Formation–structure–performance correlation of thin film composite membranes prepared by interfacial polymerization for gas separation

Formation–structure–performance correlation of thin film composite membranes prepared by interfacial polymerization for gas separation

Journal of Membrane Science 421–422 (2012) 327–341 Contents lists available at SciVerse ScienceDirect Journal of Membrane Science journal homepage: ...

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Journal of Membrane Science 421–422 (2012) 327–341

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Formation–structure–performance correlation of thin film composite membranes prepared by interfacial polymerization for gas separation Fang Yuan a,b,c, Zhi Wang a,b,c,n, Shichun Li a,b,c, Jixiao Wang a,b,c, Shichang Wang a,c a

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China b

a r t i c l e i n f o

abstract

Article history: Received 26 April 2012 Received in revised form 10 July 2012 Accepted 29 July 2012 Available online 8 August 2012

Thin film composite (TFC) membranes for CO2/N2 separation were prepared by interfacial polymerization from N-Methyldiethanolamine (MEDA) and Trimesoyl chloride (TMC) on crosslinked polydimethylsiloxane (PDMS) coating polysulfone (PS) support membrane. The structural properties of TFC membrane surfaces were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffractometer (XRD). The relationships among the skin layer formation conditions, skin layer structure, and membrane separation performance were investigated. Results show that membranes with higher CO2 permeance and good CO2/N2 selectivity appeared to consist of thinner, more crosslinked, and less crystalline skin layer structures. Such high performance gas separation membranes were obtained by (1) increasing MEDA diffusivity and decreasing MEDA solubility in the organic solvent, and (2) reducing TMC concentration in organic phase and raising MEDA concentration in aqueous phase under the circumstances of forming an integrated skin layer. Furthermore, our results also indicate that crosslinking could enhance the CO2-induced plasticization resistance of the membranes. These findings have great theoretical significance for the controlled preparation of gas separation membranes. & 2012 Elsevier B.V. All rights reserved.

Keywords: Gas separation Thin film composite Interfacial polymerization Membrane structure

1. Introduction Interfacial polymerization (IP), developed by Morgan and Kwolek [1], has been a well-established method for the preparation of the dense and thin active layers for thin film composite (TFC) membranes [2,3], encapsulation of active ingredients [4], and ultrathin responsive and catalytic films [5–7]. The major advantages of the IP process involve the formation of an ultrathin active skin layer (0.1–0.25 mm) [8], the minimization of macrovoid defects [9], and the tunable functional groups [10]. Thus, commercially TFC reverse osmosis (RO) and nanofiltration (NF) membranes are both manufactured by IP process [2]. Moreover, several efforts [11–15] have been focused on the preparation of TFC gas separation membranes using IP technique, and the results showed that IP is also an effective way for the synthesis of gas separation membranes. Several research works on interfacially polymerized TFC RO membranes have been carried out to explore the relationships

n Corresponding author at: Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Weijin Road 92#, Nankai District, Tianjin 300072, PR China. Tel.: þ 86 22 27404533; fax: þ 86 22 27404496. E-mail address: [email protected] (Z. Wang).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.07.035

among the preparation conditions, structure and performance of these membranes [16–22]. Shah et al. [20] studied the effects of polymerization conditions on membrane structure and performance, and found that an inverse correlation existed between skin layer thickness and water flux in the thickness range of 100–300 nm, while membrane selectivity was independent of skin layer thickness. Nevertheless, by investigating the impacts of reaction and curing conditions on membrane performance and skin layer structure, Hoek et al. [21] reported that the water permeability was strongly correlated with the crosslinked structure and salt rejection with the skin layer thickness and morphology. Most recently, study of He et al. [22] showed that the existence of a highly crosslinked middle layer rather than the entire skin layer thickness was mainly responsible for separation. Obviously, some of the conclusions on the relationships among the preparation conditions, structure and performance of interfacially polymerized TFC RO membranes reported were in mutual conflict, e.g., the effects of skin layer thickness on both water flux and salt rejection were inconsistent. The most important point is that the conclusions on these relationships are not well applicable to interfacially polymerized TFC gas separation membranes, which is supported by the following comments. On one hand, an inverse relationship between skin layer thickness and gas permeance was clearly presented in the literature [15], which is

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2. Experimental

distinct from the relationship between skin layer thickness and water flux (or salt rejection) for RO membranes as mentioned above. On the other hand, apart from skin layer thickness and crosslinking, crystallinity is also an important structural parameter that influences the gas transport properties [23–25]. Nevertheless, the relationship between crystallinity and separation performance has never been investigated in the field of interfacially polymerized TFC RO membranes, although it has been reported in the field of non interfaciallly polymerized membranes that decreasing crystallinity could enhance the water permeability [26–28]. Therefore, it is very necessary to understand the relationship between the preparation conditions and structural properties (such as skin layer thickness, crosslinking, and crystallinity) and investigate the effect of such structural properties on gas separation performance for interfacially polymerized TFC membranes. However, to the best of our knowledge, no information has been reported on studying the above relationships for interfacially polymerized TFC gas separation membranes, although it has great theoretical significance for the controlled preparation of TFC gas separation membranes. Based on the above consideration, in the present study, TFC membranes for CO2/N2 separation were prepared by interfacial polymerization from N-Methyldiethanolamine (MEDA) and Trimesoyl chloride (TMC) on crosslinked polydimethylsiloxane (PDMS) coating polysulfone (PS) support membrane. The highly permeable crosslinked PDMS intermediate layer functions as both an adhesive and a protective coating which prevents the aqueous or organic solution penetration into the porous PS membrane, and at the same time compensates for minor surface imperfections in the substrate [29,30]. The possible chemical structure units of the MEDA–TMC polymer are illustrated in Scheme 1. This investigation related several MEDA–TMC reaction conditions to TFC membrane skin layer structural properties (thickness, crosslinking, and crystallinity). The structural properties were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffractometer (XRD). Meanwhile, membrane separation performance, determined by both CO2 and N2 permeance, was correlated to the measured skin layer structural properties. These experiments allow us to achieve our primary objective, that is, to elucidate the various relationships among reaction conditions, structure and gas separation performance of TFC membranes.

2.1. Materials The polysulfone (PS) flat ultrafiltration membranes with an average cut-off molecular weight of 6000 Da were supplied by Vontron Technology Co. Ltd. (China). Polydimethylsiloxane (PDMS) (ShinEtsu, Japan), Tetraethoxysilane (TEOS) (AR; Aladdin, China) and ditin butyl dilautate (DBD) (95%; Aladdin, China) were used to modify PS membranes. Chemicals used in skin layer formation included monomers N-Methyldiethanolamine (MEDA) (99.0%; Aladdin, China) and Trimesoyl chloride (TMC) (99.5%; Sanli, Qingdao, China) as well as aqueous solution additive Na2CO3 (AR; Guangfu, Tianjin, China). Hexane, heptane and cyclohexane (AR; Guangfu, Tianjin, China) were the organic solvents selected for preparing TMC solutions. For the preparation of the aqueous solution, RO deionized water was used. All chemicals were used without any further purification.

