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Preparation of ultrathin polymeric membranes for gas separation by the new method of spontaneous film formation
Separation and Purification Technology 22-23 (2001) 247– 253 www.elsevier.com/locate/seppur
Preparation of ultrathin polymeric membranes for gas separation by the new method of spontaneous film formation C. Macht, G. Hinrichsen * Institute of Nonmetallic Materials-Polymer Physics, Technical Uni6ersity of Berlin, Englische Strasse 20, 10587 Berlin, Germany Received 10 October 1999; received in revised form 25 January 2000; accepted 6 March 2000
Abstract A new method for the production of nanoscaled polymeric films with a width of 10 cm and of infinite length is presented. The principle is based on a modified water casting method where a polymer solution spreads on a water surface and is wound up by a rotating cylinder. The film thickness depends on several parameters and varies from 30 to 1000 nm. In contrast to most processes for the production of thin polymeric films, the one described here can be driven continuously and allows a production speed of up to 10 m/min. Films were produced from various commercially available polymers and were investigated regarding surface quality, molecular orientation and gas permeation properties. The best results concerning film formation and surface quality were obtained with polymers containing aromatic rings such as polyether imide, polycarbonate and polysulfone. A slight chain orientation was found which increases with drawing velocity. Gas permeation experiments with oxygen and nitrogen showed maximum separation factors of 4.9 (polyether imide) and 6.1 (polysulfone) which is in good agreement with values reported in the literature. It turned out that the fabrication of ultrathin polymeric films by the method of spontaneous film forming seems to be a suitable way to get gas separation membranes at low costs and almost industrial quantities. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ultrathin polymer film; Membrane; Gas separation
1. Introduction There is great technological interest in the production of ultrathin polymeric films for various applications. Mostly thin films are used in electro* Corresponding author. Tel./fax: +49-30-31424464/21100. E-mail address: [email protected] (G. Hinrichsen).
optical devices, in sensors, or as membranes [1–4]. Several methods are already in use to produce such thin films but suffer either from limited thinness or from complicated or time consuming preparation. Known processes are PVD, CVD, spin casting, solution casting, and the Langmuir – Blodgett technique. A common feature of those techniques is that the polymeric film is not yet in existence before being brought to the substrate,
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but is formed from vapour, solution or even from monomers on the substrate itself. In contrast, only a few methods show a proper film which is applied on a substrate after its formation [5]. To date several technologies are used for gas separation. One such technology is the cleaning of industrial flue gases, were certain components have to be removed. This is mostly done by chemical or physical binding of the unwanted components [6–8]. The other field of interest in the separation of gases is the production of pure gases for technical and medical applications and for food and beverage industry [9,10]. Pure gases are obtained by cryogenic methods which are based on the different boiling temperatures of the gaseous components (inverse distillation) or by absorption –desorption mechanisms. Another possibility is to separate a gas mixture by a polymeric membrane. Generally, dense membranes of some microns thickness are in use for this purpose, where the difference in solubility and diffusivity for the gases leads to different permeation rates through the membrane. This depends on the membrane material as well as on its structure. While the permeation rate and selectivity are material constants, the flux through the membrane can be enhanced by higher pressure or lower membrane thickness. It is obvious that the latter is of great interest with respect to energy consumption. Thus, for a given membrane material, the preparation of thin membranes allowing high fluxes without the need of high pressure is a promising alternative to common ‘thick’ membranes. The objective of this work was, therefore, to prove the applicability of the film forming method, as described below, to produce ultrathin polymeric membranes for gas separation.
to surface energy reduction [11]. While spreading, the solvent evaporates and a condensed film results. The film is now taken from the water surface by a rotating cylinder. The film thickness is a function of winding speed and solution concentration and depends on the chosen polymer. As one would expect, the thickness decreases linearly with decreasing solution concentration and exponentially with increasing drawing velocity. To ensure constant solution concentration during the production process, we measure UV-absorption of the solution which is related to the polymer concentration, so that solvent may eventually be added. Fig. 1 schematically shows the apparatus, and more details have been reported elsewhere [12]. Since the polymer solution has to meet several conditions, e.g. low solubility in water, density\ 1 g/cm3, high vapour pressure of the solvent, we chose dichloromethane (CH2Cl2, solubility in water 20 g/l, z= 1.32 g/cm3, vapour pressure 475 mbar at room temperature) as solvent. For film material we took polyether imide Ultem 1000® (GE Plastics), polycarbonate Makrolon® (Bayer), and polysulfone Udel® (Amocco). All films were produced from a 5 wt.% solution. The membranes were produced at a drawing speed of 3 or 5 m/min. For the characterisation of surface quality and orientation, drawing speeds ranging from 1 to 8 m/min were used.
