poly(tetramethylene glycol) based polyurethane membranes

Journal of Membrane Science 415–416 (2012) 469–477

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Gas separation properties of poly(ethylene glycol)/poly(tetramethylene glycol) based polyurethane membranes Mohammad Mehdi Talakesh a, Morteza Sadeghi a,n, Mahdi Pourafshari Chenar b, Afsaneh Khosravi a a b

Department of Chemical engineering, Isfahan University of Technology, Isfahan 84156-8311, Iran Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, PO Box 91775-1111, Iran

a r t i c l e i n f o

abstract

Article history: Received 4 March 2012 Received in revised form 13 April 2012 Accepted 17 May 2012 Available online 26 May 2012

In this study, the effect of the structure of polyether-based polyurethane (PU) membranes on their gas separation properties has been investigated. In this regard, a series of polyurethanes based on hexamethylene diisocyanate (HDI) and 1,4-butanediol as hard segments and different soft segments such as poly(tetramethylene glycol) (PTMG, 2000 g/mol), poly(ethylene glycol) (PEG, 2000 g/mol) and PTMG/PEG mixture were synthesized. PU membranes were prepared by thermal phase inversion method and their physical properties were analyzed by FT-IR, XRD and DSC analyses. The obtained results from FT-IR and DSC analyses indicated that the phase separation of hard and soft segments decreased and chain mobility was restricted, resulting in the increase in glass-transition temperature of the soft segment, by content of the PEG in the PU structure. The permeability of pure N2, O2, CH4 and CO2 gases were determined using constant pressure method at different feed pressures and temperatures. The results of gas permeation experiments showed that by increasing the ether group content in the polymer structure, permeability of the pure gases decreased, while CO2/N2 ideal selectivity increased. The obtained results also indicated that the permeability of CO2 decrease from 132.52 barrer in PU0 (PU containing 100 wt% of PTMG in soft segment) to 20 barrer in PU100 (the PU containing 100 wt% of PEG in soft segment), respectively. CO2/N2 selectivity increased from 28 to 90. Trade-off evaluation also showed that the potential of studied membranes for commercialization in CO2/N2 and even CO2/CH4 separations increase by PEG content in polymer and the PU membrane which contains 75/25 wt% ratio of PEG/PTMG had the highest. & 2012 Elsevier B.V. All rights reserved.

Keywords: Polyurethane Membrane Gas permeation Phase separation Ether group

1. Introduction Membrane technology is one of the most important separation methods due to lower energy consumption, higher performance insurance and less space and initial cost requirements. Polymeric membranes with favorable selectivity and permeability are mostly used membranes for gas separation applications, especially for carbon dioxide separation from natural gas and flare gases. Development and application of polymeric membranes for separation of gases require more investigation and improvement in their structure and separation properties [1]. Polyurethane is a rubbery polymer which is introduced for gas separation applications vastly. Polyurethanes are formed of hard segments containing urethane/urea group, and soft segments of polyether/ polyester. The main outstanding properties of this polymer are good mechanical properties and desirable permeation properties.

n

Corresponding author. Tel.: þ98 311 3915645; fax: þ 98 311 3912677. E-mail address: [email protected] (M. Sadeghi).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.05.033

Due to the presence of alkaline and polar urethane groups, and also polar ether soft segments in the structure of polyurethane, it was considered as one of the good candidates for separation of polar and acid gases from gaseous mixtures [2–5]. Different studies were performed on gas separation properties of polyurethane membranes. The main purpose of these studies was to find the relationship between the structure of the polyurethanes and their separation properties [6–27]. A direct relationship was observed between free volumes and permeability of gases in the polyurethane membranes. That means with increasing the free volumes in the polymer structure, permeability of gases increases. It was also observed that by increasing the amount of hard segment, size and percentage of free volumes decreased [6–10]. It should be noted that in polyurethane membranes soft segments are permeable domains, while hard segments are nonpermeable [11–14], and the length and kind of the soft and hard segments and the length of the chain extender affect the gas separation properties. Semsarzadeh et al. [12] showed that permeability of N2, CH4, O2, and CO2 gases through polyurethane membrane increased by decreasing the length and amount of hard segment (the amount of soft segment increases),

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but CO2/N2, CO2/CH4 and O2/N2 selectivities decreased. It was also presented by Grabczyk et al. [13] that by increasing the content of hard segments in the structure of polyurethanes, size and percentage of free volumes and permeable domains decreased. On the other hand, soft segments, due to creation of more permeable domains in the polymer, increase the permeability of gases and decrease the CO2/N2, CO2/CH4 and O2/N2 selectivities [11–14]. Semsarzadeh et al. [15] showed that with increasing length of chain extender, the gas permeability increases. In other research, it has been observed that the polyurethane and polyurethane urea membranes with soft segments of low molecular weight and in the presence of amine groups in the functionalized polyurethane chain as chain extender (free volume increment due to more phase separation), permeability of gases increases [16–18]. Polydimethylsiloxane is one of the polymers which has been considered as a soft segment in the structure of polyurethane and polyurethane urea. CO2 permeability increased in the presence of PDMS in the polyether-based polyurethane. This could be caused by induced phase separation between the two soft and hard segments of the polymer in the presence of PDMS. Also, the selectivities of polyurethane urea containing two soft segments are higher than those containing only on soft segment polyurethane or polyurethane urea membranes. Addition of polyethers (such as PPO, PEO, PTMO) to the PDMS-based polyurethane urea resulted in reduction in gas permeability, but increased the selectivity of CO2/N2 [19–21]. With increasing PDMS content in PU, gas permeability of polyurethane urea membranes containing two soft segments, PPO and PDMS, increased because of the high weight fraction of the siloxane segment, low degree of crosslinking and low accumulation of urethane/urea groups [22]. Several studies have shown that ether-based PU or PU containing urea groups in the polymer chain are good option for separation of polar and acidic gases such as CO2 and H2S from nonpolar gases [23–27]. In none of the studies mentioned above, polyurethanes containing the aliphatic diisocyanate (HDI) have not been investigated. In this study, the ether-based PU containing HDI in the hard segment was synthesized. Because the amount of the ether groups should enhance the CO2 separation we tried to evaluate this effect in this research. So in this work the effects of polyether type, and also increasing the content of ether groups in polyurethane structure on their gas permeation properties were investigated.

