Formation of Cellulose Acetate Membranes via Phase Inversion Using

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Ind. Eng. Chem. Res. 2010, 49, 8761–8769

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Formation of Cellulose Acetate Membranes via Phase Inversion Using Ionic Liquid, [BMIM]SCN, As the Solvent Ding Yu Xing, Na Peng, and Tai-Shung Chung* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, Singapore 117576

Ionic liquids have gained worldwide attention as green solvents in the past decade. This study explores, for the first time, the fundamental science and engineering of using ionic liquids as a new generation of solvents to replace traditional organic solvents for the fabrication of flat sheet membranes and hollow fiber membranes. The fundamentals and characteristics of membrane formation of cellulose acetate (CA) membranes have been investigated using 1-butyl-3-methylimidazolium thiocyanate ([BMIM]SCN) as the solvent via phase inversion in water. For elucidation, other solvents, i.e., N-methyl-2-pyrrolidinone (NMP) and acetone, were also studied. It is found that [BMIM]SCN has distinctive effects on phase inversion process and membrane morphology compared to NMP and acetone because of its unique nature of high viscosity and the high ratio of [BMIM]SCN outflow to water inflow. Membranes cast or spun from CA/[BMIM]SCN have a macrovoidfree dense structure full of nodules, implying the paths of phase inversion are mainly nucleation growth and gelation, followed possibly by spinodal decomposition. The recovery and reuse of [BMIM]SCN have also been demonstrated and achieved. The derived flat sheet membranes made from the recovered [BMIM]SCN show morphological and performance characteristics similar to those from the fresh [BMIM]SCN. It is believed that this study could enrich the understanding of membrane formation using environmentally benign ionic liquids. 1. Introduction Ionic liquids, containing essentially only ions, have been considered as a group of environmentally friendly solvents with unique properties, such as negligible vapor pressure, thermal and chemical stability, recyclability, and nonflammability.1-3 Ionic liquids have also attracted great attention among scientists because they are versatile in cations, anions, and their combinations, which make their properties designable according to different requirements.1-3 With the fast expansion of chemical industries, environmental problems such as air pollution, waste chemicals, and water shortage have been emerging. Ionic liquids appear to be striking alternatives to replace the traditional volatile organic solvents for industrial uses because they have shown promising applications in many aspects including organic synthesis, catalysis, separations, and material preparation.4-9 Currently in polymer science, ionic liquids are not only used as the media for polymerization process but also used in preparation of functional polymer materials.3,10 For example, Snedden et al.11 have prepared porous catalytic membranes through in situ polymerization in imidazolium-based ionic liquids followed by the removal of ionic liquids which behave as the porogen. Other porous materials were also fabricated by interfacial polymerization of imidazolium-based ionic liquids and monomers12 or polymerization of microemulsions stabilized by surfactant ionic liquids that consisted of an imidazolium cation polar group and a hydrophobic tail.13 Furthermore, ionic liquids have been employed to replace the traditional solvents in supported liquid membranes, acquiring a long-term, continuous separation performance for CO2/CH4 and CO2/N2 mixed gases.14 Another major application of ionic liquids is to dissolve macromolecules which have limited solubility in common solvents. Some types of ionic liquids are very powerful to dissolve biopolymers, such as glucose, fructose, and sucrose, * To whom correspondence should be addressed. Tel: +6565166645. Fax: +65-67791936. E-mail: [email protected].

at higher concentrations compared to traditional solvent systems.3 So far, many studies have focused on dissolving cellulose, using hydrophilic imidazolium-based ionic liquids.15 This may be due to the fact that cellulose is the most abundant renewable source but very difficult to dissolve in organic solvents. In addition, these ionic liquids can simplify the dissolving process without creating environmental problems. Swatloski et al.16 were the first group who reported that ionic liquids were effective solvents for cellulose. Their ionic liquids contained 1-butyl-3methylimidazolium cations ([BMIM]+) and anions such as Cl-, SCN-, and Br-. Solutions in [BMIM]Cl containing 3 and 10 wt % cellulose were prepared at 70 and 100 °C, respectively. They have hypothesized and later confirmed by 13C nuclear magnetic resonance spectroscopy (NMR) that the high chloride concentration and activity in [BMIM]Cl can effectively break the hydrogen bonding present in cellulose and lead to the ability to dissolve a higher concentration of cellulose than the traditional solvents.17 Zhang et al.18,19 explored the solubility of cellulose in 1-allyl-3-methylimidazolium chloride ([AMIM]Cl), and prepared transparent cellulose films and cellulose/multiwalledcarbon-nanotube composite fibers from [AMIM]Cl by precipitation in water. Moreover, bioactive cellulose films incorporating enzymes20 and cellulose fibers21 were prepared from ionic liquids by phase inversion in water. The residue ionic liquids in the coagulation bath can be easily recycled by evaporation to remove the water22 or by using aqueous biphasic systems.23 Since Leob and Sourirajan developed the phase inversion process to fabricate membranes in the late 1950s,24 the issues related to membrane formation have been heavily studied and debated. There exists a rich literature on the formation of asymmetric membranes by the phase inversion process using traditional organic solvents for polymeric materials.25-30 Generally, there are four distinguished structural elements that have been addressed, e.g., nodules, cellular structure, bicontinuous structure, and macrovoids. With the in-depth exploration, scientists proposed different mechanisms of phase inversion

