Constructing CO2 transport passageways in Matrimid® membranes using nanohydrogels for efficient carbon capture

Journal of Membrane Science 474 (2015) 156–166

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Constructing CO2 transport passageways in Matrimids membranes using nanohydrogels for efficient carbon capture Xueqin Li a,b, Meidi Wang a, Shaofei Wang a,b, Yifan Li a,b, Zhongyi Jiang a,b, Ruili Guo d, Hong Wu a,b,c,n, XingZhong Cao e, Jing Yang e, Baoyi Wang e a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China d Key Laboratory for Green Process of Chemical Engineering of Xinjiang Bingtuan, School of Chemistryand Chemical Engineering, Shihezi University, Xinjiang, Shihezi 832003, China e Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

art ic l e i nf o

a b s t r a c t

Article history: Received 18 June 2014 Received in revised form 27 September 2014 Accepted 4 October 2014 Available online 14 October 2014

Composite membranes were fabricated by incorporating poly(N-isopropylacrylamide) nanohydrogels (NHs) into Matrimids 5218 matrix to improve the separation performance for CO2/CH4 and CO2/N2 mixtures. The membranes were characterized by a fourier transform infrared spectrometer (FT-IR), scanning electron microscopy (SEM), tensile test, dynamic mechanical analysis (DMA), X-ray diffraction (XRD), positron annihilation lifetime spectroscopy (PALS), the static contact angle and water content measurement. The incorporation of nanohydrogels increased the fractional free volume of the composite membranes, water uptake and water retention capacity. The composite membranes displayed better performance than the pure Matrimids membrane. The nanohydrogels homogeneously embedded in the Matrimids matrix acted as water reservoirs to not only provide more water for dissolving CO2, but also construct interconnected CO2 transport passageways. The as-prepared Matrimids/NHs-20 composite membrane showed CO2/CH4 and CO2/N2 selectivities of 61 and 52 with a CO2 permeability of 278 Barrer, surpassing or being close to the 2008 Robeson upper bound lines. & 2014 Elsevier B.V. All rights reserved.

Keywords: Matrimids Nanohydrogels Composite membranes Water content CO2 separation

1. Introduction The development of energy-efficient and scalable CO2 capture technologies has recently become an important worldwide issue [1,2]. Among various capture technologies, membrane separation is an attractive alternative to such conventional techniques as absorption, adsorption and cryogenic distillation for its high energy efficiency and environmental sustainability [3–6]. For many gas separation applications (biogas and fuel gas) involving CO2 capture, the gas mixture is often moist or sometimes even saturated with water vapor [2,7–9]. The solubility of CO2 in water is an order of magnitude higher than that of CH4 and N2, leading to a higher gas permselectivity. Moreover, water typically swells the polymer matrix and increases the polymer chain mobility, thus reducing the gas diffusion resistance [10–15]. The immobilized water retained in the polymer matrix forms transport n Corresponding author at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel./fax: þ 86 22 23500086. E-mail address: [email protected] (H. Wu).

http://dx.doi.org/10.1016/j.memsci.2014.10.003 0376-7388/& 2014 Elsevier B.V. All rights reserved.

passageways for gas permeation through the membrane. The permeability of CO2 in water has been found to be as high as 1983 Barrer, which is three to six orders of magnitude higher than that in polymer matrix [16,17]. Therefore, the presence of water vapor is expected to increase both solubility and diffusivity of the gas in the membranes. Water-swollen hydrogel membranes have been studied for CO2 separation in consideration of the favorable solubility of CO2 in water [18–21]. Park and Lee [22] prepared a water-swollen poly (vinyl alcohol) (PVA) hydrogel membrane crosslinked by glutaraldehyde. The permeation rate of CO2 through the PVA membrane was 1011 GPU with an approximate CO2/N2 selectivity of 80. Jiang and Yuan [23] prepared a water-swollen cellulose membrane which showed a high permeation rate of CO2 with a CO2/CH4 selectivity of 30 and a CO2/N2 selectivity of 50. They concluded that the permeation rate of a gas through a water-swollen cellulose membrane depended on both the gas solubility and diffusivity in the water existing in the membrane. Feng et al. [16] studied a series of hydrogel membranes including poly(vinyl alcohol), chitosan, carboxyl methyl cellulose, alginic acid and poly(vinylamine), and found that gas permeability in these water-swollen hydrogel