2. Experimental 2.1. Materials Polytetramethylene-glycol (PTMG, Mw ¼2000 gmol  1), was obtained from Arak Petrochemical Complex (Arak, Iran) and Polyethylene-glycol (PEG, Mw ¼2000 gmol  1) was obtained from Kimyagaran Emrooz (Arak, Iran). Both of them were dried at 80 1C under vacuum for 24 h to remove the residual water. 1,4butanediol (BDO), hexamethylene diisocyanate (HDI) and N,Ndimethylformamide (DMF) were purchased from Merck. The chain extender (BDO) was dried over 4 A˚ molecular sieves before use. N2, O2 and CO2 gases (purity 99.99) were purchased from Ardestan Gas Co. (Ardestan, Iran) and CH4 (purity 99.5) was purchased from Technical Gas Service (USA). 2.2. Polymer synthesis Polymerization was carried out in a three-necked roundbottomed flask equipped with a mixer shaft, nitrogen gas inlet tube and raw material entrance. Polytetramethylene-glycol

Table 1 Compositions and glass transition temperature (Tg) of synthesized polyurethanes. Sample

PU100 PU75 PU50 PU25 PU0

Soft segment content (wt%) PEG

PTMG

100 75 50 25 0

0 25 50 75 100

Tg (1C)

 49.69  54.989  56.137  63.655  71.712

contained polyurethanes were synthesized by two-step bulk polymerization technique and polyurethane containing 100% polyethylene-glycol was synthesized by two-step solution polymerization technique. Polyols reacted with hexamethylene diisocyanate for 2 h at 85–90 oC under nitrogen atmosphere to obtain macrodiisocyanate prepolymer. The chain extending of polymer occurred by addition of 1,4-butane diol after prepolymerization step at room temperature. In order to obtain linear polymer, the molar ratio of NCO and OH groups was kept 1:1. The molar ratio of the used components in the polymer synthesis was as follows: (polyol/diisocyanate/chain extender)¼1:3:2. Table 1 shows the composition and glass transition temperatures of synthesized polyurethanes.

2.3. Preparation of PU Membranes Polyurethane membranes were prepared by thermal phase inversion method. The synthesized polyurethanes were dissolved in DMF at 60 1C (10 wt% solution) and cast in petri dishes at room temperature. To prepare the homogenous dense membrane the membranes were formed by evaporating the solvent in an oven for 24 h at 40 1C. The thickness of the prepared membranes found to be around 100 mm.

2.4. Characterization The obtained functional groups in synthesized polyurethane were investigated by Tensor27 FT-IR (ATR, Bruker) in the range 4000–600 cm  1. The thermal behavior of polyurethanes was investigated by DSC (Metler-Toledo DSC822e) with heating rate of 10 1C/min. X-Ray diffraction patterns were recorded by monitoring the diffraction angle 2y from 51 to 601 on a Philips X’Pert (Netherlands) using Cu radiation under a voltage of 40 kV and a current of 40 mA.

2.5. Gas permeability measurement The permeability of pure N2, O2, CH4 and CO2 gases were determined using constant pressure/variable volume method at different feed pressures and temperatures [12]. The gas permeability (P) of membranes was determined as: P¼

ql AðP 1 =P 2 Þ

ð1Þ

where P is permeability expressed in barrer (1 barrer¼10  10 cm3(STP) cm/cm2 s cmHg), q is flow rate of the permeate gas passing through the membrane (cm3/s), ‘ is membrane thickness (cm), p1 and p2 are the absolute pressures of feed -side and permeate-side, respectively (cmHg) and A is the effective membrane area (cm2).

