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Journal of Membrane Science 385– 386 (2011) 76– 85

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

j ourna l ho me pag e: www.elsev ier .com/ locate /memsci

tudy on the morphology and gas permeation propertyf polyurethane membranes

orteza Sadeghia,∗, Mohammad Ali Semsarzadehb, Mehdi Barikanic, Behnam Ghaleib

Chemical Engineering Department, Isfahan University of Technology, Isfahan 84156-8311, IranChemical Engineering Department, Tarbiat Modares University, 14155-143, Tehran, IranIran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 15 April 2011eceived in revised form4 September 2011ccepted 15 September 2011vailable online 21 September 2011

eywords:olyurethaneas separationembrane

a b s t r a c t

This work presents the structural properties of gas permeation of a group of polyurethane membranes. Allpolymers were synthesized using a 1:3:2 molar ratio of polyol:diisocyanate:chain extender. The obtainedresults from Fourier transform infrared spectrometer (FT-IR) of polymers indicate that by changing thediisocyanate from aromatic to linear aliphatic, the microphase separation of hard and soft segmentsincreases. Study of the differential scanning calorimetery (DSC) and X-ray diffraction (WAXD) patternsconfirmed that PTMG and PCL polyols can be arranged in small crystalline structures. Furthermore, ahard ordered segment in crystal phase could be obtained, owing to the high phase separation of thepolymer based on HDI. Permeation measurements of polymers revealed that the permeability of gasesincreases with microphase separation in polymer and selectivity of gases drops down. Polymers based onPPG showed the highest phase separation and permeability. The obtained results revealed more phase

orphologytructure

mixing of the polymer based on DMPA chain extender in comparison to BDO, which lead to lower perme-ability and higher selectivity. The solubility and diffusivity of gases indicate solubility domination of gastransport in these membranes. The observed solubility domination increases by phase separation. Theresults of permeability tests indicate high permeability, up to 186 Barrer (1 Barrer = 1 × 10−10 [cm3 (STP)cm/cm2 s cmHg]), and high selectivity for carbon dioxide with respect to nitrogen (CO2/N2: 45). Our datasheds light on gas permeation properties of polyurethane membranes.

. Introduction

The use of gas separation membranes has recently become annteresting alternative to other conventional methods because ofts energy conservation and reduction of emission of the envi-onmental pollutants. Applications of polymeric membranes asas separation membranes are used in a wide variety of areasuch as air separation, separation of carbon dioxide from natu-al gas and removal of hydrogen from mixture with hydrocarbonsn petrochemical processing. The enhancement of the gas separa-ion property of the polymeric membrane is the target of the mostesearches in this area. Finding the structure and gas separationroperties of the polymeric membrane is the key factor to reachetter gas separation efficiency in these membranes. Research on

tructure and property relationship in polyimides [1–3], polysul-ones [4,5], polycarbonates [6], polydimethylsiloxanes [7,8] andther polymers are examples of these efforts.

∗ Corresponding author. Tel.: +98 311 3915645; fax: +98 311 3912677.E-mail address: m-sadeghi@cc.iut.ac.ir (M. Sadeghi).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.09.024

© 2011 Elsevier B.V. All rights reserved.

In the search for superior membranes, a number of investigatorshave studied the gas permeation properties of polyurethanes (PUs)[8–10]. Polyurethanes are materials, with possibility of tailoringtheir transport properties by varying the polymer microstructure.PUs are usually composed of a polyether or polyester soft segmentand a hard segment. They usually have a micro-phase separatedstructure due to the incompatibility between the soft and hard seg-ments [9]. The hard segment is formed by extending a terminaldiisocyanate with a low molecular weight diol or diamine. Factorssuch as the kind of the reagents, their initial composition and thesynthesis method can be used to change the structure and mor-phology of the resulting polyurethanes. This consequently affectsthe transport properties of polyurethane membranes and resultsin change of phase-separated domain morphology, polyfunctionalcross-linking, density and glass transition temperature of mem-branes. To design and obtain membrane materials with betterperformance, the structure and properties relationship have to beknown. The studies of gas transport properties of the polyurethane-

based membranes have shown that the length of polyol, the typeof soft segment, and the type and proportion of the hard segmentinfluence the permeation properties of the membranes [11–13].In addition, the type of chain extender also plays an important

brane Science 385– 386 (2011) 76– 85 77

roaegWha[maobg

iiptobHtmPetstmd

bti(

mpasotf

2

2

PSa(fbacp

2

mf

Table 1Composition and Tgs of synthesized polyurethanes.

Polymer Polyol Diisocyanate Chain extender Tg (◦C)

PPG–TDI–BDO PPG TDI BDO −40PPG–IPDI–BDO PPG IPDI BDO −52.6PPG–HDI–BDO PPG HDI BDO −60.3PTMG–TDI–BDO PTMG TDI BDO −55PTMG–IPDI–BDO PTMG IPDI BDO −75PTMG–HDI–BDO PTMG HDI BDO −78PCL–TDI–BDO PCL TDI BDO −24.3PCL–IPDI–BDO PCL IPDI BDO −38.3

M. Sadeghi et al. / Journal of Mem

ole in permeation properties of the membranes due to the changef the phase-separated domain morphology, crystallinity, density,nd glass transition of the membranes [14]. Galland and Lam havevaluated the effect of hard segment content on the polyurethaneas permeability by varying the molecular weight of the polyol [15].olinska-Grabczyk and Jankowski [11], Knight and Lyman [16]

ave also investigated the influence of hard segment on gas perme-bility of the PUs by changing the length of polyols. Freeman et al.17] and Lee et al. [18] have investigated effect of polyol type on per-

eation and microstructure of PU membranes. Huang and Lai [19]nd Ruaan et al. [10] have studied the effect of hard segment contentf hydroxyl terminated polybutadiene based polyurethane mem-ranes by measuring the permselectivity of oxygen and nitrogenases through synthesized membranes.

