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Alkylimidazolium-functionalizedcardo-basedpoly(etherketone)sasnovelpolymermembranesforO2/N2andCO2/N2separations

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ImpactFactor:3.56·DOI:10.1016/j.polymer.2013.05.006

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Polymer 54 (2013) 3534e3541

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Polymer

journal homepage: www.elsevier .com/locate/polymer

Alkyl imidazolium-functionalized cardo-based poly(ether ketone)s asnovel polymer membranes for O2/N2 and CO2/N2 separations

Irshad Kammakakama, Hyo Won Kimb, SangYong Nam c, Ho Bum Park b, Tae-Hyun Kim a,*

aOrganic Material Synthesis Laboratory, Department of Chemistry, Incheon National University, Songdo-dong 12-1, Yeonsu-gu, Incheon 406-772,Republic of KoreabWCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of KoreacDepartment of Polymer Science and Engineering, Gyeongsang National University, 900 Gazwa-dong, Chinju 660-701, Republic of Korea

a r t i c l e i n f o

Article history:Received 13 March 2013Received in revised form28 April 2013Accepted 1 May 2013Available online 9 May 2013

Keywords:Imidazolium-based ionic saltPolymer membranesGas separation

* Corresponding author. Tel.: þ82 32 835 8232; faxE-mail address: tkim@incheon.ac.kr (T.-H. Kim).

0032-3861/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.05.006

a b s t r a c t

Alkyl imidazolium-functionalized poly(ether ketone)s (Im-PEKs) were prepared as novel polymermembranes for O2/N2 and CO2/N2 separations. The Im-PEK polymers were synthesized by poly-condensation between difluoro- and dihydroxy-monomers, followed by benzylic bromination and alkylimidazolium-functionalization. The polymers with pendant imidazolium-based ionic salts showed highCO2 solubility and hence a very high CO2/N2 permselectivity of 66.1 due to their high solubility selectivity.In addition, the Im-PEK with long alkyl chain ([C12-Im-PEK][Br]) displayed an extraordinary high O2/N2

permselectivity of 15.5 due to its enhanced diffusivity selectivity. Overall, the combined O2/N2 and CO2/N2 separation properties were achieved in single polymeric membrane. The Im-PEK membranes alsodisplayed good thermal and mechanical stabilities.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction solubility of CO in ILs [7e9]. While other classes of ILs including

Polymer membrane-based gas separation technologies arepotentially more energy-efficient than conventional separationprocesses when applied to gasmixtures, such as O2/N2, CO2/CH4 andCO2/N2, etc [1e3]. For a given pair of gases, the key parameters thatcharacterize membrane separation performance are the perme-ability coefficient, PA, and the selectivity, aA/B. The permeabilitycoefficient is the product of the solubility coefficient and the diffu-sivity coefficient, and the ideal gas selectivity is the product ofthe solubility selectivity and the diffusivity selectivity. High-performance polymer membranes ideally have both a high perme-ability and a high selectivity because a high permeability reducesthe membrane area size required to treat a given amount of gas,thereby decreasing the capital costs associated with membranemanufacture. A high selectivity also increases the purity of theproduct gas. A strong trade-off between permeability and selectivityhas always existed, defining a so-called “upper bound” on the sep-aration efficiency for each gas pair [4e6].

Ionic liquids (ILs) have emerged as promising separationmaterials that rival conventional amine scrubbing technologies dueto their tunable properties, lack of volatility and thermal stability.ILs have even greater utility for CO2 separations due to the large

: þ82 32 835 0762.

All rights reserved.

2phosphonium [10,11], ammonium [10,11] and pyridinium [12] havecertainly received attention in CO2 separations, imidazolium-basedILs dominate the literature [13].

Some studies have examined CO2/N2 separation using supportedionic liquid membranes (SILMs) [13e15], in which ILs are impreg-nated into microporous polymers, due to their high permeabilityand selectivity. The use of SILMs in practical gas separation pro-cesses, however, is hindered by stability issues (leaching of ILs fromthe membranes at pressure differences beyond 0.2 atm) [13].

