Improvement in the permeation performance of hybrid membranes by the incorporation of functional multi-walled carbon nanotubes

Journal of Membrane Science 466 (2014) 338–347

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

Improvement in the permeation performance of hybrid membranes by the incorporation of functional multi-walled carbon nanotubes Ting Wang a, Jiang-nan Shen b, Li-guang Wu a,n, Bart Van der Bruggen c a

School of Environment Science & Engineering, Zhejiang Gongshang University, Hangzhou 310012, China Center for Membrane and Water Science, Ocean College, Zhejiang University of Technology, Hangzhou 310014, China c Department of Chemical Engineering, Process Engineering for Sustainable Systems (ProcESS), KU Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium b

art ic l e i nf o

a b s t r a c t

Article history: Received 29 November 2013 Received in revised form 30 March 2014 Accepted 28 April 2014 Available online 14 May 2014

The morphology and swelling performance of poly(methyl methacrylate) (PMMA) and polyurethane (PU) membranes containing pristine multi-walled carbon nanotubes (P-MWCNTs) was investigated. Next, amino (NH2) groups were introduced into the MWCNTs by chemical modification for improving their affinity to the membranes, thereby resulting in a better distribution of the MWCNTs in the membranes. The performance of the hybrid membranes was evaluated by swelling and pervaporation experiments. The Fourier transform infrared and transmission electron microscopy analyses showed that the addition of both the pristine and functionalized MWCNTs to the membranes improved their permeation performance. The dispersion of the P-MWCNTs and MWCNTs-NH2 in the PU hybrid membranes was better than that in the corresponding PMMA hybrid membranes because of the rapid polymerization rate of the monomers and the elasticity of the membranes. Thus, the P-MWCNTs/PU and MWCNTs-NH2/PU hybrid membranes performed better than the corresponding P-MWCNTs/PMMA and MWCNTS-NH2/PMMA hybrid membranes. The surface functionalization promoted the affinity of the MWCNTs to the monomer solution and the polymer membranes. Compared to the P-MWCNTs, the MWCNTs-NH2 showed an improved distribution in the PU and PMMA hybrid membranes, and the membranes containing the MWCNTs-NH2 performed better than those containing the P-MWCNTs. & 2014 Elsevier B.V. All rights reserved.

Keywords: Hybrid membranes Multi-walled carbon nanotubes Surface modification Swelling absorption Pervaporation

1. Introduction Because of their low power consumption, high separation performance and stability, membrane separation processes have a great potential for the environmental applications and energy production [1–4]. Membrane technology is a rapidly developing separation process in diverse fields such as chemical and environmental engineering, production of pharmaceuticals, and process intensification. However, there is a lot of potential in this technology that still needs to be explored or optimized. The development of new membrane materials is the key factor that enhances the application of the separation technology [5]. The separation properties of common organic polymer and inorganic membrane materials are difficult to determine because of the “trade-off” phenomenon [6,7]. In recent years, the development of nanomaterials and their use in membranes has provided a novel way to solve this problem. The preparation of organic–inorganic hybrid membranes containing inorganic nanomaterials can effectively overcome the trade-off phenomenon and significantly improve

n

Corresponding author. Tel.: þ 86 571 88071024 7017; fax: þ 86 571 88865762. E-mail address: [email protected] (L.-g. Wu).

http://dx.doi.org/10.1016/j.memsci.2014.04.054 0376-7388/& 2014 Elsevier B.V. All rights reserved.

the membrane performance [8,9]. For example, the hybrid polymer membranes containing AgCl nanoparticles did not only increase the separation performance by combining the Ag þ ions and the double bonds in the aromatic or olefinic molecules, but also decreased the effect of the Ag þ ions on the polymer–silver salt complex membranes [10,11]. In previous studies [12–14], hybrid membranes containing AgCl nanoparticles were prepared via in situ microemulsion polymerization. These hybrid membranes showed a high separation performance for benzene/cyclohexane mixtures. Hybrid membranes containing other inorganic particles, such as TiO2 [15] and SiO2 [16], have also been prepared and are commonly used for the CO2 separation and wastewater treatment processes. However, these membranes are mainly used to separate only certain types of substances, such as aromatic hydrocarbons or CO2 gas, indicating that they have a narrow application range. Therefore, the development of novel hybrid membranes with wider applications is an important research area in membrane technology. Among the many new inorganic nanomaterials suggested, carbon nanotube (CNT) is the most promising material because of its unique one-dimensional tubular structure and hydrophobic properties [17–20]. Since their discovery in 1991, CNTs, including single-walled and multi-walled CNTs (MWCNTs), have attracted significant research

