Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes

International Biodeterioration & Biodegradation xxx (2015) 1e7

Contents lists available at ScienceDirect

International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod

Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes K.C. Wong, P.S. Goh*, A.F. Ismail Advanced Membrane Technology Research Centre, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, 81310 Johor, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 December 2014 Received in revised form 8 February 2015 Accepted 8 February 2015 Available online xxx

Recently, membrane technology has gained much momentum in the field of carbon capture and storage due to its practicality and energy efficiency. In this work, thin film nanocomposite (TFN) membranes embedded with polymethyl methacrylate (PMMA) grafted multi-walled carbon nanotubes (MWNTs) were successfully fabricated. The effects of nanofillers modification on the resultant membranes morphology and gas separation performance have been highlighted. TFN incorporated with milled PMMA grafted MWNTs showed 29% increment in CO2 permeance at 70.5 GPU with 47% and 9% enhancement in CO2/N2 and CO2/CH4 selectivity, respectively compared to the thin film composite counterpart. While the improvement was mainly attributed to the presence of highly diffusive channels upon the addition of MWNTs, PMMA grafting also played an important role in ensuring well nanofillers dispersion and good compatibility with the polyamide thin film. Uncovering the construct of membrane fabrication could pave facile yet versatile way for the development of effective membranes for greenhouse gas removal. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Thin film nanocomposite Carbon nanotube Interfacial polymerization

Introduction By the 21st century, fossil fuels have become one of the main sources of energy that drive our daily activities and economy. As our civilization flourish, our over-reliance on these non-renewable resources has severely intoxicated the world through the release of large amount of greenhouse gases (GHG); mainly the carbon dioxide (CO2) (Pires et al., 2011). High atmospheric CO2 concentration can cause global warming which leads to climate changes and disturb the balance of our ecosystem (Powell and Qiao, 2006). As such, mitigation of anthropogenic carbon dioxide emission is now a critical issue that needs to be addressed promptly. In recent years, a lot of attentions have been given to the development of membrane technology for CO2 separation application due to its simplicity and modularity (Zhang et al., 2013). Additionally, membranes offer relatively energy efficient separation performance compared to the conventional chemical absorption and cryogenic approaches. Thus,

* Corresponding author. Tel.: þ60 7 5535807. E-mail address: [email protected] (P.S. Goh).

this technology holds much promises as feasible route to tackle the aforementioned issues (Sanders et al., 2013). However, the performance of traditional polymeric membranes is bounded by the Robeson's tradeoff whereby the membranes selectivity cannot be improved without sacrificing their permeability or vice versa. In view of such predicament, attempts have been made to improve these cheap and highly processable membranes in term of performance via material engineering such as thermochemical treatment, polymers blending and incorporation of inorganic fillers (Adewole et al., 2013). One of the significant breakthroughs in membrane engineering is the fabrication of composite membrane by incorporating inorganic fillers such as zeolite, carbon molecular sieve (CMS), metal organic framework (MOF), carbon nanotube (CNT), silica and titanium oxide (TiO2) which have superior separation performance into polymer matrix (Aroon et al., 2010a; Hashemifard et al., 2011). This combination overcomes the difficulties of processing the inherently brittle inorganic materials and enhances the separation performance as well as physical properties of the host polymeric membrane (Bastani et al., 2013). Mixed matrix membrane (MMM) that is fabricated via single step phase inversion technique is one of such membranes. Yet, controlling the distribution of nanofillers within

http://dx.doi.org/10.1016/j.ibiod.2015.02.007 0964-8305/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007

2

K.C. Wong et al. / International Biodeterioration & Biodegradation xxx (2015) 1e7