10.1021/ie1007085  2010 American Chemical Society Published on Web 08/12/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010 Table 1. Physicochemical Properties of Solvents and Water

3

Figure 1. Structure of (a) [BMIM]SCN and (b) [BMIM][MeSO4].

including nucleation growth, spinodal decomposition, gelation, and vitrification and even their combinations in time and in space. Some theoretical mass transfer models have also been developed to describe these processes based on simple polymer solutions.31-34 However, the formation mechanisms in many cases still remain hypothetical and experimental, especially for hollow fiber spinning, because of the complexity of two phase inversion processes taking place at the same time. In addition to having characteristics of environmentally friendliness, the good capability of ionic liquids in dissolving macromolecules and the miscibility of ionic liquids with water inspire us to employ ionic liquids as a new generation of solvents to replace the organic solvents for membrane preparation. We aim at (1) exploring the feasibility of using ionic liquids to replace the organic solvent to prepare asymmetric flat sheet membranes and hollow fiber membranes; (2) examining the differences in the fundamentals of membrane formation between using ionic liquids and traditional organic solvents, i.e., N-methyl-2-pyrrolidinone (NMP) and acetone, during the phase inversion process; and (3) studying the feasibility to recycle and reuse ionic liquids. Membrane scientists have well demonstrated that proper choice of solvents and coagulant media can affect the phase inversion pathways and hence control the membrane structure and separation performance.35-37 However, this is the first work in the literature that explores usage of ionic liquids for membrane fabrication and studies the fundamentals of phase inversion of polymer/ionic liquid solutions. It is believed that this work can provide in-depth insight of membrane formation mechanism and bring membrane research into a brand new area. 2. Experimental Section 2.1. Materials. Cellulose acetate (CA-398-30, acetyl content 39.8%) was purchased from Eastman Chemical Company, USA. The ionic liquids including 1-butyl-3-methylimidazolium thiocyanate ([BMIM]SCN, >95%) and 1-butyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO4], >95%), as shown in Figure 1, were obtained from BASF, Germany, acetone (>99.5%) was purchased from Tedia, USA, and N-methyl-2-pyrrolidinone (NMP, >99.5%) was purchased from Merck, USA. All solvents were used as received. [BMIM]SCN and [BMIM][MeSO4] were chosen as the ionic liquids being studied in this work because they have a lower melting point ( DNMP-water > D[BMIM]SCN-water. This clearly implies that the ratio of solvent outflow to the coagulant inflow defined by Yilmaz and McHugh32,33 is much greater than one and is the highest in the CA/[BMIM]SCN system, followed by the CA/NMP system, and then the CA/acetone system. In addition, the phase inversion kinetics also plays an important role in the membrane formation process. As confirmed by the light transmittance results in Figure 6, in the 10/90 wt % CA/NMP system, the spinodal decomposition happened immediately which was indicated by a strong decrease in the light transmittance, while in the CA/[BMIM]SCN/water system, the phase inversion happened more slowly than the CA/NMP system. In the CA/acetone system, there was a time delay for the onset of

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Figure 4. Cross-section morphology of flat sheet membranes prepared from different solvents (The top of membranes face upside; CA concentration: 10 wt %; thickness of casting knife: 100 µm).

Figure 5. Surface morphology of flat sheet membranes prepared from different solvents (CA concentration: 10 wt %; thickness of casting knife: 100 µm).

