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Formation of integrally skinned asymmetric polysulfone gas separation membranes by supercritical CO2
Journal of Membrane Science 320 (2008) 431–435
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Formation of integrally skinned asymmetric polysulfone gas separation membranes by supercritical CO2 b ˜ a,∗ , Gabriel Luna-Barcenas ´ Alondra Torres-Trueba a , F. Alberto Ruiz-Trevino , a,∗ Ciro H. Ortiz-Estrada a b
Chemical Science and Engineering Department, Universidad Iberoamericana, Prol. Paseo de la Reforma No. 880, Lomas de Santa Fe, M´exico, D.F., C.P. 01219, Mexico Materials Research Laboratory, CINVESTAV-Quer´etaro, Quer´etaro, Qro., C.P. 76230, Mexico
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 1 April 2008 Accepted 14 April 2008 Available online 20 April 2008 Keywords: Polysulfone Asymmetric membrane Supercritical CO2 Gas permeability
a b s t r a c t Integrally skinned asymmetric polysulfone membranes were prepared from originally dense films inducing asymmetry by the formation of the porous layer adding to one side of the membranes chloroform and supercritical CO2 (SCCO2 ), and then allowing the SCCO2 expansion to occur. The influence of the chloroform/polysulfone mass ratio (g CH3 Cl/g PSF), SCCO2 density and depressurization rate over the thickness of both the porous and the dense skin layers, the morphology of the porous support and the pure O2 and N2 permeability and selectivity performance were studied. The results show that it is possible to induce a very-controlled asymmetry in a dense film following the procedure described in this work and as expected, the thickness of the porous layer increases while the dense skin layer decreases as the chloroform/polysulfone mass ratio increases. Images of the porous layer show that the average-pore size decreases at high SCCO2 densities and slightly decreases with increasing the CO2 depressurization rates. The O2 and N2 permeability coefficients, measured at 35 ◦ C and 2 bar, for the polysulfone asymmetric membranes are practically the same of those determined in dense films, suggesting that the dense skins are essentially defect-free of pinholes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The use of CO2 supercritical fluid to produce porous polymeric membrane materials that may be useful in micro, ultra and nano filtration, dialysis and reverse osmosis has been extensively studied with different polymers such as nylon [1], polystyrene [2], cellulose acetate [3,4], polycarbonate [5], polylactide acid [6,7], polysulfone [8–10], poly(vinylidene fluoride-co-hexafluoropropylene) [11,12], poly-vinyl-alcohol [13], poly(methyl methacrylate) [14], modified poly(ether ether ketone) [15], and more recently poly(l-lactic acid) [16] and poly(vinylidene fluoride) [17], but to best of our knowledge, it has not yet been used to produce integrally skinned asymmetric membranes for the gas separation processes. A typical process to produce integrally skinned asymmetric membranes is the dry/wet phase inversion [18–24] that is based upon creating a dense skin layer of the polymer membrane by first allowing the evaporation of the solvent in a cast membrane (flat or hollow fiber), and then creating the porous support by
∗ Corresponding authors. Tel.: +52 55 5950 4389; fax: +52 55 5950 4225. ˜ E-mail addresses: [email protected] (F.A. Ruiz-Trevino), [email protected] (C.H. Ortiz-Estrada). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.04.024
solvent/nonsolvent exchange during a quench step [19]. Pinnau and Koros [19,22,24,25], Pinnau et al. [18] and Mikawa et al. [23] have produced membranes with this method obtaining essentially defect-free integrally skinned asymmetric membranes with optimum combinations of gas permeance, P(i)/L, and gas separation selectivity, P(i)/P(j). Nevertheless, many organic solvents, nonsolvents and quenching agents are used in this method; most of them are extremely volatile, flammable, toxic, expensive and hazardous for the environment and health as well. In response to this problem, a new alternative has emerged based upon the substitution of some of the solvents or nonsolvents by supercritical CO2 (SCCO2 ) which may act as a solvent and nonsolvent simultaneously in a ternary mixture [1–17,26–28]. The substitution of any conventional solvent/nonsolvent in the formation of membranes materials by SCCO2 has technological advantages since the membrane morphology can be manipulated with variables such as SCCO2 density, depressurization rates among others and, due to the rapid expansion of the CO2 during the depressurization step, a totally dried asymmetric membrane is formed without the need of further posttreatment (energy efficiency). Even more, the CO2 , a nontoxic, flammable and environmentally friendly component, can be easily recovered just changing from the supercritical conditions to the gas conditions.
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A. Torres-Trueba et al. / Journal of Membrane Science 320 (2008) 431–435
In this work an alternative to the typical dry/wet phase inversion process has been explored for the formation of asymmetric membranes using SCCO2 . The method consists of forming the porous structure layer using an already dense thick film, by contacting one face of the membrane with solvent and SCCO2 for several minutes, and then allowing the extraction of solvent/SCCO2 by reducing the CO2 pressure. The effect of solvent/polymer mass ratio, SCCO2 density and depressurization rate on the reduction of the dense skin layer, the morphology of the porous support and the O2 and N2 permeability coefficients and selectivity factor is studied using polysulfone as the membrane material.
chamber for 30 min in order to form a ternary mixture of polymer/solvent/nonsolvent. Then, the pressure was diminished and a dry polysulfone asymmetric membrane was obtained. All experiments were conducted at constant temperature in a high-pressure stainless steel (60 cm3 ) vessel where the CO2 is charged by a manual feeding device (pressure measured by a Sensotec, TJF/7039-03). The CO2 densities studied in this work were set at different temperatures (35–50 ◦ C) and pressures (140–245 bar) and calculated from the program CO2 PAC which uses the equation of state for CO2 of Wagner [29]. 2.4. Membrane structure characterization
2. Experimental
Polysulfone (Mn = 16,000) was obtained from Sigma–Aldrich, chloroform (purity 99.8%) was from J.T. Baker, CO2 (purity 99.99%) was from Praxair, and ultrahigh purity O2 and N2 were from AGA gas. All materials except chloroform were used as received and without any further purification.
