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Modelling of polysulfone membrane formation by immersion precipitation
Desalination 200 (2006) 427–428
Modelling of polysulfone membrane formation by immersion precipitation Luizildo Pitol-Filho, Carles Torras, Josep Bonet-Avalos, Ricard Garcia-Valls* Departament d’Enginyeria Química, Universitat Rovira i Virgili, Av. Països Catalans, 26, 43007, Tarragona, Spain Tel. +34 977559610; Fax +34 977559621; email: [email protected] Received 28 October 2005; accepted 3 March 2006
1. Introduction Asymmetric membranes have a wide field of application because of its unique morphology. Its high selectivity is assured when a dense layer alternates with a porous one. Immersion precipitation is one of the processes generally used for the membrane production. In this process, a polymeric solution placed on the top of a glass plate is immersed in a bath of a non-solvent. The membranes are formed as long as the solvent leaves the polymeric layer and the non-solvent takes its place. The driving force for the membrane formation is the difference of chemical potential between the bath and the polymeric layer. 2. Experimental A large number of membranes were obtained by immersion precipitation technique using several proportions of polysulfone (PSf ) in dimethyl formamide (DMF) as solvent and several compositions in the coagulation bath from pure water to 2-propanol (IPA), as non-solvent. The cross section of membranes was characterized *Corresponding author.
by scanning electron microscopy (SEM) and the micrographs obtained were interpreted using IFME® software in order to quantify the main morphology parameters. IFME® software was developed by our group, and has been described in a previous work [1]. 3. Modelling The formation of membranes may be described by equations derived from the FreeVolume theory coupled to the Flory–Huggins thermodynamics. The equations used are very similar to those derived for the obtention of membrane by solvent evaporation [2]. Just the boundary condition at the interface changes, because here the liquid–polymer equilibrium is taken into account. The continuity equations in the polymeric phase are directly written in terms of volume fraction, as well as the boundary conditions and the equation for the thickness of the membrane. 4. Results and discussion Fig. 1 shows the cross section of a membrane obtained with 20 wt.% PSf and its interpretation
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.391
428
L. Pitol-Filho et al. / Desalination 200 (2006) 427–428
Fig. 1. SEM micrograph of a membrane synthesized with 20 w% polysulfone (PSf) in dimethyl formamide (DMF) and with pure water into the coagulation bath; IFME® micrograph interpretation and symmetry analysis [1].
by using the IFME® software. After repeating this method with a large number of different membranes, a correlation was found between the polymer concentration and the membrane mean pore size [2]: Ps = 10.069 – 0.3961 [PSf]. In Fig. 1, both SEM image and IFME® interpretation show a distribution of porosity through the membrane: a very dense layer on top and regions of high porosity in the middle, and intermediate porosity near the bottom. Such asymmetric configuration may be useful for several
H2O Macrovoids
Microporous region
applications that require high selectivity, for example in fuel cells based on methanol. The initial concentrations of the polymeric solutions are shown in Fig. 2, as well as the equilibrium curve that may exist when the precipitation bath has a DMF molar fraction varying from 0.04 to 0.65. Fig. 2 also shows the spinodal curve for the system and the approximate location of the different polymer volume fractions once the membranes are formed: a dense layer on top, a region with macrovoids in the middle, and the microporous regions. The mathematical modelling of the membrane formation, by using Free-Volume theory and Flory–Huggins equilibrium approach, will provide the concentration paths that lead to the obtention of each different region within the membrane.
Dense layer
References [1] PSf
DMF Initial casting solutions Liquid – polymer equilibrium DMF Spinoidal vs PSf spinoidal
Fig. 2. Ternary diagram for the system DMF–water–PSf.
[2]
C. Torras and R. Garcia-Valls, Quantification of membrane morphology by interpretation of scanning electron microscopy images, J. Membr. Sci., 233 (2004) 119–127. S.A. Altinkaya and B. Ozbas, Modeling of asymmetric membrane formation by dry-casting method, J. Membr. Sci., 230 (2004) 71–89.