T. Wang et al. / Journal of Membrane Science 466 (2014) 338–347

attention because of their unique structure and outstanding physical, electronic, and thermal properties [21]. MWCNTs are of particular interest because of their relatively low cost and more advanced stage in commercial production. Recently, the addition of CNTs into the intermediate and final polymer products, as the CNT/polymer composites, has been studied. These composites significantly increase the separation performance of the hybrid membranes. Hinds et al. [18] reported that the aligned MWCNT membranes showed potential for application in chemical separation and sensing. Peng et al. [19] reported the preparation of novel nanocomposite membranes composed of polyvinyl alcohol (PVA) and chitosan-wrapped CNTs. Compared to pure PVA membranes, these PVA–CNT membranes showed both higher permeation flux and separation factor in the pervaporation (PV) for the separation of benzene/cyclohexane mixtures. Qiu et al. [20] reported the preparation of functionalized MWCNTs incorporated in a chitosan membrane for the separation of ethanol/water mixtures by PV. The CNT-filled membranes were easier to penetrate and exhibited a higher flux performance than the pristine membranes. Similar to the preparation and application of most nanocomposite membranes, the dispersion and alignment of CNTs is crucial in improving the quality and properties of the resulting hybrid membranes. However, it is difficult to achieve a homogeneous distribution of CNTs in the polymer matrices because of the chemical inertness of CNTs and their tendency to form bundles, which significantly compromises the membrane performance. This challenge must be overcome in the preparation of CNT/polymer membranes. In particular, it is important to ensure an adequate interfacial adhesion between the CNTs and polymers to facilitate the uniform distribution of CNTs in the polymers and avoid agglomeration. To date, the improvements in the distribution of CNTs in membranes have only focused on the CNT surface modification. The properties of the monomer and polymer are also key factors that affect the CNT distribution in a membrane. The characteristics of the dispersion of the same type of CNTs in different monomer and polymer materials are expected to vary; however, this topic has been rarely studied. In this study, polymethyl methacrylate (PMMA) and polyurethane (PU) membranes were used as the substrates for the preparation of the corresponding hybrid membranes containing different MWCNTs. This study aims to investigate the effect of different polymer materials on the distribution of pristine and functionalized MWCNTs in the corresponding hybrid membranes.

2. Experimental 2.1. Materials Pristine MWCNTs with diameters ranging from 20 to 40 nm, inner diameter of 5 nm, and 95% purity were manufactured by Nanotech Port Co., Ltd. (Shenzhen, China). H2SO4 and HNO3 acids were purchased from Shanghai Reagent Factory (Shanghai, China). 2-Azobisisobutyronitrile (AIBN), methyl methacrylate (MMA), N,N-dimethylformamide (DMF), phenyl isocyanate (MDI), 1,4-butanediol (BDO), and dibutyltin dilaurate (DBTDL) were purchased from Reagent Chemical Manufacturing (Shanghai, China) and used as received without further purification. The polysulfone film used in this manuscript with a molecular weight cutoff of 20,000 was obtained from Hangzhou Development Center of Water Treatment Technology (Hangzhou, China). 2.2. Functionalization of MWCNTS Carboxylic MWCNTs (MWCNTS-COOH) were prepared according to the method reported elsewhere [19]. First, the pristine MWCNTs

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were treated with a mixed acid solution (H2SO4/HNO3 ¼3/1) at 80 1C for 6 h and then diluted with deionized water. The minor nanotube residues and impurities were removed via a reduced pressure distillation, and the bulk MWCNTs were washed with deionized water repeatedly to remove the traces of the acid completely. The MWCNTs were dried in a vacuum drying oven at 70 1C prior to further treatment. The oxidized MWCNTs were dispersed in DMF and then refluxed in diisobutyryl peroxide, which was synthesized from butanedioic anhydride and oxydol placed in an ice bath for several hours, at 80 1C for 10 d. The resulting mixture was washed with an excess of DMF, and the MWCNTs-COOH was obtained after the reduced pressure distillation and drying in an oven. The MWCNTs-COOH were treated with 4,4-diaminodiphenylmethane dissolved in a mixture of pyridine and acetone at 140 1C for 48 h, diluted with deionized water, and then dried in a vacuum drying oven at 70 1C to afford the aminated MWCNTs (MWCNTsNH2).

2.3. Preparation of hybrid membranes containing MWCNTs 2.3.1. Preparation of MWCNTs-NH2/PMMA Certain amounts of different MWCNTs were added to 30 mL of MMA. The mixed solution was stirred for  24 h and then placed in an ultrasonic bath at a fixed frequency for  30 min in order to improve the MWCNT dispersion in the MMA and achieve a homogeneous mixture. Next, AIBN was added to this homogeneous mixture while stirring from 60 to 65 1C to initiate the polymerization reaction. When the viscosity of the reaction system reached  300 mPa s, the reaction mixture was used to coat a polysulfone film (molecular weight cut-off (MWCO)¼ 20,000). The hybrid membranes with different MWCNTs on the polysulfone film were incubated and obtained by continuing the polymerization reaction in a vacuum oven at 60–65 1C for another 12 h to allow the evaporation of the solvent.

2.3.2. Preparation of MWCNTS-NH2/PU Different MWCNTs were dispersed in 15 mL DMF using an ultrasonic probe (VC750, 150 W, 20 Hz) for 5 min and then stirred in a water bath at 45 1C for 10 min. MDI (5.75 g) was dissolved in 15 mL DMF using an ultrasonic bath (70 W, 42 Hz) and then added to the DMF containing the MWCNTs under stirring at 45 1C. After 30 min of constant stirring, 4.7 mL BDO and 50 μL DBTDL were sequentially added to the mixture while stirring vigorously at 45 1C to initiate the polymerization reaction. When the viscosity of the reaction system reached about 300 mPa s, the reaction mixture was used to coat a polysulfone film (MWCO¼20,000). The hybrid membranes with different MWCNTs on the polysulfone film were incubated and obtained by continuing the polymerization reaction in a vacuum oven at 45 1C for another 10 h to allow the evaporation of the solvent.