Membranes composed of polymerized ionic liquids, poly(IL)s,may be better options for CO2 separation due to their higher CO2sorption capacities and higher sorption and desorption ratescompared to ILs [16e18]. In poly(IL)s, the polymerizable groupswere added to mostly imidazolium-based ILs, allowing for theconversion of the molten salts into the solid-state materials. Pol-y(IL)s are, therefore, more stable than SILMs. Strictly speaking,poly(IL)s do not belong to the category of ionic liquids but ratherpolymers based on ionic salts, but they share many features of ILssuch as high CO2 solubility. Poly(IL)s have proven to be selective inCO2-based separations, but the applicability of poly(IL)s in practicalgas separation techniques is limited due to the limited physical andchemical properties of thematerials. Because poly(IL)s are preparedfrom IL-containing reactive monomers, it is usually difficult tocontrol the resultant polymer properties. Furthermore, the polymerstructures used to prepare poly(IL)s have largely been limited topolystyrenes and polyacrylates.

I. Kammakakam et al. / Polymer 54 (2013) 3534e3541 3535

Polyimides (PIs) [19,20], poly(ether sulfone)s (PESs) [21] andpoly(ether ketone)s (PEKs) [22] display moderate gas separationproperties and excellent mechanical and thermal stabilities, mak-ing them suitable as hollow fiber membranes for gas separations inthe context of industrial applications [23].

Wehave combined thebenefits of the above-mentionedpolymersand ILs, in a rigid polymer such as PEK having pendant imidazolium-based ionic bromides as novel materials for gas separation. Themembranesobtained fromthe imidazolium-functionalizedPEKs (Im-PEKs) showed excellent thermal and mechanical stabilities, and veryhighO2/N2 andCO2/N2permselectivities.We investigate theeffects ofthe alkyl chains in the imidazolium groups on the structures andproperties of the polymers, aswell as the gas separation properties ofthe corresponding polymer membranes.

2. Experimental

2.1. Materials

4,40-Difluorobenzophenone(DFBP), 2,2-bis(4-hydroxy-3-methy-lphenyl)propane (HMPP) were purchased from Tokyo ChemicalIndustry (TCI) Co., Ltd. (Tokyo, Japan) and used as obtained. 4,40-(9-Fluorenylidene)biphenol (FBP) was obtained from Fluka andrecrystallized from water and ethanol. Potassium carbonate, DFBP,HMPP and FBP were dried under vacuum at 60 �C for 24 h prior tothe polymerization.

1-Ethyl-1H-imidazole (99%) was purchased from Sigma Aldrich.1-Hexyl-1H-imidazole and 1-dodecyl-1H-imidazole were synthe-sized according to the literature procedure [13], from imidazole(TCI, 99%) reacting with iodohexane and iodododecane, respec-tively, in presence of sodium hydride (60 wt%) and DMF (Aldrich,99%) as a solvent. All other chemicals, unless otherwise noted, wereobtained from commercial sources and used as received.

2.2. Characterization

1H NMR spectra were obtained on an Agilent 400-MR(400 MHz) instrument using d6-DMSO or CDCl3 as a reference orinternal deuterium lock.

FT-IR spectra of the materials were recorded as KBr pellets usingNicolet MAGNA 560-FTIR spectrometer in the range of 4000e400 cm�1.

Molar masses were determined by Gel Permeation Chromatog-raphy (GPC) using two PLGel 30 cm� 5 mmmixedC columns at 30 �Crunning inTHF and calibrated against polystyrene (Mn¼ 600� 106 g/mol) standards using a Knauer refractive index detector.

The glass transition temperature (Tg) of each polymer wasmeasured using a PerkineElmer Pyris-1 DSC from 20 �C to 300 �Cwith a scan rate of 10 �C min�1 under nitrogen.

2.3. Synthesis of the Im-functionalized poly(ether ketone)s

2.3.1. Synthesis of the cardo-based PEK (1)Into a 250 cm3 two-neckedflask equippedwith amagnetic stirrer,

nitrogen inlet, and a DeaneStark trap, 4,40-difluorobenzophenone(5.00 g, 19.91 mmol), 4,4-(9-fluorenylidene)biphenol (3.49 g,9.96 mmol), 2,2-bis(4-hydroxy-3-methyl phenyl)propane (2.55 g,9.96 mmol), potassium carbonate (2.89 g, 20.91 mmol), DMAc(30 cm3) and toluene (35 cm3)were added. The reactionmixturewasheated at 150 �C for 4 h. After thewaterwas essentially removed fromthe reaction mixture by azeotropic distillation, toluene was distilledout. The temperatureof the reactionmixturewas then raised to170 �Cand allowed to stir for another 16e20 h and then the viscous reactionsolutionwas cooled to r.t. and dissolved in DMF (10 cm3), followed bypouring into methanol (400 cm3). White polymer beads were

