A study of gas transport properties of semifluorinated poly (ether imide) membranes containing cardo diphenylfluorene moieties

Journal of Membrane Science 362 (2010) 58–67

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A study of gas transport properties of semifluorinated poly (ether imide) membranes containing cardo diphenylfluorene moieties Barnali Dasgupta, Susanta Banerjee ∗ Materials Science Centre, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal 721302, India

a r t i c l e

i n f o

Article history: Received 11 May 2010 Received in revised form 5 June 2010 Accepted 10 June 2010 Available online 17 June 2010 Keywords: Diphenylfluorene based semifluorinated poly(ether imide)s Permeability Selectivity Activation energy

a b s t r a c t This manuscript presents the gas transport properties of a series of semifluorinated poly(ether imide) (PEI) membranes containing diphenylfluorene moiety in the main chain. The effect of diphenylfluorene moiety towards four gases (e.g., CO2 , O2 , N2 and CH4 ) at three different temperatures (35, 45 and 55 ◦ C) under an applied upstream pressure of 3.5 bar has been examined. The membranes were prepared by thermal imidization of the poly(amic acid)s, reaction products of a diphenylfluorene containing semifluorinated diamine monomer, 4,4 -bis-((2 -trifluoromethyl-4 -(4 -aminophenyl) phenoxy)-9-fluorenylidene (FBP) with five different commercial dianhydrides namely, PMDA, BTDA, 6FDA, ODPA, and BPADA in DMF. All these polymeric membranes showed higher gas permeability with comparable permselectivity. The polymer FBP-6FDA and FBP-BTDA showed better separation efficiency in comparison to many other FBP containing polymeric membrane of different classes (e.g., polysulfones, polycarbonates, polyacrylates, poly(ether ketone)s, poly(arylene ether)s, poly(arylene ether ketone)s etc.) reported earlier. Moreover highest ideal CO2 permeability of 53.09 Barrer with highest permselectivity (39.62 for CO2 /CH4 ) of the FBP-6FDA membrane containing hexafluoro-isopropylidene group made this polymer interesting for gas separation purpose among the entire diphenylfluorene based polymeric membrane investigated so far. This FBP-6FDA membrane also showed the highest permeability for O2 (13.46 Barrer) with ideal permselectivity of 6.53 whereas FBP-BTDA also showed similar ideal permselectivity of 6.57 for O2 /N2 gas pair but with lower permeability value (6.24 Barrer). The effect of temperature on the gas transport properties of these membranes was also studied. The activation energies for gas permeation and diffusion processes were calculated from the corresponding Arrhenius plots. An attempt has been taken to draw a structure property correlationship between the polymer structures and their gas transport properties. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Membranes offer an attractive alternative to cryogenic or pressure swing adsorption processes for gas separation applications [1–4]. Naturally, strong interest exists in the area of preparation of new membrane with higher separation efficiency. This means that both the permeability coefficient and the selectivity should be as large as possible. Only glassy polymers combine sufficiently high permeability and selectivity needed for consideration as next generation gas separation membrane materials. In general, structural changes which lead to increases in polymer permeability also cause losses in permselectivity. This so-called “trade-off” relationship is well described in the literature [5,6]. Former studies of structure–property relationships on gas transport behavior in glassy polymers indicate important rules for the design of improved membrane materials. The increase in gas per-

∗ Corresponding author. Tel.: +91 3222 283972; fax: +91 3222 255303. E-mail address: [email protected] (S. Banerjee). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.06.015

meability while keeping selectivity is induced by the chemical structure changes which hinder packing of relatively rigid main chain. Furthermore, improved permselectivity without a loss of permeability, results from the chemical structure changes which reduce the rotational mobility around the flexible linkage in the polymer main chain if intersegmental packing is not significantly affected [2,4]. Because the introduction of bulky moieties to the main chain leads to both the reduction of the rotational mobility and the prohibition of intersegmental packing, the bulky moieties are considered effective in improving membrane performance. The phenylfluorene based cardo polymers show high thermal stability, high solubility, low refractive index, high optical transparency and low dielectric constant, because of the relatively high free volume [7–9]. In recent years, polymers containing fluorene moieties have been interesting because of their potential applications as photoelectronic materials [10,11]. These polymers are also interesting for gas separation application. In fact, the improved gas transport properties of several cardo polymers were reported [12–15]. The bulky fluorene moiety in a diphenylfluorene based cardo polymers protrudes vertically from the polymer main chain.

