Semi-interpenetrating polymer network membranes based on a self-crosslinkable comb copolymer for CO2 capture

Accepted Manuscript Semi-interpenetrating polymer network membranes based on a self-crosslinkable comb copolymer for CO2 capture Na Un Kim, Byeong Ju Park, Min Su Park, Jung Tae Park, Jong Hak Kim PII: DOI: Reference:

S1385-8947(18)32113-2 https://doi.org/10.1016/j.cej.2018.10.152 CEJ 20215

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

4 September 2018 16 October 2018 19 October 2018

Please cite this article as: N.U. Kim, B.J. Park, M.S. Park, J.T. Park, J.H. Kim, Semi-interpenetrating polymer network membranes based on a self-crosslinkable comb copolymer for CO2 capture, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.10.152

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Semi-interpenetrating polymer network membranes based on a selfcrosslinkable comb copolymer for CO2 capture

Na Un Kim,a Byeong Ju Park,a Min Su Park,a Jung Tae Park,b Jong Hak Kima,*

a

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-gu, Seoul 03722, South Korea b

Department of Chemical Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-

gu, Seoul 05029, Korea

*To whom correspondence should be addressed Tel: +82-2-2123-5757; Fax: +82-2-312-6401 E-mail: [email protected]

1

Abstract High-performance, semi-interpenetrating polymer network (semi-IPN) membranes are prepared by incorporating a self-crosslinkable comb copolymer into the matrix of Pebax, a commercially available block copolymer with rigid polyamide blocks and soft polyethylene oxide blocks. The comb copolymer, poly(glycidyl methacrylate-g-polypropylene glycol)-copoly(oxyethylene methacrylate) (PGP-POEM) is synthesized via one-pot free-radical polymerization and undergoes epoxide-amine self-crosslinking reaction without any additional catalyst or thermal treatment. The structural, thermal, and mechanical properties as well as the gas-separation performance of the membrane are systematically investigated by varying the content of PGP-POEM in the Pebax matrix. As the PGP-POEM loading is increased, the CO2 permeability gradually increases without significant loss of CO2/N2 selectivity. The self-crosslinked PGP-POEM comb copolymer not only effectively degrades the crystalline structure of Pebax by disturbing the chain-packing but also provides numerous CO2-philic groups, resulting in both increased diffusivity and solubility of CO2. As compared to neat Pebax membrane, the semi-IPN membrane containing 40 wt% PGP-POEM exhibits approximately 2.5-fold enhancement in CO2 permeability (up to 246.6 Barrer) with similar CO2/N2 selectivity (38.8) as that of neat Pebax membrane. This work suggests that the semiIPN membrane based on a self-crosslinkable comb copolymer has great promise for application in CO2 capture owing to its high performance and simple preparation process.

Keyword: CO2 capture; comb copolymer; self-crosslinking; interpenetrating polymer network; membrane.

2

1. Introduction Greenhouse gases have caused global warming and rapid climate change and are one of the biggest problems currently faced by humanity [1]. Accordingly, various separation technologies such as absorption, adsorption, cryogenic distillation, and membrane separation have been developed to capture greenhouse gases such as CO2 [2-4]. Over the recent decades, studies on gas separation have focused on membranes owing to their many advantages such as low energy consumption, low operating costs, and small footprints [5, 6]. In particular, although polymeric membranes have many desired properties such as high processability and scalability in addition to being low cost, they suffer from a trade-off relationship between the selectivity and permeability [7, 8]. Poly(ethylene oxide) (PEO) has been considered one of the suitable polymeric materials for CO2 capture owing to its low cost and high CO2 solubility. Ethylene oxide units have specific and preferential affinity for CO2 molecules and they associate through dipolequadrupole interactions, leading to a high CO2 solubility of the membrane [9, 10]. However, neat PEO membranes have low gas permeability or structural defects, which results from the inherently high degree of crystallinity, which limits their application potential for CO2 capture [7]. Block copolymers are prepared by controlled polymerization of one monomer, followed by chain extension with a different monomer. Such polymers have facilitated the development of innovative concepts in various areas such as electronics, electrochemistry, nanomedicine, nanotechnology, and separation membranes owing to their well-defined nanostructure and microphase-separation. A membrane based on PEO-based block copolymer, poly(ether-block-amide) with the trade name of Pebax, has been attracting significant 3