2.2. Membrane preparation In order to manufacture ultra-thin defect-free skin layer, we used a highly permeable intermediate layer to modify PS membrane. This concept was first introduced by Browal and Salemme [30], and later applied by Cabasso and Lundy [31], Peinemann et al. [29], and Li et al. [32]. Modified PS support membrane was prepared by casting crosslinked PDMS solution on PS membrane by a coating applicator. Crosslinked PDMS solution was prepared by dissolving 2 wt% PDMS, 1 wt% TEOS and 1 wt% DBD in heptane, followed by stirring for 5 min and standing for 30 min at room temperature. The resulted support membrane was dried at 301 C and 40% relative humidity in an artificial climate chamber (Climacell 222R, Germany) for at least 12 h. The TFC membrane was prepared through interfacial polymerization technique in an assembly clean room. First, the modified PS support membrane was clamped between two Teflon frames. Then, the organic solution of TMC was poured onto the top surface of the support membrane and allowed to soak for 10 min, followed by removing the excess solution and blowing the coated surface with an air-knife until no residual liquid. Afterwards, the MEDA aqueous solution with 0.0379 mol/L Na2CO3 acted as the acid acceptor was

ClOC

CH3 OH-CH2-CH2-N-CH2-CH2-OH

+

COCl COCl (TMC)

(MEDA)

O

O

CH3

O

O

O-(CH2)2-N-(CH2)2-O-C-

-C

O-(CH2)2-N-(CH2)2-O-C-

-C

CH3

C=O

m

COOH

n

O (CH2)2 N -CH3 (CH2)2 O

Scheme 1. Possible polymerization reaction between MEDA and TMC to form (A) fully crosslinked polymer chain, (B) linear polymer chain with pendant–COOH.

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poured on the top of the TMC saturated support membrane and allowed to remain for 10 min at 401 C, which resulted in the formation of an ultra-thin film over the modified PS support membrane. Finally, the excess aqueous solution was removed, and the resulting composite membrane was thoroughly washed with RO deionized water to eliminate excess amine and the byproduct. The resulted membrane was kept in the artificial climate chamber for at least 12 h under the same condition mentioned above. 2.3. Monomer and solvent characterization Ultraviolet–visible (UV–vis) spectroscopy (DH-2000, Ocean Optics) was used to study MEDA solubility in each organic solvent. The partition coefficient is the ratio of MEDA concentrations in organic solvent and in water at equilibrium [33]. To obtain the partition coefficient, a total volume of 50 mL MEDA (0.0615 mol/L) aqueous solution was added to 50 mL of organic solvent in a separating funnel. After equilibrium was attained, the aqueous MEDA solution was removed. The concentration of MEDA in the aqueous solution before and after contact with the organic solvent was determined from UV–vis spectroscopic analysis. Hoek et al. [21] suggested that diffusivity of aqueous monomers in the organic solvents strongly correlates (negatively) with solvent viscosity (m). Therefore, the relative diffusion coefficient (D) of MEDA from the aqueous phase to each organic solvent can be calculated as follows: D ¼ mwater/msolvent

(1)

where mwater and msolvent represent the viscosity of water and organic solvent, respectively. The viscosity of water and each organic solvent are reported in Table 1. 2.4. Membrane characterization Chemical structure of the membrane surface was assessed by attenuated total reflectance infrared (ATR-FTIR) spectroscopy (FTS-6000, Bio-Rad of USA) using a germanium crystal at a 451 angle of incidence. Chemical composition of the TFC membrane surface was characterized by X-ray photoelectron spectroscopy (XPS, PHI1600, USA) using Mg Ka as the radiation source. Survey spectra were collected over a range of 0–1100 eV. The relative atomic concentrations and oxygen-to-nitrogen (O/N) ratios were calculated following a previously published method [34]. The skin layer has a chemical structure of the partially crosslinked polyester, which consists of the crosslinked portion (A) and the linear portion containing pendant carboxylic acid (B) (as shown in Scheme 1). The relative fractions of crosslinked portion, m, and linear portion, n, were calculated based on Scheme 1 using the following relations [34]: mþn ¼ 1

ð2Þ

O 6m þ6n ¼ N 1:5m þ n

ð3Þ

Table 1 Viscosity of water and organic solvents used to form TFC membranes. Solvent

Water

Hexane

Heptane

Cyclohexane

m (mPa s)

0.893

0.300

0.387

0.894

1. m represents the viscosity. 2. The data (25 1C) were obtained from ‘‘CRC Handbook of Chemistry and Physics’’, David R. Lide, 84th Edition (2003–2004), CRC Press.

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Theoretically, fully crosslinked and fully linear MEDA–TMC polymers have O/N ratios of 4 and 6, respectively. Generally, polymer membranes are partially crystalline, which consist of an amorphous phase and a crystalline phase [8]. Crystallinity is usually specified as a percentage of the volume of the polymer that is crystalline. Crystallinity of the TFC membrane was investigated by X-ray diffractometer (XRD, D/MAX2500, Japan) using Cu Ka radiation (l ¼0.1542 nm) with 2y scanned between 51 and 901 under an 8 kW power. XRD spectra were analyzed through Jade software package. It should be mentioned that, the thickness of IP formed skin layer is usually very thin below several hundred nanometer scale, and thus, the XRD spectra of the TFC membrane reveal diffraction peaks attributed to both skin layer and the support membrane. The impacts of the support membrane on the crystallinity of all the TFC membranes prepared in this study can be recognized as the same, and therefore, the change of crystallinity of TFC membranes can reflect that of crystallinity of TFC membrane skin layers. Surface and cross-section morphologies of the TFC membranes were visualized with scanning electron microscopy (SEM, Nova Nano430, FEI of USA). For the cross-section observation, membrane samples were prepared by peeling away the polyester backing fabric and fracturing the fabric free membrane samples after immersion in liquid nitrogen. All samples were coated with gold by a sputter-coating machine. The thermal properties of poly(MEDA–TMC) samples were measured on a TA Q100 differential scanning calorimeter (DSC, Perkin-Elmer DIAMOND, USA). The temperature range scanned was from 25 to 2501 C at a heating rate of 101 C/min. The poly(MEDA–TMC) samples were obtained as follows. First, poly (MEDA–TMC)/PS membrane was prepared through IP technique using PS membrane as the support membrane following the same procedure as that used for producing TFC membrane (Section 2.2). Then, the polyester backing fabric was carefully peeled off, and the fabric free membrane sample was immersed in dimethyl sulfoxide (DMSO) (AR; Guangfu, Tianjin, China) with stirring for 24 h to completely dissolve PS. Finally, the deposit, which is poly(MEDA–TMC), was dried in a vacuum oven at 301 C for 48 h. 2.5. Gas permeation measurements Permselectivities of the TFC membranes were tested by a homemade apparatus [15] using CO2/N2 (15/85 by volume) mixed gas. The membrane was mounted in a circular stainless steel cell (effective membrane area ¼19.26 cm2). Prior to contacting the membranes, the feed gas was saturated with water vapor by bubbling through water bottles at 30 1C and then passing an empty bottle at 25 1C to remove the condensate water. The sweep gas (H2) was humidified by passing through water bubblers at 25 1C. The flow of outlet sweep gas was measured using a soap film meter and its composition was analyzed by a gas chromatograph equipped with a thermal conductivity detector (HP7890, Porapak N). The fluxes of CO2 ðNCO2 Þ and N2 ðNN2 Þ were calculated from the outlet sweep gas flow rate and its composition. The downstream pressure in the apparatus was maintained at atmosphere pressure. The permeance of a species i, Ri, is defined as the flux (Ni) divided by the partial pressure difference (DPi) between the upstream and downstream side of the membrane, Ri ¼Ni/DPi, and the selectivity is given by aij ¼Ri/Rj (the downstream pressure is negligible compared to the upstream pressure). The gas permeance is customarily expressed in the unit of GPU [1 GPU¼10-6 cm3(STP)/(cm2 s cmHg)¼3.35  10  10 mol/(m2 s Pa)]. Permeation experiments were carried out at 251 C with a feed pressure varying from 0.11 MPa to 2.0 MPa, and steady state permeation was assumed to have reached when the outlet gas flow rate and its composition no longer changed with time. All error bars