collectedbyfiltration,washedwithdeionizedwater several timesanddried at 80 �C under vacuum for 48 h to give the cardo-based PEK 1(10.6 g, 96%); dH(400 MHz, CDCl3) 7.90e7.63 (10H, br signal, ArH),7.48e7.36 (4H,brsignal,ArH),7.31e7.28 (2H,br signal, ArH), 7.24e7.22(4H, br signal, ArH), 7.16 (2H, s, ArH), 7.09e7.07 (2H, br signal, ArH),7.00e6.82 (14H, br signal, ArH), 2.17 (6H, s, ArCH3) and 1.70 (6H, s,CH3); (KBr)/cm�1 3065, 2929, 1717, 1602, 1495, 1260, 1138,1024,932 and 841; GPC (THF, RI)/DaMn 61.2 kg/mol,Mw 157.5 kg/mol andMw/Mn 2.57.

2.3.2. Bromination of the polymer 1 to give the bromobenzylatedPEK (2)

Into a 250 cm3 two-necked flask equipped with a magneticstirrer, nitrogen inlet, and a condenser, the PEK polymer 1 (6 g,6.23 mmol), a catalytic amount of benzoyl peroxide (BPO) andtetrachloroethane (30 cm3) were added. This was heated to 85 �Cfor a complete dissolution before adding N-bromosuccinimide(2.77 g, 15.57 mmol) and stirring for 12 h at this temperature. Uponcooling to r.t., the resultant red solution was precipitated intomethanol (400 cm3). The yellow-colored polymer beads werecollected by filtration, washed with water and dried at 80 �C undervacuum for 48 h to give the bromobenzylated PEK 2 (6.1 g, 88%);dH(400 MHz, CDCl3) 7.93e7.61 (10H, br signal, ArH), 7.46e7.36 (4H,br signal, ArH),7.33e7.30 (2H, br signal, ArH), 7.24e7.22 (4H, brsignal, ArH), 7.15 (2H, s, ArH),7.06e6.77 (16H, br signal, ArH), 4.51(2H, s, ArCH2) 2.17 (4H, s, ArH) and 1.70 (6H, s, CH3); (KBr)/cm�1

3043, 2967, 1747, 1641, 1496, 1236, 1145, 1024, 939, 841 and 620.

2.3.3. Incorporation of the alkyl imidazoles to give the Im-functionalized PEKs (3)

To a solution of the brominated PEK 2 (6 g, 5.35 mmol) in DMAc(20 cm3), the corresponding alkyl imidazole in DMAc was addeddropwise. The reaction mixture was heated to 90 �C for 24 h withvigorous stirring under nitrogen. After this time, the brown solu-tionwas cooled to r.t. and precipitated into ethyl acetate (400 cm3).The brown yellow-colored polymer powders were collected byfiltration, washed with methanol and dried at 80 �C under vacuumto give the desired alkyl imidazolium-functionalized PEKs 3;

[C2-Im-PEK][Br] (5.7 g, 83%); dH(400 MHz, d6-DMSO) 9.27e9.10(1H, br signal, ArH), 8.12e7.53 (12H, br signal, ArH), 7.53e6.51 (28H,br signal, ArH), 5.55e5.35 (2H, br signal, ArCH2), 4.20e3.98 (2H, brsignal, NCH2), 2.24e1.85 (4H, br signal, ArH), 1.79e1.45 (6H, brsignal, CH3) and 1.33e1.16 (3H,br signal, CH3); (KBr)/cm�1 3051,2959, 1724,1671, 1485, 1450, 1560, 1236, 1153, 1015, 939 and 841.

[C6-Im-PEK][Br] (5.9 g, 86%); dH(400 MHz, d6-DMSO) 9.28e9.12(1H, br signal, ArH), 8.09e7.54 (12H, br signal, ArH), 7.54e6.60(28H, br signal, ArH), 5.53e5.36 (2H, br signal, ArCH2), 4.14e3.96(2H, br signal, NCH2), 2.21e1.95 (4H, br signal, ArH), 1.78e1.49 (8H,br signal, CH3), 1.24e1.00 (6H,br signal, CH2) and 0.83e0.63 (3H,brsignal, CH3); (KBr)/cm�1 3051, 2929, 1717, 1664, 1588, 1481, 1450,1236, 1160, 1015, 932 and 833.

