1. Introduction

Global warming is increasing every year and there is very high concern how to get some solution. Growing population leads to ever increasing demand on energy and fossil fuel is the main source of energy. Environment is polluted by the huge amount of carbon dioxide (CO₂) release from heavy industrialization. Natural gases contain methane (CH₄) along with CO₂ and other heavy gases. Separation of CO₂ is essential for useful uti- lization of CH₄ as combustible gas. Although there are several other processes available but membrane-based gas separation technology is one of the most efficient gas separation process[1-4]. PEBA is an block copolymer comprised of polyamide which is more hydrophobic and poly(ethylene oxide) which is more hydrophilic in nature[5-7]. The hydrophobic part which is hard one provides the mechanical strength while the ether group present in the soft part interact with the acidic CO₂ molecules. Presence of hydrogen bonding within polyamide phase leads to the much stronger domain in the block copolymer. The mechanical strength of PEBA is high enough to fabricate free-standing membranes and utilize host matrix for various fillers.

Although PEBA has appropriate CO₂ permeability, the selectivity of the membrane can be enhanced by mixing it with inorganic filler or other polymeric components. Mixed matrix membrane (MMM) are one of the excellent materials to combine the easy processability property of polymer matrix and greater selectivity of inorganic fillers[1-4]. There are several challenges in MMM. One of the problems is the compatibility of the organic-inorganic matrix that induce weak interface. Secondly, homogenous dispersion on filler in the polymer matrix is very critical to enhance the performance of the membrane. So, there are several kinds of molecular organic frame work (MOF), zeolite etc are used to fabricate MMM.

In this review, focus is on both binary and ternary kind of PEBA membrane in which the how type of filler affects the permeability as well as selectivity is discussed. Fabrication process of the composite membrane as well as the optimum loading is explained in detail.

2. PEBA mixed membrane

2.1. Inorganic filler filled composite membrane

Azizi reported the fabrication of nanocomposite membrane consisting of PEBAX-1074 and TiO₂, SiO₂ and Al₂O₃[8]. Pristine membrane was prepared by solution casting and nanocomposite membrane are prepared by blending technique. FESEM measurement show the uniform distribution of the inorganic filler though out the matrix. Presence of inorganic fillers increase the performance of the nanocomposite membrane and best properties is observed with 8 wt% filler in all the cases. Out of the three different fillers 8 wt% Al₂O₃ shows highest CO₂ permeability of 163.87 Barrer at 3 bar and 25°C. It also shows the best CO₂/CH₄ selectivity of 14.24 under similar measurement conditions. In case of TiO₂ and SiO₂ there is not much difference between CO₂ permeability as well as selectivity. The same author chooses ZnO nanoparticle (10~30 nm) as another filler to prepare nanocomposite membrane[9]. Free standing membrane were fabricated by solution casting method with a thickness of 60~80 μm. Gas permeation properties was checked at different pressure and at 25°C. Pristine membrane has CO₂ permeability, CO₂/N₂ and CO₂/CH₄ selectivities are 110.67 Barrer, 50.08 and 11.09 respectively at 3 bar and 25°C. In case of PEBAX/ZnO (8 wt%) composite membrane permeability increased to 152.27 Barrer and CO₂/N₂ and CO₂/CH₄ selectivity are increased to 62.15 and 11.09 respectively under similar conditions. The higher adsorption capacity and higher affinity of the ZnO nanoparticles leads to the higher permeability as well enhanced selectivity of the composite membrane. A mixed matrix membrane consisting of PEBAX 1657 and fume silica (FS) was reported by Gamali et al.[10]. Fig. 1 represents the schematic diagram of PEBA composite membrane. Composite membrane with 4.6 wt% FS shows the best CO₂/CH₄ selectivity. It was observed from SEM image that FS are mixed homogenously in the PEBAX matrix. CO₂ permeability of the pristine membrane increase from 80 to 162 Barrer when the upstream pressure increases from 1.0 to 3.5 MPa under mixed gases condition. PEBAX pristine polymer has a CO₂/CH₄ selectivity of 64.53 but PEBAX/4.6 wt% FS composite membrane selectivity increase to 74.5. One of the reasons is that in case of composite membrane there is no phase separation is observed whereas in pristine membrane there it is clearly visible. As a result, the crystallinity of the polymer matrix reduced, and it is well supported by AFM image that distinctly represent the integration of FS into polymer host.