2.4. Swelling and sorption measurements The pre-weighed membranes were immersed in benzene or cyclohexane in a closed bottle at room temperature for 448 h for achieving the swelling equilibrium state after dehydration in a desiccator. The membranes were periodically weighed until a constant mass was obtained. The membrane sample was then taken out from the liquid bath; the surface solution was wiped off carefully with a tissue paper; and then weighed in a tightly closed bottle. The amount of sorbed liquid in the membranes was expressed as the degree of swelling (DS), which was calculated

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equation:

by the following equation: mt  m0 DSð%Þ ¼  100 m0

ð1Þ

where m0 and mt are the weight of the dry and solvent-swollen membranes, respectively. The membrane sample was periodically taken out, wiped clean of the surface solution, and weighed until the mass became constant. The amount of absorbed benzene (or cyclohexane) in the membranes was expressed as the equilibrium swelling–sorption degree (A1;b or A1;c ). The sorption selectivity of benzene to cyclohexane (as, b/c) of the hybrid membranes was calculated using the following equation [22]:

αs;b=c ¼

A1;b A1;c

ð2Þ

First, the relationship between the amount of sorption and the sorption time of benzene and cyclohexane in the hybrid membranes containing different MWCNTs was explored, following a method published elsewhere [20] (Supporting Information, Figs. S1–S8). The diffusion coefficient D of benzene and cyclohexane in the different hybrid membranes was obtained by using the standard equation for Fickian diffusion according to the slopes of the lines in Figs. S1–S8.

βðA=BÞ ¼

Y A =Y B ; X A =X B

ð3Þ

where β is the separation factor; X and Y are the molar concentrations of the components of the feed and permeate, respectively; A and B denote the components to be separated. 2.6. Gas permeance measurement The gas permeation properties of the hybrid membranes were determined using the variable-pressure constant-volume method with a pre-calibrated permeation cell as described elsewhere. The measurements were performed at 35 1C with the pressures of up to 10 bar, and each membrane sample was measured three times. A mixture of CO2/N2 (volume ratio, 1:9) was used as the test gas. 2.7. Characterization The morphologies of the MWCNTs and MWCNT-containing membranes were characterized using a JEM-1230 transmission electron microscopy (TEM) system (Jeol Co., Ltd.). The MWCNT structure was characterized by Fourier transform infrared (FTIR) spectroscopy (Nexus-670, Nicolet Co.). During the polymerization, the viscosity of the reaction mixture was determined using a viscometer (LVDV-I Prime, Brookfield).

2.5. Pervaporation experiments of benzene/cyclohexane mixtures The membranes with an area of 19.6 cm2 were placed in a stainless-steel permeation cell in contact with the feed. The temperature of the cell was thermostatically controlled. The vacuum in the lower region of the cell was maintained at about 160 Pa using a vacuum pump. The experiments with the benzene/ cyclohexane mixture were performed at a constant temperature of 30 1C (feed composition: Xbenzene ¼50 wt%). The PV vapor emitted was condensed using liquid nitrogen. After running the PV apparatus for 2 h, the composition of the permeation liquid was analyzed using a gas chromatograph instrument (GC-950; Shanghai Haixin Chromatography Instruments Co., Ltd., China) equipped with a thermal conductivity detector. The separation performance of the membrane was characterized by two parameters in this study: the flux and separation factor, according to the literature [23]. The separation performance of the membrane was characterized by the separation factor. Flux is defined as the rate of permeation of a feed component through a membrane unit area per unit time. The two most commonly used PV process flux units are kg m  2 h  1 and g cm  2 s  1. The separation factor β for all the membranes was calculated according to the following 450

350

The polymerization of the monomer increased with the increase in the viscosity of the reaction mixture. The effect of MWCNTs on the polymerization was explored by measuring the change in the viscosity during the monomer polymerization. As shown in Fig. 1, the changes in the viscosity show that the addition of the P-MWCNTs inhibits the monomer polymerization during the formation of the PMMA or PU hybrid membranes. This is because of the formation of the P-MWCNT aggregates in the monomer solution. P-MWCNTs are difficult to disperse well in the monomer solution because of their poor wettability; instead, the particles aggregate to inhibit the polymerization of the monomer. The inhibition of polymerization increases with increasing P-MWCNT content in the monomer solution. Fig. 1 shows that the formation of the PMMA and PU hybrid membranes was affected by the addition of the P-MWCNTs in different ways. The polymerization of MDI and BDO occurred 400

0 wt% 0.2 wt% 0.5 wt% 1.0 wt% 1.5 wt%

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300

Viscosity(mPa.s)

Viscosity(mPa.s)

3.1. Effect of P-MWCNTs on polymerization

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3. Results and discussion

0

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Time (min)

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Time (min)

Fig. 1. Effect of the content of p-MWCNTS on the polymerization, (a) formation of PMMA; (b) formation of PU.

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Fig. 2. TEM micrographs of P-MWCNT incorporated PMMA and PU membranes, (A) P-MWCNTs/PMMA; (B) P-MWCNTs/PU, (The MWCNT loading in the membrane matrix was 0.5 wt%.).

