Flat sheet asymmetric Polysulfone (PSF) and Polysulfone/Polyimide (PSF/PI) membranes were prepared simultaneously by using phase inversion technique. The solvents Dichloromethane (DCM) and methyl-2-pyrrolidone (NMP) were used with premium ternary casting solution having the same ratio to control the rate of evaporation. The composition of polyimide varied from 5% to 20% with ethanol as a coagulation medium. The effects of change in the morphology of asymmetric membranes with the increase in the percentage of PI were analyzed with the help of field emission scanning electron microscopy (FESEM). The top surface of the each membrane showed well homogenized and mixed blends while the cross sectional surface showed non-porous top and diminutive porous substructure resulting from delayed de-mixing. The Fourier transform infrared microscopy (FTIR) also showed companionable bonding with PSF and PI characteristic peaks between 1140 to 1245 cm-1 and PI 1717 cm-1 respectively. Mechanical analysis of each membrane was examined that showed promising results with PSF/PI 20% and 15% with increased elastic modulus and fair tensile properties. Permeation test were carried on each membrane to evaluate the permeance and selectivity of CO2 and CH4 at 2-10 bar pressure. Experimental results showed that the permeance of CO2 increased with the increase in PI which gradually decreases with the increase in the pressure in contrary to the results showed by CH4 permeance. These results showed highest ideal selectivity for 15% PI (α = 16.736- 6.477) indicating towards the improved properties of newly developed blended gas membranes for efficient CO2 separation.
Natural gas is essential for the world's energy supply which is considered to be the safest and most useful of all energy sources. The various uses of natural gas have increased the consumption of natural gas. Consequently, natural gas production must be increased in order to meet the increasing demand of natural gas. Malaysia is one of the largest natural gas producers of about 2.48 trillion cubic meters of natural gas production from the total worldwide production (about 177.36 trillion cubic meters at the end of 2007) . Basically; methane is the major component in natural gas, comprising typically 75-90% of the total components. In addition, natural gas may also contain undesirable impurities such as carbon dioxide and hydrogen sulfide. Carbon dioxide is corrosive, acidic and decreases the heating value of natural gas [2, 3]. Gas separation through membranes has been actively practiced for more than 30 years [4-7]. Membrane technology offers advantages over other conventional CO2 removal technologies which are environmental friendly, lower capital cost, low energy consumption, space efficiency and also suitable for remote location application [8-10]. Polymeric membranes have been serving as the oldest and one of the efficient tools in the world of membrane technology. In this regard, the fabrication and designing of membranes to obtain the optimum permeability and selectivity are the key challenges faced in this quarter [11, 12]. However these factors are addressed only if the membranes are strong enough to discourage plasticization. Polyimide is considered to be the best among other glassy polymers because of having high permeability and selectivity however they are highly induce to plasticization as CO2 also act as a plasticizing agent which can swell up the membrane by making the chain of the polymer flexible and thus induces plasticization at starting pressure of 8 bar thus result in reducing the permselectivity of the membrane. This situation can be managed by cross-linking the membrane that changes the molecular orientation of the polymer and make it resistant to these affects but can influence the permselectivity of the membrane [13-16]. Polysulfone is considered to be a good candidate towards ignoring the plasticization over 30 bar pressure together with good permselectivity and low cost. So cross-linkage of these two polymers may pave towards superior properties [17-21]. Some researches on polymeric blends have been made in the past as well to investigate the properties of the membrane blends. G.C kapantaidakis at el.  studied the blending properties of polyethersulfone/ polyimide blends hollow fiber membranes for gas separation at different compositions showing high permeation properties for CO2 from 31 to 60 GPU with silicon rubber coated with thickness of 1030oA and selectivity varying between 31 to 60 GPU at room temperature. S.S.Hosseini at el.  studied effectiveness of carbon membranes from blend of poly(benzimidazole) (PBI) and matrimid 5218 and compared this blend with another blend of Torlon(a polyamide imide) and P84. Result showed good performance of PBI/matrimid blends as compared to the other blends and by further crosslinking it with 10wt% p-xylene diamine in methanol, before carbonization resulted in enhanced selectivity for N2 , CH4 and CO2 surpassing the tradeoff CO2 /CH4(α = 203.95), H2 /CO2 (α = 203.95) with improved permeability. Similar blending work has been reported by A.F.Ismail et al.  using flat sheet based polyetherimide/polyimide blends with zeolite particles. The results indicated homogenous blended developed membranes that were further improved in its structure and properties as the zeolite loading was increased.