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The ideal selectivity (permselectivity, aA/B) of membranes was calculated from the ratio of pair gas permeabilities:

aA,B ¼

PA PB

ð2Þ

3. Results and discussion 3.1. Characterization of polyurethane membranes 3.1.1. FT-IR analysis Fig. 1 shows FT-IR spectra of synthesized polyurethanes. The absorption of the N–H group in urethane unit occurs at 3319 cm  1 whereas the absorption of the free-urethane carbonyl groups occurs at 1714–1720 cm  1 [15,18]. The urethanebonded carbonyl groups (involving in the hydrogen bonds to urethane N–H groups) are recognized by absorption at 1683 cm  1. The absorption peak of urethane ether is at 1112 and 1105 cm  1 [15,18]. The results from absorption of carbonyl groups in polyurethanes provide useful information about phase separation resulted from soft and hard segments in polyurethanes. Hard segments (urethane groups) separate from the soft segments and make agglomeration because carbonyl groups of urethane bonds connect to N–H groups of urethane bonds by hydrogen bonding. In this case, the so-called phase separation is obtained between the soft and hard segments. But, because of the hydrogen bonding between the urethane N–H groups and the ether groups in polyol, the hard segments and the soft segments connect together and mixing of hard and soft segments occurs. As shown in Fig. 1, following the changes in the type and the weight percent of polyol from PTMG to PEG, the absorption region of the N–H group in polyurethane is extended and its intensity is reduced. This phenomenon could be due to the reduction in hydrogen bonds between carbonyl groups of urethanes and the N–H groups, which can somehow indicate the reduction in phase separation and less affinity of the N–H groups to create the hydrogen bonds to carbonyl groups of urethanes and more affinity to mix with ether groups in soft segments. Bonding N–H groups of urethanes to ether groups in polyol by hydrogen bonds and due to the dispersion of hard segments in soft segments of polymer, the intensity of hydrogen bonds of N–H with ether groups will be different and thus it results the absorption peak of N–H to be extended. As indicated before, the frequency of the absorption of carbonyl groups serves as a factor for how and the rate of the bonds of such groups to the N–H groups of urethanes by hydrogen bonds. Bonding carbonyl groups to the N–H groups by hydrogen bonds, the absorption of such group occurs in the lower frequencies. Therefore, such carbonyl groups are called bonded carbonyl. To examine the changes in micro phase separation in synthesized polyurethanes more exactly, the region of absorption of carbonyl groups in the spectrum of FT-IR (ATR) is shown in Fig. 2. The figure shows that changes in polyol from PTMG to PEG result in changes in the peaks of absorption of free and bonded carbonyls and the spectrum of absorption of free carbonyl is transferred to the lower frequencies and the spectrum of absorption of bonded carbonyl is relatively removed. As shown in Fig. 2, the intensity of bonded carbonyl peaks is reduced by the increase in PEG. The peak of bonded carbonyl is relatively removed in PU100 polymer, and the intensity of its bonded carbonyl peak is more than that of free carbonyl peak in PU0 polymer. Such phenomenon indicates less phase interference and phase separation in PEG-based polymer, PU100, compared to the other structures.

Fig. 2. Region of absorption of carbonyl groups in the spectrum of FT-IR. On the other hand, emerging the bonded carbonyl groups in polyurethanes having low amount of PEG as completely separated peaks with more intensity than free carbonyl peak, indicates higher phase separation occurring in such polymers. In other words, phase separation reduced and the interference of phases increased by the increase in the amount of PEG (the number of ether groups) in polyurethane structures. Table 2 shows the frequency of free and bonded carbonyl absorptions with the changes in polyol type in the studied polyurethane structures. The amounts and the variation of hydrogen bonding in polymer can be represented by hydrogen bonding index (HBI) [15]. HBI value is obtained as:

HBI ¼

AC ¼ O,bonded AC ¼ O,free

ð3Þ

where AC ¼ 0, bonded and AC ¼ 0, free are the intensity of bonded and free carbonyls, respectively. The amount of phase separations for synthesized polymers can be compared by this factor. By increasing the amount of hydrogen bonds between the carbonyl groups and N–H groups in urethanes resulting in higher phase separation in polymers, HBI increases. Here, it is also examined the changes in HBI relative to the changes in polyol. The results showed that HBI reduces by changing the type and content of polyol from PTMG to PEG. The obtained HBI of PU0 was 2.308 whereas that of PU100 was 0.974. So, it can be said that the amount of hydrogen bonds between the existing groups in hard segments in synthesized polymers will change as follows: PU100ð0:974Þ o PU75ð1:141Þ o PU50ð1:182Þ o PU25ð1:487Þ o PU0ð2:308Þ

Fig. 1. FT-IR spectra of synthesized polyether-based polyurethanes.

It would be concluded that the presence of PEG as soft segment in the polyurethane structure reduces the formation of hydrogen bonds between the hard segments and results in lower phase separation in polymer. The reason for this phenomenon is the more polarity of PEG compared to PTMG due to the presence of more ether groups in its polyol compared to PTMG which increases the possibility of formation of hydrogen bonds between the soft and hard segments. As it is obvious from the structures of PEG and PTMG, there is one ether group for every two carbon atoms in PEG, whereas there is one ether group for every four groups of carbon atoms in PTMG, so, the amount of ether group in PEG is about twice that of PTMG. Thus, the tendency of PEG to establish hydrogen bonds to the N–H groups in urethane is twice that of PTMG, and obviously its hydrogen bonds are more and finally, it can create phase interference in polyurethanes.