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present the standard errors of performances of three membranes prepared under the same condition. In addition, it has been suggested that the effect of back-diffusion of H2 on data analysis is negligible [15]. It should be mentioned that the effect of concentration polarization on data analysis is also negligible. To prove this, we chose the most permeable TFC membrane to do the permeation test under 0.11 MPa feed pressure. The flow of retentate gas (Qr) was measured using the water draining method, while the flow of outlet sweep gas (Qo) was measured using a soap film meter and its composition (yo,i) was analyzed by the gas chromatograph abovementioned. The flow of permeate gas (Qp) and the composition (by volume) on the permeate side (yi) are given by Q p ¼ Q o yo,CO2 þQ o yo,N2 yi ¼

ð4Þ

Q o yo,i Qp

ð5Þ

experimentally confirmed that tertiary amine groups could react reversibly with CO2 in wet condition, but could not in dry condition. Therefore, the sudden decrease of the separation performance when the humidification was discontinued abovementioned is attributed to the loss of water. The absence of water not only eliminates the CO2 facilitated transport [15], but also makes the membrane become rigid without swelling effect [37]. Besides, the Joule–Thomson effect might occur during gas permeation measurement, which could lead to the temperature decrease at the permeate side [8] and subsequent water condensation. It should be pointed out that, the sweep was employed at constant temperature during the measurement, which could take away the condensation water. Moreover, the outlet sweep gas was dried before injected into the gas chromatograph. Based on the analyses above, it can be suggested that no water condensation effects takes place in the porous PS sublayer, and that the effect of water on data analysis is negligible.

The material balance equations can be expressed as Q f ¼ Q p þQ r

ð6Þ

Q f xi ¼ Q p yi þ Q r xr,i

ð7Þ

where Qf represents the flow of feed gas. xi and xr,i are the composition (by volume) of the feed and retentate, respectively. The composition change of the retentate compared with the feed (Di) is given by

Di ¼

xr,i xi  100% xi

ð8Þ

The stage cut (y) is defined as the ratio of the flow of permeate gas to the flow of feed gas [35]

y ¼ Q p =Q f

ð9Þ

The data were read after steady state permeation was reached abovementioned. The raw permeation data of the TFC membrane are summarized in Table 2. Observed composition change between the retentate and feed and stage cut are small enough, indicating that concentration polarization could be negligible [36]. Furthermore, as shown in Scheme 1, the skin layer of TFC membrane prepared in this study contains tertiary amine groups which could react with CO2 reversibly [15], suggesting that this type of TFC membrane belongs to the fixed carrier membrane. To illustrate the importance of gas humidification to the membrane permselectivity, TFC membrane prepared with 0.0615 mol/L MEDA and 0.0250 mol/L TMC was tested with and without water under 0.11 MPa feed pressure. When the feed gas was humidified, the membrane showed a CO2 permeance of 1035 GPU and CO2/N2 selectivity of 87. However, when the humidification was discontinued, sudden decrease of both CO2 permeance and CO2/N2 selectivity was observed, and the dry membrane displayed a CO2 permeance of 135 GPU and CO2/N2 selectivity of 20. When the feed gas was humidified again, the separation performance of the membrane was well recovered. Our previous study [15] has

3. Results and discussion 3.1. Characterization of skin layer 3.1.1. ATR-FTIR study The minimum depth of penetration of ATR-FTIR technique used here is about 1 mm in wave number range while the thickness of IP formed skin layer is usually less than 0.5 mm. Thus, the ATR-FTIR spectrum of a composite membrane reveal bands attributed to both skin layer and the support membrane. The ATR-FTIR spectra of the TFC membrane as well as the PS membrane and the modified PS support membrane are displayed in Fig. 1. Compared with the PS membrane, the spectra of the modified PS support membrane exhibits evident absorbance signals at 1256 cm  1 (Si–CH3 bending), 799 cm  1 (Si–O–Si symmetry stretching) and around 1100–1000 cm  1 (Si–O–Si asymmetric stretching) (Fig. 1(b)) [38]. Moreover, the results shown in Fig. 1(c) indicate that the interfacial polymerization has occurred since two strong bands at 1726 cm  1 and 1240 cm  1 are present, which are characteristic of nC¼O and nC–O of ester compound, respectively [38]. 3.1.2. SEM image The changes of the surface morphologies between the modified PS support membrane and the TFC membrane can be observed distinctly from Fig. 2. Compared with Fig. 2(a), one can see that the interfacial polymerization technique generates a rough active layer on the smooth surface of the modified PS support membrane (as shown in Fig. 2(b)). 3.1.3. DSC analysis The glass transition temperature (Tg) is about 1141 C according to the DSC curve of poly(MEDA–TMC) shown in Fig. 3, indicating that poly(MEDA–TMC) is a glassy polymer at room temperature.

Table 2 Raw permeation data of the most permeable TFC membrane in this work under 0.11 MPa feed pressure. Feed gas

Permeate gas

Retentate gas

Qf (cm3(STP)/s)

xCO2

xN2

Qp (cm3(STP)/s)

yCO2

yN 2

Qr (cm3(STP)/s)

xr,CO2

xr,N2

16.0983

0.15

0.85

0.1232

0.5556

0.4444

15.9751

0.1469

0.8531

DCO2

DN2

y

 2.07%

0.36%

0.0076

1. Qf, Qp and Qr represent the flow of feed, permeate and retentate gas, respectively; xi, yi and xr,i represent the composition (by volume) of the feed, permeate and retentate, respectively; Di is the composition change between the retentate and feed; y is the stage cut. (i ¼CO2 or N2). 2. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0100 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C.