A low-vacuum scanning electron microscope (SEM JEOL 5600) at an accelerating voltage of 20 kV was used to observe the porous structure of the integrally skinned asymmetric polysulfone membranes. Approximately 300–500 pores, from the images obtained by SEM, were used to determine the pore size distribution and the average-pore size. The Digital Micrograph software (Gatan, Inc.) was used to determine the size of the pores and the thickness of both the porous and dense layers of the asymmetric membranes.
2.2. Dense membrane preparation
2.5. Gas permeability measurements
A 5 wt% solution of polysulfone in chloroform (previously distillated) was poured onto cellulose paper/glass plates and then dried for 1 day at room temperature. Thereafter, the polymer films were removed from the cellulose paper and dried in a natural oven for 24 h at 50 ◦ C. After that the polymer films were vacuum dried for 3 days by gradually increasing the temperature from 50 to 150 ◦ C. Finally the temperature was held constant at 150 ◦ C for 24 h to eliminate any residual solvent. All membranes were treated according to the same procedure and their thicknesses, measured with a Mitutoyo caliper, varied from 100 to 130 m.
A standard permeation cell was used to determine the pure gas permeance (Pi /L), at 35 ◦ C and 2 bar upstream gas pressure, for the integrally skinned asymmetric polysulfone membranes. The characteristics of a typical determination and equipment details are described elsewhere [30]. Ultrahigh purity gases O2 and N2 were used and the pure gas permeance for each gas was determined from the slope of the downstream pressure versus time plot once a steady state had been achieved. The permeability coefficients, P(i), were determined from the pure gas permeance values using the real thickness of the dense skin layer determined from the SEM images of the same membranes used for the permeation experiments.
2.1. Materials
2.3. Asymmetric membrane preparation 3. Results and discussion A typical experiment to produce the porous layer in a polysulfone dense film consists of delivering a predetermined amount of chloroform (controlled as the chloroform/polysulfone mass ratio) in a well-defined film area using aluminum adhesive to delimit the area and to fix the films into a clean glass support. Then, the glass with the membrane was immediately transferred into a high-pressure vessel and CO2 was introduced into it until the desired pressure was reached. The membranes were left into the
3.1. Effect of the solvent/polymer mass ratio Fig. 1 shows the SEM cross-section images of integrally skinned asymmetric polysulfone membranes, with a CH3 Cl/PSF mass ratio (a) 2.45 and (b) 3.07, formed from original dense films (∼101 m thickness) with a SCCO2 density of 0.7986 g/cm3 (35 ◦ C; 140 bar) and 718 bar/min depressurization rate. The original polysulfone
Fig. 1. Integrally skinned asymmetric polysulfone membranes formed treating one face of a dense film with a CH3 Cl/PSF mass ratio (a) 2.45 and (b) 3.07, and 30 min exposure with SCCO2 at a density = 0.7986 g/cm3 and depressurization rate = 718 bar/min.
A. Torres-Trueba et al. / Journal of Membrane Science 320 (2008) 431–435
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Fig. 2. Real thickness of the dense skin of integrally skinned asymmetric polysulfone membranes formed at different CH3 Cl/PSF mass ratios, 30 min exposure of SCCO2 at a density = 0.7986 g/cm3 (35 ◦ C; 140 bar) and depressurization rate = 718 bar/min.
Fig. 4. Effect of SCCO2 density on the average-pore size of a porous PSF layer formed from an already dense PSF film treated on one side with a CH3 Cl/PSF mass ratio of 7.44, 30 min SCCO2 exposure and 718 bar/min depressurization rate.
dense films have been modified into asymmetric membranes that have a sponge-like structure layer and a dense or selective skin. Moreover, the pores may be practically considered spherical. Fig. 2 shows the effect of the chloroform/polysulfone ratio on the reduction of the original thickness of the membrane. As it was expected, there is a linear decrease in the thickness of the selective or dense skin, accompanied by an increase in the thickness of the porous layer, as the CH3 Cl/PSF mass ratio increases. To explain this behavior it is necessary to understand the mechanism related to this process and take into account thermodynamic aspects. In related works where asymmetric membranes are obtained by phase inversion with SCCO2 [2,8,13,17], porous membrane formation departs from a homogeneous polymeric solution at a given concentration of polymer/solvent/nonsolvent. In these cases, the porous structure is generated by the liquid–liquid demixing obtained by nucleation and growth of the polymer lean phase; and the dense skin is formed by the solvent outflows from the solution before the SCCO2 causes the phase separation. In this work, a porous sublayer is obtained
by the dissolution of polymer when solvent is added on one side of an already formed dense membrane followed by the incorporation of SCCO2 . The dense layer is already present but its final thickness can be controlled by the solvent/polymer mass ratio and the time elapsed before the SCCO2 is incorporated. When this homogenous solvent/polymer solution membrane enters in contact with SCCO2 , the nonsolvent, a phase separation process takes place on the ternary mixture that is characterized by the formation of droplets of polymer lean phase disseminated inside the polymer rich phase. During the depressurization step, the solvent and CO2 are dragged out of the system and solidification of the polymer rich phase takes place forming a dry asymmetric membrane. These results are important because they show a different way to produce asymmetric membranes with variables that can be controlled in an easy way. To the best of our knowledge, this method is an alternative to those published in the literature since it can be used to produce asymmetric structures in an already formed film and/or hollow fiber.