attention in the field of CO2 capture owing to its excellent mechanical properties and separation performance [11-13]. Pebax is composed of two microphase-separated domains with different properties, viz., polyamide (PA) and polyethylene oxide (PEO) domains; whereas the rigid PA blocks provide good mechanical strength, the soft PEO blocks serve as the permeable phase. Pebax membranes are highly effective in separating CO2 from nonpolar gases such as N2, H2, and CH4, and the separation is based on the interaction between CO2 and PEO domains [14, 15]. Various methods have been widely reported for improving the performance of polymer membrane by using additives such as ionic liquids and/or inorganic fillers [16, 17]. Some researchers have developed mixed matrix membranes (MMMs) by introducing inorganic fillers including carbon nanotubes, graphene oxides , metal-organic frameworks, zeolites, and metal oxides into the Pebax matrix [18-22]. However, MMMs often suffer from non-selective defects at the interface between the inorganic fillers and polymer matrix, thereby resulting in decreased selectivity and long-term stability. Furthermore, the fabrication of their membranes in the form of a thin film layer (typically less than hundred nanometers) on porous supports, which is highly required for commercial applications, is difficult. Polymer blending is an attractive alternative for overcoming the drawbacks of the MMMs because it is simple, reproducible, and commercial viable [23]. A large variety of polymer blends with tailor-made properties can be produced by appropriately selecting the materials. Various blend membranes have been developed to improve the properties of Pebax-based membranes [24, 25]. Incorporating low-molecular-weight PEO can significantly increase the CO2 permeability of Pebax membrane [26, 27]. However, simple blending has often resulted in macroscopic phase separation of the polymers due to entropically 4

unfavorable long chains and thus mutual immiscibility. Interpenetrating polymer networks (IPNs) can enhance the compatibility between polymers by resisting the phase separation [28, 29]. IPNs are polymer structures with two or more physically interlaced networks without covalent bonding between or among them. In particular, a system in which one linear polymer is intertwined with another cross-linked polymer is referred to as a pseudo- or semi-IPNs [30]. Semi-IPN membranes are reported to exhibit better chemical resistance and anti-plasticization property compared to simple blend membranes [31, 32]. However, there have been no reports on IPNs or semi-IPNs membranes utilizing Pebax matrix. In this study, we report the preparation of a high-performance CO2-philic semi-IPN membrane by incorporating a self-crosslinkable poly(glycidyl methacrylate-g-polypropylene glycol)-co-poly(oxyethylene methacrylate) (PGP-POEM) copolymer into the Pebax matrix. The PGP-POEM comb copolymer was synthesized by free radical polymerization, which involves self-crosslinking of the polymers via the epoxide-amine reaction [33], to form a polymer network within the membrane. We expected the self-crosslinking nature of PGPPOEM to have a positive effect on the gas-separation performance of the membrane by substantially affecting the crystalline structure of Pebax. The Pebax/PGP-POEM semi-IPN membranes were fabricated by blending two polymers without any thermal treatment. The physical, chemical, and structural properties of the membranes were analyzed by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA), and the mechanical properties were evaluated using a universal testing machine (UTM). Pure gas separation properties of 5

the membranes were investigated using a time-lag method at 35 °C and 1 bar. The CO2 solubility and diffusivity were also evaluated using a permeation membrane analyzer (PMA) under the same conditions.

2. Experimental 2.1. Materials Glycidyl methacrylate (GMA), O-(2-aminopropyl)-O’-(2-methoxyethyl) polypropylene glycol (am-PPG, average Mn = 600 g mol-1), and poly(oxyethylene methacrylate) (POEM, poly(ethylene glycol) methyl ether methacrylate, Mn = 500 g mol-1) were purchased from Sigma-Aldrich. The initiator, 2,2’-azobis (2-methylpropionitrile) (AIBN, 98%) was purchased from Acros Organics. Pebax-1657 (40 wt% of PA and 60 wt% of PEO) was obtained from Arkema Inc. (Paris, France). Ethyl acetate, n-hexane, and ethanol were purchased from J. T. Baker. All the solvents were reagent grade and were used without further purification.