the polymer matrix of MMM is challenging and most of the fillers that remained in the support layer would have lost their separation functionality (Aroon et al., 2010a). Therefore, in recent years, interfacial polymerization (IP) technique has been adopted by many researchers to produce thin film nanocomposite (TFN) (Lau et al., 2012). TFN is similar to MMM but with the benefit of optimizing its skin and support layers discretely, thanks to the two-steps approach of the IP method (Sorribas et al., 2013). This allows a more versatile membrane to be tailored. Apart from that, due to the self-termination property of IP, at certain point of the reaction, the growing film will restricts supply of reactants to the reaction interface leading to formation of very thin (around 0.1e0.25 mm, could go down to 0.05 mm) skin layer with minimal defects (Yu et al., 2010). Moreover, penetration of the polyamide layer into the porous layer pores ensures good adhesion between the thin film and the support (Zhao et al., 2006). Contemporary, TFN is commonly fabricated for liquid separation and relevant literature in CO2 separation is rather limited (Li et al., 2013). As such, it is of great interest in this research to investigate the potential of TFN in gas separation application. In CO2 separation application, IP technique is usually applied to fabricate thin film composite (TFC) membranes. Studies done by Sridhar et al. (2007) and Albo et al. (2014) showed that the common amine monomers used in fabricating composite membranes for water separation are not well-suited for gas separation with low CO2 permeance and selectivity. In order to improve the performance of TFC for CO2 separation, monomers that can react with polar gases have been evaluated. Polymerizing trimethylene tetramine (TETA) with TMC resulted in TFC that gave CO2 permeance of 13 gas permeation unit (GPU) and CO2/CH4 selectivity of 94 (Zhao et al., 2006) whereas polymerizing tertiary amino, 3,30 -diamino-Nmethyldipropylamine (DNMDAm) with TMC resulted in TFC that showed CO2 permeance of 118 GPU and CO2/CH4 selectivity of 37 (Yu et al., 2010). In both studies, the researchers tried to assimilate the advantage of facilitated transport into TFC by using monomers containing fixed carriers. The high performance of both amine containing TFC clearly showed that selection of suitable active monomers is crucial to fabricate TFCs that are suitable for CO2 capture, hence the same should hold in TFNs fabrication. Li et al. (2013) have produced TFC membrane containing ethylene oxide (EO) groups. They performed interfacial polymerization of diethylene glycol bis(3-aminopropyl) ether (DGBAmE) with TMC atop of a polydimethylsiloxane (PDMS) coated polysulfone (PSf) support and tested the TFC using mixed gases. By optimizing the concentration of the monomers used (DGBAmE: 0.0115 mol L1, TMC: 0.0104 mol L1), the fabricated membrane showed high CO2 permeance (815 GPU in CO2/H2 gas test, 727 GPU in CO2/CH4 gas test, 973 GPU in CO2/N2 gas test) and good selectivity (CO2/H2 selectivity: 10, CO2/CH4 selectivity: 31, CO2/N2 selectivity: 84) at feed pressure of 0.11 MPa. The high permeance and selectivity of this TFC were attributed to the polar EO groups which have strong affinity toward CO2. Impressed by the potential of EO-containing material, in our attempt, the same monomer was used to produce TFN for CO2 separation. CNT filler is of particular interest in this study because this inorganic nanomaterial possesses good physical properties which could greatly enhance the strength and thermal stability of the polymer matrix (Surapathi et al., 2013). The inner pore of CNT which is formed from rolled-up sheet of graphene is extremely smooth and could allow rapid mass transport in a few orders of magnitude greater than that of zeolite with similar pore size (Kim et al., 2007). Furthermore, by manipulating the growth time and catalyst film thickness, the dimension (length and diameter) of CNT can be customized to achieve precise size exclusion capability and to suit a wide range of applications (Seah et al., 2011). Despite the

advantages offered by this nanomaterial, past studies have witnessed the poor compatibility between the CNT fillers and polymer matrix which could lead to the formation of interface voids, rigidification of polymer surrounding the nanomaterials and blockage of fillers pore (Bastani et al., 2013). Other than that, owning to their nano-sized tube-like structure and high aspect ratio, CNTs held strongly together by van der Waals forces and tend to entangle into bundles of rope-like crystalline structures (Sanip et al., 2011). All these present as major hiccups to the development of CNTs/polymer composite (Chung et al., 2007). In order to overcome the complications described above, functionalization of CNTs is necessary. Generally, covalently functionalized CNTs have more stable dispersion compared to non-covalently functionalized CNTs and a wide variety of secondary modification could be performed by reacting the preattached functional groups on the nanotubes surface with other chemical reagents (Aroon et al., 2010b). For instances, Ismail et al. (2011) have produced aminopropyltriethoxysilane (APTES) functionalized multi-walled carbon nanotubes (MWNTs) via condensation reaction of hydrolyzed APTES with oxidized MWNTs (O-MWNTs) while Surapathi et al. (2013) have attached zwitterionic groups onto O-MWNTs. Shen et al. (2013) demonstrated that polymethyl methacrylate grafted MWNTs (PMMAMWNTs) have good compatibility with polyamide which encourages the formation of dense thin film structure of the TFN selective layer. Furthermore, PMMA-MWNTs shows good solubility in organic solvent, thus ensuring well dispersion of the nanofillers in the organic phase used in interfacial polymerization (Shang et al., 2009). PMMA-MWNTs with high grafting density can be easily synthesized via in-situ emulsion polymerization as this method promotes strong polymereCNT interaction (Liu et al., 2013). Besides, the synthesis process is effective and environmentally friendly (Zhang et al., 2009). Hence, in-situ emulsion polymerized PMMA-MWNTs have been chosen as the nanofillers in this study. Based on the work done by Li et al. (2013), when DGBAmE concentration in the range of 0.15e0.60% w v1 was used to react with 0.28% w v1 TMC, the resultant polyamide thin film with thickness around 150 nm (0.15 mm) can be obtained. Since the length of the pristine MWNTs (dimension: 10 nm ± 1 nm outer diameter  4.5 nm ± 0.5 nm inner diameter  3e6 mm length) we used in this study is greater than the thickness of the thin film, some of the randomly oriented nanotubes might protrude from the thin film surface if they are directly incorporated into the skin layer. Although acid oxidation of MWNTs could shorten the nanotubes
s et al., 2009), the reaction conditions used in this length (Avile study were not sufficient to cut the MWNTs below 0.15 mm. In order to minimize protrusion, the functionalized MWNTs have been incorporated into the PDMS coating layer (sub-layer beneath the thin film) during TFNs fabrication. According to Fonseca et al. (2010), the length of nanotubes can be significantly reduced upon physical ball milling for more than 2 h. Therefore, in this work, another set of TFNs were fabricated using functionalized MWNTs that have been milled for around 8 h to be compared with the former to assess the effectiveness of ball milling in shortening the nanotubes. Material and methods Materials MWNTs (98% carbon basis, OD  ID  L: 10 ± 1 nm  4.5 ± 0.5 nm  3e6 mm) polyvinylpyrrolidone (PVP, K90) and DGBAmE (97%) were purchased from Sigma Aldrich whereas sulfuric acid (H2SO4, 95e97%), nitric acid (HNO3, 65%),

