Phase-Inversion Poly(ether imide) Membranes Prepared from Water

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Ind. Eng. Chem. Res. 1999, 38, 2650-2658

Phase-Inversion Poly(ether imide) Membranes Prepared from Water-Miscible/Immiscible Mixture Solvents Jyh-Jeng Shieh† and Tai-Shung Chung*,†,‡ Department of Chemical Engineering, and Institute of Materials Research and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

By using water-miscible and water-immiscible solvents (N-methyl-2-pyrrolidone and methlyene chloride (MC)) in the casting solution to prepare poly(ether imide) (PEI) asymmetric membranes via a phase-inversion process, we have studied the flat membrane formation, membrane morphology, and gas separation performance in terms of PEI content, MC content, and standing time. In such a system, the immiscibility of MC with water plays a very important role in determining the final membrane morphology and performance. The influence of methylene chloride was shown in two aspects: (1) change of phase-inversion course and mechanisms; (2) post membrane structure change due to the interaction between PEI and methylene chloride. The silicone-coated PEI membranes exhibited a good membrane performance with O2 permeance of (1.3-9.3) × 10-6 cm3(STP)/cm2‚s‚cmHg and O2/N2 selectivity of 5.2-6.4. It was found, from the gas permeation and membrane morphology studies, that the separation was mainly attributed to the cellular structures (closed cells) beneath the membrane surface skin. Introduction Generally speaking, asymmetric membranes are mainly prepared by a phase-inversion technique and their permeation flux is effectively improved by reducing the skin layer thickness. To achieve a good membrane performance for gas separation applications, surface defects in these asymmetric membranes have to be limited so that a high selectivity can be obtained after coating with a permeable layer1,2 or membranes with a defect-free surface skin are desirable. A significant number of phase-inversion asymmetric membranes for gas separation is prepared from a polymer/solvent/nonsolvent system. For example, Kesting et al.3,4 used a Lewis acid/base complex solvent to incorporate a high level of nonsolvent into polymer dope and prepared high-performance asymmetric membranes with a graded-density skin. Koros and co-workers5,6 developed a novel dry/wet phase-inversion process using a casting solution consisting of volatile solvent and less volatile nonsolvent to fabricate asymmetric membranes with a defect-free surface skin for gas separation. Van’t Hoff7 used a dual coagulation procedure, which is identified as a wet/wet phase-inversion process, to make hollow fiber membranes based on the delayed phase separation behavior of a polymer solution in contact with various nonsolvents. Chung et al.8 also developed ultrathin high-performance poly(ether sulfone) hollow fibers from a one-polymer/one-solvent system by controlling the chemistry of the internal coagulant and regulating the dope concentration. Shieh and Chung9 prepared polymer dopes with different liquid-liquidphase separation properties using binary solvent mixtures to fabricate hollow fibers having different membrane morphologies. All solvents or nonsolvents used * To whom all correspondence should be addressed. Tel: 65874-6645. Fax: 65-779-1936. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Institute of Materials Research Engineering.

in these approaches are miscible with coagulation media in order to extract solvent from the nascent membrane systems. Poly(ether imide) (PEI) is utilized in this study because it is an important polymer material and has very attractive features for gas separation applications, such as excellent thermal stability, a good membraneforming property, and a high gas separation performance. Reported values for the gas separation performance of a dense PEI membrane at 35 °C and 10 atm are He ) 9.5 barrer, O2 ) 0.4 barrer, CO2 ) 1.3 barrer (1 barrer ) 10-10 cm3(STP)‚cm/cm2‚s‚cmHg) with selectivities for He/CH4, O2/N2, and CO2/CH4 of 264, 7.6, and 37.4, respectively.10 The potential applications for PEI membranes in gas separation are, for example, the recovery of H2 from chemical industrial gases containing CH4, N2, or CO, the separation of acidic gases (CO2, H2S) from natural gases, and the enrichment of O2 and N2 from air for industrial and instrumentation uses. Efforts have been made in developing various PEI asymmetric membranes,3,11-14 but most of the resultant membranes suffered from either a low permeation flux or a low selectivity. Also, the solution systems for the formation of these asymmetric PEI membranes used miscible solvents and nonsolvents. In this work we choose a binary solvent mixture consisting of one water-miscible solvent (N-methyl-2pyrrolidone, NMP) and the other water-immiscible solvent (methylene chloride, MC) to prepare the PEI casting solution, while the phase-inversion process takes place by using water as the coagulant. Because methylene chloride is immiscible with water, it is left in the membrane after coagulating in water and can further modify the membrane structure through its solvency power. This membrane formation system provides us with the opportunity to fundamentally understand the phase behavior of casting solutions, the morphology of the resultant PEI asymmetric membranes, and their gas separation performance of oxygen and nitrogen as a

