European Polymer Journal 49 (2013) 4157–4166

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European Polymer Journal

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Cross-linking of Polymer of Intrinsic Microporosity (PIM-1) vianitrene reaction and its effect on gas transport property

0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.09.022

⇑ Corresponding author. Tel.: +49 (0) 4152 87 2425; fax: +49 (0) 415287 2499.

E-mail address: volkan.filiz@hzg.de (V. Filiz).

Muntazim Munir Khan, Gisela Bengtson, Sergey Shishatskiy, Bahadir N. Gacal,Md. Mushfequr Rahman, Silvio Neumann, Volkan Filiz ⇑, Volker AbetzHelmholtz-Zentrum Geesthacht, Institute of Polymer Research, Max-Planck-Str. 1, 21502 Geesthacht, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 June 2013Received in revised form 23 August 2013Accepted 17 September 2013Available online 27 September 2013

Keywords:Polymers of Intrinsic MicroporosityNitrene reactionCross-linkingMembranesGas separation

Polymer of Intrinsic Microporosity (i.e. PIM-1) has been crosslinked thermally via nitrenereaction using polyethylene glycol biazide (PEG-biazide) as a crosslinker. The crosslinkingtemperature was optimized using TGA coupled with FT-IR spectroscopy. The dense mem-branes containing different ratios of PIM-1 to PEG-biazide were cast from chloroform solu-tion. Crosslinking of PIM-1 renders it insoluble even in excellent solvents for theuncrosslinked polymer. The resulting crosslinked membranes were characterized by FT-IR spectroscopy, TGA and gel content analysis. The influence of crosslinker content onthe gas transport properties of PIM-1, its density and fractional free volume (FFV) wereinvestigated. Compared to the pure PIM-1 membrane, the crosslinked PIM-1 membranesshowed better gas separation performance especially for CO2/N2, CO2/CH4 and propyl-ene/propane (C3H6/C3H8) gas pairs and as well as suppressed penetrant-induced plasticiza-tion under high CO2 pressure.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Membrane technology has attracted great interest inparticular in gas separation since the last two decadesdue to its environmental and economical advantages, suchas reduced energy consumption, lower expenditure, andportability. Polymeric materials are promising candidatesfor fabricating membranes owing to their ease of process-ability, and low capital [1]. In the industrial gas separationsector, the gaseous feed stream may contain an amount ofover 50% carbon dioxide at a higher temperature and up toa feed pressure of 60 atm [2]. However, a major technolog-ical barrier to separate condensable gases (such as CO2)using polymeric membranes at high pressure is plasticiza-tion. Plasticization occurs when the CO2 concentration inthe polymer is high enough to increase free volume andsegmental mobility. Due to the swelling of the polymer

matrix, the permeation of other gases like CH4 is acceler-ated and as a consequence the polymer loses its selectivity[3]. This phenomenon therefore should be reduced in orderto make polymers attractive in membrane based separa-tion applications, such as natural gas upgrading, enhancedoil recovery operations (EOR), and landfill gas cleaning.Hence, it is necessary to develop a membrane for CO2/CH4 gas separation which can maintain the separation per-formance at sufficiently high driving pressures.

Furthermore, separation of olefin and paraffin gases isalso important for the petrochemical industry. Currently,the separation of olefin and paraffin components is per-formed by cryogenic distillation, which is expensive andenergy intensive due to the relative volatilities of thecomponents. Due to cost and complexity of distillation,membrane separations are being considered for propyl-ene/propane separation. Several studies have shown thatpolymeric membranes offer promising potential, but theirseparation selectivity needs to be improved to make themuseful as a cost effective separating membrane.

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In 2004, Budd et al. introduced a new class of polymers,termed ‘‘Polymers of Intrinsic Microporosity’’ (PIMs) as apromising material for polymeric membranes for gas sepa-ration [4–8]. PIMs possess unusually high free volume andhigh internal surface area, which can be mainly ascribed toits special ladder type main chain structure with contortedor kinked centres that prevent efficient space packing.PIM-1, the best known member of PIM family is widelystudied for gas transport properties due to the very prom-ising combination of its high molecular weight, solubilityin a number of organic solvents and attractive gas trans-port properties rising from stiff and contorted main chainwhich forms polymer matrix with a high degree of micro-porosity. PIM-1 has demonstrated extremely high gas per-meability with moderate to good selectivity for severalimportant gas pairs, since some separations such as O2/N2 have only moderate selectivity [6,9–12]. However, ithas been documented that, CO2 and other condensablegases at high pressure affects the CO2/N2 and CO2/CH4 sep-aration performance of PIM-1 membrane, due to ‘‘pene-trant-induced plasticization’’ [13,14]. This means at highpressure, adsorption of CO2 tends to swell the polymermatrix, leading to higher chain mobility which results incompaction and a change in cavity shape or size [15].Hence, instead of increasing permeability as observed intypical plasticization behaviour in glassy polymers, a sig-nificant decrease in CO2 permeability is observed forPIM-1 membranes [13,14].

