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Journal of Membrane Science 354 (2010) 40–47

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

he effect of urethane and urea content on the gas permeation properties ofoly(urethane-urea) membranes

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

Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-8311, IranChemical Engineering Department, Tarbiat Modares University, Jalal Al Ahmad Highway, P.O. Box 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 9 October 2009eceived in revised form 23 January 2010ccepted 27 February 2010vailable online 6 March 2010

eywords:olyurethane–ureatructure

a b s t r a c t

The effect of urethane and urea contents on gas separation properties of the groups of polyurethane–ureamembranes is studied. The membranes used in this study were prepared by polytetramethylene-glycol(PTMG) and isophorone diisocyanate (IPDI) prepolymers. The prepolymers were converted to final poly-mer using the designed proportion of 1,4-butane diol (BDO) and 1,4-butane diamine (BDA) as a chainextender. Five different BDO/BDA ratios were chosen in order to synthesize poly(urethane-urea)s withdifferent urethane/urea linkage content. The prepared polymers were characterized and the permeabil-ity of gases through them was investigated. Results obtained by Fourier transform infrared spectrometer(FT-IR) and differential scanning calorimetery (DSC) indicate that by increasing the urea linkage in the

embranehase separationas separation

polymers, the microphase separation of hard and soft segments increase. Study of the X-ray diffraction(WAXD) patterns confirmed that polyols may be arranged in small crystalline structures. Permeationmeasurements of polymers revealed that the permeability of gases decreases with increasing urethanecontent in the polymers and selectivity of gases decreases with increasing urea content. The solubilityand diffusivity of gases indicate solubility domination of gas transport in these membranes. The results ofpermeability also show high amounts up to 128 Barrer (1 Barrer = 1 × 10−10 [cm3 (STP) cm/cm2 s cm Hg]),

rbon

and high selectivity for ca

. Introduction

Efficiency improvement of polymeric membranes in the gas sep-ration field has been a considerable research subject in the lastecade [1]. The fundamental parameters characterizing gas sep-ration membranes are the permeability coefficient, PA, and theelectivity, ˛AB (= PA/PB) [2]. Enhancement of membrane operationn each separation case is obtained by increasing the permeabilitynd selectivity of membranes with respect to distinct gases. Typ-cally, more permeable polymers are less selective and vice versa2–4]. The improvement of permeability and selectivity of gaseshrough membranes can be achieved, through understanding theolymer structure and the structure–transport properties of mem-ranes [5,6]. Functional groups in the polymers chemical backboneffect gas separation properties [7].

Polyurethanes are a class of polymers with a very wide vari-bility in structures and properties and can be used in membraneas separation [8,9]. Polyurethanes are composed of alternatingrethane or urea hard segments and polyol (polyether/polyester)

∗ 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 © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2010.02.070

dioxide with respect to nitrogen (CO2/N2: 27).© 2010 Elsevier B.V. All rights reserved.

soft segment. The urethane/urea hard segments are formed byextending a terminal diisocyanate with a low molecular weightdiol/diamine. The soft segment consists of high molecular weightpolyether/polyester groups [10]. The properties of polyurethanesare easily tailored by introducing controlled changes in polyol chainlength as well as by changing the proportions and chemical natureof the constituents which make up the flexible and rigid segments ofthe polymer chain [11–13]. Polyurethanes have been investigatedparticularly for the possibility of reconciling their transport proper-ties by varying the polymer microstructure [14–16]. The obtainedresults show that the gas permeation properties of polyurethanemembranes are affected by length, type and amount of soft andhard segments [17–20]. The types of chain extenders used in thesynthesis of polyurethanes also affect the permeation properties ofthe membranes and therefore changes the phase-separated domainmorphology, crystallinity, density and glass transition tempera-ture of the membranes [21,22]. Our previous work has shown thatthe permeability increases with the length of chain extender. The

variations of structural parameters in polyurethanes also affectinteractions of these two segments [23]. Kind, length and amount ofhard and soft segments indicate the phase separation properties ofpolyurethanes and consequently change gas separation propertiesof polyurethanes [17,24–26].

M. Sadeghi et al. / Journal of Membrane Science 354 (2010) 40–47 41

Table 1Composition and thermal properties of synthesized poly(urethane-urea)s.