10.1021/ie9807912 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/06/1999

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2651

function of PEI content, methylene chloride content, and standing time. Experimental Section Materials. Aromatic PEI (ULTEM 1010, supplied by General Electric Co.) was used after vacuum-drying at 150 °C for 8 h. It has a glass transition temperature of 490 K, and its repeat unit is shown in Figure 1. A Sylgard184 silicone elastomer kit was purchased from Dow Corning. NMP and MC of reagent grade were obtained from Merck and used as received. Membrane Preparation. According to the desired composition, PEI was first dissolved in NMP using a magnetic stirrer equipped with a heating plate at 60 °C for 8 h to form a homogeneous solution. After the solution was cooled to room temperature, MC was added carefully dropwise, with stirring, to a specific amount. The resultant solution was stirred for 8 h and degassed for another 4 h before membrane casting. The membrane was prepared by casting the polymer solution on a clean Pyrex glass plate to a preadjusted thickness. The casting temperature and atmosphere were ambient (25 °C, relative humidity 60%). The casting film was kept at ambient conditions for various periods of time and, subsequently, coagulated in a water bath at 25 °C for 8 h. The solidified film was then solvent-exchanged with methanol for 4 h to remove NMP residual and MC (methanol is miscible with both solvents) and then air-dried at room temperature for 1 day. The thickness of the dry membranes was calculated as about 200-250 µm. A silicone-coated PEI membrane was prepared by spreading a 3 wt % silicone rubber solution in hexane onto the PEI membrane with the help of a dropper and a glass rod. After the PEI membrane was coated with silicone rubber, it was stored in a clean environment for 48 h to allow the silicone rubber to cure. Gas Permeation Test. The gas permeation apparatus used in our laboratory was described elsewhere.15 The flux of gas through the PEI membrane was measured using a soap bubble meter at 25 °C with a pressure range of 20-100 psig. Each data point is the average value of three to five samples. The gas permeance (cm3(STP)/cm2‚s‚cmHg), P/L, is given by

P Q ) L A∆p

Figure 1. Repeat unit of aromatic poly(ether imide). Table 1. Solution Quality and Membrane Formation Behavior of the Poly(ether imide) (PEI)/ N-Methylpyrrolidone (NMP)/Methylene Chloride (MC) Ternary Systema composition, wt %

appearance

PEI

NMP

MC

solution state

forming membrane

20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20

80 75 70 65 60 55 50 45 43 40 30 24 19 18 10 0

0 5 10 15 20 25 30 35 37 40 50 56 61 62 70 20

clear, homogeneous clear, homogeneous clear, homogeneous clear, homogeneous clear, homogeneous clear, homogeneous clear, homogeneous clear, homogeneous opaque opaque two phases gellike opaque clear, homogeneous clear, homogeneous clear, homogeneous

yes yes yes yes yes yes yes yes, but deformed later no no no no no yes, but deformed later yes, but deformed later yes, but deformed later

a Membranes were prepared by immersing the polymer solution into a water bath immediately after casting onto a glass plate.

Figure 2. Qualitative phase diagram for the PEI/NMP/MC system.

(1)

where P ) permeability of the separation layer (cm3(STP)‚cm/cm2‚s‚cmHg); L ) effective thickness of the separating layer (unknown) (cm); ∆p ) transmembrane pressure difference (cmHg); A ) membrane surface area (cm2); and Q ) gas flux (cm3(STP)/s). Gas permeation unit (GPU, 1 × 10-6 cm3(STP)/cm2‚s‚cmHg) is used in this study. The ideal selectivity of oxygen over nitrogen for the PEI membrane is given by

RO2/N2 )

(P/L)O2 (P/L)N2

(2)

Scanning Electron Microscopy (SEM). Membrane specimens for the SEM study were cryogenically fractured in liquid nitrogen and then sputtered with gold to a thickness of 200-300 Å using a JEOL JFC-1100E

Figure 3. Qualitative phase diagram for the (PEI+MC)/NMP/ H2O system.

ion sputtering device. A field emission SEM, Hitachi S-4500, was employed to investigate the membrane morphology.