Conventional methods for stabilizing polymericmembranes are either annealing or crosslinking. Severalcrosslinking methods for polymeric membranes were re-ported, which include thermal treatment [16], UV-photo-chemical [17] and chemical crosslinking. [18–21]. Thecrosslinking reaction described in this work capitalizeson the ability of alkyl biazides (N3–R–N3) to decomposeto molecular nitrogen (N2) and highly reactive divalentnitrene (— N :) radical at elevated temperatures. Since thebiazide decomposition reaction cleanly generatesnitrogen gas and reactive nitrenes intermediates as itsexclusive products, biazides are ideal candidates forcrosslinking PIM-1 because of their non-volatility and ab-sence of low molecular weight by-products during thecrosslinking reaction that could contaminate the polymer[22].

Although previous studies have shown that commer-cially available biazides are effective for increasing gas sep-aration performance and reduced plasticization of PIM-1membrane [14], olefin and carboxyl nitrenes tend to un-dergo intramolecular rearrangements which reduce oppor-tunities for bimolecular crosslinking reactions [23]. In thisstudy, the effect of using PEG-biazide (alkoxy biazide) ascrosslinker, synthesized with low molecular weight PEGchain, to crosslinked efficiently PIM-1. The permeabilityand diffusivity of H2, He, N2, O2, CO2 and CH4 in crosslinkedPIM-1 are reported, and these transport properties are cor-related with the FFV. The gas separation properties of theresulting crosslinked PIM-1 membranes were studiedincluding gas pairs such as CO2/N2, CO2/CH4. Moreover,the gas transport properties of C3H6 and C3H8 gases in highfree volume polymer PIM-1 and crosslinked PIM-1/biazidemembranes were also compared.

2. Experimental

2.1. Materials

The monomer 5,50,6,60-tetrahydroxy-3,3,30,30-tetra-methyl-1,10-spirobisindane (TTSBI, 97%) was obtained fromABCR, Germany and 2,3,5,6-tetrafluoroterephthalonitrile(TFTPN, 99%) was kindly donated by Lanxess (Germany).TFTPN was sublimated twice under vacuum prior to use.Potassium carbonate (K2CO3, >99.5%) was dried overnightunder vacuum at 120 �C in order to ensure no moisture istrapped in it and then milled in a ball mill for 15 min.Poly(ethylene glycol) (600 g/mol) (Merck), methanesulfo-nyl chloride (MsCl, P99.7%, Sigma–Aldrich), methylenechloride (P99.8%, anhydrous, Sigma–Aldrich), magnesiumsulphate (MgSO4, P99.8% (dried overnight prior to use),anhydrous, Sigma–Aldrich), triethylamine (TEA, P99.8%(GC), Fluka), hydrochloric acid (HCl, 37%, Merck), N,N-dimethylformamide (DMF, P99.8%, Merck), Sodium azide(NaN3, P99.0%, Aldrich), and tetrabutylammonium iodide(TBAI, P99.0%, Aldrich), diethylbenzene (isomeric mixture)from Sigma–Aldrich, dimethyl acetamide (DMAc, P99%),methanol (P99.9%), chloroform (CHCl3, 99.99%), dioxane(P99%), from Merck were used as received.

2.2. Synthesis of crosslinker (PEG-Biazide)

To a stirred solution of the PEG (15 g, 0.025 mol) in130 mL methylene chloride anhydrous magnesiumsulphate (MgSO4) was added for drying. After filtration toremove the MgSO4, the solution was cooled to 0 �C. Trieth-ylamine (TEA) (13.9 mL, 0.1 mol) and methanesulfonylchloride (MsCl) (7.8 mL, 0.1 mol) were added sequentially.Then the flask was kept at room temperature for 1 day. Dur-ing the reaction, the colour of the solution turned yellowishand some solid precipitated out. The reaction solution firstwas filtrated to remove the solid and then washed conse-quently with 400 mL 1 M hydrochloric acid (HCl), 400 mL1 M sodium hydroxide (NaOH), and 300 mL 1 M sodiumchloride (NaCl) solutions. The organic layer was dried overanhydrous MgSO4 overnight, and the solvent was removedin vacuum. The yield of the bifunctional mesyl PEG was ob-tained with the yield of 14.2 g (75.1%) and was character-ized by FT-IR and 13C NMR. The bifunctional mesyl PEG(14 g, 0.018 mol) was dissolved in 100 mL N,N-dimethyl-formamide (DMF). Sodium azide (4.8 g, 0.072 mol) and0.53 g tetrabutylammonium iodide (TBAI) were addedsequentially. The reaction solution was kept under stirringat 50 �C for 1 day, and DMF was removed by rotary evapora-tion. The resulting viscous liquid was dissolved in methy-lene chloride to remove undissolved solids by filtration.The organic solution was washed twice with water anddried overnight over MgSO4. Methylene chloride was evap-orated to give the PEG biazide (9.5 g, 80.9%) as a yellow vis-cous liquid. The PEG biazide was characterized by FT-IR and13C NMR spectroscopy. The infrared spectra were recordedon attenuated total reflectance (ATR) mode with a BrukerEquinox 55 FT-IR spectrometer. The transmission measure-ments were done in a spectral range of 400–4000 cm�1 witha resolution of 4 cm�1 and average of 64 scans. 13C NMR

M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166 4159

spectra were recorded in deuterated solvents (CDCl3) usingdept45 and deptq135 sequences employing a waltz-16decoupling scheme. Chemical shifts were reported in ppm.