B. Dasgupta, S. Banerjee / Journal of Membrane Science 362 (2010) 58–67

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Table 1 Properties of the poly(ether imide)s. Polymer h

FBP-PMDA FBP-BTDAh FBP-6FDAh FBP-ODPAh FBP-BPADA a b c d e f g h

Tg a (◦ C)

Td5 b (◦ C)

Mw c (Da)

PDId

TSe (MPa)

Modf (GPa)

EBg (%)

309 284 297 254 247

516 511 501 509 489

39,590 59,230 49,460 87,780 69,200

2.3 3.4 2.5 2.9 2.0

102 122 119 112 98

2.0 2.8 2.0 2.4 2.0

7 12 9 9 13

Glass transition temperature determined by DSC, heating rate at 20 ◦ C/min. 5% degradation temperature in air, TGA heating rate at 10 ◦ C/min. Mw , weight average molecular weight with respect to polystyrene standard. PDI, poly dispersity index. Tensile strength. Modulus. Percent elongation at break. Ref. [9].

This chemical structure of four phenyl rings connected to a quaternary carbon leads to severe rotational hindrance of the phenyl groups. Thus, the stiff, bulky cardo moiety must hinder the packing and reduce the rotational mobility of main chain. Therefore, the diphenylfluorene cardo moiety is supposed to have the potential of improving the gas transport properties. Polysulfones [16,17], polycarbonates [17], polyacrylates [18], poly(ether ketone)s [19], poly(arylene ether)s [20] and poly(arylene ether ketone)s [21] derived from 4,4 -(9-fluorenylidene)bisphenol have been synthesized and their gas transport properties have been reported. It has been observed that polyimides are the most attractive candidates for gas separation membranes as they show high gas selectivity for different gas pair (CO2 /CH4 , H2 /CH4 , O2 /N2 etc.) along with a number of outstanding properties such as excellent thermal and thermo-oxidative stability, solvent resistance, fire retardance, mechanical and electrical properties [22,23]. Fluorinated polyimides have attracted much attention as the basic materials for gas separation membranes due to their high gas permeability with relatively high selectivity for a pair of gas [24,25]. Accordingly, it is expected that a combination of cardo diphenylfluorene group and pendent bulky trifluoromethyl groups in a poly(ether imide) backbone can lead to highly permeable and at the same time highly permselective membrane material in the field of gas separation. In continuation of our research on membrane based separation [12,13,26–28], in this work, five structurally different semifluorinated PEI containing diphenylfluorene moieties in polymer chains were fabricated into dense membranes and their gas transport properties for CO2 , O2 , N2 , and CH4 gases were studied at three different temperatures (35, 45 and 55 ◦ C) at a fixed upstream pressure (3.5 bar). The investigation of gas separation properties for such polymers and a comparison between these polymers and those with structurally analogous polymers reported in the literature will be interesting. Temperature dependencies of gas permeation and diffusion processes enable to calculate the activation energies of the permeation and diffusion processes for the four gases through these five PEI membranes.

The dianhydrides were recrystallized from acetic anhydride, dried and heated to 120 ◦ C for overnight under vacuum prior to use. N,Ndimethylformamide (DMF, Merck, India) was dried over anhydrous phosphorus pentoxide (P2 O5 ) and distilled under reduced pressure. All the four gases e.g., CH4 , O2 , N2 and CO2 were of XL grade and purchased from BOC Gases, India. 2.2. Membrane preparation The synthesis and characterization of four structurally different PEIs containing PMDA, BTDA, 6FDA and ODPA as dianhydride have already been reported in our previous publication [9]. A new PEI membrane using BPADA as dianhydride was prepared following the same procedure and has been described here. All the PEIs were prepared from equimolar amounts of the bis(ether amine), 4,4 -bis-((2 -trifluoromethyl4 -(4 -aminophenyl) phenoxy)-9-fluorenylidene (FBP) with five structurally different aromatic dianhydrides like PMDA, BTDA, 6FDA, ODPA and BPADA in DMF by conventional two stage process i.e., solution polymerization to polyamic acids followed by thermal imidization of the polyamic acids. All the membranes were prepared by spreading viscous polyamic acid solution in DMF (10% W/V) onto Petri dishes and kept in an oven initially at 80 ◦ C (overnight) for the slow removal of the solvent. Finally, the thermal imidization of the polyamic acids to PEIs was achieved by sequential heating at 120, 150, 180, 200 and 220 ◦ C, each for half an hour and at 250 ◦ C for 15 min in an oven. All the dense planar membranes were removed from the Petri dishes after soaking in hot water. The complete imidization and removal of solvent was confirmed by FT-IR and thermogravimetric (TGA) analysis. The stretching frequencies near 3200 cm−1 (–O–H stretch) and 3350 cm−1 (>N–H stretch) originally present in poly(amic acid), disappeared completely for all the membranes. Molecular structures of the PEIs are shown in Fig. 1. Detailed spectral characterizations of FBP-BPADA were done and are given in experimental section. Typical physical properties of this PEI membrane studied in this work are summarized in Table 1 along with the reported PEI membranes.