Fig. 3. Effect of SCCO2 density over the average-pore size of a porous layer formed from an already polymer thick film treated on one side with a CH3 Cl/PSF mass ratio of 7.44, 30 min exposure of SCCO2 and 718 bar/min depressurization rate. (a) 0.6660 g/cm3 (50 ◦ C; 140 bar), 0.7986 g/cm3 (35 ◦ C; 140 bar), (c) 0.8456 g/cm3 (35 ◦ C; 175 bar), (d) 0.8980 g/cm3 (35 ◦ C; 245 bar).
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Fig. 5. Effect of SCCO2 depressurization rate on the pore size distribution of a PSF porous layer formed on an already dense film treated on one side with a CH3 Cl/PSF mass ratio of 7.44 and 30 min SCCO2 exposure.
3.2. Effect of SCCO2 density Fig. 3 shows cross-section images of integrally skinned asymmetric polysulfone membrane morphologies produced at different SCCO2 densities and with a 7.44 solvent/polymer mass ratio and 718 bar/min depressurization rate that were held constants. It is important to mention that PSF thick films (130 m) and a really high CH3 Cl/PSF mass ratio were selected to study the effect of SCCO2 density and depressurization rate since bigger cross-section areas facilitate the measurement of the porous size to statistically justify the determination of the average-pore size and its distribution. As the SCCO2 density increases, the sponge-like polysulfone porous layer presents smaller pore sizes as it is shown in Fig. 4. Similar behaviors were observed by Reverchon and Cardea [8] for the PSF–NMP–SCCO2 and PSF–CH3 Cl–SCCO2 systems but at different densities values. This behavior is due to the change of the solvent power of SCCO2 . At high densities there is major solvent power of the SCCO2 which means a larger amount of solvent in the polymer lean phase (SCCO2 –CH3 Cl) and a higher viscosity of the polymer rich phase (PSF–CH3 Cl). The higher viscosity of the polymer rich phase prevents the nuclei growth of the polymer lean phase generating pores of minor size. When density decreases, its solvent power decreases as well, this translates into a smaller amount of solvent in the polymer lean phase and a lower viscosity of the polymer rich phase. Lower viscosities of the polymer rich phase allows the growth of the nuclei of the polymer lean phase generating pores of major size.
size in the porous layer shows a relatively small decrease as the depressurization rate increases. This observation is in accordance to that reported by Xu et al. [7] for the system PLA–THF–SCCO2 and opposite from the results observed by Temtem et al. [9] for the PSF–CH3 Cl–SCCO2 and PSF–N,N-dimethylacetamide–SCCO2 systems. Temtem et al. [9] observed the average-pore size decreases with prolonged depressurization rates but such differences may be due to the mode the porous are formed since they operated at a continuous mode with a CO2 constant flow, while the results reported here are obtained in a batch mode just as in the case of Xu et al. A possible explanation may found in the foaming action of the CO2 during depressurization as it was pointed out by Xu et al. When pressure is diminished the SCCO2 present on the nuclei of polymer lean phase becomes in gaseous CO2 and the nuclei has more time to grow at lower depressurization rates, thus major pores are obtained. In contrast, at higher depressurization rates the nuclei have less time to grow and smaller pores are obtained. However, an additional observation is that for the PSF/CH3 Cl/CO2 system studied in this work, the effect of the depressurization rate of CO2 shows a relatively small influence on the average-pore size probably indicating that the foaming action of CO2 does no play an important role on generating different average-pore sizes. This could be an interesting result because it says that the porous structure is formed, and it is relatively stable, before the depressurization step is applied. Of course, a more detailed study addressing this point could support this statement but it is not the subject of this paper.
