 ) and α(O2 /N2 ) values defined the commercially attractive region some time ago – are also included for comparison purposes. It can be appreciated that PEKs have P(O2 ) values a little smaller than PSF but they are more selective; whereas CP-1 has a P(O2 ) very similar to that of PSF but its ideal selectivity is significantly larger. Analyzing the permeability–selectivity
C. Camacho-Zu˜niga et al. / Journal of Membrane Science 283 (2006) 393–398
397
Table 2 Gas permeability coefficients and ideal selectivity factors, measured at 35 ◦ C and 2 atm, for the cardo poly(aryl ether ketone)s membranes studied in this work Polymer
P(H2 )a
P(O2 )a
P(CO2 )a
α(H2 /N2 )
α(H2 /CH4 )
α(O2 /N2 )
α(CO2 /CH4 )
CP-1 PEK-2 PEK-3 PEK-4
74 9.2 7.5 5.3
11 0.68 0.51 0.27
72 3.10 2.14 0.91
31 94 103 171
26 92 106 189
4.6 6.9 7.0 8.7
25 31 31 33
a
Permeability in Barrers (1 Barrer = 10−10 cm3 (STP) cm/cm2 s cmHg), with an associated uncertainty of 4%.
Fig. 4. Gas permeability-FFV relationship shown by the cardo polymers reported in this work.
relationship for the four pairs of gases studied, it is important to notice that any increment either in permeability or selectivity coefficients lowers the other gas transport parameter following the so-called “trade-off” rule. Former studies of gas transport–structure relationships in glassy polymers indicate that high permeability is mainly caused by more fractional free volume, while significant increase in gas selectivity may be due to restricted segmental motion [14,16,18]. Even though the presence of more cardo groups inside the main chain, that results from a decrease in the length of the repeating unit, accomplishes
Fig. 5. O2 /N2 selectivity–permeability combination of properties determined for the cardo polymers reported in Figs. 1 and 2. Robeson’s Upper Bound [26] limit is included as a reference line.
both of these requirements, this series of materials does not show a simultaneous increase in permeability and selectivity. Other literature, however, specifies that such restricted segmental motion has to be achieved through intermolecular forces while trying to keep intersegmental packing almost unaffected [7,10]. This last statement could explain the permeability–selectivity behavior observed here since the hindrance to the segmental motion of these macromolecules is imposed mainly by steric impediments. Nevertheless, a deeper work most be done to determine if the
Table 3 Gas diffusivity and solubility coefficients, and their contribution to the selectivity factor at 35 ◦ C and 2 atm, calculated by Eqs. (1) and (2), for the cardo poly(aryl ether ketone)s membranes Polymer
D(H2 )a
D(O2 )a
D(CO2 )a
D(H2 )/D(N2 )
D(H2 )/D(CH4 )
D(O2 )/D(N2 )
D(CO2 )/D(CH4 )
CP-1 PEK-2 PEK-3 PEK-4
244 74 65 57
7.02 1.16 1.02 0.75
2.98 0.31 0.29 0.18
121 222 212 336
442 1433 1392 1885
3.48 3.49 3.31 4.42
5.42 6.02 6.09 5.91
Polymer
S(H2 )b
S(O2 )b
S(CO2 )b
S(H2 )/S(N2 )
S(H2 )/S(CH4 )
S(O2 )/S(N2 )
S(CO2 )/S(CH4 )
CP-1 PEK-2 PEK-3 PEK-4
0.231 0.095 0.090 0.070
1.19 0.44 0.38 0.28
18.3 7.5 5.7 3.8
0.259 0.422 0.529 0.500
0.058 0.064 0.075 0.100
1.33 1.97 2.24 1.98
4.60 5.08 4.72 5.45
a b
D: diffusivity (10−8 cm2 /s) with an experimental uncertainty of 1.5%. S: solubility (cm3 (STP)/cm3 atm) with an experimental uncertainty of 5%.