Yu et al. prepared composite membrane of PEBAX- 1657 with silica, polystyrene (PS) colloids, and carbon nanotubes (CNTs)[11]. Nanocomposite membrane with different composition were prepared by solution casting and evaporation at higher temperature to prepare self-standing dense membrane with thickness of 50~90 μm. Nanocomposite membrane of silica or PS colloids reduce the CO₂ permeability as well as selectivity but not for N₂ or H₂ as compared to pristine membrane. Two different types of CNT like single wall and multiple wall CNT were used for preparation of nanocomposite membrane. PEBAX/CNT (5 wt%) showed the best CO₂ permeability and SWCNT shows 43% and MWCNT has 24% increase, although there is no enhancement in CO₂/N₂ or CO₂/H₂ selectivity for pure gas at 21°C and 2.27 bar pressure. The solubility of the composite membrane remains intact, but the diffusivity increases due to the presence of CNT. A composite membrane consisting of PEBAX-1657 and graphene oxide (GO) flakes decorated iron oxide (Fe₃O₄) was reported by Zhu et al.[12]. Flat sheet mixed matrix membrane was prepared by solution casting method with film thickness of about 45~65 μm. PEBAX/Fe₃O₄-GO membrane were characterized by SEM that indicate the uniform dispersion of the filler through the polymer matrix. Out of various composition 3 wt% loading results in highest gas separation and selectivity performance. CO₂ permeability is 538 Barrer at 0.2 MPa and room temperature. CO₂/N₂ and CO₂/CH₄ gas selectivity are 47 and 75 respectively. Presence of hydroxyl group on the surface of GO leads to better interaction with CO₂ and hence there is higher solubility selectivity. Instead of decorated GO, porous reduced graphene oxide (PRG) was selected as a filler to check its effect on gas separation performance[13]. PRG was prepared by wet chemical process. PEBAX was mixed with PRG in different ratio and composite free-standing membrane was prepared by casting of the solution onto Teflon petri dish. The well dried membrane thickness is around 45~60 μm. There is homogenous distribution of PRG in the polymer matrix until 5 wt%, but further enhancement leads to the agglomeration. T_g of the composite membrane increased with PRG quantity due to the hydrogen bonding interaction between the hydroxyl group on the PRG with polymer matrix and membrane rigidity increases and consequently its selectivity. PEBAX/PRG (5 wt%) composite membrane shows CO₂ permeability of 119 Barrer and CO₂/N₂ selectivity of 104.