X. Li et al. / Journal of Membrane Science 474 (2015) 156–166

membranes was three to six orders of magnitude higher than that in dry membranes. It was found that water offered transport passageways for gas permeation through the membrane. Therefore, the gas permselectivity can be improved significantly by increasing water content in the membranes. However, the presence of a large amount of water in hydrogel membranes usually causes such problems as low mechanical property (Young's modulus,  103 Pa), poor operation stability and low CO2/gas selectivity, thus limiting their industrial application [16,24,25]. The commercial polyimides, one of the most widely used glassy polymer membranes [26], are attractive materials for gas separation owing to their excellent mechanical property (Young's modulus, 106 Pa), operation stability and gas separation performance, especially for CO2 separation [27,28]. However, the hydrophobic aromatic backbone of polyimide restricts the amount of water in polymer matrix, and the increase of permeability is thus limited under wet conditions. Exploring new membranes with high water content, good mechanical properties and operation stability simultaneously is highly desirable. Composite membranes comprising a polymer bulk phase (continuous phase) and a filler phase (dispersed phase) provide a promising solution for the above-mentioned sticky problems [29,30]. The use of appropriate fillers with multifunctional properties provides the possibility to better design membrane structure. For example, the incorporation of versatile hydrophilic fillers into polymer matrix can increase water content and the fractional free volume, providing more gas transport passageways [31–33]. Nanohydrogels have triggered considerable attention in recent years owing to their high water content, biocompatibility and desirable mechanical properties [34–36]. Nanohydrogels can absorb and retain extremely high water content of up to 10–1000 times of their original weight or volume without being dissolved [37–39]. The water-absorbing ability of nanohydrogels arises from the abundant hydrophilic functional groups on polymer backbone, while their indissolubility in water is due to cross-links between polymer chains. However, nanohydrogels have not been considered as multifunctional fillers in composite membranes for efficient CO2 capture. In this work, nanohydrogels were exploited as versatile fillers to enhance CO2 separation performance of the composite membranes. To be specific, poly(N-isopropylacrylamide) nanohydrogels (NHs) with a diameter of  250 nm were synthesized by precipitation polymerization method. The NHs were then embedded into Matrimids matrix to fabricate a series of composite membranes. The water content and the gas separation performance of composite membranes were systematically investigated. In addition, the physicochemical properties of the membranes including microstructure, mechanical properties and free volume characteristics were evaluated in detail. This study may offer a generic strategy to construct ideal membrane structure and fabricate highly permeable and selective membrane for CO2 capture.

2. Experimental and characterization method 2.1. Materials Polyimide resin (Matrimids5218) powder was purchased from Alfa Aesar. N-isopropylacrylamide (NIPAM, 98 wt%) was purchased from Aladdin Chemistry Co. Ltd. N,N0 -Methylenebisacrylamide (BIS, 99 wt%) was purchased from Tianjin Bodi Chemical Industry Co. Ltd. Potassium peroxodisulfate (KPS, 99.5 wt%) and N,NDimethylformamide (DMF, 99.5 wt%) were purchased from Tianjin Guangfu Technology Development Co. Ltd. All reagents were of analytical grade and used without further purification. Deionized water was used throughout the experiments.

157

2.2. Synthesis of poly(N-isopropylacrylamide) nanohydrogels The crosslinked poly(N-isopropylacrylamide) nanohydrogels were prepared by a precipitation polymerization method [40]. Briefly, NIPAM and BIS (NIPAM: BIS ¼14:9 weight ratio) were dissolved in deionized water (2.8 mg/mL NIPAM solution) in a four-neck flask equipped with a nitrogen inlet and a thermometer. Nitrogen was bubbled into the solution to remove oxygen and the flask was kept at 90 1C for 30 min. Then, KPS (NIPAM: KPS ¼ 14:1 weight ratio) was dissolved in deionized water (10.0 mg/mL KPS solution) and was added into the flask to initiate polymerization. After being stirred for 6 h, the resultant NHs were purified by three cycles of centrifugation followed by drying in a vacuum oven at 50 1C until constant weight was reached. 2.3. Membrane preparation The Matrimids/NHs composite membranes and pure Matrimids membrane were fabricated by a solution-casting method. Prior to membrane fabrication, Matrimids and NHs were dried in a vacuum oven at 50 1C for 24 h in order to remove the water completely. Matrimids powder was dissolved in DMF under magnetic stirring at room temperature for 12 h to obtain a 6 wt% homogeneous solution. A specified amount of NHs was dispersed into DMF under ultrasonic treatment for 30 min. After being stirred vigorously for another 12 h, the mixed solution was cast onto a flat glass plate, dried first at 50 1C for 12 h and then 80 1C for another 12 h. The pure Matrimids membrane was also fabricated by exactly the same procedure without the addition of NHs. For simplicity, the obtained composite membranes were designated as Matrimids/NHs-X, where X in the range of 0–20 wt% referred to the weight percentage of NHs relative to the weight of Matrimids. The membrane thickness was measured by a digital micrometer and varied from 70 to 100 μm. 2.4. Nanohydrogels and membrane characterization The morphology of nanohydrogels was observed with a Tecnai G2 F20 transmission electron microscopy (TEM). The infrared spectra of nanohydrogels and membranes in the range of 4000– 400 cm  1 were recorded on a BRUKER Vertex 70 fourier transform infrared spectrometer (FT-IR) equipped with a horizontal attenuated transmission accessory for nanohydrogels and a horizontal attenuated total reflectance accessory for membranes. The cross-sectional morphology of all membranes was observed by scanning electron microscopy (SEM). The images were collected using a Hitachi S-4800 instrument with an accelerating voltage of 15 kV. The mechanical properties of membranes were measured by a Changchun Kexin WDW-02 tensile test machine. The tests were conducted at a crosshead speed of 5 mm/min at 25 1C. Dynamic mechanical properties of membranes were measured by a PerkinElmer dynamic mechanical analyzer (DMA). The samples were measured at a frequency of 1 Hz and a heating rate of 5 1C/min in the range of 25–400 1C. The crystalline structure of membrane was investigated using an X-ray diffraction (XRD) with a Rigaku D/max 2500 v/pc in the range of 3–651 at the scan rate of 101/min. The average d-spacing of Matrimids matrix was evaluated based on Bragg's law as follows: nλ ¼ 2d sin θ