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magnitude of the intermolecular forces inside these materials is comparable. Table 3 shows the apparent diffusivity and solubility coefficients of the studied gases and their corresponding contribution to the overall diffusivity and solubility selectivity. The CP-1 membranes show higher diffusivity and solubility coefficients than PEKs and this behavior is expected since both of the coefficients show the same trend as the FFV. On concerning the diffusivity and solubility selectivity, it is clearly seen that for H2 /N2 and H2 /CH4 the solubility selectivity is small compared to that of diffusivity selectivity; meanwhile, for O2 /N2 and CO2 /CH4 both contributions are comparable. This observation could be explained by a better interaction of O2 and CO2 with the polymer matrix due to the presence of the ether and carbonyl linkages. 4. Conclusions The thermal, volumetric and gas transport properties of new cardo poly(aryl ether ketone)s have been studied. They have high glass transition temperatures (up to 420 ◦ C), high thermostability (above 500 ◦ C) and enough mechanical toughness to allow their permeation properties to be measured. Their different lengths in the chemical structures yield evident effects on the polymeric segmental mobility and chain packing so as to change the gas transport properties of their membranes. A decrease in the length of the aryl ether ketone connectors of the cardo groups, to increase the frequency of the appearance of the cardo group, enhance chain rigidity, fractional free volume and permeability, but the expected increase in selectivity is not observed. Therefore, to increase fractional free volume and to restrict segmental motions is not enough to avoid the “trade-off” rule as it was established in previous literature. Finally, a very good correlation between the gas permeability coefficients and FFV was observed. Acknowledgements Thanks are due to M.A. Canseco and G. Cedilllo for their assistance with thermal and spectral analysis. Financial support from DGAPA (project PAPIIT IN101405-3) and CONACYT (Project No. 42477) are appreciated. References [1] M. Freemantle, Membranes for gas separation, Chem. Eng. News 83 (2005) 49. [2] W.J. Koros, Evolving beyond the thermal age of separation processes: membranes man lead the way, AIChE J. 50 (2004) 2326. [3] V.V. Korshak, S.V. Vinogradova, Ya.S. Vygodskii, Cardo polymers, J. Macromol. Sci. Rev. Macromol. Chem. C1 (1974) 45. [4] S.V. Vinogradova, V.A. Vasnev, Y.S. Vygodskii, Cardo polyheteroarylenes, Synth. Prop. Peculiarities Uspekhi Khimii 65 (1996) 266. [5] A.Y. Houde, S.S. Kulkarni, M.G. Kulkarni, Sorption, transport and history effects in phenolphthalein-based polysulfone, J. Membr. Sci. 95 (1994) 147.
[6] Y. Hiarayama, S. Kazama, E. Fujisawa, M. Nakabayashi, N. Matsumiya, K. Takagi, K. Okabe, H. Mano, K. Haraya, C. Kamizawa, Novel membranes for carbon dioxide separation, Energy Convers. Manage. 36 (1995) 435. [7] Z. Wang, T. Chen, J. Xu, Novel poly(aryl ether ketone)s containing various pendant groups. II. Gas-transport properties, J. Appl. Polym. Sci. 64 (1997) 1725. [8] S. Hsiao, C. Yang, S. Wang, M. Chuang, Synthesis and properties of aromatic polyamides containing the cyclohexane structure, J. Polym. Sci. Part A: Polym. Chem. 37 (1997) 3575. [9] S. Hsiao, C. Yang, Chuang, S. Lin, Synthesis and characterization of novel aromatic polyamids with polyalicyclic cardo groups, J. Polym. Sci. Part A: Polym. Chem. 37 (1999) 4510. [10] Z. Wang, T. Chen, J. Xu, Gas transport properties of novel cardo poly(aryl ether ketones)s with pendant alkyl groups, Macromolecules 33 (2000) 5672. [11] A.P. Korikov, Y.S. Vygodskii, Y.P. Yampol’skii, Transport properties of cardo poly(imides): Homo- and copolymers, Polym. Sci. Ser. A 43 (2001) 638. [12] J.F. Blanco, Q.T. Nguyena, P. Schaetzel, Novel hydrophilic membrane materials: sulfonated polyethersulfone cardo, J. Membr. Sci. 186 (2001) 267. [13] Y.L. Liu, S.H. Tsai, Synthesis and properties of new organosoluble aromatic polyamides with cyclic bulky groups containing phosphorus, Polymer 43 (2002) 5757. [14] Z. Wang, T. Chen, J. Xu, Gas transport properties of a series of cardo polyarylethers, J. Appl. Polym. Sci. 83 (2002) 791. [15] D.R. Godhani, M.R. Sanariya, Y.V. Patel, P.H. Parsania, Synthesis and characterization of cardo polysulfonates of phenolphthalein, Eur. Polym. J. 38 (2002) 2171. [16] C. Garcia, P. Tiemblo, A.E. Lozano, J. de Abajo, J.G. de la Campa, Gas separation properties of new poly(aryl ether