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Table 2 Frequency of bonded and free carbonyls and HBI index of the synthesized polyurethanes. Polymer Frequency

Free carbonyl Bonded carbonyl

HBI

PU0

PU25

PU50

PU75

PU100

1720.44 1683.79 2.308

1718.52 1683.79 1.487

1716.58 1683.79 1.182

1714.66 1683.79 1.141

1714.66 1683.79 0.974

Table 3 The crystalline melting temperature of the soft segments in synthesized polyurethane. Polyurethane

PU0

PU25

PU50

PU75

PU100

Tm (1C)

12.3

17.1

27.3

42.5

51.1

PU100 4PU75 4 PU504 PU25 4PU0

As shown in Fig. 3, it is observed an endothermic peak in synthesized PU at temperature of 10–50 1C. This peak can be accounted for the crystals formed in the soft segment. It was cleared in previous studies that PEG and PTMG have the potential to be crystallized. A more accurate evaluation of endothermic peaks of the crystalline regions related to the soft segments, brings to mind two points: First, by increasing the amount of PEG, the intensity of crystalline peak increases. It would evaluate that more crystalline regions creates in soft segment regions. Second, by increasing the amount of PEG, melting temperature of the crystalline regions related to the soft segments increases. In the previous section it was observed that by increasing the amount of PEG in polymer, phase interference increases. Therefore, increasing amount of crystals in high level of PEG (while phase interference increased) can be attributed to more polar bonds between the PEG chains in the crystalline regions due to the more ether groups. In high amounts of PEG, more polar ether groups present in the soft chain and cause the more and stronger links between the chains in the crystalline regions and ultimately lead to an increment in the amount of the crystallization in the polymer. On the other hand, as reported in Table 3, the melting temperature of the soft segments increases in the polymer structures containing more PEG. According to the melting thermodynamic, by increasing the amount of polar bonds between the polymer chains in crystalline regions, the melting enthalpy (DH) increases. As a result, the melting temperature also increases, which is directly related to enthalpy [29]. It is observed that increasing of the polar ether linkages in the PEG based polymer, in addition to increasing the amount of crystals in the soft segment, also resulting in increased melting temperature of crystals in this region. As shown in Fig. 3, there is a broad endothermic peak at temperature of 150– 162 1C in both PU0 and PU25, the low PEG amount. Considering the figure, the emerged peak and its temperature region are the same for both polymers, and it can be related to the crystallization of hard segments in such polymers. In HDIbased synthesized polymers, the hard segment of polymer finds the potential to be regulated in the crystallized structure due to the linearity of diisocyanate and the strong hydrogen bonds in hard segment enhances also this phenomenon [28]. Emerging the endothermic peak related to the crystallization of hard segment in polyurethane PU0 and PU25 indicates the presence of the strong hydrogen bonds between the hard segments as well as high phase separation in polymer. Increasing the phase separation in such polymers provides the possible regulation of the chains of hard segments in crystal cells and the crystal regions are observed in the segments. This can confirm the results from FT-IR analysis about the increase in phase separation in polymer following the decrease in PEG in the polymer structure. The possibility of crystallization of hard segment will be reduced and the endothermic peak won’t emerge following the increase in PEG percentage in the structure (more than 50 wt%).

It was observed that the micro phase separation in synthesized PU reduced with the reduction in the amount of PTMG in soft segment. In structures with lower phase separation, the dynamics of polyol chain is reduced and the movement of the chain is limited because of more bonds between the soft, flexible chains of polyol and hard, motionless segments. So, it is observed that following the more phase mixing, glass transition temperature which is a good indication for flexibility and movement of polymer chains increased. Accordingly, lower glass transition temperature is shown by polymer PU0 with higher phase separation relative to polymer PU100 with the least phase separation and most phase interference. As it is known, the glass transition temperature in polymers is also affected by the crystalline regions. So it can be said that increasing the glass transition temperature of the soft segment in PEG-based polymer can be affected by further crystallization of polymer in these regions.

3.1.3. X-ray analysis Fig. 4 shows the X-ray spectra of the synthesized polymers. All polymers show a broad peak around 201 and according to the results from DSC test, the emerged broad peak can be accounted for small crystals distributed in the polymer or for the diffraction from large crystals [18]. Considering the spectrum of polymer PU0, it can be observed that the dispersion of crystallization is more in this structure which is observable at 201, 241, 281, 29.51, and 421. It is consistent with the observed peaks in DSC test of this polymer, indicating the presence of crystal regions in the soft and hard segments. Increasing the amount of PEG in the structure results in the reduction in the number of peaks related to the crystal regions and in other words dispersion of crystals decreases. Considering Fig. 4, it has been evaluated that the broad and great peaks in the range of 20–251 approaches together and finally they overlap at 231. In other words, consistent

Fig. 3. DSC thermograms of different polyol based polyurethanes. 3.1.2. DSC analysis Thermal behavior of synthesized PU, was studied by DSC method, Fig. 3. Because of the increase in phase separation in polymer, described in the previous section related to studying FT-IR, the glass transition temperature changes for polymers are as follows:

M.M. Talakesh et al. / Journal of Membrane Science 415–416 (2012) 469–477

Fig. 4. XRD pattern obtained for synthesized polyurethanes.

473

Fig. 6. CO2/N2, CO2/CH4, and O2/N2 selectivities versus amount of PEG in soft segment at 25 1C and 10 bar.