Figure 6. Phase inversion kinetics of films cast from 10/90 wt % CA/[BMIM]SCN and 10/90 wt % CA/NMP and precipitated in water.

liquid-liquid demixing which was related to the nascent membrane thickness.47 As a result, the phase separation to fix

the membrane outer contour should be fastest in the CA/NMP system and slowest in the CA/[BMIM]SCN system, thus the

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Table 4. Comparison of Various Parameters and PWP of CA Flat Sheet Membranes

solvent

porosity (%)

testing pressure (bar)

fresh [BMIM]SCN NMP acetone recycled [BMIM]SCN

6.21 ( 2.76 84.30 ( 0.71 50.83 ( 1.54 7.03 ( 1.89

1.5 1.5 4 1.5

time for the nascent membrane to adjust its thickness according to the ratio of solvent outflow to water inflow should follow the order CA/NMP < CA/acetone < CA/[BMIM]SCN. Influenced by these competing factors, flat asymmetric membranes cast from CA/[BMIM]SCN have the thinnest thickness (8.72 µm), followed by those from CA/acetone (11.61 µm), and then from CA/NMP (55.7 µm). Because water diffuses very slowly into the nascent CA/ [BMIM]SCN membrane and because the binodal curve for the CA/[BMIM]SCN/water system is much closer to the polymerwater axis compared to the CA/NMP and CA/acetone systems, nucleation growth and gelation may dominate the phase inversion paths in the beginning, and followed possibly by the spinodal decomposition, thus resulting in a membrane crosssection structure full of nodules. In addition, the low water inflow rate and high viscosity of the CA/[BMIM]SCN solution play important roles to retard the macrovoid formation even though the system has a very low polymer concentration. It has been known that macrovoids can be formed by various mechanisms.26-29,35-38,49-55 However, surface instability, nonsolvent intrusion, and localized supersaturation have been often cited as the main causes. As in the CA/[BMIM]SCN system, the low water inflow rate and high dope viscosity prevent the rapid intrusion of the external coagulant into the nascent membrane and thus eliminate any chance of localized supersaturation for the macrovoid formation. 3.2.2. Porosity, Pure Water Permeability, Pore Size and Its Distribution of CA Flat Sheet Membranes. Table 4 shows the porosities of membranes cast from various systems. Consistent with the membrane morphology discussed in the previous section, membranes cast from CA/[BMIM]SCN have the smallest porosity (6.21%), followed by CA/acetone (50.84%) and then CA/NMP (84.30%). Moreover, they have quite different pure water permeabilities (PWP). As illustrated in

pure water permeability (L/(m2 h bar)) 114.14 983.49 0 119.68

mean pore size µ p (nm)

standard deviation σp

39.16 41.01

2.428 1.818

Table 4 and Figure 7, the CA/NMP membrane has a much larger PWP value than the CA/[BMIM]SCN membranes. It is known that the PWP values of membranes are not determined only by their pore sizes, but also by other pore characteristics, such as porosity and pore interconnectivity.56 Compared to the membranes cast from CA/[BMIM]SCN, the much higher PWP of membranes cast from CA/NMP is not only due to the bigger pore size and broader pore size distribution, but also due to its higher porosity, as listed in Table 4. In addition, the CA/NMP membrane has formed a much more open cell structure compared to the CA/[BMIM]SCN membrane because of different precipitation paths during the phase inversion process, which can be indicated by the morphology in Figure 4 and 5. Interestingly, under the same casting conditions, the CA/ acetone membrane has no water permeability even when the trans-membrane pressure is elevated to 4 bar. This is due to the highly volatile nature of acetone and the delayed demixing, which leads to form the densest top surface among these three kinds of membranes. SEM pictures shown in Figure 5 confirm our hypothesis. 3.3. Fabrication of CA Hollow Fiber Membranes from [BMIM]SCN and the Morphology Study. Even though the CA/[BMIM]SCN solution has a reasonably high viscosity, it is difficult to fabricate hollow fibers from the CA/[BMIM]SCN solution owing to its low precipitation rate as shown in the phase diagrams in Figure 3. Transparent and white hollow fibers can be fabricated only by carefully adjusting the spinning parameters. First, several bore fluids with different NMP content were tried. It is known that as the NMP content increases in the bore fluid, delayed demixing occurs at the lumen side and a more porous structure can be achieved.41,57,58 However, a high NMP content in bore fluid may lower the viscosity and strength of the nascent fiber and thus induce spinning instability. A mixture

Figure 7. Pore size distribution probability density curve for CA/[BMIM]SCN and CA/NMP flat sheet membranes.