Figs. 3 and 4 show the addition of P-MWCNTs in PMMA and PU membranes. P-MWCNTs could increase the DS of the hybrid membranes in benzene. This is because of the π complexation between the benzene molecules and the MWCNTs in the membranes. The addition of the P-MWCNTs also slightly affected the DS of the membranes in cyclohexane. Thus, the sorption selectivity of both the hybrid membranes increased after the addition of the P-MWCNTs. Figs. 3 and 4 show that the DS of the PU hybrid membranes in benzene are close to that of the PMMA membranes with the same added quantity of P-MWCNTs. In contrast, the DS of the PU hybrid membranes was significantly lower than that of the PMMA hybrid membranes in cyclohexane because of the stronger polarity of PU. Therefore, the sorption selectivity of the PU hybrid

3.0

60.0

DS at equilibrium (%)

50.0 2.0 40.0 1.5 30.0 1.0 20.0

10.0

Sorption Selectivy, α S,b/c

2.5

0.5

Benzene Cyclohexane

0.0 0.0

Sorption selectivity

0.5

1.0

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Mass percentage of MWCNT (%)

Fig. 3. Variation in degree of swelling with different contents of P-MWCNTs incorporated in PMMA membranes. 6.0

70.0

60.0

5.0

50.0 4.0 40.0 3.0 30.0 2.0 20.0

10.0

Sorption Selectivy, α S,b/c

3.2. Effect of P-MWCNTs on the swelling performance of hybrid membranes

70.0

DS at equilibrium (%)

rapidly, and the viscosity of the reaction system reached 300 mPa s after the polymerization for 10 min. In this system, the P-MWCNTs were dispersed in the monomer solution for a shorter period and fewer aggregates were formed. Therefore, the inhibition of the polymerization process was weaker. Fig. 1b shows the inhibition of MDI and BDO polymerization when the added quantity of the P-MWCNTs reaches 1.0 wt% and beyond. In contrast, the viscosity of the MMA reaction system reached 300 mPa s only after the polymerization for 4 150 min, even when no P-MWCNTs was added to the system. Thus, the P-MWCNTs were dispersed in the MMA solution for a longer period, and more aggregates of PMWCNTs that inhibit the polymerization of the monomer were formed. The MMA polymerization became severely inhibited after the addition of 4 0.5 wt% the P-MWCNTs. Fig. 2 shows the TEM images of the hybrid membranes. The distribution of the P-MWCNTs in the PU hybrid membranes was significantly better than that in the corresponding PMMA hybrid membranes after the addition of 0.5 wt% the P-MWCNTs to the monomer solutions. The P-MWCNT aggregation in the PMMA and PU hybrid membranes occurred significantly and minimally, respectively, thus confirming the findings in Fig. 2. The variations in the polymerization rates of the monomers also determined the degree of different distributions of the P-MWCNTs in the two hybrid membranes. In addition, PU is an elastic material, and this property may promote the dispersion of the P-MWCNTs in PU. In contrast, PMMA is a glassy material; therefore, it may be more difficult for the P-MWCNTs to distribute homogeneously in the PMMA polymer matrices.

1.0

Benzene Cyclohexane

0.0 0.0

Sorption Selectivity

0.5

1.0

1.5

2.0

0.0

Mass percentage of MWCNT (%)

Fig. 4. Variation in degree of swelling with different contents of P-MWCNTs incorporated in PU membranes.

membranes is much higher than that of the corresponding PMMA hybrid membranes. The DS curves further indicate that the sorption selectivity of the PU and PMMA hybrid membranes initially increased but eventually decreased with increasing content of P-MWCNTs.

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The decrease in the sorption selectivity is because of the aggregation of the P-MWCNTs in the hybrid membranes. Similar to the results in the literature and our previous work [13,19,22], the separation performance would depress due to aggregation of nanomaterials formed in the hybrid membranes. The results in Section 3.1 showed a considerable agglomeration of the PMWCNTs in the PMMA membranes, when 0.5 wt% P-MWCNT was added. The aggregation of P-MWCNTs would first weaken the π complexation between the benzene molecules and the MWCNTs in the membranes. In addition, the formation of PMWCNTs aggregations caused some obvious regions without nanotubes in the membrane and these regions became more with increase of P-MWCNTs aggregations (as shown in Supporting Information, Fig. 9S). These regions decreased the DS of the hybrid membrane in benzene. The two negative effects caused by the PMWCNTs aggregation both decreased the sorption selectivity. In addition, relatively few MWCNT aggregates were formed in the PU membranes containing 0.5 wt% P-MWCNTs. Thus, the sorption selectivity of the PU hybrid membranes continued to increase with the continued addition of the CNTs. A significant effect of the aggregation was observed in the PU and PMMA hybrid membranes at high P-MWCNT contents, and the separation performance of the membranes decreased. The diffusion coefficients of benzene (Db) and cyclohexane (Dc) in the hybrid membranes are shown in Figs. 5 and 6. Both the Db and Dc values in the PMMA or PU membranes increased after the addition of P-MWCNTs. This is because of the fast molecular transportation in the cavity of MWCNTs. Thus, both the Db and Dc values in the PU and PMMA hybrid membranes increased with increasing addition of P-MWCNTs. The comparison of the changes in the Db and Dc values in both the figures indicate that the increase in the benzene transport in the membranes due to the addition of P-MWCNTs was more significant than that in the cyclohexane transport. This is because of the π complexation between the benzene molecules and MWCNTs in the membranes. The increase in the Db value in the hybrid membrane was more significant after 0.2 wt% P-MWCNTs was added. However, the formation of MWCNT aggregates in the hybrid membranes slightly restricted the transportation of benzene molecules. Thus, the increase in the Db values in the hybrid membranes became slow when 40.2 wt% P-MWCNT was added. The Db value of the PU hybrid membranes was much higher than that of the PMMA hybrid membranes with the same added quantity of the CNTs. This difference may be attributed to the 0.7

0.7 Benzene Cyclohexane

0.6

Diffusion coefficient (×107m2·s-1)