ketone)s with pendant groups, J. Membr. Sci. 205 (2002) 73. [17] G. Fan, L. Chu, G. Liu, Gas separation properties of polyether-ketones with cardo group based on phenolphthalein derivatives, Polym. Mater. Sci. Eng. 86 (2002) 252. [18] S. Kazama, T. Teramoto, K. Haraya, Carbon dioxide and nitrogen transport properties of bis(phenyl)fluorene-based cardo polymer membranes, J. Membr. Sci. 207 (2002) 91. [19] S. Kazama, M. Sakashita, Gas separation properties and morphology of asymmetric hollow fiber membranes made from cardo polyamide, J. Membr. Sci. 243 (2004) 59. [20] S. Kazama, T. Teramoto, K. Haraya, Oxygen separation properties of bis(phenyl)fluorene-based cardo polymer membranes, High Perform. Polym. 17 (2005) 3. [21] M.G. Zolotukhin, D.R. Rueda, F.J. Balta Calleja, M.E. Cagiao, M. Bruix, E.A. Sedova, N.G. Gileva, Aromatic polymers obtained by precipitation polycondensation. 4. Synthesis of poly(ether ketone ketone)s, Polymer 38 (1997) 1471. [22] S.N. Salazkin, V.V. Shaposhnikova, K.I. Donetskii, G.V. Gorshkov, P.V. Petrovskii, L.I. Komarova, M.M. Genina, A.S. Tkachenko, Synthesis of phthalide-containing dihalogeno derivatives and polyarylene ether ketones based on them, Russ. Chem. Bull. 49 (2000) 1093. [23] M.G. Zolotukhin, V.A. Kovardakov, S.N. Salazkin, S.R. Rafikov, Some regularities of poly(diphenylenephthalide) synthesis by p-(3-chloro-3phthalidyl)biphenyl homopolycondensation, Vysokomol. Soedin. A26 (1984) 1212. [24] W.J. Koros, M.R. Coleman, D.R.B. Walker, Controlled permeability polymer membranes, Annu. Rev. Mater. Sci. 22 (1992) 47. [25] F.A. Ruiz-Trevi˜no, D.R. Paul, Gas permselectivity properties of high free volume polymers modified by a low molecular weight additive, J. Appl. Polym. Sci. 68 (1998) 403. [26] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165.
Gas transport properties of new aromatic cardo poly(aryl ether ketone)s C. Camacho-Zu˜niga a , F.A. Ruiz-Trevi˜no a,∗ , M.G. Zolotukhin b , L.F. del Castillo b , J. Guzman b , J. Chavez b , G. Torres b , N.G. Gileva c , E.A. Sedova c a
Universidad Iberoamericana, Departamento de Ingenier´ıas, Prol. Paseo de la Reforma No. 880, M´exico, D.F. 01210, Mexico b Instituto de Investigaciones en Materiales, Universidad Nacional Aut´ onoma de M´exico, Apartado Postal 70-360, CU, Coyoac´an, 04510 M´exico, D.F., M´exico c Institute of Organic Chemistry of Russian Academy of Sciences, UFA, Prospekt Oktuabrya, 71, Russia Received 6 March 2006; received in revised form 30 June 2006; accepted 8 July 2006 Available online 14 July 2006
Abstract New cardo poly(aryl ether ketone)s containing side phthalide groups and aryl ether ketones in different lengths have been synthesized and characterized in terms of their thermal, volumetric and gas transport properties to H2 , O2 , N2 , CH4 and CO2 . The polymers show high glass transition temperature (218–420 ◦ C), good solubility in chlorinated solvents and strong acids as well as excellent thermal stability (decomposition temperatures above 510 ◦ C). The most permeable membrane studied shows permeability coefficients of 11 to O2 and 72 to CO2 , with ideal selectivity factors of 4.6 for the pair O2 /N2 and 25 for CO2 /CH4 . The results, interpreted in terms of chain rigidity and chain packing ability, show that decreasing the length of the connector moieties between the cardo groups increases the fractional free volume, the glass transition temperature and the gas permeability coefficients. © 2006 Elsevier B.V. All rights reserved. Keywords: Cardo polymers; Transport properties; Gas separation; Membranes; Poly(aryl ether ketone)s
1. Introduction Gas separation membrane unit operations are recognized as clean technology, being the petrochemical and chemical industry one of their most promising applications. In such cases, they acquire greater interest especially when gas and oil prices increase as a consequence of the natural disasters caused by the severe global climate changes [1]. Since it can be a non-thermal process, it can reduce the amount of fuel burned around the world. However, to see its generalized application in the wild environments (aggressive chemical surroundings and high pressure and temperature) typically found in these industries, better polymer membranes should be achieved [2]. In this address, polymers that may possess high glass transition temperature, excellent mechanical toughness and thermo oxidative stability may be considered good candidates if their permeability–selectivity relationship is better or similar to the one shown by commercial membranes.