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007

K.C. Wong et al. / International Biodeterioration & Biodegradation xxx (2015) 1e7

methyl methacrylate (MMA, stabilized for synthesis), N-methyl-2pyrrolidone (NMP, 99%), n-hexane (99%) and analytical grade potassium persulfate (KPS), sodium hydrogen carbonate (NaHCO3) and sodium carbonate (Na2CO3) were purchased from Merck Milipore. Other chemicals include cetyltrimethylammonium bromide (CTAB, 99%, Acros Organics), trimesoyl chloride (TMC, 98%, Acros Organics), methanol (99.8%, Fisher Scientific), ethanol (96%, Fisher Scientific), polydimethylsiloxane coating solution (PDMS, Sylgard 184, Dow Corning) and polysulfone (PSf, Udel P-3500, Solvay). Chemical oxidation of MWNTs 3 g of MWNTs was added into 600 mL of H2SO4:HNO3 (each 3 M) mixed acids solution with volume ratio of 3:1 at 80  C for 6 h to introduce carboxylic acid functional groups onto the MWNTs surface. The dispersion solution was then filtered and the MWNTs residue was rinsed with deionized (DI) water until pH 6 was obtained. Lastly, the oxidized MWNTs were rinsed with 600 mL ethanol and left to dry overnight at 60  C.

3

The PSf supports were post-treated by immersing the membranes in ethanol solutions for 5 min followed by in n-hexane for 1 min and left to dry at ambient condition for 12 h. Preparation of composite membrane TFNs Fabricated PSf membranes were sandwiched in between a glass plate and a rubber frame followed by coating with PDMS solution for 10 min (Step 1) then dried in ambient condition for 12 h. After that, the coated PSf supports were sandwiched within the same setup and impregnated with organic phase for 10 min (Step 2). Excess solution was carefully removed from the surface before polymerizing the impregnated supports with aqueous phase for 3 min (Step 3) to form the polyamide thin film. The resultant composite membranes were dried in ambient condition for 12 h. By using the same procedure, TFC and TFNs embedded with PMMAMWNTs or milled PMMA-MWNTs (m-PMMA-MWNTs) were produced by varying the solutions composition as presented in Table 1. All solutions containing nanofillers were sonicated for 1 h before used.

Synthesis of PMMA-MWNTs 10 g of CTAB, 10 g of MMA, 1.5 g O-MWNTs and 300 mL deionized (DI) water were mixed and sonicated for 20 min. The solution was then poured into a 500 mL three-neck round bottom flask that was equipped with a condenser, a mechanical stirrer and a nitrogen inlet and placed in a 70  C oil bath. The air in the flask was replaced by a stream of nitrogen gas and the mixture was stirred at 500 rpm. 3 g of KPS and 2 g of NaHCO3 were gradually added into the mixture and the reaction was allowed to take place for 21 h with continuous stirring until a grayish odorless (all MMA reacted) solution was produced. The latex dispersion was drawn out from the flask and added into methanol solution and allowed to stand overnight. Finally, the precipitation was filtered via vacuum filtration and washed with excess methanol and DI water before dried in oven at 60  C. Ball milling of functionalized MWNTs 1 g of Functionalized MWNTs were placed in a 100 mL Schott bottle containing around 200 g of 0.25 inch steel balls. The bottle was then capped tightly and place inside a bench-top laboratory roller mill and allowed to roll for 8 h. Preparation of PSf supports PSf (15 wt. %) and PVP (3 wt. %) were dissolved in NMP (82 wt. %) with stirring until a homogenous dope solution was obtained. The two opposing ends of a clean dried glass plate were each taped with three layers of cellulose tapes to act as guide during casting of the PSf support. The dope solution was poured onto the prepared glass plate slowly and casted using a clean glass roller. Then the glass plate was immediately immersed into coagulation bath (water) at room temperature and allowed to sit for 10 min to ensure a complete phase inversion process. After that, the fabricated PSf membranes were stored in DI water for 24 h to remove solvent residue.

