Silicon (2016) 8:75–85 DOI 10.1007/s12633-015-9312-9
ORIGINAL PAPER
Characterization and Gas Permeation Properties of Synthesized Polyurethane-Polydimethylsiloxane / Polyamide 12-b-Polytetramethylene Glycol Blend Membranes Eshagh Vakili1 · Mohammad Ali Semsarzadeh2 · Behnam Ghalei1,2 · Morteza Khoshbin3 · Hadi Nasiri3 Received: 14 September 2014 / Accepted: 10 June 2015 / Published online: 30 July 2015 © Springer Science+Business Media Dordrecht 2015
Abstract Blend membranes of synthesized polyurethane (PU) based on toluene diisocyanate (TDI), polydimethylsiloxane (PDMS) and polytetramethylene glycol (PTMG) with polyamide 12-b- polytetramethylene glycol (PA12-bPTMG) were prepared by a solution casting technique. The heterogeneous microstructures of the blend membranes (PU /PA12-b-PTMG) were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Gas transport properties were determined for O2 , N2 , CH4 , and CO2 gases and the obtained permeabilities were correlated with polymer properties and morphology of the membranes. Comparison of the results with that of the pure PU membrane indicates that the blend membranes had higher permeability to CO2 , but lower permeability to O2 , N2 and CH4 gases, and, therefore, had higher values of CO2 /N2 and CO2 /CH4 ideal gas pair selectivities. The blend membrane with 20 % (wt) PA12-b-PTMG showed the highest CO2 permeability (≈105 Barrer) compared to the PU and other blend membranes. In the blend membranes with 5–20 % (wt) PA12-b-PTMG contents an enhancement of CO2 /CH4 (≈ 10) and CO2 /N2 (≈ 52)
Behnam Ghalei
[email protected] 1
Polymer group, Chemical Engineering Department, Tarbiat Modares University, Tehran, Iran
2
Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida-Honmachi, Sakayo-ku, Kyoto 606-8501, Japan
3
Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
selectivities was observed. The experimental permeabilities of the blend membranes were compared with the calculated permeabilities based on a modified additive logarithmic model. Keywords Gas separation · Membrane · Polyamide-12-b-PTMG · Polyurethane
1 Introduction Gas separation processes by means of membranes have become an interesting alternative to other conventional methods. Polymeric gas separation membranes are used in a wide variety of areas, such as air separation, separation of carbon dioxide from natural gas and removal of hydrogen from mixtures with hydrocarbons in petrochemical processing [1, 2]. Blending different polymers to achieve a membrane material with superior properties compared to those of the initial constituting components has been considered as an attractive method to attain better separation characteristics in membrane materials. The gas separation performance of polymeric blends is mainly related to composition, transport properties and morphology of the phases [3–6]. In miscible blends, the diffusion process is influenced by the interaction between the constituting polymers, while for heterogeneous blends, interfacial phenomena and the rubbery or glassy nature of the phases are important. Also, in heterogeneous polymer systems, microphase and/or macrophase separation occurs. The degree of heterogeneity and the preparation method may influence the transport properties of the final membranes [7–9]. The most desirable properties of polyurethanes (PUs) are their advantages in tailor-made adjustment of phases and
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the ability of making a great variety of microstructures. In PU multiblock polymers, the hard segments act as fillers or physical crosslinks. These segments are in the crystalline or amorphous glassy state while the soft segments are in a rubbery state with flexibility and high elasticity. The hard to soft segment ratio can be changed in the synthesis to achieve PUs with specific properties. These polymers may exhibit very interesting mechanical and thermal properties due to both their chemical structure and the extent of phase separation states between hard and soft blocks. For membrane uses, polysiloxanes have been one of the most attractive membrane materials due to their unique properties that arise mainly from the nature of the siloxane bond and high permeability to gases and organic vapors. PUs with siloxane based soft segments are considered as high performance materials with good oxidative and thermal stability, gas permeability and biocompatibility [10–13]. Polyamide 12-b-polytetramethylene glycol (PA12-bPTMG) is a block copolymer containing ether oxygen units as poly (amide-b-ether) that has attracted significant attention as a promising rubbery polymer for making gas separation membranes. This block copolymer consists of a regular linear chain of rigid polyamide (nylon 12) as hard segments, interspaced with the flexible polyether, poly (tetramethylene glycol) (PTMG), as soft segments. The hard polyamide crystalline domains provide mechanical strength and the soft polyether amorphous domains offer a high permeability due to the chain mobility of the ether linkages and methylene groups. The gas permeability of PA12-b-PTMG has been studied for various gases, and the PA12-b-PTMG block copolymers, in general have good permselectivity for polar/non-polar gas pairs [14–16]. In this study thermal characterization and gas transport properties of PU/PA12-b-PTMG blend membranes were investigated. In particular, the influence of PA12b-PTMG content on the gas permeation properties of PTMG/PDMS based polyurethane membranes is reported. The obtained experimental permeabilities data were compared with that calculated on the basis of the blend composition.