ð1Þ

where n was an integer (1, 2, 3, …), λ denotes the X-ray wavelength, d represented the intersegmental spacing between two polymer chains and θ stands for the X-ray diffraction angle of the peak. Thermogravimetric analysis (TGA) of the membranes was conducted by a NETZSCH TG 209 F3 TGA instrument from the

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room temperature to 800 1C under nitrogen flow at a heating rate of 10 1C/min. The static contact angle of the membrane was measured at room temperature by a JC2000C Contact Angle Meter contact angle goniometer. Positron annihilation lifetime spectroscopy (PALS) experiment was determined by an EG&GORTEC fast–fast coincidence system (resolution 201 ps) at room temperature. We used a 22Na (5  105 Bq) radioactive positron source which was sandwiched by two pieces of sample (thickness 1.0 mm). More than 2  106 coincidences were collected for each sample. On assumption that the location of o-Ps occurs in a sphere potential well surrounded by an electron layer of a constant thickness Δr (0.1656 nm), the radius of free volume cavity (r3, r4) is calculated from the pick-off annihilation lifetime of o-Ps (τ3, τ4) by the following semiempirical equation:     1   1 r 1 2π r þ sin τ ¼ 1 ð2Þ 2 r þ Δr 2π r þ Δr The fractional free volume (Fv) was determined by multiplying the volume of equivalent sphere and the intensity of o-Ps (I3, I4): 4 4 F v ¼ π r 33 I 3 þ π r 34 I 4 3 3

ð3Þ

In order to obtain the fractional free volume of pure Matrimids membrane in humidified state, the fractional free volume was calculated based on the density of membranes [41]. The density of soaked membrane was determined by the buoyancy method. Silicon oil with known density (ρ0 ¼ 0.971 g/cm3) was selected as the auxiliary liquid. The values of density (ρp) were calculated by the following equation:

ρp ¼

MA ρ MA  ML 0

ð4Þ

where MA and ML were the membrane weights in the air and in the auxiliary liquid, respectively. The fractional free volume of soaked pure Matrimids membrane (Fh) was estimated by the following equation, F h ¼ 1  1:3υw ρp

ð5Þ

where ρp was the density of pure Matrimid membrane, νw was van der Waal's volume of the repeat unit of Matrimids in g/cm3 which was calculated from Bondi radii [42]. s

2.5. Measurement of total water, free water, bound water and water retention The water uptake (total water) and water state in the membrane were determined according to the method in literatures [11,41]. Each membrane was weighed to determine the “humidified” weight (Wh) after soaking in deionized water to achieve complete hydration at constant temperature. Free water had essentially the same property as bulk water and bound water bonded to the polymer matrix via hydrogen bonds. The membranes were first heated at 100 1C in a vacuum oven for 6 h to remove free water and reweighed (Wb), and then dried in a vacuum oven at 150 1C for 6 h and reweighed again (Wd). Immediately after water uptake measurement, the hydrated membrane was maintained at 40 1C and 20% RH in a climate box and was weighed (Wht) at time t. The content of total water, free water, bound water and water retention were calculated using the formulas W Wd Total waterð%Þ ¼ h  100 Wd Free waterð%Þ ¼

W h W b  100 Wb

ð6Þ ð7Þ

Bound waterð%Þ ¼

Wb Wd  100 Wd

Water retentionð%Þ ¼

Wh t Wd  100 Wd

ð8Þ ð9Þ

2.6. Swelling property of membranes The swelling property was determined by measured the membrane area differences before and after soaking in water. The swelling degree with an error within 73% was calculated based on the following equation: Swelling degree ð%Þ ¼

As  Ad  100 Ad

ð10Þ

where As and Ad were the areas of the swollen and dry membranes, respectively. 2.7. Gas permeation tests The gas permselectivity of the membranes was measured by a set of test equipments described in the previous literatures [43,44]. Pure gas (CO2, CH4 and N2) and mixed-gas (CO2/ CH4 ¼ 30/70, vol%; CO2/N2 ¼ 10/90, vol%) permeation experiments were conducted using the conventional constant pressure/variable volume method under both humidified and dry conditions [43,45]. The kinetic sorption behavior of water in the Matrimids membrane showed that water sorption reached equilibrium after 2 weeks (Fig. S5). Therefore, the humidified membranes samples had been soaked in water for 2 weeks to absorb sufficient water before they were tested. In a typical measurement, the feed gas was saturated with water vapor through a humidifier and then passed through an empty bottle to remove the condensate water. Meanwhile, the sweep gas was humidified by passing through a water-containing bottle. For comparison, dry state gas permeation experiments were also conducted, in which case the feed gas and sweep gas were directly introduced into membrane cell. N2 was used as sweep gas when the feed gas was CO2, CH4 or CO2/CH4 mixtures, while CH4 was used as the sweep gas when the feed gas was N2 or CO2/N2 mixtures. The sweep gas was kept at atmospheric pressure. The flow of sweep gas was measured using a mass flowmeter. The compositions of the feed and permeate gases were analyzed by an Agilent 6820 gas chromatograph equipped with a thermal conductivity detector (TCD). The effective area of composite membranes is 12.56 cm2. The permeability (Pi, Barrer, and 1 Barrer¼10  10 cm3 (STP) cm/ (cm s cmHg)) of each gas was obtained from the average value of at least thrice measurements by using Eq. (8) Pi ¼