[C12-Im-PEK][Br] (5.8 g, 82%); dH(400 MHz, d6-DMSO); 9.30e9.11 (1H, br signal, ArH), 8.08e7.53 (12H, br signal, ArH), 7.53e6.57(28H, br signal, ArH), 5.52e5.36 (2H, br signal, ArCH2), 4.13e3.95(2H, br signal, NCH2), 2.21e1.95 (4H, br signal, ArH), 1.78e1.46(8H, br signal, CH3), 1.32e0.93 (18H, br signal, CH2) and 0.86e0.67(3H,br signal, CH3); (KBr)/cm�1 3051, 2913, 1717, 1656, 1580, 1481,1450, 1236, 1160, 1015, 932 and 833.

2.4. Membrane preparation

All membranes were prepared in a DMF solution of the corre-sponding polymers using the solution-casting method. The corre-sponding polymers 3 (1.0 g) were dissolved in 5.0 cm3 of dry DMFand stirred at r.t. overnight. The solution was filtered through a

I. Kammakakam et al. / Polymer 54 (2013) 3534e35413536

short column filled with cotton wool before casting directly ontoglass plates. The plates were then covered with aluminum foilshaving small holes and dried at 60 �C for 48 h and 90 �C for 24 h indry oven. The membrane thickness was controlled to be 70e90 mm.

2.5. Membrane characterization

The densities of the membranes (g cm�3) were determinedexperimentally using a top-loading electronic Mettler Toledo bal-ance (XP205, Mettler-Toledo, Switzerland) coupled with a densitykit based on Archimedes’ principle. The samples were weighed inair and a known-density liquid, high purity water. The measure-ment was performed at room temperature by the buoyancymethod and the density was calculated as follows

rpolymer ¼ W0

W0 �W1� rliquid

Scheme 1. Synthesis of the alkyl imidazoliu

where, W0 and W1 are the membrane weights in air and waterrespectively. The water sorption of the Im-PEK membranes was notconsidered due to their extremely low water uptake property.

The X-ray diffraction patterns of themembranes weremeasuredusing a Rigaku DMAX-2200H diffractometer by employing a scan-ning rate of 4�/min in a 2q range from 5� to 30� with a Cu Ka1 X-ray(l ¼ 0.1540598). The d-spacings were calculated using the Bragg’slaw (d ¼ l/2 sin q).

Tensile strength and elongation at break of themembranes weremeasured on a Shimadzu EZ-TEST E2-L instrument benchtop ten-sile tester using a crosshead speed of 1 mm/min at 25 �C under 50%relative humidity. Engineering stress was calculated from the initialcross sectional area of the sample and Young’s modulus (E) wasdetermined from the initial slope of the stressestrain curve. Themembrane samples were cut into a rectangular shape with80 mm � 8 mm (total) and 80 mm � 4 mm (test area), and fivespecimens were used for the measurements.

m-functionalized poly(ether ketone)s 3.

I. Kammakakam et al. / Polymer 54 (2013) 3534e3541 3537

2.6. Gas separation measurements

The gas permeation measurements were performed using ahigh-vacuum time lag measurement unit based on constant-volume/variable-pressure method. All the experiments were per-formed at a feed pressure of 1500 Torr and a feed temperature of30 �C in the order of different kinetic diameters of gas moleculessuch as CO2 (0.33 nm), O2 (0.346 nm) and N2 (0.364 nm) [24].Before the gas permeation measurements, both the feed and thepermeate sides were thoroughly evacuated to below 10�5 Torr untilthe readout showed zero values to remove any residual gases. Thedownstream volume was calibrated using a Kapton membrane andwas found to be 50 cm3 [25]. The upstream and downstreampressures were measured using a Baraton transducer (MKS; modelno. 626B02TBE) with a full scale of 10,000 and 2 Torr, respectively.The pressure rise versus time transient of the permeate side,equipped with a pressure transducer, was recorded and passed to adesktop computer through a shield data cable. The permeabilitycoefficient was determined from the linear slope of the down-stream pressure rise versus time plot (dp/dt) according to thefollowing equation:

P ¼ 27376

VlATp0

dpdt

(1)

where P is the permeability expressed in Barrer (1barrer ¼ 10�10 cm3 (STP)cm/cm2 s cmHg); V (cm3) is the down-stream volume; l (cm) is the membrane thickness; A (cm2) is theeffective area of the membrane; T (K) is the temperature of mea-surement; p0 (Torr) is the pressure of the feed gas in the upstreamchamber and dp/dt is the rate of the pressure rise under the steadystate. All gas permeation tests were performed more than threetimes, and the standard deviation from the mean values of

9 8 7 6 5

Chemical

15

16

16

15

1516

(a)

(b)

(c)