2.2. Membrane preparation The membrane specimens were prepared by fixing a support on the rotating cylinder (see Fig.
2. Experimental
2.1. Film formation The principle of this method is based on a modified water casting method. This means, a polymer solution which is in steady contact with a water reservoir spreads on the water surface due
Fig. 1. Design scheme of the laboratory device for the production of ultrathin polymeric films (spontaneous film formation technique).
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1). By turning it, the support is covered with the film and a given number of layers can be applied so that the total membrane thickness can be varied. A polyacrylonitrile membrane for ultrafiltration, provided by GKSS Teltow, with an effective area of 7.1 cm2 was used as support for the ultrathin films. Resistance to N2 permeation is 104 times smaller than that of the specimen, therefore, it can be neglected. Membranes were produced with single layer thickness ranging from 21 to 70 nm and consisting of 3 to 6 layers.
2.3. Permeability measurements Permeability measurements were performed using an experimental setup in which two chambers coupled to pressure sensors are separated by the film to be tested. The experimental device together with the gas reservoir is placed in a thermostat at constant temperature (30°C). The membrane and the low pressure chamber are evacuated to a pressure of 8×10 − 3 mbar. After evacuation, the gas to be tested is applied with a defined constant pressure to the high pressure side of the membrane. The gas flow through the membrane is monitored by measuring the pressure in the low pressure chamber as a function of time, and the permeability is calculated from: d J P0 = Ap Dt
(1)
where P0 is the permeability (or permeability coefficient), d the membrane thickness, A its area, p the upstream pressure and J/Dt the gas flow rate. These experiments were carried out at seven pressure steps ranging from 1 to 4 bar for N2, O2 and CO2, and the permeability was measured before and after every stage to detect eventual ageing of the membrane. CO2 was only measured during the last stage in order to avoid structural changes in the polymer due to CO2 solution.
2.4. Film surface and chain orientation
Fig. 2. Permeability of N2, O2 and CO2 in dependence on pressure difference. Polysulfone membrane, thickness 97 nm, three layers.
The orientation of the spontaneously formed films was also a subject of investigation and was examined by birefringence measurements using a polarized microscope. This method is commonly used to determine molecular orientation from the relationship between oriented chain segments and macroscopical birefringence [13]. The specimen were cut from the centre of a 12× 23 cm sheet of a multilayer film to avoid border effects which may occur during film formation.
3. Results and discussion
3.1. Gas permeation and separation ability The results of permeability measurements in dependence on pressure, Dp, for a polysulfone membrane of 97 nm thickness (three layers) and a polycarbonate membrane of 348 nm thickness (five layers) are shown in Figs. 2 and 3, respectively. In the initial region the curves for the test gases N2, O2 and CO2 decrease slightly with increasing feed pressure, as it is predicted following the well known dual-mode transport model for nonporous membranes [14,15]:
P0 = kDDD 1+ To determine the film formation capability of a polymer and to see how it depends on production parameters, the film surfaces were investigated by scanning electron microscopy.
FK 1+ bp
(2)
where P0 is the permeability, kD is the Henry’s law constant, F=DH/DD, DD and DH are the diffusivities for the gas molecules dissolved in the Henry’s
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results agree with the well-known result that for a large number of glassy polymers the permeability for N2 is lower than that for O2 [17,18]. The ability of a membrane to separate the components in a mixture of two gases is defined as selectivity hA/B by [15,19]: XA,permeate XA,feed hA/B = XB,permeate XB,feed Fig. 3. Permeability of N2 and O2 in dependence on pressure difference. Polycarbonate membrane, thickness 348 nm, six layers.