2. Experimental 2.3. General considerations 2.1. Materials The synthesis and characterization of the four PEI containing diphenylfluorene were reported in a previous journal [9]. All the dianhydrides namely, benzene-1,2,4,5-tetracarboxylic dianhydride (PMDA, 97%), benzophenone-3,3 ,4,4 -tetracarboxylic dianhydride (BTDA, 98%), 4,4 -(hexafluoro-isopropylidene)diphthalic anhydride (6FDA, 99%), 4,4 -oxydiphthalic anhydride (ODPA, 97%) and 4,4 -(4,4 -isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) were purchased from Aldrich, USA. The PEI using BPADA as dianhydride, not previously reported was newly synthesized.

The elements, carbon, hydrogen, and nitrogen were analyzed by pyrolysis method using vario EL (Elementar, Germany) elemental analyzer. 1 H NMR was recorded on a Bruker 500 MHz instrument (Switzerland) using CDCl3 as solvent [reference zero ppm with respect to TMS]. IR spectra of the polymer film was recorded with a Bruker IFS 55 spectrophotometer. DSC measurements was made on a NETZSCH DSC 200PC instrument at a heating/cooling rate of 20 ◦ C min−1 in nitrogen. Glass transition temperature (Tg ) was taken at the middle of the step transition in the second heating run. Thermogravimetric measurement was done on NETZSCH TG

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Fig. 1. Molecular structures and denotations of the poly(ether imide)s.

209 F1 thermal analyzer instrument. A heating rate of 10 ◦ C min−1 was used for determination of the decomposition temperature under nitrogen. Viscosity was measured by Ubbelohde viscometer at 34 ◦ C in DMF as solvent. Mechanical properties such as tensile strength and elongation at break of the thin polymer films (30 mm × 10 mm × 0.1 mm) were measured at room temperature on a Hounsfield (UK) H10KS-0547 instrument under strain rate

of 5% min−1 of the sample length. GPC was performed in Waters instrument. Tetrahydrofuran (THF) was used as eluent, and Styragel HR-4 columns were employed. The molecular weight and polydispersity are reported vs. monodisperse polystyrene standard. X-ray diffaction study was conducted by Rigaku, Ultema III X-ray diffractometer with a Cu K␣ ( = 0.154 nm) source, operated at 40 kV and 40 mA.

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Polyimide FBP-BPADA: Anal. Calcd for (C82 H50 O8 F6 N2 )n (1305.27 gmol −1 )n : C, 75.45%; H, 3.86%; N, 2.15%; Found: C, 74.89%; H, 3.75%; N, 2.10%. IR (KBr) (cm−1 ): 3495 (–N< stretching), 3040 (aromatic C–H stretching), 2930, 2855 (aliphatic C–H stretching), 1777 (C O asym. stretching), 1718 (C O sym. stretching), 1599 (aromatic C C ring stretching band), 1482 (C − F absorption), 1366 (asymmetric C–O–C stretching), 1124, 1047 (symmetric C–O–C stretching), 816 (C–N bending). 1 H NMR (CDCl3 ): ı (ppm) 7.97–7.90 (m, 4H, H14, H11); 7.82 (d, J = 7.6 Hz, 2H, H3); 7.71–7.67 (m, 6H, H12, H1); 7.55 (d, J = 8.4 Hz, 4H, H2); 7.48–7.43 (m, 6H, H4, H8, H10); 7.39–7.36 (m, 8H, H13, H9, H16); 7.27 (d, J = 7.2 Hz, 2H, H5); 7.10–6.97 (m, 12H, H7, H15, H6); 1.80 (s, 6H, H17).