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Figure 5. Morphology of the PEI membrane: (a) top surface; (b) cross section near the top layer. Casting conditions: PEI ) 25 wt %; NMP/MC ) 3/1 by weight; standing time ) 3 min.

Figure 4. Cross sections of PEI membranes prepared from casting solutions with various PEI contents: (a) 15 wt %; (b) 20 wt %; (c) 25 wt %. Casting conditions: NMP/MC ) 3/1 by weight; standing time ) 1 min.

Results and Discussion Phase Behavior of Casting Solution. (a) PEI/ NMP/MC System. Although both NMP and MC are

solvents for PEI, PEI only dissolves in a specific concentration range of the mixture of NMP and MC. Table 1 shows that PEI will separate from the bulk solution when the MC content is between 37 and 61 wt %. It has been reported that in a solvent/solvent/polymer system, although no phase separation occurs on each composition axis, phase separation may occur inside the composition triangle. In such a case, the binodal describes a loop and has two critical points.16 The physical meaning explanation for this is that if the two solvents are near critical mixing, addition of the third component can decrease the solvent power of component 1 for component 2 sufficiently for the two liquids not to be completely miscible anymore.17 A hypothetical phase diagram for the PEI/NMP/MC system is shown in Figure 2. There may be a closed twophase region lying entirely within the triangle, near the solvent/solvent axis. MC is a volatile solvent, while NMP is not. If the casting solutions have compositions on the left-hand side of the phase separation region as indicated in point A, the casting solutions, after a short standing for solvent evaporation, will remain in the homogeneous state and will not enter the phase separa-

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2653

Figure 6. Cross sections of the PEI membranes prepared from casting solutions with various MC contents: (a) 0 wt %; (b) 10 wt %; (c) 20 wt %; (d) 30 wt %. Casting conditions: PEI ) 20 wt %; standing time ) 3 min.

tion region. On the other hand, if we prepare a casting solution having a composition on the right-hand side of the phase separation region, as indicated in point A′, the composition path, in most cases, will enter into the phase separation region after solvent evaporation. The solvent-evaporation-induced phase-inversion process may therefore play an interesting role in asymmetric membrane preparation. (b) (PEI + MC)/NMP/H2O System. Because MC is immiscible with water, it may be reasonable to view PEI and MC as one constituent to construct the ternary phase diagram studied here. A qualitative triangle phase diagram of the (PEI + MC)/NMP/H2O system is described schematically in Figure 3. From the thermodynamic point of view, when the MC content in the casting solution is increased, the (PEI + MC)-H2O interaction parameter, χ(PEI+MC)-H2O, will decrease because of the large difference in the solubility parameter

between H2O and methylene. Therefore, the binodal boundary will shift toward the (PEI + MC)-H2O axis.5,18 Also, the binodal boundary will move toward the (PEI + MC)-H2O axis as the standing time is increased because of the vaporization of MC, which reduces the MC content in the casting solution. When the solution is cast on a glass plate, the stepby-step phase-inversion process of the (PEI + MC)/NMP/ H2O system can be described briefly as follows: 1. Vaporization of MC and absorption of moisture occur concurrently for the casting film during the standing period. 2. NMP flows out of and H2O flows into the casting film after immersing into the water bath, while most MC remains in the casting film because of its immiscibility with water. 3. Phase inversion occurs and the casting film is

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Figure 7. Top surfaces of the PEI membranes prepared from casting solutions with various MC contents: (a) 0 wt %; (b) 10 wt %; (c) 20 wt %; (d) 30 wt %; (e) the cross section near the top layer with MC content ) 30 wt %. Casting conditions: PEI ) 20 wt %; standing time ) 1 min.

solidified when the coagulation path enters into the phase separation region.

4. The solidified film undergoes a further structural modification by MC remaining in the film.

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2655

Figure 8. Cross sections of the PEI membranes prepared from casting solutions with various standing times: (a) 0 min; (b) 1 min; (c) 3 min, (d) 5 min. Casting conditions: PEI/NMP/MC ) 20/60/20 by weight.