3.4. Gas separation properties 3.3. Effect of SCCO2 depressurization rate The effect of the depressurization rate over the pore size distribution and the average-pore size on the porous layer was also investigated at least for two depressurization rates and using membranes formed with a 7.44 chloroform/polysulfone mass ratio and a SCCO2 density of 0.7989 g/cm3 (35 ◦ C; 140 bar). The results are reported in Fig. 5 where it can be seen that the average-pore
Table 1 shows the gas permeance, P(i)/L, and permeability P(i) coefficients for O2 and N2 , as well as the O2 /N2 selectivity, measured for the integrally skinned asymmetric polysulfone membranes formed under the alternative proposed in this work. The gas permeance coefficients increase as the thickness of the dense or selective layer decreases as it would be expected, but more importantly, their O2 /N2 selectivity remains constant leading to the conclusion that
Table 1 Thickness, gas permeance and permeability coefficients for O2 and N2 , at 35 ◦ C and 2 bar, in integrally skinned asymmetric polysulfone membranes formed from an already dense film (∼101 m) Final dense skin thickness (m)
RO2 = PO2 /L (GPU)
RN2 = PN2 /L (GPU)
PO2 (Barrer)
PN2 (Barrer)
Selectivity O2 /N2
51.13 43.72 40.57 38.58 35.05 31.08 26.16
0.0253 0.0276 0.0332 0.0359 0.0381 0.0419 0.0532
0.00497 0.00528 0.00653 0.00654 0.00741 0.0075 0.00987
1.293 1.207 1.347 1.385 1.335 1.302 1.391
0.254 0.230 0.265 0.252 0.259 0.233 0.258
5.28 5.23 5.08 5.49 5.15 5.59 5.39
A. Torres-Trueba et al. / Journal of Membrane Science 320 (2008) 431–435
their dense thicknesses are defect-free of any pinhole that could be created by the diffusion of the solvents involved during their formation. 4. Conclusions Integrally skinned asymmetric polysulfone membranes useful for gas separation were prepared by an alternative method to the dry/wet phase inversion processes using chloroform and supercritical CO2 on one side of an already formed polysulfone dense film which avoids the need of the preparation of a typical ternary casting solution. The results proved the success of the method since the appropriate handling of variables such as the solvent/polymer mass ratio, the SCCO2 density and the depressurization rate can lead to the formation of a totally dried defect-free dense film supported on well-defined porous morphology when an already polymer dense film has been processed. It is also shown that the averagepore size of the porous layer decreases with increasing the density of the SCCO2 and slightly decreases as the depressurization rate increases. Acknowledgments Authors acknowledge the financial support from CONACYT ´ (Mexico) throughout project 52631-Y. Alondra Torres Trueba thanks Universidad Iberoamericana and CONACYT for their financial support of her master degree studies. Authors also acknowledge the technical support from the Central Microscopy Laboratory of the Physical Institute-UNAM and CINVESTAV-Queretaro. References [1] Y.W. Kho, D.S. Kalika, B.L. Knutson, Precipitation of nylon 6 membranes using compressed carbon dioxide, Polymer 42 (2001) 6119. [2] H. Matsuyama, H. Yano, T. Maki, M. Teramoto, K. Mishima, K. Matsuyama, Formation of porous flat membrane by phase separation with supercritical CO2 , J. Membr. Sci. 194 (2001) 157. [3] H. Matsuyama, A. Yamamoto, H. Yano, T. Maki, M. Teramoto, K. Mishima, K. Matsuyama, Effect of organic solvents on membrane formation by phase separation with supercritical CO2 , J. Membr. Sci. 204 (2002) 81. [4] E. Reverchon, S. Cardea, Formation of cellulose acetate membranes using a supercritical fluid assisted process, J. Membr. Sci. 240 (2004) 187. [5] M.S. Kim, S.J. Lee, Characteristics of porous polycarbonate membrane with polyethylene glycol in supercritical CO2 and effect of its porosity on tearing stress, J. Supercrit. Fluid 31 (2004) 217. [6] Q. Xu, M. Pang, Q. Peng, Y. Jiang, J. Li, Application of supercritical carbon dioxide in the preparation of biodegradable polylactide membranes, J. Appl. Polym. Sci. 94 (2004) 2158. [7] Q. Xu, M. Pang, Q. Peng, Y. Jiang, J. Li, H. Wang, M. Zhu, Effect of different experimental conditions on biodegradable polylactide membranes prepared with supercritical CO2 as nonsolvent, J. Appl. Polym. Sci. 98 (2005) 831. [8] E. Reverchon, S. Cardea, Formation of polysulfone membranes by supercritical CO2 , J. Supercrit. Fluids 35 (2005) 140.
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[9] M. Temtem, T. Casimiro, A. Aguilar-Ricardo, Solvent power and depressurization rate effects in the formation of polysulfone membranes with CO2 -assisted phase inversion method, J. Membr. Sci. 283 (2006) 244. [10] M. Temtem, T. Casimiro, J.F. Mano, A. Agular-Ricardo, Preparation of membranes with polysulfone/polycaprolactone blends using a high pressure cell specially designed for a CO2 -assisted phase inversion, J. Supercrit. Fluids 43 (2008) 542. [11] J.-H. Cao, B.-K. Zhu, G.-L. Ji, Y.-Y. Xu, Preparation and characterization of PVDF–HFP microporous flat membranes by supercritical CO2 induced phase separation, J. Membr. Sci. 266 (2005) 102. [12] E. Reverchon, S. Cardea, PVDF–HFP membrane formation by supercritical CO2 processing: elucidation of formation mechanisms, Ind. Eng. Chem. Res. 45 (2006) 8939. [13] E. Reverchon, S. Cardea, C. Rapuano, Formation of poly-vinyl-alcohol structures by supercritical CO2 , J. Appl. Polym. Sci. 104 (2006) 3151. [14] E. Reverchon, E. Schiavo Rappo, S. Cardea, Flexible supercritical CO2 -assisted process for poly(methyl methacrylate) structure formation, Polym. Eng. Sci. 46 (2006) 188. [15] S. Cardea, A. Gugliuzza, E. Schiavo Rappo, M. Aceto, E. Drioli, E. Reverchon, Generation of PEEK-WC membranes by supercritical fluids, Desalination 200 (2006) 58. [16] I. Tsivintzelis, E. Pavlidou, C. Panayiotou, Porous scaffolds prepared by phase inversion using supercritical CO2 as antisolvent i. poly(l-lactic acid), J. Supercrit. Fluids 40 (2007) 317. [17] S. Huang, G. Wu, S. Chen, Preparation of microporous poly(vinylidene fluoride) membranes via phase inversion in supercritical CO2 , J. Membr. Sci. 293 (2007) 100. [18] I. Pinnau, J. Wind, K.