Synthesis and gas permeation properties of amphiphilic graft copolymer membranes

Synthesis and gas permeation properties of amphiphilic graft copolymer membranes

Journal of Membrane Science 345 (2009) 128–133 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 345 (2009) 128–133

Contents lists available at ScienceDirect

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

Synthesis and gas permeation properties of amphiphilic graft copolymer membranes Sung Hoon Ahn a , Jin Ah Seo a , Jong Hak Kim a,∗ , Youngdeok Ko b , Seong Uk Hong b,∗∗ a b

Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, South Korea Department of Chemical Engineering, Center for Appropriate Technology, Hanbat National University, Yuseong-gu, Daejeon 305-719, South Korea

a r t i c l e

i n f o

Article history: Received 11 July 2009 Received in revised form 18 August 2009 Accepted 23 August 2009 Available online 29 August 2009 Keywords: Graft copolymer Atom transfer radical polymerization (ATRP) Membrane Gas separation Carbon dioxide

a b s t r a c t Amphiphilic graft copolymers comprising poly(vinyl chloride) (PVC) main chains and poly(oxyethylene methacrylate) (POEM) side chains, i.e. PVC-g-POEM, were synthesized via atom transfer radical polymerization (ATRP) using direct initiation of the secondary chlorines of PVC. Successful synthesis of the graft copolymer was confirmed using 1 H NMR and FT-IR spectroscopy. TEM and DSC analysis revealed the welldefined microphase-separated structure of the graft copolymer into hydrophobic PVC and hydrophilic POEM domains. All the membranes exhibited amorphous structures and the intersegmental d-spacing were increased with the grafting degree, as characterized by XRD analysis. Permeation experimental results using a CO2 /N2 (50/50) mixture indicated that as an amount of POEM in a copolymer increased, CO2 permeability increased dramatically without the sacrifice of selectivity. For example, the CO2 permeability [1 × 10−8 cm3 (STP) cm/cm2 s cmHg (100 Barrer)] of PVC-g-POEM with 70 wt% of POEM at 25 ◦ C was about 70 times higher than that of the pure PVC membrane [1.45 × 10−10 cm3 (STP) cm/cm2 s cmHg (1.45 Barrer)]. © 2009 Elsevier B.V. All rights reserved.

1. Introduction For many years, global warming due to the increased atmospheric CO2 concentration has been a global issue. Therefore, development of economically feasible CO2 capture process is becoming increasingly important [1]. Polymeric membranes have been widely used to separate gas mixtures, such as O2 /N2 , CO2 /CH4 , CO2 /N2 , olefin/paraffin, etc. It has been known that polymers containing polar groups, such as ether or amine groups, have a high affinity for CO2 because of dipole–quadrupole interactions [2,3]. Therefore, there have been many studies about CO2 separation using polymeric membranes containing poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) [3–10]. However, one critical drawback of a neat PEO membrane is its strong tendency to crystallize due to the helical structure of chains and consequently it presents low gas permeability [11,12]. Thus, researches on modification of a PEO membrane have recently received great attention based on controlling the structures of polymer such as blending [3], crosslinking [13], block copolymerization [14] and random copolymerization [15]. As far as we know,

∗ Corresponding author. Tel.: +82 2 2123 5757; fax: +82 2 312 6401. ∗∗ Corresponding author. Tel.: +82 42 821 1536; fax: +82 42 821 1593. E-mail addresses: [email protected] (J.H. Kim), [email protected] (S.U. Hong). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.08.037

however, there has been no report on the gas permeation properties based on an amphiphilic graft copolymer containing PEO units although there have been some reports on the use of graft copolymer as a membrane material for liquid–liquid separation [16], ultrafilatration [17], microfiltration [18] and pervaporation [19]. Poly(vinyl chloride) (PVC) is one of the most widely used vinyl polymers in the world because of its low cost and good chemical and mechanical properties [20]. However, study on the gas permeation properties of PVC membrane was very limited because of its intrinsic low gas permeation properties resulting from high chain compactness and low segmental motion of polymeric chains [21]. On the other hand, poly(oxyethylene methacrylate) (POEM) is a well-known amorphous PEO, which possess a “liquid-like” poor mechanical property [22] and thus cannot be readily applied to separation membranes. If these properties of PVC and POEM are combined together, the membranes possessing both high separation properties and good mechanical properties are expected to be prepared successfully. In this study, therefore, we synthesized amphiphilic graft copolymers consisting of poly(vinyl chloride) main chains and poly(oxyethylene methacrylate) side chains (PVCg-POEM) via atom transfer radical polymerization (ATRP) and investigated their gas permeation properties. The resultant membranes were also characterized using nuclear magnetic resonance (1 H NMR), FT-IR spectroscopy, transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD).

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Scheme 1. Synthesis of PVC-g-POEM graft copolymer by ATRP.