In our previous works, we have shown that permeabilityncreases with the length of chain extender [13] and that byncreasing the urea linkage and increasing chain mobility ofolymers, diffusivity and permeability of gases increase and selec-ivity decreases [20]. Particularly, many studies on the polyetherr polyester-based PU membranes have found the relationshipetween the structure and gas property of PU membranes [12–20].owever, there were not a complete investigation on the simul-

aneous changes of soft and hard segments and their effects onicro-phase separation and consequently transport properties of

U membranes. In this study we tried to find a comprehensivexplanation for the relationship between gas permeation proper-ies and the microstructure of PU membranes specifically phaseeparation between hard and soft domains. In this way, we inves-igated the permeability of oxygen, nitrogen, carbon dioxide and

ethane through series of PUs varying in the nature of the macro-iol and diisocyanate segments.

The effect of diisocyanate on the gas permeation properties of PUased membranes was studied by use of an aromatic diisocyanate,oluene diisocyanate (TDI), and two types of aliphatic diisocyanate:sophorone diisocyanate (IPDI) and hexamethylene diisocyanateHDI).

The effect of polyol on gas permeation properties of PUembranes was also tested by means of polyether polyols, i.e.

oly(propylene glycol) (PPG), poly(tetra methylene glycol) (PTMG)nd a polyester polyol, polycaprolactone (PCL). All polyols had theame molecular weight of 2000 g/mol. The influence of the presencef a 2, 2-dimethylol propanoic acid (DMPA) as a chain extender onhe performance of membranes to polar gases has also been studiedor selected PUs.

. Experimental and theoretical

.1. Materials

PTMG (Mw: 2000 g/mol, Arak petrochemical complex, Iran),PG (Mw: 2000 g/mol, Sigma–Aldrich) and PCL (Mw = 2000 g/mol,olvay, UK) were dried at 80 ◦C under vacuum for 48 h to removeny trace of water. DMPA, TDI, HDI and N, N-dimethylformamideDMF) and the chain extenders (BDO and DMPA) were purchasedrom Merck. BDO and DMPA were dried over 4 A molecular sievesefore use and TDI, HDI and DMF were used as received. CO2, N2nd O2 gases (purity 99.99) used for gas permeation tests were pur-hased from Roham Gas Co. (Tehran, Iran) and CH4 (purity 99.9) wasurchased from Air Products Co.

.2. Polymer synthesis

All polyurethanes were synthesized by bulk two-step poly-erization method [13]. Polyol was incubated with diisocyanate

or 2 h at 85–90 ◦C under nitrogen atmosphere to obtain

PCL–HDI–BDO PCL HDI BDO −56.35PPG–TDI–DMPA PPG TDI DMPA −24.8

macrodiisocyanate prepolymer. The chain extension of prepoly-mer was performed by addition of chain extenders (BDO or DMPA)at room temperature. In order to obtain linear polymer, themolar ratio of NCO:OH was kept at 1:1. The molar ratio of theused components was as follows: polyol:diisocyanate:chain exten-der = 1:3:2. Differential scanning calorimeter (DSC) with heatingrate of 10 ◦C min−1 was used to determine the glass transition tem-perature (Tg). Table 1 shows the synthesized polyurethanes withtheir components and Tgs.

2.3. Membrane preparation

Polyurethane membranes were prepared by solution castingand solvent evaporation technique. 10 g of synthesized thermo-plastic PUs was dissolved in 90 g DMF to obtain a 10 wt.% solutionat 70 ◦C. The mixture was then stirred for 30 min to form a homo-geneous solution. The bubble free polymer solution was cast to thedesired thickness on clean glass plates and incubated at 60 ◦C for48 h to allow the evaporation of the solvent. For complete removalof the solvent, prepared membranes were removed from the glassplates and dried in a vacuum oven at 70 ◦C for 12 h. The aver-age thickness of the prepared membranes was measured using amicrometer caliper and found to be around 100 �m. The averagethickness of an individual membrane was calculated based on theresults of several separate thickness measurements.

2.4. Membrane characterization

The obtained functional groups and their interactions in syn-thesized polyurethanes were investigated by BIO-RAD FTS-7Fourier Transform Infrared spectrometer (FT-IR) in the range of4000–500 cm−1. All the films used for FT-IR measurement wereprepared by casting the 2 wt.% PU solution on KBr disc. The thermalbehavior of polyurethanes was investigated by differential scan-ning calorimeter, Metler-Toledo DSC822e (DSC), with heating rateof 5 ◦C min−1. X-ray diffraction patterns were recorded by monitor-ing the diffraction angle 2� from 5◦ to 60◦ on a Philips X’Pert usingcupper radiation under a voltage of 40 kV and a current of 40 mA.