2.3. Synthesis of polymer (PIM-1)

PIM-1 was prepared by using the fast synthesis methodproposed by Guiver et al. [23]. A slightly modified proce-dure for PIM-1 [24–26] was carried out as follows: TTSBIand TFTPN in equimolar ratio were dissolved in DMAc (or-ange–red solution). Addition of K2CO3 (2.3 times with re-spect to tetrahydroxy monomer concentration) under acontinuous flow of argon caused a colour change to yellow.The reaction mixture was immersed in an oil bath main-tained at 150 �C. After 5 min precipitate appeared and thereaction mixture became viscous, then diethylbenzene(DEB; same amount with respect to DMAc amount) wasadded into the reaction mixture, otherwise stirring couldnot be done easily. Stirring continued for a further onehour and the polymer was isolated by precipitation inmethanol and filtered off. The polymer was boiled in waterfor several hours to remove salts and solvent residues, andthen filtered off and dried overnight at 60 �C. The driedpolymer was dissolved in chloroform and precipitatedagain in methanol and then dried overnight at 60 �C in vac-uum to give PIM-1 in 90% yield.

2.4. Membrane preparation

Dense membranes of PIM-1 were prepared from a chlo-roform solution containing 2.5 wt.% of polymer. The PEGbiazide blend membranes were prepared by addingdifferent wt.% (on solid basis) of PEG-biazide crosslinkerto PIM-1 solution The reported CL concentrations (wCL)on total solid basis and thus in the final membrane arecalculated according Eq. (1):

wCL ¼mCL

mCL þmP� 100 ð1Þ

where mCL is the CL mass and mP is the polymer mass. Thesolutions were magnetically stirred for 24 h at room tem-perature and cast in a Teflon dish, which was covered to re-duce evaporation rate under nitrogen atmosphere. Thesemembranes were not treated with methanol (because ofsolubility of PEG biazide) and dried in high vacuum for16 h at 120 �C. The thickness of the membranes was mea-sured by a digital micrometer (Deltascope� MP2C), andwas in the range of 100–120 lm.

2.5. Thermal crosslinking treatment

The crosslinking reaction temperature was investigatedand optimized using thermogravimetric analysis coupledwith FT-IR (Bruker Equinox 55, TGA-IR). The thermal cross-linking procedure was conducted under a nitrogen envi-ronment in a tube furnace, which was fitted with aninternal thermocouple for temperature control. Thefurnace was purged with nitrogen prior to the thermalcrosslinking reaction. The crosslinking reaction was donein the absence of oxygen because nitrene oxidizes tonitroso compound, which reduces the effectiveness of the

crosslinking reaction. The membranes were placed in aceramic boat and furnace temperature was raised to250 �C at a rate of 1 K/min and held for a period of 1 h.After a thermal crosslinking treatment, the membraneswere cooled naturally in the furnace to room temperatureand stored in desiccators for further studies. A clear color-ation change from yellow to brown and dark brown wereobserved for the thermally treated membranes dependingon the composition of PEG-biazide in the PIM-1. The densemembrane films were labelled as ‘‘PIM-composition-uncrosslinked’’ and ‘‘PIM-1-composition-crosslinked’’, forexample, PIM-5PB-UnX and PIM-5PB-X, respectively.

2.6. Membrane characterization

The average molecular weight (Mw) and molecular dis-persity (Mw/Mn) of the prepared PIM-1 is 2.21 � 105

(g mol�1) and 4.8, respectively, determined by size exclu-sion chromatography (SEC) in chloroform against polysty-rene standards. A possible structural change of PIM-1crosslinked membranes before and after the thermal cross-linking process has been monitored by FTIR analysis. Themeasurements were conducted using attenuated totalreflectance (ATR) mode of the Bruker Equinox 55 FT-IRspectrometer. The measurements were done in a spectralrange of 400–4000 cm�1 with a resolution of 4 cm�1 andaverage of 64 scans. The gel content of a crosslinked mem-brane with respect to the PEG-biazide content was deter-mined by soaking the respective membranes in CHCl3

(solvent for film casting) for 24 h. The insoluble membraneportion was dried under high vacuum at 120 �C for 16 h toremove the residual solvent before weighing. The gelcontent (GC) was calculated by Eq. (2)

GCð%Þ ¼ M1

Mo� 100 ð2Þ

where M0 and M1 are the weights of the crosslinked mem-brane before and after soaking in CHCl3, respectively.