∗
Corresponding author. Tel.: +52 55 5950 4000; fax: +52 55 5950 4225. E-mail address: [email protected] (F.A. Ruiz-Trevi˜no).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.07.013
The term “cardo polymers”, introduced by the beginning of 1960 [3,4], refers to polymers containing bulky side groups. It has been shown for different polymer families that the presence of cardo groups inside the polymer structure disrupts crystallinity (if any), increases their glass transition temperature and improves their solubility. Generally, cardo groups in aromatic polymers twist the phenylene moieties of the main chain out of a planar conformation, thereby affecting the polymer free volume as well. All of these characteristics make aromatic cardo polymers suitable candidates for gas separation applications [5–20]. In this work, new cardo poly(aryl ether ketone)s (PEKs) polymers containing side phthalide groups were synthesized and characterized for their thermal, volumetric and gas transport properties. For comparison, another cardo polymer containing a biphenylene connector in its repeating unit was also studied (See Figs. 1 and 2). The systematic changes in the length of the chemical structure were expected to result in variations of their physical properties, e.g., packing density and segmental motion of the polymer chain. This work investigates these effects on the gas permeation behavior of their membranes.
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Fig. 1. Reaction scheme, chemical structure and nomenclature for cardo-PEKs synthesized in this work.
2. Experimental 2.1. Materials Dichloroethane and anhydrous aluminum chloride were obtained from Aldrich and purified by distillation and sublimation, respectively. Aromatic hydrocarbons reported on Fig. 1: 4,4 -diphenoxybenzophenone (for PEK-2), 1,4bis (4-phenoxybenzoyl)benzene (for PEK-3) and 1,3-bis(4phenoxybenzoyl)benzene (for PEK-4) were prepared according to the published method [21]. Dichloride of 3,3bis(4,4 -carboxyphenyl)phthalide was synthesized as previously reported [22]. 2.2. Polymer synthesis Three different cardo poly(aryl ether ketone)s (PEK2-4) were prepared via electrophilic polycondensation according to the scheme outlined in Fig. 1. Every synthesis was carried out by the reaction of aromatic electrophilic substitution of Friedel-Crafts of the acid dichloride with the corresponding aromatic hydrocarbon in presence of aluminum chloride. Other conditions were similar to those described elsewhere [21].
Fig. 2. Cardo polymer repeating unit based on phthalide groups and biphenylene.