Sample code Mole ratio of chain extenders (diol/diamine) Urea linkage (%) Tg (◦C) (soft segment)

PUU0 1/0 0 −75.12

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2

2

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4uCP

2

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2

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2

ws

PUU25 75/25PUU50 50/50PUU75 25/75PUU100 0/1

In the present study the effect of urethane and urea linkagesontent on the gas separation properties of the groups of synthe-ized poly(urethane-urea) membranes with different amounts ofiol and diamine chain extenders are investigated.

. Experimental

.1. Materials

Polytetramethylene-glycol (PTMG, Mw = 2000 g mol−1), wasbtained from Arak petrochemical complex (Arak, Iran) and driedt 80 ◦C under vacuum for 48 h to remove the residual water.,4-butanediol (BDO), 1,4-butanediamine (BDA), isophorone diiso-yanate (IPDI) and N,N-dimethylformamide (DMF) were purchasedrom Merck. The chain extenders (BDO and BDA) were dried over

´A molecular sieves before use. CO2, N2 and O2 gases (purity 99.99)sed for gas permeation tests were purchased from Roham Gaso. (Tehran, Iran) and CH4 (purity 99.9) was purchased from Airroducts Co. (Tehran, Iran).

.2. Polymer synthesis

All polyurethanes were synthesized by bulk two-step polymer-zation method [20]. PTMG was incubated with IPDI for 2 h at5–90 ◦C under nitrogen atmosphere to obtain macrodiisocyanaterepolymer. The chain extension of prepolymer was performed byddition of BDO, BDA or selected ratio of BDO/BDA at room temper-ture. In order to obtain linear polymer, the molar ratio of NCO: OHnd NCO:NH2 were kept 1:1. The molar ratio of the used compo-ents was as follows: PTMG:IPDI:BDO–BDA = 1:3:2. Table 1 showshe synthesized polyurethane–ureas with their components molaratio and the amount of urea linkage (%). The molecular structuref synthesized polyurethane and polyurethane–urea are shown incheme 1. As shown in this scheme by synthesis of polymer in theresence of BDA, urea linkages form in hard segment.

.3. Membrane preparation

Polyurethane–urea membranes were prepared by solutionasting and solvent evaporation technique. 10 g of synthesizedhermoplastic poly(urethane-urea)s was dissolved in 90 g DMF tobtain a 10 wt.% solution at 70 ◦C. The mixture was then stirred forperiod of half an hour to form a homogeneous solution. The bub-le free polymer solution was cast to the desired thickness on cleanlass plates and incubated at 60 ◦C for 48 h to allow the evaporationf the solvent. For complete removal of the solvent, prepared mem-ranes were removed from the glass plates and dried in a vacuumven at 70 ◦C for 12 h. The thickness of the prepared membranesas measured using a micrometer caliper and found to be around

00 �m.

.4. Membrane characterization

The obtained functional groups in synthesized polyurethanesere investigated by BIO-RAD FTS-7 Fourier Transform Infrared

pectrometer (FT-IR) in the range 4000–500 cm−1 (All the films

16.7 −75.4733.4 −77.150 −78.2666.6 −79

used for FT-IR measurement were prepared by casting the 2 wt.% PUsolution on KBr disc). The thermal behavior of polyurethanes wasinvestigated by differential scanning calorimeter Metler-ToledoDSC822e (DSC) with heating rate of 5 ◦C/min. X-ray diffraction pat-terns were recorded by monitoring the diffraction angle 2� from 5◦

to 60◦ on a Philips X’Pert using cupper radiation under a voltage of40 kV and a current of 40 mA.

2.5. Gas permeation

The permeability of oxygen, nitrogen, methane and carbondioxide were determined using constant pressure/variable volumemethod at 10 bar pressure and 25 ◦C [23]. The gas permeability ofmembranes was determined using the following equation:

P = q�

A(p1 − p2)(1)

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

(STP) cm/cm2 s cm Hg), q is flow rate of the permeate gas passingthrough the membrane (cm3/s), � is membrane thickness (cm), p1and p2 are the absolute pressures of feed side and permeate side,respectively (cm Hg) and A is the effective membrane area (cm2).