2.5. Gas permeability measurement

The permeability of oxygen, nitrogen, methane and carbon diox-ide were determined using a constant pressure/variable volumemethod at 10 bar pressure at 25 ◦C [14]. The permeate side wasmaintained at atmospheric pressure. The flux of the permeated gaswas measured by a U-shape flow meter. The flux of the permeatedgas versus time was used to calculate the permeability coefficientfrom the slope of the linear section of flux–time curve. Permeabil-

ity coefficient calculated three times for each membrane. The errorfor the absolute values of the permeability coefficients can be esti-mated to about ±5%, due to uncertainties of the determination ofthe gas flux and the effective membrane area and thickness, while

78 M. Sadeghi et al. / Journal of Membrane Science 385– 386 (2011) 76– 85

tiw

P

w(pba(tc

˛

3

3

tddtT

Fig. 1. FT-IR spectra of PUs based on PTMG–TDI, PTMG–IPDI and PTMG–HDI.

he reproducibility is better than ±4%. The typical membrane arean the test cell was 11.34 cm2. The gas permeability of membranes

as determined using the following equation:

= qL

A(p1 − p2)(1)

here P is permeability expressed in Barrer (1 Barrer = 10−10 cm3

STP) cm/(cm2 s cmHg), q is the flow rate of the permeate gasassing through the membrane (cm3 (STP)/s), L is the average mem-rane thickness (cm) for individual membrane, p1 and p2 are thebsolute pressures of feed side and permeate side, respectivelycmHg), and A is the effective membrane area (cm2). The ideal selec-ivity, ˛A/B (the ratio of single gas permeabilities) of membranes wasalculated from pure gas permeation experiments:

A/B = PA

PB(2)

. Results and discussion

.1. FT-IR characterization

The FT-IR spectrum of the PU samples based on three differentypes of polyols namely PTMG, PPG and PCL and three different

iisocyanates, i.e. TDI, IPDI and HDI and BDO chain extender isepicted in Figs. 1–3. The disappearance of NCO stretching vibra-ion at 2270 cm−1 is used to show the completion of the reaction.he N–H stretching vibration of urethane occurs approximately at

Fig. 2. FT-IR spectra of PUs based on PPG–TDI, PPG–IPDI and PPG–HDI.

Fig. 3. FT-IR spectra of PUs based on PCL–TDI, PCL–IPDI and PCL–HDI.

3313 cm−1 and the stretching vibration of free carbonyl groupsat around 1730 cm−1. The carbonyl group involved in hydrogenbonding is known to absorb at about 1620–1670 cm−1. The peak ofurethane ether linkage is at 1112 and 1106 cm−1. The FT-IR data ofthe carbonyl stretching vibration of poly(urethane-urea)s providesuseful information on the microphase separation resulting from thehydrogen bonding of the urethane hard segments. As known thephase separation of the hard and soft segments in polyurethaneshas a significant effect on the polymer properties [11,15]. There-fore, in the following parts the effect of each structure componentof polymer has been evaluated on the microphase separation ofhard and soft segments.

3.1.1. Effect of polyolThe FT-IR spectrums of the PU samples based on HDI, BDO and

three different types of polyols include PTMG, PPG and PCL aredepicted in Fig. 4. It is evident that variation of the type of polyolcauses substantial changes in the free and bonded carbonyl groupabsorption peak. Changing polyol type from PPG to PTMG and PCLcould decrease the intensity of bonded carbonyl group adsorptionpeak and increase the height of free carbonyl group adsorptionpeak. This observation is due to increasing phase interactions in PCLbased polyurethanes in comparison with other samples. In order toinvestigate the interaction of phases, HBI parameter was calculatedfor these samples. Polyurethane based on PPG showed the highestHBI value (HBI = 6.7), PCL based polyurethane the least HBI value(HBI = 0.29) and the HBI value for polyurethane based on PTMGwas 5.28. The microphase separation of the studied polyurethanesvaried in the following order:

PCL–TDI–BDO < PTMG–TDI–BDO < PPG–TDI–BDO

This result is in accordance with the amount and type of polargroups in each repeating unit of polyols. PPG macromoleculeshave a methyl side group in their repeating units and these sidegroups cause creation of spatial hindrances and prevent formationof hydrogen bonds between hard and soft segments. As there arepolar esters groups (COO−) in PCL structure, it can create hydrogenbonds with hard segments. Due to existence of ether groups andlack of spatial hindrance, PTMG has medium ability in formation ofhydrogen bonds.

3.1.2. Effect of diisocyanateFT-IR peaks of PUs based on PTMG, PPG and PCL and three

different kinds of diisocyanate are shown in Figs. 1–3, respec-tively. Changing diisocyanate structure from aromatic or alicyclic to

M. Sadeghi et al. / Journal of Membrane Science 385– 386 (2011) 76– 85 79

aapabIemb

Table 2Frequency amounts of free and bonded carbonyl stretching vibrations.

Polyol TDI IPDI HDI

PPGFree carbonyl 1730 1721 1716Bonded carbonyl 1705 1699 1684

PCLFree carbonyl 1740 1737 1735Bonded carbonyl 1715 1710 1685

Fig. 4. FT-IR spectra of PUs with different type of polyols.

liphatic causes uniform hydrogen bonding with N–H groups ands a result the areas corresponding to the N–H adsorption in FT-IReak become narrower with higher intensity. This change can belso related to raising phase separation and formation of hydrogenonds between amine and carbonyl groups of urethane compound.

n this situation reduction in hydrogen bonding between ether andster groups of soft segments and urethane groups of hard seg-ents occurs and as a result increasing micro phase separation can

e observed.

Fig. 5. Carbonyl vibration stretchin

PTMGFree carbonyl 1731 1722 1730Bonded carbonyl 1708 1695 1683

For precise investigation of microphase separation in syn-thesized PUs, carbonyl group absorption ranges were measured(Fig. 5). It is evident that changing diisocyanate structure fromaromatic or alicyclic to aliphatic shifts the free and bonded car-bonyl adsorptions to lower frequencies. Also decreasing intensity offree carbonyl vibration and increasing intensity of bonded carbonylvibration can be seen. Frequencies of free and bonded carbonyladsorptions in synthesized PU based on three kinds of polyols withvariation of diisocyanate are reported in Table 2.