Characterization of functionalized MWNTs and TFNs The prepared samples were examined by Fourier transform infrared spectroscopy (FTIR, Nicolet 5700, Thermo Electron Corporation) to identify the functional groups present in the samples upon the CNT functionalization and thin film formation. Thermogravimetric analysis (TGA, DSC822e, Mettler Toledo) conducted in the temperature range of 25e600  C and under nitrogen gas flow at heating rate of 10  C/min was used to determine the degree of MWNTs functionalization. Transmission electron microscopy (TEM, HT7700, Hitachi) was used to inspect the structure of the pristine and modified MWNTs. The surface and cross section morphologies of the fabricated membranes were investigated using field emission scanning electron microscopy (FESEM, SU8020, Hitachi). In order to observe the cross section of the membranes, the samples were fractured in liquid nitrogen to obtain a clean break across the membranes. Gas permeation measurement All membrane samples were saturated with water for 5 min prior to the testing. The pressure-normalized fluxes of the membranes were determined by a variable volume method using pure (N2), methane (CH4) and CO2 at room temperature. The measurement was conducted with input stream pressure of 2 bar while the outlet stream was exposed to atmospheric conditions. Circular membrane disc with effective permeation area of 13.5 cm2 was used. Volumetric gas permeation rates were determined with a simple soap bubble flow meter. The upstream of the chamber was always purged with the test gas prior to the permeation measurement. The gas permeation rate was measured after the steady state was reached and each set of data was determined as an average of three replicates. The gas permeance was determined using the following expression:

Table 1 Membrane samples description. Sample ID TFC TFN-P TFN-mP

Step 1: coating (10 min) 2 wt % PDMS in hexane 0.5 g L1 PMMA-MWNTs þ2 wt % PDMS in hexane 0.5 g L1 m-PMMA-MWNTs þ 2 wt % PDMS in hexane

Step 2: organic phase (10 min) 1

0.28% w v TMC in hexane 0.28% w v1 TMC in hexane 0.28% w v1 TMC in hexane

Step 3: aqueous phase (3 min) 0.35% w v1 DGBAmE þ 0.4% w v1 Na2CO3 in water 0.35% w v1 DGBAmE þ 0.4% w v1 Na2CO3 in water 0.35% w v1 DGBAmE þ 0.4% w v1 Na2CO3 in water

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007

4

K.C. Wong et al. / International Biodeterioration & Biodegradation xxx (2015) 1e7

ðP=lÞi ¼ Q i =ðDp$AÞ where, Qi is the volumetric flow rate of gas ‘i’ or ‘j’ at standard temperature and pressure, l is the membrane thickness (cm), A is the effective membrane area (cm2) and Dp is the trans-membrane pressure difference (cm Hg). Permeance is expressed in gas permeation unit (GPU) where:


1 1 GPU ¼ 1  106 cm3 ðSTPÞ cm2 $s$cm$Hg The pure gas selectivity was obtained by taking the ratio of the pure gas permeabilities:

. aij ¼ ðP=lÞi ðP=lÞj Fig. 2. TGA profiles of pristine MWNTs, O-MWNTs and PMMA-MWNTs.

Results and discussion Characterization of functionalized MWNTs As depicted in Fig. 1, all the spectra show broad absorption bands around 3200 cm1 and 3600 cm1 (area shaded in gray color) which are attributed to eOH stretching, indicating the presence of hydroxyl groups on all the MWNTs. For PMMA-MWNTs, absorption peak at 1726 cm1 which corresponds to the carbonyl groups (C]O) in PMMA is detected along with two characteristic peaks of CeO stretching in PMMA at 1053 cm1 (Wu et al., 2007). The spectrum obtained clearly indicates that PMMA has been successfully grafted onto the nanotube surface. Based on Fig. 2, at temperature below 100  C, there was mass loss of less than 2% in all samples due to the removal of surface absorbed water. At decomposition temperature below 600  C, the pristine MWNTs experienced a minute mass loss of about 1.7% due to the breakdown of carboxylic and hydroxyl groups whereas the
s et al., oxidized sample undergone more than 6% loss in mass (Avile 2009). This is a good indication that the pristine MWNTs were successfully oxidized. The mass of PMMA-MWNTs sample decreased rapidly from 200  C to 450  C due to decomposition of grafted PMMA. By quantifying the difference in mass loss between PMMA-MWNTs and O-MWNTs at 450  C, taking into account their respective mass loss from evaporated moisture content, the PMMA grafting degree of 11.9% was estimated, which is in good agreement with the literature that employed similar functionalization procedure (Liu et al., 2013).