A standard procedure for the preparation of cardo poly(aryl ether ketone)s is described below. For PEK-2, a 300 ml threeneck flask equipped with magnetic stirrer, nitrogen inlet, thermometer, solids addition funnel and gas outlet, was purged with dry nitrogen and charged with 3.66 g (0.01 mol) of 4,4 -diphenoxybenzophenone, 4.11 g (0.01 mol) of dichloride of 3,3-bis(4,4 -carboxyphenyl)phthalide and 125 ml of 1,2dichloroethane. The addition funnel was charged with 5.05 g (0.038 mol) of high purity A1C13 . The transparent colorless solution was cooled to −15 ◦ C, aluminum chloride was added and then the reaction mixture was stirred for 30 min. Thereafter, the temperature was raised to 20 ◦ C over 2 h and the reaction was allowed to continue at this temperature for 20 h. The suspension obtained was filtered. The precipitate was washed with methanol, extracted with boiling methanol for 20 h and allowed to dry in air. The air-dried product was heated at 100 ◦ C overnight under vacuum to give 7.04 g of the polymer (95% yield). Similar preparations were carried out to obtain the polymers PEK-3 and PEK-4. Cardo polymer CP-1 was prepared according to the previously reported method [23]. 2.3. Polymer characterization The inherent viscosities of 0.2% polymer solutions in symmtetrachloroethtane were measured at 25 ◦ C using an Ubbelohde viscometer. The 1 H and 13 C NMR spectra were recorded using a Bruker Avance 400 spectrometer, operating at 400.13 and 100 MHz for 1 H and 13 C, respectively. Thermogravimetric analyses (TGA) were carried out in air and under nitrogen at a heating rate of 10 ◦ C min−1 on a TGA 2950 Thermogravimetric Analyzer, TA Instruments. The glass transition temperature for every polymer was evaluated by differential scanning calorimetry (DSC) measurements at 20 ◦ C min−1 on a DSC 2910 TA Instruments. Finally, the polymer densities were determined from a dry film sample in a density gradient column containing aqueous zinc chloride solution at 30 ◦ C.
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2.4. Membrane preparation
S(i) =
Dense polymer films were prepared by casting CHCl3 solutions containing 10% (w/v) polymer onto glass plates and dried for 24 h at 40 ◦ C in an oven. They were removed from the glass plates and dried for another 24 h at 100 ◦ C under vacuum in order to evaporate residual solvent. All membranes were treated according to the same procedure and their thickness, measured with a Mitutoyo caliper, varied from 20 to 30 m. 2.5. Gas permeability measurements A standard permeation cell was used to determine the pure gas permeability coefficients at 35 ◦ C and 2 atm. Before measurements were performed, vacuum was pull overnight in order to remove any residual gas dissolved in the membrane. During the permeation experiment, the increase in pressure in the downstream side of the membrane was followed by an absolute pressure transducer, MKS Baratron type 627B, and kept below 1 mmHg while the upstream pressure was maintained at 2 atm. Ultrahigh purity gases H2 , O2 , N2 , CH4 and CO2 were used in that order to avoid the plasticization effect of CO2 , the most condensable gas. Gas permeability coefficients for each gas, P(i), were determined from the slope of the downstream pressure versus time plot once steady state had been achieved. Permeability coefficients were evaluated for at least two different membranes of the same polymer and their difference was smaller than 3%. This fact confirms reproducibility of the reported values.The apparent diffusion coefficients, D(i), and apparent solubility coefficients, S(i), for each gas were calculated from the time lag θ(i) using the equations: D(i) =
L2 6θ(i)
(1)
P(i) D(i)
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(2)
In Eq. (1) L is the film thickness. The ideal separation factor, α(i/j) = P(i)/P(j), was calculated from the ratio of the pure gas permeability coefficients. 3. Results and discussion The cardo polymers synthesized are not soluble in alcohols, aromatic hydrocarbons or ethers but fully soluble in chlorinated solvents (such as chloroform and tetrachloroethane) and strong acids. Their good solubility would allow the formation of an ultra thin active layer as asymmetric or composite membrane. Such arrangements can yield a high gas flux and therefore these materials fulfill the requirement of processability for their application in gas separation membrane technology. It is well known that the type of reactions used in these syntheses is not very selective and, frequently, leads to isomer mixtures. For that reason, it is necessary to analyze the obtained chemical structures. Their solubility allowed performing reliable 1 H and 13 C NMR spectral studies to define their structures. As an example, the 1 H NMR spectrum of polymer PEK-2 is given in Fig. 3. It is well resolved: the aromatic resonances anticipated for the diphenoxybenzophenone (doublets at 7.15, 7.50, 7.78, and 7.00 ppm) and biphenyl fragments (two doublets near 7.85 ppm) are all evident and no other resonances can be observed. Similarly, its 13 C NMR spectrum, not included for simplicity purposes, showed a highly resolved pattern with no evidence of ortho- or meta-substitution. Similar to that of PEK-2, spectra of PEK-3 and PEK-4 are also well resolved and confirm the corresponding substitution in the aromatic fragments of the main chain. Table 1 summarizes the cardo polymers properties in terms of their inherent viscosity of their solutions, the glass transition temperature, the thermal degradation temperature at a 10 wt.%
Fig. 3. 1 H NMR spectrum determined for polymer PEK-2 (solution in CDCl3 ).
