Ind. Eng. Chem. Res. 2004, 43, 1553-1556
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RESEARCH NOTES Thickness Dependence of Macrovoid Evolution in Wet Phase-Inversion Asymmetric Membranes Dongfei Li,†,‡ Tai-Shung Chung,*,† Jizhong Ren,§ and Rong Wang§ Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Institute of Material Research and Engineering, 3 Research Link, Singapore 117602, and Institute of Environmental Science & Engineering, Nanyang Technological University, Innovation Center, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723
Following an inspiration from Vogrin et al. (J. Membr. Sci. 2002, 207, 139), we have investigated the thickness dependence of macrovoid evolution during the phase inversion of asymmetric flat membranes and observed, for the first time, a critical structure-transition thickness, Lc, which indicates a transition of the membrane morphology from a spongelike to a fingerlike structure with an increase in membrane thickness. Below Lc, the membranes show a fully spongelike cross-sectional structure, whereas above Lc, the membranes exhibit a mainly fingerlike macrovoid structure. Two binary solutions are employed in this study: 20 wt % polyethersulfone/N-methyl2-pyrrolidone (NMP) and 20 wt % BTDA-MDI/TDI co-polyimide/NMP solutions. Their corresponding critical thicknesses, Lc, are 1.5 ( 0.4 and 11 ( 2 µm, respectively. Introduction Macrovoids often appear in phase-inversion membranes, and their formation mechanisms have been studied and heavily debated in the past 4 decades since Loeb and Sourirajan developed asymmetric cellulose acetate membranes for seawater desalination in the late 1950s.1-22 More importantly, the phenomenon of macrovoid formation is of great significance even for normal textile fibers made by wet spinning.2,23-25 Membrane scientists are divided on the origins of macrovoid formation. Several believe that it most likely originates from thermodynamics, so they have investigated the subject from the perspective of chemical potential gradients and phase diagrams with the aid of the Flory-Huggins theory.7,8,10-12,14,15,17 Others consider that it more likely starts from local surface instabilities and material and stress imbalances that result in weak points and induce solvent intrusion; thus, their studies emphasize convective flow or nonsolvent penetration, from the aspects of kinetics and dynamics.2-6,18,20-22 Other mechanisms have also been proposed, such as those based on Marangoni effects3,18 and osmosis pressure.19 Generally, two different structures, namely, spongelike and macrovoid (including fingerlike) configurations, have often been observed. Membranes that experience instantaneous liquid-liquid demixing tend to exhibit macrovoids, whereas membranes that experience de* To whom all correspondence should be addressed. E-mail:
[email protected]. Tel.: 65-6874-6645. Fax: 656779-1936. † National University of Singapore. ‡ Institute of Material Research and Engineering. § Nanyang Technological University.
layed demixing tend to exhibit spongelike structures. Recently, Vogrin et al.26 studied a ternary cellulose acetate/acetone/water system and reported that macrovoid formation is dependent on the membrane thickness. In their study, macrovoids appeared on membranes prepared from a 12.5 wt % casting solution at the thickness of 500 µm, but not at thicknesses of 150 and 300 µm. In this communication, we report, for the first time, that a critical structure-transition thickness has been observed for the transition of membrane morphology from a spongelike to a macrovoid-type structure. This critical structure-transition thickness exists for membranes prepared from polyethersulfone and polyimide. Experimental Section The membrane materials, polyethersulfone (PES, Radel A-300P, CAS# 25667-42-9) and BTDA-MDI/TDI co-polyimide of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA)-4,4′-diphenylmethane diisocyanate (MDI, 80%)-2,4-toylene diisocyanate (TDI, 20%) copolymer (BTDA-MDI/TDI or P-84, CAS# 58698-66-1) powder, were supplied by Amoco and Lenzing, respectively. N-Methyl-2-pyrrolidone (NMP, >99.5%, CAS# 872-504), supplied by Merck, was used as a solvent. Water as a nonsolvent additive was produced by Milli-Q ultrapure water system. All reagents were used as received without further purification. Two dope solutions were used to fabricate flat sheet membranes. One was a PES/NMP system, and the other was a BTDA-MDI/TDI (P84)/NMP system with the same polymer concentration of 20 wt % each. The membrane thickness varied from 0.5 to 50 µm. The polymer materials were dried at 110 °C in a vacuum oven for 2 days. They were then dissolved in NMP
10.1021/ie034264g CCC: $27.50 © 2004 American Chemical Society Published on Web 02/25/2004
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Figure 1. Effect of membrane thickness on PES membrane structures (dope,PES/NMP; PES 20 wt %; casting temperature, 25 °C; coagulant, water).