While TGA is a fair method to determine the success in functionalizing the MWNTs, covalently introducing functional groups onto the nanotubes will inevitably affects the MWNTs structural integrity (Sahoo et al., 2010; Kim et al., 2012). Results from the TEM analysis shall be used to evaluate the extent of the structural changes on the nanotubes after undergone the functionalization. Fig. 3 illustrates the TEM images of the pristine, oxidized and PMMA grafted MWNTs. TEM images revealed that amorphous carbon was deposited (indicated by arrow) along the surface of the pristine MWNTs (Fig. 3b) and most of the tips of the nanotubes remained closed (indicated by red box, in the web version, in Fig. 3a). After subjected to acid oxidation, the amorphous deposition was removed and the nanotubes tips were opened (indicated by red circles, in the web version, in Fig. 3c). The layer of PMMA deposition on the grafted MWNTs in Fig. 3f and h cannot be visibly observed, probably due to the low concentration of the PMMA monomer used for the functionalization. Nevertheless, the grafted nanotubes (Fig. 3e and g) appeared to be less entangled compared to the pristine and oxidized samples, which has indicated the effectiveness of PMMA to enhance the dispersion of the grafted MWNTs. In overall, the modified nanotubes still retained their tubular structure integrity with no visible wall damages can be observed. Based on the images shown in Fig. 4, it is obvious that oxidized MWNTs dispersed poorly in PDMS coating solution due to agglomeration of the nanotubes whereas the PMMA grafted MWNTs showed good dispersion. Here, the PMMA chains on the nanotubes have reduced the van der Waals forces among the nanotubes and prevented them from aggregating. Characterization of TFN

Fig. 1. FTIR spectra of pristine MWNT, O-MWNT and PMMA-MWNT.

FTIR spectra of all the composite membranes in Fig. 5 show that interfacial polymerization has successfully taken place since strong characteristic peaks of C]O at 1655 cm1 and CeN at 1243 cm1 which correspond to the amide groups in polyamide thin film were detected. From Fig. 6a, the surface of TFC is covered by dense nodular structures which are commonly reported for polyamide thin films (Yu et al., 2010) whereas the surface morphology of the TFNs are similar as those reported by Ma et al. (2012). Generally, the incorporation of nanofillers into the thin film has resulted in the increase of the nodule size with broadened ridge and valley features (Fig. 6b and c). Nevertheless, cross section images revealed that the polyamide layer TFN-P (Fig. 6e) are filled with coarse sized aggregates whereas the samples containing milled functionalized MWNTs have more refined structure (Fig. 6f). This observation suggests that

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007

K.C. Wong et al. / International Biodeterioration & Biodegradation xxx (2015) 1e7

5

Fig. 3. TEM images of (a and b) pristine MWNT, (c and d) O-MWNTs, (e and f) PMMA-MWNTs and (g and h) m-PMMA-MWNTs at 120k and 600k magnifications respectively.

milling the nanotubes can suppress the formation of aggregates and defects on the thin film thus led to the enhancement in gas separation performance of the TFNs. Gas separation performance of TFNs Gas separation performance of the resultant TFNs was evaluated using pure gas of nitrogen (N2), methane (CH4) and CO2 at room temperature using feed pressure of 2 bar. All membrane samples were saturated with water prior to the testing to simulate the separation of damp membrane when moist gases are fed. Table 2 tabulates the separation performance of TFNs with TFC. It should be noted that gas permeation test was also performed on the dry polyamide composite membranes and the separation behavior of these dry samples was similar to the one reported by Andrew Lee et al. (2013) in which their performance is not as good as the wetted ones. Once the composite membranes are wetted, the permeance of CO2 and CO2/N2 selectivity increased while the N2 permeance decreased. It is believed that the enhancement in performance is due to the facilitated transport mechanism of CO2 rendered by the amide and free-amine groups of the membranes

Fig. 4. Dispersion of O-MWNTs, milled O-MWNTs, PMMA-MWNTs and milled PMMAMWNTs in PDMS coating solution after 1 h sonication and left to stand for 24 h in sample bottles.

which is activated in the presence of moisture content (Saedi et al., 2014). Overall, experimental results showed that the TFN embedded with MWNTs experienced significant permeance improvement for all tested gases compared to the TFC. The increase in permeance was primarily resulted from the addition of nanotubes that allowed faster gas flow through the hollow cavity but it could also indicate the formation of defects which is commonly related to the deterioration in selectivity. TFN-P showed the reduction in CO2/N2 and CO2/CH4 selectivity which could be related to the formation of aggregates observed from the FESEM images in the previous discussion. As mentioned earlier, ball milling can reduce the length of the MWNTs that might suppress the protrusion of the nanofillers. Indeed, instead of suffering the same fate like the TFN-P, the TFN that were incorporated with milled functionalized MWNTs at the coating layer experienced increment in CO2 and CH4 permeance without sacrificing the selectivity. As a matter of fact, the TFN-mP exhibited higher CO2/N2 and CO2/CH4 selectivity than TFC. In this case, the enhancement in CO2 and CH4 permeance of TFN-mP can be concluded to be a direct result of nanotubes incorporation. It is postulated that the EO, amide and free-amine groups in the defectfree polyamide thin film work in favor of CO2 by interacting with

Fig. 5. FTIR spectra of fabricated composite membranes.