Fig. 1. 1H NMR spectra of the (a) [C2-Im-PEK][Br

permeabilities was within ca. �3%. Sample to sample reproduc-ibility was high and within �2%. The effective membrane areaswere 15.9 cm2. The ideal permselectivity, aA/B, of the membrane fora pair of gases (A and B) is defined as the ratio of the individual gaspermeability coefficients as, PA

aA=B ¼ PAPB

The diffusivity and solubility were obtained from the interceptaccording to the following equations, l2

D ¼ l2

6q(2)

S ¼ PD

(3)

where D (cm2 S�1) is the diffusivity coefficient, l is the membranethickness (cm) and q is the time lag (s), obtained from the interceptof the linear steady state part of downstream pressure rise versustime plot. Solubility, S, was calculated from Eq. (3) with perme-ability and diffusivity obtained from Eqs. (1) and (2).

3. Results and discussion

3.1. Synthesis of the imidazolium-functionalized poly(ether ketone)s with different alkyl chains

The cardo-based poly(ether ketone) 1 was first synthesized bythe polycondensation of FBP and HMPP with DFBP, followed bybromination at the benzyl position and alkyl imidazole incorpora-tion (Scheme 1). A bis(phenyl)fluorene unit was used as a cardo-

4 3 2 1

Shift (ppm)

17b

1,18b19b

20b

2

17a

17c

2

2

1,18c

19c

20c

18a

1

], (b) [C6-Im-PEK][Br], (c) [C12-Im-PEK][Br].

Fig. 2. FT-IR spectra (400e1700 cm�1) of the Im-functionalized PEKs 3.

Fig. 3. DSC graphs of the [C2-Im-PEK][Br], [C6-Im-PEK][Br] and [C12-Im-PEK][Br]polymers.

I. Kammakakam et al. / Polymer 54 (2013) 3534e35413538

moiety with the hope that the bulky moiety in the main chainwould both reduce the rotational mobility and prohibit interseg-mental packing. These effects would improve the overall mem-brane performance for gas separation [26,27].

The cardo-based PEK 1 had a high molecular weight(Mw ¼ 157 KDa as confirmed by GPC). Bromination of the ArCH3unit, using 1.5 equiv of NBS, was conducted in a tetrachloroethanesolution of polymer 1 to produce the bromobenzylated PEK 2.Comparative 1H NMR spectroscopic analysis of polymers 1 and 2revealed that the intensity of the benzylic proton (Ha) decreasedand new bromobenzyl proton peak (Hb) appeared (Fig. S1 in Sup-porting Information). No particular changes were observed amongthe other aromatic peaks, indicating the selective bromination ofthe benzyl group. The degree of bromination in polymer 2 wasestimated based on the ratio of the integrals of the bromobenzylprotons (Hb) in 2 to the benzylic protons (Ha) in polymer 1, andfound to be 42%.

Further functionalization to the Im-PEKs 3 was carried out bytreating a DMF solution of polymer 2 with the alkyl imidazoles(C2, C6, and C12) to give the corresponding Im-functionalized PEKs

Table 1Physical parameters of the Im-PEKs.

Polymer Tg (�C) d-spacing (�A) Density

[C2-Im-PEK][Br] 233 4.8 1.282[C6-Im-PEK][Br] 222 4.8 1.318[C12-Im-PEK][Br] 210 4.8 1.343

Fig. 4. Wide-angle X-ray diffraction plots of the [C2-Im-PEK][Br], [C6-Im-PEK][Br] and[C12-Im-PEK][Br] membranes.

Table 2Pure gas permeabilitiesa and selectivities (a) of the Im-PEK membranes at 30 �C and2 atm.

PCO2PO2

PN2aCO2=N2

aO2=N2

[C2-Im-PEK][Br] 1.89 0.621 0.082 23 7.6[C6-Im-PEK][Br] 1.11 0.295 0.023 48.3 12.8[C12-Im-PEK][Br] 1.19 0.28 0.018 66.1 15.5

a Permeabilities in barrers, where 1 barrer ¼ 10�10 cm3 (STP) cm/cm2 s cm Hg.

Table 3Gas diffusivity-selectivity and solubility-selectivity.

DCO2=N2DO2=N2

SCO2=N2SO2=N2

[C2-Im-PEK][Br] 2.1 1.2 11.0 6.1[C6-Im-PEK][Br] 2.1 6.3 22.8 2.0[C12-Im-PEK][Br] 2.1 6.8 31.8 2.2

Table 4Gas diffusivity coefficientsa and solubility coefficientsb at 30 �C.