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Figure 8. Morphology of the CA/[BMIM]SCN hollow fiber membranes (free-fall wet-spun hollow fibers with a bore fluid of NMP/water ) 5/5).

Figure 9. Comparison of the morphology of flat sheet membranes prepared from (a) fresh [BMIM]SCN and (b) recovered [BMIM]SCN (CA concentration: 10 wt %; thickness of casting knife: 100 µm).

of 50 wt % NMP in water was found suitable to maintain a stable spinning process of the CA/[BMIM]SCN solution. The wet spinning process is preferred for the fabrication of hollow fiber membranes from the CA/[BMIM]SCN solution because it has a very slow phase inversion process. Figure 8 displays the SEM pictures of the entire hollow fiber morphology prepared from [BMIM]SCN. Similar to the flat sheet mem-

branes, these hollow fibers exhibit a macrovoid-free structure with an extremely thin wall because of a low polymer concentration, a high ratio of solvent outflow to water inflow, and an extremely low precipitation rate. The resultant hollow fiber has an asymmetric structure consisting of a porous inner surface and a relative dense outer surface. However, the whole cross section shows a looser interconnected nodular structure

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compared to that of the flat sheet membrane cast from 10/90 wt % CA/[BMIM]SCN. This difference may arise from the following facts. First, since the hollow fiber faces two coagulants at the outer and lumen sides after exiting from the spinneret and the two coagulants are of various compositions in this work, the coagulation rates in the outer and lumen sides must be different, which would affect the membrane formation.53 Second, compared with the flat sheet membranes, the hollow fibers are subjected to more shear stress within the spinneret and elongation stresses induced by gravity and its own weight. The polymer chains may be under different states of shear and elongation stresses before the phase inversion, and these factors may contribute to the looser interconnected nodular structure as discussed in the literature.54 A high dope viscosity alone cannot eliminate macrovoids in CA hollow fibers. In Peng et al.’s previous work,29 the hollow fibers were fabricated from a 18/82 wt % CA/NMP solution which has a viscosity value comparable with that of the 10/90 wt % CA/[BMIM]SCN solution. However, Peng et al.’s fibers still have macrovoids on the cross-section even at a take-up speed of 10 m/min and an air-gap distance of 1 cm. This may be additional proof that [BMIM]SCN has unique characteristics to facilitate the formation of macrovoid-free hollow fibers at a fairly low CA concentration due to its high viscosity and the high ratio of [BMIM]SCN outflow to water inflow. However, the thin wall fibers must be carefully handled because of their relatively poor mechanical strength. Future work will be aimed to overcome these issues. 3.4. Recovery and Reuse of [BMIM]SCN for Membrane Fabrication. The coagulation bath for flat membranes was collected and water was evaporated from the water and [BMIM]SCN mixture. The recycled [BMIM]SCN was reused for CA flat sheet membranes. Figure 9 shows a morphological comparison of CA flat sheet membranes prepared from the fresh and recycled [BMIM]SCN, while Table 4 compares their porosity and PWP values. The morphology, porosity, and pure water permeability are all quite comparable, indicating ionic liquids are truly environmentally benign solvents that can be recovered and reused. 4. Conclusions We have conducted a pioneering study of the fundamentals of membrane formation for flat asymmetric and hollow fiber membranes using environmentally benign ionic liquids as the solvent and CA as the polymer via phase inversion. The following conclusions can be made: 1. Key factors affecting the membrane formation have been explored. CA flat membranes cast from the 10/90 wt % CA/ [BMIM]SCN solution exhibit a macrovoid-free and relatively dense structure full of nodules, which is quite dissimilar to the membranes cast from 10/90 wt % CA/acetone or 10/90 wt % CA/NMP. The results suggest that the phase inversion of the CA/[BMIM]SCN system most likely occurs through nucleation growth and gelation followed possibly by spinodal demixing and then solidification due to a high ratio of the solvent outflow to the coagulant inflow, and a high viscosity of the CA/ [BMIM]SCN solution. The resultant CA flat sheet membranes from the CA/[BMIM]SCN solution have a mean pore size of 39.2 nm and pure water permeability of 114.1 L/(m2 h bar). 2. Under the current experimental setup, the wet spinning process is preferred for the fabrication of hollow fiber membranes made from CA/[BMIM]SCN because of a very slow phase inversion process. The resultant hollow fiber has an asymmetric structure consisting of a porous inner surface and