Thermogravimetric analysis (TGA) measurements weredone using Netzsch TG209 F1 Iris, under constant argonflow of 20 mL/min at a constant heating rate of 10 K/min.The density of the membranes was determined by buoy-ancy method following Eq. (3)

q ¼ MA

MA �MLqo ð3Þ

where q and qo are the densities of the blend membraneand perfluorinated liquid (FC 77) and MA and ML the weightof membrane in air and perfluorinated liquid, respectively.All the density measurements were done at 23 �C.

The FFV of the pure PIM-1 and the thermally cross-linked membrane were evaluated based on measured den-sity values and Bondi’s group contribution method [27–29]using Eq. (3)

FFV ¼ 1� Vo

V

� �ð4Þ

where V is the specific volume (cm3/g) of the polymercalculated from the measured density, Vo is the volume(cm3/g) occupied by the polymer molecules and equals to1.3 times of the polymer’s van der Waals volume (Vw).

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2.7. Gas transport properties

Permeabilities of single gases (O2, N2, CH4 and CO2)were measured using a constant volume variable pressuretime lag apparatus at 30 �C. Due to lack of facility of mixed-gas measurement for dense film, only single gas measure-ments were done for present work. The permeability (P),diffusivity (D), solubility (S) and selectivity (a) for gases Aand B were determined under steady state by the followingequations [30,31]:

P ¼ D:S ¼Vplðpp2 � pp1Þ

ARTDtðpf � ðpp2 þ pp1Þ=2Þ ð5Þ

D ¼ l2

6hð6Þ

aA=B ¼PA

PB¼ DASA

DBSBð7Þ

where Vp is the constant permeate volume, R the gas con-stant, l the film thickness, A the effective area of mem-brane, Dt the time for permeate pressure increase frompp1 to pp2, pf is the feed pressure, and h is called time-lag.The solution–diffusion transport model [32] was appliedfor discussing the gas transport properties of PIM-1 cross-linked membranes, and the selectivities of membranes forgas ‘‘A’’ relative to another one ‘‘B’’ which is the ratio oftheir permeabilities were determined by using Eq. (7).

Diffusion of small molecules in the dense polymericfilms is also understood to be a function of polymer freevolume, and the following Eq. (8) is often used to describethis dependence [33].

D ¼ C exp � chVf i

V�A

� �ð8Þ

where C is a pre-exponential factor, c is a numerical factorintroduced to account for overlap of free volume, V�A is theminimum free volume element size needed to accommo-date a gas molecule, and hVf i is the average free volumein the polymer. hVf i is usually replaced by fractional freevolume (FFV) in gas diffusion studies and calculated bygroup contribution method described by Bondi’s method(see above).

Because solubility is typically a weak function of freevolume, permeability often exhibits a similar dependenceon free volume as diffusivity, and given by the followingEq. (9).

P ¼ SC exp � chVf i

V�A

� �ð9Þ

3. Results and discussion

3.1. Synthesis of polymer

PIM-1 was synthesized by reacting molar equivalents of5,50,6,60-tetrahydroxy-3,3,30,30-tetramethyl-1,10-spirobisin-dane (TTSBI) and 2,3,5,6-tetrafluoroterephthalonitrile(TFTPN) with an excess of K2CO3 (Scheme 1). PIM-1 synthe-sis procedure was reported elsewhere [25,26,34], which is

different from previous polymerization protocols [35,36].Diethylbenzene (DEB) was introduced to this reaction sys-tem instead of toluene, because of its higher boiling point(150 �C) [34,37]. DEB was added into the reaction mixture,otherwise it could not be stirred easily. The polymer wascharacterized by 1H NMR spectroscopy and SEC.

3.2. Synthesis of crosslinker and its characterization

In this study, we synthesized a PEG-Biazide by a simpletwo-step reaction performed on PEG [38]. In a first step,mesylation of PEG with methanesulfonyl chloride (MsCl;1:4; in terms of OH-moieties) was conducted as describedin experimental section to obtain PEG with mesyl endgroups. And in a second step, the resulting mesylatedPEG polymer was then converted into PEG biazide in pres-ence of NaN3/DMF at 50 �C [Scheme 2].

The extent of the modification was determined using13C NMR spectra (CDCl3). The signal of neighbouring car-bons of hydroxyl end groups appeared at 61.89 ppm. Aftermesylation and azidation, the migrations of this signal to38.03 ppm and 50.62 ppm, respectively, give reliable evi-dence about the successful reaction (Fig. 1). The effectivemesylation and azidation were further evidenced by thedisappearance of the hydroxyl stretching band at3456 cm�1 and the appearance of the azide stretchingband at 2098 cm�1, respectively, in the FT-IR spectra ofPEG, PEG-bimesyl, and PEG-biazide (Supplementary info.Fig. 1).