61

Table 2 Density and fractional free volume of poly(ether imide)s. Polymer

Vf a (CO2 )

Vf (O2 )

Vf (N2 )

Vf (CH4 )

FBP-PMDA FBP-BTDA FBP-6FDA FBP-ODPA FBP-BPADA

0.1860 0.1569 0.1890 0.1524 0.1490

0.1850 0.1564 0.1896 0.1521 0.1474

0.1840 0.1510 0.1860 0.1472 0.1432

0.1872 0.1553 0.1834 0.1520 0.1486

a Fractional free volume [Vf = (Vs − V0 )/Vs ], where Vs : specific volume; V0 : specific ␥k (Vw )k ]). volume at 0 K (from increments by Park and Paul [31]; [V0 =

3. Results and discussion

2.4. Permeability measurements

3.1. Synthesis and properties of PEI

The gas transport properties of these PEI membranes were measured at 3.5 atm of applied gas pressure and at 35, 45 and 55 ◦ C using the automated Diffusion Permeameter (DP-100-A) manufactured by Porous Materials, Inc., USA. The gas transport properties of CO2 , O2 , N2 and CH4 were investigated for all the PEI membranes. The permeation equipment was placed in a thermostatically controlled housing for isothermal measurement conditions. All the membranes were degassed for at least 12 h at the operating temperature within the permeation cell prior to the experiments. The effective permeation area (A) was 5.069 cm2 . To the upstream side of the film the gas pressure (pi = 3.5 atm) was applied instantaneously and in the downstream side a reservoir of constant volume (119 cm3 ) was connected with a pressure transducer to monitor the total amount of gas which passed through the polymer film. The timelag method was employed for the gas transport measurements. This technique allows the determination of the mean permeability coefficient P from the steady state gas pressure increment (dp/dt)s in the calibrated volume V of the product side of the cell. The permeability coefficients are reported in Barrer and were calculated from Eq. (1), where T0 and p0 are the standard temperature and pressure (T0 = 273.15 K, p0 = 1.013 bar), T is the temperature of measurement, d is the thickness of the film and (dp/dt)s was obtained from the slope of the increments of downstream pressure vs. time plot. The reproducibility of the measurements was checked from three independent measurements and it was better than ±5%. The mean permeability was calculated from the following equation.

The new PEI membrane was prepared using the synthesized semifluorinated fluorene containing diamine monomer and BPADA in DMF under nitrogen atmosphere adapting the standard protocol of polyamic acid synthesis [9]. Properties of the FBP-BPADA are summarized in Table 1. Elemental analysis, IR and NMR spectroscopic data support the formation and structure of the polymer. FBP-BPADA showed lowest Tg (247 ◦ C) in the series. The 5% weight loss temperature (489 ◦ C) is also low in comparison to the other PEIs of this series. This is because of the presence of relatively thermally labile aliphatic methyl groups, coming from the dianhydride moiety. This PEI showed 98 MPa of tensile strength and highest elongation at break (13%) due to its additional flexible ether linkage. All the polymers are amorphous in nature as observed from X-ray diffraction study.

P=

VdT0 Api p0 T

 dp  dt

(1) s

The effective diffusion coefficient D that is an apparent one for the glassy polymers [29] is calculated from the time-lag  according to the following equation (2): D=

d2 6

(2)

The solubility coefficient S of each gas for all the polymers were calculated from their corresponding permeability coefficient (P) and diffusion coefficient (D) values obtained using the following equation (3): S=

P D

(3)

The ideal permselectivity of the membranes toward a gas ‘A’ relative to another gas ‘B’ were calculated from the individual gas permeabilities using the following equation (4): ˛P(A/B) =

PA PB

(4)