2 Experimental 2.1 Materials Polytetramethylene glycol (PTMG, Mw =2000 gmol−1 , Mn =1000 gmol−1 ) was purchased from Arak Petrochemical Co. Iran. Poly (dimethylsiloxane bis hydroxyalkyl terminated) (PDMS, Mn =5600 gmol−1 ) and the PA12-bPTMG diblock (comprised of 70 and 30 wt. % of PTMG
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and PA12, respectively) were obtained from Sigma-Aldrich, UK. The PTMG was dried at 80 ◦ C under vacuum for 48 h in order to remove the residual water. 1,4-butanediol (BDO), toluene diisocyanate (TDI), n-propanol and nbutanol were purchased from Merck, Germany. The chain ˚ molecular sieves before extender (BDO) was dried over 4A use. CO2 , N2 and O2 gases (purity 99.99 %), used for gas permeation tests, were purchased from Roham Gas Co. Iran and CH4 (purity 99.9 %) was obtained from Air Products Co. Iran. 2.2 Polymer Synthesis The PDMS/PTMG based PU was prepared via a twostep condensation reaction [17]. The mixtures of PDMS and PTMG were incubated with TDI for 2 h at 85–90 ◦ C under nitrogen atmosphere to obtain macrodiisocyanate prepolymer. The pre-polymer was examined for NCO content using the standard method of n-butyl amine titration (ASTM D2572). The chain extension of the pre-polymer was performed by addition of BDO at room temperature. In order to obtain linear polymer, the molar ratio of NCO: OH was kept at 1:1. The molar ratios of the used components were as follows: PTMG/PDMS: TDI: BDO = 1:3:2 and PTMG/ PDMS= 2:1. The basic synthesis mechanism of PU-PDMS is schematically illustrated in Fig. 1. 2.3 Membrane Preparation PU-based blend membranes were prepared by a solution casting and solvent evaporation technique. For this purpose 5 wt. % of PA12-b-PTMG was dissolved in a mixture of npropanol / n-butanol (1/3 wt. %) and the polymer solution (5 wt. % of PA12-b-PTMG) stirred under reflux at 80 ◦ C for 2 h. n-propanol / n-butanol (1/3 wt. %) as a good solvent does not show problems of gelation during polymer solution preparation and homogeneous multicomponent membranes are obtained. After cooling the solution to room temperature, different amounts of PDMS/PTMG based PU were added and stirred for 2 h. The two polymers were mixed in the following proportions: PU/ PA12-b-PTMG: 100/0, 95/5, 90/10, 85/15, 80/20 % (wt). The obtained homogeneous solution was poured into a Teflonized glass Petri dish and left at room temperature under atmospheric pressure for 48 h. The films were then dried in a vacuum oven overnight at 60 ◦ C to remove residual solvent. Membrane thickness was measured by a micrometer caliper; it varied from 100 to 120 μm. The average thickness of an individual membrane was calculated based on the results of five separate thickness measurements at different points on the membrane surface.