A variety of commercially available biazide had beenconsidered for these studies. But so far, no literature isavailable on using PEG-biazide as a crosslinker. Also a fac-ile synthetic approach was used here for the synthesis ofPEG-biazide. Moreover, PIM-1 films crosslinked with PEGbiazide were less soluble in CHCl3 (a good solvent forPIM-1) than films crosslinked with equivalent amounts ofother biazide considered, such as 2,6-di(p-azidobenzyl)-4- methylcyclohexanone and 4-azido phenylsulfone [14].

3.3. Dense membrane characterization

To investigate the crosslinking process in PIM-1/biazidemembranes, TGA measurements coupled with FTIR of theevolved gases were conducted on the PIM-20PB-UnXmembranes. Fig. 2 shows that the three dimensional (3D)view of maximum intensity of FTIR signals is connectedto maximum gradient of weight loss in the TGA trace.

TGA mass loss analysis for PIM-20PB-UnX subjected toa thermal ramp up to 600 �C under argon atmosphere ata rate of 5 K/min. The two dimensional version of Fig. 2is shown in Fig. 3. The initial mass loss of 1.5 wt.% occur-ring at 200 �C to 280 �C is attributed to almost completedecomposition of azide group, compared with the calcu-lated response for decomposition of –N3 groups in PIM-20PB-UnX membrane and does not show an gaseousdecomposition product in IR spectra because of IR inactiveN2 gas (Fig. 3 at 240 �C and shaded region in Fig. 1). Thethermal decomposition of the biazides proceeds with aninitial degradation of the –N3, releasing nitrogen gas (N2)and leaving behind highly reactive divalent nitrene — N :

radical. After azide decomposition, the resulting nitrenes

Scheme 1. Synthesis of PIM-1, a – Reagent and condition, K2CO3, DMAc, DEB, 150 �C, 1 h.

Scheme 2. Synthesis of PEG-biazide.

Fig. 1. 13C-NMR spectra of (1) PEG, (2) PEG-bismesyl and (3) PEG-Biazide in CDCl3.

M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166 4161

insert into an alkyl group (such as >CH2 and CH3) in thePIM-1 spirobisindane segment to form a covalent network.Moreover, PEG-biazide is also capable of reacting with it-self to form a network through the nitrene and alkyl group(such as >CH2 of PEG chains). Hence, under a variety of dif-ferent reactive sites, PIM-1/biazide membranes are capableof forming crosslinked polymer network. The significantsecond mass loss between 300 and 425 �C, approximately15–16 wt.% loss was observed in the mixtures, which cor-responds to decomposition of PEG chains in biazide cross-linker. Analysis of gaseous decomposition product by FTIRspectra (Fig. 3 at 390 �C) shows a band at 1000 cm�1,1700 cm�1 and 3000 cm�1 which fitted with PEG referencegas spectra (from Merck). Furthermore, the weight lossoccurring after 450 �C is caused mainly by the decomposi-tion of the polymer backbone of PIM-1. Moreover the gas-eous decomposition product by FTIR spectra at 480 �Cshows a band around 2150 cm�1 attributed to CO2, a band

at 2310 cm�1 corresponding to CO2 and a band at3000 cm�1 corresponding to CH4 (Fig. 3 at 480 �C). Sincethe crosslinking reaction temperature observed at or below300 �C, carbon evolution from polymer main chain degra-dation which begins at 400 �C does not appear to beresponsible for observed crosslinking.

During the whole process of crosslinking, a relativelygradual degradation rate with increasing temperaturewas observed. From these TGA-FTIR results, we can con-clude that the PIM-1 polymer is a suitable precursor formaking crosslinked membranes via nitrene reaction.

3.4. Characterization of thermally crosslinked PIM-1/biazidemembranes

From the TGA-FTIR data, it is confirmed that the initialmass loss of �1.5% observed between 230 and 250 �C inuncrosslinked membranes attributed to N2 decomposition

Fig. 2. 3D TGA-FTIR spectra and TGA trace of uncrosslinked PIM1-20-PB-UnX.

Fig. 3. Comparative TGA-FTIR spectra of PIM-1 an uncrosslinked PIM-20PB membranes.

Fig. 4. Comparative FTIR spectra of PIM-1, uncrosslinked PIM-20PB andcross-linked PIM-20PB membranes.

4162 M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166

and generation of nitrene radical for crosslinking (Fig. 3 at240 �C). Therefore PIM-1/biazide membranes were cross-linked at 250 �C and its procedure was described abovein Section 2.5. From the previous studies, Du et al. reportedan azide based crosslinking approach with PIM-1, usingcommercial biazide, which required 7.5 h at 175 �C [14].In this study, we explore the same crosslinking approach,using a different biazide, which is easily synthesized inan efficient way and crosslinked at 250 �C for 1 h only. Alsowe did not observe any crosslinking below 250 �C whichmost likely due to the insufficient annealing temperature.