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3.2. Properties of membranes formed in different organic solvents Generally, IP reaction occurs in a reaction zone which lies on the organic side of the interface due to the low solubility of acid chloride in water and relatively good solubility of amines in the organic phase [9,39–42]. When the two monomer solutions are brought into contact, the aqueous monomer partitions across the liquid–liquid interface and reacts with the organic monomer to form a polymer in the reaction zone [9]. The choice of the organic solvent is critical since it governs the aqueous monomer solubility and diffusivity in the reaction zone, and thus the separation performance of the resulting TFC membranes [21,43]. In this section, hexane, heptane and cyclohexane were selected as the organic solvent for the preparation of TFC membranes.

331

3.2.1. Relevant diffusion and partitioning of MEDA in each organic solvent Calculated partition coefficient (s) and relative diffusivity (D) of MEDA in each organic solvent are summarized in Table 3. Relative diffusivity of MEDA in the organic solvents is observed from highest to lowest according to hexane4heptane4cyclohexane. The partition coefficient (s) provides insights into the relative availability of MEDA and the thickness of the reaction zone in each organic solvent [9,44]. It can be seen from Table 3 that solubility of MEDA in the organic solvents increases in the order of hexane oheptaneocyclohexane. 3.2.2. Membrane structural properties The structural properties of TFC membrane skin layers investigated in this study involve thickness, crosslinking and crystallinity.

Fig. 3. DSC curve of poly(MEDA–TMC). Poly(MEDA–TMC) preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C.

Table 3 Relevant diffusion and partitioning of MEDA in each organic solvent.

Fig. 1. ATR-FTIR spectra of (a) PS membrane, (b) modified PS support membrane, and (c) TFC membrane. TFC membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C.

Solvent

s (dimensionless)

D (dimensionless)

Hexane Heptane Cyclohexane

0.213 0.331 0.507

2.98 2.31 1.00

s represents the partition coefficient of MEDA in the organic solvent and in water, and D represents the relative diffusivity of MEDA in the organic solvent, similarly hereinafter.

Fig. 2. SEM images of (a) modified PS support membrane and (b) TFC membrane. TFC membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C.

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Representative SEM images of the modified PS support membrane and TFC membranes prepared using each organic solvent are shown in Fig. 4. The interface between the skin layer and intermediate PDMS layer cannot be distinguished, but by subtracting the thickness of the intermediate PDMS layer, the thickness of the skin layer can be estimated. Image analysis suggests the average values of TFC membrane skin layer thicknesses on the order of 138 nm, 141 nm, and 144 nm for hexane, heptane, and cyclohexane, respectively. It should be mentioned that the thickness difference of skin layers of TFC membranes prepared using different organic solvents is too close with such big deviations (Fig. 4). To verify the validity of the above thickness change, TFC membranes prepared with different organic solvents were characterized by ATR-FTIR technique, and these results are displayed in Fig. 5. The characteristic band at 1726 cm  1 which is ascribed to nC¼O of ester compound as mentioned in Section 3.1.1 was used for the determination of skin layer thickness by measurements of peak areas (A). The peak area is proportional to the thickness of the skin layer since the IR beam penetrates through the skin layer and enters the modified PS support membrane [20]. The results show that the areas of peak (1726 cm  1) of TFC membranes on the order of 280.3, 331.8, and 413.0 for hexane, heptane, and cyclohexane, respectively (Fig. 5). Moreover, the correlation coefficient between the area of peak (1726 cm  1) and the skin layer thickness measured by SEM was calculated to be 0.97. These results suggest that the skin layer thickness judged by SEM is reliable. Experimentally determined values of crystallinity and crosslinking extent of TFC membrane skin layers are plotted in Fig. 6 with average values indicated by the column height and standard deviation of measured values indicated by the error bars. Observed crystallinity increases as hexaneoheptaneocyclohexane, whereas

observed extent of crosslinking decreases in order of hexane4 heptane4cyclohexane. Average values of skin layer thickness (d), extent of crosslinking (m), and crystallinity (C) for TFC membranes prepared using different organic solvents are reported in Table 4.

Fig. 5. ATR-FTIR spectra of TFC membranes prepared in (a) hexane, (b) heptane, and (c) cyclohexane. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C.

Fig. 4. SEM images of (a) modified PS support membrane and TFC membranes prepared in (b) hexane, (c) heptane, and (d) cyclohexane. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C.

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extent of crosslinking in the TFC membrane skin layer increases with increasing MEDA diffusivity. In addition, it has been reported that crosslinking a polymer can inhibit its ability to crystallize [23,28,46]. This is why an inverse relationship between extent of crosslinking and crystallinity is found in Table 4.

Fig. 6. Properties of TFC membranes formed using different organic solvents. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C.

Table 4 Correlation of TFC membrane structural properties with MEDA solubility and diffusivity. Solvent Hexane Heptane Cyclohexane

d (nm) 138 141 144

Correlation coefficients s 0.97 D  0.93

m (dimensionless) 0.752 0.526 0.317  0.96 0.92

C (dimensionless) 0.403 0.455 0.466 0.59  0.46

1. d, m and C represent the thickness, crosslinking extent, and crystallinity of TFC membrane skin layers, respectively, similarly hereinafter. 2. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C.

Correlations of membrane structural properties with the relative solubility (s) and diffusivity (D) of MEDA in the solvents are also summarized in Table 4. Correlations are classified as strong, moderate, and weak for the absolute values of the coefficients greater than 0.8, between 0.8 and 0.5, and less than 0.5, respectively [45]. Skin layer thickness is strongly correlated with MEDA solubility (positively) and diffusivity (negatively). Moreover, crosslinking extent also correlates strongly with MEDA solubility (negatively) and diffusivity (positively), while crystallinity correlates moderately with MEDA solubility (positively) and weakly with diffusivity (negatively). Following the logic developed above, higher MEDA solubility (in isolation) should produce thicker, less crosslinked skin layers, while higher MEDA diffusivity (in isolation) should produce thinner, more crosslinked skin layers. Higher MEDA solubility, on one hand, facilitates MEDA molecules penetrating deeper into the organic phase [34], which makes the skin layer thicker; on the other hand, promotes more MEDA available to form the polymer and consequent hydrolysis of TMC [21], which inhibits crosslinking of the skin layer. While higher MEDA diffusivity makes contiguous skin layer form more quickly and subsequent termination of the reaction [21], which leads to a thinner skin layer. Besides, the quicker skin layer formation facilitates the monomers to form multiple amide linkages more easily [21,34], and thus, the