2.2. Synthesis of self-crosslinkable PGP-POEM comb copolymer The self-crosslinkable PGP-POEM comb copolymer was synthesized via one-pot free-radical polymerization [33]. First, 1 g of GMA, 1 g of POEM, and 4.2 g of am-PPG were dissolved in 6 mL of ethyl acetate containing 0.02 g of AIBN. The mixed solution was then heated in an oil bath at 70 °C for 18 h with magnetic stirring. After polymerization, the solution was poured into excess n-hexane with vigorous stirring to eliminate the unreacted monomers. This purification process was repeated thrice. The resultant comb copolymer was immediately dissolved in 70 vol.% ethanol/water to prevent further self-crosslinking. 6

2.3. Preparation of Pebax/PGP-POEM semi-IPN membranes The Pebax/PGP-POEM semi-IPN membranes were prepared via a solution casting method. A 10 wt% PGP-POEM solution was prepared by dissolving the copolymer in 70 vol.% ethanol/water at room temperature. Separately, a 10 wt% Pebax solution was prepared in the same solvent by stirring at 70 °C for 3 h. After that, the PGP-POEM and Pebax solutions were mixed to obtain 3 mL volumes of mixed solutions with different Pebax:PGP-POEM ratios (10:0, 8:2, 6:4, 4:6, 2:8) and stirred at 50 °C for 3 h. The prepared solutions were cast onto circular Teflon dishes and slowly dried at room temperature over three days. Then, the membranes were further dried at 50 °C in a vacuum oven to completely remove the residual solvents to obtain free-standing Pebax/PGP-POEM membranes with a thickness of approximately 70–80 μm. A pure PGP-POEM membrane could not be prepared owing to its poor mechanical strength.

2.4. Gas permeation measurements The pure gas permeation properties of the membranes were evaluated by a time-lag method using a constant volume/variable pressure apparatus (Airrane Co. Ltd., Korea), according to a previously reported procedure [34]. The downstream pressure was maintained at less than 2 Torr, which is much lower than the upstream pressure (760 Torr). Five replicates of each membrane were tested and the average estimated error of permeability was approximately ±5%. The gas permeability (P) was calculated using the following equation: 2

(1)

where, P is the gas permeability in Barrer (1 Barrer = 10-10 cm3 (STP) cm cm-2 s-1 cmHg-1), A 7

is the effective membrane area (cm2), V is the volume of the chamber (cm3), T is the experimental temperature (K), the membrane thickness (cm), and

is the transmembrane pressure difference (cmHg),

is

is the steady-state rate of pressure rise (mmHg/s)

in the downstream side. The CO2/N2 selectivity ( ) of the membrane was calculated by the ratio of two pure gas permeabilities under the same conditions: (2) In addition, the diffusivity and solubility coefficients were determined by a continuous-flow method using a permeation membrane analyzer (PMA) provided by SepraTek Inc. (South Korea) [35]. The parameters could be obtained by detecting an electrical signal, which is proportional to the amount of gas permeating through the membrane. All measurements were conducted at a constant temperature of 35 °C.

2.5. Characterization The molecular interactions between Pebax and PGP-POEM copolymer were identified by FT-IR spectroscopy (Spectrum 100, Perkin Elmer, USA) in the frequency range of 4000–500 cm-1. The thermal properties of the membranes were investigated by DSC (Discovery DSC, TA Instrument, USA) at the heating rate of 20 °C min−1 under nitrogen atmosphere. A second heating profile in the temperature range of −75 to 250 °C was used for characterization. The WAXS patterns of the membranes were acquired from a high-resolution X-ray diffractometer (SmartLab, Rigaku, Japan) using Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 40 mA. The diffractograms were collected in the 2θ range of 5–40° at a scanning speed of 2° min-1. SAXS data were obtained from the 4C SAXSII beamline at the Pohang Light Source (PLS), Korea. The surface and cross-sectional morphologies of the membranes were 8

characterized via FE-SEM (JSM-7001F, JEOL Ltd., Japan). The thermal stabilities of the membranes were analyzed by TGA (TGA/DSC1, Mettler Toledo, Korea) at the heating rate of 20 °C min−1 in air atmosphere. The tensile properties of the membranes were determined using a UTM (LR10KPlus Series) at a crosshead speed of 30 mm min-1. To confirm the cross-linking within the membrane, the gel contents of the semi-IPN membranes were determined. The films were immersed in an excess amount of ethanol/water at 70 °C for 3 days to extract the non-crosslinked portion. The remaining insoluble portion was dried at room temperature and then in a vacuum oven at 50 °C for 24 h. The gel fraction was calculated using the following equation: % gel fraction =

%

(3)

where m1 is the mass of the insoluble fraction after extraction and m0 is the original mass of the membrane.

3. Results and Discussion 3.1. Formation and interactions of semi-IPN membranes

9

Scheme 1. Chemical structures of (a) Pebax, (b) self-crosslinkable PGP-POEM comb copolymer and (c) schematic illustration of Pebax/PGP-POEM semi-IPN membrane structure. PA: polyamide, PEO: poly(ethylene oxide), PGP-POEM: poly(glycidyl methacrylate-gpolypropylene glycol)-co-poly(oxyethylene methacrylate).

