Effect of copolymer microphase-separated structures on the gas separation performance and aging properties of SBC-derived membranes

Journal of Membrane Science 529 (2017) 63–71

Contents lists available at ScienceDirect

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

Effect of copolymer microphase-separated structures on the gas separation performance and aging properties of SBC-derived membranes

MARK

⁎

Guo-Liang Zhuanga, Ming-Yen Weya, , Hui-Hsin Tsengb,c,⁎⁎ a b c

Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC School of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan, ROC Department of Occupational Medicine, Chung Shan Medical University Hospital, Taichung 402, Taiwan, ROC

A R T I C L E I N F O

A BS T RAC T

Keywords: SBC copolymer Microphase-separated morphology Gas separation Aging

Three styrene-butadiene copolymer (SBC)-derived membranes were prepared using different polystyrene/ polybutadiene (PS/PB) ratios, including a graft copolymer with 4 wt% PB (PS-4BR) and block copolymers with 55 and 70 wt% PB (PS-55BR and PS-70BR). The SBC-derived membranes were confirmed by Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA); the CO2/N2 separation performances of the membranes were studied under long-term operation. The SBC-derived membranes adopted different microphase-separated structures, including spherical, lamellar and cylindrical structures for PS-4BR, PS-55BR and PS-70BR, respectively. This result indicated that the CO2 separation performance of the SBC-derived membranes was affected by the CO2 solubility in the PB domain and the crystalline PS domain in the microphase-separated structure. The cylindrical PS-70PB membrane that contained a γ crystalline PS domain showed a high CO2 permeability of 50–60 Barrer, with a CO2/N2 selectivity of 15. Moreover, after 28 days, the cylindrical PS-70BR membrane showed greater stability in CO2 separation performance compared to the pure PS and PB membranes due to the high stability of the microphase-separated structure.

1. Introduction Carbon capture and storage (CCS) has become an important strategy for controlling the emission of greenhouse gases from large CO2-emitting industrial sources [1–3]. Compared to typical physical/ chemical sorption, adsorption [4,5], and cryogenic separation [6], membrane separation technologies are considered to be of tremendous importance for CO2 separation due to the expected advantages, which include high efficiency and selectivity for CO2 even under low partial pressure [7] and low cost of operation and maintenance [8]. To date, various materials have been rapidly developed as gas separation membranes over the past years [9–11]. Among them, organic polymer membranes have been recognized as the most promising membrane material because they offer advantages over inorganic materials, such as large scale industrial manufacture and cost-effectiveness. Rubbery polymers with low glass transition temperatures (Tg) are well known to demonstrate high CO2-selective solubility due to high molecular chain mobility and an excellent affinity for CO2. Examples of these polymers include poly(ethylene oxide) (PEO), natural rubber (NR), butadiene rubber (BR) and polydimethylsiloxane (PDMS). ⁎

However, most rubber materials are in a liquid-like state at room temperature, which is problematic for preparing a stabile membrane. Thus, many researchers have focused on exploiting CO2-selective rubber-based membranes by using processing techniques such as cross-linking or blending to improve the mechanical strength [12– 16]. However, several disadvantages of the above methods exist, including a requirement of compatibility between the two phases of a blended polymer [17,18], and a limit in the gas transport from the structure of cross-linked polymer membranes [16,19], which are limited in commercial availability. In general, the gas transport performance of a membrane depends on its morphological properties [20,21]. Panapitiya et al. [22] studied the morphological properties of a diphthalic anhydride (6FDD)/polybenzimidazole (PBI) (50/50) blend polymer membrane. This study revealed an inconsistent size in the 6FDD dispersed phase within the PBI matrix (continuous phase) due to immiscibility of the polymer blends, which prevented improvement of the gas separation performance of the blend membrane. Based on this result, the structures of blend polymer membranes must be further improved by modifications that include the incorporation of compatibilizer [23,24] and the coordination templates [25]. George et al. [26]

Corresponding author at: Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC. Corresponding author at: Department of Occupational Safety and Health, Chung Shan Medical University, Taichung 402, Taiwan, ROC. E-mail addresses: [email protected] (M.-Y. Wey), [email protected] (H.-H. Tseng).

⁎⁎

http://dx.doi.org/10.1016/j.memsci.2017.01.060 Received 27 December 2016; Received in revised form 27 January 2017; Accepted 28 January 2017 Available online 03 February 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

Table 1 A list of copolymer membrane performance for CO2/N2 separation. Copolymer membrane

Permeability (Barrer)

Poly(styrene-b-butadiene-b-styrene) (SBS) Polystyrene-b-pol(ethylene oxide) (PS-b-PEO) Polycarbonate Z-r-poly(ethylene glycol) (PCZ-r-PEG) Polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SEBS) Poly(ethyleneglycol) behenyl ether methacrylate-g- poly(oxyethylene methacrylate) (PEGBEM-g-POEM) comb copolymer Polystyrene-b-poly(ethylene oxide) diblock/polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer (SO/SOS) P(PEGMA-co-DEAEMA-co-MMA)

CO2/N2 selectivity

Thickness (μm)

Ref.

CO2

N2

237–289 21.8 50.7 170.6 43.8

5.8–13 0.6 1.2 13.8 0.5

20–50 36.3 42.25 12.4 84.7

66–120 45–50 80 – 1.5–2.3

[27] [33] [34] [35] [36]

970–996

–

21.3–59.8

55–190

[37]

308

8.1

38

30–40

[38]

CO2 plasticization and maintaining high CO2 separation. Wijayasekara et al. [37] examined the long-term stability of an ionic liquid composite membrane based on polystyrene-b-poly(ethylene oxide) diblock/polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer (SO/ SOS) for CO2 separation. The CO2 permeation performance of the membrane was consistent after 28 days and 16 applications, even with the presence of PS. In general, copolymer membranes with microphase-separated structures of rubbery and glassy polymer segments have significant potential for CO2 separation. However, to our knowledge, there have been no reports on the effect of the micro-structure of copolymer membranes on the durability of gas separation performance. In this work, we compared SBC-derived membranes with different PS/PB ratios, including a graft copolymer–high impact polystyrene (HIPS, 96/ 4)–and two block copolymers: styrene-butadiene-styrene (SBS, 45/55 and 30/70). The copolymers were directly coated onto α-Al2O3 substrate to fabricate composite membranes. Fig. 1 summarizes the fracture mechanisms of the microphase-separated structure during membrane performance. Unique microphase-separated morphologies were found in the SBC-derived membranes with different PS/PB ratios. The structures can provide high thermal stability and a favorable diffusion pathway, which is expected to result in better gas separation property. The gas transport properties of the membranes were tested by the time-lag method using a single gas permeation system. High gas separation can be achieved from the synergistic effects of the microphase-separated structure. To further examine the effect of aging on the membrane performance, the CO2/N2 separation performance of the SBC-derived membranes were evaluated over a period of 28 days, and the influence of the microphase-separated structure on aging was examined through analysis of the structural properties.