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Fig. 1 Synthesis of PU-PDMS
3 Membrane Characterization
3.3 Morphology
3.1 Fourier Transform Infrared (FT-IR)
The cross-section morphology of the membranes was examined using scanning electron microscopy (SEM). Crosssections of the membranes were obtained by fracturing in liquid nitrogen. All samples were coated with gold/palladium and were observed with a Philips XL30 (Netherlands) SEM.
A Perkin–Elmer Spectrum One FT-IR spectrophotometer (USA) was used to characterize the synthesized PU, PA12-b-PTMG and PU/PA12-b-PTMG blends using the KBr method at room temperature. The scanning frequency range was 4000–400 cm−1 .
3.4 Gas Permeation Measurements 3.2 Thermal Analysis Thermal properties of the samples were characterized in the temperature range from −100 to 190 ◦ C by using a Netzsch DSC 204 calorimeter (Germany). Before performing DSC, the test samples were heated to 50 ◦ C for 48 h in a vacuum oven to eliminate prior thermal history. Measurements, including baseline determinations, were performed at the scanning rate of 10 ◦ C /min, and the experiments were conducted using a nitrogen purge gas stream. Crystallization and glass-transition temperature values were obtained from cooling scans.
The pure gas (N2 , O2 , CH4 , and CO2 ) permeation properties of PU and PU/PA12-b-PTMG membranes were measured using a constant pressure method at room temperature (25 ◦ C) [1]. Figure 2 shows the schematic representation of the gas permeation equipment. The feed side pressure of the gases ranged from 4 to 10 bar. The permeate side was maintained at atmospheric pressure. The gas permeability was determined from (1): P =
q A(p1 − p2 )
(1)
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Fig. 2 Schematic of single-gas permeation apparatus
Where P is permeability expressed in Barrer (1 Barrer = cm3 (STP) cm/(cm2 s cm-Hg), q is the flow rate of the permeate gas passing through the membrane (cm3 (STP)/s), l the membrane thickness (cm), p1 and p2 the absolute pressures of feed side and permeate side, respectively (cmHg), and A the effective membrane area (cm2 ). The ideal selectivity,αA/B (the ratio of single gas permeabilities), of the membranes was calculated from the pure gas permeation experiments: 10−10
αA/B =
PA PB
1020, 1100 cm−1 (–Si–O–Si– and –C–O–C– bending) and 803 cm−1 (–CH3 rocking of –Si(CH3 )2 –O–) attributable to the PDMS and PTMG. Several characteristic absorption peaks of PTMG and PDMS overlap, but the absorption peaks of the urethane hard segment resulting from the reaction between isocyanate (–NCO) and hydroxyl (–OH) groups verified the synthesis of PDMS/PTMG based PU.
(2)
All permeability measurement uncertainties were estimated using a standard propagation of errors analysis, unless specified otherwise. The error of the calculated permeability mainly originated from the variation of membrane thickness; for this study, the uncertainties of gas permeability at the moment of test are within ± 7 %.
4 Results and Discussion 4.1 FT-IR Characterization The FT-IR spectra of synthesized PU (Fig. 3-curve a), PA12b-PTMG (Fig. 3-curve b) and PU/PA12-b-PTMG blend membranes (Fig. 3-curve c) are depicted in Fig. 3. As can be seen in Fig. 3-curve a, the absorption peaks due to the urethane hard segments are seen at around 3320 cm−1 (–NH stretching) and 1750 cm−1 (–CO stretching) [18]. Other absorption peaks due to the soft segments are seen at about 2920 and 2850 cm−1 (–CH2 symmetric and asymmetric stretching), 1260 cm−1 (–CH3 bending),
Fig. 3 FT-IR spectra of (a) synthesized PDMS/PTMG based PU, (b) PU/PA12-b-PTMG (10 % wt.) and (c) PA12-b-PTMG in (400–4000 cm−1 ) spectral region
Silicon (2016) 8:75–85 Table 1 The density and thermal properties of PU and PU/ PA12-b-PTMG membranes measured using DSC