-V. Peinemann, Ultrathin multicomponent poly(ether sulfone) membranes for gas separation made by dry/wet phase inversion, Ind. Eng. Chem. Res. 29 (1990) 2028. [19] I. Pinnau, W.J. Koros, Structures and gas separation properties of asymmetric polysulfone membranes made by dry, wet, and dry/wet phase inversion, J. Appl. Polym. Sci. 43 (1991) 1491. [20] I. Pinnau, Skin formation of integral-asymmetric gas separation membranes made by dry/wet phase inversion, PhD Dissertation, The University of Texas at Austin, 1991. [21] P.H. Pfromm, I. Pinnau, W.J. Koros, Gas transport through integral-asymmetric membranes: a comparison to isotropic film transport properties, J. Appl. Polym. Sci. 48 (1993) 2161. [22] I. Pinnau, W.J. Koros, Relationship between substructure resistance and gas separation properties of defect-free integrally skinned asymmetric membranes, Ind. Eng. Chem. Res. 30 (1991) 1837. [23] M. Mikawa, N. Seki, S. Nagaoka, H. Kawakami, Structure and gas permeability of asymmetric polyimide membranes made by dry-wet phase inversion: influence of alcohol as casting solution, J. Polym. Sci. B: Polym. Phys. 45 (2007) 2739. [24] I. Pinnau, W.J. Koros, A qualitative skin layer formation mechanism for membranes made by dry/wet phase inversion, J. Polym. Sci. B: Polym. Phys. 31 (1993) 419. [25] I. Pinnau, W.J. Koros, Gas permeation properties of asymmetric polycarbonate, polyestercarbonate and fluorinated polyimide membranes prepared by the generalized dry/wet phase inversion process, J. Appl. Polym. Sci. 46 (1992) 1195. [26] C.-F. Zhang, B.-K. Zhu, G.-L. Ji, Y.-Y. Xu, Supercritical carbon dioxide extraction in membrane formation by thermally induced phase separation, J. Appl. Polym. Sci. 103 (2007) 1632. [27] E. Reverchon, S. Cardea, E.S. Rappo, Production of loaded PMMA structures using the supercritical CO2 phase inversion process, J. Membr. Sci. 273 (2006) 97. [28] A.I. Cooper, Porous materials and supercritical fluids, Adv. Mater. 15 (2003) 1049. [29] J.F. Ely, NIST CO2PAC: a computer program to calculate physical properties of pure CO2 , in: Proceedings of the Annual Convention of the Gas Processors Association, San Antonio, Texas, 1986. ˜ ˜ M.G. Zolotukhin, L.F. del Castillo, J. Guz[30] C. Camacho-Zuniga, F.A. Ruiz-Trevino, man, Gas transport properties of new aromatic cardo poly(aryl ether ketone)s, J. Membr. Sci. 283 (2006) 393.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Formation of integrally skinned asymmetric polysulfone gas separation membranes by supercritical CO2 b ˜ a,∗ , Gabriel Luna-Barcenas ´ Alondra Torres-Trueba a , F. Alberto Ruiz-Trevino , a,∗ Ciro H. Ortiz-Estrada a b
Chemical Science and Engineering Department, Universidad Iberoamericana, Prol. Paseo de la Reforma No. 880, Lomas de Santa Fe, M´exico, D.F., C.P. 01219, Mexico Materials Research Laboratory, CINVESTAV-Quer´etaro, Quer´etaro, Qro., C.P. 76230, Mexico
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 1 April 2008 Accepted 14 April 2008 Available online 20 April 2008 Keywords: Polysulfone Asymmetric membrane Supercritical CO2 Gas permeability
a b s t r a c t Integrally skinned asymmetric polysulfone membranes were prepared from originally dense films inducing asymmetry by the formation of the porous layer adding to one side of the membranes chloroform and supercritical CO2 (SCCO2 ), and then allowing the SCCO2 expansion to occur. The influence of the chloroform/polysulfone mass ratio (g CH3 Cl/g PSF), SCCO2 density and depressurization rate over the thickness of both the porous and the dense skin layers, the morphology of the porous support and the pure O2 and N2 permeability and selectivity performance were studied. The results show that it is possible to induce a very-controlled asymmetry in a dense film following the procedure described in this work and as expected, the thickness of the porous layer increases while the dense skin layer decreases as the chloroform/polysulfone mass ratio increases. Images of the porous layer show that the average-pore size decreases at high SCCO2 densities and slightly decreases with increasing the CO2 depressurization rates. The O2 and N2 permeability coefficients, measured at 35 ◦ C and 2 bar, for the polysulfone asymmetric membranes are practically the same of those determined in dense films, suggesting that the dense skins are essentially defect-free of pinholes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The use of CO2 supercritical fluid to produce porous polymeric membrane materials that may be useful in micro, ultra and nano filtration, dialysis and reverse osmosis has been extensively studied with different polymers such as nylon [1], polystyrene [2], cellulose acetate [3,4], polycarbonate [5], polylactide acid [6,7], polysulfone [8–10], poly(vinylidene fluoride-co-hexafluoropropylene) [11,12], poly-vinyl-alcohol [13], poly(methyl methacrylate) [14], modified poly(ether ether ketone) [15], and more recently poly(l-lactic acid) [16] and poly(vinylidene fluoride) [17], but to best of our knowledge, it has not yet been used to produce integrally skinned asymmetric membranes for the gas separation processes. A typical process to produce integrally skinned asymmetric membranes is the dry/wet phase inversion [18–24] that is based upon creating a dense skin layer of the polymer membrane by first allowing the evaporation of the solvent in a cast membrane (flat or hollow fiber), and then creating the porous support by
∗ Corresponding authors. Tel.: +52 55 5950 4389; fax: +52 55 5950 4225. ˜ E-mail addresses: [email protected] (F.A. Ruiz-Trevino), [email protected] (C.H. Ortiz-Estrada). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.04.024
solvent/nonsolvent exchange during a quench step [19]. Pinnau and Koros [19,22,24,25], Pinnau et al. [18] and Mikawa et al. [23] have produced membranes with this method obtaining essentially defect-free integrally skinned asymmetric membranes with optimum combinations of gas permeance, P(i)/L, and gas separation selectivity, P(i)/P(j). Nevertheless, many organic solvents, nonsolvents and quenching agents are used in this method; most of them are extremely volatile, flammable, toxic, expensive and hazardous for the environment and health as well. In response to this problem, a new alternative has emerged based upon the substitution of some of the solvents or nonsolvents by supercritical CO2 (SCCO2 ) which may act as a solvent and nonsolvent simultaneously in a ternary mixture [1–17,26–28]. The substitution of any conventional solvent/nonsolvent in the formation of membranes materials by SCCO2 has technological advantages since the membrane morphology can be manipulated with variables such as SCCO2 density, depressurization rates among others and, due to the rapid expansion of the CO2 during the depressurization step, a totally dried asymmetric membrane is formed without the need of further posttreatment (energy efficiency). Even more, the CO2 , a nontoxic, flammable and environmentally friendly component, can be easily recovered just changing from the supercritical conditions to the gas conditions.