Abbreviation DCO2DO2

DN2SCO2

SO2SN2

[C2-Im-PEK][Br] 1.73 1.01 0.815 1.1 0.61 0.10[C6-Im-PEK][Br] 0.27 0.82 0.130 4.1 0.36 0.18[C12-Im-PEK][Br] 0.44 1.44 0.212 2.7 0.19 0.085

a Diffusivity coefficient (10�8 cm2/s).b Solubility coefficient (10�2 cm3 (STP)/cm3 cm Hg).

I. Kammakakam et al. / Polymer 54 (2013) 3534e3541 3539

with bromide anions ([C2-Im-PEK][Br], [C6-Im-PEK][Br], and [C12-Im-PEK][Br], respectively). It should be mentioned that althoughanions of the ILs can also play an important role on CO2 dissolutionand hence on the gas separation properties of the correspondingpolymer membranes [28], the anion effects are not investigatedhere.

The 1H NMR spectra of the alkyl imidazolium-functionalizedPEKs 3 displayed the characteristic peaks of the imidazolium pro-tons (Hd) at 9.2 ppm and benzylic protons (Hc) of alkyl imidazolesat 5.5 ppm, indicating the successful incorporation of the imida-zolium groups (Fig. 1). The degree of functionalization was calcu-lated based on the integral ratio of the Hb protons in 2 to the Hc

Fig. 5. “Robeson upper bound 2008” plot (a) for comparing the O2/N2 separation performanccomparing the CO2/N2 separation performances of the Im-PEKs with the SILM and poly(ILs

protons in the polymer 3, and was found to be 100%. The Im-functionalized PEK structure was further verified by the FT-IRspectra by observing the peaks at 1450 cm�1, 1481 cm�1 and1560 cm�1, which correspond to the vibrational modes of theimidazolium cations and the benzene ring (Fig. 2) [29].

3.2. Preparation of the Im-functionalized PEK membranes

All Im-functionalized PEKs showed high solubility in commonorganic solvents, suggesting that thematerials could be processablefor preparing ultra-thin active layers, and they formed transparenttough and dense membranes adequate for gas permeation testing.

3.3. DSC and WAXS studies

The Tg values of all three Im-PEKs exceeded 200 �C (Table 1 andFig. 3) even in the presence of the pendant imidazolium groups,indicating that they were glassy polymers at r.t. The high Tg valuesare desirable for gas separation and aremuch higher than values forthe poly(IL)s and other typical glassy polymers such as polysulfonesor polyimides.

The fluorene unit is considered to increase the barrier to mainchain motion and increase the Tg of these cardo-based polymers.The Tg values of the three Im-PEKs decreased as the chain length ofthe imidazolium cation increased due to the plasticizing effects ofthe longer chains.

By contrast, the wide-angle X-ray scattering (WAXS) datarevealed that the intersegmental (d-) spacings between the poly-mer chains in themembranes remained constant irrespective of thealkyl chain length (Table 1 and Fig. 4). These effects should lead tohigher Im-PEK densities with longer alkyl chain substituents, andhence, lower free volumes. The [C12-Im-PEK][Br] yielded the largestdensity among the three polymers.

3.4. Gas separation properties

The pure gas permeabilities and permselectivities of the Im-PEKs at 2 atm and 30 �C are summarized in Table 2. The perme-abilities toward O2 and N2 decreased as the chain length of theimidazolium salts increased. The presence of the flexible alkyl

es of the Im-PEKs with previously described SILMs; data taken from Refs. [6,14], (b) for) performances reported previously; data taken from Refs. [6,14,17,30,31].

Table 5Tensile properties of the Im-PEK membranes 3.

Membrane Maximum tensilestrength, MPa

Elongationat break, %

Young’smodulus, GPa

[C2-Im-PEK][Br] 70.61 5.33 2.02[C6-Im-PEK][Br] 82.98 4.3 2.01[C12-Im-PEK][Br] 87.38 3.97 1.96

I. Kammakakam et al. / Polymer 54 (2013) 3534e35413540

chains in the Im-PEKs reduced the free volumes in the polymers,sharply reducing the permeability of the Im-PEKs that containedlonger alkyl chains (Table 2).