law domains and the Langmuir domains, respectively, K=C%Hb/kD with C%H and b being the Langmuir constant and the affinity constant, respectively, and p is the pressure. If there were capillaries or pore-like defects in the membrane, the permeability should rise with pressure because of [16]: J
Dp d
(3)
where J is the gas flow, Dp the pressure difference across the membrane and d its thickness. Therefore, the slight increase in permeability at elevated pressures makes clear that permeation through the membrane is mainly controlled by the sorption –diffusion mechanism and that, for the polysulfone membrane, only very little contribution of porous flow (Knudsen or Poiseuille or slip-flow) through the membrane exists. Furthermore, our
(4)
where Xi,permeate/feed is the mole fraction of the component i in the permeate and in the feed gas, respectively. If the downstream pressure (which means the pressure at the outgoing side of the permeation cell) is negligible compared to the feed pressure, hA/B can be approximated by [15,16]: hA/B =
P0,A P0,B
(5)
with P0 meaning the permeability coefficient. The determination of hA/B through the ratio of the permeabilities of the single components is a common procedure which in most cases yields the same results as a two-molecules assessment [20]. Table 1 shows the selectivities for different polysulfone membranes, calculated from Eq. (5), together with the experimentally determined permeabilities for N2, O2 and CO2. There is some scatter for the tested membranes which is neither related to the total membrane thickness, nor to the number of layers. The results are in the range of values of selectivity reported in the literature [21], exceeding it in one case (hO2/N2 = 6.1).
Table 1 Permeability and resulting O2/N2-selectivity of polysulfone membranes of different thickness and number of layers d (nm)
Nlayer
hO2/N2 at 1 bar
P0 (N2/1 bar)
P0 (O2/1 bar)
P0 (CO2/1 bar)
97 97 97 97 126 126 126 126
3 3 3 3 6 6 6 6
4.1 2.7 3.6 3.1 6.1 4.6 3.6 3.3
4.40E−12 9.61E−12 5.52E−12 9.13E−12 3.82E−12 6.14E−12 7.50E−12 8.13E−12
1.81E−11 2.57E−11 1.98E−11 2.86E−11 2.32E−11 2.88E−11 2.67E−11 2.71E−11
1.05E−10 1.27E−10 1.17E−10 1.33E−10 1.20E−10 1.37E−10 1.35E−10
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Fig. 4. Permeability and separation factor of N2 and O2 in dependence on number of previous measurement cycles. Polysulfone membrane, thickness 97 nm, pressure difference distance 1 bar.
To determine the presence or the growth of pore-like defects in the membrane, experiments were carried out where permeability values for N2 and O2 and the resulting separation factors at 1 bar pressure difference were measured depending on the number of previous measurement cycles on the same membrane. Fig. 4 shows the results for a polysulfone membrane of 97 nm thickness. As mentioned above, permeation through a membrane follows different mechanisms depending on whether it is porous or dense. However, both cases are idealized. So, for an imperfect nonporous membrane one would expect a contribution of some pore-like defects to the overall permeation, increasing the gas flux through the membrane. Following [16]: hA/B =
JA = JB
'
MB MA
251
defects, aA/B decreases, thus changing disadvantageously for the separation of N2 and O2. The results in Fig. 4 demonstrate that there is an increase in gas flow with the test number leading to a higher permeability. The decrease of hA/B indicates that the rise in permeability is caused by pore-like defects. Therefore, one can deduce that by imposing pressure on the membrane, the number and/or the size of pore-like defects increases. Furthermore, there is a relatively great scatter of the permeability values for the tested membranes, and membranes consisting of only one layer all failed upon testing. Therefore, it is assumed that defects are already present in the membranes. That means, for multi-layered membranes, defects in one layer are covered by another layer. The results have to be interpreted such that in most cases it is sufficient to use three layers and that there is no further increase in selectivity with more layers. Experiments with only two layers have still to be carried out. Also, it is not yet proven whether the film thickness influences the number of defects in the layer.
3.2. Surface quality and orientation Fig. 5 presents a scanning electron micrograph of the surface of a polyether imide film on a PAN support produced at a drawing speed of 6 m/min.