2. Experimental 2.1. Materials Poly(vinyl chloride) (PVC, Mw ∼ 97,000 g/mol, Mn ∼ 55,000 g/mol), poly(oxyethylene methacrylate) (POEM, poly(ethylene glycol)methyl ether methacrylate, Mn 475 g/mol), 1,1,4,7,10,10hexamethyltriethylene tetramine (HMTETA,99%), and copper(I) chloride (CuCl, 99%) were purchased from Aldrich. Tetrahydrofuran (THF), N-methyl pyrrolidone (NMP) and methanol were obtained from J.T. Baker. All solvents and chemicals were regent grade and were used as received. 2.2. Synthesis of graft copolymer Six grams of PVC was dissolved in 50 mL of NMP by stirring at 90◦ C for 4 h. After cooling the solution to room temperature, the various amounts (9 g and 18 g) of POEM, 0.1 g of CuCl and 0.23 mL of HMTETA were added to the solution. The green mixtures were stirred to produce homogeneous solutions and purged with nitrogen for 30 min. The reaction was carried out at 90◦ C for 18 h. After polymerization, the resultant mixtures were diluted with THF. The solutions were precipitated into methanol. The grafted copolymers were obtained as a powder form and dried in a vacuum oven overnight at room temperature. 2.3. Membrane preparation First, as-synthesized graft copolymers were dissolved in 3 wt% of THF. After dissolving completely, the polymer solutions were cast into Teflon flat-bottomed Petri dishes. After air drying at ambient conditions for one day, films were removed from the dish and placed on a Teflon plate. The films were stored under vacuum in an oven at room temperature for 24 h to assist in removal of residual THF and then annealed at 80◦ C for 24 h. The thickness of membranes was in the range of 50–80 ␮m.

area of membranes was 28 cm2 . The measuring temperature range was 35–45 ◦ C and the feed composition of CO2 and N2 was 50:50. CO2 and N2 concentrations were detected using a thermal conductivity detector. By measuring the permeate gas concentration, the permeation selectivity was calculated as a ratio of CO2 mole fraction to N2 mole fraction of a permeate gas. 3. Results and discussion 3.1. Synthesis of graft copolymer The reaction scheme for the synthesis of PVC-g-POEM graft copolymer via ATRP is illustrated in Scheme 1. It has been established that ATRP is an efficient polymerization method for preparing a well-defined graft copolymer [23,24]. Various side chains can be grafted from the main chain backbone using the ‘grafting from’ ATRP method, which is initiated by pendant initiating group [25–28]. It was reported that the labile chlorines of PVC resulting from structural defects formed during the radical polymerization of vinyl chloride act as initiation sites for the direct grafting of PVC [29,30]. Because of the amphiphilic properties, PVC-g-POEM graft copolymer is expected to molecularly self-assemble into nanophase domains of hydrophobic PVC interweaved with hydrophilic domains of POEM brush layer. These membranes are expected to exhibit both high separation properties and good mechanical properties due to the microphase-separated properties [31]. The successful graft copolymerization of PVC-g-POEM has been confirmed using 1 H NMR spectroscopy. The 1 H NMR spectra for PVC-g-POEM graft copolymers with different amounts POEM are presented in Fig. 1. The three peaks at around 4.6–4.4 ppm are attributed to the CHCl group in PVC [30]. Upon grafting of POEM side chains from PVC mainchains, additional peaks at around 4.2,

2.4. Characterization 1 H NMR measurements were performed with a 600 MHz, high resolution NMR spectrometer (AVANCE 600 MHz FT NMR, Germany, Bruker). FT-IR spectra of the samples were collected using an Excalibur Series FT-IR (DIGLAB Co.) instrument between the frequency ranges of 4000 and 400 cm−1 using an ATR facility. TEM pictures were obtained from a Philips CM30 microscope operating at 300 kV. For TEM measurements, the samples were dissolved in THF, and then a drop of this solution was placed onto a standard copper grid. The XRD experiment was carried out on a Rigaku 18 kW rotating anode X-ray generator with Cu-Ka radiation operated at 40 kV and 300 mA. DSC measurements (DSC 2920, TA Instruments, Inc.) were carried out to characterize the materials at a heating rate of 10 ◦ C/min under N2 environment. Mixed gas permeability coefficients were measured using a GTR-W30 gas permeation apparatus equipped with a gas chromatography (Yanaco, Japan). The effective

Fig. 1. 1 H NMR spectra for PVC-g-POEM graft copolymers with different amounts POEM.

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Fig. 2. FT-IR spectra of pristine PVC, POEM monomer and PVC-g-POEM graft copolymers.