3.2. Gas transport properties The mean gas permeability of pure CO2 , O2 , N2 and CH4 and ideal permselectivity value for this different gas pairs through the polyimide membranes measured at 35 ◦ C and 3.5 bar are summarized in Table 3. It was not possible to measure the gas transport properties of FBP-PMDA membrane as this membrane could not withstand the pressure difference during the measurements and hence, the gas transport properties of the four PEI membranes were measured. It is well accepted that the permeation properties of a polymeric membrane are dependent on the casting protocol and the measured permeability coefficients in different laboratories may differ. However, the ideal permselectivity for a pair of gas can be considered as a standard parameter to compare with other polymeric membranes measured in other laboratories [30]. The observed permeability order for four gases through these PEI membranes was as P (CO2 )  P (O2 ) > P (N2 ) > P (CH4 ) which is in the same order to their kinetic diameter (Å), CO2 , 3.3; O2 , 3.46; N2 , 3.64; CH4 , 3.8 [4]. In general, all these membranes showed excellent permeability for all the gases. This is attributed to the presence of bulky diphenylfluorene moiety and pendent trifluoromethyl groups in the polymer backbone. Among all the PEIs, the FBP-6FDA showed the highest gas permeability among the series. The order of permeability for O2 , N2 and CH4 is FBP-6FDA > FBP-ODPA > FBP-BPADA > FBP-BTDA. The polymer FBP-BTDA exhibited lowest permeability among the series. The lower gas permeability of BPI-BTDA is due to the additional carbonyl group present in the dianhydride, which leads to close packing of the polymer by more charge transfer complex formation. The order of permeability for CO2 gas is FBP-6FDA > FBPBTDA > FBP-ODPA > FBP-BPADA, which is in accordance with their fractional free volumes (Vf , Table 2) calculated from density values utilizing the incremental method developed by Park and Paul [31]. Moreover, the presence of carbonyl group in BPI-BTDA leads to

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Table 3 Gas permeability coefficients (P) and permselectivities (˛p ) of the poly(ether imide)s and their comparison with other indane based polyimides reported earlier. Molecular structure with designation

P (CO2 )

P (CH4 )

˛P (CO2 /CH4 )

P (O2 )

P (N2 )

˛P (O2 /N2 )

Ref.

FBP-BTDA FBP-6FDA FBP-ODPA FBP-BPADA

36.07 53.09 25.91 22.52

0.94 1.34 1.04 1.01

38.37 39.62 24.91 22.30

6.24 13.46 7.86 7.01

0.95 2.06 1.22 1.19

6.57 6.53 6.44 5.89

– – – –

1.51

0.06

26.03

3.18

0.59

5.39

[17]

14.00

0.54

25.93

2.76

0.48

5.75

[17]

12.40

0.62

20.00

3.03

0.57

5.32

[18]

36.80

2.37

15.53

9.55

1.93

4.95

[18]

-

-

-

1.56

0.24

6.50

[19]

25.70

1.59

16.16

6.22

2.06

3.02

[20]

35.60

1.80

19.78

8.40

1.99

4.22

[20]

2.40

0.08

30.77

0.72

0.10

7.42

[21]

B. Dasgupta, S. Banerjee / Journal of Membrane Science 362 (2010) 58–67

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Table 3 (Continued ) Molecular structure with designation

P (CO2 )

P (CH4 )

˛P (CO2 /CH4 )

P (O2 )

P (N2 )

˛P (O2 /N2 )

Ref.

1.87

0.05

34.00

0.53

0.06

8.28

[21]

4.97

0.19

26.16

1.42

0.22

6.45

[21]

8.70

0.24

36.00

1.90

0.27

7.00

[34]

P = gas permeability coefficient in Barrer. 1 Barrer = 10−10 cm3 (STP) cm/cm2 s cm Hg.

The critical volume (cm3 /mol) of these gases are O2 , 73.4; N2 , 90.1; CO2 , 94.1; CH4 , 98.6 [25]. The order of diffusivity coefficient for all the gases are FBP-6FDA > FBP-BTDA > FBP-ODPA > FBP-BPADA, which is in good agreement with their fractional free volumes (Vf , Table 2). Based on the solution-diffusion mechanism, gas permeability is the product of gas solubility and gas diffusivity. Accordingly the solubility coefficient values was calculated from S = P/D. The solubility coefficient and solubility selectivity value are presented in Table 4. Due to limited instrumental facility, we were not able to perform the direct sorption measurements of the polymers. The incremental order of the gas solubility coefficients calculated from permeability and diffusivity values is as S (CO2 ) > S (CH4 ) > S (O2 ) > S (N2 ). The high permeation rate of CO2 through these PEI membranes is dominated by high solubility coefficient of CO2 . It is evident in the literature that in case of various polyimides the solubility coefficients for CO2 is exceptionally high in comparison to other gases like CH4 , O2 , N2 etc. [24]. On close examination of Table 3, it is found that overall selectivities (˛) for CO2 /CH4 are mainly because of the solubility selectivity as the diffusion selectivity values are relatively small for this gas pair.