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007

6

K.C. Wong et al. / International Biodeterioration & Biodegradation xxx (2015) 1e7

Fig. 6. Surface morphologies of (a) TFC, (b) TFN-P and (c) TFN-mP obtained at 5k magnification and cross section of (d) TFC, (e) TFN-P and (f) TFN-mP obtained at 600 magnification.

Table 2 Gas permeance, CO2/N2 and CO2/CH4 selectivity of resultant composite membranes. Sample ID

TFC TFN-P TFN-mP

Permeance, P (GPU)

Percentage permeance change, (%)

Selectivity, a

Percentage selectivity change (%)

N2

CH4

CO2

DPN2

DPCH4

DPCO2

CO2/N2

CO2/CH4

2 DaCO N2

2 DaCO CH4

1.20 1.67 1.05

2.06 2.82 2.43

54.87 58.09 70.54

e 39.17 12.5

e 36.89 17.96

e 5.87 28.56

45.73 34.78 67.18

26.64 20.60 29.03

e 23.93 46.92

e 22.66 8.98

Percentage change in permeance of specific gas through TFN compared to TFC, DPi ¼ ðPiTFN  PiTFC Þ=PiTFC  100%Þ. 2 2 2 2 Percentage change in selectivity of TFN compared to TFC, DaCO ¼ ðaCO TFN  aCO TFCÞ=aCO TFC  100%. i i i i Where i refers to N2, CH4 or CO2.

the gas and assisted in its transport through the membrane (Ge et al., 2011). Besides, the kinetic diameter of CO2 (3.3 Å) is smaller than that of CH4 (3.8 Å) (Zhang et al., 2013) which allows the former to diffuse faster through the polymer matrix as it requires less activation energy (Singh and Koros, 1996). Thus, the enhancement in CO2 permeance is greater compared to CH4, and eventually resulted in improvement of CO2/CH4 selectivity. Surprisingly, upon the addition of the milled functionalized MWNTs, a decrease in N2 permeance has been observed. Here we

assumed that the selective surface flow mechanism has taken place within the nanotubes which have much larger pore size (4e5 nm equivalent to 40e50 Å) than N2 (3.6 Å) (Ismail et al., 2011) and water (2.6 Å) (ten Elshof et al., 2003). The transport of N2 (noncondensable component) through the nanotubes was hindered by the presence of water (condensable polar component) attributed by the selective adsorption of water molecules on the MWNTs pore walls (Sircar et al., 1999). Another possible explanation of this phenomena would be due to rigidification of EO-containing

Fig. 7. Gas separation performance of resultant composite membranes. Red and blue dotted lines correspond respectively to the selectivity and permeance baseline using TFC as reference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007

K.C. Wong et al. / International Biodeterioration & Biodegradation xxx (2015) 1e7