reported the effect of the level of cross-linking on the gas separation performance, with an enhancement of cross-link density increasing the selectivity but decreasing the permeability. Thus, competing effects limit the efficacy of cross-linked polymer membranes. Furthermore, the cross-linking process needs to control the addition of cross-linker as well as reaction conditions, which would increase the cost and difficulty of the manufacturing process. Recently, the application of a copolymer in gas membrane fabrication has gained attention as a CO2-selective separation membrane, as shown in Table 1. A copolymer is a polymer made up of two monomers that cluster together to form 'block', 'random', 'alternating' or 'graft' repeating units, which is widely used to modify the properties of a manufactured polymer to meet specific needs, such as crystallinity or glass transition temperature, or to improve solubility. In a previous study, a copolymer that consisted of soft segments (rubbery domain) and hard segments (glassy domain) was directly made into a membrane by either pressing or solution casting [27,28]. Copolymers can spontaneously form microphase-separated structures with mirco- or nanoscale in both phases due to the incompatible chemical components [29]. The microphase-separated structures include the spherical, cylinders, bicontinuous and lamellar structure, which are controlled by the annealing process in bulk [30,31] or the composition of concentrated solution (the ratio of components in a copolymer chain, solvent and concentration) [27,32]. The various structures in membrane would play an important role in gas separation applications. Buonomenna et al. [27] investigated the CO2 separation performance of a block copolymer membrane based on poly(styrene-b-butadiene-bstyrene) (SBS, 72–79% butadiene). An SBS membrane that was derived from a 20 wt% casting solution in toluene showed the best results with a CO2/N2 selectivity of 20–50, and a of CO2 permeability of 237–289 Barrer that resulted from microphase separation of the hard and soft segments. Minelli et al. [33] prepared and characterized a membrane made of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) that had a lamellar morphology with a domain spacing (d) of 96 nm. The CO2 permeability and selectivity for CO2/N2 at 30 °C was 21.8 Barrer and 36.5, respectively. Patel et al. [34] compared pure polycarbonate Z (PCZ) with a copolymer containing 80% polyethylene glycol (PEG, PCZ-r-PEG). The PCZ-r-PEG copolymer exhibited individual homophases due to partial miscibility between the two polymer segments, which imparted not only high CO2 separation performance but also increased both the thermal stability and the mechanical properties of the membrane. Thus, a copolymer membrane can obtain high gas separation performance by combining glassy and rubber polymers. The durability of a membrane is an important property for its practical application. In general, the glassy polymer membrane must overcome the following negative effects on gas separation performance: (1) CO2 plasticization at high operating pressure [39–41] and (2) aging behavior in long-term operation [42–45]. In our previous work, we determined the plasticization behavior by comparing pure PS to SBC (high impact polystyrene (HIPS) with a PS/PB ratio of 96/4) [46]. The trace of butadiene rubber in the PS matrix was beneficial for inhibiting

2. Experimental 2.1. Materials and general procedures As shown in Table 2, the prepared pure and copolymer membranes that were investigated include: pure polymer (polystyrene (PS) and polybutadiene (PB)), high impact polystyrene (HIPS) (PS/PB =96/4, PS-4BR) and styrene-black-butadiene (SBS) (PS/PB =45/55 and 30/ 70, PS-55BR and PS-70BR), which were purchased from SigmaAldrich. The above polymers were added into toluene and stirred continually at 50 °C for 12 h. The concentration of the casting solutions was controlled at 21 wt% for PS, PB and HIPS (a concentration of 11 wt % was used for SBS to match the viscosity). Thereafter, the solution was degassed for 6 h. After complete degassing, the solution was coated on the surface of a porous Al2O3 disk (support) 23 mm in diameter (Ghana Fine Ceramics, Taiwan) by spin coating with a spinning speed of 2400 rpm for 16 s to obtain a mechanically stabile polymer membrane that was then dried for 24 h at 120 °C in a vacuum oven. 64

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

Fig. 1. Schematic illustration of the fracture mechanisms of SBC-derived membranes for CO2 separation.

porous α-Al2O3 substrate and a thin selective layer on top, the thickness of the selective layer is 11.6 µm. The microphase-separated structures of the membranes were analyzed by microscopy (AFM; Bruker Dimension Icon) and X-ray diffraction (XRD, M18XHF, Mac Science Company). AFM surface images and roughness were simultaneously collected in a 1 µm scan box. The XRD diffraction spectra were obtained using CuK source (λ=1.54 Å) operating in a 2θ range of 5‒40° with scan rate of 2°/min. The thickness of the prepared membranes were measured by the observation of SEM cross-section image. The mechanical strength of the membranes were determined by three-point bending strength method with universal testing machine (tensile tester, Ju-Yen Machinery Co., Ltd.). The bending strength (σb) of flat-sheet sample is calculated using the following equation:

Table 2 Composition and decomposition temperature (Td) of the polymers used in this study. Types

Sample

Styrene/butadiene weight ratio

Pure polymer

Graft copolymer Block copolymer

PS PB PS-4BR PS-55BR PS-70BR

100/0 0/100 96/4 45/55 30/70

Decomposition temperature (Td, °C) 424.13 477.07 439.11 456.49 472.63

2.2. Spectroscopic and thermal analysis The chemical structures and compositions of the samples were analyzed by attenuated total reflectance-Fourier transforms infrared spectroscopy (ATR-FTIR, JASCO-4100 spectrophotometer). The thermal stability and relative crystallinity of the samples was determined using thermal analysis instrument (Perkin Elmer, STA 6000). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were simultaneously run and operated under a continuous N2 flow (20 ml/min) with a heating rate of 10 °C/min from 50 to 650 °C.

σb =

3FL 2ωl 2

(1)

where σb is bending strength, F the fracture force, L the membrane support span length (7 mm), ω the membrane support diameter (16 mm) and l the membrane support thickness.

