Thickness Effect on the Morphology and Permeability of CO2/N2 Gases in Asymmetric Polyetherimide Membrane

The effect of membrane thickness on the morphology and performance of polyetherimide (PEI) flat sheet membranes for gas separation has been investigated. An asymmetric PEI flat sheet membrane was fabricated using phase inversion with N-methyl2-pyrrolidone (NMP) and water-isopropanol as solvent and coagulant, respectively. Scanning electron microscopy (SEM) was used to analyse the morphology of the prepared membrane. The results showed that the membrane with high casting thickness of 300 μm yielded a lower permeability compared to the fabricated membrane with lower casting thickness at 1 bar and 25oC for pure carbon dioxide (CO2) and nitrogen (N2) as test gas. It was found that the permeability strongly depended on the cast solution thickness. It was also found out that the macrovoid became open and appeared larger as the casting thickness increased.


INTrODUCTION
The use of membranes for gas separation has been recognised for many decades since the first successful development of integrally skinned asymmetric membrane prepared by Loeb and Sourirajan in 1960. 1 These membranes were limited to a certain degree of perfection and thickness of the selective layer affixed on the porous support in form of composite membrane.Besides the Robeson trade-off, there is usually another trade-off relationship between permeance (related to the skin thickness) and selectivity (related to the integrity of the skin).The membrane sub-layer serves as a mechanical support for the skin, with negligible effect on separation.Macrovoids can usually be observed in asymmetric membrane fabricated by phase inversion process.In high-pressure operation during gas separation, the presence of macrovoid is not generally favorable because this usually leads to mechanical weakness. 2,3The performance of membrane-based gas separation process strongly depends on permeability and selectivity of the membrane.A membrane with higher permeability yields higher productivity and low capital cost, whereas membrane with higher selectivity leads to more efficient separation, high recovery as well as low power cost.The membrane that simultaneously possesses a high value of selectivity and permeability would lead to the most economical gas separation process. 4ase inversion process induced by immersion precipitation is a well-known technique to fabricate asymmetric membranes.During immersion precipitation, the substrate in the coagulation bath and the solvent in the casting solution film are exchanged with non-solvent (NS) in precipitation media which results in phase separation.This process yields the formation of asymmetric membrane with a dense top layer as well as a porous sub-layer containing macrovoids, pores and micropores.The sub-layer formation is controlled by numerous variables in the polymer dope solutions such as composition, coagulant temperature and organic/ inorganic additives.To attain a desired membrane morphology and performance, the phase inversion process must be carefully controlled. 5[8][9][10] The liquid-liquid demixing by nucleation and growth of a polymer-lean phase is responsible for generating porous morphology. 9 CO 2 /N 2 separation is one of the most interesting processes in the energy industry because of gas recovery and decreasing flaring emissions.Membrane separation is one of the new perspective methods for such applications.This method has more advantages than other traditional gas separation techniques such as cryogenic distillation and pressure swing adsorption.These include energy saving, easy installation, operation as well as maintenance, modular and low space requirements, and environmentally friendly. 11The polymeric membrane should be able to offer significant improvements in CO 2 permeability and selectivity with excellent thermal and chemical stability, resistance to plasticisation, resistance to aging, and ease of scale-up in order to be commercially competitive. 12ommercially available polyetherimide (PEI) has several important advantages as a membrane material.This polymer has good chemical and thermal stability.
The aromatic imide units provide stiffness and heat resistance, while the swivel groups such as -O and -C(CH3) 2 form flexible macromolecular chains that allow for good processability. 13A study by Wang et al. 13 on gas permeation of the PEI dense films revealed that PEI exhibits an impressively high selectivity for many important gas pairs especially He and H 2 separation from other gasses.In 2013, Shamsabadi et al. 14 prepared flat sheet asymmetric PEI membrane for the separation of H 2 from CH 4 which resulted in an ideal selectivity of 27.8.Meanwhile, Saedi et al. 15 fabricated asymmetric flat sheet polyethersulfone (PES) membrane for separation of CO 2 from CH 4 using PEI as a polymeric additive which led to CO 2 permeance of 11.1 at 5 bars.Ren et al. 7 also prepared the PEI flat sheet membrane with N-methyl-2-pyrrolidone (NMP)/non-solvent system.In their study, they investigated the influence of various non-solvents on the membrane morphology.
Kurdi and Tremblay 16 produced a defect-free PEI asymmetric flat sheet membrane for the separation of oxygen from nitrogen using LiNO 3 and isopropanol as dual bath coagulation method.Simons et al. 17 investigated CO 2 and CH 4 sorption as well as transport behaviour using the PEI membrane which produced CO 2 permeability of 1.4 at 5 bars.In 1987, Pinnau et al. 18 prepared PEI flat-sheet asymmetric membranes for CO 2 /CH 4 separation using a halogenated hydro-carbon as solvent and reported a selectivity of about 30-40.
In this study, we fabricated structural transition of asymmetric PEI flat sheet membranes prepared with different thicknesses of cast solution.The morphology and the performances for CO 2 /N 2 separation of the produced membrane were investigated.