a relatively dense outer surface, but the cross-section is macrovoid-free and full of nodules. 3. The recovery and reuse of [BMIM]SCN has been demonstrated and the derived flat asymmetric membranes made from the recovered [BMIM]SCN show morphological, porosity, and flux characteristics similar to those from the fresh [BMIM]SCN. Based on this work, future work will be aimed at the optimization of spinning conditions to fabricate hollow fiber membranes with desirable properties for different applications. In addition, methods to design and fabricate more porous membranes using polymer/ionic liquid solutions are underway. Acknowledgment We are grateful to and would like to thank the NUS initiative grant for life science (R-279-000-249-646) and the NRF CRP grant for energy development (R-279-000-261-281) for funding this research. Special thanks are due. We also would like to thank BASF for the provision of 1-butyl-3-methylimidazolium thiocyanate and other ionic liquids and to Eastman for the CA material. Special thanks are due to Prof. Da-Ming Wang for his valuable suggestions on membrane formation. Thanks are also due to Dr. May May Teoh, Dr. Qian Yang, and Mr. Panu Sukitpaneenit for their useful suggestions. Note Added after ASAP Publication: After this paper was published online August 12, 2010, the footnote was deleted from Table 3. The corrected version was published August 17, 2010. Literature Cited (1) Earle, M. J.; Seddon, K. R. Ionic liquids. Green solvents for the future. Pure Appl. Chem. 2000, 72, 1391. (2) Rogers, R. D.; Seddon, K. R. Ionic liquids - Solvents of the future. Science 2003, 302, 792. (3) El Seoud, O. A.; Koschella, A.; Fidale, L. C.; Dorn, S.; Heinze, T. Applications of ionic liquids in carbohydrate chemistry: a window of opportunities. Biomacromolecules 2007, 8, 2629. (4) Cao, Y.; Wu, J.; Zhang, J.; Li, H.; Zhang, Y.; He, J. Room temperature ionic liquids (RTILs): A new and versatile platform for cellulose processing and derivatization. Chem. Eng. J. 2009, 147, 13. (5) Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. ReV. 1999, 99, 2071. (6) Parvulescu, V. I.; Hardacre, C. Catalysis in ionic liquids. Chem. ReV. 2007, 107, 2615. (7) Lewandowski, A.; Swiderska, A. New composite solid electrolytes based on a polymer and ionic liquids. Solid State Ionics 2004, 169, 21. (8) Lu, J.; Yan, F.; Texter, J. Advanced applications of ionic liquids in polymer science. Prog. Polym. Sci. 2009, 34, 431. (9) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Room temperature ionic liquids as novel media for ‘clean’ liquid-liquid extraction. Chem. Commun. 1998, 16, 1765. (10) Ueki, T.; Watanabe, M. Macromolecules in ionic liquids: progress, challenges and opportunities. Macromolecules 2008, 41, 3739. (11) Snedden, P.; Cooper, A. I.; Scott, K.; Winterton, N. Cross-linked polymer-ionic liquid composite materials. Macromolecules 2003, 36, 4549. (12) Zhu, L.; Huang, C. Y.; Patel, Y. H.; Wu, J.; Malhotra, S. V. Synthesis of porous polyurea with room-temperature ionic liquids via interfacial polymerization. Macromol. Rapid Commun. 2006, 27, 1306. (13) Yan, F.; Texter, J. Surfactant ionic liquid-based microemulsions for polymerization. Chem. Commun. 2006, 25, 2696. (14) Scovazzo, P.; Havard, D.; Mcshea, M.; Mixon, S.; Morgan, D. Long-term, continuous mixed gas dry fed CO2/CH4 and CO2/N2 separation performance and selectivities for room temperature ionic liquid membranes. J. Membr. Sci. 2009, 327, 41. (15) Zhu, S.; Wu, Y.; Chen, Q.; Yu, Z.; Wang, S.; Jin, S.; Ding, Y.; Wu, G. Dissolution of cellulose with ionic liquids and its application: a mini-review. Green Chem. 2006, 8, 325. (16) Swatloski, R. P.; Spear, S. K.; John, D.; Holbrey, J. D.; Rogers, R. D. Dissolution of cellulose with ionic liquids. J. Am. Chem. Soc. 2002, 124, 4974.

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ReceiVed for reView March 23, 2010 ReVised manuscript receiVed July 3, 2010 Accepted July 28, 2010 IE1007085