The Pebax/PGP-POEM semi-IPN membranes were prepared by introducing a selfcrosslinkable PGP-POEM comb copolymer into the Pebax matrix (the structures are shown in Scheme 1). A Pebax block copolymer has a characteristic microphase-separated structure consisting of a hard PA phase and a soft PEO phase, which results in good mechanical stability and separation performance. The PGP-POEM is a comb or graft copolymer composed of GMA, PPG, and POEM chains. The copolymer has a distinctive network structure owing to its self-crosslinking properties and high CO2 solubility resulting from a large number of ethylene oxide units. Therefore, introducing PGP-POEM copolymer into the Pebax matrix to form a semi-IPN structure is expected to not only reduce the crystallinity of Pebax but also increase the CO2 solubility. We carefully designed the self-crosslinkable PGP-POEM comb copolymer with the expectation of simultaneously enhancing the CO2 separation performance and the thermal stability due to the following reasons. First, the strongly self-crosslinked network formed by the epoxide-amine reaction may increase the gas selectivity as well as the thermal stability. Second, the am-PPG chains possessing the amine groups that participate in self-crosslinking also have a number of propylene oxide groups with high CO2 affinity [36], which would enhance the CO2/N2 selectivity of the membrane. Third, the introduction of POEM units possessing ethylene oxide segments with high chain mobility could not only prevent 10

excessive densification of the polymer matrix, but also improve the CO2 solubility of the membranes. As shown in Fig. S1, all the Pebax-based semi-IPN membranes containing PGPPOEM comb copolymer were homogeneous and translucent. The transparency of the membrane indicates the degree of compatibility between the components. Although blending of two long-chain polymers is entropically disfavored, the membranes did not show macrolevel phase separation at any component ratio. The absence of macro-level phase separation indicates that the two polymers in the membrane are highly compatible, likely due to its semiIPN structure. The membrane with 80% PGP-POEM had sufficient mechanical strength to be a free-standing membrane, however, not strong enough to withstand high vacuum conditions required for the permeation tests. 1638

1096 1542

3299

1731

Absorbance

Pebax 20% 40% 60% 80% PGP-POEM

4000

3500

3000

2500

2000

1500

Wavenumber (cm

1000

500

)

-1

Fig. 1. FT-IR spectra of Pebax, PGP-POEM, and Pebax/PGP-POEM semi-IPN membranes with different PGP-POEM loadings (20–80%).

11

The intermolecular interaction between Pebax and PGP-POEM comb copolymer in the membranes was analyzed by FT-IR spectroscopy (Fig. 1). The characteristic absorption bands at 3299 and 1542 cm-1 in the FT-IR spectra of Pebax are assigned to the stretching and bending vibrations of the amide N–H groups of Pebax, respectively. The absorption band of the amide carbonyl group (N–H–C=O) appears at 1638 cm-1, while that of the O–C=O groups in both Pebax and PGP-POEM copolymer appear at ~1731 cm-1. Strong absorption bands between 1093 and 1096 cm-1 observed in the FT-IR spectra of all samples correspond to the ether groups (C–O–C) in both polymers [27]. No significant band shift is observed between the different spectra, which indicates their chemical similarity, demonstrating that the semiIPN of Pebax and PGP-POEM is a weakly interacting system [24]. To confirm the formation of a semi-IPN structure, the gel fraction of Pebax/PGPPOEM semi-IPN membranes was determined, as shown in Fig. S2. The presence of insoluble fractions in the membrane demonstrates the successful formation of crosslinked networks [37]. In semi-IPNs, the constituent linear polymers can be separated from the crosslinked polymer structure by extraction without breaking the chemical bonds. The gel content of each membrane was almost identical to that of the self-crosslinkable PGP-POEM copolymer in the blend, revealing that the cross-linked PGP-POEM comb copolymer physically interlocked the linear Pebax chains without forming chemical bonds with them, that is, a semi-IPN structure was successfully obtained.

3.2. Thermal, structural, and mechanical properties

12

-47.0 PGP-POEM

Heat Flow (W/g)

-62.1

8.4

-60.7

198.1

80%

199.8

60%

200.4

40%

201.4

20%

204.0

Pebax

9.2 -57.8 9.9 -56.2 11.2 -54.5 12.7

exo.

-50

0

50

100

150

200

250

Temperature (C)

Fig. 2. DSC thermograms of Pebax/PGP-POEM semi-IPN membranes with different PGPPOEM contents.