Table 4 Condensability and kinetic diameter of studied gases [18]. Gas

˚ Kinetic diameter (A)

Condensability (K)

Carbon dioxide Oxygen Nitrogen Methane

3.30 3.46 3.64 3.80

195 107 71 149

from PU0 to PU100 for all studied gases changed as follows: CH4 ð96:06%Þ 4 N2 ð95:32%Þ4 O2 ð94:81%Þ4 CO2 ð84:71%Þ

Fig. 5. Permeability of pure gases versus amount of PEG content in soft segment at 25 1C and 10 bar. with the results from previous analyses, increasing the amount of PEG in the polymer structure adds the interference of phases and so the synthesized polymer becomes more crystalline. 3.2. Gas permeation In this study, the permeability of pure N2, O2, CH4, and CO2 gases through prepared PU membranes were investigated at different operating conditions (temperature and pressure). Therefore the effects of ether groups were investigated on the gas permeation properties of the synthesized membranes. 3.2.1. Effect of ether groups on the gas permeability Figs. 5 and 6 show the permeability and selectivity of dense PU membranes (at 25 and 10 bar) versus the amount of PEG in soft segment, respectively. As shown in these figures the permeability of pure gases changes as follows in all membranes: P(CO2) b P(CH4) 4P(O2) 4P(N2) High permeability of CO2 compared to other pure gases in polyurethane is because of high solubility of CO2 in polyurethanes. Because of having polar ether groups of C–O–C in the soft segment, the polyurethanes provide suitable sites for attracting the molecules contain polar bonds like CO2 in the matrix of polymer and the increase in its permeability in the membrane is resulted. Moreover, CO2 is smaller than other gases in terms of molecular size (Table 4) [18]. It facilitates the penetration of this gas in the membrane. The more permeability of CH4 compared to the smaller molecules of N2 and O2, indicates the predominance of solution mechanism in the process of gas permeation in the studied polyurethane membranes. As shown in Fig. 5, permeability of all pure gases decreased by increasing the amount of PEG and ether groups in PU structure. The decrement of permeability

The order of reduction in gas permeability is followed on the molecular size of gases (Table 4). This trend shows that increasing the ether groups in polymer matrix results in the more reduction in permeability of bigger molecule. As evaluated in characterization of membranes, by increasing the amount of PEG, the mixing of soft and hard phase increases and also the crystallization of soft segment increases. The more soft and hard phase mixing leads to lower chain mobility and also the crystal regions act as non-permeable regions in membranes. These two phenomena lead to lower permeability of gases in polyurethanes containing higher amount of PEG. Also these phenomena may increase the screening ability of the membranes. Accordingly, molecules with larger size will have more limitation and their permeability will be reduced more than the smaller ones. In fact, the polyurethane matrix will become more polar by increasing the polar ether groups in the structure, so the absorption and penetration of polar molecules will increase [12–18]. Also, according to the previous studies, it is clear that the amount of and the size of free volumes in polyurethane is reduced by increasing the phase interference and crystallization of soft segment, because of this, gases with greater molecular size will face with more limitation in passing through the polyurethane membrane [18]. As mentioned before, the less movement of polymer chains, the molecular screening of polymer increases and it is obvious that the selectivity of gases also increases. Changes in the selectivity of gases by increasing PEG in polymer structure from 0 to 100 are as follows: CO2/CH4 (287.99% 4 CO2/N2 (226.59%) bO2/N2 (11.02%) As shown the CO2/CH4 and CO2/N2 selectivities are considerably more than O2/N2 selctivity (about 22–28 times). It is because of the higher deference between molecular size of the CO2 in comparison to N2 and CH4 and also more affinity of polar CO2 gas in polymer due to presence of more ether groups in PEG. The molecular size of the O2 and N2 gases are close together and also they have no special interaction with polymer matrix, so the changes formed in the selectivity of such gases is only resulted from the increase in molecular screening of polymer. This effect will be presented in the penetrative selectivity. Whereas the pair gases of CO2/CH4 and CO2/N2 will face the increase in the penetrative selectivity, in addition to the small size of molecule CO2 compared to CH4 and N2, high affinity of CO2 to ether groups results in the considerable increase in solubility of this gas in the polymer, and so its solubility selectivity will considerably increase. So, the total selectivity which is product of diffusivity and solubility selectivity will be increased considerably in CO2/CH4 and CO2/N2 pair gases in comparison to O2/N2.

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Table 5 Effect of pressure on permeability of pure gases and selectivity of PU membranes at 25 1C. Sample Pressure (bar) Permeability (barrer)

PU0

PU25

PU50

PU75

PU100

6 8 10 6 8 10 6 8 10 6 8 10 6 8 10

Selectivity

N2

O2

CH4

CO2

O2 / N2

CO2/ CH4

CO2/ N2

4.68 4.69 4.70 4.88 4.77 4.15 3.56 3.48 3.31 3.12 2.53 2.32 0.39 0.31 0.22

11.90 11.93 11.95 11.60 11.40 11.06 9.18 8.81 8.18 6.51 6.38 5.73 0.84 0.68 0.62

15.73 15.77 16.24 14.59 14.67 14.88 10.18 10.27 11.13 8.20 8.06 7.46 0.77 0.73 0.64

118.35 123.25 132.52 123.24 124.81 129.40 121.29 122.15 126.07 96.83 102.63 106.03 24.21 23.04 20.26

2.54 2.54 2.54 2.38 2.39 2.67 2.58 2.53 2.47 2.09 2.52 2.47 2.12 2.19 2.82

7.52 7.82 8.16 8.45 8.51 8.70 11.91 11.89 11.33 11.81 12.73 14.21 31.44 31.56 31.66

25.29 26.28 28.19 25.25 26.17 31.18 34.07 35.10 38.08 31.04 40.57 45.70 62.08 74.32 92.09

Fig. 7. Effect of temperature on the permeation of N2 gas through synthesized polyurethanes.