2.4. Gas permeation measurements 2.3. Morphological measurement

2.4.1. Single gas permeation test The gas permeability of the pure polymer and copolymer membranes was evaluated through single gas permeation tests using labmade circular cells with stainless steel 47 mm diameter membrane discs and the constant-volume method (time-lag method). The constant-volume method was conducted at 30 °C and 1 bar for five representative gases, CO2, O2, N2 and CH4, and was defined by the following equation:

The cross-sectional image and the thickness of the membranes were characterized by scanning electron microscopy (SEM, model JEOL JSM-6700F). The membranes were fractured in liquid nitrogen and sputter-coated with gold before analyzed. Fig. 2 demonstrates typical micrographs of the cross section of the PS-77BR membrane. The membrane shows an anisotropic structure consisting of a thick micro-

⎛ dp ⎞ 273 Vl Pi = ⎜ ⎟ × × ⎝ dt ⎠ 76 ATΔp

(2)

where Pi is the gas permeability (1Barrer =10−10 (cm3(STP) cm/s−1 cm−2 cmHg−1) of component i, V is the downstream volume (cm3), l and A are the film thickness (μm) and the effective area of membranes (0.785 cm2), respectively, T is the temperature (K), and Δp is the pressure difference between the feed and permeate sides (cmHg). The ideal selectivity (αij) of membranes is defined as the ratio of the individual gas permeabilities (Pi and Pj) and can be calculated using the following equation:

αi / j = Fig. 2. The cross-sectional image of PS-70BR.

65

Pi Pj

(3)

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

Fig. 4. DTG curves for the pure PS and PB polymers and the copolymers with different PS/PB ratios.

Fig. 3. FT-IR spectra of the pure PS and PB polymers and the copolymers with different PS/PB ratios.

polymer.

2.4.2. Long-term aging test The long-term aging characteristics of the membranes with different PS/PB ratios were tested over 28 days. To avoid the possible effect of atmospheric moisture on the membrane properties, the samples were stored at 27 °C and below 50% relative humidity in a test chamber before testing.

3.2. Morphological structure properties of the membranes AFM and XRD studies were performed to characterize the microphase-separated structural properties of the PS, PB and SBC-derived membranes. The surface topographies of the membranes were observed in a 1 µm×1 µm 3-D AFM scan (Fig. 5). The roughness parameters, Ra, for the surfaces of the membranes are listed in Table 3. The pure PS and PB polymers exhibited smoother surfaces than the SBCs. This observation could be due to the occurrence of microphase separation in the copolymers. Different microphase-separated structures were detected on the surfaces of the copolymer membranes with varying PS/PB ratios. As illustrated in Fig. 5(b), a minor component composition of 4 wt% PB promoted the formation of cubic lattice spheres of PB in a continuous-phase PS matrix. Meanwhile, the 55 wt% PB composition contained lamellae parallel to the membrane surface (Fig. 5(c)). An increase in the PB component composition to 70 wt% (Fig. 5(d)) caused the minor-phase PS to adopt a cylindrical structure within the continuous-phase PB matrix. The microphase separation properties of the copolymer membranes were further analyzed by XRD analysis. Fig. 6 presents the XRD patterns of the pure polymer (PS and PB) and copolymer (PS-4BR, PS55BR and PS-70BR) membranes. In the case of the pure polymer membranes, the PS membrane exhibited typical amorphous X-ray diffraction curves with two characteristic peaks at 2θ=9.47° and 19.39°. The first and second peaks have d-spacing values of 9.33 Å and 4.57 Å, respectively, which are consistent with those reported by Murhty et al. [52] The PB membrane showed a single amorphous peak at 19.16° with a d-spacing of 4.63 Å. The X-ray diffraction curve of the PS-4BR membrane (PS/PB =96/4) is similar with that of the PS membrane. With an increase in the PS/PB ratio to 45/55, the broadest and highest diffraction peak was observed of all the membranes. The reason is due to an overlap between the diffraction peaks of PS and PB. The PS-70BR membrane showed an amorphous diffraction peak at 19.24° resulting from the continuous major-phase PB matrix. In addition, PS-55BR showed a low-angle peak at 6.68° for the α crystalline phase of the PS domain (d-spacing = ca. 13.22 Å), and PS-70BR showed two diffraction peaks at 9.42° and 28.61°for the γ crystalline phase of the PS domain (d-spacing = ca. 9.38 Å and 3.12 Å) [53–56]. The crystallinity degrees of PS, PB and copolymer membranes were further measured by DSC analysis. As shown in Table 4, the similar results were obtained compared to the XRD result. The relative crystallinity of pure PS membrane (0.17%) is very low and smaller than that of block copolymer membranes (1.29–7.34%). In general, the crystalline property of the glassy PS polymer depended on the

3. Results and discussion 3.1. Spectroscopic and thermal analyses Fig. 3 depicts the FT-IR spectra of the pure polymers (PS and PB) and the copolymers (PS-4BR, PS-55BR and PS-70BR). The FT-IR spectrum of PS contained absorption bands at 1601, 1493 and 1451 cm−1 due to the stretching vibrations of carbon (v(C=C), vC-H) in the aromatic ring. The absorption bands at 1028 and 905 cm−1 are assigned to the out-of-plane hydrogen atom from the aromatic ring [47–50]. The FT-IR spectrum of the PB polymer showed an absorption band at 1654 cm-1 that is assigned to the stretch vibrations v(C=C) in the cis-PB group. The peak at 1451 cm-1 is assigned to the vinyl-PB CH2=CH group. In the region of 900–1000 cm−1, the infrared peaks for C-H bending of PB are located at 993, 966 and 911 cm−1; these peaks correspond to cis-alkene, trans-alkene and terminal-alkene, respectively [48]. The copolymer contains these same features, which is consistent with the literature [51]. Thus, the PB segment in SBC contains a trans-alkene whereas a cis-alkene is present in pure PB. The FT-IR spectra of the PS-b-PB copolymers exhibited regular changes with increasing PB content in which the infrared peaks for PS decreased and those for PB increased. With the increase in the PB content of the SBC polymer, the band at 966 cm−1 that is attributed to the trans-alkene of the PB segment increased significantly. In contrast, the band at 993 cm−1 that corresponds to the cis-alkene of PB increased only slightly. The derivative thermogravimetric (DTG) curves obtained from the TGA analysis of the PS, PB and SBC polymers containing 4, 55, and 70 mol% of PB are represented in Fig. 4. The decomposition temperature (Td) of the polymers was determined from the temperature of maximum degradation in the derivative curves, and the values are summarized in Table 2. The Td value of the PS polymer (424.13 °C) was lower than that of the PB polymer (477.07 °C). The Td values of the PS-xPB copolymers (x=4%, 55% and 70%) were 439.11, 456.49 and 472.63 °C, respectively. The introduction of the PB segment within the SBC backbone clearly increased the Td of the copolymer. According to the TGA results, the thermal stability of PB is higher than that of PS. Moreover, the copolymer synthesized by copolymerization of PS and PB exhibited a moderate thermal stability compared to the pure 66