Table 1. Thermal properties and the degree of crystallinity (Xc) of Pebax/PGP-POEM semiIPN membranes obtained from DSC thermograms PGP-POEM loading (wt%)

Tg, PEO (°C)

Tm, PEO (°C)

Tm, PA (°C)

Xc, PEO (%)

Xc, PA (%)

0

−54.5

12.7

204.0

26.1

28.3

20

−56.2

11.2

201.4

19.4

21.8

40

−57.8

9.9

200.4

14.7

17.8

60

−60.7

9.2

199.8

8.5

10.2

80

−62.1

8.4

198.1

3.9

5.3

100

−47.0

-

-

-

-

DSC analysis was carried out to investigate the melting temperature (Tm) and glass transition temperature (Tg) of the Pebax/PGP-POEM semi-IPN membranes (Fig. 2). Strong endothermic peaks of neat Pebax at 12.7 and 204.0 °C are attributed to the melting of soft 13

PEO and hard PA crystalline phase, respectively [38]. The presence of two distinct Tm values demonstrates that neat Pebax has a well-defined microphase-separated morphology. The degree of crystallinity of each microphase i (Xc,i) can be calculated using the following equation: m

(4)

m

where,

m

indicates the heat of melting of microphase crystals determined by integrating

the areas under the corresponding melting peaks, and

m

is the weight fraction of the segment,

is the enthalpy of melting of the 100% crystalline phase.

m

O

and

m

are taken to be 166.4 and 230.0 J/g, respectively, from previous reports [39, 40]. As shown in Fig. 2 and Table 1, increasing the PGP-POEM content of the membrane resulted in a decrease in the Tm value of both the PEO and PA segments of Pebax, demonstrating that both PA and PEO microphases interacted effectively with the added PGPPOEM copolymer segments [41]. Furthermore, it is clear that the normalized area of the melting peaks of PEO and PA significantly decrease with increasing PGP-POEM content, implying reduced crystallinity of both microphases. Therefore, one can conclude that the PGP-POEM comb copolymer is highly compatible with the PA chains as well as PEO chains. The comb polymer acts as a spacer or plasticizer for the block copolymer, and it degrades the crystalline structure of Pebax into smaller crystallites. The self-crosslinking of the PGPPOEM copolymer generates a network structure within the membrane, leading to a relatively loose structure [42]. For example, the addition of 40% of PGP-POEM decreases the crystalline structure of neat Pebax by almost half. At 80% content, Xc of each microphase is lowered to 4.9 and 5.3%, indicating that the crystallites of Pebax almost disappeared and the membrane became almost amorphous. 14

Intensity (a.u.)

Pebax 20% 40% 60% 80% PGP-POEM

5

10

15

20

25

30

35

40

Two theta (degree)

Fig. 3. WAXS patterns of Pebax/PGP-POEM semi-IPN membranes with different PGPPOEM contents.

Additionally, a single Tg was observed for all membranes, which corresponds to the soft PEO phase. The Tg of the PA segment could not be observed because of its low chain mobility [25]. Neat Pebax and PGP-POEM exhibited Tg at −54.5 °C and −47 °C, respectively, which is consistent with the values reported previously [24, 33]. The presence of a single Tg indicates good miscibility and homogeneity between PGP-POEM and the PEO phase in Pebax. It is noteworthy that although the Tg of PGP-POEM comb copolymer is higher than that of Pebax, it shifted to a lower value as the PGP-POEM content increased. In general, the Tg of a blend membrane appears between those of the pure components, which could be predicted by well-known models such as the Fox equation and Gordon-Taylor equation [43, 44]. The higher Tg value of the neat PGP-POEM comb copolymer can be attributed to its selfcrosslinking nature, because crosslinking can decrease the chain mobility. However, the 15