5. The final membrane structure is obtained after immersing in methanol for solvent exchange and being dried. The last column in Table 1 shows the effect of the MC content on the quality of casting solutions and on the feasibility of membrane formation. No useful films can be formed for those solutions exhibiting phase separation behavior. Membranes formed in step 3 will be destroyed or deformed when the MC content equates to or exceeds 35 wt %. For those casting films with a MC content of less than or equal to 30 wt %, they remain a useful membrane form throughout the entire membrane preparation procedure, but a microscopically structural modification resulting from PEI-MC interaction is expected to occur. Membrane Morphology. (a) Effect of PEI Content. Figure 4 shows the cross sections of PEI membranes prepared from casting solutions with various PEI

contents. The macrovoids disappear as the PEI content increases. A membrane formation system with an instantaneous liquid-liquid phase separation mechanism generally produces macrovoid membrane morphology.19 However, increasing the polymer concentration may prohibit the formation of macrovoids, and such a phenomenon has been reported in cellulose acetate/ dimethyl sulfoxide/water20 and Nomex/N,N-dimethylacetamide/water21 systems. The reason for the elimination of macrovoids in a membrane system with an instantaneous liquid-liquid phase separation property may be due to the fact that a greater viscoelasticity of a more concentrated polymer solution prohibits the immediate convective type of solvent exchange in an instantaneous liquid-liquid phase separation process. Therefore, the liquid-liquid phase separation process in a highly concentrated polymer solution is delayed, and a spongelike morphology, which is usually observed

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Figure 9. Effect of the PEI content on the O2 permeance. Casting conditions: NMP/MC ) 3/1 by weight; standing time ) 3 min.

Figure 12. Effect of the MC content on the O2/N2 selectivity. Casting conditions: PEI ) 20 wt %; standing time ) 1 min.

Figure 10. Effect of the PEI content on the O2/N2 selectivity. Casting conditions: NMP/MC ) 3/1 by weight; standing time ) 3 min.

Figure 13. Effect of the standing time on the O2 permeance. Casting conditions: PEI/NMP/MC ) 20/60/20 by weight.

Figure 11. Effect of the MC content on the O2 permeance. Casting conditions: PEI ) 20 wt %; standing time ) 1 min.

in a delayed liquid-liquid phase separation process, is produced.9,19,22 The membrane morphologies of the top surface as well as the cross section near the top layer are similar for this series of PEI membranes, which are shown in parts a and b of Figure 5, respectively. Pores with diameters ranging from 2 to 0.5 µm scatter over the membrane surface. Cellular structures, mainly closed cells, are observed on the cross section near the top layer. The closed-cell-type cellular structures are evidence of binodal nucleation and growth.22 (b) Effect of Methylene Chloride Content. Figure 6 shows the cross sections of PEI membranes prepared from casting solutions with various MC contents. The

Figure 14. Effect of the standing time on the O2/N2 selectivity. Casting conditions: PEI/NMP/MC ) 20/60/20 by weight.

elongated macrovoids diminish gradually as the MC content increases, and no macrovoids are formed when the MC content is 30 wt %. The PEI/NMP/H2O system exhibits an instantaneous liquid-liquid phase separation property, but the phase separation rate reduces with the addition of MC into the casting solution. The decrease in the phase separation rate may be mainly due to the difference in solubility parameters;23 water is 47.9 (MPa)1/2, while MC is 19.8 (MPa)1/2. Figure 6 also indicates that the spongelike top layer becomes thicker with an increase in the MC content. Parts a-d of Figure 7 show the top surfaces of the PEI membranes prepared from casting solutions with various MC contents. Interestingly, the surface defects diminish as the MC content increases. The smoother

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2657 Table 2. Some Literature Data of Gas Separation Performance of Phase-Inversion Poly(ether imide) Membranes membrane type

solution/coagulant

dense, flat asymmetric, flat asymmetric, hollow fiber asymmetric, hollow fiber asymmetric, flat asymmetric, hollow fiber asymmetric, flat

melt extruded CH2Cl2 + xylene/acetone NMP + propionic acid/H2O NMP + γ-butyrolactone/H2O NMP/1-propanol NMP + ethanol/H2O NMP + CH2Cl2/H2O

a

permeance, GPU He H2 O2 9.5 93 14 68 40

0.4 71 0.5 2.8 0.2 30

1.3 9.3 1.3

He/N2 181 144 167 170 220

selectivity H2/N2 O2/N2 7.6 7.9 7.0

10000 800 11300 1300

7.0 5.2 6.4

2800 300 2600

113

160

skin layera thickness, Å

reference 10 11 3 12 13 14 this study this study

Calculated on the basis of the N2 permeance.