exceptionally higher solubility coefficient for CO2 and hence higher permeability [24] (Table 3). It is well known that selectivity of the membrane decreases as permeability increase for a pair of gas [5]. High chain stiffness, as indicated by high glass transition temperature, is expected to result in relatively high selectivity and the same trend was observed. The permselectivity order of the polymer for CO2 /CH4 gas pair is FBP6FDA > FBP-BTDA > FBP-ODPA > FBP-BPADA, which is in the same order of their glass transition temperature. FBP-6FDA exhibited highest CO2 gas permeability (53.09 Barrer) with highest CO2 /CH4 selectivity (39.62) in the series. The higher permeability of the polymer is coming from the higher free volume due to the presence of bulky –CF3 group and at the same time the high gas selectivity of this polymer is due to the restricted local segmental mobility attributed to the restricted torsional motion of phenyl rings around a >C(CF3 )2 linkage [24]. On the other hand, FBP-BTDA shows lowest O2 and N2 gas permeability and highest selectivity for O2 /N2 gas pair. The overall selectivities (˛) for CO2 /CH4 gas pair in all the PEIs are mainly because of the solubility selectivity of these two gases into the polymer. The diffusion coefficient and the diffusivity selectivity value are summarized in Table 4. The diffusion coefficients were calculated from the time-lag plot of gas flow vs. time. Determination of diffusion coefficients from the time-lag values is unreliable, when dual mode transport characteristics apply [32]. However, within a series of the polymers it can be used for the comparison of the general behavior. The order of the gas diffusion coefficients through these polymers were observed to be D (O2 ) > D (N2 ) > D (CO2 ) > D (CH4 ). This is in the same order to the critical volume of the gas molecules.

3.3. Comparison with other fluorene based polymeric membranes The gas permeability and permselectivity data obtained for the gas pair are compared with other diphenylfluorene based polymeric membrane from different classes of polymer and summarized in Table 3. Robeson plots [6] are presented in Figs. 2 and 3, for better comparison of the gas permeation performance of

Table 4 Gas diffusion coefficients, D × 108 (cm2 /s), solubility coefficients (S), diffusivity selectivities (˛D ) and solubility selectivities (˛S ) of the poly(ether imide)s at 3.5 bar and 35 ◦ C. Polymer

FBP-BTDA FBP-6FDA FBP-ODPA FBP-BPADA

CO2

O2

N2

CH4

D

S

D

S

D

S

D

S

4.02 4.83 4.01 3.79

8.97 10.99 6.46 5.94

8.22 13.11 8.08 6.99

0.76 1.03 0.97 1.00

4.11 4.98 3.61 3.34

0.23 0.41 0.34 0.36

1.04 1.24 1.02 0.95

0.90 1.08 1.02 1.06

Gas solubility coefficients calculated from S = P/D, in cm3 (STP)/cm3 cm Hg.

˛D (CO2 /CH4 )

˛D (O2 /N2 )

˛S (CO2 /CH4 )

˛S (O2 /N2 )

3.87 3.90 3.93 3.99

2.00 2.63 2.24 2.09

9.93 10.17 6.34 5.59

3.28 2.48 2.88 2.81

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Fig. 2. Robeson plot [6] for a comparison of CO2 /CH4 selectivity vs. CO2 permeability coefficients of the poly(ether imide)s with known polyimides, values taken from Refs. [17,18,20,21].

these PEI membranes for permeability coefficient of CO2 vs. its selectivity over CH4 and permeability coefficient of O2 vs. its selectivity over N2, respectively. In comparison to previously reported polymer membrane containing diphenylfluorene moiety the membranes under this investigation exhibited higher permeability as well as higher gas selectivity. Aguilar-vega et al. [17] reported diphenylfluorene containing polycarbonate and polysulfone which show lower permeability with moderate permselectivity. Pixton et al. [18] observed the gas transport properties of the diphenylfluorene containing polyarylates based on isophthalic acid and t-butyl substituted isophthalic acid. Substitution with –C(CH3 )3 group on the benzene ring increases the gas permeability remarkably (36.8 Barrer for CO2 and 9.55 Barrer for O2 ) with little reduction of selectivity (15.5 for CO2 /CH4 and 4.95 for O2 /N2 ). The gas permeabilities of these membranes were further enhanced by tetrabromination of the bisphenol [33]. Wang et al. [21] reported gas transport properties for a series of poly(arylene ether ketone)s containing diphenylfluorene moiety which showed very low permeability but higher selectivity. Among that series FBP-3,3 -DBBP showed highest selectivity for both the gas pair (34 for CO2 /CH4 and

Fig. 4. Temperature dependence of gas permeability for BPI-6FDA.