On the other hand, the Im-PEKs with longer alkyl chainsexhibited a dramatic increase in the permselectivity due to theenhanced diffusivity selectivity properties (1.2 for [C2-Im-PEK][Br],6.3 for [C6-Im-PEK][Br] and 6.8 for [C12-Im-PEK][Br].) (Table 3). The[C12-Im-PEK][Br] showed an extraordinary high O2/N2 permse-lectivity of 15.5. To the best of our knowledge, this is the highestselectivity value yet reported for polymer membrane-based sys-tems. These membrane properties are clearly useful for O2/N2separation in the context of an appropriate structural design.

As expected, the CO2 solubility in the Im-PEKs was much higherthan the O2 or N2 solubilities (Table 4). The cation with a C6-alkylchain showed a higher CO2 solubility than the C2-containing cation.The C12-alkyl substituent, however, reduced the CO2 solubility,possibly because the long alkyl groups on the cation stericallyhindered the interactions between CO2 and the cation [17]. Despitethe lower solubility of CO2, the diffusivity of CO2 in the [C12-Im-PEK][Br] was higher than that of C6 due to an increased chainflexibility (or mobility). As a result, the permeability of CO2 in thispolymer was slightly higher than in the C6-containing polymer.Nevertheless, the [C12-Im-PEK][Br] displayed a high CO2/N2 perm-selectivity of 66 due to its solubility selectivity (11 for [C2-Im-PEK][Br], 22.8 for [C6-Im-PEK][Br] and 31.8 for [C12-Im-PEK][Br]), ratherthan diffusivity selectivity (Table 3).

3.5. Permeability vs. selectivity for Im-PEKs

The O2 permeability versus O2/N2 selectivity values (Fig. 5a), andthe CO2 permeability versus CO2/N2 selectivity values (Fig. 5b) ofthe Im-PEKs with different alkyl chains are compared in the upperbound of the Robeson plot [4,6]. Data from the SILMs [14] andpoly(IL)s [17,30,31] are also included for comparison. The Robesonplot of the poly(IL)s for O2/N2 separation is not included here as ithas not been reported elsewhere. Although most membranes fallbelow the upper bound line for O2/N2, the [C12-Im-PEK][Br] lay onthe Robeson upper line for O2/N2, indicating that this polymer hasremarkable potential for achieving air separation.

None of the Im-PEKs developed here outperformed either theSILMs or the poly(IL)s for CO2 separation, and their performancesfell within the general range of the published data for poly(IL)s.A direct comparisonwith the poly(IL)s, however, could not bemade

Fig. 6. Stressestrain behavior of the [C2-Im-PEK][Br] (dotted curve), [C6-Im-PEK][Br](dashed curve) and [C12-Im-PEK][Br] (bold curve) membranes.

due to differences in the polymer backbone. Furthermore, thepolymer backbone in Im-PEKs 3 can potentially be fine-tuned byintroducing more permeable polymers, such as PESs or PIs, toimprove the gas separation performance while preserving thephysical properties of the material. The relatively low gas perme-abilities of these Im-PEKs might be due to either too much orinsufficient amounts of the cardo moiety in the polymer backbone.Similar results have revealed that an optimal cardo-moiety loadingis required to obtain large microvoid volumes and hence anenhanced permeability [32]. This factor can be further optimized.The Im-PEKmembranes prepared here provide a novel approach tointroducing alkyl imidazolium side groups into a variety of polymerstructures, including rigid backbones, such as those present inPEKs.

3.6. Mechanical properties

The mechanical properties of the Im-PEK membranes at 50% RHshowed excellent tensile strengths up to 87.38 MPa with Young’smoduli as high as 2.02 GPa (Fig. 6 and Table 5). The mechanicalstrengths of the Im-functionalized PEKs are sufficient for use inindustrial applications, such as hollow fiber membranes for gasseparations.

4. Conclusions

In conclusion, we prepared a series of PEKs, having pendantimidazolium groups with different alkyl chains for achievingenhanced gas separation. Several previously reported polymersprepared from IL-based monomers, such as poly(IL)s, and ionenes,in which ILs were incorporated into the polymer backbone, havedemonstrated promising CO2 separation due to high CO2 solubil-ities, however, none of these studies attempted to introducependant ILs, or ionic salts such as imidazolium salts presented inthis work, into the rigid polymers such as PEKs. In addition, the useof these IL-containing polymers, other than the SILMs, in O2/N2separation has not been previously investigated, whereas the Im-functionalized PEKs demonstrated here displayed the combinedO2/N2 and CO2/N2 separation properties in single polymericmembrane.