surface may be attributed to the slower liquid-liquid phase separation property of the casting solutions after adding more MC or to the solvency power of MC remaining in the membranes. The membrane morphologies of the cross section near the top layer for this series of PEI membranes are shown in Figure 7e (because this set of membranes has similar crosssectional morphology near the top, we only take one membrane as an example). The bicontinuous structure in the top layer probably results from spinodal decomposition. (c) Effect of Standing Time. Figure 8 shows the cross sections of the PEI membranes prepared from casting solutions with various standing times. Surprisingly, round microvoids are found within a membrane if the film was immediately immersed into water bath, i.e., standing time is 0 min, while the elongated microvoids appear for the casting solutions with standing times of 1 and 3 min and diminish when the standing time is 5 min. It is difficult to explain the exact mechanisms for the above phenomenon. As discussed above, MC and water are not miscible, and the addition of MC in the casting solution reduces the liquid-liquid phase separation rate; therefore, there are no elongated macrovoids formed for the PEI membrane with 0 min of standing time. However, the liquid-liquid phase separation rate of a membrane formation system will increase rapidly because of the evaporation of MC during the membrane standing. The increase in the phase separation rate may induce flow instability,24,25 and thus the elongated macrovoids are formed. When the standing time is 5 min, no obvious macrovoids are found. This may arise from the fact that, because of the evaporation of a considerable amount of MC and the absorption of moisture (nonsolvent) from the ambient, the casting film becomes opaque (phase separation) with 5 min of standing time before immersing into the water bath. A nascent, relatively strong outer skin is formed; thus, it is possible to avoid the formation of macrovoids in an instantaneous phase separation system. As for the top surfaces of PEI membranes prepared from casting solutions with various MC contents, a defective surface with cracks is observed when the standing time is equal to or shorter than 1 min, like one shown in Figure 7; large surface pores exist when the standing time is equal to or longer than 3 min, like one shown in Figure 5a. The morphology of the cross sections near the top layer for this series of PEI membranes changes from bicontinuous structures to closed-cell cellular structures as the standing time increases (similar to the pictures shown in Figures 7e and 5b, respectively). This indicates the phase separation mechanism near the outer skin shifts from spinodal decomposition to binodal nucleation and growth with an increase of the standing time.

Gas Separation Performance. (a) Effect of PEI Content. Figures 9 and 10 show the effect of the PEI content on the O2 permeance and O2/N2 selectivity, respectively. The permeance decreases as the PEI content in the casting solution increases, which may be due to either the denser skin layer or the more compact substructure structure. As seen in the SEM pictures (Figure 5; similar morphology in the top and bottom layers for this series of membranes), the membranes have obvious surface pores; therefore, the decrease in gas permeance should result from the more compact substructure (higher substructure resistance). The silicone-coated PEI membranes exhibit lower permeance than the uncoated ones because the silicone coating layer seals the surface defects of the membranes. The selectivity remains low for the uncoated membranes, while it improves significantly for the silicone-coated ones with an increase in the PEI content. Basically, the uncoated PEI membranes exhibit a Knudsen flow type transport mechanism, while the silicone-coated ones exhibit a solution-diffusion transport mechanism. On the other hand, because the membrane surface is highly defective, the separation should mainly take place in the closed-cell cellular structure beneath the surface skin. Furthermore, the drop in selectivity for a 25 wt % PEI membrane may attribute to a too high substructure resistance which will deteriorate the separation performance.26 (b) Effect of Methylene Chloride Content. Figures 11 and 12 show the effect of the MC content on the O2 permeance and O2/N2 selectivity, respectively. The permeance drops abruptly when the MC content exceeds 20 wt % for both uncoated and silicone-coated PEI membranes. The selectivity remains almost unchanged for an MC content below 25 wt %. As observed in SEM micrographs of Figures 6 and 7, the membrane structure becomes more compact as the MC content increases; therefore, the membrane performance changes accordingly. The presence of MC in casting solutions not only changes the ternary phase diagram but also provides a function to posttreat the PEI membranes after the phase-inversion process. MC retained in PEI membranes when coagulating in water may rearrange or redissolve the polymer chains to form a denser structure. As a result, the permeance decreases and the selectivity increases. (c) Effect of Standing Time. The standing time is the time allowed for the casting solution to stand in the ambient before immersing into the water bath. During the standing time, the volatile solvent will vaporize from and the moisture will be absorbed into the casting solution. The effects of standing time on the O2 permeance and O2/N2 selectivity are shown in Figures 13 and 14, respectively. The permeance first increases and then decreases as the standing time decreases for both