A filler with both organic ligand and inorganic part was selected to have better interaction with the organic PEBAX matrix. Li et al. reported composite membranes made up of PEBAX MH 1657 and octa (3-hydroxy-3- methyl butyldimethylsiloxy) (POSS (POSS-OH)) or Octa amic acid (POSS (POSS-acid))[14]. Flat sheet self-standing dense mixed matrix membrane with thickness of 80~100 μm were prepared from above materials by solution casting process as presented in Fig. 2. POSS is miscible at the molecular level as observed by SEM. CO₂ permeability of PEBAX/POSS mixed matrix membrane initially increase and then decrease with the in crease in the both types POSS as compared to pristine PEBAX. The initial increase in the permeability is due to presence of large pore in POSS. As the percentage of POSS increase there is blockage of the pore due to the chain entanglement from PEBAX. Finally, the glass transition temperature of the polymer initially decreases and then increase with the increase in the filler loading which is in opposite trend to CO₂ permeability. This may be due to the enhanced pore volume of the MMM at the beginning by disruption in ordering of the polymer chain. Hydrogen bonding is the additional factor between the hydroxyl or carboxyl group of POSS with the polymer matrix that lead to increase in rigidity of the polymer and reduce the permeability but enhance the selectivity of CO₂ over H₂. Important factor is that the effect on T_g is more by POSS-acid filler than POSS-OH which is reflected in better permeation and selectivity behavior of PEBAX/POSS-acid then PEBAX/ POSS-OH. The mixed gas permeability as well as the selectivity of PEBAX/POSS membrane is higher than the pure gas permeability which is very unusual case in general. One of the reasons may be due to the enhancement of interaction between CO₂ with the polymer matrix at the cost of interaction of H₂ with the matrix polymer. PEBAX/POSS shows the best CO₂ permeability of 135 Barrer at 35°C and 8 atm pressure with CO₂/H₂ selectivity 52.3 for mixed gas. A composite membrane was prepared by selecting two different grades of PEBAX and poly(ethylene glycol) functionalized polyoctahedral oligomeric silsesquioxanes (PEGPOSS) reported by Rahman et al.[15]. PEBAX MH 1657 and PEBAX 2533 composite membrane single gas permeation properties of N₂, O₂, CH₄, H₂ and CO₂ were checked by time lag method. The chain length of PEG present in POSS is similar to the PEO length of PEBAX 1657 as a result of which the compatibility of the PEBAX 1657/PEG-POSS is much better than PEBAX 2533/PEG-POSS. T_g of the PEBAX 1657 blend reduced a lot due to the presence of low mol. Wt. PEG that enhance the permeability of the membrane unlike PEBAX 2533 blend where there is no significance decrease in T_g. Gas permeability of composite membrane of PEBAX 1657 increase gradually with the increase in PEG-POSS upto 30 wt%. This is due to the increase in CO₂ diffusion in 30~70°C temperature range. Another factor that plays a critical role is the surface topography of the membrane. In case of PEBAX 1657/PEG-POSS composite membrane the surface roughness increases with loading of 30 wt% filler but it decreases in PEBAX 2533/PEG-POSS composite membrane. The same author linked PEG with glycidyl terminated POSS (PEG-GLY-POSS) and glycidyldimethylsilyl terminated POSS (PEG-GDMS-POSS) and used as a filler with PEBAX MH 1657 to check the CO₂ permeability behavior of the composite membrane[16]. 40 wt% of the filler was mixed with the PEBAX 1657 in different solvents like tetrahydrofuran (THF), chloroform or toluene and self-standing membrane was prepared by solution blending process (Fig. 3). The morphology of the composite membrane changed drastically depending on the type of solvent which was checked by atomic force microscopy (AFM). Loading percent of the nanoparticle filler was fixed at 40 wt%. There is highest reduction of T_g for composite containing PEG-GDMS-POSS due to higher mobility of the polyether segment of PEBAX. The presence of the additional Si-O linkage in the spacer complementing the higher polyether mobility. This leads to the higher diffusion coefficient of the various gas molecules. As a result, these membranes have highest CO₂, N₂, H₂ permeability. Composite membrane made with THF shows better performance irrespective of type of spacer. CO₂/N₂ and CO₂/H₂ selectivity of the composite remain intact but permeability increases as compared to the pristine membrane.

2.2. Metal organic framework (MOF) composite membrane

T. Khosravi et al. reported fabrication of composite membrane made up of PEBAX and metal organic frameworks[17]. Two different type of MOF are copper benzene-1,3,5-tricarboxylate (CuBTC) and NH₂-CuBTC. NH₂-CuBTC is a modified version MOF-199[18]. Appropriate quantity of both components is mixed and casted on a teflon petri dish to prepare the composite membrane with thickness roughly between 60~90 μm. BET surface are of CuBTC is 1,261 m²/g which is almost double that of NH₂-CuBTC (644 m²/g). T_g of the mixed membrane increased due to enhancement in rigidity of PEBAX in the presence of MOF. In case of PEBAX/NH₂-CuBTC membrane, the presence of hydrogen bonding between the ether group of PEBAX and amine group of MOF results in more appreciable increase in T_g. PEBAX/NH₂-CuBTC (20 g/g) membrane shows highest CO₂ permeability of 328.4 Barrer at 30 MPa and 30°C. The CO₂/CH₄ selectivity of this composite membrane is 43.2 under similar condition which is highest value due to the presence of the hydrogen bonding interaction between membrane matrix and the MOF resulting in better interface. At the same time there is interaction between acidic CO₂ and basic amine group that enhance the performance of the membrane. CuBTC is replaced by iron benzene-1,3,5- tricaboxylate (Fe-BTC) to prepare the composite membrane by phase inversion method[19] and structures are presented in Fig. 4. The interaction between MOF filler with the polymer matrix was checked by SEM. There is agglomeration of filler when the it is around 30 wt% that affect the selectivity of the membrane. The CO₂ diffusivity and solubility of the composite membrane was measured by time lag study. It shows that diffusivity increase with MOF content having very negligible decrease in solubility indicating high porosity and specific interaction of filler with the CO₂. As a result, the diffusivity is the dictating factor for the better selectivity and permeability of the composite membrane. In that case composite membrane with 40 wt% showed CO₂ permeability of 425.4 Barrer at 3 bar pressure and room temperature but has the lowest CO₂/CH₄ selectivity may be due to filler agglomerations. But 30 wt% filler content has the best CO₂ permeability of 402.69 Barrer and 21.5 CO₂/CH₄ selectivity among all the composite membrane.