3.2.2. Effect of pressure on the gas permeability Pure gas permeation test were carried out at 25 1C and 6, 8 and 10 bar on all PU membranes to investigate the effect of pressure on the gas permeability. The obtained results are reported in Table 5. As shown in Table 5, in PU0 polyurethane, permeability of almost pure gases increase slightly by increasing pressure from 6 to 10 bar. The permeability coefficients in PU0 increase as follow: O2 (0.42%) ZN2 (0.43%)o CH4 (3.24%)5CO2 (11.97%) This process shows that the permeability of condensable gases CO2 and CH4 increase more than the one of non-condensable gases, O2 and N2 gases. This observation indicates the dominating solution mechanism in the synthesized polymers. The gas penetration into the polymer is increased by increasing the pressure. So, the condensable gases with more solubility in rubbery urethane polymers will gain more solubility, thus their permeability which is product of solubility and permeability will increase more than the non-condensable gases. On the other hand, the more increase in the permeability of CO2 compared to CH4 resulted from its more solubility in polymer matrix. Therefore, it is observed that the O2/N2 selectivity remains unchanged by pressure whereas the CO2/N2 and CO2/CH4 selectivities increased. Incorporating PEG into the polyurethane structure as a whole or part of the soft segment and also with increasing pressure, the increasing trend of the pure gases permeability are undergoing considerable changes. So that the permeability of non-condensable gases O2 and N2 is decreased in polyurethane PU25 by 14.96 and 4.96%, respectively. But, the permeability of condensable gases CO2 and CH4 increased by 4.99 and 1.99%, respectively. As a result, because of the more increase in the permeability of CO2 compared to CH4 and the more reduction in the permeability of N2 compared to O2, the selectivity of all pair gases will increase. The trend of permeability in polyurethane PU50 is the same as polyurethane PU25, but only its CO2/N2 selectivity is high. By increasing the share of PEG to more than 50%, some changes in the permeability of gases were observed. In polyurethane PU75, in addition to the non-condensable gases O2 and N2, the permeability of the condensable CH4 gas is also reduced, as follows: N2 (25.64%) c O2 (11.98%)4CH4 (9.20%) Consequently, by increasing the permeability of condensable gas CO2 (9.5%) in this structure, the selectivity of all gases increased. By removing PTMG completely from soft segment, in polyurethane PU100 (with 100 wt% of PEG), the trend of reduction in the permeability of pure gases (by increasing the pressure) involves the polar and condensable CO2 gas, and the trend of this reduction is as follows: N2 (43.59%) b O2 (26.19%) 4CH4 (16.88%) 4CO2 (16.32%) Considering this process and more reduction in the permeability of N2 compared to other gases, it is observed that the selectivity of all gases increase so that the CO2/CH4 and CO2/N2 selectivities increase up to 92.10 and 31.66, respectively. In this regard and confirming the results stated in the previous parts, it can be concluded that by incorporating and increasing the amount of PEG in the polyurethane structure, this polymer presents the behavior of a glass polymer against increasing the pressure. In other words, by increasing the pressure, the permeability of pure gases decreases and the rate of matrix screening and gas

Fig. 8. Effect of temperature on the permeation of O2 gas through synthesized polyurethanes. selectivity increase. As described before, incorporating PEG in the soft segment decreases the phase separation and the movement of flexible and soft chains of polyol because of bonding to hard segment. As a result, the rate of permeability is reduced and finally, penetrative selectivity is increased in polymer. Considering the greater molecular size of CH4, more reduction in permeability of N2 compared to CH4 is because of more condensability of CH4 in polymer. Therefore, it is observed that contrary to the greater size of its molecule, the permeability of CH4 has less reduction compared to N2. Also, it is seen that the selectivity of two non-condensable gases O2 and N2 increases less than CO2/N2 and even CO2/CH4 in some cases, which is due to the penetrative selectivity created in polymer occurred in all pair gases.

3.2.3. Effect of temperature on the gas permeability To investigate the effect of temperature on the permeability of pure polyurethane membranes, pure gas permeability test was conducted at room temperature 35 and 45 1C (at 10 bar). The effect of temperature on permeability and selectivity of pure gases are given in Figs. 7, 10, 11, and 13. As shown in Figs. 7–10, the increase in the temperature, results in the increase in the permeability of gases. The increase in permeability by temperature is because of the higher molecular movement of polymer chains and also higher free volume at higher temperatures. Investigating the results obtained for polyurethane structure indicates that by increasing the temperature, there is a similar trend in increasing the permeability of pure gases. The trend of increasing in the gas permeability of PU0, PU25, PU50 and PU 75 polyurethanes is as follow: N2 4CH4 4 O2 b CO2 This trend in PU100 changes as follow: CH4 (2226.56%) 4 N2 (1959.10%)4O2 (1674.19%) 4CO2 (571.42%)

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475

Fig. 9. Effect of temperature on the permeation of CH4 gas through synthesized polyurethanes.

Fig. 12. Effect of temperature on CO2/CH4 selectivity in the synthesized polyurethanes.

Fig. 10. Effect of temperature on the permeation of CO2 gas through synthesized polyurethanes.

Fig. 13. Effect of temperature on CO2/N2 selectivity in the synthesized polyurethanes.