342

0.5

0.4

0.3

0.2

0.1

0

0

0.2

0.5

1

1.5

2

Mass percentage of MWCNT (%)

Fig. 6. Diffusion coefficient with different contents of P-MWCNTs incorporated PU membranes in benzene and cyclohexane.

different properties of PU and PMMA membranes. The elasticity property of PU is beneficial to the molecular transport, whereas the glassy property of PMMA hinders the molecular transport. The improved distribution of the P-MWCNTs in the PU hybrid membranes also favors the benzene and cyclohexane transports in the membranes. The stronger polarity of PU hinders the cyclohexane transport in the membranes, even though the elasticity of PU improved the molecular transport. As such, the Dc value of the PU hybrid membrane was close to that of the PMMA hybrid membranes. The addition of P-MWCNTs could significantly increase the separation performance of the hybrid membranes. However, the poor surface wettability of the P-MWCNTs formed the P-MWCNT aggregates in the monomer solution or the polymer material, thereby inhibiting the membrane performance by the addition of the CNTs. Therefore, the MWCNT surface was chemically modified in order to introduce polar groups to their surface. These experiments aimed to improve the surface wettability of the P-MWCNTs and MWCNTs dispersion in the monomer solution and the polymer materials. 3.3. The distribution of functionalized MWCNTs in the monomer solution

Benzene Cyclohexane

Diffusion coefficient (× 107 m2·s -1)

0.6

The distribution of MWCNTs-NH2 and P-MWCNTs in MMA or MDI was characterized by TEM analysis, as shown in Fig. 7. The two monomers (MMA and MDI) are both polar organic compounds. Thus, the modification of MWCNT with polar groups could improve the dispersion of MWCNT in two monomers. The TEM micrographs clearly show that the distribution of the MWCNTsNH2 in two monomers is significantly better than that of P-MWCNTs, indicating that the surface wettability of the MWCNTs was improved by the chemical modification with the polar groups. Moreover, the distribution of the MWCNTs-NH2 in MDI is slightly better than those distributed in MMA. The isocyanate groups of MDI might have benefited the dispersion of MWCNTs-NH2.

0.5

0.4

0.3

0.2

0.1

0

3.4. Effect of functionalized MWCNTs on polymerization 0

0.2

0.5

1

1.5

2

Mass percentage of MWCNT (%)

Fig. 5. Diffusion coefficient with different contents of P-MWCNTs incorporated PMMA membranes in benzene and cyclohexane.

Fig. 8a shows that the addition of the MWCNTs-NH2 accelerates the MMA polymerization when the added quantity is below 1.5 wt%. The viscosity of the reaction system reached 300 mPa s within

T. Wang et al. / Journal of Membrane Science 466 (2014) 338–347

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Fig. 7. TEM micrographs of MWCNTs in the monomer solution, (the MWCNT loading in the membrane matrix was 1.0 wt%.), (a) P-MWCNTs in MMA; (b) P-MWCNTs in MDI; (c) MWCNTs-NH2 in MMA; (d) MWCNTs-NH2 in MDI.

450

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Viscosity(mPa.s)

350 300

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400

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Time (min)

0

0

2

4

6

8

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12

Time (min)

Fig. 8. Effect of the content of MWCNTs-NH2 on the polymerization, a. Formation of PMMA; b. Formation of PU.

100 min after the addition of 0.2 wt% MWCNTs-NH2. This is because of the well dispersion of the MWCNTs-NH2 in the monomer solution. Owing to the rapid polymerization rate, the acceleration of the polymerization of MDI and BDO by the addition of MWCNTs-NH2 was also not significant, as shown in Fig. 8b. However, the addition of excess MWCNTs-NH2 (41.5 wt%) forms the aggregates of the MWCNTs, restricting the polymerization.

monomers and the elasticity of PU, the bonding between the amino groups on the MWCNTs-NH2 surface and the isocyanate groups of the MDI appeared to affect the dispersion of the CNTs. The bonding between amino (NH2) and isocyanate (NCO) groups formed stable covalent bonds (urea, NHCONH) between the MWCNTs-NH2 and PU. Therefore, the MWCNTs-NH2 remained well distributed in the PU hybrid membranes even at 1.0 wt% added MWCNTs-NH2.

3.5. Morphology of hybrid membranes containing functionalized MWCNTs

3.6. Characterization of the functionalized MWCNTs

The TEM micrographs also show the improvements on the dispersion of the modified MWCNTs in the hybrid PU or PMMA membranes. After the surface functionalization, the MWCNTs-NH2 (0.5 wt%) dispersed well without aggregating in both the PMMA and PU hybrid membranes. PMMA and PU are both polar polymer materials. Therefore, the increase in the surface wettability of the MWCNTs after the modification with polar groups improved the affinity between the MWCNTs and the monomer/polymer materials. The introduction of grafted points on the walls of the MWCNTs by the modification also increased the binding between the CNTs and polymers. Thus, the dispersion of the MWCNTs-NH2 was significantly better than that of the P-MWCNTs in the membranes when the same quantity of CNTs was added. With 0.5 wt% MWCNTs-NH2, the distribution of the MWCNTsNH2 in the PMMA hybrid membranes was similar to that in the PU hybrid membranes, as shown in Fig. 9A and B. However, the dispersion of the MWCNTs-NH2 in the PU and PMMA hybrid membranes differed significantly with 1.0 wt% MWCNTs-NH2 added quantity, as shown in parts C and D in Fig. 10. Some aggregation of the CNTs was observed in the PMMA membranes containing 1.0 wt% MWCNTs-NH2; in contrast, MWCNT-NH2 was homogenously distributed in the PU membranes with the same added quantity. Besides the rapid polymerization rate of the