Q il

ΔP i A

ð11Þ

where Qi is the volumetric flow rate of gas ‘i’ (cm3/s) at standard temperature and pressure (STP), l is the membrane thickness (cm), Δpi is the transmembrane partial pressure difference of gas ‘i’ (cmHg) between the feed side and permeate side partial pressures, and A is the effective membrane area. The CO2/CH4 and CO2/N2 selectivities (αij) were calculated by the following equation:

αij ¼

Pi Pj

ð12Þ

The diffusion coefficient and solubility coefficient of pure gas (CO2, CH4 and N2) were measured at steady state conditions by the “timelag” method [45]. All experiments were performed at a constant temperature of 30 1C. The cell exposed an effective membrane area of 12.56 cm2 to permeation. In all gas permeation experiments, permeability results were recorded at steady state conditions. The feed

X. Li et al. / Journal of Membrane Science 474 (2015) 156–166

159

pressure was 2 bar for all gases (CO2, CH4 and N2), and the measurement was conducted 3 times for each membrane.

3. Results and discussion 3.1. Characterization of the poly(N-isopropylacrylamide) nanohydrogels The TEM image in Fig. 1 illustrated the as-synthesized nanohydrogel particles with a diameter of about 250 nm in their dried state. The chemical composition of NHs was characterized by FT-IR as shown in Fig. 2. The two characteristic peaks appeared at 1545 cm  1 (N–H bending) and 1647 cm  1 (C–O stretching) in FT-IR spectrum confirmed that NHs were successfully synthesized via precipitation polymerization. 3.2. Membrane characterization

Transmittance (%)

Fig. 1. TEM image of NHs.

1545 1647

4000

3600

3200

2800

2400

2000

Wavenumber

1600

1200

800

400

(cm-1)

Fig. 2. FT-IR spectrum of NHs.

The FT-IR spectra of all membranes are presented in Fig. 3. Compared with pure Matrimids membrane, two new absorption bands at 1647 cm  1 and 1545 cm  1 were observed for the composite membranes, suggesting that NHs were successfully incorporated into the polymer matrix. In composite membranes doped with NHs, the adsorbed water was detectable by O–H stretching bond which appeared as a broad at 3000–3600 cm  1. The characteristic absorbance bands appeared at 3050–3100 cm  1 were due to the C–H stretching vibrations of aromatic rings, while the ones observed at 2863 cm  1 and 2956 cm  1 were attributed to the C–H stretching vibrations of aliphatic rings. The band at 1778 cm  1 and 1683 cm  1 were assigned to C–O stretching bands of ketone groups and imidic groups, respectively. Bending vibrations of aliphatic C–H bonds presented at 1395 and 1426 cm  1. The bending vibrations of C–CO–C groups could be observed at 1297 cm  1. The peak of 1360 cm  1 was due to the stretching vibration of C–N in the imide group. The peak for Matrimids/NHs membrane at different loading displayed similar signals. Therefore, it could be interpreted that there was no chemical interaction between polymer matrix and NHs. The SEM micrographs in Fig. 4 show the dispersion of NHs within the membrane. Fig. 4(a)–(d) reveals that the NHs maintained their pristine structure and dispersed homogeneously in Matrimids matrix

2956 2863

Matrimid/PHMs-20

1778

1683

1545

1647

1426

1395

1297 1360

Transmittance (%)

Transmittance (%)

Matrimid/PHMs-15 Matrimid/PHMs-10

Matrimid/PHMs-5 Matrimid

4000

3500

3000

2000

Wavenumber Fig. 3. FT-IR spectra of (a) pure Matrimid (1800–1200 cm  m).

s

1500

1000

500

1800

1700

1600

(cm-1)

1500

Wavenumber s

membrane and Matrimid /NHs composite membranes (4000–400 cm

m

1400

1300

1200

(cm-1)

) and (b) the amplification of the low wavenumber zone

160

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Fig. 4. SEM images of the cross-section of (a) Matrimids/NHs-5, (b) Matrimids/NHs-10, (c) Matrimids/NHs-15, (d) Matrimids/NHs-20, (e) pure Matrimids and (f) Matrimids/NHs-M membranes.