3.6 and 3.4 ppm appeared, resulting from bonding environments in the ethylene oxide units of POEM [25,32]. The grafting ratio of POEM in the PVC-g-POEM copolymers was calculated by comparing integral areas. As a result, we obtained the PVC-g-POEM graft copolymers with two kinds of composition, i.e. 30 wt% and 70 wt% of POEM, which corresponds to 35 vol% and 75 vol%, respectively, on a volume basis. Fig. 2 shows the FT-IR spectra of pristine PVC, POEM monomer and PVC-g-POEM graft copolymers. Upon graft copolymerization, the sharp absorption bands at 2870, 1725 and 1101 cm−1 were additionally observed for both membranes containing 30 wt% and 70 wt% of POEM compared to pristine PVC. These bands result from the stretching vibration modes of methyl (CH2 ), carbonyl (C O) and ether bonds (C O C) of POEM, respectively. Upon comparison of the graft copolymer with POEM monomer, the weak absorption band at 1638 cm−1 assigned to the C C stretching mode of POEM monomer completely disappeared, indicating successful graft copolymerization via ATRP. In addition, the stretching band of carbonyl (C O) in POEM monomer at 1717 cm−1 shifted to a higher wavenumber at 1725 cm−1 upon graft copolymerization, presumably due to loss of ␲ conjugation in POEM monomer [32,33]. These FT-IR spectroscopic results provide strong evidence that the graft copolymerization via ATRP from secondary chlorine atoms on the PVC backbone was performed successfully.

Fig. 3. TEM pictures of PVC-g-POEM graft copolymers: (a) 30 wt% and (b) 70 wt% of POEM.

3.2. Structural properties of graft copolymer TEM analysis was carried out to characterize the morphology of graft copolymers. Fig. 3 shows the TEM images of unstained PVC-g-POEM graft copolymers with 30 wt% (35 vol%) and 70 wt% (75 vol%) of POEM. Because of the large difference of electron densities between PVC and POEM chains, the unstaining TEM samples are sufficient to provide clear image contrast between the two domains. Dark regions are due to the hydrophobic domains of PVC main chains while bright regions come from the hydrophilic POEM side chains. As seen in the pictures, both membranes exhibited well-defined microphase-separated morphology between PVC main chains and POEM side chains. At higher POEM concentration (70 wt%), the area of bright regions was increased due to the increase of volume in POEM domains. These TEM results reveal that the amphiphilic PVC-g-POEM graft copolymers molecularly self-assemble into continuous nanophase domains of POEM interweaved with hydrophobic domains of PVC, providing transport mechanism of gas molecules in the graft polymer membranes.

DSC measurements were carried out to determine the thermal properties of graft copolymers such as glass transition temperature (Tg ). The DSC heating curves for PVC-g-POEM with two different compositions were presented in Fig. 4. The DSC results of Fig. 4 indicates that the materials are in amorphous state and microphase-separated, showing two distinct Tg at around −68 and 32 ◦ C that can be attributed to the POEM and PVC domains, respectively [31]. However, the Tg of PVC in the graft copolymers was to some degree lower than that of PVC homopolymer (80 ◦ C). It may arise from plasticization of rigid PVC main chains by rubbery POEM side chains due to the partial phase mixing between POEM and PVC segments [34]. XRD has been established as a powerful tool to investigate the structural changes of polymer membranes [35]. The XRD patterns for the pristine PVC and PVC-g-POEM graft copolymers are presented in Fig. 5, where the intensity of X-ray scattering is plotted against the diffraction angle, 2. Pristine PVC is an amorphous polymer to exhibit some weak peaks centered at a diffraction angle

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Fig. 6. Temperature dependence of gas permeabilities and selectivity of the PVC-gPOEM (70%) film. Fig. 4. DSC curves of PVC-g-POEM graft copolymers.

of 18.1◦ , 24.5◦ and 40.0◦ . Upon grafting of POEM from the PVC main chains, the molecular structure and physical network were significantly changed, leading to more ‘structureless’ amorphous morphology, consistent with DSC results of Fig. 4. This behavior was more pronounced at higher POEM concentration. This is explained by the fact that the randomness of the amorphous phase in graft copolymers is more increased by introducing graft side chains, resulting in a perturbation of long-ranged spacing between the chains [32]. Another observation is that the main amorphous peak shifted to a lower diffraction angle at higher POEM content, i.e. 21.5◦ at 70 wt% of POEM. According to the Bragg equation, the interchain d-spacing was increased upon POEM grafting, indicating loosely packed structure of the graft copolymer membranes [35]. 3.3. Gas permeation properties The performance test was done for CO2 /N2 separation, which presents the greatest challenge for membrane systems due to the green house effect of CO2 . The mixture gas permeabilities of CO2 and N2 were measured for all of the membranes at 2 atm (total upstream pressure) and 35–45 ◦ C. The composition of feed gas was CO2 :N2 = 50:50. The temperature dependence of gas permeabilities and selectivity of the PVC-g-POEM (70%) membrane was shown in

Fig. 5. XRD patterns of pristine PVC and PVC-g-POEM graft copolymers.