Fabrication of PEI Asymmetric Flat sheet Membrane
PEI was dried in a vacuum oven at 110°C overnight in order to remove any trace of adsorbed humidity.The desired polymer solution (w/w) was prepared by dissolving a certain weight of PEI in a pre-determined weight of NMP in a closed glass jar.A stirring speed of 400 rpm at 60°C was applied for efficient mixing for 24 h until a clear yellowish uniform viscous solution was achieved and left overnight.Then the solution was degassed for 30 min in order to remove the air bubbles from the solution.After the degassing process, the polymer solution was cast on a glass plate with a different thickness of 150-300 µm with the aid of casting knife by a motorised film applicator (Elcometer 4340, Dutech Instruments Malaysia).The cast film along with the glass plate were immediately immersed in a water-isopropanol (4:1 of water: isopropanol) bath to accomplish the phase inversion for 5 min, followed by immersing the membrane in distilled water for 24 h.Subsequently, the membrane was placed in between two dried glasses filled with sheets of filter paper and dried for 24 h at room temperature.Finally, the PEI membrane was dried in an oven at 70°C ± 2°C for 12 h for the complete removal of solvents and adsorbed humidity.The dope composition was represented in Table 2.

Gas Permeation Test
A gas permeation experiment was performed using an assembled setup as shown in Figure 2. A cross-flow membrane cell made from stainless steel was used to perform the experiments.The cell consists of two detachable circular compartments clamped together by six screws.The upper part was connected to the feed and the retentate streams.The permeate streams were exhausting from the lower part.Circular membrane discs with effective permeation area of 7.0 cm 2 were used.The pressure-normalised fluxes of the fabricated flat sheet membranes were measured for pure CO 2 and N 2 at 25°C and 1 bar.The feed gas was supplied to the shell side of the module.The standard permeate volumetric flow rate at atmospheric conditions was measured by a soap bubble flow meter.Each set of data represents an average of 6 replicates.The pressure-normalised flux (permeance) which flows through the membranes was calculated using Equation 1: where Q is the measured volumetric flow rate (at standard pressure and temperature) of the permeated gas (cm 3 (STP)/s), p is permeance, l is membrane thickness (cm), A is the effective membrane area (cm 2 ) and δP is the pressure difference across the membrane (cmHg).The common unit of permeance is GPU where GPU is equal to cm 3 (STP)/cm 2 s cmHg.Also, the ideal gas separation factor (α) can be calculated using Equation 2:

Characterisation of Flat sheet Polyetherimide Membrane
The morphology of fabricated flat sheet membranes was investigated using scanning electron microscope (SEM) (TM 3000 Hitachi, High-Tech TOKYO) with an accelerating voltage of 25.0 kV.Prior to scanning, small pieces of the membrane samples were immersed into liquid nitrogen carefully for a minimum of 120 s.Then, it was fractured to obtain a clear cross section.After drying, the fractured membrane sheet was placed on a disc for sputtering with a thin film of platinum prior to SEM testing.

Effect of Thickness on Membrane structure
Figure 3 shows the SEM micrograph for the cross-sectional morphologies of 25 wt.%PEI membrane fabricated under different casting thicknesses.The presence of densed skin layer with porous substructure was observed from the three membranes cast with a thickness of 150, 250 and 350 µm (Figure 3 (a), (b) and (c)).The skin layer (selective layer) of these three membranes increases as the casting thickness increases.The thickness of these skin layers was measured to be approximately 7.268 µm, 18.853 µm, and 26.330 µm, respectively.On the other hand, the sub-layer of these three membranes has a finger-like macrovoid but it was observed that the macrovoid of the membrane cast with a thickness of 300 µm is significantly larger than the membranes cast with lower thickness.This reveals that macrovoid size increases with membrane casting thickness.Li et al. 19 reported similar behaviour which was further attributed to the existence of critical structure-transition thickness.The transition was initiated with a fully sponge-like structure forming in the first region, followed by some fluctuations into the fingerlike structure and ended with the fully formed finger-like structure as the thickness increased.
Mulder 2 described the macrovoid formation mechanism during membrane precipitation as a phenomenon that originates from the formation of nuclei at the polymer lean region which is near to the membrane surface.Due to further diffusion and displacement of the solvent/non-solvent pair, the macrovoid becomes larger.The Monte Carlo diffusion by Termonia also supported that the macrovoid initiation was through non-solvent penetration via skin defects and the faster exchange rate between solvent and non-solvent aided the growth. 20For lower membrane thickness (i.e., 150 µm), the presence of the thin layer of macrovoid was due to insufficient space for the membrane to be formed between the film and the bottom surface.However, the formation of finger-like macrovoid exhibited in thicker membrane thickness was as a result of the non-solvent inflow which dominated the exchange rate during the membrane precipitation.
Zhang et al. 21reported that finger-like macovoid of cellulose acetate membrane prepared disappeared as the thickness is lowered down when the critical thickness is at 150 µm.From Figure 3(a), it was observed that the macrovoid formed at the skin layer of the membrane prepared with lower casting thickness of 150 µm is in compact that it cannot be easily seen, unlike the other two membranes prepared from higher casting thicknesses of 250 and 300 µm in Figure 3(b) and 3(c) respectively, where the macrovoid at the skin layer becomes open and larger.This can be attributed to the fact that the speed of demixing point in skin layers is extremely faster because of the higher concentration gradient in such thickness.Similar reason was reported by Safi et al. 22 that the thickness of the membrane is in relation to the macrovoid.In their study, they concluded that finger-like structures and the pores were far away from top layer in a lower thickness of cast solution, while at higher thickness values, they existed just beneath the top layer.
Tsay and McHugh 23 explained that an increase in the skin layer thickness was a result of the decrease in asymmetric porous sub-layer due to increase in the membrane thickness.This reason was attributed to the sponge-like formation which was established as can be seen in the membrane prepared with 250 and 300 µm.
Besides, the macrovoid-free structure could prevent the mechanically weak point in the membrane structures and this can be attained below the critical thickness. 19,24