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Table 1 Inherent viscosity, glass transition temperature, thermal degradation temperature, specific and fractional free volume measured for the cardo polymers synthesized in this work Polymer
ηinh , dl/g (TCE)
Tg (◦ C)
Tdecom (◦ C, onset)
V (cm3 /g)
FFVa
CP-1 PEK-2 PEK-3 PEK-4
0.86 1.24 1.16 1.35
420 238 227 218
510 517 520 517
0.8309 0.7927 0.7921 0.7857
0.182 0.152 0.149 0.142
◦ 0 FFV = V (30)−V V (30) , where V(30) is the specific volume at 30 C, V0 = 1.3Vw and Vw is the van der Waals volume of the repeating unit estimated by the method outlined by Bondi. a
loss in air and the specific and fractional free volume. Their inherent viscosities with an average of 1.15 dl/g indicate, at least from a qualitative point of view, that the polymers have a high molecular weight. This fact allowed the formation of flexible membranes with enough mechanical strength for gas permeation measurements. Moreover, their flexibility suggests an amorphous nature, which can also be inferred from the optical transparency of their films and the typical metallic sound when they are waved in air [25]. Two more attributes of the polymers studied here are their high Tg (above 218 ◦ C and up to 420 ◦ C) and high thermal decomposition temperatures (above 510 ◦ C). This high thermal oxidative stability makes them good candidates for applications in the chemical industries, besides offering a wide range of temperatures to process them. Their Tg and FFV (above 0.142) rank in the following order: PEK-4 < PEK-3 < PEK-2 < CP-1. Such trend for the Tg can be understood in terms of chain rigidity. The Tg of CP-1 is 420 ◦ C, a lot higher than PEKs’. Its repeating unit contains a cardo group connected by a byphenylene moiety. It is an extremely rigid structure with severe rotational hindrance due to the phenylene groups connected to a quaternary carbon. Analyzing PEKs’ chemical structures it can be seen that the rigidity of CP-1 is effectively lowered by increasing the length of the repeating unit with aryl ether ketones. In this case, the chain rigidity of the polymer chains is also related to the behavior of the FFV. CP-1 has the highest FFV. The associated rigidity of CP-1’s repeating unit, its non planar conformation and the high frequency of appearance of the cardo group seriously affect the packing ability of the chains. Including the phenylene, carbonyl and ether connectors in PEK’s repeating unit not only softens the chains but also reduces the frequency of appearance of the cardo group resulting in a better packing of the chains. This effect can also be appreciated in the specific volume that follows the same behavior as FFV and Tg and varies between 0.7857 and 0.8309 cm3 /g. The gas permeability coefficients to the more permeable gases, as well as the corresponding selectivity for the gas pairs H2 /N2 , H2 /CH4 , O2 /N2 and CO2 /CH4 , of the membranes synthesized here are summarized in Table 2. The poly(aryl ether ketone) membranes result with relatively high permeability coefficients for all the gases measured since they are practically in the same order of magnitude as the coefficients reported for other families such as polysulfones (see Fig. 5), polycarbonates
and some polyarylates [24]. Similar chemical structures to the ones studied in this paper have been published before [14] showing permeability values in the same order of magnitude as those reported in Table 2. The gas permeability coefficients of the PEKs show the following order: PEK-4 < PEK-3 < PEK-2 < CP-1, the same trend followed by Tg , V and FFV. These results can be understood by thinking of the highly restricted segmental motion of an opened polymer matrix due to the presence of the cardo group. CP-1 has very rigid polymer chains with the highest free volume; consequently it is the most permeable material (11 Barrer to O2 ). In contrast, PEK-4, with less rigid polymer chains and