The uncrosslinked and crosslinked PIM-1/biazide mem-branes were characterized by FTIR and the only significantdifference between the spectra of uncrosslinked(PIM-20PB-UnX) and crosslinked (PIM-20PB-X) is the dis-appearance of the azide peak at 2120 cm�1 in PIM-20PB-X, indicating the conversion of biazide crosslinker to areactive nitrene radical (Fig. 4).

PIM-1/biazide crosslinked and uncrosslinked mem-branes (PIM-20PB-X) were also characterized by TGA,and the results compared with pure PIM-1 membranewere shown in Fig. 5. Like PIM-1, none of the crosslinked

and uncrosslinked films had an observable Tg in the rangeup to 400 �C likely due to absence of rotational freedom oflinkage in polymer chain. The resulting crosslinked mem-brane has a lower thermal stability than the pure PIM-1.As a result, the initial weight loss of the crosslinked mem-brane observed below 450 �C is mainly attributed to thedegradation of PEG chains in crosslinker and decomposi-tion of –N–C, which resulted from the nitrene reaction.

Moreover, the crosslinked polymer was also analysedfor its gel content and calculated according to Eq. (2).PIM-1 is readily soluble in common solvents such as chlo-roform, THF, dichloromethane, dichlorobenzene. Differentfrom the PIM-1, the resulting crosslinked PIM-1/biazidefilms were insoluble in common solvents mentionedabove, even at high temperature, which was in contrastwith PIM-1b (PIM-1 membrane annealed at 250 �C for1 h) subjected to the same thermal treatment. Withincreasing the PEG biazide concentration on polymer ma-trix, the gel content of the crosslinked film increases andwe observed that for high crosslinker content crosslinkedfilm (i.e PIM-20PB-X) has a remaining gel content >95%.The conclusions drawn from the FTIR and gel content anal-ysis is the nitrene reaction generated by PEG-biazide inpresence of PIM-1 led to an insoluble crosslinked network.

3.5. Gas transport properties of thermally crosslinked PIM-1/PEG biazide membranes

3.5.1. Effect of crosslinker on pure gas separation performanceSingle gas permeation test of PIM-1 and crosslinked

PIM-1/biazide (without methanol treated) membraneswith different ratios of crosslinker were carried out bythe time-lag method, measured at 30 �C and 1 bar feedpressure. The crosslinked membranes were not treatedwith methanol due to solubility of PEG chains in methanol.Therefore all data shown in this work were without meth-anol treated membranes. For single gases (i.e. O2, N2, CH4,and CO2), the permeability decreases with the content ofthe crosslinker. This leads to the decrease of density andfraction free volume too. Fig. 6 shows a typical exampleof such results for PIM-1 and crosslinked PIM-1/biazide

Fig. 5. TGA of PIM-1, crosslinked PIM-20PB (inset-red) and uncrosslinkedPIM-20PB (inset-blue) membranes. (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version ofthis article.)

Fig. 6. Effect of crosslinker content on initial permeability and FFV ofPIM-1 and crosslinked PEG-biazide membranes.

M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166 4163

with different ratios of crosslinker. N2 permeability de-creases from 548 to 14 barrer as the crosslinker content in-creases from 0 to 20 wt.%, while the FFV decreases from0.208 to 0.153. The FFV values of the crosslinked samplesare calculated using measured density values and Bondi’sgroup contribution method [27–29]. It is of interest tounderstand whether the decrease in FFV associated withhigher crosslinker content is due to the densification in-duced by the thermal annealing protocol or related to thepresence of crosslinker in PIM-1. In this regard, the FFVof a PIM-1 film containing 10 wt.% crosslinker is the samebefore and after exposure to the crosslinking condition(250 �C for 1 h in N2 atmosphere). Therefore, the reductionin free volume is due to the mixing of high free volumePIM-1 with crosslinker, which is expected to have a muchlower inherent FFV than PIM-1.

The decrease in permeability can be correlated with thedecrease in FFV [39]. The relationship between He, H2, N2,O2, CO2 and CH4 permeabilities and FFV is shown in Fig. 7.The N2 permeability decreases from 548 to 14 barrer, andthe CO2 permeability decreases from 10667 to 433 barreras the biazide content increases from 0 to 20 wt.%. A strongrelationship between permeability and 1/FFV is observedfor He, H2, O2 and CH4 which is consistent with Eq. (9).

Gas solubility is not measurably influenced by cross-linking, the decrease in gas permeability in crosslinkedPIM-1 might be due to a decrease in gas diffusivity. Fig. 8shows the relationship between gas diffusivity and 1/FFV.The diffusivity decreases as FFV decreases and crosslinkercontent increases. This result is expected from Eq. (8).According to Eq. (8), the diffusion coefficient is a functionof penetrant size and polymer average free volume. Asthe average free volume increases, the effect of penetrantsize on diffusivity diminishes, so the polymer becomes lesssize-sieving [15]. One more reason for the decrement ofFFV and gas permeabilities could be the flexible PEG chainsoccupy the rigid and contorted PIM-1 polymer chains. Thistrend is especially valid for high free volume polymers andthe same behaviour was found by Kleman et al. for the high

free volume glassy polymer PTMSP [40]. The H2 and He dif-fusion coefficients are an order of magnitude larger thanthose of CH4, CO2, N2 and O2, since the kinetic diameterof these gases is at least 1 ÅA

0

larger than that of He and H2

[41] .Fig. 9 shows the CO2/N2, CO2/CH4 and O2/N2 selectivi-

ties, measured for single gases, for PIM-1 and the curedPIM-1/biazide membranes of different crosslinker contentmeasured at 303 K. These selectivities were enhancedcould be due to an increase in the diffusivity selectivities.