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

Fig. 5. AFM micrographs of (a) PS, (b) PS-4BR, (c) PS-55BR, (d) PS-70BR and (e) PB.

arrangement of the molecular structure [56]. The amorphous and crystalline PS are defined by atactic (no regular order) and syndiotactic structures (regular order). Moreover, the crystallinity of glassy polymers is often affected by the molecular weight [57]. Therefore, the low molecular weight PS segments in the copolymer could easily form a high order structure in the matrix compared to the pure PS polymer with a high molecular weight. According to the AFM results, the molecular assembly of both block copolymer membranes contained regions of a lamellar-like pattern in the matrix. In contrast, the PS membrane did not exhibit any crystallinity. Based on the results of the AFM and XRD analyses, the three copolymers exhibited different microphase-separated structures. Moreover, two of the block copolymers (PS-55BR and PS-70BR) contained crystalline regions in the PS domain. Molecular structure and polymer composition are known to affect the gas transport mechanism. As a result, the gas permeation performance of the copolymers was investigated. Table 5 lists the bending strength of the PS, PB, and copolymer membranes with different PS/PB ratio. The results indicate that the membranes have microphase-separated structure can improve the bending strength of PS and PB membranes. The reason is that there are some crystalline phases formed in the PS domain homogeneously, which resulted in the rigid structure in the copolymer membrane. Therefore, more energy is needed to break down the microphaseseparated structure. Consequently, the mechanism strength of copolymer is improved.

Table 3 Roughness (Ra), micro-structure and PS crystalline type of the prepared membranes. Membrane

Roughness (Ra, nm)

Micro-structure

PS crystalline type

PS PS-4BR PS-55BR PS-70BR PB

0.25 1.21 2.10 1.59 0.81

Amorphous Sphere Lamellar Cylinder Amorphous

– – α form γ form –

3.3. Influence of the PB content on the permeation properties

Fig. 6. X-ray diffraction of the pure polymers (PS and PB) and the copolymers (PS-4BR, PS-55BR and PS-70BR).

3.3.1. Gas transport mechanism Fig. 7 presents the relationship between the gas permeability and the gas critical temperature for the PS and PB polymer membranes and the SBC-derived membranes. The PB and SBC-derived membranes displayed a greater correlation between the permeability and gas

Table 4 DSC results obtained for pure PS and PB and PSPB membranes. Membrane

Heat of fusion (ΔHf, J/g)

Relative crystallinity (%)a

PS PS-4BR PS-55BR PS-70BR PB

0.14 0.25 1.07 6.07 –

0.17 0.30 1.29 7.34 –

Table 5 Bending strength of PS, PB, and copolymer prepared in this study.

a The relative crystallinity (%) is ΔHf /ΔH f0 × 100%; the theoretical heat of fusion (ΔHf0) computed for 100% crystalline PS is 82.6 J/g [58].

67

Membrane

Bending strength (MPa)

Membrane

Bending strength (MPa)

PS without support PS PS-4BR

0.91 1.19 1.19

PS-55BR PS-70BR PB

1.69 1.67 1.28

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

Fig. 8. CO2/N2 separation performances of the membranes with different PS/PB ratios.

Fig. 7. Relationship between gas permeability and the gas critical temperature.

[59]. The PS-55BR and PS-70BR membranes have lamellar and cylindrical microphase structures, respectively. They contain a PS crystal phase, which could affect gas transport due to the stable and regular molecular structure of the PS domain. The kinetic diameters of CO2 and N2 are 3.3 Å and 3.6 Å, respectively, which are able to permeate the PS-55BR membrane because of the bigger d-spacing of 13.22 Å in the α crystalline phase of the PS domain. Thus, the gas transport would increase by the diffusion pathway, resulting in an increase in the gas permeability further within the PS-55PB membrane. Moreover, the PB domain still offers the effective soluble diffusion of CO2. As a result, the CO2 permeability is significantly increased compared to the N2 permeability and the CO2/N2 selectivity is simultaneously enhanced. Thus, the PS-55BR membrane exhibits synergistic effects on the separation performance by (1) CO2 affinity in the PB domain and (2) the gas diffusion pathway in PS domain. A similar result was also observed for the PS-70BR membrane, which contains a γ crystalline phase. The two d-spacing of 9.38 Å and 3.12 Å size were formed in the γ crystalline phase of the PS domain. The former is higher than the kinetic diameter of CO2 (3.3 Å) and N2 (3.6 Å), leading to an increase in the gas permeability. However, the latter is smaller than the kinetic diameter of CO2 and N2, leading to a decrease in the gas permeability, especially of N2 gas. Further, CO2 has high critical temperature also can pass through PS domain by solution mechanism It's mean that the PS domain with γ crystalline phase can restrict the N2 transport but CO2 transport. Thus, there are not significantly affect the CO2 permeability. According to the above results, the PB and PS domain in copolymer can improve the CO2 permeability and decrease N2 permeability, respectively, leading to achieve a high CO2/N2 selectivity for the PS-70BR membrane.

critical temperature than the PS membrane. Moreover, the R-squared (R2) value increased with an increase in the PB content of the SBC. The enhanced relationship between the gas permeability and gas critical temperature is attributed to the rubbery PB segments in the matrix. In general, the gas with high critical temperature is easy to transport in rubbery polymer membrane via solution-diffusion mechanism due to the high solubility property [16]. Moreover, the critical temperature of CO2 (304.19 K) is higher than that of N2 (126.2 K). Therefore, the SBCderived membranes would be capable of transporting CO2 gas through the PB segment, resulting in a high CO2/N2 separation performance.