2000

Matrimid

Storage modulus (MPa)

without obvious agglomeration. The whole cross-section of the composite membrane was shown in Fig. S1. Fig. 4(f) (Matrimids/ NHs-M) clearly exhibited the cross-section image of Matrimids/NHs20 composite membrane at high magnification. The image showed that NHs have favorable compatibility with Matrimids matrix, implying a good affinity between the filler and the polymer matrix. The macroscopic appearance of the pure Matrimids and Matrimids/NHs20 composite membranes exhibited transparent yellow and semitransparent yellow, respectively (Fig. S2). The storage modulus is an indication of the stiffness of the material. DMA was used to elucidate the effect of NHs on the mechanical properties of the composite membranes. The storage modulus of all membranes was plotted as a function of temperature in Fig. 5. The pure Matrimids membrane displayed a decreasing storage modulus with increasing temperature, with a transition from a glassy state to a rubbery state around 300– 340 1C. The storage modulus for Matrimids/NHs-20 composite membrane was slightly lower than that of pure Matrimids membrane. It was indicated that Matrimids/NHs membranes still maintained their superior mechanical properties after the incorporation of the abundant flexible NHs in Matrimids matrix. In addition, the increase of Young's modulus suggested the strong interfacial adhesion between fillers and polymer chains (Table S1).

Matrimid /NHs-5 1500

Matrimid /NHs-10 Matrimid /NHs-15 Matrimid /NHs-20

1000

500

0 0

50

100

150

200

250

300

350

400

Temperature (°C) Fig. 5. Storage modulus for pure Matrimids membrane and Matrimids/NHs composite membranes.

Although he thermal stability of the composite membranes displayed a slight decrease, overall composite membranes showed high thermal resistance (Fig. S3).

X. Li et al. / Journal of Membrane Science 474 (2015) 156–166

To evaluate the influence of fillers on the arrangement of polymer chains, the crystalline structure of the membranes was characterized by XRD (Fig. 6). The average inter-chain spacing (d-spacing) of each sample was calculated by Bragg's Law using the 2θ value of the most predominant peak. The pure Matrimids membrane showed a dspacing of 0.563 nm, which was in agreement with the literatures reported for Matirmids (0.580 nm) [46]. The d-spacing of composite membranes decreased with increasing NHs loading, and the minimum value of 0.412 nm was achieved for Matrimids/NHs-20 composite membrane. The close integration of NHs with the polymer chains led to this densification of the membrane matrix. The free volume characteristics of the membranes were determined by positron annihilation lifetime spectroscopy (PALS). The results are presented in Table 1. The network cavities (τ3) referred to the small interstitial cavities, and the aggregate cavities (τ4) referred to the free volume among the network clusters and at the interface between polymer matrix and filler. Table 1 displays that Matrimids membranes composed of two types of cavities having radii in the ranges of 0.05–0.13 nm from τ3, and 0.33–0.35 nm from τ4. The radius of a CO2 molecule was 0.33 nm, which was in the range of the aggregate cavities. The radii of a CH4 and N2 were 0.38 and 0.36 nm, respectively, which was larger than the aggregate cavities. The selectivity of composite membranes can be increased by a size-selective mechanism. The PALS data confirmed that the fractional free volume of Matrimids/NHs composite membranes increased from 0.79% to 0.98% as NHs content increased from 5 to 20 wt%. The increase of fractional free volume was beneficial to increase water uptake and facilitate permeation of gases.

3.3. The contact angle, water uptake and water state analysis Contact angle analysis is utilized to probe the hydrophilicity of the membrane surface and the results are listed in Table 2.

d=0.412

161

The contact angle of pure Matrimids membrane was 86.631. Compared to pure Matrimids membrane, the contact angle of the composite membranes decreased in the range of 84.92–73.571, suggesting an enhancement in surface hydrophilicity by the incorporation of nanohydrogels. The water uptake of NHs and all membranes at 25 1C are also presented in Table 2. The NHs exhibited high water uptake about 108.05% due to the presence of abundant hydrophilic amide groups. The water uptake of pure Matrimids membrane was 10.38%. Due to the incorporation NHs, the composite membranes showed an enhancement in water uptake compared with pure Matrimids membrane. Meanwhile, the water uptake of the composite membranes gradually increased from 16.67% to 27.00% as the NHs content increased from 5 to 20 wt%. The amounts of the free water and the bound water were calculated and summarized in Table 2. The NHs contained a larger amount of bound water (70.03%) than pure Matrimids membrane (3.40%). Obviously, the incorporation of the NHs would increase the amounts of free water and bound water in the composite membranes. With the NHs content increasing from 5 to 20 wt%, the amount of free water in composite membrane increased from 9.92% to 15.03%, and the amount of bound water increased from 6.75% to 12.97%. The high water content can increase the amount of dissolved CO2 in membranes. In addition, water itself can create additional passageways for gas transport since CO2 permeability in water is as high as 1983 Barrer [16]. The water retention capacity of all membranes was characterized by measuring its water uptake as a function of time, and the results are illustrated in Fig. 7. The water uptake of the membranes decreased sharply at first and then levelled off, indicating that the free water was evaporated at first and then the bound water. When the time was 180 min, the water uptake of pure Matrimids membrane declined to 1.5%, while the presence of the NHs reduced the water loss and the final water uptake of the composite membrane was increased to 4.0% with NHs content of 20 wt%. The above results suggested that the water retention capacity of the membrane was well enhanced by incorporating NHs into the composite membranes.

Intensity (counts)

3.4. Effect of water content in the membranes on gas separation performance d=0.537 d=0.546

Water vapor can permeate easily through most polymeric membranes. Water absorbed in polymer matrix can swell the

Matrimid /NHs-20

d=0.561 Matrimid d=0.563

/NHs-15

Table 2 The contact angle, water uptake, free water and bound water of the hydrated NHs, pure Matrimids membrane and Matrimids/NHs composite membranes.