Table 1 Activation energies (kJ/mol) for CO2 and N2 in PVC and PVC-g-POEM graft copolymer membranes.

CO2 N2

PVC

PVC-g-POEM (30%)

PVC-g-POEM (70%)

14.1 32.9

45.2 62.1

23.5 39.8

Fig. 6. Since it follows the Arrhenius behavior very well, i.e., plot of log P vs. 1/T is linear, data at 25 ◦ C are extrapolated from the plot. The r2 value was always higher than 0.999 for all the regression. The activation energies of gases were also estimated and presented in Table 1. The activation energies for both gases reached the maximum at 30% POEM content. The reason for this behavior is not clear at this stage. Fig. 7 represents the gas separation performance of the polymer membranes at 25 ◦ C. The solid line is an empirical upper bound relation [36] and numerical values are also provided in the Table 2. The CO2 permeability of pure PVC membrane was only 1.45 × 10−10 cm3 (STP) cm/cm2 s cmHg (1.45 Barrer). When the POEM amount in the copolymer was 30%, the CO2 permeability was increased by 6.5 times with a little increase of selectivity. This is probably due to the presence of the ether linkage of POEM in

Fig. 7. The relationship between the CO2 permeability and the CO2 /N2 selectivity for polymer films. The solid line is an empirical upper bound relation [36]. The dashed line is a guide for the eyes.

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Table 2 CO2 permeability, N2 permeability and permeation selectivity of membranes at 25 ◦ C using a CO2 /N2 (1/1) mixture. Data were calculated by extrapolating the experimental results at 35–45 ◦ C (1 Barrer = 1 × 10−10 cm3 (STP) cm/cm2 s cmHg). Membranes

P(CO2 ) (Barrer)

P(N2 ) (Barrer)

Selectivity (CO2 /N2 )

PVC PVC-g-POEM (30%) PVC-g-POEM (70%)

1.45 9.47 100

0.04 0.18 2.04

38 52 49

the graft copolymer membranes. It has been known that polymers containing polar groups, such as ether groups, have a high affinity for CO2 because of dipole–quadrupole interactions [11,12]. When we increased the POEM amount further, the CO2 permeability increased to 1 × 10−8 cm3 (STP) cm/cm2 s cmHg (100 Barrer) without the large sacrifice of selectivity, approaching the “upper bound”. This CO2 permeability value is about 70 times higher than that of pure PVC. Although the permeation properties did not overcome the “upper bound” limit, this is the first report on gas separation membranes based on the amphiphilic graft copolymer containing PEO units, which was synthesized by ATRP process. Therefore, if we prepare a copolymer of POEM with a polymer having much higher CO2 permeability than that of PVC, we can get the copolymer membrane which may overcome the “upper bound”. 4. Conclusions This work has demonstrated that the amphiphilic PVC-gPOEM graft copolymer membranes synthesized via an ATRP process exhibit good separation properties for CO2 . PVC backbone was directly used as a macroinitiator for the synthesis of the graft copolymer. Successful synthesis of graft copolymers was confirmed by 1 H NMR and FT-IR spectroscopy. TEM analysis showed the PVC-g-POEM graft copolymers exhibited microphase-separated morphology and self-assembled into continuous nanophase domains of hydrophilic POEM interweaved with the domains of hydrophobic PVC. All the graft copolymer membranes exhibited ‘structureless’ amorphous morphology and the interchain d-spacing was increased upon the graft copolymerization, as revealed by XRD analysis. Permeation experimental results using a CO2 /N2 (50/50) mixture indicated that as an amount of POEM in a copolymer increased, CO2 permeability increased dramatically without the sacrifice of selectivity, approaching the “upper bound”. Therefore, if we prepare a copolymer of POEM with a polymer having much higher CO2 permeability than that of PVC, we can get the copolymer membrane to overcome the “upper bound”, which is in progress. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (R11-2007-050-04003-0) and by the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement in Technology (KIAT) through the Workforce Development Program in Strategic Technology. References [1] J.D. Figueroa, T. Fout, S. Plansynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology – The U.S. Department of Energy’s carbon sequestration program, Int. J. Greenhouse Gas Control 2 (2008) 9. [2] K. Ghosal, R.T. Chen, B.D. Freeman, W.H. Daly, I.I. Negulescu, Effects of basic substituents on gas sorption and permeation in polysulfone, Macromolecules 29 (1996) 4360. [3] A. Car, C. Stropnik, W. Yave, K.-V. Peinemann, PEG modified poly(amide-bethylene oxide) membranes for CO2 separation, J. Membr. Sci. 307 (2008) 88.

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