3.3.2. CO2/N2 separation performance The effect of the PB content on the CO2/N2 separation performance was investigated, as illustrated in Table 6 and Fig. 8. The results indicated that the separation performance of the PB membrane (PCO2 =49.75 Barrer; αCO2/N2 =10.69) was better than that of the PS membrane (PCO2 =18.34 Barrer; αCO2/N2 =5.24). The SBC-derived membranes also demonstrated higher separation performances than the PS membrane. As predicted in Section 3.3.1, the presence of the PB segment can effectively increase the CO2 permeability via solutiondiffusion mechanism, resulting in better CO2/N2 separation performance compared to the homopolymer-derived membranes (PS and PB). The SBC-derived membranes exhibited higher performances than the pure PB membrane. Furthermore, the SBC-derived membranes demonstrated a significant increase in the CO2 permeability as the PB content was increased in addition to a slight enhancement in the N2 permeability compared to the PS membrane. The CO2 permeability increased to 86.23 Barrer with a 70 wt% PB content in the matrix. Compared to CO2, the N2 permeability decrease from 7.67 to 5.51 Barrier upon increasing the PB content from 4 to 70 wt%. The irregular change in the gas permeability was due to the morphological structure properties of the membranes. As discussed in Section 3.2., the SBC-derived membranes contained unique microphase-separated structures in the matrix that could affect gas transportation. The PS-4BR polymer with a spherical microphase structure has a rough surface that can enhance the gas permeability

3.4. Aging properties and measurement Fig. 9 depicts the relative gas permeability versus the operating time from the 1st day to the 28th day for the pure and copolymer membranes. As illustrated in Fig. 9(a), CO2 permeability increased after 28 days for the membranes with the lowest PB content (PS and PS-4BR) and decreased for the membranes with the highest PB content (PS-55BR, PS-70BR and PB). The N2 permeability also exhibited a similar trend, as shown in Fig. 9(b). Based on this result, the change in gas permeability upon aging depends on whether the PS or PB polymer is the dominant matrix. In general, the aging behavior of the polymer membranes can be characterized by two terms: (1) plasticization, which generality occurs in the glassy polymer membrane. The rigid backbone of the intrapolymer chain can be plasticized, resulting in an increase in the free volume [60–62]. This case would increase the gas permeability of the membrane after aging, particularly for the CO2 permeation process [46]. CO2 induces the plasticization of the glassy polymer because of the high gas critical temperature; (2) densification, which is an adverse effect of the aging process on the polymer membrane. As a result of

Table 6 CO2/N2 separation performances of the prepared membranes. Membrane

PS PS-4BR PS-55BR PS-70BR PB

Gas permeability (Barrer) PCO2

PN2

18.34 58.85 64.39 86.23 49.75

3.50 7.67 6.83 5.51 4.6

CO2/N2 selectivity (α)

5.24 7.67 9.43 15.65 10.69

Relative crystallinity

68

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

Fig. 11. CO2/N2 selectivity of the membranes with different PS/PB ratios from the 1st day (closed point) to the 28th day (open point).

(2θ=19.16°) decreased slightly and broadened as a result of an increase in the randomness of the amorphous phase after aging. The same result was observed for both the PS-55BR and PS-70BR membranes (Fig. 10(c) and (d)). Moreover, the linear PB polymers without side chains have smaller rotation dimensions compared to the PS polymer. Thus, the aged PB matrix would enhance the gas transport resistance due to the formation of a dense molecular structure, resulting in a decrease in the gas permeability of the PB membrane and the SBCderived membranes with high PB contents. Fig. 11 illustrates the change in the CO2/N2 separation performance of the membranes with different PS/PB ratios from the 1st day to the 28th day. The increase in the gas permeability of the membranes with high glassy PS contents is due to plasticization, whereas a decrease in the gas permeability of the membranes with high rubbery PB contents is due to the densification. However, the CO2/N2 selectivity significantly increased for the membranes with high PB content (PB, PB55BR and PB-70BR). This result is consistent with previous literature [16]. Because the molecular size of N2 is greater than that of CO2, N2 would have more transport resistance in the densified membrane compared to CO2. Therefore, the CO2/N2 selectivity would increase after long-term operation. Conversely, Fig. 11 illustrates that the gas separation performance of the PS-70BR membrane changed only slightly after long-term aging compared to the other copolymer membranes. The stable separation properties of PS-70BR can be attributed to the microphase-separated structure in the matrix. Fig. 10(d) reveals that the XRD curve of aged PS-70BR changed only slightly, and the characteristic peak for the γ crystalline phase in the PS domain was maintained. Moreover, PS70BR contains a cylindrical micro-structure within the matrix, as described in Section 3.2. This suggests that the cylindrical microstructure of PS-70BR is responsible for the high aging resistance. Table 7 tabulates the different polymer precursor-derived membrane and their effect on membrane aging properties and performance efficiency. In general, the glassy polymer and rubbery polymer occurred the aging behavior, resulting in the decline of gas permeability [16,43,66]. Thus, there are some study to prevent or retard the aging behavior for gas separation performance. For example, Kim et al. [67] treated the polyimide (PI) membrane by crosslinking. The result showed that there are a decrease in the decline of CO2 permeability from 92% to 80% over 40–41 day. Recently, the polymers of intrinsic microporosit (PIM) is as the novel material for gas separation membrane because it has the high porous to exhibit the higher gas permeability than general polymer membrane. However, PIM membranes also have the serious decline for gas separation performance after aging. The main reason is that the occurrence of polymer chains relax with aging time reduce the free volume of gas diffusion, resulting in an increase in the restriction of gas transport in matrix [62]. Mitra et al. [68] used the cost-effective hyper-crosslinked nanoparticle fillers to improve the negative effect of aging. This method can slow down the decline of CO2 permeability from 58.5% to 47.8% over 150 days.

Fig. 9. The relative permeability of (a) CO2 and (b) N2 versus operating time for PS, PS4BR, PS-55BR, PS-70BE and PB at 25 °C.

Fig. 10. X-ray diffraction of the (a) PS, (b) PS-4BR, (c) PS-55BR, (d) PS-70BR and (e) PB membranes after the 28th day.

densification, the free volume of the membrane decreases with aging, resulting in a decrease in the gas permeability [16,63–65]. Fig. 10 depicts the XRD patterns of the pure polymer (PS and PB) and copolymer (PS-4BR, PS-55BR and PS-70BR) membranes after the 28th day. Fig. 10(a) reveals that in the aged PS membrane, one characteristic peak (2θ=9.47°) increased while the other characteristic peak (2θ=19.39°) decreased. This indicates that the packing of the PS polymer chains is influenced by aging, likely due to plasticization. The glassy PS polymer chains become softer from the swelling of the polymer matrix, resulting in an increase in the intermolecular spaces. The same result was also observed for the PS-4BR membrane, as shown in Fig. 10(b). Thus, the aged PS and PS-4BR membranes showed higher gas permeability. A similar result was observed for the PB membrane as the PS membrane (Fig. 10(e)). The characteristic peak 69

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

MY3).