Shift of carbonyl peak to lower wave numbers imposes strongerhydrogen bonding between carbonyls and amines in urethanegroups. Thus existence of aliphatic diisocyanate could intensify for-mation of hydrogen linkages in hard segments and cause furthermicrophase separation. Stretching vibration of carbonyls in HDIbased polyurethanes appeared as a distinct peak. In these poly-mers intensity of bonded carbonyls is more than free ones. Thisindicates high phase separation. It can be therefore concluded thatchanging the type of diisocyanate from aromatic or alicylic struc-tures to aliphatic structures in synthesized PU, causes an increasein micro phase separation between hard and soft segments. AlsoHydrogen bonding between hard segment molecules improves inthis case.

Finally it can be noticed that the order of microphase separation

of synthesized polyurethanes changes with type of the diisocyanateas follow:

HDI based PU > IPDI based PU > TDI based PU

g peaks of synthesized PUs.

80 M. Sadeghi et al. / Journal of Membrane Science 385– 386 (2011) 76– 85

F

3

diDbcwasgcTP

Tt

Fa

ig. 6. FT-IR peaks of synthesized PUs based on PPG–TDI–BDO and PPG–TDI–DMPA.

.1.3. Effect of chain extenderFT-IR peaks of two kind of PUs based on PPG and TDI with two

ifferent types of chain extenders, BDO and DMPA, are depictedn Fig. 6. Comparison of these peaks revealed that in PUs based onMPA chain extender, vibration stretching of N–H group is slightlyroader than adsorption peak of N–H group in PUs based on BDOhain extender. This is related to distribution of hydrogen bondsith urethane N–H groups and more phase interactions. For more

ccurate investigation of hydrogen bonding between hard and softegments in these polymers, FT-IR adsorption peaks of carbonylroups were measured (Fig. 7). Existence of BDO chain extenderauses more separation between free and bonded carbonyl groups.his leads to better dispersion between hard and soft segments inUs based on DMPA chain extender.

HBI parameters for mentioned synthesized PUs were calculated.he HBI parameter value for PUs based on BDO was 0.687 whilehe amount of this parameter was 0.566 in PUs based on DMPA.

1640169017401790Wavenumber (cm-1 )

Tran

smita

nce

(%)

PPG-TDI-BDO

PPG-TDI-DMPA

ig. 7. Carbonyl stretching vibration of synthesized PUs based on PPG–TDI–BDOnd PPG–TDI–DMPA.

Fig. 8. DSC thermograms of synthesized PUs based on PCL–TDI–BDO, PPG–TDI–BDOand PTMG–TDI–BDO.

These data indicate that the PU samples based on BDO have morehydrogen bonds between their hard segments and less interactionbetween their hard and soft segments. This is due to the existence ofCOOH polar groups in side chains of DMPA. These groups may formhydrogen bond with carbonyl groups of urethane in hard segmentor ether groups in soft segments. As COOH groups in DMPA interactwith carbonyl groups of hard segment the urethane groups couldinteract to soft segments more and as a result dispersion of hard andsoft segments may occur. This phenomenon leads to lower phaseseparation of hard and soft segments in DMPA based polyurethanes.Presence of COOH groups also disturb the packing of chains in thehard segment and consequently the hard and soft segments arebetter mixed.

3.2. DSC analysis

3.2.1. Effect of polyolThe thermal properties of the synthesized polyurethanes were

investigated by differential scanning calorimetery. Thermal behav-iors of three kinds of synthesized PUs based on HDI and BDO andthree different types of polyols containing PCL, PTMG and PPG areshown in Fig. 8. The transition temperatures with respect to softsegments of PUs were measured by DSC (Table 1). The results indi-cate the following order for Tgs:

PCL–TDI–BDO > PPG–TDI–BDO > PTMG–TDI–BDO

The glass transition temperature is one of the best criterions forcomparing the chain mobility of the polymers. According to our FT-IR results, the amount of phase separation in synthesized PUs basedon PPG is more than PTMG-based PUs while the value of Tg is less.This may be due to structure of PTMG and PPG chains. Existenceof methyl group in side chains of PPG prevents the chains mobilityand therefore the Tg value increases in comparison with the PTMGchains. The more phase separation in the structure of PPG-basedpolyurethane compared to PTMG- and PCL-based polyurethaneswhich was observed in FT-IR analysis was seen in the DSC resultsas well. As shown in Fig. 8 the slope of glass transition of the softsegments change by polyol as follows:

PPG based PU > PTMG based PU > PCL based PU

It is known that the more interacted chains show the broader

transition region. Though, the chains which are not related to orhave not interacted with other chains show a narrower transition.Therefore, PUs based on PPG have a narrower transition becauseof more phase separation and lower interaction of soft to hard

M. Sadeghi et al. / Journal of Membrane Science 385– 386 (2011) 76– 85 81

sb

ipw

3

sarPoctsc

iiHglboatmmiaHach

3

PDip

37027017070-30-130Temperature (ºC)

Endotherm

ic

PPG-TDI-BDO

PPG-TDI-DMPA

DSC thermograms. All synthesized samples showed broad peak at2� = 20◦. The broadened peak might be due to the presence of smallcrystalline structures or diffraction from large crystals in PCL and

Fig. 11. XRD pattern of synthesized PUs based on PCL–TDI–BDO, PPG–TDI–BDO andPTMG–TDI–BDO.

Fig. 9. DSC thermograms of synthesized PUs.

egments. Whereas PUs based on PCL have a broader transitionecause of their higher soft to hard interaction.

An endothermic peak in the range of −2 to −10 ◦C was observedn thermogram of synthesized PUs based on PCL and PTMG. Thiseak is related to formation of crystals in soft segment. This resultas further confirmed by WAX.