432
A. Torres-Trueba et al. / Journal of Membrane Science 320 (2008) 431–435
In this work an alternative to the typical dry/wet phase inversion process has been explored for the formation of asymmetric membranes using SCCO2 . The method consists of forming the porous structure layer using an already dense thick film, by contacting one face of the membrane with solvent and SCCO2 for several minutes, and then allowing the extraction of solvent/SCCO2 by reducing the CO2 pressure. The effect of solvent/polymer mass ratio, SCCO2 density and depressurization rate on the reduction of the dense skin layer, the morphology of the porous support and the O2 and N2 permeability coefficients and selectivity factor is studied using polysulfone as the membrane material.
chamber for 30 min in order to form a ternary mixture of polymer/solvent/nonsolvent. Then, the pressure was diminished and a dry polysulfone asymmetric membrane was obtained. All experiments were conducted at constant temperature in a high-pressure stainless steel (60 cm3 ) vessel where the CO2 is charged by a manual feeding device (pressure measured by a Sensotec, TJF/7039-03). The CO2 densities studied in this work were set at different temperatures (35–50 ◦ C) and pressures (140–245 bar) and calculated from the program CO2 PAC which uses the equation of state for CO2 of Wagner [29]. 2.4. Membrane structure characterization
2. Experimental
Polysulfone (Mn = 16,000) was obtained from Sigma–Aldrich, chloroform (purity 99.8%) was from J.T. Baker, CO2 (purity 99.99%) was from Praxair, and ultrahigh purity O2 and N2 were from AGA gas. All materials except chloroform were used as received and without any further purification.
A low-vacuum scanning electron microscope (SEM JEOL 5600) at an accelerating voltage of 20 kV was used to observe the porous structure of the integrally skinned asymmetric polysulfone membranes. Approximately 300–500 pores, from the images obtained by SEM, were used to determine the pore size distribution and the average-pore size. The Digital Micrograph software (Gatan, Inc.) was used to determine the size of the pores and the thickness of both the porous and dense layers of the asymmetric membranes.
2.2. Dense membrane preparation
2.5. Gas permeability measurements
A 5 wt% solution of polysulfone in chloroform (previously distillated) was poured onto cellulose paper/glass plates and then dried for 1 day at room temperature. Thereafter, the polymer films were removed from the cellulose paper and dried in a natural oven for 24 h at 50 ◦ C. After that the polymer films were vacuum dried for 3 days by gradually increasing the temperature from 50 to 150 ◦ C. Finally the temperature was held constant at 150 ◦ C for 24 h to eliminate any residual solvent. All membranes were treated according to the same procedure and their thicknesses, measured with a Mitutoyo caliper, varied from 100 to 130 m.
A standard permeation cell was used to determine the pure gas permeance (Pi /L), at 35 ◦ C and 2 bar upstream gas pressure, for the integrally skinned asymmetric polysulfone membranes. The characteristics of a typical determination and equipment details are described elsewhere [30]. Ultrahigh purity gases O2 and N2 were used and the pure gas permeance for each gas was determined from the slope of the downstream pressure versus time plot once a steady state had been achieved. The permeability coefficients, P(i), were determined from the pure gas permeance values using the real thickness of the dense skin layer determined from the SEM images of the same membranes used for the permeation experiments.
2.1. Materials
2.3. Asymmetric membrane preparation 3. Results and discussion A typical experiment to produce the porous layer in a polysulfone dense film consists of delivering a predetermined amount of chloroform (controlled as the chloroform/polysulfone mass ratio) in a well-defined film area using aluminum adhesive to delimit the area and to fix the films into a clean glass support. Then, the glass with the membrane was immediately transferred into a high-pressure vessel and CO2 was introduced into it until the desired pressure was reached. The membranes were left into the
3.1. Effect of the solvent/polymer mass ratio Fig. 1 shows the SEM cross-section images of integrally skinned asymmetric polysulfone membranes, with a CH3 Cl/PSF mass ratio (a) 2.45 and (b) 3.07, formed from original dense films (∼101 m thickness) with a SCCO2 density of 0.7986 g/cm3 (35 ◦ C; 140 bar) and 718 bar/min depressurization rate. The original polysulfone
Fig. 1. Integrally skinned asymmetric polysulfone membranes formed treating one face of a dense film with a CH3 Cl/PSF mass ratio (a) 2.45 and (b) 3.07, and 30 min exposure with SCCO2 at a density = 0.7986 g/cm3 and depressurization rate = 718 bar/min.