B.C. NG, A.F. Ismail et al.  carried out lot of work on flat sheet asymmetric polymeric membranes that consist up of non-porous skin and porous substructure. The key is to develop a defect free membrane by controlling the skin thickness to minimum and avoiding the macropores in the substructure. In these types of membranes, one of the dominant technique is dry/wet phase inversion involving the evaporation or liquid exchange with the non-solvent in the coagulation bath to form membrane after precipitation. The membrane molecular orientation is effected by the exchange rates between the polymer and the non-solvents due to solubility difference. Investigation regarding the phase inversion was carried out by H.Kawakami et al.  who prepared asymmetric membranes by dry/wet phase inversion with ultrathin skin with sponge structures. The selectivity increased with the decrease in the skin layer. A.F.Ismail and P.Y.Lai  prepared asymmetric polysulfone membranes defect free with skin thickness of 6950-11330 oA by controlling various factors involving polymer concentration , solvent ratio, forced convection evaporation time and casting share rate. M.J. Han and D.Bhattacharyya  studied the effect of non solvents on the development of polysulfone membranes by following Flory-Huggins theory in phase inversion by calculating tenary phase equilibra of the components that showed that demixing of Polysulfone with NMP will be faster in water that in isopropanol bath. Through several investigations he concluded that membranes that were developed in isopropanol bath considerable low pore volume and surface area with thick dense skin structure compared with the membrane developed in water bath. I. Pinnau and J. Koros  prepared and compared integrally skinned asymmetric polysulfone membranes by dry, wet and dry/wet phase inversion methods. Convective evaporation system was used in the dry/wet technique that showed the optimum performance with ultra-thin skin with sponge-like substructure that showed high flux compared with the membranes developed by free standing evaporation with membranes prepared with dry phase showed lowest flux.
The rising research trend in the field of blending technology is the reason of the advantages associated by such membranes that combine the properties of the blended polymers yielding high performance along with emergence of new family, however it still require lot of attention for improvement . In this respect, the present study is focused towards the blending of different glassy polymers to modify the developed membrane for enhanced results which is demanding in terms of blending properties and structure. For this purpose, selection of suitable polymers is of particular interest to form homogenous miscible blends in order to accomplish our requirements. Keeping this in view, polysulfone and polyimide are selected for investigation purposed to study the blending morphology of the developed membrane, their molecular orientation, mechanical strength and CO2/CH4 separation performances. It is conceived that this field of study will provide multipurpose maturation in the formulation of blended morphology as well as naive group that will be much more effective and durable in the quarter of gas separation.
Blends of asymmetric homogenous membranes were prepared from Polysufone (PSU) (B.P 185 oC) Udel® P-1800, Solvay advanced polymers and Polyimide (Matrimid® 5218) (B.P 302oC), supplied by Huntsman, are selected as the polymer matrix for this study. The polymer blends are soluble in several common organic solvents, such as dichloromethane (DCM) (B.P 40oC, Merck) and 1-methyl-2-pyrrolidone (NMP) (B.P 204.3°C, Merck) is used to prepare films from solution casting.
2.2. Membrane preparation
Polysulfone and polyimide was dried for 24 hours prior to use. The casting solution was prepared from 25gms of total dope solution to develop membranes of different compositions. The percentage of blends of polysulfone and polyimide are shown in the Table 1.
4.25gms (20%) of DCM was added as more volatile solvent and 17 grams (80%) of NMP was added to solution to control the rate of evaporation. The mixture was stirred in a round bottom reaction vessel with magnetic stirrer at 35 oC for 24 h to prepare a clear solution followed by bath sonicated for 5h with water to remove gas bubbles or any other particles that may had left while preparing dope solution. Asymmetric flat sheet membranes were then prepared by the dry/wet phase inversion process. This clear solution was used to cast membrane onto a glass plate using a casting knife with a gap opening of 150µm. Ethanol was used as a coagulation medium.
2.3. Field emission scanning electron microscopy (FESEM) structural analysis
The morphology of each membrane was analysed by FESEM. Random specimens from the developed asymmetric membranes were drawn carefully with a sharp blade in order to examine the morphology of surface and cross-sections. Cross-section of the membranes was obtained by liquid nitrogen freeze fracturing, followed by gold sputtering and finally the samples were tested for FESEM (make: Zeiss, model: SUPRA 55VP).
2.4. Fourier transform infrared spectroscopy(FTIR) analysis
To measure the molecular interactions between the polymer blends of different composition, a drop from each different dope solutions prepared were casted on potassium bromide pellets and then the thin coated pellets were dried under vacuum before they were analyzed through FTIR (Perkin Elmer 2000 spectrometer).
2.5. Mechanical analysis
The mechanical properties of the membrane samples were determined by ASTM D882-02 standard test method for tensile properties using universal testing machine (UTM) LR 5K Lloyd Instruments. For each membrane, five samples of 100mm x 9mm was prepared and tested to keep constancy.