.2.2. Effect of diisocyanateThermal properties of nine kinds of synthesized PUs are pre-

ented in Fig. 9. Changing diisocyanate structure from aromaticnd alicyclic to aliphatic causes increasing phase separation andeduction in Tgs. There is an endothermic peak at −2.5 ◦C inTMG–HDI-based PU thermogram; that is related to the formationf crystalline structures in soft segment. This result was furtheronfirmed by XRD. In HDI based PUs due to more phase separa-ion, hard segments have no interfere with soft segments thus softegments have enough space and mobility to create ordered andrystalline structures.

Figs. 8 and 9 show that there is a broad similar endothermic peakn all PU samples based on HDI in the range of 100–170 ◦C. This peaks related to crystalline structures in hard segment area. Because ofDI linearity, a tendency to crystallization and formation of hydro-en bonds was observed in this compound. Due to voluminous andarge electron density of convoluted structures in TDI- and IPDI-ased PUs, unlike HDI no crystalline and ordered structures werebserved. The main and fundamental reason for more phase sep-ration in PUs containing IPDI and HDI in comparison with TDI isheir ability to pack and find ordered structures in their hard seg-

ents. Existence of endothermic peak at 50 ◦C in PPG-based PUsight be related to incomplete hard segment crystals that appear

n lower temperature because of the interactions between hardnd soft segments. The existence of hard segment crystals in theDI-based polymers is respected to high phase separation of hardnd soft segments. In the phase separated polyurethane the polyolhains could not prevent the chain regularity in hard segments. Theard segment chains could order in the crystalline cells.

.2.3. Effect of chain extenderDSC thermograms of two synthesized samples based on

PG–TDI with different kinds of chain extenders including BDO andMPA are shown in Fig. 10. Based on FT-IR analyses, more phase

nteraction occurs in Pus based on DMPA chain extender in com-arison with PUs based on BDO. Therefore, Tgs of DMPA-based PUs

Fig. 10. DSC thermograms of synthesized PUs based on PPG–TDI–DMPA andPPG–TDI–BDO.

are higher than Tgs of PUs based on BDO. Also it is evident in Fig. 10that the slope of the glass transition temperature in BDO-basedPU is more than DMPA-based one. This is respected to more phaseseparated structure of the BDO-based PU.

3.3. X-ray analysis

The X-ray diffraction patterns of synthesized PUs based on TDI,IPDI and HDI are depicted in Figs. 11–13, respectively. Accordingto DSC results, PCL and PTMG showed crystalline structures. PPGis liquid at room temperature (25 ◦C) and therefore the PPG-basedPUs neither show any crystalline structure nor any melting point in

Fig. 12. XRD pattern of synthesized PUs based on PCL–IPDI–BDO, PPG–IPDI–BDOand PTMG–IPDI–BDO.

82 M. Sadeghi et al. / Journal of Membran

Fa

PactsiTbcw

3

3

epi

N

wptp

TG

TG

TG

ig. 13. XRD pattern of synthesized PUs based on PCL–HDI–BDO, PPG–HDI–BDOnd PTMG–HDI–BDO.

TMG and amorphous structures of PPG. The sharp peak appearedt 2� = 24.5◦ in all HDI-based PUs is related to the formation ofrystalline structures in hard segments. As the soft segment ofhe PPG-based polymer cannot crystallize at all due to its irregulartructure, it can be concluded that the appeared peak at 2� = 24.5◦

n all HDI-based PUs refers to the crystal cells in hard segments.his peak was not observed in PUs based on other diisocyanatesecause of their unwillingness to crystallinity. The presence of therystalline structure in hard segments of the HDI-based polymersas also observed in the DSC results.

.4. Gas permeation properties

.4.1. Effect of polyol and diisocyanateO2, N2, CO2 and CH4 permeation and ideal permselectivity prop-

rties of TDI-, IPDI- and HDI-based PUs with different polyols areresented in Tables 3–5, respectively. Gas permeability sequence

n TDI-based PU with all of the polyols is as follows:

2 < CH4 < O2 � CO2

The order of gas permeabilities in PPG- and PTMG-based PUs,

hich are synthesized with IPDI and HDI, are different and methaneermeability in these cases is more than oxygen. As known,he soft segment domains in polyurethane membranes are theermeable regions in the process of the transport of permeant

able 3as permeation and ideal pemselectivity properties of TDI based PUs.

Polyol Permeability (Barrer) Selectivity

PCO2 PCH4 PO2 PN2 ˛O2/N2˛CO2/N2

˛CO2/CH4

PPG 79.50 7.39 7.96 3.79 2.10 20.97 10.76PTMG 82.00 6.35 7.06 2.57 2.74 31.90 12.91PCL 51.80 3.08 3.50 1.27 2.76 40.22 16.58

able 4as permeation and ideal pemselectivity properties of IPDI based PUs.

Polyol Permeability (Barrer) Selectivity

PCO2 PCH4 PO2 PN2 ˛O2/N2˛CO2/N2

˛CO2/CH4

PPG 105.60 9.16 8.17 3.42 2.54 30.87 11.50PTMG 92.78 8.67 7.76 3.07 2.52 30.22 10.70PCL 70 4.47 6.04 1.55 3.89 45.16 15.66

able 5as permeation and ideal pemselectivity properties of HDI based PUs.