(6)
for porous membranes, where Mi are the molecular masses for the components with MA BMB, for N2 and O2 one obtains hA/B $1. If hA/B,total is assumed to result from both porous and nonporous membrane regions: hA/B,total =xhA/B,nonporous +(1 −x)hA/B,porous
(7)
where hA/B,total is the separation factor of the whole membrane, aA/B,nonporous/porous are the separation factors of the nonporous and porous regions, respectively, and x is the area fraction of the nonporous membrane regions, it is obvious that for an increasing area fraction of pore-like
Fig. 5. Scanning electron micrograph of the surface of a polyether imide film on a PAN support. Film drawing velocity 6 m/min.
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4. Conclusions
Fig. 6. Birefringence of polysulfone films produced from a 5 wt.% solution in dependence on winding velocity.
A certain number of fine creases (A) are observed which are typical for the so produced films and probably result from vibrations induced by the rotating cylinder. Also, some defects (B) can be seen which are caused by the surface roughness of the support material. The number as well as the size of the creases depend on the production parameters of the film as it does the surface quality in general [12]. The tested polymers showed different abilities to form films spontaneously which also influence their appearance. As described above, the polymer solution spreads on the water surface without the application of an external force but due to a reduction of surface energy [11]. The geometrical conditions of the device allow spreading in only one direction. It is in this direction that the condensed film is pulled upon winding of the cylinder. Fig. 6 shows the birefringence of a PSU film produced from a 5 wt.% solution at winding speeds of 1, 3, 5, and 8 m/min. The thicknesses of the single layers are 45, 29, 16, and 11 nm. An increasing birefringence in the drawing direction was measured for specimens produced at higher speeds, even though the absolute values remain small. We assume that the molecules are already slightly oriented by the spreading process, while the evaporation of the solvent hinders the orientation relaxation of the macromolecules.
A new technique has been presented for the manufacture of thin polymeric membranes in the nanometer thickness range. It could be shown for membranes from polysulfone, polycarbonate and polyether imide, that their gas separation ability for O2/N2 was comparable to results reported in the literature. A decreasing separation ability of the membranes was found after repeated exposure to higher feed pressures. A molecular orientation in the films was observed caused by the solution spreading on the water surface and the drawing effect of the winding process. The quality of the film showed rather smooth surfaces in the case of films produced at medium drawing speeds. Creases as well as small defects were found which are suspected to arise from device vibrations and roughness of the support, respectively. Considering the nonideal laboratory conditions we are convinced that even more defect free films could be produced. To date, membranes can only be produced discontinuously because of the need for more than one layer. The process might be driven continuously either with defect free films on smooth membrane supports, where only one film layer might be sufficient to obtain a dense membrane, or by modifying the technique so that two cylinders are used for alternating film winding.
Acknowledgements The authors wish to thank the Deutsche Forschungsgemeinschaft for the financial support.
References [1] S. Singh, A. Kapoor, K.N. Tripathi, S. Misra, S. Chandra S, in: Proc. SPIE 2733, Narosa Publishing House, New Delhi, India, 1996, pp. 229– 231. [2] P. Story, D. Galipeau, R. Mileham, in: Proc. 27th Int. Symp. on Microelectronics, ISHM Microelectronic Society, Reston, 1994, pp. 548– 553. [3] L. Robeson, W. Buroyne, M. Langsam, A. Savoca, C. Tien, Polymer 35 (1994) 4970– 4978.
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[13] M. Hoffmann, H. Kro¨mer, R. Kuhn, Polymeranalytik II’, Georg Thieme, Stuttgart, 1977. [14] S. Kimura, T. Hirose, in: N. Toshima (Ed.), Polymers for Gas Separation, VCH, Weinheim, 1992. [15] H. Odani, T. Masuda, in: N. Toshima (Ed.), Polymers for Gas Separation, VCH, Weinheim, 1992. [16] E. Staude, Membranen und Membranprozesse, VCH, Weinheim, 1992. [17] S.T. Hwang, C.K. Choi, K. Kammermeyer, Sep. Sci. 9 (6) (1974) 461– 478. [18] S. Pauly, in: J. Brandrup (Ed.), Polymer Handbook, 3rd edn, Wiley, New York, 1989. [19] A. Tabe Mohammadi, T. Matsuura, S. Sourirajan, Gas Sep. Purif. 9 (3) (1995) 181– 187. [20] J. Hong, P. Anderson, J. Qian, C. Martin, Chem. Mater. 10 (1998) 1029– 1033. [21] J.D. Le Roux, O.G. Van Schalkwyk, J. Appl. Polym. Sci. 71 (1999) 163– 175.