crosslinking of PGP-POEM is relatively weak in the semi-IPN membranes owing to the interpenetrating Pebax chains, and thus, the comb copolymer can act as plasticizer leading to lower Tg values. The decrease in the Tg and crystallinity indicates an increase in the free volume and chain mobility of the membrane. This can affect the permeability of gas through the membranes by increasing both diffusivity and solubility, which will be discussed later. The results of WAXS (Fig. 3) indicate a change in the crystallinity of the membranes with the PGP-POEM content. The WAXS pattern of the PGP-POEM comb copolymer does not have any sharp crystalline peaks, indicating that it is completely amorphous. In contrast, the WAXS pattern of neat Pebax exhibits two strong crystalline peaks at 2θ = 20.0 and 24.1, corresponding to the crystalline domains of PA and PEO, respectively [45]. As the content of PGP-POEM increased, the intensity of these two peaks gradually decreased and the area of the amorphous peak increased. This indicates reduced crystallinity and increased amorphous domains in the membrane, which could be advantageous for gas transport through it. Therefore, it can be concluded that amorphous and self-crosslinkable PGP-POEM comb copolymer effectively destroys the crystalline structure of Pebax, which is in good agreement with the above DSC analysis. SAXS analysis was conducted to further investigate the microstructure of Pebax/PGP-POEM semi-IPN membranes (Fig. 4). The presence of the scattering maximum (qmax) is an important indicator of nanoscale phase separation between the soft PEO and rigid PA domains in the membrane. In addition, the mean correlation distance between the nanodomains can be calculated from the peak maximum using the Bragg relation, d = 2π/qmax [46]. The pristine Pebax membrane showed a broad peak with its maximum at q = 0.48 nm-1, suggesting a microphase-separated structure with a nanodomain size of 13.1 nm. As the 16

content of PGP-POEM increased, the position of the scattering peak maximum shifted to a higher q value, indicating a gradual decrease in the nanodomain distance. The decreased dspacing is presumably due to the crosslinked structure of the membranes with PGP-POEM as polymer chains pull on each other. Almost no characteristic peak was observed above 60% PGP-POEM in the membrane when Pebax becomes a minor component. This result is consistent with the DSC and WAXS data, suggesting that the microphase-separated structure of Pebax gradually collapses after the addition of the PGP-POEM comb copolymer.

log(I) (a.u.)

13.1nm 12.6nm 12.2nm

Pebax 20% 40% 60% 80%

0.1

-1

q (nm )

1

Fig. 4. SAXS profiles of Pebax/PGP-POEM semi-IPN membranes with different PGP-POEM contents.

17

Fig. 5. Cross-sectional (left) and surface (right) SEM images of Pebax/PGP-POEM semi-IPN membranes: (a, b) Pebax, (c, d) Pebax with 20% PGP-POEM, and (e, f) Pebax with 40% PGP-POEM.

The cross-sectional and surface morphologies of the membranes with different PGPPOEM contents were observed by SEM (Fig. 5 and Fig. S3 (high-magnification images)). The cross-sectional SEM images reveal that all the membranes have a homogeneous dense structure without obvious defects or aggregation, which confirms the excellent compatibility and miscibility between Pebax and the PGP-POEM comb copolymer. In addition, the neat Pebax membrane (Fig. 5b and Fig. S3b) shows a characteristic microphase-separated surface structure with some amount of nanofibril-like PA crystalline phase dispersed in an amorphous 18

PEO phase, which has been reported by other groups [25, 47]. With increasing PGP-POEM content, the crystalline PA domains gradually disappeared and the membranes became amorphous.

Pebax 20% 40% 60% 80% PGP-POEM

100

Weight (%)

80 60 40 20 0 100

200

300

400

500

600

700

Temperature (C) Fig. 6. TGA thermograms of Pebax/PGP-POEM semi-IPN membranes with different PGPPOEM contents.

The thermal stability of the Pebax/PGP-POEM semi-IPN membrane was characterized by TGA (Fig. 6). The thermal degradation temperature (Td), which is defined as the temperature at which 10% weight loss occurs, represents the beginning of the degradation of the polymer. With increasing PGP-POEM content, Td of the membrane gradually decreased from 360 °C corresponding to neat Pebax to 270 °C corresponding to neat PGPPOEM copolymer. This indicates that the thermal stability of the membrane is reduced by the addition of PGP-POEM. However, it is worth noting that all the Pebax/PGP-POEM semi-IPN 19

membranes are thermally stable up to 250 °C, which makes this membrane attractive even for high-temperature commercial applications.

140 Pebax 20% 40% 60% 80%

Stress (MPa)

120 100 80 60 40 20 0 0

200

400

600

800

1000

1200

Strain (%) Fig. 7. Stress-strain curves of Pebax/PGP-POEM semi-IPN membranes with different PGPPOEM contents.

Table 2. Mechanical properties of Pebax/PGP-POEM semi-IPN membranes obtained from stress-strain analysis PGP-POEM content [wt%]

Tensile stress (MPa)

Y ung’s m ulus (MPa)

Maximum elongation (%)

0

110.7

4.6

963

20

78.6

2.5

493

40

68.2

2.1

478

60

44.4

0.9

202

80

12.2

0.5

60

20

The mechanical properties of the Pebax/PGP-POEM semi-IPN membranes were characterized using a UTM and the results are shown in Fig. 7 and Table 2. The addition of PGP-POEM resulted in the decline of both the tensile modulus and strain of the membranes compared to those of the pristine Pebax membrane. The decrease in the mechanical strength is mainly due to the amorphous and rubbery nature of the PGP-POEM comb copolymer. The membrane with 80% PGP-POEM loading was too weak to endure the high vacuum used during the time-lag measurement; however, all other membranes maintained sufficient mechanical strength during the permeation test.