Table 7 Comparison of our result with other investigation in literatures. Membrane

Aging period (day)

Change trend

Ref.

PS

30–40

[43]

PIa

280

Crosslinked PIa

40–41

NRa

188

PIMa

15

HCP/PIM with ethanol treateda

150

SBC-derived membrane

28

O2 (−20–25%) N2 (−18–20%) O2/N2 (similar) CO2 (−30.8%) N2 (−40%) CO2/N2 (+13.7%) Untreated: CO2 (−92%) Treated: CO2 (−80%) CO2 (−22.8%) N2 (−40.2%) CO2/N2 (+29.2%) O2 (−76.3%) N2 (−79.5%) O2/N2 (+13.6%) Untreated: CO2 (−58.5%) Treated: CO2 (−47.8%) CO2 (−10.8%) N2 (−3.6%) CO2/N2 (−7.5%)

References [1] M. Songolzadeh, M. Soleimani, M. Takht Ravanchi, R. Songolzadeh, Carbon dioxide separation from flue gases: a technological review emphasizing reduction in greenhouse gas emissions, Sci. World J. 2014 (2014) 34. [2] Y. Tan, W. Nookuea, H. Li, E. Thorin, J. Yan, Property impacts on carbon capture and storage (CCS) processes: a review, Energy Convers. Manag. 118 (2016) 204–222. [3] M. Zaman, J.H. Lee, Carbon capture from stationary power generation sources: a review of the current status of the technologies, Korean J. Chem. Eng. 30 (2013) 1497–1526. [4] P. Luis, Use of monoethanolamine (MEA) for CO2 capture in a global scenario: consequences and alternatives, Desalination 380 (2016) 93–99. [5] B. Lv, B. Guo, Z. Zhou, G. Jing, Mechanisms of CO2 capture into monoethanolamine solution with different CO2 loading during the absorption/desorption processes, Environ. Sci. Technol. 49 (2015) 10728–10735. [6] G. Xu, F. Liang, Y. Yang, Y. Hu, K. Zhang, W. Liu, An improved CO2 separation and purification system based on cryogenic separation and distillation theory, Energies 7 (2014) 3484–3502. [7] P. Luis, T. Van Gerven, B. Van, der Bruggen, Recent developments in membranebased technologies for CO2 capture, Prog. Energy Combust. Sci. 38 (2012) 419–448. [8] T. Kuramochi, A. Ramírez, W. Turkenburg, A. Faaij, Techno-economic prospects for CO2 capture from distributed energy systems, Renew. Sust. Energy Rev. 19 (2013) 328–347. [9] X.Y. Chen, H. Vinh-Thang, A.A. Ramirez, D. Rodrigue, S. Kaliaguine, Membrane gas separation technologies for biogas upgrading, RSC Adv. 5 (2015) 24399–24448. [10] H.-H. Tseng, C.-T. Wang, G.-L. Zhuang, P. Uchytil, J. Reznickova, K. Setnickova, Enhanced H2/CH4 and H2/CO2 separation by carbon molecular sieve membrane coated on titania modified alumina support: effects of TiO2 intermediate layer preparation variables on interfacial adhesion, J. Membr. Sci. 510 (2016) 391–404. [11] T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Corma, F. Kapteijn, F.X. Llabrés i Xamena, J. Gascon, Metal–organic framework nanosheets in polymer composite materials for gas separation, Nat. Mater. 14 (2015) 48–55. [12] L.M. Robeson, Polymer blends in membrane transport processes, Ind. Eng. Chem. Res. 49 (2010) 11859–11865. [13] A. Mokdad, A. Dubault, Transport properties of carbon dioxide through singlephase polystyrene/poly(vinylmethylether) blends, J. Membr. Sci. 172 (2000) 1–8. [14] Y. Chen, W.S.W. Ho, High-molecular-weight polyvinylamine/piperazine glycinate membranes for CO2 capture from flue gas, J. Membr. Sci. 514 (2016) 376–384. [15] C. Lamlong, W. Taweepreda, Coating of porous PVC-PEG memebrane with crosslinkable XSBR for O2/N2 and CO2/N2 separation, Polymer 96 (2016) 205–212. [16] G.-L. Zhuang, H.-H. Tseng, M.-Y. Wey, A novel technique using reclaimed tire rubber for gas separation membranes, J. Membr. Sci. 520 (2016) 314–325. [17] W.F. Yong, T.-S. Chung, Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and Matrimid, Polymer 59 (2015) 290–297. [18] W.F. Yong, Z.K. Lee, T.-S. Chung, M. Weber, C. Staudt, C. Maletzko, Blends of a Polymer of Intrinsic Microporosity and Partially Sulfonated Polyphenylenesulfone for Gas Separation, Chem. Sus. Chem. 9 (2016) 1953–1962. [19] Z.-A. Qiao, S.-H. Chai, K. Nelson, Z. Bi, J. Chen, S.M. Mahurin, X. Zhu, S. Dai, Polymeric molecular sieve membranes via in situ cross-linking of non-porous polymer membrane templates, Nat. Commun. 5 (2014) 3705. [20] S.C. George, S. Thomas, Transport phenomena through polymeric systems, Prog. Polym. Sci. 26 (2001) 985–1017. [21] N. Panapitiya, S. Wijenayake, D. Nguyen, C. Karunaweera, Y. Huang, K. Balkus, I. Musselman, J. Ferraris, Compatibilized immiscible polymer blends for gas separations, Materials 9 (2016) 23. [22] N.P. Panapitiya, S.N. Wijenayake, D.D. Nguyen, Y. Huang, I.H. Musselman, K.J. Balkus, J.P. Ferraris, Gas separation membranes derived from high-performance immiscible polymer blends compatibilized with small molecules, ACS Appl. Mater. Inter. 7 (2015) 18618–18627. [23] N.P. Panapitiya, S.N. Wijenayake, Y. Huang, D. Bushdiecker, D. Nguyen, C. Ratanawanate, G.J. Kalaw, C.J. Gilpin, I.H. Musselman, K.J. Balkus Jr, J.P. Ferraris, Stabilization of immiscible polymer blends using structure directing metal organic frameworks (MOFs), Polymer 55 (2014) 2028–2034. [24] T. Kwon, T. Kim, Fb Ali, D.J. Kang, M. Yoo, J. Bang, W. Lee, B.J. Kim, Sizecontrolled polymer-coated nanoparticles as efficient compatibilizers for polymer blends, Macromolecules 44 (2011) 9852–9862. [25] T. Uemura, T. Kaseda, Y. Sasaki, M. Inukai, T. Toriyama, A. Takahara, H. Jinnai, S. Kitagawa, Mixing of immiscible polymers using nanoporous coordination templates, Nat. Commun. 6 (2015). [26] S.C. George, K.N. Ninan, S. Thomas, Permeation of nitrogen and oxygen gases through styrene–butadiene rubber, natural rubber and styrene–butadiene rubber/ natural rubber blend membranes, Eur. Polym. J. 37 (2001) 183–191. [27] M.G. Buonomenna, G. Golemme, C.M. Tone, M.P. De Santo, F. Ciuchi, E. Perrotta, Nanostructured poly(styrene-b-butadiene-b-styrene) (SBS) membranes for the separation of nitrogen from natural gas, Adv. Funct. Mater. 22 (2012) 1759–1767. [28] D.S. Marques, U. Vainio, N.M. Chaparro, V.M. Calo, A.R. Bezahd, J.W. Pitera, K.V. Peinemann, S.P. Nunes, Self-assembly in casting solutions of block copolymer membranes, Soft Matter. 9 (2013) 5557–5564. [29] Y. Matsushita, Microphase separation (of Block Copolymers), in: S. Kobayashi,