2.3. Zeolite composite membrane

F. Karamouz et al. reported the preparation of PEBAX (1074)/Deca-dodecasil-3-rhombohedral (DD3R) composite membrane[20]. The BET surface area of DD3R is around 304 m²/g. Homogenous solution with different composition were casted Teflon petri dish and solvent was removed to prepare the free-standing membrane. FTIR measurement shows there is good interaction between the zeolite filler with the polymer matrix and T_g of the composite membrane increase with the enhancement of the zeolite loading. PEBAX/DD3R composite membrane with 5 wt% zeolite exhibit highest selectivity for CO₂/CH₄ and increases carbon dioxide permeability. ZIF-8 filler is introduced to prepare a dual layer mixed matrix membrane consisting of ZIF-8/PEBAX-2533[21]. Free standing dense MMM was prepared by solution casting method consisting of various ratio of ZIF-8 to PEBAX and thickness around 40~60 μm. Bottom layer of the membrane is rich in the filler layer whereas the top layer is rich in polymer matrix which is named as dual layer MMM which is clearly observed in SEM (Fig. 5) DSC analysis shows that there is hardly any difference in T_g and melting transition temperature (T_m) of MMM and pristine membrane. Loading of ZIF-8 was varied from 5 to 35 wt% in MMM. Single CO₂ gas permeability increased from 351 to 1287 Barrer from pristine to 35% wt% MMM. There is not much change in the CO₂/N₂ and CO₂/CH₄ selectivity in case of pristine and MMM for single gas measurement. In case of mixed gas measurement for both dry and moist there is significant drop in the CO₂/N₂ selectivity for the dual layer MMM.

2.4. Polymer blend membrane

Didden et al. reported blended membrane of PEBAX with two pluronics named L-61, L-81 and poly(propylene glycol) (PPG) with different compositions by solution blending process[22]. Mixed solution is casted on the Teflon petri dishes and the free-standing membrane are prepared by evaporation of solvent at higher temperature. Pluronics are copolymer of PEG and PPG and L-81 contain one extra PPG then L-61. The addition of PPG enhances the polyether melting temperature compared to L-61, L-81 and pristine PEBAX. The polyether domain is immiscible with the additive and the degree of crystallinity increase with PPG as com pared with pluronics. Hydroxyl end group present in PPG leads to hydrogen bonding interaction that leads to further increase in crystallinity and phase separation (Fig. 6). Blending increase the gas permeation properties of the membrane but the mixed gas selectivity is reduced.