3.3. Gas separation performance

Table 3. Pure gas permeability and CO2/N2 selectivity of Pebax/PGP-POEM semi-IPN membranes at 35 °C and 1 bar. Permeability (Barrer) CO2

N2

Selectivity (CO2/N2)

0

106.1

2.8

37.5

20

188.5

4.8

39.3

40

236.6

6.1

38.8

60

271.8

9.3

29.2

PGP-POEM content [wt%]

Figure 8 and Table 3 present the pure gas permeability and CO2/N2 selectivity of the Pebax/PGP-POEM semi-IPN membranes as a function of the PGP-POEM content, which were determined by the time-lag method at 35 °C and 1 bar. The permeability of CO2 and N2 21

of the pristine Pebax membrane are 106.1 Barrer (1 Barrer = 10-10 cm3 (STP) cm cm-2 s-1 cmHg-1) with a selectivity of 37.5. Further, both the CO2 and N2 permeability increased with an increase in the content of PGP-POEM.

Permeability (Barrer)

(a) 1000

CO2 N2

100

10

1 0

20

40

60

PGP-POEM content (wt%)

Selectivity (CO2/N2)

(b)

50

40

30

20

10

0 0

20

40

60

PGP-POEM content (wt%)

Fig. 8. (a) Pure gas permeability and (b) CO2/N2 selectivity of Pebax/PGP-POEM semi-IPN membranes with different PGP-POEM contents at 35 °C and 1 bar.

The increment in the gas permeability is mainly due to the increase in the diffusivity through the membrane with increased PGP-POEM loading, as shown in Fig. 9 and Table 4. 22

PGP-POEM copolymer can act as a spacer at the interface of the PEO and PA crystalline phases owing to its good compatibility with both microphases, as confirmed by DSC analysis. Therefore, the comb copolymer chains can interfere with the interactions between the Pebax chains and reduce the rigidity of the chains [27]. Furthermore, semi-IPN structures could effectively reduce the chain regularity, thereby leading to more free volume in the membrane. Moreover, the addition of PGP-POEM can also increase the CO2 solubility of the membrane. The decline in the crystallinity of the PEO domain indicates that the impermeable and non-sorbing parts of the membrane are reduced, which can result in the increased gas solubility [40]. In addition, the large amount of ethylene oxide units in the PGP-POEM comb copolymer can enhance favorable interaction of the membrane with CO2 molecules. Note that the PGP-POEM comb copolymer showed higher CO2 uptake than neat Pebax under the same condition, as reported in our previous research [33].

8

2

-1

Diffusivity Solubility

-8

6

6 4

-1

CO2 diffusivity (x10 cm s )

10

CO2 solubility (x10 cm (STP) cm cmHg )

8

3

4 2

2

-3

0

0 20

40

60

-1

0

PGP-POEM content (wt%)

Fig. 9. CO2 diffusivity and CO2 solubility of Pebax/PGP-POEM semi-IPN membranes with different PGP-POEM contents. 23

Table 4. CO2 diffusivity and solubility of Pebax/PGP-POEM semi-IPN membranes with different PGP-POEM contents. PGP-POEM content [wt%]

CO2 diffusivity

CO2 solubility

CO2 permeability

(×10 )

(×10 )

(Barrer)