the lowest free volume, has a permeability coefficient smaller than CP-1 by two orders of magnitude (0.27 Barrer to O2 ). One can conclude that in a structure consisting mainly of cardo groups, like that of CP-1, the inclusion of the phenylene, carbonyl and ether groups, in similar lengths to the ones studied, softens the chain and improves chain packing diminishing the permeability of the materials. Concerning PEKs’ chemical structure, the only difference between PEK-3 and PEK-4 is the position of the two carbonyl groups, the first one being in para and the second one in meta. The cardo polymer based on the para-position is more permeable than the one based on the meta-position; similar behaviors have been reported with polyarylates and polyamides. The difference between PEK-2 and PEK-3 is the length of the branch (aryl ether ketone). PEK-3 is larger in one phenylene-carbonyl group than PEK-2 and the former is less permeable than the latter. The gas permeability coefficients increase with smaller (aryl ether ketone) branches and this effect can be explained by the associated increment in FFV as it was mentioned. Besides the simultaneous increase in FFV and permeability characteristic of amorphous polymers, the permeability coefficients correlate very well with the FFV according to the well known empirical equation [25]: P = Ae−B/FFV
(3)
where A and B are characteristic parameters for each gas that may also depend to some degree on the polymer family. Fig. 4 shows these relationships and reports trend lines equations for each gas. Ideal selectivity factors (see Table 2) rank in the following order: CP-1 < PEK-2 < PEK-3 < PEK-4 for the four pairs of gases studied. In general, the poly(aryl ether ketone)s membranes are more selective than PSF. For example, PEK-4, the most selective material, has a factor of 171 for H2 /N2, 189 for H2 /CH4 , 8.7 for O2 /N2 and 33 for CO2 /CH4 . Fig. 5 shows O2 /N2 selectivity versus O2 permeability of the polymers analyzed here. The experimental “upper bound” limit observed by Robeson [26] as well as the properties of PSF – whose P(O2 ) and α(O2 /N2 ) values defined the commercially attractive region some time ago – are also included for comparison purposes. It can be appreciated that PEKs have P(O2 ) values a little smaller than PSF but they are more selective; whereas CP-1 has a P(O2 ) very similar to that of PSF but its ideal selectivity is significantly larger. Analyzing the permeability–selectivity
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397
Table 2 Gas permeability coefficients and ideal selectivity factors, measured at 35 ◦ C and 2 atm, for the cardo poly(aryl ether ketone)s membranes studied in this work Polymer
P(H2 )a
P(O2 )a
P(CO2 )a
α(H2 /N2 )
α(H2 /CH4 )
α(O2 /N2 )
α(CO2 /CH4 )
CP-1 PEK-2 PEK-3 PEK-4
74 9.2 7.5 5.3
11 0.68 0.51 0.27
72 3.10 2.14 0.91
31 94 103 171
26 92 106 189
4.6 6.9 7.0 8.7
25 31 31 33
a
Permeability in Barrers (1 Barrer = 10−10 cm3 (STP) cm/cm2 s cmHg), with an associated uncertainty of 4%.
Fig. 4. Gas permeability-FFV relationship shown by the cardo polymers reported in this work.
relationship for the four pairs of gases studied, it is important to notice that any increment either in permeability or selectivity coefficients lowers the other gas transport parameter following the so-called “trade-off” rule. Former studies of gas transport–structure relationships in glassy polymers indicate that high permeability is mainly caused by more fractional free volume, while significant increase in gas selectivity may be due to restricted segmental motion [14,16,18]. Even though the presence of more cardo groups inside the main chain, that results from a decrease in the length of the repeating unit, accomplishes
Fig. 5. O2 /N2 selectivity–permeability combination of properties determined for the cardo polymers reported in Figs. 1 and 2. Robeson’s Upper Bound [26] limit is included as a reference line.