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uncoated and silicone-coated membranes. Knudsen flow selectivity is observed for the uncoated membranes, suggesting the membranes are defective, while after coating with silicone rubber, the selectivity increases significantly for the standing time and is equal to or more than 3 min. When the standing time increases, the layer facing the ambient becomes more concentrated because of the vaporization of MC and the resultant membrane has a more compact structure; thus, the permeance decreases and the selectivity increases for the silicone-coated membranes. Because the surface pores are observed when the standing time is equal to or more than 3 min, this suggests that the separation takes place mainly in the closed-cell cellular structure beneath the surface skin. Table 2 lists the gas separation performance of various phase-inversion PEI membranes in the literature and this work. Most membranes have fairly low gas permeance. The calculated skin layer thicknesses for these asymmetric PEI membranes almost all exceed 1000 Å, while the PEI membrane prepared in this study has the thinnest skin layer thickness, 300 Å (highest gas permeance), with a little loss in selectivity. Conclusions Phase-inversion PEI asymmetric membranes have been prepared from the PEI/(NMP + MC)/H2O system. In this casting system, NMP is a water-miscible solvent, while MC is not. The presence of MC not only changes the coagulation paths and the ternary phase diagram of the PEI/NMP/H2O system but also employs a microscopic modification on the post membrane structure because of its retention within the polymeric matrix. It was also found that the NMP/MC/PEI system exhibits an interesting solution behavior; i.e., a closed two-phase region lies within the composition triangle. PEI membranes prepared in this study are defective. The high permselectivity is obtained only after coating with a permeable silicone rubber layer. Generally speaking, increasing the PEI content, the MC content, and the standing time produces a more compact membrane structure and results in lower permeance and higher selectivity. Closed-cell cellular structures beneath the skin surface are the major location providing separating function. A poly(ether imide) membrane with O2 permeance of 9.3 GPU and O2/N2 selectivity of 5.2 is obtained after a silicone coating. This membrane was cast from a PEI/NMP/MC 20/60/20 (weight ratio) solution with 3 min of standing time. Acknowledgment The authors thank the National University of Singapore (NUS) for a research fund (RP960609A) and ET Enterprise in NUS for a research fund (RP 3602037). Special thanks are due to Prof. J. Phang at Image Transform Limited Inc. for the use of their SEM. Literature Cited (1) Henis, J. M. S.; Tripode, M. K. Multicomponent Membranes for Gas Separation. U.S. Patent 4,230,463, 1980. (2) Henis, J. M. S.; Tripodi, M. K. Composite Hollow Fiber Membranes for Gas Separation: The resistance model approach. J. Membr. Sci. 1981, 8, 233. (3) Kesting, R. E.; Frizsche, A. K.; Murphy, M. K.; Handermann, A. C.; Cruse, C. A.; Malon, R. F. Process for Forming Asymmetric Gas Separation Membranes Having Graded Density Skins. U.S. Patent 4,871,494, 1989.