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PA12-b-PTMG (wt %)
ρ gcm−3
Tg (◦ C)
Tc(◦ C)
Hf (J/g)
XaC (%)
ϕab (%)
5 10 15 20
1.23 1.21 1.20 1.19 1.18
−79.1 −75.3 −71.3 −68.2 −65.3
N/A 136.7 137.4 138.8 139.9
N/A .189 .388 .590 .798
N/A 5.12 5.25 5.33 5.40
100 94.00 93.90 93.86 93.83
a Degree of crystallinity was calculated by using Eq. 3, the final crystallinity is normalized by PA12 weight fractions b The volume fraction of amorphous phase was calculated by using Eq. 4
The disappearance of the NCO stretching vibration at 2270 cm−1 showed the completion of the reaction [18, 19]. Figure 3-curve c shows that the spectrum of pure PA12b-PTMG has a characteristic peak at 1730 cm−1 which is attributed to the -C=O stretching vibration. Another peak at 1640 indicates the presence of H–N–C=O groups. By adding PA12-b-PTMG to PU, the N-H stretching vibration at 3320 cm−1 in PU shifted to a lower frequency region (Fig. 3-curve b). Therefore, in the blends, more highenergy hydrogen bonds could be formed than in the virgin component (PU) [20]. 4.2 DSC Analysis DSC cooling thermograms of PU, PA12-b-PTMG and PU/PA12-b-PTMG blend membranes are presented in Fig. 3. Also, Table 1 summarizes the thermal transitions from these DSC scans. The synthesized PU exhibits only a glass transition temperature at −79 ◦ C (Fig. 4 curve a). There were no observations of crystallization transitions of the soft and hard segment regions. This is desirable for preparing highly permeable polymers, since crystallinity
Fig. 4 DSC curves of synthesized PU, PA12-b-PTMG and PU/ PA12-b-PTMG blend membrane. (a) PU, (b) blend with 5 wt% PA12-b-PTMG, (c) blend with 10 wt% PA12-bPTMG, (d) blend with 15 wt% PA12-b-PTMG, (e) blend with 20 wt% PA12-b-PTMG, (f) PA12-b-PTMG
reduces gas permeability [21]. As can be seen in Fig. 4curve f, an endothermic peak at 142.1 ◦ C for the pure PA12b-PTMG shows the crystallization of the polyamide-12 (PA12) crystalline phase. The DSC curves of the PU/PA12b-PTMG blends, illustrate the crystallization temperature (Tc ) in PA shifted to lower values, and the enthalpy of formation of the PA12 crystalline phase (Hf ) and as a result the degree of crystallinity, XC (g crystals/g blend) and the volume fraction of crystalline polymer in the blends, ϕC , estimated from Eqs. 3 and 4 respectively, decreased with reducing PA12-b-PTMG content in the PU matrix [23]. Hf (3) XC = wH0f where Hf is the enthalpy of formation of the crystalline PA12 (J/g), H0f is the enthalpy of formation of the pure crystal (246 J/g for PA12 [24]), and w is the weight fraction of PA12 in the blend. Note that according to the DSC curves, presumably the crystalline phase contained only PA12 structures. ρ XC (4) ϕC = ρC
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Fig. 5 SEM photographs of cross sections of PU and PU/ PA12-b-PTMG blend membranes. (a) PU cross section, (b) cross section of PU/ PA12-b-PTMG (95/5) wt%, (c) cross section of PU/ PA12-bPTMG (90/10) wt%, (d) cross section of PU/ PA12-b-PTMG (85/15) wt%, (e) cross section of PU/ PA12-b-PTMG (80/20) wt%
where ρ is the measured blend density and ρc is the density of crystalline PA12 (1.034 g cm−3 [25]) at room temperature. This value is used to calculate the amorphous phase volume fractions in the blends. The degree of crystallinity and amorphous volume fraction values obtained from DSC analysis are presented in Table 1. These results can be due to a plasticizing effect of the elastomeric polyurethane in the polymer blend, which results in a deteriorated crystal structure [1]. As shown in Table 1, the Tg values of the PU in the blends, compared to pure PU, shifted to higher temperature, indicating an interaction between PA12-b-PTMG and the flexible segments.