Matrimid /NHs-10 Matrimid

/NHs-5

Sample

Contact angle (1)

Water uptake (%)

Free water (%)

Bound water (%)

NHs Matrimids Matrimids/NHs-5 Matrimids/NHs-10 Matrimids/NHs-15 Matrimids/NHs-20

– 86.63 84.92 81.29 76.93 73.57

108.05 10.38 16.67 19.76 23.02 27.00

38.02 7.98 9.92 10.28 12.83 15.03

70.03 3.40 6.75 9.48 10.19 12.97

Matrimid 5

10

15

20

25

30

35

40

45

50

55

60

2 theta (degrees) Fig. 6. XRD patterns of pure Matrimids membrane and Matrimids/NHs composite membranes.

Table 1 Free volume parameters of pure Matrimids membrane and Matrimids/NHs composite membranes. Membrane

I3 (%)

τ3 (ns)

I4 (%)

τ4 (ns)

r3 (nm)

r4 (nm)

F3

F4

Fv

Matrimids Matrimids/NHs-5 Matrimids/NHs-10 Matrimids/NHs-15 Matrimids/NHs-20

8.30 9.90 10.10 17.00 6.53

0.678 0.648 0.662 0.548 0.818

3.78 4.45 4.86 5.48 5.54

2.852 2.726 2.691 2.557 2.686

0.099 0.091 0.095 0.054 0.132

0.353 0.344 0.341 0.331 0.341

0.035 0.031 0.036 0.011 0.063

0.696 0.759 0.807 0.832 0.920

0.731 0.790 0.843 0.843 0.983

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polymer chains and form passageways for gas permeation. In pure Matrimids membrane, the CO2 permeability increased with increasing soaking time at humidified state due to the increase in swelling degree and fractional free volume of the membrane (Table S2). The effect of water content on gas separation performance was studied to further elucidate the role of water in membrane. Fig. 8 clearly reveals the correlations between water uptake and CO2 selective permeation. It can be seen that CO2 permeability and CO2/gas selectivity monotonically increased with increasing water uptake. The results corresponded to the positive effect of water on CO2 transport. On one hand, water swells the polymer chains, thus increasing the CO2 permeability. On the other hand, water constructs interconnected CO2 transport passageways, thus facilitating CO2 transport. Both bound water and free water could significantly increase the CO2 permeability. The membrane with large amount of free water often shows low selectivity, while bound water can maintain a preferable selectivity. It was found that CO2 permeability, CO2/CH4 30

Matrimid Matrimid /NHs-5

Water retention (%)

25

Matrimid /NHs-10 20

Matrimid /NHs-15 Matrimid /NHs-20

15

10

5

0 0

20

40

60

80

100

120

140

160

180

Time (min) Fig. 7. Water retention of pure Matrimids membrane and Matrimids/NHs composite membranes as a function of time at 40 1C and 20% RH.

and CO2/N2 selectivities were positively correlated with the bound water portion in these membranes (Fig. S4). High bound water resulted in relatively lower transport resistance of CO2 than that of CH4 and N2, because diffusion coefficient of CO2 in bound water was higher than that of CH4 and N2 in free water [47,48]. Furthermore, the selectivity was dominated by the difference in the gas solubility, and the solubility of CO2 was an order of magnitude higher than that of other gases (CH4 and N2) in water. Therefore, a higher water content led to a larger amount of dissolved CO2, and hence higher ideal CO2/CH4, CO2/N2 selectivities. 3.5. Membrane separation performance 3.5.1. Pure gas permeation performance Pure gas permeability and ideal selectivities of all membranes under dry state and humidified state are listed in Table 3. In order to further explain gas transport mechanism, the diffusion coefficients (D), the solubility coefficients (S) of CO2, CH4 and N2, and their respective solubility selectivity and diffusion selectivity are shown in Table 4. Table 4 shows that CO2 and N2 diffusion coefficients of composite membranes increased with increasing NHs loading, while the CH4 diffusion coefficient exhibited an opposite trend. The introduction of the flexible filler phase will lead to more flexible polymer chains, thus resulting in a high gas diffusivity. Since the critical diameter increases in the order of CO2 (0.33 nm)oN2 (0.36 nm) oCH4 (0.38 nm), the decrease of CH4 diffusivity was due to the molecular sieving effect. Compared to the separation performance of all dry membranes, both the CO2 permeability and the selectivities of all humidified membranes were significantly enhanced as shown in Table 3. For pure Matrimids membrane, CO2 permeability increased from 8.8 Barrer at dry state to 67 Barrer at humidified state, increasing by 661%. Water played a crucial role for gas transport in the humidified Matrimids membrane. Firstly, water can swell and plasticize Matrimids matrix, enhancing the inter-segmental motion of polymer chains and increasing gas diffusivity. Secondly, water molecules have a tendency to form clusters. These water clusters will more effective in swelling the polymeric matrix, intensifying swelling that increased gas diffusivity. Thirdly, water

300

70 65

250

200

Ideal selectivity

P(CO2) (Barrer)

60

150

100

CO2/CH4

55

CO2/N2

50 45 40 35

50 8

10

12

14

16

18

20

22

Water uptake (%)

24

26

28

30

30 8

10

12

14

16

18

20

22

24

26

28

Water uptake (%)

Fig. 8. Correlations between (a) water uptake and pure CO2 permeability and (b) water uptake and ideal CO2/CH4, CO2/N2 selectivities of the membranes.