(4) Kesting, R. E.; Frizsche, A. K.; Cruse, C. A.; Murphy, M. K.; Handermann, A. C.; Malon, R. F.; Moore, M. D. The SecondGeneration Polysulfone Gas Separation Membranes. I. The Use of Lewis Acid:Base Complexes as Transient Templates to Increase Free Volume. J. Appl. Polym. Sci. 1990, 40, 1557. (5) Pesek, S. C.; Koros, W. J. Aqueous Quenched Asymmetric Polysulfone Hollow Fibers Prepared by Dry/Wet Phase Separation. J. Membr. Sci. 1994, 88, 1. (6) Pinnau, I.; Koros, W. J. Defect-Free Ultrahigh Flux Asymmetric Membranes. U.S. Patent 4,902,422, 1990. (7) Van’t Hoff, J. A. Wet Spinning of Asymmetric Hollow Fiber Membranes for Gas Separation. Ph.D. Thesis, Twente University, Twente, The Netherlands, 1988. (8) Chung, T. S.; Teoh, S. K.; Hu, X. Formation of Ultrathin High-Performance Polyethersulfone Hollow Fiber Membranes. J. Membr. Sci. 1997, 133, 161. (9) Shieh, J.-J.; Chung, T. S. Effect of Liquid-Liquid Demixing on the Membrane Morphology, Gas Permeation, Thermal and Mechanical Properties of Cellulose Acetate Hollow Fibers. J. Membr. Sci. 1998, 140, 67. (10) Barbari, T. A.; Koros, W. J.; Paul, D. R. Polymeric Membranes Based on Bisphenol A for Gas Separations. J. Membr. Sci. 1989, 42, 69. (11) Peinemann, K.-V. Method for Producing an Integral, Asymmetric Membranes and the Resultant Membrane. U.S. Patent 4,673,418, 1987. (12) Kneifel, K.; Peinemann, K.-V. Preparation of Hollow Fiber Membranes from Polyetherimide for Gas Separation. J. Membr. Sci. 1992, 65, 295. (13) Perez, S.; Merlen, E.; Robert, E.; Cohen Addad, J. P.; Viallat, A. Characterization of the Surface Layer of Integrally Skinned Polyimide Membranes: Relationship with Their Mechanism of Formation. J. Appl. Polym. Sci. 1993, 47, 1621. (14) Wang, D.; Li, K.; Teo, W. K. Preparation and Characterization of Polyetherimide Asymmetric Hollow Fiber Membranes for Gas Separation. J. Membr. Sci. 1998, 138, 193. (15) Chung, T. S.; Hu, X. Effect of Air-Gap Distance on the Membrane and Thermal Properties of Polyethersulfone Hollow Fibers. J. Appl. Polym. Sci. 1997, 66, 1067. (16) Kurata, M. Thermodynamics of Polymer Solutions; Harwood Academic Publishers: Lausanne, Switzerland, 1982; Chapter 2. (17) Tompa, H. Polymer Solutions; Butterworth Publications Ltd.: London, 1956; Chapter 7. (18) Mulder, M. H. V.; Oude Hendrikman, J.; Wijmans, J. G.; Smolders, C. A. A Rationale for the Preparation of Asymmetric Pervaporation Membranes. J. Appl. Polym. Sci. 1985, 30, 2805. (19) Smolders, C. A.; Reuvers, A. J.; Boom, R. M.; Wienk, I. M. Microstructures in Phase-Inversion Membranes. Part 1. Formation of Macrovoids. J. Membr. Sci. 1992, 72, 259. (20) Frommer, M. A.; Messalam, R. M. Mechanism of Membrane Formation. VI. Convective Flows and Large Void Formation during Membrane Precipitation. Ind. Eng. Chem., Prod. Res. Dev. 1973, 12, 328. (21) Strathmann, H.; Kock, K.; Amar, P.; Baker, R. W. The Formation Mechanism of Asymmetric Membranes. Desalination 1975, 16, 179. (22) van de Witte, P.; Dijkstra, P. J.; van de Berg, J. W. A.; Feijen, J. Review: Phase Separation Processes in Polymer Solutions in Relation to Membrane Formation. J. Membr. Sci. 1996, 117, 1. (23) Lide, D. R. Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995. (24) Ray, R. J.; Krantz, W. B.; Sani, R. L. Linear Stability Theory Model for Finger Formation in Asymmetric Membranes. J. Membr. Sci. 1985, 23, 155. (25) Paulsen, F. G.; Shojaie, S. S.; Krantz, W. B. Effect of Evaporation Step on Macrovoid Formation in Wet-Cast Polymeric Membranes. J. Membr. Sci. 1994, 91, 265. (26) Pinnau, I.; Koros, W. J. Relation Between Substructure Resistance and Gas Separation Properties of Defect-Free IntegrallySkinned Asymmetric Membranes. Ind. Eng. Chem. Res. 1991, 20, 1837.

Received for review December 18, 1998 Revised manuscript received April 14, 1999 Accepted April 16, 1999 IE9807912