Fig. 11. Effect of temperature on O2/N2 selectivity in the synthesized polyurethanes.

The considerable point in increasing the permeability of pure gases is that by increasing PEG in soft segment of polyurethane, the increment in permeability of gases will increase. It is because that increasing the temperature results in the increase in the movement of flexible ether groups which is intensified by the

increasing the ether groups in polymer matrix, so the permeability is increased severely. In addition to what was observed, because of more interaction between soft and hard segments at the presence of PEG in polymer, the movement of polyol chains is limited more by bonding to the hard segment. So, increasing the temperature and the movement in polymer, the bonds are more likely to be disbanded and the PEG chain shows more motion. Thus, the effect of increasing the temperature is more obvious in polyurethanes containing higher degree of PEG. The other point to be noted here is the amount of crystallization of soft phase in polymer. As observed in DSC test, there is a crystal peak for all polymers at 45–50 1C. So, by approaching to the melting temperature of such crystals, it is possible for polymer chains to leave crystal cells and the crystal structure would be opened. On the other hand, because the crystal parts are impermeable segments against the gas molecules, the amorphous segments and consequently the space needed for passing gas will increase and finally the permeability of gases will enhance by reducing the crystal regions. As shown in Figs. 11–13, the gas selectivity would decrease by temperature. Also, it has been shown that increasing the temperature leads to more reduction in the CO2/CH4 and CO2/N2 selectivities in comparison to O2/N2 selectivity. It is clear that the reduction in the selectivities of gases by temperature in PU100 is more than the other synthesized polymers because of good mixing of phases in this polymer. Because of favorable mixing of phases in such polymer, the phases are disbanded more likely by increasing the temperature and molecular movement. Therefore the permeability was increased and finally the selectivity was reduced more because of the reduction in molecular screening. Moreover, the reasons described for increased permeability of gases in polymer can be represented. The relationship of permeability with temperature is assumed to obey Arrhenius equation as follows [30]:   Ep P ¼ PO exp  RT

ð4Þ

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Table 6 Activation Energies (EP; J/mol) of synthesized polyurethanes for each gas (feed pressure: 10 bar). Gas Polymer

CO2

CH4

O2

N2

PU0 PU25 PU50 PU75 PU100

4.01 8.81 5.87 12.81 75.39

20.71 24.29 25.25 37.04 124.29

18.69 21.00 23.17 34.98 113.65

22.81 25.53 26.97 35.85 119.56

where P0 is the pre-exponential factor, EP the permeation activation energy, R the universal gas constant, and T is the absolute temperature. The dependence of the permeability of condensable gases on temperature is similar to that of noncondensable and light gases whose transport in a dense polymer membrane is controlled by the same solution-diffusion mechanism. Then it depends to the diffusivity (D) and solubility (S) coefficients. Diffusion coefficient always increases with a increase in temperature; that is, the energy of activation for diffusion (Ed) is positive. While, sorption coefficient increases with decreasing temperature; that is, the heat of solution (DHS) is negative. The permeation activation energy (EP) is the sum of Ed and DHS. Then it can be either positive or negative [26,30]. Activation energy of permeation of gases for investigated polymers is presented in Table 6. As is evident, by increasing the amount of PEG in the polymer structure, activation energy increases which indicates the higher dependency of polymer to temperature in more PEG-based polymer. As shown in Fig. 7, the permeabilities of pure gases increase exponentially with increasing temperature. The results indicate that the temperature dependence of pure gases permeabilities in polyurethane membranes is dominated by diffusion (Ed 4 DHS). Gases such as CO2 that present low EP values have a high permeability. Higher EP values were observed for CH4 and N2, the two highest size gas molecules. Two main factors govern the behavior of EP of different gases are molecular size and interaction with the polymer (solubility). The reason why N2 is less permeable than CH4 in spite of its being a smaller molecule is its low solubility in the polyurethane. Hydrogen interaction may take place between CO2 and N–H groups. N2 has the highest EP value due to its small solubility. As shown in Table 6, by increasing the amount of PEG up to 50 wt% in the soft segment of polyurethanes, increment trend of the activation energy of pure gases is as follows:

Fig. 14. CO2/CH4 Separation performance of polyether-based polyurethane membranes compared to the Robeson’s upper bound and other polyurethanes reported in literature.

EP (CO2) 4EP (O2) 4EP (CH4) 4EP (N2) With more than 50 wt% PEG, the activation energy of pure gases will change as follows: EP (CO2) 4EP (O2) 4EP (N2) 4EP (CH4) These trends confirm that by increasing the ether groups, the dominant mechanism is penetration (diffusion) and therefore the size of gas molecules will be decisive. On the other hand, it is clear from the results that by increasing the amount of PEG in soft segment of polyurethane, the activation energy of each gases increases. So that the lowest and highest increase from PU100 to PU0 is 424 and 1780% for N2 and CO2, respectively. The permeation activation energy depends on the structure and morphology of the polymers. Semi-crystalline polymers have high permeation activation energy. The low gas permeability is caused to the presence of crystalline domains as hinder the transport of gases [30].