The ideal selectivity, ˛A/B (the ratio of single gas permeabilities)of membranes was calculated from pure gas permeation experi-ments:

˛A/B = PA

PB(2)

The diffusion coefficient (D) was determined by the time lagmethod, represented as:

D = �2

6�(3)

where � is the time lag (s), i.e. the intercept obtained fromextrapolating the linear region of the P2 versus the time plot tothe time axis. D is the diffusion coefficient (cm2/s).

The solubility coefficient (S) was then calculated from Eq. (4):

S = P

D(4)

3. Results and discussion

The composition of the hard and soft segments and their effectson each other are one of the most important factors in control-ling the structure and properties of poly(urethane-urea)s [17–19].Quantity and state of the urea and urethane linkage are the keyparameters to change the hard segment composition and phaseseparation of hard and soft segments in polymer [21,22]. To eval-uate the effect of urethane and urea linkage on structure and gas

separation properties of poly(urethane-urea)s, a set of polymerswith different contents of urethane and urea linkages were synthe-sized. To avoid changing the other structural factors, only the dioland diamine linkage in chain extender was varied and the otherfactors was kept the same in all synthesized poly(urethane-urea)s.

42 M. Sadeghi et al. / Journal of Membrane Science 354 (2010) 40–47

S f BDOs r chem

3

Tuvs[tetuhaoftttacaPo

and 1695 cm , while the poly(urethane-urea) series only showbonded urethane carbonyl at around 1700 cm−1. Appearance ofthe weak absorption at 1702 cm−1 in PUU0 confirms more hardand soft segment connections via hydrogen bonding of the N Hgroup to ether oxygen of PTMG. Increasing the urea linkages in

cheme 1. The molecular structure of synthesized PU and PUUs, in the presence oegment of PU and the urethane/urea linkages in PUU are shown in upper and lowe

.1. FT-IR characterization

The FT-IR spectrum of the PUU samples is depicted in Fig. 1.he disappearance of NCO stretching vibration at 2270 cm−1 issed to show the completion of the reaction. The N H stretchingibration of urethane occurs approximately at 3313 cm−1 and thetretching vibration of free carbonyl groups at around 1730 cm−1

23]. The carbonyl group involved in hydrogen bonding is knowno absorb at about 1620–1670 cm−1 [27,28]. The peak of urethanether linkage is at 1112 and 1106 cm−1 [27,28]. The FT-IR data ofhe carbonyl stretching vibration of poly(urethane-urea)s providesseful information on the microphase separation resulting from theydrogen bonding of the urethane/urea hard segments [27,28]. Thebsorption of carbonyl groups is depicted in Fig. 2. The strong peakf carbonyl stretching occurred in the range of 1695–1722 cm−1

or pure polyurethane (PUU0). By increasing the urea linkage inhe polymer structure, the intensity of this peak decreased andhe new peak appeared in the range of 1635–1651 cm−1. In PUU0he free carbonyl absorption is at 1722 and 1717 cm−1 without

ny significant separated peak for bonded carbonyls. The freearbonyl absorption of urethane linkage in poly(urethane-urea)sppears at 1717 cm−1 with the small absorption at 1722 cm−1 whileUU100 just shows a weak absorption at 1717 cm−1. As previ-usly shown [29], the peaks at 1722 and 1717 cm−1 are assigned

Fig. 1. FT-IR spectra of synthesized poly(urethane-urea)s.

and BDO/BDA mixture chain extender. The presence of urethane linkages in hardical formula respectively.

to the stretching vibration of free carbonyls. However, for the lat-ter the urethane group is hydrogen bonded to ether oxygen viaan N H group (C O group remaining free), i.e. (NH O ) bonding.This indicates that some urethane/urea moieties dissolve in PTMGsoft segment domains and form hydrogen bonding with ether oxy-gen which leads to stronger interaction of soft and hard segmenttogether. The bonded carbonyl groups for PUU0 appeared at 1702

−1

Fig. 2. FT-IR spectra in the range of 1600–1800 cm−1 for synthesized poly(urethane-urea)s.