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Fig. 2. Real thickness of the dense skin of integrally skinned asymmetric polysulfone membranes formed at different CH3 Cl/PSF mass ratios, 30 min exposure of SCCO2 at a density = 0.7986 g/cm3 (35 ◦ C; 140 bar) and depressurization rate = 718 bar/min.
Fig. 4. Effect of SCCO2 density on the average-pore size of a porous PSF layer formed from an already dense PSF film treated on one side with a CH3 Cl/PSF mass ratio of 7.44, 30 min SCCO2 exposure and 718 bar/min depressurization rate.
dense films have been modified into asymmetric membranes that have a sponge-like structure layer and a dense or selective skin. Moreover, the pores may be practically considered spherical. Fig. 2 shows the effect of the chloroform/polysulfone ratio on the reduction of the original thickness of the membrane. As it was expected, there is a linear decrease in the thickness of the selective or dense skin, accompanied by an increase in the thickness of the porous layer, as the CH3 Cl/PSF mass ratio increases. To explain this behavior it is necessary to understand the mechanism related to this process and take into account thermodynamic aspects. In related works where asymmetric membranes are obtained by phase inversion with SCCO2 [2,8,13,17], porous membrane formation departs from a homogeneous polymeric solution at a given concentration of polymer/solvent/nonsolvent. In these cases, the porous structure is generated by the liquid–liquid demixing obtained by nucleation and growth of the polymer lean phase; and the dense skin is formed by the solvent outflows from the solution before the SCCO2 causes the phase separation. In this work, a porous sublayer is obtained
by the dissolution of polymer when solvent is added on one side of an already formed dense membrane followed by the incorporation of SCCO2 . The dense layer is already present but its final thickness can be controlled by the solvent/polymer mass ratio and the time elapsed before the SCCO2 is incorporated. When this homogenous solvent/polymer solution membrane enters in contact with SCCO2 , the nonsolvent, a phase separation process takes place on the ternary mixture that is characterized by the formation of droplets of polymer lean phase disseminated inside the polymer rich phase. During the depressurization step, the solvent and CO2 are dragged out of the system and solidification of the polymer rich phase takes place forming a dry asymmetric membrane. These results are important because they show a different way to produce asymmetric membranes with variables that can be controlled in an easy way. To the best of our knowledge, this method is an alternative to those published in the literature since it can be used to produce asymmetric structures in an already formed film and/or hollow fiber.
Fig. 3. Effect of SCCO2 density over the average-pore size of a porous layer formed from an already polymer thick film treated on one side with a CH3 Cl/PSF mass ratio of 7.44, 30 min exposure of SCCO2 and 718 bar/min depressurization rate. (a) 0.6660 g/cm3 (50 ◦ C; 140 bar), 0.7986 g/cm3 (35 ◦ C; 140 bar), (c) 0.8456 g/cm3 (35 ◦ C; 175 bar), (d) 0.8980 g/cm3 (35 ◦ C; 245 bar).
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Fig. 5. Effect of SCCO2 depressurization rate on the pore size distribution of a PSF porous layer formed on an already dense film treated on one side with a CH3 Cl/PSF mass ratio of 7.44 and 30 min SCCO2 exposure.
3.2. Effect of SCCO2 density Fig. 3 shows cross-section images of integrally skinned asymmetric polysulfone membrane morphologies produced at different SCCO2 densities and with a 7.44 solvent/polymer mass ratio and 718 bar/min depressurization rate that were held constants. It is important to mention that PSF thick films (130 m) and a really high CH3 Cl/PSF mass ratio were selected to study the effect of SCCO2 density and depressurization rate since bigger cross-section areas facilitate the measurement of the porous size to statistically justify the determination of the average-pore size and its distribution. As the SCCO2 density increases, the sponge-like polysulfone porous layer presents smaller pore sizes as it is shown in Fig. 4. Similar behaviors were observed by Reverchon and Cardea [8] for the PSF–NMP–SCCO2 and PSF–CH3 Cl–SCCO2 systems but at different densities values. This behavior is due to the change of the solvent power of SCCO2 . At high densities there is major solvent power of the SCCO2 which means a larger amount of solvent in the polymer lean phase (SCCO2 –CH3 Cl) and a higher viscosity of the polymer rich phase (PSF–CH3 Cl). The higher viscosity of the polymer rich phase prevents the nuclei growth of the polymer lean phase generating pores of minor size. When density decreases, its solvent power decreases as well, this translates into a smaller amount of solvent in the polymer lean phase and a lower viscosity of the polymer rich phase. Lower viscosities of the polymer rich phase allows the growth of the nuclei of the polymer lean phase generating pores of major size.