solutions by mechanical stirring for 2 days at room temperature. These prepared homogeneous casting solutions were degassed before use. The relative humidity in the casting environment was 70-80%. The flat sheet membranes were fabricated on a glass plate with a casting knife. After casting, the nascent membranes were immediately immersed into a water bath together with the glass plate. Later, the membranes were washed with water for 2 days at room temperature. The crosssectional morphologies of the prepared membranes were examined by scanning electron microscopy (SEM) using a JSM-6700F instrument. Results and Discussion Effect of Membrane Thickness on Membrane Morphology. Figure 1 shows the evolution of the morphology of the PES flat membranes with thickness. The PES flat membranes have a loose nodular-like structure when the thickness is extremely low at about 0.57 µm (Figure 1A). The membrane evolves into a spongelike structure as the membrane thickness increases to 0.76 µm (Figure 1B). However, with a further increase in membrane thickness, macrovoids gradually appear. When the membrane thickness is above 2 µm, a fingerlike structure is fully developed (Figure 1D, E). Similar phenomena were observed for the BTDA-MDI/ TDI/NMP system, as shown in Figure 2. The membrane has a spongelike structure when its thickness is about 8.1 µm (Figure 2A). When the thickness reaches 9.5 µm, some macrovoids form, as illustrated in Figure 2B1. Both the number and the size of the macrovoids increase
with increasing membrane thicknesses, as shown in Figure 2B2, B3, and C. A fingerlike structure is fully developed at the thickness of 22 µm (Figure 2D). Clearly, the structure of membranes prepared by the phase-inversion process shows a strong dependency on the membrane thickness. Critical Structure-Transition Thickness. Membranes with various thicknesses were cast, and their cross-sectional morphologies were examined by SEM. The thicknesses of both spongelike portions and entire membranes were measured and are summarized in Figures 3 and 4. Three regions could be identified along the abscissa corresponding to different membrane structures. In region I, the thickness of the spongelike portions is the same as the thickness of the entire membranes. It means that the cross sections of the membranes have a fully spongelike structure. The membrane morphology transitions from a spongelike to a fingerlike structure with some degrees of fluctuation in sponge thickness in region II. In region III, the membranes are of mainly fingerlike structure with an almost constant thickness of spongelike portion, which is independent of overall membrane thickness. On the basis of the above analysis, it is reasonable to deduce that there exists a critical structure-transition thickness, Lc, that reflects the transition of membrane morphology from a spongelike to a fingerlike structure during the formation of asymmetric flat membranes. Lc is about 1.5 ( 0.4 µm for the 20% PES/NMP dope solution and about 11 ( 2 µm for the 20% BTDA-MDI/ TDI/NMP dope solution.
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Figure 2. Effect of membrane thickness on BTDA-MDI/TDI co-polyimide membrane structures (dope, BTDA-MDI/TDI/NMP; BTDAMDI/TDI, 20 wt %; casting temperature, 25 °C; coagulant: water).
Figure 3. Effect of membrane thickness on the thickness of spongelike portion of PES membranes (dope, PES/NMP; PES, 20 wt %; casting temperature, 25 °C; coagulant, water).
The critical structure-transition thickness Lc is different for different dope solutions possibly because the solutions have different viscosities, surface energies, phase diagrams, and many other characteristics. Future
Figure 4. Effect of membrane thickness on the thickness of spongelike portion of BTDA-TDI/MDI membranes (dope, BTDAMDI/TDI/NMP; BTDA-MDI/TDI, 20 wt %; casting temperature, 25 °C; coagulant, water).
studies will be focused on the relationship between Lc and the physical chemistry of the dope solution properties.
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Conclusion The thickness dependence of macrovoid evolution during the phase-inversion process of asymmetric flat membranes was studied using 20% PES/NMP and 20% BTDA-MDI/TDI/NMP dope solutions. It was found that the membrane morphology strongly depends on the membrane thickness. A critical structure-transition thickness, Lc, was observed for the two systems, indicating the transition of the membrane morphology from a spongelike to a fingerlike structure with an increase in membrane thickness. Acknowledgment The authors thank NUS for funding this research through Grant R-279-000-108-112. Special thanks are due to Professor D. R. Paul at University of Texas at Austin for the provision of valuable references and comments. Literature Cited (1) Loeb, S.; Sourirajan, S. Sea Water Demineralization by means of an Osmotic Membrane. Adv. Chem. Ser. 1963, 38, 117. (2) Graig, J. P.; Knudsen, J. P.; Holland, V. F. Characterization of Acrylic Fiber Structure. Text. Res. J. 1962, 32, 435. (3) Levich, V. G.; Krylov, V. S. Surface-Tension-Driven Phenomena. Annu. Rev. Fluid Mech. 1969, 1, 293. (4) Matz, R. The Structure of Cellulose Acetate Membranes 1. The Development of Porous Structures in Anisotropic Membranes. Desalination 1972, 10, 1. (5) Strathmann, H.; Kock, K.; Amar, P.; Baker, R. W. The Formation Mechanism of Asymmetric Membranes. Desalination 1975, 16, 179. (6) Strathmann, H.; Kock, K. The Formation Mechanism of Phase Inversion Membranes. Desalination 1977, 21, 241. (7) Cohen, C.; Tanny, G. B.; Prager, S. Diffusion-Controlled Formation of Porous Structures in Ternary Polymer Systems. J. Polym. Sci. B: Polym. Phys. 1979, 17, 477. (8) Broens, L.; Altena, F. W.; Smolders, C. A.; Koenhen, D. M. Asymmetric Membrane Structures as a Result of Phase Separation Phenomena. Desalination 1980, 32, 33. (9) Uragami, T.; Ohsumi, Y.; Sugihara, M. Studies on Syntheses and Permeabilities of Special Polymer Membranes. 40. Formation Conditions of Finger-like Cavities of Cellulose Nitrate Membranes. Desalination 1981, 37, 293. (10) Altena, F. W.; Smolders, C. A. Calculation of LiquidLiquid Phase Separation in a Ternary System of a Polymer in a Mixture of a Solvent and a Nonsolvent. Macromolecules 1982, 15, 1491. (11) McHugh, A. J.; Yilmaz, L. The Diffusion Equation for Polymer Membrane Formation in Ternary Systems. J. Polym. Sci. B: Polym. Phys. 1985, 23, 1271.