Modelling of polysulfone membrane formation by immersion precipitation Luizildo Pitol-Filho, Carles Torras, Josep Bonet-Avalos, Ricard Garcia-Valls* Departament d’Enginyeria Química, Universitat Rovira i Virgili, Av. Països Catalans, 26, 43007, Tarragona, Spain Tel. +34 977559610; Fax +34 977559621; email: [email protected] Received 28 October 2005; accepted 3 March 2006
1. Introduction Asymmetric membranes have a wide field of application because of its unique morphology. Its high selectivity is assured when a dense layer alternates with a porous one. Immersion precipitation is one of the processes generally used for the membrane production. In this process, a polymeric solution placed on the top of a glass plate is immersed in a bath of a non-solvent. The membranes are formed as long as the solvent leaves the polymeric layer and the non-solvent takes its place. The driving force for the membrane formation is the difference of chemical potential between the bath and the polymeric layer. 2. Experimental A large number of membranes were obtained by immersion precipitation technique using several proportions of polysulfone (PSf ) in dimethyl formamide (DMF) as solvent and several compositions in the coagulation bath from pure water to 2-propanol (IPA), as non-solvent. The cross section of membranes was characterized *Corresponding author.
by scanning electron microscopy (SEM) and the micrographs obtained were interpreted using IFME® software in order to quantify the main morphology parameters. IFME® software was developed by our group, and has been described in a previous work [1]. 3. Modelling The formation of membranes may be described by equations derived from the FreeVolume theory coupled to the Flory–Huggins thermodynamics. The equations used are very similar to those derived for the obtention of membrane by solvent evaporation [2]. Just the boundary condition at the interface changes, because here the liquid–polymer equilibrium is taken into account. The continuity equations in the polymeric phase are directly written in terms of volume fraction, as well as the boundary conditions and the equation for the thickness of the membrane. 4. Results and discussion Fig. 1 shows the cross section of a membrane obtained with 20 wt.% PSf and its interpretation
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.391
428
L. Pitol-Filho et al. / Desalination 200 (2006) 427–428
Fig. 1. SEM micrograph of a membrane synthesized with 20 w% polysulfone (PSf) in dimethyl formamide (DMF) and with pure water into the coagulation bath; IFME® micrograph interpretation and symmetry analysis [1].
by using the IFME® software. After repeating this method with a large number of different membranes, a correlation was found between the polymer concentration and the membrane mean pore size [2]: Ps = 10.069 – 0.3961 [PSf]. In Fig. 1, both SEM image and IFME® interpretation show a distribution of porosity through the membrane: a very dense layer on top and regions of high porosity in the middle, and intermediate porosity near the bottom. Such asymmetric configuration may be useful for several
H2O Macrovoids
Microporous region
applications that require high selectivity, for example in fuel cells based on methanol. The initial concentrations of the polymeric solutions are shown in Fig. 2, as well as the equilibrium curve that may exist when the precipitation bath has a DMF molar fraction varying from 0.04 to 0.65. Fig. 2 also shows the spinodal curve for the system and the approximate location of the different polymer volume fractions once the membranes are formed: a dense layer on top, a region with macrovoids in the middle, and the microporous regions. The mathematical modelling of the membrane formation, by using Free-Volume theory and Flory–Huggins equilibrium approach, will provide the concentration paths that lead to the obtention of each different region within the membrane.
Dense layer
References [1] PSf
DMF Initial casting solutions Liquid – polymer equilibrium DMF Spinoidal vs PSf spinoidal
Fig. 2. Ternary diagram for the system DMF–water–PSf.
[2]
C. Torras and R. Garcia-Valls, Quantification of membrane morphology by interpretation of scanning electron microscopy images, J. Membr. Sci., 233 (2004) 119–127. S.A. Altinkaya and B. Ozbas, Modeling of asymmetric membrane formation by dry-casting method, J. Membr. Sci., 230 (2004) 71–89.