[66]

[67] [16]

[62]

[68]

This study

a Polyimide (PI); Nature rubber (NR); polymers of intrinsic microporosit (PIM); hyper-crosslinked polymer (HCP) as filler.

Despite the different modification method were developed, there are a limitation of improving on the decline of gas separation performance as well as the complication of preparation processes. In this study, the PSPB block copolymer (PS/PB =30/70) membrane exhibits the high CO2 separation performance and the high aging resistance for longterm use through the microphase separated structure in bulk. We believe that this represents a useful membrane material for gas separation. 4. Conclusions Understanding the influence of aging on the gas separation performance of copolymer membranes is important. Membranes with different microstructures were prepared using SBCs with different PS/ PB ratios. The morphologies of the SBC-derived membranes were confirmed by AFM, SEM and XRD analysis. In the pure gas permeation test, the SBC-derived membranes exhibited higher CO2/N2 separation performances compared to the pure PS and PB membranes due to the synergistic effects of the PS and PB phases. In particular, the cylindrical micro-structure of the PS-70BR membrane that contains a γ crystalline PS domain within the 70% PB domain effectively enhanced the CO2 permeability and CO2/N2 selectivity to 57.88 Barrer and 15.07, respectively. Comparison of the effects of aging on the gas separation performance of the different SBC-derived membranes was performed over 1–28 days. An increase in gas permeability was observed for the aged membranes with PS as the dominant matrix due to the CO2induced plasticization of the PS domain. In contrast, the gas permeability decreased in the aged membranes with PB as the dominant matrix due to the densification of the PB domain. The aged SBCderived membranes maintained or increased the CO2/N2 selectivity. Furthermore, only the PS-70BR membrane exhibited high aging resistance due to the stable cylindrical micro-structure. Thus, the present work clearly identified the aging performance of the SBCderived membranes, indicating that the main polymer matrix and the micro-structure play important roles in the aging process. Acknowledgments The authors would like to acknowledge support for this study from Ministry of Science and Technology (MOST- 104–2221-E-005-00570

Journal of Membrane Science 529 (2017) 63–71

G.-L. Zhuang et al.

[30]

[31] [32] [33]

[34]

[35]

[36]

[37]

[38]

[39] [40] [41]

[42]

[43] [44]

[45]

[46] [47]

[48]

(2005) 3135–3148. [49] R.A. Nyquist, M. Malanga, Infrared study of styrene-4-methylstyrene copolymers, Appl. Spectrosc. 43 (1989) 442–444. [50] S. Jitian, I. Bratu, Determination of optical constants of polymethyl methacrylate films from IR reflection-absorption spectra, AIP Conference Proceedings 1425 2629, 2012. [51] J.F. Masson, L. Pelletier, P. Collins, Rapid FTIR method for quantification of styrene-butadiene type copolymers in bitumen, J. Appl. Polym. Sci. 79 (2001) 1034–1041. [52] N.S. Murthy, H. Minor, C. Bednarczyk, S. Krimm, Structure of the amorphous phase in oriented polymers, Macromolecules 26 (1993) 1712–1721. [53] G. Guerra, V.M. Vitagliano, C. De Rosa, V. Petraccone, P. Corradini, Polymorphism in melt crystallized syndiotactic polystyrene samples, Macromolecules 23 (1990) 1539–1544. [54] Y.S. Sun, E.M. Woo, M.C. Wu, Thermal-induced crystal/conformation transformations in syndiotactic polystyrene films treated with different solvents, Macromol. Chem. Phys. 204 (2003) 1547–1556. [55] A.R. Albunia, P. Musto, G. Guerra, FTIR spectra of pure helical crystalline phases of syndiotactic polystyrene, Polymer 47 (2006) 234–242. [56] N. Brun, P. Bourson, S. Margueron, M. Duc, Study of the Thermal Behavior of Syndiotactic and Atactic Polystyrene By Raman Spectroscopy, AIP Conference Proceedings 1353 856-861, 2011. [57] L.A. Baldenegro-Perez, D. Navarro-Rodriguez, F.J. Medellin-Rodriguez, B. Hsiao, C.A. Avila-Orta, I. Sics, Molecular weight and crystallization temperature effects on poly(ethylene terephthalate) (PET) homopolymers, an isothermal crystallization analysis, Polymers 6 (2014) 583–600. [58] G. Gianotti, A. Valvassori, Fusion enthalpy and entropy of syndiotactic polystyrene, Polymer 31 (1990) 473–475. [59] G.-L. Zhuang, H.-H. Tseng, M.-Y. Wey, Preparation of PPO-silica mixed matrix membranes by in-situ sol–gel method for H2/CO2 separation, Int. J. Hydrog. Energy 39 (2014) 17178–17190. [60] J. Xia, T.-S. Chung, D.R. Paul, Physical aging and carbon dioxide plasticization of thin polyimide films in mixed gas permeation, J. Membr. Sci. 450 (2014) 457–468. [61] W. Qiu, C.-C. Chen, L. Xu, L. Cui, D.R. Paul, W.J. Koros, Sub-Tg cross-linking of a polyimide membrane for enhanced CO2 plasticization resistance for natural gas separation, Macromolecules 44 (2011) 6046–6056. [62] R. Swaidan, B. Ghanem, E. Litwiller, I. Pinnau, Physical aging, plasticization and their effects on gas permeation in “rigid” polymers of intrinsic microporosity, Macromolecules 48 (2015) 6553–6561. [63] N. Müller, U.A. Handge, V. Abetz, Physical ageing and lifetime prediction of polymer membranes for gas separation processes, J. Membr. Sci. 516 (2016) 33–46. [64] A.A. Shamsabadi, R.M. Behbahani, F. Seidi, M. Soroush, Physical aging of polyetherimide membranes, J. Nat. Gas. Sci. Eng. 27 (Part 2) (2015) 651–660. [65] C.-C. Hu, Y.-J. Fu, S.-W. Hsiao, K.-R. Lee, J.-Y. Lai, Effect of physical aging on the gas transport properties of poly(methyl methacrylate) membranes, J. Membr. Sci. 303 (2007) 29–36. [66] W.-H. Lin, T.-S. Chung, Gas permeability, diffusivity, solubility, and aging characteristics of 6FDA-durene polyimide membranes, J. Membr. Sci. 186 (2001) 183–193. [67] J.H. Kim, W.J. Koros, D.R. Paul, Effects of CO2 exposure and physical aging on the gas permeability of thin 6FDA-based polyimide membranes: part 2. with crosslinking, J. Membr. Sci. 282 (2006) 32–43. [68] T. Mitra, R.S. Bhavsar, D.J. Adams, P.M. Budd, A.I. Cooper, PIM-1 mixed matrix membranes for gas separations using cost-effective hypercrosslinked nanoparticle fillers, Chem. Commun. 52 (2016) 5581–5584.