2.6. Permeation properties
For the permeability evaluation of the developed membranes their efficiency was checked on membrane permeation system. The assembly unit consists up of permeation cell having stainless steel paired disk tightened together with nut bolts having lower one fixed in which the circular sample to be tested is placed with an area of 14.54 cm2. To start with the experiment, the system should be fully evacuated from residual gases or dust which may had been settled earlier by using vacuum pump for at least half an hour. The permeation of the gases CO2 and then CH4 at ambient conditions was calculated by bubble flow meter attached to the assembly . In order to verify the constancy of the values, all readings were repeated for each gas. The permeance, for species i was calculated by using the following equation.
where, Ji is flux of species i, ΔPi is differential partial pressure of species i across the membrane, and l is membrane thickness. Similarly, the permeance for species j can also calculated by following the same equation (1). The ideal selectivity (α) is the ratio of permeance of the two species i, j is calculated by using the formula :
3. Results and discussion
3.1. Morphology of PSF-PI asymmetric membranes
The surface and cross sectional views of the development membranes are shown in the Figure 1 (a), (b), (c), (d), (e) and Figure 2. (a), (b), (c), (d), (e) respectively. FESEM pictures of the membranes were prepared by immersion into ethanol bath after evaporation under controlled condition for 30 seconds. The structures show homogenous surfaces of pure and blended asymmetric membranes indicating towards the compatibility between the two glassy polymers. In comparison to the pure polysulfone surface, the membrane blends show no phase separation. Various morphological asymmetric membranes were developed with the increase in polyimide percentage by fracturing the asymmetric flat sheet membranes in liquid nitrogen. The cross sectional views which are obtained show fairly dense skin and porous sublayer owing to the process of phase separation which come within reach of our vision. These microvoids are supposed to be produced by the involvement of non-solvent during the wet phase separation. As DCM has low boiling nature that can cause fast evaporation with the development of diffused skin layer, NMP was used along with DCM in the ratio of 80/20 to control the rate of rapid evaporation. The formation of skin layer is evidence of 30 seconds evaporation time that show in the least nucleation at the top layer of the film. During the immersion step, top skin which is more intense as shown in the images, act as a barrier against the non-solvent and will resist the incoming solution to affect the substructure, thus slowing down the precipitation step causing the delayed demixing [33-37]. Hence the chances of more macrovoids and pores will be reduced with this delayed demixing mechanism. However more macrovoids appear at a higher concentration of PI for 20%, this is because at higher concentration and relatively higher glass transition of PI, the evaporation of solvent is slower and formation of a skinny structure at the top thus causing fast demixing compared to less percentages of PI and hence more macrovoids. Studies to reduce the macrovoid in the membrane structure have been done in the past. Z. Li et al.  studied morphological parameters of membranes and prepared macrovoid-free sponge-like membranes by increasing the ratio of NMP upto 80% as compared to butyrolactone in water/NMP-GBL/CA systems.
3.2. Spectral analysis
In order to find the compatible nature of the polymeric blends, FTIR spectroscopy is employed. The spectra of PSF and PSF/PI blended asymmetric membranes are shown in figure3 (a), (b), (c), (d) and (e). It is noticed that due to the blending of PSF and PI, the frequency shift, rightly so, takes place, indicating towards the miscibility of the polymers. PSF stem is made up of diaryl sulfone (Ar-SO2-Ar) which is the defining feature of sulphone group and diaryl ether (Ar-O-Ar) groups, the frequency shift of the characteristic peak from 1150 cm-1 to 1151 cm-1 and from 1245 cm-1 to (1248 cm-1 - 1244 cm-1 ). The frequencies for bond stretching with benzene ring which changes from 1485 cm-1 to (1488 cm-1 -1489 cm-1 ) show its bond strength till 1655 cm-1 . In polysulfone structure, the presence of two bending CH3 groups associated with one carbon at carbon atom to form a propane is found to change from 1375 cm-1 to (1405-1406 cm-1 ) for mixed blends. With the inclusion of PI, its characteristic peak is found in the stretching region of (1690 cm-1- 1640 cm-1) that changes to 1717 cm-1 at various intensities along with C-N stretching from 1025 cm-1 to 1014 cm-1 .These spectral frequencies shifts and intensity changes put forward the miscibility of PSF and PI blends.
3.3. Mechanical Analysis
The properties of asymmetric membrane films are further evaluated on the basis of mechanical analysis performed over each membrane in order to check there elastic modulus, strength at maximum load, stiffness, percentage break at maximum load and extension at break. Data for mechanical properties are presented in Table.2. Elastic modulus is increases as the composition of PI is increased which suggest the rigidity of the membrane. This can be verified with the increasing value of stiffness property where the membrane stiffness is increased with the increase in the PI percentage. Strength at maximum load is maximum at maximum PI percentage which satisfy the elastic modulus, however this property of the membrane material depict the performance of the membranes in combination with extension at break, which shows the attractive elastic nature of the membrane with PI up to 15%. So this membrane, not only show its decent properties for not only elastic modulus but also extension at break in comparison to other membranes. However the properties of PSF/PI 20% show the least percentage of strain at maximum load with all other promising results, thus making it attractive too. These results can eventually lead towards the permeation properties in favour of selecting the most suitable membrane for efficient CO2 separation.