2.5. Ternary composite membrane

A flat sheet dense membrane was prepared by mixing PEBAX, silver nanoparticles and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) ionic liquid[ 23]. Silver nanoparticle (~100 nm) were dispersed in poly(vinyl pyrrolidone) (PVP). Three components are mixed homogenously and casted on a teflon plate to fabricate a free-standing membrane. The thermal property of the mixed matrix membrane was studied by differential scanning calorimetery (DSC). Presence of ionic liquid and silver nanoparticle reduce the crystallinity of PEBAX. The MMM shows only one T_g indicating good compatibility among each component. Further there is no change in T_g of the composite membrane shows that it is devoid of cross linking and rigidification due to increasing concentration of silver nanoparticle (Fig. 7). There is significant increase in the CO₂ permeability to 325 Barrer in case of PEBAX/ 1% Ag binary membrane at 35°C and 10 bar. In case of the ternary blend membrane PEBAX/0.5%Ag/50%IL the CO₂ permeability increased from 110 to 180 Barrer under similar condition as compared to pristine mem- brane. The CO₂/CH₄ and CO₂/N₂ selectivity of the same composite membrane increased from 20.8 to 61.0 and 78.6 to 187.5 respectively. Silver nanoparticle is replaced by ZIF 8 to prepare a ternary blend[24]. Jomekian et al. reported the preparation of 1,3-Di-n-butyl-2-methylimidazolium chloride ionic liquid (DnBMCl) by following Gholap et al.[25]. ZIF 8 was synthesized by varying the ratio of 2-methyl Imidazole (MeIm) to Zinc nitrate hexahydrate (Zn(NO₃)₂H₂O) to check the effect of Zn²⁺ on the gas permeation properties. MeIm/Zn²⁺ with molar ration 32/1 shows type I type of isotherm. It has higher surface area of 1,792 m²/g compared to other composition due to higher microporosity as well as micropore volume. PEBAX was modified with DnBMCl and solution blended at 70°C to prepare ionic liquid modified PEBAX (IL-PEBAX). ZIF-8/IL-PEBAX mixed matrix membrane selective layer on the poly(ether sulfone) (PES) substrate was prepared by coating the homogenous solution. Ionic liquid modified PEBAX mixed matrix membrane shows higher CO₂/CH₄, CO₂/N₂ and CO₂/H₂ selectivity than unmodified PEBAX/ZIF-8 and pristine PEBAX membrane for pure gas. In case of mixed gas, selectivity decreases due to the plasticization of the soft segment present in PEABX. The performance of the MMM reduced as well at higher pressure due to the similar reason.

Sanaeepur et al. synthesized copper nanoparticle and mixed with glycerol (Gl) modified PEBAX 1657 to prepare self-standing mixed matrix membrane by sol ution casting method with average thickness of 240 μm[26]. PEBAX modification was optimized with 15 wt% of glycerol and copper nanoparticle was varied between 0~2 wt%. CO₂ permeability of the PEBAX/Gl (15 wt%) blended membrane reduced from 50.42 Barrer from 65.71 Barrer for pristine membrane but CO₂/N₂ selectivity of the blended membrane increased drastically to 227.7 from 81.9 for pristine membrane at 25°C and 10 bar. PEBAX/Gl (15 wt%)/Cu (1.5 wt%) MMM showed CO₂ permeatility of 63.5 Barrer and CO₂/N₂ selectivity of 200 under similar condition which is lower than blended membrane, but permeability is higher than the blended membrane. Presence of Cu nanoparticle disrupt the hydrogen bonding of the blended membrane which results in the enhance permeability of the MMM without affecting much CO₂/N₂ selectivity. PEBAX 1657 and poly(ethylene glycol) methyl ethyl acrylate (PEG-MEA) was blended together by solution blending to prepare membrane. PEBAX/PEG-MEA with 50 wt% (PEBAX-PEG-MEA-50) showed the best performance [27]. Instead of copper nanoparticle GO was added as filler. GO was synthesized and disperses in solvent and mixed in polymer blend of PEBAX-PEG- MEA-50 to prepare mixed matrix membrane. PEBAX-PEG-MEA-50 blend membrane showed very high CO₂ permeability of 572 Barrer as compared to 80 Barrer for pristine membrane. The same blend composition with 0.3 wt% GO showed highest CO₂/N₂ selectivity of 55.8 which is higher than the Robeson’s upper bound without compromising much the CO₂ permeability. It is due to the higher CO₂ sorption due to the CO₂-philic oxygen functional group on the GO surface and edge. The MMM has good anti plasticization until 9 bar pressure and it has long term stability up to 100 days without affecting the performance of the membrane. The CO₂ permeability and CO₂/N₂ selectivity of various PEBA-based membranes were summarized in Table 1 and Figure 8. The Robeson upper limit is used for evaluating the separation performance of these membranes.

3. Conclusions

There is growing necessity of CO₂ separation membrane due to every increasing demand of energy which leads to the global warming due to the CO₂ emission. Composite gas separation membrane brings in the advantage of both organic and inorganic components in the membrane that leads to better permeability as well as selectivity. PEBA has both hard as well soft component and loading of fillers into become easier due to the presence of rubbery components. But phase separation of inorganic components is one of the disadvantages of composite membrane. In this review preparation of composite membrane along with their disadvantage is discussed in detail. Keeping in mind the compatibility issue there is the scope of various modification of in- organic filler so that it will be well integrated to the polymeric matrix at molecular level.