30

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163

Table 3 Pure gas permeability and ideal selectivity of the dry membranes and humidified membranes. Sample

Dry membrane

Matrimids Matrimids/NHs-5 Matrimids/NHs-10 Matrimids/NHs-15 Matrimids/NHs-20

Humidified membrane

P CO2

P CH4

P N2

αCO2 =CH4

αCO2 =N2

P CO2

P CH4

8.84 70.40 10.26 70.51 12.28 70.49 17.58 70.65 18.46 70.53

0.26 70.01 0.37 70.02 0.37 70.01 0.29 70.01 0.27 70.01

0.27 70.01 0.39 70.02 0.37 70.01 0.3570.01 0.31 70.01

33.987 1.70 27.54 7 1.38 33.327 1.67 61.047 1.90 69.177 1.71

33.07 71.64 26.42 71.33 32.83 71.62 50.81 71.82 59.30 71.58

677 2.1 1367 6.0 1847 9.4 2247 11.3 2787 13.6

1.39 70.07 2.61 70.13 3.41 70.17 3.93 70.19 4.56 70.22

P N2

αCO2 =CH4

αCO2 =N2

1.677 0.08 3.167 0.15 4.007 0.20 4.577 0.22 5.34 7 0.27

48 71.8 52 72.6 54 72.7 57 72.9 61 73.1

407 2.0 437 2.1 467 2.3 497 2.5 527 2.6

Table 4 Gas diffusivity and solubility coefficients of the dry membranes. (all membranes were tested at 2 bar, 30 1C). D (  10  8 cm2/s)

Membrane

s

CO2

CH4

N2

CO2

CH4

N2

2.85 3.14 3.31 4.59 4.64

1.04 1.01 0.97 0.90 0.89

1.67 1.69 1.70 1.73 1.73

3.10 3.27 3.71 3.83 3.98

0.25 0.37 0.38 0.32 0.30

0.16 0.23 0.22 0.20 0.18

400

70

400

350

60

350

250 40 200 30 150 20

100 50 0 0

5

10

15

SCO2 =SCH4

SCO2 =SN2

2.74 3.11 3.41 5.10 5.21

1.71 1.85 1.95 2.65 2.68

12.40 8.84 9.76 11.97 13.27

19.37 14.22 16.86 19.15 21.11

60

300 40 250 200

30

150 20 100 10

10

50

0

0

20

0 0

5

10

15

20

Content of filler (wt%)

Content of filler (wt%) Fig. 9. Effect of filler content on gas separation performance of pure Matrimid and (b) CO2/N2 separation performance.

DCO2 =DN2

CO2/N2 selectivity

50

DCO2 =DCH4

50

P(CO2) (Barrer)

300

CO2//CH4 selectivity

P(CO2) (Barrer)

Matrimid Matrimids/NHs-5 Matrimids/NHs-10 Matrimids/NHs-15 Matrimids/NHs-20

S (  10  2 cm3(STP)/cm3 cmHg)

s

s

membrane and Matrimid /NHs composite membranes: (a) CO2/CH4 separation performance

itself can create additional passageways for gas transport. In summary, the positive effects of water outweighs the negative effect of water and results in an increase in gas permeability. The results were consistent with the reported references [49,50]. For composite membranes, CO2 permeability and selectivities continually increased with increasing NHs loading in polymer matrix under humidified state. Compared to pure Matrimids membrane, the CO2 permeability, CO2/CH4 selectivity and CO2/N2 selectivity of Matrimids/NHs-20 membrane increased by 315%, 27%, 30%, respectively. The introduction of the highly hydrophilic nanohydrogels increased the water content of the composite membranes, which increased the amount of dissolved CO2, and simultaneously constructed interconnected CO2 transport passageways in the membranes, increasing CO2 permeability and selectivity. The increases in permeability and selectivity were attributed to the increased water content in the membrane as described in Section 3.4.

3.5.2. Mixed-gas permeation performance Fig. 9 shows the mixed-gas separation performance of all membranes. The mixed-gas selectivity was lower than the corresponding ideal selectivity for pure Matrimids membrane. In contrast, the Matrimids/NHs composite membranes exhibited similar real selectivities with their ideal values, indicating the absence of competitive adsorption among CO2 and CH4 (N2) gas molecules in composite membranes. It is well known that solubility of CO2 is an order of magnitude higher than that of other gases (CH4 and N2) in water. Since Matrimids/NHs composite membranes held more water and the CO2 transport passageways were multiplied, the competitive adsorption caused by CH4 or N2 was not obvious.