3.2.3. Membrane performances Representative CO2/N2 permeance and selectivity of TFC membranes prepared in different organic solvents are plotted in Fig. 7. As can be seen from Fig. 7(a), CO2 permeance of TFC membranes prepared with different solvents drops rapidly within the pressure range of 0.11–0.5 MPa. With increasing feed pressure, the concentration of CO2 dissolved into the membrane increases, thus some carriers may be tied-up with the CO2 molecules and cannot in combination with other CO2 any more, resulting in a decrease in CO2 permeance [15]. However, CO2 permeance of TFC membranes prepared with different solvents decreases gently at a higher pressure ranging from 0.5 to 2.0 MPa (Fig. 7(a)), which is because the carriers in the membranes are saturated with CO2 to their maximum capacity when the pressure is high enough [15]. As shown in Fig. 7(b), with increasing feed pressure, N2 permeance of TFC membranes prepared using both hexane and heptane declines continuously, while N2 permeance of the TFC membrane prepared with cyclohexane goes down slightly within the range of 0.11–0.5 MPa and then increases beyond 0.5 MPa. To explain the above phenomena, these membranes were tested with pure N2, and the results show that pure N2 permeance of TFC membranes prepared with all the solvents decreases gradually with increasing feed pressure (Fig. 8). These results suggest that, the decrease of N2 permeance of TFC membranes prepared using both hexane and heptane with increasing feed pressure of CO2/N2 mixed gas (see Fig. 7(b)), and the decrease of N2 permeance of the TFC membrane prepared using cyclohexane within the CO2/N2 mixed gas pressure range of 0.11–0.5 MPa (see Fig. 7(b)), is not due to the presence of CO2, but is due to the membrane compaction which could decrease the amount of free volume and subsequently reduce the mobility of the penetrating molecules [47]; and that the increase of N2 permeance of the TFC membrane prepared with cyclohexane when the feed pressure of CO2/N2 mixed gas is higher than 0.5 MPa (see Fig. 7(b)) is attributed to the presence of CO2. At high feed pressure, the dissolution of a large quantity of CO2 within the polymer matrix will disrupt polymer chain packing, enhance inter-segmental mobility, and increase free volume of the polymer, which was also called CO2-induced plasticization in the literature [48–50]. Besides, as presented in Fig. 7(b) and Fig. 8, N2 permeance of the TFC membrane prepared with heptane under mixed gas test is a little higher than that under pure N2 test when the feed pressure is higher than 0.5 MPa, indicating that CO2-induced plasticization slightly exists in the TFC membrane prepared with heptane. However, this phenomenon does not happen for the TFC membrane prepared with hexane. The above results suggest that the CO2-induced plasticization resistance of TFC membranes decreases according to hexane4 heptane4cyclohexane. It should be mentioned that, because CO2 is less sensitive to plasticization than N2 [50], the promotion of CO2 permeance caused by the CO2-induced plasticization is not observed in Fig. 7(a). Moreover, as can be seen from Fig. 7(b), N2 permeance decreases as hexane 4heptane4cyclohexane at a lower feed pressure (0.11 and 0.5 MPa), but increases according to hexaneoheptaneocyclohexane when the feed pressure exceeds 0.5 MPa. The variation of N2 permeance with organic solvent at

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Fig. 7. CO2/N2 permeance and selectivity of TFC membranes prepared in different organic solvents: (a) CO2 permeance; (b) N2 permeance; (c) CO2/N2 selectivity. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C. Feed gas: CO2/N2 mixed gas (15/85 by volume).

Table 5 Correlation of performances with structural properties for TFC membranes prepared in different organic solvents. Solvent

Feed pressure 0.11 MPa RCO2 (GPU)

Hexane Heptane Cyclohexane

1035 812 493

Correlation coefficients  0.98 d m 0.97 C  0.60

1.0 MPa RN2 (GPU) 11.83 9.98 8.87  0.96 0.97  0.91

RCO2 (GPU) 308 287 208  0.80 0.77  0.21

RN2 (GPU) 6.70 7.53 8.26 0.99  0.99 0.80

1. RCO2 and RN2 represent CO2 and N2 permeance, respectively, similarly hereinafter. 2. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C.

Fig. 8. Pure N2 measurement for TFC membranes prepared in different organic solvents. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 0.0250 mol/L concentration of TMC solution, 10 min interfacial polymerization at 401 C.

a higher feed pressure is attributed to the different CO2-induced plasticization resistance of the membranes abovementioned. Furthermore, as shown in Fig. 7(c), the membrane prepared using hexane showed the highest CO2/N2 selectivity in this section because of the variation of CO2 and N2 permeance. Average values of CO2 permeance (RCO2 ) and N2 permeance (RN2 ) at the feed pressure of 0.11 and 1.0 MPa for TFC membranes

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prepared using different organic solvents are reported in Table 5. CO2/N2 selectivity is not contained in Table 5, because it depends on both CO2 and N2 permeance. Correlation coefficients between each membrane performance and structural property are also summarized in Table 5. At a lower feed pressure (0.11 MPa), CO2 permeance strongly correlates with skin layer thickness (negatively) and crosslinking (positively), but moderately with crystallinity (negatively). N2 permeance is strongly correlated with skin layer thickness (negatively), crosslinking (positively) and crystallinity (negatively). At a higher feed pressure (1.0 MPa), CO2 permeance correlates strongly with skin layer thickness (negatively), moderately with crosslinking (positively), but weakly with crystallinity (negatively). While N2 permeance is strongly correlated with skin layer thickness (positively), crosslinking (negatively) and crystallinity (positively). These relationships suggest that the variation of CO2 and N2 permeance is determined by different structural properties at lower and higher feed pressure. Commonly, membrane structural properties influence gas separation performance in the following ways. First, thinner skin layer thicknesses could lead to higher gas permeance [15]. Second, an inverse relationship between crystallinity and gas permeance was clearly presented in the literature [23–25]. Crystalline region in polymer acts as impermeable barriers to gas molecules, forcing the penetrants to travel a tortuous path through the polymer, thus decreasing the diffusion coefficient [25]. Finally, increasing the crosslinking extent could decrease the gas permeability [51–53] due to the decrease of the gas diffusivity inside the crosslinked network [23,54]. However, it should be mentioned that TFC membrane skin layer with higher crosslinking extent possesses higher N content (see Section 2.4) in this study, and all the N atoms in the skin layer stem from the tertiary amino groups (see Scheme 1) which are the carriers of the membranes. In other words, the higher crosslinking extent of the skin layer, the higher total carrier content in the membrane. The increase in the total carrier content will facilitate more CO2 molecules across the membrane [55]. Both the two effects contribute to the variation of CO2 permeance with crosslinking extent. Nevertheless, increasing crosslinking could decrease N2 permeance due to no chemical reaction between N2 and the carriers. As illustrated in Table 5, at a lower feed pressure (0.11 MPa), the nearly perfect correlations between CO2 permeance and skin layer thickness (negatively) as well as crosslinking (positively) indicate that the variation of CO2 permeance is mainly determined by the thickness and crosslinking of skin layers. Moreover, skin layer thickness and crystallinity dominates N2 permeance according to the strong negative correlation. Nevertheless, the calculated strong positive correlation (0.97) between N2 permeance and crosslinking extent is not logical, which may be due to the fact that both N2 permeance and crosslinking are influenced by skin layer thickness and crystallinity. To eliminate the effects of both skin layer thickness and crystallinity, the partial correlation coefficient [56] between N2 permeance and crosslinking extent when conditioning on skin layer thickness ðr RN ,m9d Þ and the partial 2 correlation coefficient when conditioning on crystallinity ðr RN ,m9C Þ 2 were calculated. Results show that the values of ðr RN ,m9d Þ and 2 ðr RN ,m9C Þ are 0.50 and 1.00, respectively, indicating that the 2 correlation between N2 permeance and crosslinking is mainly affected by skin layer thickness, and that crosslinking does not govern N2 permeance. Furthermore, as shown in Table 5, at a higher feed pressure (1.0 MPa), the moderate-to-strong correlations between CO2 permeance and skin layer thickness (negatively) together with crosslinking extent (positively) suggest CO2 permeance is also mainly determined by the thickness and crosslinking of the skin layers. Besides, the calculated strong positive correlations between N2