To better understand the effect of the chemical modification on the distribution of MWCNTs in the monomer solution and the polymer materials, their surface properties and morphologies before and after the modification were analyzed by the FTIR and TEM analyses. As shown in Fig. 10, the spectrum of the P-MWCNTs exhibited weak sp2 and sp3 C–H stretching bands at 2935 and 2860 cm  1, respectively, which are attributed to the defects at the sidewalls and the open ends of the MWCNTs. In the FTIR spectrum of the MWCNTs-NH2, the band at  1600 cm  1 is attributed to the characteristic peak of aromatic CQC stretching vibrations of the benzene rings. The weak band at 3300 cm  1 is the characteristic peak of N–H stretching vibrations of the amino groups (shown as circles). Both the sp2 and sp3 C  H stretching bands at 2935 and 2860 cm  1, respectively, became stronger after the functionalization because of the increase in the MWCNT defects. All the existing bands indicate that amino groups had been grafted onto the MWCNTs. The morphology of the non-functionalized and functionalized MWCNTs was also indicated by the TEM micrographs, as shown in Fig. 11. The P-MWCNTs are shown as the bundles or individual tubes with some degree of entanglement in the solvent and were difficult to observe as single particles/tubes. However, the aminated MWCNTs were well dispersed after the functionalization.

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Fig. 9. TEM micrographs of MWCNTs-NH2 incorporated PMMA and PU membranes, (A) MWCNTs-NH2/PMMA (0.5 wt% loading); (B) MWCNTs-NH2/PU (0.5 wt% loading); (C) MWCNTs-NH2/PMMA (1.0 wt% loading); (D) MWCNTs-NH2/PU (1.0 wt% loading).

3.7. Effect of MWCNTs-NH2 on the swelling performance of hybrid membranes P-MWCNT

2935

2860

Transmittance (%)

MWCNT-NH 2

3150

2850

2550

Wavelength (cm )

3800

3300

2800

2300

1800

1300

Wavelength (cm-1) Fig. 10. FTIR spectra of MWCNTs.

Compared to the smooth wall of pristine MWCNTs, that of MWCNTs-NH2 was rough, as shown in TEM images. According to the literature [20,24], it was suggested that the rough walls of MWCNTs-NH2 were caused by the formation of some grafted points, which complemented the results of FTIR to ascertain the structure of MWCNTs-NH2.

From the TEM images, the MWCNTs-NH2 distributed homogeneously both in the monomer and in the polymer materials, which would improve the separation performance of MWCNTs incorporated hybrid membranes [13,19,22]. Therefore, Figs. 12 and 13 show that the hybrid membranes containing the MWCNTs-NH2 have high sorption selectivity and DS in benzene. In addition, the amino groups on the MWCNTs-NH2 contain benzene rings, which increase the absorption of benzene by the hybrid membranes containing the MWCNTs-NH2, as predicted by the principle of “like dissolves like.” Figs. 12 and 13 also indicate that the MWCNTs-NH2/ PU hybrid membranes show better swelling selectivities than the MWCNTs-NH2/PMMA hybrid membranes with the same added quantity of the CNTs because of the improved distribution of the MWCNTs-NH2 in the PU hybrid membranes. Similar to the effect of the P-MWCNTs on the membrane performance, the addition of excess MWCNTs-NH2 to the polymer membranes also formed the CNT aggregates in the hybrid membranes and thereby inhibited the membrane performance. Thus, the swelling selectivities of the PU and PMMA hybrid membranes initially increased but eventually decreased with the addition of the MWCNTs-NH2. The Db and Dc values of the hybrid membranes were also calculated by using the standard equation for Fickian diffusion, as shown in Figs. 14 and 15. The homogenous distribution of the MWCNTs-NH2 in the polymers favors the benzene and cyclohexane transports in the hybrid membranes. Thus, both the Db and Dc values in the membranes containing the MWCNTs-NH2 were

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Fig. 11. TEM micrographs of MWCNTs, (a) P-MWCNTs; (b) MWCNTs-NH2. 90.0

5.0

80.0

4.5

3.0

50.0

2.5 40.0

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Cyclohexane Sorption selectivity

0.5

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0.8 Diffusion coefficient (× 10 7 m2·s -1)

3.5

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Sorption Selectivy, α S,b/c

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4.0

70.0

0.0 0.0

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0.7 0.6 0.5 0.4 0.3 0.2

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Fig. 12. Variation in the degree of swelling with different contents of MWCNTsNH2 incorporated PMMA membranes.

0

0

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Mass percentage of MWCNT (%) 100.0

15.0

90.0

Fig. 14. Diffusion coefficient with different contents of MWCNTs-NH2 incorporated PMMA membranes in benzene and cyclohexane.