3.5.3. Effect of feed gas pressure on gas separation performance Fig. 10 shows the mixed-gas separation performance as a function of feed gas pressure. As the feed gas pressure

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X. Li et al. / Journal of Membrane Science 474 (2015) 156–166

500

500

160

450

Selectivity Matrimid

140

400

Selectivity Matrimid /NHs-20

120

P(CO2) (Barrer)

300

100

250 60

80

50 60

40 30

40

20

CO2//CH4 selectivity

350

2

4

6

8

100

Selectivity Matrimid /NHs-20

300

80 250 60

60

50 40

40

20

20

10

0

0

Selectivity Matrimid

30

20

10

120

PCO Matrimid /NHs-20

350

P(CO2) (Barrer)

PCO Matrimid /NHs-20 400

140

PCO Matrimid

CO2/N2 selectivity

180

PCO Matrimid 450

0

10

2

4

Feed pressure (bar)

6

8

10

0

Feed pressure (bar)

Fig. 10. Effect of feed gas pressure on gas separation performance of pure Matrimids membrane and Matrimids/NHs composite membranes: (a) CO2/CH4 separation performance and (b) CO2/N2 separation performance.

PCO2 Matrimid

PCO2 Matrimid

500

400

Selectivity Matrimid

350

Selectivity Matrimid

100

80

300 60 250 200

40

150 100

20

P(CO2) (Barrer)

PCO2 Matrimid /NHs-20

450

PCO2 Matrimid /NHs-20

400

Selectivity Matrimid

350

Selectivity Matrimid /NHs-20

80

60

300 250 40

200 150

20

100

CO2/N2 selectivity

450

CO2//CH4 selectivity

P(CO2) (Barrer)

500

100

550

120

550

50

50 0 20

30

40

50

60

70

0 20

0 80

30

40

50

60

70

0 80

o

o

Temperature ( C)

Temperature ( C)

Fig. 11. Effect of operating temperature on gas separation performance of pure Matrimids membrane and Matrimids/NHs composite membranes: (a) CO2/CH4 separation performance and (b) CO2/N2 separation performance.

1000

1000 Humidified Matrimid /NHs

Humidified Matrimid /NHs

Humidified Matrimid

Humidified Matrimid

Dry Matrimid

Dry Matrimid

CO2/N2 selectivity

CO2/CH4 selectivity

Robeson upper bound 2008

Dry Matrimid /NHs

100

10 Robeson upper bound 1991

1

Dry Matrimid /NHs

100

Robeson upper bound 2008

10

1

1

10

100

1000

10000

CO2 permeability (Barrer)

1

10

100

1000

10000

CO2 permeability (Barrer)

Fig. 12. Robeson plots for: (a) CO2/CH4 separation and (b) CO2/N2 separation.

increased from 2 bar to 10 bar, the decrement degree of permeability was smaller for Matrimids/NHs-20 composite membrane than that for pure Matrimids membrane. This less reduction

for composite membranes was derived from the interconnected CO2 transport passageways constructed in the composite membranes.

X. Li et al. / Journal of Membrane Science 474 (2015) 156–166

3.5.4. Effect of operating temperature on gas separation performance Operating temperature is a crucial parameter that determines the applicability of membrane. As shown in Fig. 11, CO2 permeability gradually increased with increasing temperature, which was attributed to the increase of gas diffusivity and polymer chain flexibility. The CO2/CH4 and CO2/N2 selectivities decreased with increasing temperature, but the relative decrease in selectivities of Matrimids/NHs-20 composite membrane was less than that of pure Matrimids membrane. The incorporation of NHs into Matrimids matrix increased the amount of dissolved CO2, thus reducing the decrease in selectivity.

3.6. Comparison of results to Robeson's upper bound curve Fig. 12 shows a comparison of gas transport properties against Robeson's upper bound [51,52]. The gas separation performance of the humidified composite membranes surpassed or was close to the 2008 Robeson upper bound line, while those of dry membranes fell far below. Both the permeability and the selectivity were significantly enhanced for Matrimids/NHs composite membranes, confirming the benefits of the nanohydrogel-containing membranes in enhancing the CO2 separation performance.

4. Conclusions Poly(N-isopropylacrylamide) nanohydrogels with a diameter of 250 nm were synthesized and then incorporated into Matrimids matrix to fabricate the composite membranes for the potential application of CO2 separation. The influence of nanohydrogels on the structures and performances of the Matrimids membranes was systematically explored. The favorable compatibility between the nanohydrogels and the Matrimids matrix allowed a homogeneous dispersion of the nanohydrogels within the polymer matrix. The introduction of nanohydrogels increased the water content of the composite membranes, which increased the amount of dissolved CO2, and simultaneously constructed interconnected CO2 transport passageways in the membranes, facilitating the transport of CO2. The CO2 separation performance of the composite membranes was remarkably improved with increasing NHs loading. Compared to pure Matrimids membrane, the CO2 permeability of Matrimids/NHs-20 composite membrane increased by  315%. The performance of the as-prepared composite membranes surpassed or was close to the 2008 Robeson upper bound line.

Acknowledgments The authors gratefully acknowledge the support from the National High Technology Research and Development Program of China (2012AA03A611), Program for New Century Excellent Talents in University (NCET-10-0623), the National Science Fund for Distinguished Young Scholars (21125627), and the Programme of Introducing Talents of Discipline to Universities (No. B06006).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2014.10. 003.

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