M. Sadeghi et al. / Journal of Membrane Science 354 (2010) 40–47 43

pbusmscwiailaWpd(btrfeasagdolp

P

3

wbttrs(

foc

was done for PTMG and the synthesized membranes based onthis polyol. As shown in Fig. 4, there are two crystalline points in2� = 20◦ and 2� = 24.5◦ for PTMG polyol. The X-ray spectra of syn-thesized poly(urethane-urea)s is depicted in Fig. 5. The broadened

Fig. 3. DSC thermograms of polyurethanes.

olymer results in better separation of the free and bonded car-onyl peaks (Fig. 2) which is due to increasing the more bondedrethane carbonyls in the polymer. This confirms a better phaseeparation of the system by raising the urea linkage in poly-er. Furthermore, the poly(urethane-urea)s of PUU25 to PUU100

how two absorption bonds at about 1651 and 1636 cm−1 that areaused by stretching vibrations of bonded urea carbonyl groups. Aeak absorption is observed around 1682–1680 cm−1, which may

ndicate traces of free urea carbonyl. The intensity of these freebsorption peaks decreases by increasing the urea linkage contentn polymer. Thus, some urea carbonyls of the poly(urethane-urea)seft as free carbonyls and by increasing the urea linkage disappearnd consequently the intensity of bonded carbonyl bands increase.ang and Cooper [30] suggested that the bonded urea carbonyl in

olyether poly(urethane-urea)s occurs in a mixed state with three-imensional hydrogen bonding and conventional inter urea bondsone urea carbonyl is bonded to one NH group) which indicatedy an FT-IR absorption band around 1645 to 1635 cm−1. The lowerhe bond frequency, the tighter the bonded urea carbonyl is. Ouresults indicate that by increasing the urea linkage in polymer, theree urethane and urea carbonyls decrease. This reduction causes anlevation in hydrogen bonding of the hard to hard segment, shrink-ge of the connection and thus a better mixing of the soft and hardegments. By raising the urea linkages in polymer backbone, due topplying diamine chain extender in polymer synthesis, the hydro-en bonding between carbonyl and NH groups increase and theissolved hard segment in PTMG decrease. The phase separationf soft and hard segments mainly occurred by increasing the ureainkage in the polymer. The microphase separation of the studiedoly(urethane-urea)s varied as the follow order:

UU100 > PUU75 > PUU50 > PUU25 > PUU0.

.2. DSC analysis

The thermal properties of the synthesized poly(urethane-urea)sere investigated by DSC. The transition temperatures reported

y DSC analysis are listed in Table 1. The obtained results indicatehat by increasing the urea linkages in polymer, the glass transitionemperature related to soft segments in polymer decreases. Ouresults indicate that the first glass transition temperature related tooft segments decreased from −75.1 to −79 ◦C for PUU0 to PUU100

Fig. 3).

The glass transition temperature is one of the best criterionsor comparing the chain mobility of the polymers. Looking at thebtained Tgs of the synthesized poly(urethane-urea)s, it can be con-luded that the chain mobility of the polymers raises by increasing

Fig. 4. XRD pattern obtained for pure PTMG2000 polyethers.

the urea linkage in polymer. Since the type and length of polyolsin all synthesized poly(urethane-urea)s are the same, the increasein chain mobility of the polymer can be attributed to the increasedphase separation of its hard and soft segments. By increasing thephase separation in polymer and decreasing the physical linkagebetween hard and soft segments, the polyether soft segments caneasily move and mostly lead to reduction of glass transition temper-ature. The variation of polymers glass transition temperatures arein good agreement with the results of hydrogen bonding betweenhard and soft segments investigated by FT-IR.

The polymers with more dissolving hard segments in soft seg-ments showed higher glass transition temperature of the softsegments. Comparison of the DSC curves of the poly(urethane-urea)s indicated an increase in the slope of glass transition stateby increasing urea linkages in polymer. This is probably due to anincrease of microphase separation of poly(urethane-urea)s. Fig. 3shows that the decrease in the amount of urea linkages broadensthe polymers transition states. The reason for this phenomenonis the interaction of hard and soft segments and therefore thedecrease of microphase separation in polymers [23]. The smallendothermic peaks appeared in DSC results may be related to melt-ing of ordered structure of soft segments. It can be suggested thatthe polyol in synthesized poly(urethane-urea)s is configured incrystalline structure but there is no clear trend in variation of melt-ing points in these poly(urethane-urea)s.