8.3 for O2 /N2 ), but the permeability value were too low (1.87 Barrer for CO2 and 0.53 Barrer for O2 ). On the other hand, diphenylfluorene based poly(arylene ether)s reported by Xu et al. [20] showed relatively higher permeability with reasonable permselectivity. The polymer FBP-6FPPr showed a higher permeability 35.6 Barrer for CO2 and 8.40 Barrer for O2 with selectivity, 19.8 for CO2 /CH4 and 4.22 for O2 /N2 gas pair. Higher flux along with higher permselectivities is always highly desired for practical commercial application [3]. The polymers reported in this journal showed much improvement in gas transport properties. FBP-6FDA and FBP-BTDA showed better separation efficiency not only for this series but also among the entire diphenylfluorene containing polymer reported earlier. These polymer also showed much higher permeability with improved permselectivity in comparison to commercially available Matrimid® [34]. Especially, FBP-6FDA is of the most interest in respect to its highest permeability coefficient (53.09 Barrer for CO2 and 13.46 Barrer for O2 ) along with highest permselectivity (39.62 for CO2 /CH4 and 6.53 for O2 /N2 ) among entire diphenylfluorene containing polymer develop so far. These high permeability coefficients along with high permselectivity have been achieved by the incorporation of pendent bulky trifluromethyl groups in both the diamine and dianhydride part of the polymer along with the cardo diphenylfluorene moiety in the main chain of this PEI. 3.4. Effect of temperature on gas permeabilities, diffusivities, and selectivities

Fig. 3. Robeson plot [6] for a comparison of O2 /N2 selectivity vs. O2 permeability coefficients of the poly(ether imide)s with known polyimides, values taken from Refs. [17–21].

Temperature dependence of permeability was evaluated and the effect of temperature on permeability coefficient is shown in Fig. 4. From the P vs. T plots it has been seen that permeability coefficients increases linearly with temperature. The permselectivity decreases as the temperature increases (Figs. 5 and 6). With increase in temperature the chain mobility and the frequency of the intersegmental jumps of the gas molecules increases, accordingly the diffusion rate as well as permeation rate also increases. At the same time, the chain segment motions may become wider, and low selectivity for a gas pair is the consequence. The diffusion coefficients also showed temperature dependence. However, within the same temperature range the D vs. T plots (Fig. 7) are not linear like the P vs. T plots, especially diffusion coefficient of O2 increased substantially for all these PEIs. The diffusion selectivities for a pair of gas also decrease with increase in temperature (Figs. 8 and 9). Interestingly, in case of FBP-ODPA, the

B. Dasgupta, S. Banerjee / Journal of Membrane Science 362 (2010) 58–67

Fig. 5. Effect of temperature on gas permselectivities of CO2 /CH4 .

65

Fig. 8. Effect of temperature on gas diffusivity selectivities for CO2 /CH4 .

diffusion selectivity value for CO2 /CH4 gas pair slightly increases at 45 ◦ C. This may be due to the fact that at this particular temperature the intersegmental gap opening is sufficient enough to permeate smaller sized CO2 but not bigger sized CH4, which ultimately results more diffusion of CO2 through the polymer matrix compared to CH4 resulting in increase in the diffusion selectivity. However, at higher temperature i.e., at 55 ◦ C the diffusion selectivity again decreases which indicates that at this temperature the thermally activated motion of the intersegments create enough gap to permeate CH4 too. Within a temperature range in which no significant thermal transitions of the polymer occurs, the temperature dependence of diffusivity and permeability can be expressed by the Arrhenius equations: D = D0 exp

Fig. 6. Effect of temperature on gas permselectivities for O2 /N2 .

Fig. 7. Temperature dependence of gas diffusion coefficient for BPI-6FDA.

 −E  D

RT

and

P = P0 exp

 −E  P

RT

where P0 and D0 are the pre-exponential coefficients; ED and EP are the apparent activation energy for the diffusion and permeation process, respectively; R is the gas constant; and T is the absolute temperature. The permeability and diffusion coefficients at different temperatures were utilized to calculate the activation

Fig. 9. Effect of temperature on gas diffusivity selectivities for O2 /N2 .