Table 2 Gases critical temperature, kinetic diameter and critical volume at 25 ◦ C
In semi-crystalline polymers, gas molecules are assumed to permeate only through amorphous phases [22]. Thus, it is of interest to determine the crystallinity of samples by Eqs. 3 and 4 which may have some crystallinity at the temperature of the gas permeation studies. 4.3 Morphology of the Polymer Blends The morphologies of the PU and PU-based blend membranes were investigated by SEM and are shown in Fig. 5. Because of the incompatibility between the hard and soft segment, polyurethanes undergo phase separation
Gas
Critical temperaturea (K)
Kinetic diametera (A ◦ )
critical volumea , Vc (cm3 /mol)
N2 O2 CH4 CO2
126.2 154.6 190.6 304.2
3.6 3.5 3.8 3.3
89.3 73.5 98.6 91.9
a Ref.
[1]
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Table 3 Pure gas permeation properties of PU, PU/PA12-b-PTMG and PA12-b-PTMG membranes at 25 ◦ C and 8 bar Membrane PU (wt %) /PA12-b-PTMG (wt %)
100/0 95/5 90/10 85/15 80/20 0/100
Permeability (Barrer) N2
O2
CH4
CO2
3.17 ± .19 2.96 ± .21 2.63 ± .18 2.31 ± .15 1.99 ± .11 2.00 ± .13
11.60 ± .69 10.96 ± .61 9.87 ± .73 8.72 ± .58 7.62 ± .49 7.00 ± .58
13.27 ± .74 12.51 ± .72 11.72 ± .66 11.01 ± .68 10.29 ± .63 8.50 ± .75
68.40 ± 4.31 76.00 ± 5.16 82.03 ± 5.20 88.37 ± 5.60 105.00 ± 6.90 134.00 ± 9.30
resulting in a hard segment domain, soft segment matrix and urethane-bonded interphase (Fig. 5a) [1]. The results show that the PA12-b-PTMG was dispersed in PU according to the mode of microphase separation. With increasing PA12b-PTMG content in the blend membranes, the dispersion of the PA12-b-PTMG improved in the PU. 4.4 Effect of PU/ PA12-b-PTMG Blend Composition on Gas Permeation Properties Gas permeability of PU membranes with different chemical characteristics has been extensively studied [10, 13, 26–29]. In this research the solution–diffusion transport model was applied to the gas permeation properties of the PU/ PA12-b-PTMG blend membranes [15]. The membrane permeability for PU, PA12-b-PTMG and PU/ PA12b-PTMG blends with 5, 10, 15 and 20 % (wt.) PA12b-PTMG against CO2 , CH4 , O2 and N2 are listed in Table 3. For all PU/ PA12-b-PTMG blend membranes, the permeability values were in the following order: P (CO2 ) >> P(CH4 ) > P(O2 ) > P(N2 ). This permeability order is mainly due to the interplay between the kinetic diameter, solubility and the critical temperature for the gas molecules [30]. CO2 has a small molecular size and high critical temperature in comparison
Table 4 Ideal permselectivities of PU, PU/ PA12-b-PTMG and PA12-b-PTMG membranes at 25 ◦ C and 8 bar
to the other gases (Table 2). It is noteworthy that CO2 also is a polar gas that can interact with polar chain polymers [1, 31, 32]. Hence, the higher CO2 permeability is related to the higher solubility of CO2 in the membranes compared to O2 , CH4 , and N2 , that was mainly attributed to the presence of the polar ether oxygen in PTMG and PDMS components of the PU/PA12-b-PTMG blend membrane and favorable interactions between the polyol components of blend and CO2 . Also, it can be seen in Table 3, that the O2 , N2 and CH4 permeability of the PU/PA12-b-PTMG blend membranes decreased by adding PA12-b-PTMG to the PU membrane. This can be attributed to the fact that the crystalline PA12 blocks in the PA12-b-PTMG thermoplastic elastomer act as the impermeable phase [16, 32] and this causes a decrease in average chain mobility and diffusivity of the blend membrane. To the contrary, the CO2 permeability of the blend membranes increased with the addition of PA12-b-PTMG content in the PU membranes which is mainly attributed to increasing the polar ether oxygens in the PU/PA12-b-PTMG blend that can interact with the CO2 polar gas and increase the CO2 solubility. Table 4 presents CO2 /N2 , CO2 /CH4 , and CO2 /O2 and O2 /N2 pair ideal gas selectivities for PU and PU/PA12-bPTMG. As can be seen, the CO2 /N2 , CO2 /CH4 and CO2 /O2 selectivities improved with the addition of PA12-b-PTMG
Membrane PU (wt %) /PA12-b-PTMG (wt %)
100/0 95/5 90/10 85/15 80/20 0/100
Ideal selectivity O2 /N2
CO2 /CH4
CO2 / O2
CO2 / N2