K. Müllen (Eds.), Encyclopedia of Polymeric Nanomaterials, Springer Berlin Heidelberg, Berlin, Heidelberg, 2014, pp. 1–6. T. Miyata, S. Obata, T. Uragami, Annealing effect of microphase-separated membranes containing poly(dimethylsiloxane) on their permselectivity for aqueous ethanol solutions, Macromolecules 32 (1999) 8465–8475. Y. Mai, F. Zhang, X. Feng, Polymer-directed synthesis of metal oxide-containing nanomaterials for electrochemical energy storage, Nanoscale 6 (2014) 106–121. J.M.G. Swann, P.D. Topham, Design and application of nanoscale actuators using block-copolymers, Polymers 2 (2010) 454. M. Minelli, M.G. Baschetti, D.T. Hallinan Jr., N.P. Balsara, Study of gas permeabilities through polystyrene-block-poly(ethylene oxide) copolymers, J. Membr. Sci. 432 (2013) 83–89. R. Patel, S.J. Kim, D.K. Roh, J.H. Kim, Synthesis of amphiphilic PCZ-r-PEG nanostructural copolymers and their use in CO2/N2 separation membranes, Chem. Eng. J. 254 (2014) 46–53. W.S. Chi, S. Hwang, S.-J. Lee, S. Park, Y.-S. Bae, D.Y. Ryu, J.H. Kim, J. Kim, Mixed matrix membranes consisting of SEBS block copolymers and size-controlled ZIF-8 nanoparticles for CO2 capture, J. Membr. Sci. 495 (2015) 479–488. C.H. Park, J.H. Lee, J.P. Jung, B. Jung, J.H. Kim, A highly selective PEGBEM-gPOEM comb copolymer membrane for CO2/N2 separation, J. Membr. Sci. 492 (2015) 452–460. D.B. Wijayasekara, M.G. Cowan, J.T. Lewis, D.L. Gin, R.D. Noble, T.S. Bailey, Elastic free-standing RTIL composite membranes for CO2/N2 separation based on sphere-forming triblock/diblock copolymer blends, J. Membr. Sci. 511 (2016) 170–179. L. Dong, Y. Wang, M. Chen, D. Shi, X. Li, C. Zhang, H. Wang, Enhanced CO2 separation performance of P(PEGMA-co-DEAEMA-co-MMA) copolymer membrane through the synergistic effect of EO groups and amino groups, RSC Adv. 6 (2016) 59946–59955. N.R. Horn, D.R. Paul, Carbon dioxide plasticization and conditioning effects in thick vs. thin glassy polymer films, Polymer 52 (2011) 1619–1627. N.R. Horn, D.R. Paul, Carbon dioxide sorption and plasticization of thin glassy polymer films tracked by optical methods, Macromolecules 45 (2012) 2820–2834. G.-L. Zhuang, M.-Y. Wey, H.-H. Tseng, The density and crystallinity properties of PPO-silica mixed-matrix membranes produced via the in situ sol-gel method for H2/CO2 separation. II: effect of thermal annealing treatment, Chem. Eng. Res. Des. 104 (2015) 319–332. P.H. Pfromm, The impact of physical aging of amorphous glassy polymers on gas separation membranes, in: Y. Yampolskii, I. Pinnau, B. Freeman (Eds.), Materials Science of Membranes for Gas and Vapor Separation, John Wiley & Sons, Ltd, Chichester, UK, 2006, pp. 293–306. T.M. Murphy, B.D. Freeman, D.R. Paul, Physical aging of polystyrene films tracked by gas permeability, Polymer 54 (2013) 873–880. J. Zhou, A.T. Haldeman, E.H. Wagener, S.M. Husson, CO2 plasticization and physical aging of perfluorocyclobutyl polymer selective layers, J. Membr. Sci. 454 (2014) 398–406. H. Wang, T.-S. Chung, D.R. Paul, Physical aging and plasticization of thick and thin films of the thermally rearranged ortho-functional polyimide 6FDA–HAB, J. Membr. Sci. 458 (2014) 27–35. G.-L. Zhuang, H.-H. Tseng, M.-Y. Wey, Feasibility of using waste polystyrene as a membrane material for gas separation, Chem. Eng. Res. Des. 111 (2016) 204–217. K. Kaniappan, S. Latha, Certain investigations on the formulation and characterization of polystyrene /poly(methyl methacrylate) blends, Int. J. Chem. Tech. Res. 3 (2011) 708–717. S.B. Munteanu, C. Vasile, Spectral and thermal characterization of styrenebutadiene copolymers with different architectures, J. Optoelec. Adv. Mater. 7

71