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B. Dasgupta, S. Banerjee / Journal of Membrane Science 362 (2010) 58–67

Table 5 Activation energies (KJ/mol) for the permeation (EP ) and diffusion (ED ) processes of the poly(ether imide)s. Polymer

FBP-BTDA FBP-6FDA FBP-ODPA FBP-BPADA

EP

ED

CO2

O2

N2

CH4

CO2

O2

N2

CH4

3.28 2.70 4.44 5.14

10.65 8.72 9.01 10.68

16.26 13.47 14.65 14.96

18.20 14.89 16.76 16.89

20.44 18.99 20.96 20.91

29.69 27.93 30.43 29.89

30.07 29.79 30.65 30.71

27.49 26.70 27.87 28.01

energies for permeation and diffusion processes of these four gases through the four PEI membranes. The activation energies for the diffusion and permeation processes have been summarized in Table 5. The trend of activation energies for the permeation process for all the PEIs, EP (CO2 ) < EP (O2 ) < EP (N2 ) < EP (CH4 ) are in good agreement with their permeability coefficient values: P (CO2 ) > P (O2 ) > P (N2 ) > P (CH4 ). However, the trend of activation energies for the diffusion process ED (CO2 ) < ED (CH4 ) < ED (O2 ) < ED (N2 ), are not in good agreement with their diffusion coefficient values, D (O2 ) > D (N2 ) > D (CO2 ) > D (CH4 ). Activation energy for diffusion process is the reverse order of their diffusion coefficient in case of O2 and N2 , but anomalous behavior is found in case of CO2 and CH4 . At present, the exact cause of this anomaly behavior of CO2 and CH4 with these polymers is not well understood but high physical interaction of CO2 molecules and to some extent with CH4 with these polar PEI structures probably leads to a decrease in diffusivity. Since the diffusion of the gas molecules through the polymer matrices are directly related to the chemical structures of the polymer backbones, the diffusion processes are more affected by the physical interaction of gas molecules with the polymer matrix. On the other hand as the permeation process is basically a cumulative effect of diffusion and solubility of different gases through different polymers, there is not much difference between their trends of permeation coefficients and activation energies. These activation energy values can be of interest per se as physicochemical characteristics of energy barriers in mass transport of small molecules through a polymeric matrix. Practical importance of these parameters arises from the fact that they are needed to calculate the gas permeation and diffusion coefficients at any temperature of interest. The representative plots for ln P (CO2 ) vs. 1/T of FBP-6FDA poly(ether imide) membrane is shown in Fig. 10.

4. Conclusions Gas transport properties for CO2 , CH4 , N2 and O2 through five PEI membranes containing diphenylfluorene moiety in the backbone and pendent trifluoromethyl groups have been successfully investigated. All the polymer membranes exhibited excellent permeabilities and moderate selectivities for a pair of gas. The presence of rigid diphenylfluorene cardo group and the bulky pendent trifluoromethyl group in a poly(ether imide) backbone help to achieve excellent permeability as well as reasonably good selectivity in comparison to the diphenylfluorene based polysulfones, polycarbonates, polyacrylates, poly(ether ketone)s, poly(arylene ether)s, poly(arylene ether ketone)s etc. membranes of different class developed earlier. The polymers FBP-6FDA and FBP-BTDA showed better separation efficiency among the series. FBP-BTDA showed highest ideal permselectivity of 6.57 for O2 /N2 gas pair but with 6.24 Barrer of O2 permeability whereas FBP-6FDA membrane showed the highest permeability for O2 (13.46 Barrer) with similar ideal permselectivity of 6.53. Moreover highest ideal CO2 permeability (53.09 Barrer) along with highest permselectivity for CO2 /CH4 gas pair (39.62 for CO2 /CH4 ), made BPI-6FDA membrane interesting for gas separation purpose among the entire diphenylfluorene based polymeric membrane of different classes investigated so far. This diphenylfluorene based semifluorinated PEI membrane can be a better candidate in the field of membrane material for gas separation application. Acknowledgement The authors thank to Department of Science and Technology, Government of India for financial support in terms of a sponsored project (Grant No. SR/S3/ME/020/2006-SERC-Eng.) to carry out this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.memsci.2010.06.015. References

Fig. 10. Representative plot of ln P vs. 1/T for BPI-6FDA.

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