3.3. X-ray analysis

Crystalline structure in polyurethanes arises from soft segmentsbecause of their ordered structures [31]. Therefore, the WAXD study

Fig. 5. XRD pattern obtained for synthesized poly(urethane-urea)s.

44 M. Sadeghi et al. / Journal of Membrane Science 354 (2010) 40–47

Table 2Diffusivity and solubility coefficients of studied gases through poly(urethane-urea) membranes.

Sample code Diffusivity × 10−7 (cm2/s) Solubility × 10−3 (cm3 (STP)/cm3 of polym cm Hg)

N2 O2 CH4 CO2 N2 O2 CH4 CO2

PUU0 2.01 2.55 1.45 3.89 1.53 3.04 5.98 23.94.45 1.50 2.55 5.65 23.85.52 1.53 3.31 5.83 20.75.90 1.42 2.85 5.56 20.26.20 1.37 2.76 5.96 20.6

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PUU25 2.15 3.07 1.92PUU50 2.32 3.09 1.96PUU75 2.89 3.61 2.38PUU100 3.45 4.20 2.57

eak around 2� = 20◦ might be due to amorphous structure, pres-nce of small crystalline structure or diffraction from large crystals32]. The thermal properties of the polymers from DSC also indicatehe endothermic peak around 60 to 80 ◦C related to the melting ofrdered structure of soft segments in polyurethanes. Therefore, it isuggested that the broadened peaks that appeared in WAXD studyre related to small crystalline regions. Comparisons of the obtainedrystalline peaks did not show a significant difference. It is there-ore concluded that there is not a considerable difference betweenolymer crystallinity in synthesized poly(urethane-urea)s. In addi-ion, these peaks in comparison to neat polyol in Fig. 4 show a lowerntensity. This might be due to the fact that hard segments formedre strengthened by the hydrogen bonding and act as reinforcingllers in the polymer and disturb the chain order needed for therystallization process [33].

.4. Gas diffusivity and permeability

According to equation 4, permeability and selectivity may beoverned by either the kinetics of diffusivity coefficient (D) or thehermodynamics of solubility coefficient (S) or both [4]. In theresent work, diffusion coefficient of gases through polymers wasalculated by the time lag method using Eq. (3). Then the solubilityoefficient for each gas was obtained from Eq. (4). The diffusiv-ty and solubility of gases in synthesized poly(urethane-urea)s areisted in Table 2. The permeability of gases through poly(urethane-rea) membranes versus the amount of urea linkage in polymersre shown in Fig. 6. For all membranes, the permeabilities decreasen the order of P(CO2) � P(CH4) > P(O2) > P(N2). The higher CO2 per-

eability is related to the higher solubility of CO2 in the membranesompared to O2, CH4, and N2. Carbon dioxide has small molecu-

ar size and high condensation temperature in comparison to thether gases. This is noteworthy that CO2 is a polar gas that cannteract with polar chain polymers. Hence, the permeability ofO2 in comparison with other gases in poly(urethane-urea) thatontain polar groups in the main chain of polymer is consider-

ig. 6. Permeability of N2, O2, CH4 and CO2 gases versus amount of urea linkageshrough poly(urethane-urea) membranes.

Fig. 7. Correlation of diffusion coefficient (D) with kinetic diameter.