3.65 3.70 3.75 3.77 3.82 3.50
5.15 6.07 6.99 8.02 10.20 15.76
5.89 6.93 8.31 10.13 13.77 19.14
21.58 25.67 31.19 38.25 52.76 67.00
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in the PU matrix. The highest selectivity was observed for the CO2 /N2 pair. This is mainly due to the higher solubility of CO2 and lower solubility of N2 than other light gases for both the PU and the blend membranes. The selectivity of pairs O2 /N2 does not change significantly with blend composition because the solubility selectivity of O2 /N2 was almost the same [27]. Considering that these measurements were performed at room temperature, presumably the presence of a higher crystalline phase content was responsible for this behavior. Comparing the increasing permselectivity of the pair gases showed increasing CO2 /N2 and CO2 /CH4 (in the blend membrane with 20 % (wt) PA12-b-PTMG) that were 244 and 198 %, respectively. As methane gas is more condensable than N2 , it may further adsorb in the interface area. Therefore, the selectivity of CO2 /CH4 will increase less than the selectivity of CO2 /N2 [1]. In homogeneous blend membranes the gas transport properties, such as permeability and selectivity, can be estimated by different models. For instance, the permeability of miscible blends can be expressed empirically by the logarithmic additive rule: ln Pb = ϕ1 ln P1 + ϕ2 ln P2
(5)
where Pb is the homogeneous blend gas permeability, ϕ1 and ϕ2 are the volume fractions of polymers 1 and 2 in the blend, and P1 and P2 are the permeabilities of the pure polymers 1 and 2 [8]. Gas transport properties in a semi-crystalline polymer are usually modeled assuming that the crystals act as an impermeable, dispersed phase imbedded in an amorphous phase. Models are then developed for the influence of crystallinity on solubility and diffusivity. In the polymer, the effect of crystallinity on penetrant sorption is typically represented by Eq. 6 [21]: SA = SA,a ϕa
(6)
where SA is the observed solubility coefficient, SA,a is the solubility coefficient in the amorphous polymer, and φa is the amorphous phase volume fraction. The influence of crystallinity on diffusivity is commonly described by Eq. 7 [22]: DA =
DA,a γτ
(7)
where DA,a is the diffusion coefficient in the amorphous polymer, τ is a tortuosity factor, and γ is the effective chain immobilization factor. τ characterizes the tortuosity of the amorphous phase caused by the presence of dispersed impermeable crystallites. Simple models from composites
theory, such as the one given below, are often used to describe the influence of crystallinity on tortuosity [21]: τ=
1 ϕa
(8)
The chain immobilization factor, γ , accounts for the restricted segmental mobility in the amorphous phase by the crystallites. In the simplest case, when γ = 1 (i.e., no chain immobilization), the gas permeability is given by [21]: PA = SA DA = PA,a ϕa2
(9)
The experimental gas permeabilities in PU/PA12-b-PTMG blend membranes predicted with the logarithmic additive rule Eq. 5 are shown in Fig. 6. Experimental permeability data reveal a negative deviation in the miscible blends. In such cases, a volume contraction is sometimes observed [33]. For microscopically immiscible blends such as the ones containing polyvinyl pyridine and ethyl cellulose (PVP–EC), a positive experimental data deviation is observed [33].The permeabilities estimated by this model are higher than the experimental data except for the CO2 permeabilities. The additive model does not consider structural changes of the polymers; this behavior could give some guidelines to explain the variation of the polymer blend properties attributed to the gas permeability, such as crystallinity and polymer blend solubility. Thus, the lower permeability in the blend membranes can be attributed to an increase in crystallinity [1]. The additive logarithmic model predicts permeabilities when the mobility of the amorphous phase is not reduced by the presence of a crystalline phase. According to Eq. 9, the predicted permeabilites could be modified as follows[1]: Pb,c = Pb ϕa2
(10)