polyamide chains around the nanotubes. Rigidification might reduce the polymer chains mobility and increase the resistance toward mass transport (Ismail et al., 2011). Consequently, the permeance of N2 which is neither condensable nor attractable by the EO groups (Li et al., 2013) could not benefit from the nanotubes while the increase in local EO and amide concentration (more EO containing polyamide chains compacted together) around the MWNTs might induce greater affinity towards CO2, thus enhancing its permeance. Furthermore, the addition of nanofillers within the coating layer could add to the tortuosity of the diffusion path. This inevitably slowed down the transport of N2 as the gas molecules need to move across lengthier channels (Aroon et al., 2010a). Ultimately, all the factors discussed above have led to the enhancement in CO2/N2 selectivity. Fig. 7 summarizes the changes in gas separation performance when the functionalized MWNTs were incorporated into the TFN. Conclusion TFNs incorporated with PMMA grafted MWNTs hold vast potential in gas separation application. This study has demonstrated that TFNs can be easily fabricated using the well-established interfacial polymerization technique. PMMA-MWNTs exhibited good dispersibility in PDMS coating solution which enabled the fabrication of TFN with enhanced gas separation performance compared to TFC. TFN with milled PMMA-MWNTs embedded in the coating layer has CO2 permeance of 70.54 GPU with CO2/N2 selectivity of 67.18 and CO2/CH4 selectivity of 29.03 which translate to around 29%, 47% and 9% improvement respectively compared to the TFC counterpart. Material selection and functionalization of the nanotubes play an important role for successful fabrication of high performance TFN. Acknowledgments Authors would like to acknowledge the financial support obtained from Research University Grant (vot number: 04H86) and Fundamental Research Grant Scheme (vot number: 4F306). References Adewole, J.K., Ahmad, A.L., Ismail, S., Leo, C.P., 2013. Current challenges in membrane separation of CO2 from natural gas: a review. Int. J. Greenh. Gas Control 17, 46e65. Albo, J., Wang, J., Tsuru, T., 2014. Gas transport properties of interfacially polymerized polyamide composite membranes under different pre-treatments and temperatures. J. Membr. Sci. 449, 109e118. Andrew Lee, S., Stevens, G.W., Kentish, S.E., 2013. Facilitated transport behavior of humidified gases through thin-film composite polyamide membranes for carbon dioxide capture. J. Membr. Sci. 429, 349e354. Aroon, M.A., Ismail, A.F., Matsuura, T., Montazer-Rahmati, M.M., 2010a. Performance studies of mixed matrix membranes for gas separation: a review. Sep. Purif. Technol. 75, 229e242. Aroon, M.A., Ismail, A.F., Montazer-Rahmati, M.M., Matsuura, T., 2010b. Effect of chitosan as a functionalization agent on the performance and separation properties of polyimide/multi-walled carbon nanotubes mixed matrix flat sheet membranes. J. Membr. Sci. 364, 309e317.
s, F., Cauich-Rodríguez, J.V., Moo-Tah, L., May-Pat, A., Vargas-Coronado, R., Avile 2009. Evaluation of mild acid oxidation treatments for MWCNT functionalization. Carbon 47, 2970e2975. Bastani, D., Esmaeili, N., Asadollahi, M., 2013. Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: a review. J. Ind. Eng. Chem. 19, 375e393. Chung, T.-S., Jiang, L.Y., Li, Y., Kulprathipanja, S., 2007. Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation. Prog. Polym. Sci. 32, 483e507. Fonseca, A., Reijerkerk, S., Potreck, J., Nijmeijer, K., Mekhalif, Z., Delhalle, J., 2010. Very short functionalized carbon nanotubes for membrane applications. Desalination 250, 1150e1154. Ge, L., Zhu, Z., Rudolph, V., 2011. Enhanced gas permeability by fabricating functionalized multi-walled carbon nanotubes and polyethersulfone nanocomposite membrane. Sep. Purif. Technol. 78, 76e82.