3.4. Gas Permeation Properties
The gas permeation properties for the developed membranes were calculated on the gas permeation cell attached with the bubble flow meter for the CO2 and CH4 gases at feed pressures of 2,4,6,8 10 bar. These permeation properties enable to develop the correlation of the permeance of CO2 and CH4 gases with the membrane structures against various pressures.
Figures 4 and 5 show that the permeance of CO2 and CH4 where the gases shows different behaviour according to the blending composition of individual membranes. The permeance of CO2 increases as the PI percentage in the polymer blend is increased and in the order of PSF/PI 20%> PSF/PI 15%> PSF/PI 10%> PSF/PI 5% > PSF, however for the slow moving gas CH4 figure 5 shows an inverted trend with the permeance increases with the increase in the pressure along with the rising trends for PSF and PSF/PI 5% and monotonously varying trend of PSF/PI 20%. The high permeance values of CO2 for 20% PI followed 15% PI is attributed towards the soaring affinity for CO2 in the membrane matrix at low feed pressure of 2 bar and show a declining trend with the increase in the feed pressure upto 10 bar with shows the absence of plasticization in the polymeric blend. In the presence of plasticization, the figure 4 would be showing the high values of permeance just after achieving the lowest value at high pressure which could be an indication towards the membrane swelling. The increasing permeance of both CO2 and CH4 gases for PSF/PI 20% may be due to more macrovoids present at in the substructure which give you an idea about membrane flaws which show quite different appearance compared to PSF/PI 15%, where macrovoids are appearing at the lowest part of membrane with well defined skin layer.
The ideal selectivity of CO2/CH4 scheme at various feed pressures is shown in figure 6. The decreasing trends of all membranes are noticed with the increase in the pressure upto 10 bar in which PSF/PI 15% showing the peak value of selectivity compared to other membranes. The comparatively low selectivity value of blended PSF/ PI 20% membrane indicates towards the minor defects in its structure with similar comparable low selectivities for other developed membranes. The decreasing trends of selectivity with rising feed pressures have been addressed in the past with dual-mode theory being the basis of research [39-42]. These observed permeation properties and morphological studies of the structures of miscible blends of PSF/ PI point towards development of membranes which are more efficient in their approach with limiting plasticisation and improved stability.
Asymmetric flat sheet membranes were fabricated from PSF with various compositions of PI for CO2 separation applications using phase inversion technique. Structural morphology of the developed membranes were examined during FTIR analysis that showed the frequency shifts occur in the blended PSF and PI characteristic peaks involving diaryl sulfone and ether groups of polysulfone along with C-N group of PI. These molecular orientations in the membrane structure suggested the compatible nature of the polymer blends. Mechanical analysis of the blended membranes showed improved performance in comparison to pure PSF membrane with PSF/PI20% followed PSF/PI15% showing high elastic modulus and improved tensile properties. Morphological analysis of the developed membranes showed homogenous and uniform membrane blends that are quite beneficial for gas separations. The permeation properties of the developed membranes showed the highest permeation value of CO2 for PSF/PI 20% at lowest pressure that decreased down with rising pressure up to 10bar. These dropping down of permeation values were noticed in all other developed membranes however inverted trends were observed for CH4 permeance. The structural morphology of this enhanced permeation of PSF/PI 20% was due to the fact of more macrovoids that appeared in the membrane substructure. This indicated towards slower evaporation rate of the solvent at high percentage of PI with skinny structure at the top thus causing fast demixing compared to less percentages of PI and hence more macrovoids. For PSF/PI 15% blended membrane showed promising performance in terms of CO2/CH4 selectivity (α = 16.736-6.477) in comparison to other membrane blends involving PSF (α = 7.497-1.432), PSF/PI 5% (α = 10.041-1.867), PSF/PI 10% (α = 13.983-4.887) and PSF/PI 20% (α = 10.327- 4.009). So by blending PSF and PI with in different compositions, the chances of plasticization at moderate pressure may shift up to elevated pressures and hence membrane swelling can be avoided at moderate pressures. Moreover this technique not only provides improved chemical and mechanical stability but is efficient enough to improve the permselective properties with economical viability. Hence, the blended PSF/PI membranes are suggested to give a better future for CO2 removal from natural gas.
[ - ]
[ Angstrom ]
[ Celsius ]
Permeance of any species i,j
Flux of species i
Differential partial pressure of species i
[ - ]
Ideal selectivity of i,j