ably higher. As shown in Table 3 the kinetic diameter of methaneis greater than nitrogen and oxygen molecules but the reportedpermeability in Fig. 6 indicates more permeation of methane com-pare to oxygen and nitrogen. This discrepancy can be explainedby known solution–diffusion mechanism [3,4]. Due to this mech-anism the higher condensation temperature of methane promotessolution of methane in polymer and leads to more permeability ofthis gas in the synthesized poly(urethane-urea)s compare to nitro-gen and oxygen. As shown in Table 2 the diffusivity of gases variesaccording to order CO2 > O2 > N2 > CH4. The order of diffusivity isthe reverse of the kinetic diameter for each gas. By increasing themolecular size, the diffusivity of gas decreases. The variation of dif-fusion coefficient (ln D) of penetrants as a function of their kineticdiameter is linear and as plotted in Fig. 7 showed a good corre-lation for all synthetic polymers (R2 > 0.95) except PUU50 whichshows weaker correlation (R2 = 0.92). The gas solubility in polymersvaried as CO2 > CH4 > O2 > N2. This order is similar to condens-ability of gases. It shows that by increasing the condensability ofgases the interaction and solubility of gases increases. Comparisonbetween diffusivity and solubility of methane, oxygen and nitro-gen revealed that the diffusivity of oxygen is 1.5–1.76 times greaterthan methane and diffusivity of nitrogen is 1.12–1.39 times greaterthan methane, due to their lower molecular size. The solubility ofmethane is 1.77–2.22 times greater than oxygen and is 3.76–4.36

times greater than nitrogen due to its higher condensation temper-ature. These results indicate domination of solubility mechanism inpermeation of gases in the studied poly(urethane-urea)s.

Table 3Condensability and kinetic diameter of studied gases [3].

Gas Kinetic diameter (Å) Condensability (K)

Carbon dioxide 3.3 195Oxygen 3.46 107Nitrogen 3.64 71Methane 3.8 149

embrane Science 354 (2010) 40–47 45

uihlrhashurwraasmpmpBgiasuep3qi

C

g

C

c

C

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siadrs

for the CO2 gas. The obtained results from these synthesizedpoly(urethane-urea) membranes were compared with Robeson’supper bound line in Fig. 11 [36]. As shown in this figure thesynthesized poly(urethane-urea) series investigated, show good

M. Sadeghi et al. / Journal of M

As shown in Fig. 6, the permeation of gases in poly(urethane-rea) membranes increases linearly with the urea linkage content

n the polymer. In addition, the reported results in Table 2 indicateigher levels of gas diffusivity by increasing the amount of urea

inkage in polymer. As investigated by FT-IR and DSC analyses, byaising the urea linkage in polymer, the hydrogen bonding betweenard and soft segments decreases and subsequently the phase sep-ration of hard and soft segments increases. Higher microphaseeparated systems lead to lower interaction and connection ofard and soft segments together. It is suggested in poly(urethane-rea) membranes that soft segment domains are formed as aesult of a microphase separation and permeable to molecules,hereas the hard segment domains act as an impermeable bar-

ier. Although the hard segment domains are not expected to beccessible to permeating molecules, they can influence the over-ll gas transport properties of a polymer due to their ability toerve physical crosslinks and change the dynamics of the soft seg-ents [23,24]. Therefore, by increasing the phase separation in

oly(urethane-urea) membranes, the permeable domains in poly-er, space created for diffusion of permeant molecules and thus the

athways for diffusing molecules through membrane increased.esides of increasing the pathways accessible for permeation ofases in membrane, the long segmental mobility of soft segmentsncreases with respect to reduction in physical bonds between hardnd soft segments. The result of permeability tests of gases in theynthesized poly(urethane-urea)s, showed that by increasing therea linkage in polymer from pure polyurethane, PUU0, to high-st degree content urea linkage poly(urethane-urea),PUU100,theermeabilities of CO2, O2, N2 and CH4 increase from 92.78, 7.76,.07 and 8.67 to 128, 11.58, 4.74 and 15.33 Barrer, respectively. Theuantity of increasing the gas permeability of the mentioned gases

s as follow:

H4 (76.8%) > N2 (54.4%) > O2 (49.2%) > CO2 (37.9%)

In addition, as reported in Table 2 increasing in diffusivity ofases changed in the following order:

H4 (77.2%) > N2 (71.6%) > O2 (64.7%) > CO2 (59.3%).

As shown in Table 3 the kinetic diameters of the studied gaseshanged as follow:

H4 > N2 > O2 > CO2

Wang et al. have shown the free volume sizes and frac-ional free volume increase by increasing the phase separation inoly(urethane-urea) membranes [34]. They established a directelationship between the gas permeability and the free-volumeased on the free-volume parameters and the measured gas dif-usivity. They have shown that the free volume and fractional freeolume increase by decreasing the phase connection and mixingn polymers [34]. Given the results of Wang et al, our experimentuggests that increasing the phase separation in poly(urethane-rea)s increases the size of free volume. In this case, diffusivity andermeability of larger molecular size gases increases more thanhe smaller ones [35]. Fig. 8 shows a brief correlation betweennhancements of transport properties, diffusion and permeabilityoefficients, versus molecular size of gases. As shown in this figureases with larger molecular size enhanced more in diffusivity andermeability.