(12) Yilmaz, L.; McHugh, A. J. Analysis of NonsolventSolvent-Polymer Phase Diagrams and Their Relevance to Membrane Formation Modeling. J. Appl. Polym. Sci. 1986, 31, 997. (13) McDonogh, R. M.; Fell, C. J. D.; Fane, A. G. Characteristics of Membranes Formed by Acid Dissolution of Polyamides. J. Membr. Sci. 1987, 31, 321. (14) Reuvers, A. J.; Van den Berg, J. W. A.; Smolders, C. A. Formation of Membranes by Means of Immersion Precipitation. Part I: A Model to Describe Mass Transfer during Immersion Precipitation. J. Membr. Sci. 1987, 34, 45. (15) Reuvers, A. J.; Smolders, C. A. Formation of Membranes by means of Immersion Precipitation. Part II: The Mechanism of Formation of Membranes Prepared from the System Cellulose Acetate-Acetone-Water. J. Membr. Sci. 1987, 34, 67. (16) Yao, C. W.; Burford, R. P.; Fane, A. G.; Fell, C. J. D. Effect of Coagulation Conditions on Structure and Properties of Membranes from Aliphatic Polyamides. J. Membr. Sci. 1988, 38, 113. (17) Smolders, C. A.; Reuvers, A. J.; Boom, R. M.; Wienk, I. M. Microstructures in Phase-Inversion Membranes. 1. Formation of Macrovoids. J. Membr. Sci. 1992, 73, 259. (18) Shojaie, S. S.; Krantz, W. B.; Greenberg, A. R. Dense Polymer Film and Membrane Formation via the Dry-Cast Process. 1. Model Validation and Morphological Study. J. Membr. Sci. 1994, 94, 281. (19) McKelvey, S. A.; Koros, W. J. Phase Separation, Vitrification, and the Manifestation of Macrovoids in Polymeric Asymmetric Membranes. J. Membr. Sci. 1996, 112, 29. (20) Chung, T. S.; Kafchinski, E. R. The Effects of Spinning Conditions on Asymmetric 6FDA/6FDAM Polyimide Hollow Fibers for Air Separation. J. Appl. Polym. Sci. 1997, 65, 1555. (21) Chung, T. S.; Hu, X. D. Effect of Air-Gap Distance on the Morphology and Thermal Properties of Polyethersulfone Hollow Fibers. J. Appl. Polym. Sci. 1997, 66, 1067. (22) Pekny, M. R.; Zartman, J.; Krantz, W. B.; Greenberg, A. R.; Todd, P. Flow Visualization during Macrovoid Pore Formation in Dry-Cast Cellulose Acetate Membranes. J. Membr. Sci. 2003, 211, 71. (23) Knudsen, J. P. The Influence of Coagulation Variables on the Structure and Physical Properties of an Acrylic Fiber. Text. Res. J. 1963, 33, 13. (24) Takahashi, M.; Nukushina, Y.; Kosugi, S. Effect of FiberForming Conditions on the Microstructure of Acrylic Fiber. Text. Res. J. 1964, 34, 87. (25) Epstein, M. E.; Rosenthal, A. J. Spinning of Polyamides from Sulfuric Acid Solution. Polymer Solubility and Coagulation Mechanisms. Text. Res. J. 1966, 36, 813. (26) Vogrin, N.; Stropnik, C.; Musil, V.; Brumen, M. The Wet Phase Separation: the Effect of Cast Solution Thickness on the Appearance of Macrovoids in the Membrane Forming Ternary Cellulose Acetate/Acetone/Water System. J. Membr. Sci. 2002, 207, 139.
Received for review November 22, 2003 Revised manuscript received February 6, 2004 Accepted February 11, 2004 IE034264G