[4] G. Eastmond, J. Paprotny, I. Webster, Macromol. Rep. A31 (Suppl. 6/7) (1994) 1177–1184. [5] K. Shuto, Y. Oishi, T. Kajiyama, C.C. Han, Macromolecules 26 (1993) 6589–6594. [6] P.V. Mercea, S.-T. Hwang, J. Membr. Sci. 88 (1994) 131– 144. [7] R. Enick, T. Gale, J. Klara, J. Jones, Env. Progr. 18 (1) (1999) 60 – 68. [8] J.T. Yeh, R.J. Demski, J.P. Strakey, J.I. Joubert, Env. Progr. 4 (4) (1985) 223–228. [9] K.A. Glass, K.M. Kaufman, A.L. Smith, E.A. Johnson, J.H. Chen, J. Hotchkiss, J. Food Prod. 62 (8) (1999) 872– 876. [10] N. Penney, R.G. Bell, S.M. Moorhead, Food Res. Int. 31 (6/7) (1998) 521– 527. [11] T. Schleeh, Thesis, TU Berlin, 1991. [12] A. Hoffmann, Thesis, TU Berlin, 1999.
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Preparation of ultrathin polymeric membranes for gas separation by the new method of spontaneous film formation C. Macht, G. Hinrichsen * Institute of Nonmetallic Materials-Polymer Physics, Technical Uni6ersity of Berlin, Englische Strasse 20, 10587 Berlin, Germany Received 10 October 1999; received in revised form 25 January 2000; accepted 6 March 2000
Abstract A new method for the production of nanoscaled polymeric films with a width of 10 cm and of infinite length is presented. The principle is based on a modified water casting method where a polymer solution spreads on a water surface and is wound up by a rotating cylinder. The film thickness depends on several parameters and varies from 30 to 1000 nm. In contrast to most processes for the production of thin polymeric films, the one described here can be driven continuously and allows a production speed of up to 10 m/min. Films were produced from various commercially available polymers and were investigated regarding surface quality, molecular orientation and gas permeation properties. The best results concerning film formation and surface quality were obtained with polymers containing aromatic rings such as polyether imide, polycarbonate and polysulfone. A slight chain orientation was found which increases with drawing velocity. Gas permeation experiments with oxygen and nitrogen showed maximum separation factors of 4.9 (polyether imide) and 6.1 (polysulfone) which is in good agreement with values reported in the literature. It turned out that the fabrication of ultrathin polymeric films by the method of spontaneous film forming seems to be a suitable way to get gas separation membranes at low costs and almost industrial quantities. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ultrathin polymer film; Membrane; Gas separation
1. Introduction There is great technological interest in the production of ultrathin polymeric films for various applications. Mostly thin films are used in electro* Corresponding author. Tel./fax: +49-30-31424464/21100. E-mail address: [email protected] (G. Hinrichsen).
optical devices, in sensors, or as membranes [1–4]. Several methods are already in use to produce such thin films but suffer either from limited thinness or from complicated or time consuming preparation. Known processes are PVD, CVD, spin casting, solution casting, and the Langmuir – Blodgett technique. A common feature of those techniques is that the polymeric film is not yet in existence before being brought to the substrate,
1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 2 5 - 8
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C. Macht, G. Hinrichsen / Separation/Purification Technology 22-23 (2001) 247–253
but is formed from vapour, solution or even from monomers on the substrate itself. In contrast, only a few methods show a proper film which is applied on a substrate after its formation [5]. To date several technologies are used for gas separation. One such technology is the cleaning of industrial flue gases, were certain components have to be removed. This is mostly done by chemical or physical binding of the unwanted components [6–8]. The other field of interest in the separation of gases is the production of pure gases for technical and medical applications and for food and beverage industry [9,10]. Pure gases are obtained by cryogenic methods which are based on the different boiling temperatures of the gaseous components (inverse distillation) or by absorption –desorption mechanisms. Another possibility is to separate a gas mixture by a polymeric membrane. Generally, dense membranes of some microns thickness are in use for this purpose, where the difference in solubility and diffusivity for the gases leads to different permeation rates through the membrane. This depends on the membrane material as well as on its structure. While the permeation rate and selectivity are material constants, the flux through the membrane can be enhanced by higher pressure or lower membrane thickness. It is obvious that the latter is of great interest with respect to energy consumption. Thus, for a given membrane material, the preparation of thin membranes allowing high fluxes without the need of high pressure is a promising alternative to common ‘thick’ membranes. The objective of this work was, therefore, to prove the applicability of the film forming method, as described below, to produce ultrathin polymeric membranes for gas separation.