size in the porous layer shows a relatively small decrease as the depressurization rate increases. This observation is in accordance to that reported by Xu et al. [7] for the system PLA–THF–SCCO2 and opposite from the results observed by Temtem et al. [9] for the PSF–CH3 Cl–SCCO2 and PSF–N,N-dimethylacetamide–SCCO2 systems. Temtem et al. [9] observed the average-pore size decreases with prolonged depressurization rates but such differences may be due to the mode the porous are formed since they operated at a continuous mode with a CO2 constant flow, while the results reported here are obtained in a batch mode just as in the case of Xu et al. A possible explanation may found in the foaming action of the CO2 during depressurization as it was pointed out by Xu et al. When pressure is diminished the SCCO2 present on the nuclei of polymer lean phase becomes in gaseous CO2 and the nuclei has more time to grow at lower depressurization rates, thus major pores are obtained. In contrast, at higher depressurization rates the nuclei have less time to grow and smaller pores are obtained. However, an additional observation is that for the PSF/CH3 Cl/CO2 system studied in this work, the effect of the depressurization rate of CO2 shows a relatively small influence on the average-pore size probably indicating that the foaming action of CO2 does no play an important role on generating different average-pore sizes. This could be an interesting result because it says that the porous structure is formed, and it is relatively stable, before the depressurization step is applied. Of course, a more detailed study addressing this point could support this statement but it is not the subject of this paper.
3.4. Gas separation properties 3.3. Effect of SCCO2 depressurization rate The effect of the depressurization rate over the pore size distribution and the average-pore size on the porous layer was also investigated at least for two depressurization rates and using membranes formed with a 7.44 chloroform/polysulfone mass ratio and a SCCO2 density of 0.7989 g/cm3 (35 ◦ C; 140 bar). The results are reported in Fig. 5 where it can be seen that the average-pore
Table 1 shows the gas permeance, P(i)/L, and permeability P(i) coefficients for O2 and N2 , as well as the O2 /N2 selectivity, measured for the integrally skinned asymmetric polysulfone membranes formed under the alternative proposed in this work. The gas permeance coefficients increase as the thickness of the dense or selective layer decreases as it would be expected, but more importantly, their O2 /N2 selectivity remains constant leading to the conclusion that
Table 1 Thickness, gas permeance and permeability coefficients for O2 and N2 , at 35 ◦ C and 2 bar, in integrally skinned asymmetric polysulfone membranes formed from an already dense film (∼101 m) Final dense skin thickness (m)
RO2 = PO2 /L (GPU)
RN2 = PN2 /L (GPU)
PO2 (Barrer)
PN2 (Barrer)
Selectivity O2 /N2
51.13 43.72 40.57 38.58 35.05 31.08 26.16
0.0253 0.0276 0.0332 0.0359 0.0381 0.0419 0.0532
0.00497 0.00528 0.00653 0.00654 0.00741 0.0075 0.00987
1.293 1.207 1.347 1.385 1.335 1.302 1.391
0.254 0.230 0.265 0.252 0.259 0.233 0.258
5.28 5.23 5.08 5.49 5.15 5.59 5.39
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their dense thicknesses are defect-free of any pinhole that could be created by the diffusion of the solvents involved during their formation. 4. Conclusions Integrally skinned asymmetric polysulfone membranes useful for gas separation were prepared by an alternative method to the dry/wet phase inversion processes using chloroform and supercritical CO2 on one side of an already formed polysulfone dense film which avoids the need of the preparation of a typical ternary casting solution. The results proved the success of the method since the appropriate handling of variables such as the solvent/polymer mass ratio, the SCCO2 density and the depressurization rate can lead to the formation of a totally dried defect-free dense film supported on well-defined porous morphology when an already polymer dense film has been processed. It is also shown that the averagepore size of the porous layer decreases with increasing the density of the SCCO2 and slightly decreases as the depressurization rate increases. Acknowledgments Authors acknowledge the financial support from CONACYT ´ (Mexico) throughout project 52631-Y. Alondra Torres Trueba thanks Universidad Iberoamericana and CONACYT for their financial support of her master degree studies. Authors also acknowledge the technical support from the Central Microscopy Laboratory of the Physical Institute-UNAM and CINVESTAV-Queretaro. References [1] Y.W. Kho, D.S. Kalika, B.L. Knutson, Precipitation of nylon 6 membranes using compressed carbon dioxide, Polymer 42 (2001) 6119. [2] H. Matsuyama, H. Yano, T. Maki, M. Teramoto, K. Mishima, K. Matsuyama, Formation of porous flat membrane by phase separation with supercritical CO2 , J. Membr. Sci. 194 (2001) 157. [3] H. Matsuyama, A. Yamamoto, H. Yano, T. Maki, M. Teramoto, K. Mishima, K. Matsuyama, Effect of organic solvents on membrane formation by phase separation with supercritical CO2 , J. Membr. Sci. 204 (2002) 81. [4] E. Reverchon, S. Cardea, Formation of cellulose acetate membranes using a supercritical fluid assisted process, J. Membr. Sci. 240 (2004) 187. [5] M.S. Kim, S.J. Lee, Characteristics of porous polycarbonate membrane with polyethylene glycol in supercritical CO2 and effect of its porosity on tearing stress, J. Supercrit. Fluid 31 (2004) 217. [6] Q. Xu, M. Pang, Q. Peng, Y. Jiang, J. Li, Application of supercritical carbon dioxide in the preparation of biodegradable polylactide membranes, J. Appl. Polym. Sci. 94 (2004) 2158. [7] Q. Xu, M. Pang, Q. Peng, Y. Jiang, J. Li, H. Wang, M. Zhu, Effect of different experimental conditions on biodegradable polylactide membranes prepared with supercritical CO2 as nonsolvent, J. Appl. Polym. Sci. 98 (2005) 831. [8] E. Reverchon, S. Cardea, Formation of polysulfone membranes by supercritical CO2 , J. Supercrit. Fluids 35 (2005) 140.
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