Where Pb,c and Pb are calculated permeabilities of polymer blends based on the additive logarithmic model with and without considering crystalline domains in the membranes. The experimental and calculated permeability of gases with different PA12-b-PTMG content and γ = 1 are depicted in Fig. 6. As can be seen, this model is in good agreement with the trends of our experimental data except for the CO2 permeabilities, the deviation from this predictive model could be attributed to the interactions between the polymer phases and their effects on variations of the polymer blend solubility which cause high CO2 solubility due to the strong affinity of the polar ether linkages for CO2 . As stated by other researchers, for commercial and industrial use of membranes in gas separation processes, membranes must have a maximum amount of permeability and selectivity. They use the Robeson upper bound line to characterize the improvement of their membrane performance. Figure 7 compares the relationship between the permeability of CO2 and the selectivity of CO2 /N2
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Fig. 6 Comparison of permeabilities for four gases between (a) additive logarithmic model with effect of crystallinity with γ = 1
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additive logarithmic model, (b)
Fig. 7 Robeson upper bound line between selectivity of CO2 /N2 and CO2 permeability [3]
experimental data, and (c)
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Fig. 8 Pure gas permeabilities of membranes as a function of upstream pressure at 25 ◦ C
with Robeson’s upper bound line [34]. As shown, the prepared PU/PA12-b-PTMG blend membranes lie close to Robeson’s upper bound line and this means prepared blend membranes present better CO2 /N2 separation performance than the pure PU membrane. 4.5 Effects of the Upstream Pressure on the Gas Permeation Properties The effect of feed pressure on the PU and PU/PA12b-PTMG blend membranes pure gas permeabilities was studied at 25 ◦ C, with results shown in Fig. 8. The permeability coefficients of the permanent gases, such as N2 , O2 , and CH4 , were essentially independent of upstream pressure, but the permeability of CO2 increased somewhat with increasing upstream pressure. The presence of strong sorbing penetrants (such as CO2 ) can plasticize the polymer matrix and increase polymer local segmental motion, resulting in an increase in solubility and diffusivity [35]. With increasing pressure, the CO2 solubility and diffusivity and, consequently, the permeability can increase as the CO2 concentration in the polymer increases.
5 Conclusions PU and PU/PA12-b-PTMG blend membranes were successfully prepared using n-propanol /n-butanol (binary) as solvents. The n-propanol /n-butanol mixture (1/3 wt.%) was a good solvent without gelation problems. PA12-b-PTMG chains contain polar ether oxygen units that can interact with the CO2 polar gas and increase the CO2 solubility in the PU/ PA12-b-PTMG blend membranes. The comparative increase in permselectivity of pair gases showed an increase in CO2 /N2 and CO2 /CH4 (in the membrane with 20 % (wt) PA12-b-PTMG) by up to 244 % and 198 % (in the membrane with 20 % (wt) PA12-b-PTMG), respectively while the O2 /N2 ratio did not change much because the solubility selectivity of O2 /N2 is almost the same. Thermal results with DSC indicated that with PA12-bPTMG content increasing in the PU matrix, an increase of the glass transition temperature of PU in the blend occurred, indicating an interaction of PA12-b-PTMG with the flexible polyurethane segment. The experimental permeability was compared with that calculated on the basis of the blend composition by the
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additive logarithmic model. The permeability of the gases through the blends was lower than estimated by this model except for the CO2 permeabilities. The lower experimental permeability in the blend membranes than the calculated permeability of blend membranes based on the additive logarithmic model can be attributed to the increase in the crystallinity of the membrane.
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