both of these requirements, this series of materials does not show a simultaneous increase in permeability and selectivity. Other literature, however, specifies that such restricted segmental motion has to be achieved through intermolecular forces while trying to keep intersegmental packing almost unaffected [7,10]. This last statement could explain the permeability–selectivity behavior observed here since the hindrance to the segmental motion of these macromolecules is imposed mainly by steric impediments. Nevertheless, a deeper work most be done to determine if the
Table 3 Gas diffusivity and solubility coefficients, and their contribution to the selectivity factor at 35 ◦ C and 2 atm, calculated by Eqs. (1) and (2), for the cardo poly(aryl ether ketone)s membranes Polymer
D(H2 )a
D(O2 )a
D(CO2 )a
D(H2 )/D(N2 )
D(H2 )/D(CH4 )
D(O2 )/D(N2 )
D(CO2 )/D(CH4 )
CP-1 PEK-2 PEK-3 PEK-4
244 74 65 57
7.02 1.16 1.02 0.75
2.98 0.31 0.29 0.18
121 222 212 336
442 1433 1392 1885
3.48 3.49 3.31 4.42
5.42 6.02 6.09 5.91
Polymer
S(H2 )b
S(O2 )b
S(CO2 )b
S(H2 )/S(N2 )
S(H2 )/S(CH4 )
S(O2 )/S(N2 )
S(CO2 )/S(CH4 )
CP-1 PEK-2 PEK-3 PEK-4
0.231 0.095 0.090 0.070
1.19 0.44 0.38 0.28
18.3 7.5 5.7 3.8
0.259 0.422 0.529 0.500
0.058 0.064 0.075 0.100
1.33 1.97 2.24 1.98
4.60 5.08 4.72 5.45
a b
D: diffusivity (10−8 cm2 /s) with an experimental uncertainty of 1.5%. S: solubility (cm3 (STP)/cm3 atm) with an experimental uncertainty of 5%.
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magnitude of the intermolecular forces inside these materials is comparable. Table 3 shows the apparent diffusivity and solubility coefficients of the studied gases and their corresponding contribution to the overall diffusivity and solubility selectivity. The CP-1 membranes show higher diffusivity and solubility coefficients than PEKs and this behavior is expected since both of the coefficients show the same trend as the FFV. On concerning the diffusivity and solubility selectivity, it is clearly seen that for H2 /N2 and H2 /CH4 the solubility selectivity is small compared to that of diffusivity selectivity; meanwhile, for O2 /N2 and CO2 /CH4 both contributions are comparable. This observation could be explained by a better interaction of O2 and CO2 with the polymer matrix due to the presence of the ether and carbonyl linkages. 4. Conclusions The thermal, volumetric and gas transport properties of new cardo poly(aryl ether ketone)s have been studied. They have high glass transition temperatures (up to 420 ◦ C), high thermostability (above 500 ◦ C) and enough mechanical toughness to allow their permeation properties to be measured. Their different lengths in the chemical structures yield evident effects on the polymeric segmental mobility and chain packing so as to change the gas transport properties of their membranes. A decrease in the length of the aryl ether ketone connectors of the cardo groups, to increase the frequency of the appearance of the cardo group, enhance chain rigidity, fractional free volume and permeability, but the expected increase in selectivity is not observed. Therefore, to increase fractional free volume and to restrict segmental motions is not enough to avoid the “trade-off” rule as it was established in previous literature. Finally, a very good correlation between the gas permeability coefficients and FFV was observed. Acknowledgements Thanks are due to M.A. Canseco and G. Cedilllo for their assistance with thermal and spectral analysis. Financial support from DGAPA (project PAPIIT IN101405-3) and CONACYT (Project No. 42477) are appreciated. References [1] M. Freemantle, Membranes for gas separation, Chem. Eng. News 83 (2005) 49. [2] W.J. Koros, Evolving beyond the thermal age of separation processes: membranes man lead the way, AIChE J. 50 (2004) 2326. [3] V.V. Korshak, S.V. Vinogradova, Ya.S. Vygodskii, Cardo polymers, J. Macromol. Sci. Rev. Macromol. Chem. C1 (1974) 45. [4] S.V. Vinogradova, V.A. Vasnev, Y.S. Vygodskii, Cardo polyheteroarylenes, Synth. Prop. Peculiarities Uspekhi Khimii 65 (1996) 266. [5] A.Y. Houde, S.S. Kulkarni, M.G. Kulkarni, Sorption, transport and history effects in phenolphthalein-based polysulfone, J. Membr. Sci. 95 (1994) 147.
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