Polyol Permeability (Barrer) Selectivity

PCO2 PCH4 PO2 PN2 ˛O2/N2˛CO2/N2

˛CO2/CH4

PPG 186.47 19.50 16.26 7.47 2.17 24.96 9.56PTMG 120.34 14.35 11.17 4.77 2.34 25.22 8.38PCL 96.03 5.59 6.27 2.24 2.79 42.87 17.17

e Science 385– 386 (2011) 76– 85

molecules through the membrane. However, each parameter thatcould change the chain mobility and interaction of the soft seg-ment to permeant molecules would effect on the gas permeationof the PUs [15,20]. Hence, as mentioned in previous section thesechanges might be due to further phase separation in HDI- and IPDI-based PUs compared to TDI-based PUs. Increasing phase separationleads to an increase in rubber properties of the PUs that causes theincrease of the free volumes and gas solubility in these polymers. Itis well known in solution-diffusion mechanism of gas permeationin polymeric membranes that the solution mechanism is dominantmechanism in rubbery polymers [21]. Though, in all of the syn-thesized polyurethanes in this research it is well established thatthe permeability of methane is more than nitrogen in despite of itshigher molecular size. In the above mentioned structures becauseof the more rubbery properties of PUs, the permeability of methaneincreases and reaches a greater value than that of the oxygen withlower molecular size. As the solubility of the more condensablemethane gas would increase in PUs, the permeability of methanewould therefore increase more than that of oxygen.

Study on the gas permeation properties of PCL-based PUsrevealed that oxygen permeability is more than methane in allspecimens. This might be due to presence of COO− polar groups insoft segments that leads to decreasing phase separation betweensoft and hard segments which in turn reduces the rubbery proper-ties of the polymer. We have shown previously that by increasingthe rubbery properties of the polyurethane-urea membranes byincreasing the phase separation in PUs, the permeability of themethane increases more than oxygen and in more phase separatedPUs the methane gas shows more permeability [23].

According to Tables 3–5 the gas permeability of different polyolbased polyurethane are as follows:

PCL-based PUs < PTMG-based PUs < PPG-based PUs

The FT-IR results showed that the PPG-based PUs have higherphase separation between hard and soft segments than the PCL-based PUs. Also the thermal behavior of polymers showed thatPTMG-based PUs have more molecular mobility and consequentlyless glass transition temperature. However, the results presentedin the permeability of gases show that the PU based on PTMG withhigher molecular mobility has less permeability than the PU basedon PPG. This contradiction might be due to the lack of crystal for-mation in the soft phase of PPG based PU. Crystalline regions inpolymers structures act as barrier fillers and reduce available spacesfor passing gas molecules through the polymers [22]. Thus, becauseof the formation of crystalline regions in the soft segment of PTMG-based PUs, their permeabilities are lower than that of PUs based onPPG.

Comparison of gas permeability of different types of diiso-cyanate based polyurethane shows that gas permeability of PUsbased on HDI is at maximum and permeability of PUs based onTDI is at minimum. As discussed in the previous section, by chang-ing diisocyanate structure from aromatic and alicyclic to aliphaticthe large aromatic ring groups of hard segment will be removed.So the compaction density of polymer chains in the hard segmentwould increase. Moreover, due to reduction of the steric hindrancethe hydrogen bonding between hard segments increases. Conse-quently, more phase separation would occur in the PU, whichwould increases the permeability of gases.

Evaluation of the ideal gas permselectivity in PUs based on PPG,PTMG and PCL showed that the PU based on PCL has the highest andthe PU based on PPG has the lowest selectivity. This is related to less

phase separation and crystalline structure in PCL-based PU in com-parison with PPG-based PU. There are also regions with crystallinestructures in the PTMG-based PU in comparison with the PPG-based PU. Both factors mentioned above lead to decreased mobility

M. Sadeghi et al. / Journal of Membrane Science 385– 386 (2011) 76– 85 83

Ft

oscpeip

tssdistaos

C(ppCt

3

Ptmr

a

C

Table 7R2 values for linear relationship between permeability and −ln(−1/Tg).

Gases PPG based PUs PTMG based PUs PCL based PUs

CO2 0.781 0.621 0.985CH 0.702 0.621 0.995

TG

ig. 14. The CO2/N2 separation performance of the synthesized polyurethanes (�) inhis study in comparison to Robbeson’s upper bond and other studied polyurethanes.

f polyol chains in soft segment and ultimately increased diffu-ion selectivity in polymers. By reduction of phase separation andhain mobility of soft segments, which are the permeable regions inolyurethane membranes [15], the effect of molecular sieve prop-rty of the polymers and domination of the diffusion mechanismncrease. These changes lead to the more diffusivity selectivity inolymers.

Comparison of CO2/N2 selectivity in synthesized PUs based onhree kinds of diisocyanates showed that in spite of the more phaseeparated structure of the HDI- and IPDI-based PUs, the CO2/N2electivity in these PUs is higher than the PUs based on the TDIiisocyanate. Because the permeation of more condensable CO2 gas

n comparison to nonpermeable nitrogen increase in more phaseeparated structures, though the effect of the solution selectivity onhe separation of condensable and noncondensable gases increasend these polymers show the higher CO2/N2 selectivity. In the casef IPDI and HDI based PUs the IPDI based PUs have higher diffusivityelectivity of the CO2/N2 than HDI based ones.

The CO2/N2 selectivity values are presented as a function ofO2 permeability in Fig. 14 along with Robbeson’s upper bonds1991 and 2008 upper bonds) and reported values from otherolyurethanes [24,25]. As shown in this figure the synthesizedolyurethanes in this study have the highest performance inO2/N2 separation in comparison to the other studied PU struc-ures.

.4.2. Effect of DMPA chain extenderGas permeability and selectivity of PPG–TDI–BDO- and

PG–TDI–DMPA-based PUs are depicted in Table 6. It is evidenthat by changing the chain extender from BDO to DMPA, gas per-

eability is reduced and gas selectivity is increased. Permeabilityeduction is due to the decrease of phase separation.