3.5.2. Effect of pressure on pure gas separation performance ofPIM-1 and crosslinked PIM-1/biazide membranes

Pressure effects on membrane performance were stud-ied over a feed pressure range of 5–30 atm at 25 �C. Fig. 10shows the pure gas selectivities of PIM-1 and crosslinkedPIM-1/biazide membrane as a function of the feed pressure(up to 30 atm) for CO2/N2 and CO2/CH4 gas pairs. The indi-vidual permeabilities of CO2, CH4 and N2 for Fig. 10 areshown in the Supporting Information Fig. 2(A–C).

The sorption and transport behaviour of especially CO2

in glassy polymers has been extensively studied because ofits potential application in several areas. However, it isrecognised that CO2 acts as a plasticizer at sufficiently highlevel of concentration [42]. The typical plasticizationbehaviour of polymer membrane was observed by de-crease in permeability with lower CO2 pressure, followedby an increase in permeability at higher CO2 pressure[2,18]. In contrast with the majority of the glassy polymers,PIM-1 does not suffer serious plasticization and show a de-crease in permeability with increasing of CO2 feed pres-sure. A similar phenomenon was observed by Du et.al[13,14]. This might be due to the microporous structuresare affected by adsorbed CO2, which led to a less increasein segmental mobility. With increasing CO2 pressure, theintrinsically microporous structure begin to collapse, dueto the initially large free volume and increased overallchain movement, though no rotational chain mobility ispossible in ladder type polymers [13]. In comparison tocrosslinked PIM-1/biazide membrane, the CO2 permeabil-ity decrement of PIM-1 by 20% at 30 atm while N2 andCH4 permeability of PIM-1 declined by 7–8% at the samepressure [see Supporting information Fig. 2(a–c)].

Fig. 7. Permeability of N2, CH4, O2, He, H2, CO2 in PIM-1 and crosslinkedPIM-1 PEG-Biazide membranes Vs 1/FFV.

Fig. 8. Diffusivity of N2, CH4, O2, He, H2, CO2 in PIM-1 and crosslinkedPIM-1 PEG-Biazide membranes Vs 1/FFV.

Fig. 9. Gas pair selectivity of PIM-1 and crosslinked PIM-1/Biazidemembranes at 30 �C.

Fig. 10. Pure gas selectivity (a) CO2/N2 and (b) CO2/CH4 for pure PIM-1and crosslinked. PIM-1/biazide membranes as a function of different feedpressure (s PIM-20PB-XL; D PIM-10PB-XL; e PIM-5PB-XL; h pure PIM-1).

4164 M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166

Compared to PIM-1, the permeability decrement of cross-linked PIM-1/biazide membrane at high pressure was less

and also the selectivity for the crosslinked PIM-1/biazidemembrane decreased to a lesser degree than pure PIM-1for CO2/N2 and CO2/CH4 gas pair, which indicates that the‘‘penetrant-induced plasticization’’ was suppressed.

3.5.3. Pure gas permeation of C3H6, C3H8 and its separation inPIM-1 and crosslinked PIM-1/biazide membranes

Table 1 shows the separation performance of PIM-1a

(without methanol treated), PIM-1b (without methanoltreated and annealed at 250 �C for 1 hr), PIM-1c (withmethanol treated) and crosslinked PIM-1/biazide mem-branes (without methanol treated) for single gas C3H6

and C3H8 measured by time lag apparatus at 1 atm feedpressure and 30 �C. PIM-1a,b,c membranes shows the sepa-ration factor �7.4 and the permeability of propylene in dif-ferent order, but highest among all other glassy polymersuch as PPO, 6FDA-6FpDA [43,44]. The crosslinked PIM-1/biazide membranes with different crosslinker contentshow the separation factor 8.3 to 9 and stable for longterm. Therefore, PIM-1a,b,c and crosslinked PIM-1/biazidemembranes in this study can meet both the selectivityand permeability requirement for a membrane process tobe economically viable.

Separation of propylene (C3H6) from propane (C3H8) hasbeen studied by many types of membranes such as Matrimid,polysulfone (PSF), poly(2,6-dimethyl- 1,4-phenyleneoxide)(PPO) and 4,4’-(Hexafluoroisopropylidene)diphthalic anhy-dride-4,40-Oxydianiline (6FDA-ODA) [43–45]. However, the

Table 1Pure gas C3H6 permeability and C3H6/C3H8 selectivity for PIM-1 andcrosslinked PIM-1/biazide membranes.