7

Hashemifard, S.A., Ismail, A.F., Matsuura, T., 2011. Effects of montmorillonite nanoclay fillers on PEI mixed matrix membrane for CO2 removal. Chem. Eng. J. 170, 316e325. Ismail, A.F., Rahim, N.H., Mustafa, A., Matsuura, T., Ng, B.C., Abdullah, S., Hashemifard, S.A., 2011. Gas separation performance of polyethersulfone/multiwalled carbon nanotubes mixed matrix membranes. Sep. Purif. Technol. 80, 20e31. Kim, S., Chen, L., Johnson, J.K., Marand, E., 2007. Polysulfone and functionalized carbon nanotube mixed matrix membranes for gas separation: theory and experiment. J. Membr. Sci. 294, 147e158. Kim, S.W., Kim, T., Kim, Y.S., Choi, H.S., Lim, H.J., Yang, S.J., Park, C.R., 2012. Surface modifications for the effective dispersion of carbon nanotubes in solvents and polymers. Carbon 50, 3e33. Lau, W.J., Ismail, A.F., Misdan, N., Kassim, M.A., 2012. A recent progress in thin film composite membrane: a review. Desalination 287, 190e199. Li, S., Wang, Z., Zhang, C., Wang, M., Yuan, F., Wang, J., Wang, S., 2013. Interfacially polymerized thin film composite membranes containing ethylene oxide groups for CO2 separation. J. Membr. Sci. 436, 121e131. Liu, X.-Q., Shen, S.-Q., Wen, R., Yang, W., Xie, B.-H., Yang, M.-B., 2013. Reinforcement and plasticization of PMMA grafted MWCNTs for PVDF composites. Compos. Part B 53, 9e16. Ma, N., Wei, J., Liao, R., Tang, C.Y., 2012. Zeolite-polyamide thin film nanocomposite membranes: towards enhanced performance for forward osmosis. J. Membr. Sci. 405e406, 149e157. ~es, M., 2011. Recent dePires, J.C.M., Martins, F.G., Alvim-Ferraz, M.C.M., Simo velopments on carbon capture and storage: an overview. Chem. Eng. Res. Des. 89, 1446e1460. Powell, C.E., Qiao, G.G., 2006. Polymeric CO2/N2 gas separation membranes for the capture of carbon dioxide from power plant flue gases. J. Membr. Sci. 279, 1e49. Saedi, S., Madaeni, S.S., Seidi, F., Shamsabadi, A.A., Laki, S., 2014. Fixed facilitated transport of CO2 through integrally-skinned asymmetric polyethersulfone membrane using a novel synthesized poly (acrylonitrile-co-N, N-dimethylaminopropyl acrylamide). Chem. Eng. J. 236, 263e273. Sahoo, N.G., Rana, S., Cho, J.W., Li, L., Chan, S.H., 2010. Polymer nanocomposites based on functionalized carbon nanotubes. Prog. Polym. Sci. 35, 837e867. Sanders, D.F., Smith, Z.P., Guo, R., Robeson, L.M., McGrath, J.E., Paul, D.R., Freeman, B.D., 2013. Energy-efficient polymeric gas separation membranes for a sustainable future: a review. Polymer 54, 4729e4761. Sanip, S.M., Ismail, A.F., Goh, P.S., Soga, T., Tanemura, M., Yasuhiko, H., 2011. Gas separation properties of functionalized carbon nanotubes mixed matrix membranes. Sep. Purif. Technol. 78, 208e213. Seah, C.-M., Chai, S.-P., Mohamed, A.R., 2011. Synthesis of aligned carbon nanotubes. Carbon 49, 4613e4635. Shang, S., Li, L., Yang, X., Wei, Y., 2009. Polymethylmethacrylateecarbon nanotubes composites prepared by microemulsion polymerization for gas sensor. Compos. Sci. Technol. 69, 1156e1159. Shen, J.n., Yu, C.C., Ruan, H.M., Gao, C.J., Van der Bruggen, B., 2013. Preparation and characterization of thin-film nanocomposite membranes embedded with poly(methyl methacrylate) hydrophobic modified multiwalled carbon nanotubes by interfacial polymerization. J. Membr. Sci. 442, 18e26. Singh, A., Koros, W.J., 1996. Significance of entropic selectivity for advanced gas separation membranes. Ind. Eng. Chem. Res. 35, 1231e1234. Sircar, S., Rao, M.B., Thaeron, C.M.A., 1999. Selective surface flow membrane for gas separation. Sep. Sci. Technol. 34, 2081e2093.
llez, C., Coronas, J., Livingston, A.G., 2013. High flux thin Sorribas, S., Gorgojo, P., Te film nanocomposite membranes based on metaleorganic frameworks for organic solvent nanofiltration. J. Am. Chem. Soc. 135, 15201e15208. Sridhar, S., Smitha, B., Mayor, S., Prathab, B., Aminabhavi, T.M., 2007. Gas permeation properties of polyamide membrane prepared by interfacial polymerization. J. Mater. Sci. 42, 9392e9401. Surapathi, A., Chen, H.-y., Marand, E., Karl Johnson, J., Sedlakova, Z., 2013. Gas sorption properties of zwitterion-functionalized carbon nanotubes. J. Membr. Sci. 429, 88e94. ten Elshof, J.E., Abadal, C.R., Sekuli
c, J., Chowdhury, S.R., Blank, D.H.A., 2003. Transport mechanisms of water and organic solvents through microporous silica in the pervaporation of binary liquids. Microporous Mesoporous Mater. 65, 197e208. Wu, H.-X., Qiu, X.-Q., Cao, W.-M., Lin, Y.-H., Cai, R.-F., Qian, S.-X., 2007. Polymerwrapped multiwalled carbon nanotubes synthesized via microwave-assisted in situ emulsion polymerization and their optical limiting properties. Carbon 45, 2866e2872. Yu, X., Wang, Z., Wei, Z., Yuan, S., Zhao, J., Wang, J., Wang, S., 2010. Novel tertiary amino containing thin film composite membranes prepared by interfacial polymerization for CO2 capture. J. Membr. Sci. 362, 265e278. Zhang, F.-A., Lee, D.-K., Pinnavaia, T.J., 2009. PMMA e mesocellular foam silica nanocomposites prepared through batch emulsion polymerization and compression molding. Polymer 50, 4768e4774. Zhang, Y., Sunarso, J., Liu, S., Wang, R., 2013. Current status and development of membranes for CO2/CH4 separation: a review. Int. J. Greenh. Gas Control 12, 84e107. Zhao, J., Wang, Z., Wang, J., Wang, S., 2006. Influence of heat-treatment on CO2 separation performance of novel fixed carrier composite membranes prepared by interfacial polymerization. J. Membr. Sci. 283, 346e356.

Please cite this article in press as: Wong, K.C., et al., Gas separation performance of thin film nanocomposite membranes incorporated with polymethyl methacrylate grafted multi-walled carbon nanotubes, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/ 10.1016/j.ibiod.2015.02.007