Effect of Thickness on Gas Permeation Performance
As can be seen from the results presented in Table 3, the membrane with a thickness of 150 µm in N-1 has a permeability of 10.365 Barrer for CO 2 gas with a separation factor of 1.7.However, permeability was drastically reduced to 2.588 Barrer for CO 2 with thickness increment to 300 µm in N-3.Meanwhile, the selectivity increased to 2.6 with a thickness of 300 µm and this was due to the thickness of the selective layer and the formation of the most sponge-like structure at the sublayer of this membrane.From Table 4, one can see that the permeability of CO 2 is encouraged when compared to other works.This result was in tandem with the report by Firpo et al. 25 that the permeability shows a strong decrease with increase in thickness but the selectivity does not change significantly in its value.Pakizeh et al. 26 reported that CO 2 molecules are absorbed at a high level on the skin layer surface and can then diffuse through the dense layer of the membrane.Therefore, the CO 2 permeability is governed by the skin layer thickness.However, when the thickness of the densed skin layer decreases, the permeability of CO 2 increases.As can be seen from the permeability results in Table 3, the permeability of CO 2 is of higher values compared to that of N 2 gas.This may be due to two mechanisms.Firstly, the high potential of CO 2 to diffuse through the polymer matrix due to its small kinetic diameter (3.3Å) than N 2 (3.64Å). 27econdly, a quadrupolar moment of CO 2 in combination with glassy nature of PEI increases its permeability for CO 2 than N 2 .Indeed, CO 2 is a condensable and interactive gas which can interact with the polymeric chains and the available functional groups.This leads to a high solubility for CO 2 , and consequently, being more permeable than N 2 gas.Besides, the lower permeability of N 2 might be due to slow diffusivity resulting from the larger kinetic diameter and solubility in the PEI polymer. 28It was also reported by Alsari et al. 29 in their study, where they laid much emphasis on the importance of membrane thickness to the separation of gas as the thickness increased, the permeability drastically reduced.
In summary, the accumulated data gives an insight that membrane thickness was directly related to the permeability of gas.It was observed that a thick layer was formed in membrane fabricated with a thickness of 300 µm which caused the lower permeability and an improvement in the selectivity of the gas.This signifies that the higher the membrane thickness, the lower the permeability is with no significant changes in the separation factor.

CONCLUsION
Asymmetric PEI membranes were successfully fabricated through phase inversion process for removal of CO 2 in single gas permeation test.The membrane casting thickness was explored and has been identified as one of the parameters in controlling the final morphologies and performances of the prepared membranes.
A study on the casting membrane thickness was carried out and its correlation was attributed to the formation of macrovoid in the structure and permeability in the gas permeation test.SEM images of the cross-sectioned membrane with varied casting thickness (150, 250 and 300 µm) revealed that the macovoid is easily formed near the skin layer in the membrane with a high thickness of 300 µm which caused the low permeability.
As observed, gas permeation properties of the selective layers with different thickness defined that the permeability of the gases decreases upon the increase in the film thickness.However, the CO 2 /N 2 selectivities remained insignificantly changed.Nevertheless, a trade-off relationship between permeability and selectivity was normally observed in the manipulation of the asymmetric membrane using thickness as an affecting parameter.

Figure 1 :
Figure 1: The molecular structure of used PEI where n represents the degree of polymerisation.

Figure 2 :
Figure 2: Schematic diagram of gas permeation set up.

Table 2 :
The composition of the polymer solution.

Table 3 :
Effect of casting thickness on membrane permeability and selectivity.

Table 4 :
Correlation of permeability and selectivity with other polymer.