Membrane (C3H6) Permeability(Barrer)

Selectivity(C3H6/C3H8)

PIM-1a 1051 7.5PIM-1b 439 7.3PIM-1c 288 7.4PIM-5PB-XL 58 8.3PIM-10PB-XL 35 8.8PIM-20PB-XL 18 9

1 Barrer ¼ 1� 10�10:cm3

ðSTPÞ � cm

cm3 � sec �cmHga Without methanol treatment.b With methanol treatment.c Without methanol treatment annealed at 250 �C for 1h.

Fig. 11. Pure gas C3H6 permeability and C3H6/C3H8 selectivity for PIM-1and crosslinked PIM-1/biazide membranes in comparison with literaturedata. (awithout methanol treated, bwithout methanol treated andannealed at 250 �C for 1 h, cwith methanol treated membranes).

Table 2Pure gas permeability, diffusivity of PIM-1 and kinetic diameter of C3H6 andC3H8.

Gas Permeability(Barrer)

Diffusivity(cm2/sec) � 10�8

Kinetic

diameter (ÅA0

)

Propylene(C3H6)a

1051 1.27 4.0

Propane(C3H8)a

140 0.17 4.3

a Without methanol treatment PIM-1 membrane.

Table 3Ideal separation characteristics of PIM-1 and comparison with literaturedata for C3H6/C3H8 separation.

Polymer Temperature(�C)

FeedPressure

Permeability (Barrer)

Propylene(C3H6)

Propane(C3H8)

PIM-1 30 1 atm 1051 140PPOa 30 2–3 atm 2.3 0.256FDA-6FpDAb 35 3 atm 0.89 0.05

a Ref. [43].b Ref. [44].

M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166 4165

performance is often limited and shows an ‘‘upper boundtrade off curve’’ between permeability and selectivity.

A plot of literature data [46] shows an upper bound forC3H6/C3H8 selectivity (straight line in Fig. 11). Compared toprevious reported glassy polymers PIM-1a,b,c and cross-linked PIM-1/biazide membrane exhibit high permeabilitycoupled with moderate selectivity that combines to givevalues that exceed the observed upper bound for theC3H6/C3H8 gas pair, indicating that the overall performance

of these membranes is superior and thus more commer-cially attractive.

In order to explain the permeability of C3 hydrocarbonsin glassy polymers, the resulting data were correlated withthe shape of the penetrating molecules. Table 2 shows theC3H6 and C3H8 permeability, diffusivity as well as theirrespective kinetic diameter of PIM-1 membrane (withoutmethanol treated).

Table 2 depicts that, the pure PIM-1 permeability anddiffusivity for C3H6 is higher than C3H8. Due to the smallerkinetic diameter, C3H6 penetrates and diffuses faster thanC3H8. The permeability of C3 hydrocarbon in glassy poly-mer also depends on the polymer structure, i.e. the freevolume. For high free volume polymer PIM-1, the C3H6

and C3H8 permeability was found much higher comparedto the PPO, 6FDA-6FpDA glassy polymer as shown inTable 3.

These results revealed that PIM-1 and PIM-1/biazidecrosslinked membranes have potential for industrial pur-pose in C3H6/C3H8 gas pair separation and the gas transportproperties can be tuned up by controlling the degree ofcrosslinking.

4. Conclusions

The low molecular weight PEG-biazide was successfullysynthesized, characterized and employed as a crosslinkerfor PIM-1 polymer. Crosslinked membranes structure de-rived from a blend of PIM-1 and biazide precursors wasachieved via nitrene reaction by annealing the membranesat 250 �C for 1 h. Based on this method, a highly reactivenitrene radical generated in situ and forms a covalent net-work within the PIM-1 polymer chains. The gas separationproperties such as permeability and diffusivity of cross-linked PIM-1 membrane decreased as crosslinker contentincreased and were proportional to the exponential of 1/FFV, consistent with the free volume theory. The resultsof CO2/CH4 and CO2/N2 gas pair separation over a feedpressure range (5–30 atm) indicate that, crosslinked PIM-1 membrane exhibit suppressed penetrant-induced plasti-cization in presence of condensable gases. Compared topure PIM-1 membrane, crosslinked PIM-1 membrane showhigher O2/N2, CO2/N2 and CO2/CH4 selectivities, coupledwith an unsurprising reduction in gas permeabilities. Inaddition, PIM-1 and crosslinked PIM-1 membranes en-hanced selectivity for the gas pair propylene/propane

4166 M.M. Khan et al. / European Polymer Journal 49 (2013) 4157–4166

(C3H6/C3H8) which exceeds the experimentally observedC3H6/C3H8 upper bound.

Acknowledgments

The authors would like to thank Silke Dargel and PetraMerten for technical support. This work was financiallysupported by Helmholtz Association of German ResearchCentres through the Helmholtz Portfolio MEM-BRAIN.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2013.09.022.

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