to surface energy reduction [11]. While spreading, the solvent evaporates and a condensed film results. The film is now taken from the water surface by a rotating cylinder. The film thickness is a function of winding speed and solution concentration and depends on the chosen polymer. As one would expect, the thickness decreases linearly with decreasing solution concentration and exponentially with increasing drawing velocity. To ensure constant solution concentration during the production process, we measure UV-absorption of the solution which is related to the polymer concentration, so that solvent may eventually be added. Fig. 1 schematically shows the apparatus, and more details have been reported elsewhere [12]. Since the polymer solution has to meet several conditions, e.g. low solubility in water, density\ 1 g/cm3, high vapour pressure of the solvent, we chose dichloromethane (CH2Cl2, solubility in water 20 g/l, z= 1.32 g/cm3, vapour pressure 475 mbar at room temperature) as solvent. For film material we took polyether imide Ultem 1000® (GE Plastics), polycarbonate Makrolon® (Bayer), and polysulfone Udel® (Amocco). All films were produced from a 5 wt.% solution. The membranes were produced at a drawing speed of 3 or 5 m/min. For the characterisation of surface quality and orientation, drawing speeds ranging from 1 to 8 m/min were used.
2.2. Membrane preparation The membrane specimens were prepared by fixing a support on the rotating cylinder (see Fig.
2. Experimental
2.1. Film formation The principle of this method is based on a modified water casting method. This means, a polymer solution which is in steady contact with a water reservoir spreads on the water surface due
Fig. 1. Design scheme of the laboratory device for the production of ultrathin polymeric films (spontaneous film formation technique).
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249
1). By turning it, the support is covered with the film and a given number of layers can be applied so that the total membrane thickness can be varied. A polyacrylonitrile membrane for ultrafiltration, provided by GKSS Teltow, with an effective area of 7.1 cm2 was used as support for the ultrathin films. Resistance to N2 permeation is 104 times smaller than that of the specimen, therefore, it can be neglected. Membranes were produced with single layer thickness ranging from 21 to 70 nm and consisting of 3 to 6 layers.
2.3. Permeability measurements Permeability measurements were performed using an experimental setup in which two chambers coupled to pressure sensors are separated by the film to be tested. The experimental device together with the gas reservoir is placed in a thermostat at constant temperature (30°C). The membrane and the low pressure chamber are evacuated to a pressure of 8×10 − 3 mbar. After evacuation, the gas to be tested is applied with a defined constant pressure to the high pressure side of the membrane. The gas flow through the membrane is monitored by measuring the pressure in the low pressure chamber as a function of time, and the permeability is calculated from: d J P0 = Ap Dt
(1)
where P0 is the permeability (or permeability coefficient), d the membrane thickness, A its area, p the upstream pressure and J/Dt the gas flow rate. These experiments were carried out at seven pressure steps ranging from 1 to 4 bar for N2, O2 and CO2, and the permeability was measured before and after every stage to detect eventual ageing of the membrane. CO2 was only measured during the last stage in order to avoid structural changes in the polymer due to CO2 solution.
2.4. Film surface and chain orientation
Fig. 2. Permeability of N2, O2 and CO2 in dependence on pressure difference. Polysulfone membrane, thickness 97 nm, three layers.
The orientation of the spontaneously formed films was also a subject of investigation and was examined by birefringence measurements using a polarized microscope. This method is commonly used to determine molecular orientation from the relationship between oriented chain segments and macroscopical birefringence [13]. The specimen were cut from the centre of a 12× 23 cm sheet of a multilayer film to avoid border effects which may occur during film formation.