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permeance and skin layer thickness together with crystallinity are illogical, because N2 permeance, skin layer thickness, and crystallinity are all impacted by crosslinking. Hence, the partial correlation coefficient between N2 permeance and skin layer thickness when conditioning on crosslinking ðr RN , d9m Þ, as well as the partial 2 correlation coefficient between N2 permeance and crystallinity when conditioning on crosslinking ðr RN ,C9m Þ were calculated as 2 mentioned above. Results show that the values of r RN , d9m and 2 r RN ,C9m are 0.50 and 0.31, respectively. These results suggest that 2 skin layer thickness and crystallinity do not dominate N2 permeance at a higher feed pressure any more. However, the nearly perfect negative correlation between crosslinking extent and N2 permeance indicates that the crosslinking of skin layer governs the variation of N2 permeance. Additionally, as mentioned in the first of this section, the variation of N2 permeance at a higher feed pressure with organic solvent arises from the different CO2induced plasticization resistance of the membranes. From the above analyses, we can infer that crosslinking governs the CO2-induced plasticization resistance of the membranes, i.e., increasing the extent of crosslinking could enhance the CO2induced plasticization resistance of the membranes. Based on the analyses above, CO2 permeance is determined by both thickness and crosslinking of the skin layers at the whole feed pressure range, while N2 permeance is dominated by skin layer thickness and crystallinity at the lower feed pressure but by crosslinking at the higher feed pressure. 3.3. Properties of membranes formed with different reactants concentrations 3.3.1. Effect of TMC concentration Fig. 9 shows the representative SEM images of membranes prepared with various TMC concentrations. By visual inspection, skin layer thicknesses increase rapidly within the concentration range of 0.0100–0.0350 mol/L and then increase gently at a higher concentration ranging from 0.0350 to 0.0750 mol/L. By subtracting the thickness of the intermediate PDMS layer (see Fig. 4(a)), the average skin layer thicknesses are measured on the order of 49 nm, 105 nm, 138 nm, 273 nm, 301 nm, and 308 nm for Fig. 9(a), (b), (c), (d), (e), and (f), respectively. Besides, to investigate whether any kind of changes in PDMS layer affect the total thickness of the dense layer (both skin layer and PDMS layer), TFC membranes prepared with different TMC concentrations were characterized by ATR-FTIR technique described in Section 3.2.2, and these results are shown in Fig. 10. It can be seen that the areas of peak (1726 cm  1) of TFC membranes increases gradually with increasing TMC concentration. Moreover, the correlation coefficient between the area of peak (1726 cm  1) and the skin layer thickness measured by SEM was calculated to be 0.88. These results suggest that the effect of PDMS layer on the total thickness of the dense layer is negligible. Average calculated values of skin layer thickness (d), extent of crosslinking (m), and crystallinity (C) for TFC membranes prepared using different TMC concentrations are reported in Table 6. Correlations of TFC membrane structural properties with TMC concentration (cTMC) are also summarized in Table 6. It can be seen that there is a moderate-to-strong, positive correlation between skin layer thickness and cTMC at the whole TMC concentration range. Besides, observed extent of crosslinking correlates moderately with cTMC at a lower TMC concentration range of 0.0100–0.0250 mol/L (positively), but strongly with cTMC at a higher TMC concentration range of 0.0350–0.0750 mol/L (negatively). However, observed crystallinity correlates strongly (negatively and positively) with cTMC at a lower and higher TMC concentration, respectively. These relationships suggest that TMC concentration in organic phase governs TFC membrane skin layer thickness at the whole TMC

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Fig. 9. SEM images of TFC membranes prepared with TMC concentration of (a) 0.0100, (b) 0.0150, (c) 0.0250, (d) 0.0350, (e) 0.0500, and (f) 0.0750 mol/L. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 10 min interfacial polymerization at 401 C.

Table 6 Correlation of TFC membrane structural properties with TMC concentration. cTMC (mol/L)

d (nm) m (dimensionless) C (dimensionless)

0.0100 0.0150 0.0250 0.0350 0.0500 0.0750

49 105 138 273 301 308

Correlation coefficients cTMC (0.0100–0.0750 mol/L) 0.74 cTMC (0.0100–0.0250 mol/L) cTMC (0.0350–0.0750 mol/L)

0.609 0.706 0.752 0.400 0.324 0.267

0.449 0.446 0.403 0.461 0.511 0.553

0.71  0.90

 0.85 0.93

1. cTMC represents TMC concentration. 2. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 10 min interfacial polymerization at 401 C.

Fig. 10. ATR-FTIR spectra of TFC membranes prepared with TMC concentration of (a) 0.0100, (b) 0.0150, (c) 0.0250, (d) 0.0350, (e) 0.0500, and (f) 0.0750 mol/L. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 10 min interfacial polymerization at 401 C.

concentration range; and that higher TMC concentration facilitate the formation of more crosslinked and less crystalline skin layer at a lower TMC concentration range, but formation of less crosslinked and more crystalline skin layer at a higher TMC concentration range.

At a low TMC concentration (0.0100 mol/L), the rate of reaction is very low which produced limited polymer [15] and an extremely thin skin layer (49 nm) is obtained. With increasing TMC concentration from 0.0100 to 0.0350 mol/L, the rate of polymerization reaction becomes higher which produces a thicker skin layer (from 49 to 273 nm) at the same reaction time [15]. When the IP film thickness increases to a certain extent, MEDA molecules could not diffuse into the organic phase for reaction any more due to the too thick film layer, reported as ‘‘self-limiting’’ phenomena [10]. This results in a slight change of skin layer thickness with further increasing TMC concentration. Generally, TMC molecules participate in two competing processes, to react with MEDA and to hydrolyze. At a low TMC concentration (0.0100 mol/L), the polymerization reaction rate is very low [15], leading to the hydrolysis of a large proportion of TMC to yield –COOH groups. Hence, a lower crosslinking extent is obtained at a lower TMC concentration. With increasing TMC concentration from 0.0100 to 0.0250 mol/L, the polymerization