13.0

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DS at equilibrium (%)

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60.0 50.0

7.0

40.0

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Benzene 3.0 Cyclohexane Sorption selectivity

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Sorption Selectivy, α S,b/c

11.0 70.0

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Fig. 13. Variation in the degree of swelling with different contents of MWCNTs-NH2 incorporated PU membranes.

higher than those in the hybrid membranes containing the P-MWCNTs with the same added quantity of the CNTs. The benzene rings attached to the amino groups introduced into the MWCNTs-NH2 further improved the benzene transport and increased the Db value of the membranes. The formation of the MWCNTs-NH2 aggregates in the hybrid membranes decreased the benzene and cyclohexane transport. Thus, the increase in both the Db and Dc values leveled off as the added quantity of the

MWCNTs-NH2 increased. The elasticity of PU benefitted molecular transport in the membranes, and the Db value of the PU hybrid membranes was much higher than that of the PMMA hybrid membranes with the same added quantity of the CNTs. 3.8. Effect of MWCNTs-NH2 on the separation performance of PU hybrid membranes To explore the effects of non-functionalized and functionalized CNTs on the performance of the hybrid membranes, the performance of the PU membranes containing different MWCNTs was evaluated for the PV of benzene/cyclohexane mixtures (Fig. 16) and for the gas separation of CO2/N2 gas mixtures (Fig. 17). The addition of the non-functionalized and functionalized MWCNTs increased the separation performance of the PU hybrid membranes for the benzene/cyclohexane mixtures (up to 0.5 wt% for the non-functionalized membranes, and up to 1 wt% for the functionalized membranes) and for CO2/N2 gas mixtures. This is because of the π-π complexation between the benzene or CO2 molecules and the MWCNTs. According to the literature [19,25,26], carbon nanotube has a 1D structure with conjugated π bonds, which shows affinity towards aromatics and CO2. The effective π–π stacking interaction between MWCNT and benzene or CO2 generated in the hybrid membranes, helped to capture benzene and CO2 molecules. Therefore, the separation performance of hybrid for

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1 0.9

benzene/cyclohexane or CO2/N2 mixtures was improved with addition of MWCNTs. The chemical modification resulted in a more homogeneous distribution of the MWCNTs-NH2 in the PU hybrid membranes and significantly improved their separation performance. The separation performance of the MWCNTs-NH2/PU membranes was much higher than that of the P-MWCNTs/PU membranes with the same added quantity of CNTs. Aggregates of the CNTs were formed in the hybrid membranes, and the membrane performance decreased by the addition of excess MWCNTs. Thus, the separation performance of the PU hybrid membranes containing the P-MWCNTs and MWCNTs-NH2 initially increased but eventually decreased with increasing addition of the CNTs. Although the separation factor of the PU hybrid membranes containing 1.0 wt% MWCNTs-NH2 was the highest for separating the benzene/cyclohexane liquid mixtures and CO2/N2 gas mixtures, the maximum performance was already found at 0.50 wt% for the P-MWCNTs.

Benzene Cyclohexane

Diffusion coefficient (× 10 7 m2·s -1)

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4. Conclusions

Fig. 15. Diffusion coefficient with different contents of MWCNTs-NH2 incorporated PU membranes in benzene and cyclohexane. 45 40

MWCNT-NH2 P-MWCNT

Separation factor β

35 30 25 20 15 10 5 0

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Fig. 16. Effect of the addition of MWCNTs on the separation factor in the pervaporation performance of the hybrid membranes.

80 70

Acknowledgment

MWCNT-NH2 P-MWCNT

Financial support from the National Natural Science Foundation of China Grants (Contracts 21076190 and 21376218) is gratefully acknowledged.

60 CO2/N2 selectivity

The separation performance of the hybrid membranes improved after the addition of both pristine or functionalized MWCNTs because of the π complexation between the benzene rings and the MWCNTs in the membranes. The functionalized and non-functionalized MWCNTs showed a different distribution of the CNTs in the PU and PMMA hybrid membranes, leading to the differences in the separation performances of the two types of hybrid membranes. Because of the rapid monomer polymerization rate and elasticity of PU, the dispersion of the P-MWCNTs and MWCNTs-NH2 in the PU hybrid membranes was more homogeneous than that in the corresponding PMMA hybrid membranes for the same added quantity of the CNTs. Both the P-MWCNTs/PU and MWCNTs-NH2/PU hybrid membranes showed higher separation performance than the corresponding P-MWCNTs/PMMA and MWCNTs-NH2/PMMA hybrid membranes. After the functionalization, the increase in the surface wettability of the MWCNTs improved the affinity of the MWCNTs to the monomer solution and the polymer material. The distributions of the MWCNTs-NH2 in the PU and PMMA hybrid membranes were better than those of the P-MWCNTs. Thus, the membranes containing the MWCNTsNH2 performed better than the corresponding membranes containing the P-MWCNTs.

50

Appendix A. Supporting information 40

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2014.04.054.

30 20

References

10 0

0

0.2

0.5

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Mass percentage of MWCNT (%)

Fig. 17. Effect of the addition of MWCNTs on the selectivity of CO2/N2 gas mixture of the hybrid membranes.

[1] E. Zondervan, B. Roffel, Evaluation of different cleaning agents used for cleaning ultra filtration membranes fouled by surface water, J. Membr. Sci. 304 (2007) 40–49. [2] F. Pithan, C. Staudt, S. Hess, R.N. Lichtenthaler, Polymeric membranes for aromatic/aliphatic separation processes, Chem. Phys. Chem. 3 (2002) 856–862. [3] E. Zondervan, B. Blankert, B.H.L. Betlem, B. Roffel, Development of a multiobjective coagulation system for long-term fouling control in dead-end ultrafiltration, J. Membr. Sci. 325 (2008) 823–830.