0

4.06

2.61

106.1

20

5.82

3.24

188.5

40

6.37

3.37

236.6

60

-8

-1

6.86

3.96

271.8

* Permeability (cm (STP)∙cm∙cm ∙s ∙cmHg ) = solubility (cm (STP)∙cm ∙cmHg-1)  3

-2 -1

-1

3

-3

diffusivity (cm2∙s-1). As shown in Fig. 8b, the Pebax/PGP-POEM semi-IPN membranes exhibit almost similar CO2/N2 selectivity as that of the neat Pebax membrane up to a PGP-POEM content of 40%, and then the selectivity decreased slightly. In general, the increase in the diffusivity leads to a decrease in the gas selectivity since this increment is higher for larger gas molecules [48]. In the case of Pebax/PGP-POEM semi-IPN membranes, the decrease in the selectivity can be compensated by the increase in the CO2 solubility. As a result, the CO2 permeability could be more than doubled without the loss of selectivity up to a PGP-POEM content of 40%. At higher PGP-POEM loadings, however, the effect of the diffusivity is dominant, resulting in a slight decrease in the gas selectivity. Figure 10 exhibits a Robeson plot showing the trade-off relationship between the selectivity and permeability of the polymer membrane [8]. The Pebax/PGP-POEM semi-IPN membranes exhibit better separation performance compared to neat Pebax membrane, which is closer to the upper limit. The best performance of the Pebax/PGP-POEM membrane was achieved at 40% loading of PGP-POEM, with CO2 permeability of 236.6 Barrer and CO2/N2 24

selectivity of 38.8. Therefore, it can be concluded that the introduction of the selfcrosslinkable PGP-POEM comb copolymer into the Pebax matrix to form a semi-IPN membrane is effective for CO2/N2 separation. The Pebax/PGP-POEM semi-IPN membranes show higher or comparable separation performance to those of Pebax-based MMMs reported previously [49-53]. Please note that the separation properties of the Pebax/PGP-POEM semiIPN membranes were measured at a standard condition (i.e. 35 °C, 1bar) while others used higher pressures or lower temperatures (Table S1). Furthermore, these previous reports [4650] were all based on the incorporation of inorganic fillers and thus the membranes might still suffer from high cost, difficulty in thin-film fabrication, the aggregation of the fillers and interfacial defects in large scale applications. Thus, we believe that the Pebax/PGP-POEM semi-IPN membranes are better suited for commercialization, considering their high performance, low cost and ease of fabrication.

10

3

Selectivity (CO2/N2)

Uppe

10

r Bou

nd (2

008)

2

20% 40% 60%

10

1

10

0

10

Pebax/ATP Pebax/MCM41 Pebax/ZIF-300 Pebax/NaX Pebax/ZIF-7 Pebax/PGP-POEM 0

10

1

10

2

10

3

CO2 permeability (Barrer) Fig. 10. CO2 permeability vs. CO2/N2 selectivity for Pebax/PGP-POEM semi-IPN 25

membranes prepared in this study and other Pebax-based membranes reported previously [49–53].

4. Conclusion We reported a new approach for the fabrication of a high-performance, semi-IPN membrane for CO2 separation by introducing a self-crosslinkable PGP-POEM comb copolymer into the Pebax matrix. Pebax-based membranes with different weight percentages of PGP-POEM copolymer were prepared by a solution casting method. The microphase structures, thermal and mechanical properties, and gas separation properties of these membranes were systematically studied. The PGP-POEM comb copolymer showed good miscibility with both PA and PEO microphases in the Pebax matrix, leading to a less crystalline structure of the membranes. In particular, the self-crosslinked PGP-POEM comb copolymer within the semiIPN membranes increased the chain mobility and free volume of the membrane by disturbing the chain packing of Pebax. As a result, the gas permeability of the membrane increased gradually with increasing PGP-POEM loading owing to the improved gas diffusivity through the membrane. Furthermore, the CO2 permeability could be improved without the loss of selectivity up to a PGP-POEM loading of 40%, which is also attributed to the increase in the CO2 solubility of the membrane. The best performance of the Pebax/PGP-POEM semi-IPN membrane was achieved at 40% loading of the PGP-POEM comb copolymer, with CO2 permeability of 246.6 Barrer and selectivity of 38.8, which represents higher or comparable separation performance to those of Pebax-based MMMs reported previously. The selectivity slightly decreased at 60% loading, but the membrane still had a comparable selectivity of 29.2 with CO2 permeability of 271.8 Barrer, which is a 256% increase over that of neat Pebax 26

(106.1 Barrer). This work suggests that blending Pebax with a self-crosslinkable copolymer can be a suitable approach to prepare high-performance CO2 separation membranes.

Acknowledgements This work was financially supported by a grant from the National Research Foundation (NRF) of South Korea funded by the Ministry of Science, ICT and Future Planning (NRF2017R1D1A1B06028030, NRF-2017M1A2A2043448, NRF-2017R1A4A1014569).

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32

Graphical Abstract

33

Research highlights

▶ Semi-IPN membranes were prepared using self-crosslinkable PGP-POEM comb copolymer. ▶ The PGP-POEM copolymer showed good miscibility with the Pebax matrix. ▶ Both diffusivity and solubility of CO2 increased with PGP-POEM loading. ▶ The best performance show higher or comparable separation performance to those of Pebax-based MMMs reported previously.

34