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reaction rate is largely accelerated [57], which results in a larger proportion of TMC to react with MEDA to obtain a more crosslinked polymer. With further increasing TMC concentration, the extent of crosslinking decreases due to the hydrolysis of an excess of acyl chloride groups [22]. Moreover, an inverse relationship between crystallinity and extent of crosslinking has been clearly presented in Section 3.2.2. This is why the crystallinity decreases within the TMC concentration range of 0.0100–0.0250 mol/L and then increases with increasing TMC concentration from 0.0250 to 0.0750 mol/L as shown in Table 6. Additionally, as can be calculated from Table 6, the variation of skin layer thickness, extent of crosslinking, and crystallinity are 529%, 182% and 37% within the TMC concentration range of 0.0100–0.0750 mol/L, respectively. These results suggest that TMC concentration in organic phase mainly governs TFC membrane skin layer thickness. CO2/N2 permeance and selectivity for TFC membranes prepared with different TMC concentrations are presented in Fig. 11. It is found that both CO2 and N2 permeance are reduced with increasing TMC concentration. Moreover, Fig. 11(b) shows that N2 permeance of TFC membrane prepared with 0.0100 mol/L TMC concentration is extremely high, which results in the lowest CO2/N2 selectivity (see Fig. 11(c)). From Fig. 11(c), it can be seen that CO2/N2 selectivity increases first and then changes slightly with increasing TMC concentration. As shown in Fig. 11, the change of both CO2 and N2 permeance with TMC concentration at the whole feed pressure range are nearly the same, and hence, only the performance properties at the feed pressure of 0.11 MPa are correlated with TFC membrane structural

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properties. These results are summarized in Table 7. As illustrated in Table 7, both CO2 and N2 permeance moderately-to-strongly correlate with skin layer thickness (negatively), but negligibly correlate with crosslinking (positively) and crystallinity (negatively), indicating that the variations of both CO2 and N2 permeance is determined by the skin layer thickness. This arises from the pronounced change of the skin layer thickness compared with the variation of crosslinking extent and crystallinity within the TMC concentration range of 0.0100–0.0750 mol/L as mentioned above. According to Section 3.2.3, thinner skin layer thickness could lead to higher gas permeance, and thus, both CO2 and N2 permeance are reduced with increasing TMC concentration.

3.3.2. Effect of MEDA concentration Experimentally determined values of skin layer thickness (d), extent of crosslinking (m), crystallinity (C), CO2 permeance ðRCO2 Þ, N2 permeance ðRN2 Þ, and CO2/N2 selectivity for TFC membranes prepared using different MEDA concentrations are plotted in Fig. 12. Average values are indicated by the column height with standard deviations of measured values displayed by the error bars. Observed skin layer thickness fluctuates slightly around 140 nm at the MEDA concentration range of 0.0309–0.1836 mol/ L. In addition, with increasing MEDA concentration, observed extent of crosslinking increases continuously, whereas observed crystallinity increases first and then decreases. Besides, as shown in Fig. 12, with increasing MEDA concentration, CO2 permeance decreases first and then increases, while N2 permeance firstly decreases, then increases, and finally keeps nearly unchanged.

Fig. 11. CO2/N2 permeance and selectivity of TFC membranes prepared with different TMC concentrations in hexane: (a) CO2 permeance; (b) N2 permeance; (c) CO2/N2 selectivity. Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 10 min interfacial polymerization at 401 C. Feed gas: CO2/N2 mixed gas (15/85 by volume).

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Table 7 Correlation of performances with structural properties for TFC membranes prepared with different TMC concentrations. Correlation coefficients

d (nm)

m (dimensionless)

C (dimensionless)

RCO2 RN2

 0.84  0.76

0.29 0.18

 0.11  0.02

Membrane preparation: 0.0615 mol/L concentration of MEDA solution, 10 min interfacial polymerization at 401 C.

Table 8 Correlation of TFC membrane structural properties with MEDA concentration. Correlation coefficients

d (nm)

m (dimensionless)

C (dimensionless)

cMEDA

0.18

0.89

0.23

1. cMEDA represents MEDA concentration. 2. Membrane preparation: 0.0226 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C.

Table 9 Correlation of performances with structural properties for TFC membranes prepared with different MEDA concentrations. Correlation coefficients

d (nm)

m (dimensionless)

C (dimensionless)

RCO2 RN2

 0.23  0.94

0.22  0.30

 0.25  0.86

Membrane preparation: 0.0226 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C.

Fig. 12. Properties of TFC membranes formed with different MEDA concentrations. Membrane preparation: 0.0226 mol/L concentration of TMC solution in hexane, 10 min interfacial polymerization at 401 C. CO2/N2 permeance and selectivity were obtained at the feed pressure of 0.11 MPa. Feed gas: CO2/N2 mixed gas (15/85 by volume).

Moreover, CO2/N2 selectivity is found to increase with an increase in MEDA concentration. Correlations of TFC membrane structural properties with MEDA concentration are reported in Table 8. There is a strong, positive correlation between crosslinking extent and MEDA concentration (cMEDA). However, skin layer thickness and crystallinity correlate weakly with MEDA concentration. These results suggest that MEDA concentration mainly influences the crosslinking of TFC membrane skin layer. Correlations of performances with structural properties for TFC membranes prepared with different MEDA concentrations are reported in Table 9. CO2 permeance is weakly correlated with skin layer thickness (negatively), crosslinking (positively), and crystallinity (negatively). While N2 permeance correlates strongly with skin layer thickness (negatively) and crystallinity (negatively), but weakly with crosslinking (negatively). The relationship between structural and performance properties for TFC membranes prepared with different MEDA concentrations can be explained as follows.

Our previous study has proved that the monomers in both aqueous and organic phase play important roles in determining skin layer thickness of TFC membranes [15]. However, an excess of MEDA over TMC is used in this study, which drives IP process to be controlled by the diffusion of TMC through the organic phase into the reaction region. Thus skin layer thickness is determined by TMC concentration rather than MEDA concentration [15]. This is why skin layer thickness changes slightly with increasing MEDA concentration. Consequently, the impact of skin layer thickness on both CO2 and N2 permeance is negligible. When the MEDA concentration in the aqueous phase is very low (0.0309 mol/L), there is not enough MEDA to form an integrated network structure [22], leading to a relatively low crystallinity. Meanwhile, the excess acyl chloride groups are hydrolyzed to form a loose structure with a very low crosslinking extent [22]. Both the above two effects contribute to the relatively high CO2 and N2 permeance at a lower MEDA concentration. With increasing MEDA concentration from 0.0309 to 0.0918 mol/L, the integrated network forms continuously, resulting in the increasing crosslinking extent and crystallinity. This gives rise to the decrease in both CO2 and N2 permeance within the MEDA concentration range of 0.0309–0.0918 mol/L. As the MEDA concentration is further increased, nearly fully crosslinked skin layer gradually forms, contributing to the further increase of the crosslinking extent. The increase of the crosslinking extent could result in the decrease of the crystallinity according to the inverse relationship between crystallinity and crosslinking extent as mentioned in Section 3.2.2. The increased total carrier content caused by the increased extent of crosslinking as mentioned in Section 3.2.3 and the decreased crystallinity could enhance CO2 permeance. Additionally, according to Section 3.2.3, increasing crosslinking could decrease N2 permeance, while decreasing crystallinity could increase N2 permeance. The comprehensive effect eventually leads to the insignificant change of N2 permeance. 3.4. Comparison with other membranes The performance of TFC membranes obtained in this work and the performance of other membranes reported elsewhere are presented in Fig. 13 with data points listed in Table 10. The TFC-a membrane prepared with 0.0615 mol/L MEDA and 0.0250 mol/L TMC showed the best selectivity of this work. For example, the TFC-a membrane displayed a CO2/N2 selectivity of 87 and CO2 permeance of 1035 GPU at 0.0165 MPa CO2 partial pressure.