All of the membranes prepared here from the Im-functionalizedPEK displayed excellent thermal andmechanical stabilities. The Im-PEK polymer containing long alkyl chains, [C12-Im-PEK][Br],showed superior CO2/N2 and O2/N2 selectivities, however, the Im-PEKs must be further examined to elucidate the transport mecha-nism and to optimize the structural variation to improve thepermeability. The plasticization effect by CO2 on the gas separationproperties of the Im-PEK membranes should be investigated aswell. These will be the focus of our future studies. This simplestrategy may be readily applied toward preparing extraordinarypolymer membranes.

Acknowledgments

This work was supported by the Korea Carbon Capture andSequestration R&D Center under the Korea CCS2020 Program of the

I. Kammakakam et al. / Polymer 54 (2013) 3534e3541 3541

Ministry of Education and Science and Technology, Republic ofKorea. Part of this work was also supported by the University ofIncheon in 2011.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2013.05.006.

References

[1] Baker RW. Ind Eng Chem Res 2002;41:1393e411.[2] Chiappetta G, Clarizia G, Drioli E. Chem Eng J 2006;124:29e40.[3] Bernado P, Drioli E, Golemme G. Ind Eng Chem Res 2009;48:4638e63.[4] Robeson LM. J Membr Sci 1991;62:165e85.[5] Freeman BD. Macromolecules 1999;32:375e80.[6] Robeson LM. J Membr Sci 2008;320:390e400.[7] Bates ED, Mayton RD, Ntai I, Davis JH. J Am Chem Soc 2002;124:926e7.[8] Baltus RE, Counce BM, Culbertson BH, Lou H, DePauli DW, Dai S, et al. Sep Sci

Technol 2005;40:525e41.[9] Raeissi S, Peters CJ. Green Chem 2009;11:185e92.

[10] Kilaru PK, Scovazzo P. Ind Eng Chem Res 2008;47:910e9.[11] Kilaru PK, Condemarin RA, Scovazzo P. Ind Eng Chem Res 2008;47:900e9.[12] Anderso JL, Dixon JK, Brennecke JF. Acc Chem Res 2007;40:1208e16.

[13] Bara JE, Carlisle TK, Gabriel CJ, Camper D, Finotello A, Gin DL, et al. Ind EngChem Res 2009;48:2739e51.

[14] Scovazzo P. J Membr Sci 2009;343:199e211.[15] Cserjési P, Nemestóthy N, Belafi-Bakó K. J Membr Sci 2010;349:6e11.[16] Supasitmongkol S, Styring P. Energy Environ Sci 2010;3:1961e72.[17] Bara JE, Lessmann S, Gabriel CJ, Hatakeyama ES, Noble RD, Gin DL. Ind Eng

Chem Res 2007;46:5397e404.[18] Hu X, Tang J, Blasig A, Shen Y, Radosz M. J Membr Sci 2006;281:130e8.[19] Xiao Y, Low BT, Hosseini SS, Chung TS, Paul DR. Prog Polym Sci 2009;34:

561e80.[20] Hirayama Y, Yoshinaga T, Kusuki Y, Ninomiya K, Sakakibara T, Tamari T.

J Membr Sci 1996;111:169e82.[21] Camacho-Zuñiga C, Ruiz-Treviño FA, Hernández-López S, Zolotukhin MG,

Maurer FHJ, Gonzalez-Montiel A. J Membr Sci 2009;340:221e6.[22] Wang ZY, Moulinié PR, Handa YP. J Polym Sci Part B: Polym Phys 1998;36:

425e31.[23] Aitken CL, Koros WJ, Paul DR. Macromolecules 1992;25:3651e8.[24] Scholes CA, Kentish SE, Stevens GW. Recent Patents Chem Eng 2008;1:52e66.[25] Park HB, Han SH, Jung CH, Lee YM, Hill AJ. J Membr Sci 2010;359:11e24.[26] Kazama S, Teramoto T, Haraya K. J Membr Sci 2002;207:91e104.[27] Aguilar-Vega M, Paul DR. J Polym Sci Part B: Polym Phys 1993;31:1599e610.[28] Bara JE, Gin DL, Noble RD. Ind Eng Chem Res 2008;47:9919e24.[29] Qiu B, Lin B, Qiu L, Yan F. J Mater Chem 2012;22:1040e5.[30] Bara JE, Gabriel CJ, Hatakeyama ES, Carlisle TK, Lessmann S, Noble RD, et al.

J Membr Sci 2008;321:3e7.[31] Li P, Paul DR, Chung TS. Green Chem 2012;14:1052e63.[32] Yeong YF, Wang H, Pramoda KP, Chung TS. J Membr Sci 2012;397e398:

51e65.