3.3. Selection the best structure of synthesized polyurethanes for CO2/N2 and CO2/CH4 gas separation To use the polymer membrane commercially in the processes of gas separation, the polymers which have high permeability and selectivity are more attractive. According to the results from permeability tests for various polyurethane structures, it is clear that the polyurethane PU0 has the maximum permeability for the pure CO2 gas, but lower CO2/N2 and CO2/CH4 selectivity compared to the other structures, whereas the polyurethane PU100 has the maximum CO2/N2 and CO2/CH4 selectivity but lower CO2 permeability compared to the other structures. Considering the reverse relation between the permeability and the selectivity, it is not possible to select the suitable option for commercialization. Accordingly, to select the best polyurethane membrane, the graphs and curves by Robeson is applied as the proper benchmark [31].

Fig. 15. CO2/N2 Separation performance of polyether-based polyurethane membranes compared to the Robeson’s upper bound and other polyurethanes reported in literature. Fig. 14 shows the CO2/CH4 selectivity versus the CO2 permeability for the synthesized polyurethanes in this study and also the results from the synthesized polyurethanes by other researches which were compared with Robeson’s upper bound in 2008 to determine the desirable polymers to be commercialized. According to Robeson’s upper bond, the membranes with permeability and selectivity near or higher than this line are suitable for commercialization [31]. As shown in the figure, by increasing the amount of PEG up to 100 wt%, separation of CO2/CH4 gases is improved and approach to the Robeson’s upper line. Therefore, it would be concluded that PU75 and PU100 have the best performance in CO2/CH4 separation. Fig. 15 shows the changes in the selectivity of CO2/N2 pair gases relative to the permeability of CO2 for the synthesized polyurethane in this study and also the results from the synthesized polyurethane by other researches. It can be seen that by increasing the amount of PEG up to 100 wt%, separation of the CO2/N2 gases is improved and approach to the Robeson’s upper line. It is apparent very well in this figure the advantage of PU75 and PU100 polyurethanes to other structures and the structures synthesized before by authors [31].

4. Conclusions In this study, behavior of pure gas permeability of polyurethanes based on PEG at different pressure and temperature was investigated. The results obtained from characterization tests like FT-IR, DSC and XRD showed that the mixing of soft and hard segment phases and crystallinity of soft segments increase by increasing the ether polar groups in polymer structure. Gas permeation results indicated that solubility mechanism was

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dominant in the gas transport through the prepared membranes. Also, it was obtained that by increasing the number of ether groups in the soft segment of polyurethane, gas permeability decreases and selectivity increases. The CO2 permeability decreased from 132.52 to 20.26 barrer in polyurethane containing 100 wt% PTMG to polyurethane containing 100 wt% PEG. While the CO2/N2 and CO2/CH4 selectivities increased from 28.20 and 8.16 to 92.10 and 31.66, respectively. With increasing pressure, the permeabilities of gases increased in polyurethane based on PTMG. With increasing amount of PEG in the structure, first the noncondensable gas permeability decreased but with increasing amount of PEG to more than 50 percent, the condensable gas permeability decreased also. So in the polyurethane based on PEG, the permeability of all gases decreased with increasing pressure. In other words, the permeability of gases versus pressure in polyurethane synthesized with PEG changes as a glassy polymer. With increasing pressure, CO2/N2 and CO2/CH4 selectivity of the synthesized polyurethane increased. With increasing temperature, gas permeability of the synthesized structures increased and selectivity of the synthesized structures decreased. Comparing the performance of the synthesized polyurethanes in this study in separation of the CO2/CH4 and CO2/N2 to previous works shows that these structures have the best performance in the above mentioned separations. References [1] S. Sridharab, B. Smithab, T.M. Aminabhavia, Separation of carbon dioxide from natural gas mixtures through polymeric membranes, Sep. Purif. Rev. 36 (2007) 113–174. [2] M.C. Porter, Handbook of Industrial Membrane Technology, Noyes Publications, 1990. [3] R.W. Baker, Membrane Technology and Applications, 2nd ed., John Wiley & Sons, Inc., 2004. [4] S.L. Cooper, A.V. Tobolsky, Properties of linear elastomeric polyurethanes, J. Appl. Polym. Sci. 31 (1986) 1837. [5] C. Hepburn, Polyurethane Elastomers, Elsevier Applied Science, 2nd ed., 1992. [6] Z.F. Wang, B. Wang, Y.R. Yang, C.P. Hu, Correlations between gas permeation and free-volume hole properties of polyurethane membranes, Eur. Polym. J. 39 (2003) 2345–2349. [7] Z.F. Wang, B. Wang, N. Qi, X.M. Ding, J.L. Hu, Free volume and water vapor permeability properties in polyurethane membranes studied by positrons, Mater. Chem. Phys. 88 (2004) 212. [8] S. Mondal, J.L. Hu, Z. Yong, Free volume and water vapor permeability of dense segmented polyurethane membrane, J. Membr. Sci. 280 (2006) 427–432. [9] M.F.F. Marques, C. Lopes Gil, P.M. Gordo, Zs. Kajcsos, A.P. de Lima, D.P. Queiroz, M.N. de Pinho, Free-volume studies in polyurethane membranes by positron annihilation spectroscopy, Radium. Phys. Chem. 68 (2003) 573–576. [10] M.F. Ferreira Marques, P.M. Gordo, A.P. de Lima, D.P. Queiroz, M.N. de Pinho, P. Major, Zs. Kajcsos, Free-volume studies in polycaprolactone/poly(propylene oxide) urethane/urea membranes by positron lifetime spectroscopy, ACTA Phys. Pol. A 113 (No. 5) (2008).

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