Gas permeability of PU-based on DMPA to PU based on BDO iss follows:

O2(37%) < O2(46.6%) < CH4(49%) < N2(55.7%)

able 6as permeation and ideal pemselectivity properties of PPG–TDI–BDO and PPG–TDI–DMP

Chain extender Permeability (Barrer)

PCO2 PCH4 PO2

BDO 79.5 7.39 7.96

DMPA 50.10 3.76 4.25

4

O2 0.651 0.502 0.992N2 0.485 0.561 0.880

It was shown before that by increasing the phase mixing inpolyurethanes the size and amount of the free volume decrease.Therefore, the permeability of the higher molecular size gasesreduces [22]. Considering the larger molecular size of methane,further reduction in nitrogen permeability might be due to morecondensability of methane in the polymer. COOH groups of DMPAchain extender can be located in the interface of the soft and hardsegments and provide suitable place for dissolution of gases. Unlikelarger molecular size of methane, methane gas permeability isreduced less than nitrogen and is closed to oxygen. The order ofincreasing the selectivity of gases in PU based on DMPA in compar-ison to BDO based PU is as follows:

O2/N2 (1.2) < CO2/CH4 (1.238) < CO2/N2 (1.42)

Due to the above mentioned arguments it is expected that theselectivity of gases with larger molecular sizes increase more thanthe smaller ones. As known the molecular size of methane is biggerthan nitrogen, but can be seen that the selectivity of nitrogen to car-bon dioxide increased more than selectivity of methane to carbondioxide. These phenomena can be justified according to previousarguments. COOH groups are placed in interfaces of hard and softsegments, increase interactions of carbon dioxide with the polymerand form suitable area for the dissolution of gases. As condens-ability of methane gas is more, so it further adsorbs in interfacearea. Therefore, selectivity of CO2/CH4 will increase less than selec-tivity of CO2/N2. Also it can be seen that the selectivity of O2/N2,because of their non-condensable nature, had no significant incre-ment. Though it would be concluded that by changing the chainextender to DMPA the diffusivity selectivity and solubility selectiv-ity increase and the pair gases with difference in condensation andsize show the highest change in selectivity.

3.4.3. Correlation between Tg and gas permeationIt is clear from the generally accepted solution-diffusion model

of transport of gases in polymers that the glass transition temper-ature of a polymer is an important factor controlling the diffusionprocess. In general, as Tg of a related series of polymers increases,the diffusivity decreases [20]. The same trend has been basicallyobserved for the PUs studied in this work. An almost linear relation-ship can be found between gas permeability and Tg in Figs. 15–17,showing the increase of permeability on the decrease of the Tg

values. Amounts of linear correlation factors (R2), between per-meability and −ln (−1/Tg) for synthesized PUs were presented inTable 7. It is clear that correlation for PUs based on PCL is morethan other synthesized PUs. This might be due to less phase sepa-

ration between hard and soft segments in these PUs. In the case ofthe PCL based PUs the change of the diisocyanate from aromatic toaliphatic does not significant effect on Tg, though, the linearity of thechanges to Tg is the most. In this case linear relationship between

A based PUs.

Selectivity

PN2 ˛O2/N2˛CO2/N2

˛CO2/CH4

3.79 2.10 20.97 10.701.68 2.50 29.82 13.30

84 M. Sadeghi et al. / Journal of Membran

1

10

100

4.243.83.63.43.23-ln (-1/Tg)

Gas

Per

mea

bilit

y (b

arre

r)

CO2

O2

CH4

N2

Fig. 15. Permeability variations of gases versus −ln(−1/Tg) in synthetic PUs basedon PCL.

1

10

100

1000

4.24.143.93.83.73.6-ln (-1/Tg)

Gas

Per

mea

bilit

y (b

arre

r)

CO2

CH4O2

N2

Fo

pbms

Fo

ig. 16. Permeability variations of gases versus −ln(−1/Tg) in synthetic PUs basedn PPG.

ermeability and −ln (−1/Tg) can be observed. Also comparisonetween R2 values of condensable gases such as carbon dioxide andethane and non-condensable gases such as oxygen and methane

howed linear relationship with condensable gases. As mentioned

1

10

100

1000

4.44.354.34.254.24.154.14.0543.95-ln (-1/Tg)

Gas

Per

mea

bilit

y (b

arre

r)

CO2

CH4O2

N2

ig. 17. Permeability variations of gases versus −ln(−1/Tg) in synthetic PUs basedn PTMG.

[

[

[

[

[

e Science 385– 386 (2011) 76– 85

the change of the diisocyanate from aromatic to aliphatic show thesignificant effect on the soft segment mobility and changing theelastic properties of the PUs. Though, the more condensable gaseswhich could affect from the rubbery properties have close relationto the Tg of the polymer and as a result its relationship to Tg changeis more linear than the noncondensable ones.

4. Conclusions

The physical and permeation properties of a systematic seriesof PU membranes with different polyols, diisocyanate and chainex-tenders were determined. Results obtained by FT-IR and DSCindicate that by changing the diisocyanate from aromatic to lin-ear aliphatic and changing the polyol from PCL to PTMG and PPG,the microphase separation of hard and soft segments increase.Study on the XRD patterns confirmed that PTMG and PCL poly-ols can be arranged in small crystalline structures and also becauseof high phase separation of the polymer based on HDI the hardsegment ordered in crystal phase was observed. Permeation mea-surements of polymers revealed that the permeability of gasesincreases with phase separation in polymers and selectivity of gasesdecreases. Also PUs based on PPG showed the highest permeabil-ity. The obtained results from FT-IR and DSC show more phasemixing on the polymers based on DMPA chain extender whichlead to lower permeability and higher selectivity of the polymer.The solubility and diffusivity of gases indicate solubility domina-tion of gas transport in these membranes and this dominationincrease by phase separation. The synthesized polymers show thehighest Permeability of CO2 up to 186 Barrer (1 Barrer = 1 × 10−10

[cm3 (STP) cm/cm2 s cmHg]), and high selectivity for carbon dioxidewith respect to nitrogen (CO2/N2: 45) in the polyurethane mem-branes.