T. Wang et al. / Journal of Membrane Science 466 (2014) 338–347

[4] X.G. Li, I. Kresse, Z.K. Xu, J. Springer, Morphology and gas permselectivity of blend membranes of polyvinylpyridine with ethylcellulose, Polymer 42 (2001) 6859–6869. [5] P. Luis, J. Degreve, B.V. Bruggen, Separation of methanol-n-butyl acetate mixtures by pervaporation: potential of 10 commercial membranes, J. Membr. Sci. 429 (2013) 1–12. [6] J.H. Kim, E.J. Moon, C.K. Kim, Composite membranes prepared from poly (manimostyrene-co-vinyl alcohol) copolymers for the reverse osmosis process, J. Membr. Sci. 216 (2003) 107–120. [7] C.K. Yeom, S.H. Lee, J.M. Lee, Pervaporative permeations of homologous series of alcohol aqueous mixtures through a hydrophilic membrane, J. Appl. Polym. Sci. 79 (2001) 703–713. [8] F.B. Peng, L.Y. Lu, H.L. Sun, Y.Q. Wang, H. Wu, Z.Y. Jiang, Correlations between free volume characteristics and pervaporation permeability of novel organic– inorganic hybrid membranes, J. Membr. Sci. 275 (2006) 97–104. [9] B.P. Tripathi, M. Kumar, A. Saxena, V.K. Shahi, Bifunctionalized organic– inorganic charged nanocomposite membrane for pervaporation dehydration of ethanol, J. Colloid Interface Sci. 346 (2010) 54–60. [10] J.H. Koh, S.W. Kang, J.T. Park, J.A. Seo, J.H. Kim, Y.S. Kang, Synthesis of silver halide nanocomposites templated by amphiphilic graft copolymer and their use as olefin carrier for facilitated transport membranes, J. Membr. Sci. 339 (2009) 49–56. [11] Y.S. Kang, S.W. Kang, H.S. Kim, J. Won, C.K. Kim, K. Char, Interaction with olefins of the partially polarized surface of silver nanoparticles activated by p-benzoquinone and its implications for facilitated olefin transport, Adv. Mater. 19 (2007) 475–479. [12] L.G. Wu, J.N. Shen, C.H. Du, T. Wang, Y.T., B.V. Bruggen, Development of AgCl/ poly(MMA-co-AM) hybrid pervaporation membranes containing AgCl nanoparticles through synthesis of ionic liquid microemulsions, Sep. Purif. Technol. 114 (2013) 117–125. [13] L.G. Wu, T. Wang, Z. Jiang, Formation of AgCl nanoparticle in reverse microemulsion using polymerizable surfactant and the resulting copolymer hybrid membranes, J Membr. Sci. 429 (2013) 95–102. [14] L.G. Wu, T. Wang, W. Xiang, Regulation of AgCl in reverse microemulsion and its effect on the performance of AgCl/PEO–PPO–PEO/PMMA hybrid membranes, Compos. Sci. Technol. 80 (2013) 8–15.

347

[15] A. Sotto, A. Boromand, R.X. Zhang, P. Luis, J.M. Arsuaga, J. Kim, B.V. Bruggen, Effect of nanoparticle aggregation at low concentrations of TiO2 on the hydrophilicity, morphology, and fouling resistance of PES–TiO2 membranes, J Colloid Interface Sci. 363 (2011) 540–550. [16] S.L. Yu, X.T. Zuo, R.L. Bao, X. Xu, J. Wang, J. Xu, Effect of SiO2 nanoparticle addition on the characteristics of a new organic–inorganic hybrid membrane, Polym 50 (2009) 553–559. [17] D. Sieffert, C. Staudt, Preparation of hybrid materials containing copolyimides covalently linked with carbon nanotubes, Sep. Purif. Technol. 77 (2011) 99–103. [18] B.J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Gavalas, L.G. Bachas, Aligned multiwalled carbon nanotubes membranes, Science 303 (2004) 62–65. [19] F.B. Peng, F.S. Pan, H.L. Sun, L.Y. Lu, Z.Y. Jiang, Novel nanocomposite pervaporation membranes composed of poly(vinyl alcohol) and chitosan-wrapped carbon nanotube, J. Membr. Sci. 300 (2007) 13–19. [20] S. Qiu, L.G. Wu, G.Z. Shi, L. Zhang, H.L. Chen, C.J. Gao, Preparation and pervaporation property of chitosan membrane with functionalized multiwalled carbon nanotubes, Ind. Eng. Chem. Res. 49 (2010) 11667–11675. [21] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [22] L.G. Wu, J.N. Shen, C.H. Du, T. Wang, Y. Teng, B.V. Bruggen, Development of AgCl/poly(MMA-co-AM) hybrid pervaporation membranes containing AgCl nanoparticles through synthesis of ionic liquid microemulsions, Sep. Purif. Technol. 114 (2013) 117–125. [23] P. Luis, J. Degreve, B.V. Bruggen, Separation of methanol-n-butyl acetate mixtures by pervaporation: potential of 10 commercial membranes, J. Membr. Sci. 429 (2013) 1–12. [24] A.L. Martínez-Hernández, C. Velasco-Santos, V.M. Castaño, Carbon nanotubes composites: processing, grafting and mechanical and thermal properties, Curr. Nanosci. 6 (2010) 12–39. [25] J. Zhao, J.P. Lu, J. Han, C.K. Yang, Noncovalent functionalization of carbon nanotubes by organic molecules, Appl. Phys. Lett. 82 (2003) 3746. [26] A.F. Ismaila, P.S. Goha, S.M. Sanipa, M. Aziz, Transport and separation properties of carbon nanotube-mixed matrix membrane, Sep. Purif. Technol. 70 (2009) 12–26.