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While TFC-b membrane prepared with 0.0615 mol/L MEDA and 0.0100 mol/L TMC showed the best CO2 permeance of this work. For example, the TFC-b membrane displayed a CO2 permeance of 2905 GPU and CO2/N2 selectivity of 64 at 0.0165 MPa CO2 partial pressure. CO2 permeability of some commercial polymeric membranes is reported in literature [58]. The thickness of the dense layer (both skin layer and PDMS layer) of TFC membranes in this study is in the range of 0.439–0.698 mm. Hence, for comparison with

Fig. 13. Relationship of CO2 permeace and CO2/N2 selectivity of various membranes. The theoretical trade-off limit, calculated according to a model proposed by Robeson in 2008 [59], is also shown.

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TFC membranes, the CO2 permeance of the commercial polymeric membranes is calculated by dividing CO2 permeability by the thickness of the dense layer (about 0.439 mm). The trade-off curve shown in Fig. 13 is calculated according to a model proposed by Robeson in 2008 [59] for a 0.439 mm thick membrane. It can be seen from Fig. 13 that the CO2 permeance and CO2/N2 selectivity of TFC membranes in this work are higher than both the trade-off curve and the commercial polymeric membranes (data points a–d in Fig. 13). Moreover, as shown in Fig. 13 and Table 10, the CO2 permeance and CO2/N2 selectivity of the high performance state-ofthe-art polymeric membranes of physical separation mechanisms in the laboratory level (data points e–j and o–q in Fig. 13) are close to the trade-off curve, and the CO2 permeance and CO2/N2 selectivity of TFC membranes in this work are higher than these membranes of physical separation mechanisms, which is attributed to the reversible reactions between tertiary amino groups and CO2. In addition, as shown in Fig. 13 and Table 10, although TFC membranes in this work displayed lower CO2/N2 selectivity than that of the liquid membranes (data points u and v in Fig. 13), the CO2 permeance of TFC-a and TFC-b membrane in this work is near 17.0–37.6 and 47.6–105.6 times as much as that of the abovementioned liquid membranes, respectively. Finally, as shown in Fig. 13 and Table 10, the CO2 permeance and CO2/N2 selectivity of most high performance state-of-the-art fixed carrier membranes in the laboratory level are in the range of 170–1700 and 60–180, respectively (data points k–n, r, and s in Fig. 13). The CO2/N2 selectivity of TFC membranes in this work are within the above CO2/N2 selectivity range, which is not special. The CO2 permeance of TFC-b membrane in this work is much higher than the CO2 permeance range above, which is mainly attributed to the thin skin layer thickness (49 nm). However, both CO2 permeance and CO2/N2 selectivity of TFC membranes in this work are lower than PVAm-PIP/PS membrane (data point t in Fig. 13). It should be pointed out that the objective of the choice of

Table 10 Performance comparison of TFC membranes obtained in this work with other membranes. Number

Membrane

RCO2 (GPU)

ab

Permselective mechanisms

Reference

a b c d e f g h i j k l m n o p q r s t u v w x

Cellulose acetate Polyimide (Matrimid) PS PDMS Poly[bis(2-(2-methoxyethoxy)ethoxy) phosphazene] PIM-7 Modified PDMS PIM-1 Poly(trimethylgermylpropyne) Poly(trimethylsilylpropyne) DNMDAm-TMC/PS PVAm/PVA PVAm/PPO PVAm-EDA/PS PEO-PBT/PEG-DBE (PAN-PDMS) PEO-PBT (PAN-PDMS) PolarisTM PANI-PVAm/PS DNMDAm-DGBAmE-TMC/PDMS/PS PVAm-PIP/PS 2.25 M Glycine-Na-Glycerol PAMAM dendrimer TFC-ac TFC-bd

14.4a 24.4a 12.8a 6150a 569a 2506a 4556a 5239a 31891a 66059a 173 212 365 607 730 896 1000 1200 1601 6500 27.5 61 1035 2905

30 33.4 22.4 10.8 62.5 26.2 34.2 25 14 10.7 70 174 60 106 40 55 50 120 138 277 3980 230 87 64

Physical separation Physical separation Physical separation Physical separation Physical separation Physical separation Physical separation Physical separation Physical separation Physical separation Facilitated transport Facilitated transport Facilitated transport Facilitated transport Physical separation Physical separation Physical separation Facilitated transport Facilitated transport Facilitated transport Facilitated transport Facilitated transport Facilitated transport Facilitated transport

[58] [58] [58] [58] [59] [59] [59] [59] [59] [59] [15] [60] [61] [55] [62] [29] [63] [64] [32] [65] [66] [67] This work This work

a

The data is calculated by dividing CO2 permeability by the thickness of the dense layer (about 0.439 mm). a represents CO2/N2 selectivity. c TFC-a represents the membrane prepared with 0.0615 mol/L MEDA and 0.0250 mol/L TMC in hexane. d TFC-b represents the membrane prepared with 0.0615 mol/L MEDA and 0.0100 mol/L TMC in hexane. b

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MEDA as the aqueous monomer is only to study the formationstructure-performance relationship of interfacially polymerized TFC gas separation membranes for controlled preparation of these membranes. TFC membranes with better separation performance will be obtained by the selection of other aqueous monomers together with the choice of preparation conditions according to the findings in this work. The related work mentioned above will be studied in the future.

4. Conclusions Herein, TFC membranes for CO2/N2 separation were prepared by interfacial polymerization from MEDA and TMC on crosslinked PDMS coating PS support membrane. The relationships among the skin layer formation conditions, skin layer structure, and membrane separation performance were investigated. The main conclusions were obtained as follows. Generally, higher MEDA solubility in the organic solvent (in isolation) could produce thicker, less crosslinked, and more crystalline skin layers, while higher MEDA diffusivity in the organic solvent (in isolation) could produce thinner, more crosslinked, and less crystalline skin layers. The choice of organic solvent with higher MEDA diffusivity and lower MEDA solubility could produce membranes with higher CO2 permeance, higher CO2/N2 selectivity, and better CO2-induced plasticization resistance. TMC concentration in organic phase determines TFC membrane skin layer thickness, whereas MEDA concentration in aqueous phase governs the crosslinking extent of TFC membrane skin layer. Overall, under the circumstances of forming an integrated skin layer, membranes with high CO2 permeance and high CO2/N2 selectivity could be obtained by decreasing TMC concentration in organic phase and increasing MEDA concentration in aqueous phase. The above findings have great theoretical significance for the controlled preparation of gas separation membranes.

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