The variation of CO2/N2, CO2/CH4 and O2/N2 selectivity ver-us urea linkage content in poly(urethane-urea)s are represented

n Fig. 9. As shown in this figure by increasing the urea link-ge in polymer, selectivity of CO2/N2, CO2/CH4 and O2/N2 gasesecreased linearly from 32 to 27, 10.7 to 8.34 and 2.52 to 2.44espectively. Considering the solution–diffusion mechanism, theelectivity of gases in polymers is specified by diffusivity- and

Fig. 8. Enhancement of transport properties versus molecular size of studied gases.

solubility-selectivities which enable the polymer chains to sepa-rate small molecules from large ones, condensable molecules fromnon-condensable ones and polar molecules from non-polar ones. Inthe studied polymers because the functional groups in permeablesoft segment domains are the same, the selectivity of gases changedin polymer dominantly by variation of the mobility of the polymerchains and the molecular sieving ability of the polymer. By increas-ing the hard and soft segments connection and increasing the hardto soft segment dissolution the size of free volume decreased andhence the separation of large and small molecules pursued moreefficient. Restricting the mobility of the soft segments increasedthe possibility of molecular sieving of polymer [34].

It is clear from the generally accepted solution–diffusion modelof 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 [17]. The same trend has been basicallyobserved for the PUUs studied in this work. An almost linear rela-tionship can be found between gas permeability and Tg in Fig. 10,showing the increase of permeability on the decrease of the Tgvalues.

Since the mole fraction of carbon dioxide in industrial stream-lines is low, the candidate membranes for separation of CO2in exhaust gases should have high selectivity and permeability

Fig. 9. Selectivity of CO2/N2, CO2/CH4, and O2/N2 gases versus amount of urea link-ages through poly(urethane-urea) membranes.

46 M. Sadeghi et al. / Journal of Membra

Fig. 10. Permeability variations of gases versus −ln(−1/Tg) in syntheticpoly(urethane-urea)s.

Fig. 11. CO /N separation performance of various polyurethane membranes(r

sabh

4

pbTsswgmmstddpiia

[

[

[

[

[

[

[

[

[

[

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[

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[

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[diene based polyurethane membranes, J. Membr. Sci. 105 (1995) 137.

[30] C.B. Wang, S.L. Cooper, Morphology and properties of segmented polyether

2 2

�) in comparison with poly(urethane-urea) membranes synthesized in thisesearch ( ).

eparation ability, high permeation and selectivity, in CO2/N2 sep-ration case. All of the synthesized polymers lie close to the upperond line. The good separation ability of these polymers indicatesigh potential of these polymers for commercial applications.

. Conclusion

Poly(urethane-urea) elastomers were synthesized usingolytetramethylene-glycol, isophorone diisocyanate and 1,4-utane diol/1,4-butane diamine with different BDO/BDA ratios.he effect of urea linkage content on the structure and gaseparation properties of polymers was investigated. The gaseparation properties of the synthesized poly(urethane-urea)sere studied using nitrogen, oxygen, methane and carbon dioxide

ases. The obtained results from DSC and FT-IR analyses showedore microphase separation of hard and soft segments andore flexibility for high urea linkage content polymers. The gas

eparation properties of polymers indicated that by increasinghe urea linkage and increasing chain mobility of polymers, theiffusivity and permeability of gases increase and selectivityecrease. The obtained results in diffusion and solution of gases inoly(urethane-urea)s confirm domination of solution mechanism

n permeation of gases through membranes. The obtained resultsn the CO2/N2 separation case, makes these poly(urethane-urea)sgood candidate for commercial applications.

[

ne Science 354 (2010) 40–47

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[2] B.D. Freeman, Basis of permeability/selectivity tradeoff relations in polymericgas separation membranes, Macromolecules 32 (1999) 375–380.

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