References

[1] T.S. Chung, E.R. Kafchinski, P.J. Foley, Development of asymmetric hollow fibersfrom polyimides for air separation, Membr. Sci. 75 (1992) 181–195.

[2] A.S. Stern, Y. Liu, W.A. Feld, Structure/permeability relationships of polyimideswith branched or extended diamine moieties, J. Polym. Sci. Part B: Polym. Phys.31 (1993) 939–951.

[3] J. Kruse, J. Kanzow, K. Ratzke, F. Faupel, M. Heuchel, J. Frahn, D. Hofmann,Free volume in polyimides: positron annihilation experiments and molecularmodeling, Macromolecules 38 (2005) 9638–9643.

[4] J.S. McHattie, W.J. Koros, D.R. Paul, Gas transport properties of polysulphones.1. Role of symmetry of methyl group placement on bisphenol rings, Polymer32 (1991) 840–850.

[5] C.L. Aitken, W.J. Koros, D.R. Paul, Gas transport properties of biphenol polysul-fones, Macromolecules 25 (1992) 3651–3658.

[6] L.M. Costello, W.J. Koros, Effect of structure on the temperature dependence ofgas transport and sorption in a series of polycarbonates, J. Polym. Sci. Part B:Polym. Phys. 32 (1994) 701–713.

[7] L. Abdellah, B. Boutevin, F. Guida-Pietrasanta, M. Smaihi, Evaluation ofphotocrosslinkable fluorinated poly-dimethylsiloxanes as gas permeationmembranes, J. Membr. Sci. 217 (2003) 295–298.

[8] M. Pegorado, L. Zanderighi, A. Penati, F. Severini, F. Bianchi, N. Cao, R. Sisto, C.Valentini, Polyurethane membranes from polyether and polyester diols for gasfractionation, J. Appl. Polym. Sci. 43 (1991) 687–697.

[9] D.P. Queiroz, M.N. de Pinho, Structural characteristics and gas permeation prop-erties of polydimethylsiloxane/poly (propylene oxide) urethane/urea bi-softsegment membranes, Polymer 46 (2005) 2346–2353.

10] R.C. Ruaan, W.C. Ma, S.H. Chen, J.Y. Lai, Microstructure of HTPB basedpolyurethane membranes and explanation of their low O2/N2 selectivity, J.Appl. Polym. Sci. 82 (2001) 1307–1314.

11] A. Wolinska-Grabczyk, A. Jankowski, Gas transport properties of segmentedpolyurethanes varying in the kind of soft segments, Sep. Purif. Technol. 57(2007) 413–417.

12] A. Wolinska-Grabczyk, Effect of the hard segment domains on the per-meation and separation ability of the polyurethane-based membranes inbenzene/cyclohexane separation by pervaporation, J. Membr. Sci. 282 (2006)225–236.

13] M.A. Semsarzadeh, M. Sadeghi, M. Barikani, H. Moadel, The effect of hard seg-ments on the gas separation properties of polyurethane membranes, IPJ 16(2007) 819–827.

14] M.A. Semsarzadeh, M. Sadeghi, M. Barikani, Effect of polyol and chain extenderlength on the gas separation properties of polyurethane, IPJ 17 (2008) 431–440.

brane

[

[

[

[

[

[

[

[

M. Sadeghi et al. / Journal of Mem

15] G. Galland, T.M. Lam, Permeability and diffusion of gases in segmentedpolyurethanes: structure properties relations, J. Appl. Polym. Sci. 50 (1993)1041–1058.

16] P.M. Knight, D.J. Lyman, Gas permeability of various block copolyether-urethanes, J. Membr. Sci. 17 (1984) 245–254.

17] H. Li, B.D. Freeman, O.M. Ekinerb, Gas permeation properties of poly(urethane-urea)s containing different polyethers, J. Membr. Sci. 369 (2011) 49–58.

18] H.B. Park, C.K. Kim, Y.M. Lee, Gas separation properties of polysilox-

ane/polyether mixed soft segment urethane urea membranes, J. Membr. Sci.204 (2002) 257–269.

19] S.L. Huang, J.Y. Lai, On the gas permeability of hydroxyl terminatedpolybutadiene based polyurethane membranes, J. Membr. Sci. 105 (1995)137–145.

[

[

[

Science 385– 386 (2011) 76– 85 85

20] M. Sadeghi, M.A. Semsarzadeh, M. Barikani, B. Ghalei, The effect of urethaneand urea content on the gas permeation properties of poly (urethane-urea)membranes, J. Membr. Sci. 354 (2010) 40–47.

21] Z.F. Wang, B. Wang, Y.R. Yang, C.P. Hu, Correlations between gas permeationand free-volume hole properties of polyurethane membranes, Eur. Polym. J. 39(2003) 2345–2349.

22] R.E. Kesting, A.K. Fritzsche, Polymeric Gas Separation Membranes, John Wiley& Sons, New York, 1993, Chapter 2.

23] D.R. Paul, Y. Yampolski, Polymeric Gas Separation Membrane, CRC press Inc.,Florida, 1994, Chapter 3.

24] L.M. Robeson, Correlation of separation factor versus permeability of polymericmembranes, J. Membr. Sci. 62 (1991) 165.

25] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400.