3. Results and discussion
3.1. Gas permeation and separation ability The results of permeability measurements in dependence on pressure, Dp, for a polysulfone membrane of 97 nm thickness (three layers) and a polycarbonate membrane of 348 nm thickness (five layers) are shown in Figs. 2 and 3, respectively. In the initial region the curves for the test gases N2, O2 and CO2 decrease slightly with increasing feed pressure, as it is predicted following the well known dual-mode transport model for nonporous membranes [14,15]:
P0 = kDDD 1+ To determine the film formation capability of a polymer and to see how it depends on production parameters, the film surfaces were investigated by scanning electron microscopy.
FK 1+ bp
(2)
where P0 is the permeability, kD is the Henry’s law constant, F=DH/DD, DD and DH are the diffusivities for the gas molecules dissolved in the Henry’s
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250
results agree with the well-known result that for a large number of glassy polymers the permeability for N2 is lower than that for O2 [17,18]. The ability of a membrane to separate the components in a mixture of two gases is defined as selectivity hA/B by [15,19]: XA,permeate XA,feed hA/B = XB,permeate XB,feed Fig. 3. Permeability of N2 and O2 in dependence on pressure difference. Polycarbonate membrane, thickness 348 nm, six layers.
law domains and the Langmuir domains, respectively, K=C%Hb/kD with C%H and b being the Langmuir constant and the affinity constant, respectively, and p is the pressure. If there were capillaries or pore-like defects in the membrane, the permeability should rise with pressure because of [16]: J
Dp d
(3)
where J is the gas flow, Dp the pressure difference across the membrane and d its thickness. Therefore, the slight increase in permeability at elevated pressures makes clear that permeation through the membrane is mainly controlled by the sorption –diffusion mechanism and that, for the polysulfone membrane, only very little contribution of porous flow (Knudsen or Poiseuille or slip-flow) through the membrane exists. Furthermore, our
(4)
where Xi,permeate/feed is the mole fraction of the component i in the permeate and in the feed gas, respectively. If the downstream pressure (which means the pressure at the outgoing side of the permeation cell) is negligible compared to the feed pressure, hA/B can be approximated by [15,16]: hA/B =
P0,A P0,B
(5)
with P0 meaning the permeability coefficient. The determination of hA/B through the ratio of the permeabilities of the single components is a common procedure which in most cases yields the same results as a two-molecules assessment [20]. Table 1 shows the selectivities for different polysulfone membranes, calculated from Eq. (5), together with the experimentally determined permeabilities for N2, O2 and CO2. There is some scatter for the tested membranes which is neither related to the total membrane thickness, nor to the number of layers. The results are in the range of values of selectivity reported in the literature [21], exceeding it in one case (hO2/N2 = 6.1).
Table 1 Permeability and resulting O2/N2-selectivity of polysulfone membranes of different thickness and number of layers d (nm)
Nlayer
hO2/N2 at 1 bar
P0 (N2/1 bar)
P0 (O2/1 bar)
P0 (CO2/1 bar)
97 97 97 97 126 126 126 126
3 3 3 3 6 6 6 6
4.1 2.7 3.6 3.1 6.1 4.6 3.6 3.3
4.40E−12 9.61E−12 5.52E−12 9.13E−12 3.82E−12 6.14E−12 7.50E−12 8.13E−12
1.81E−11 2.57E−11 1.98E−11 2.86E−11 2.32E−11 2.88E−11 2.67E−11 2.71E−11
1.05E−10 1.27E−10 1.17E−10 1.33E−10 1.20E−10 1.37E−10 1.35E−10
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Fig. 4. Permeability and separation factor of N2 and O2 in dependence on number of previous measurement cycles. Polysulfone membrane, thickness 97 nm, pressure difference distance 1 bar.
To determine the presence or the growth of pore-like defects in the membrane, experiments were carried out where permeability values for N2 and O2 and the resulting separation factors at 1 bar pressure difference were measured depending on the number of previous measurement cycles on the same membrane. Fig. 4 shows the results for a polysulfone membrane of 97 nm thickness. As mentioned above, permeation through a membrane follows different mechanisms depending on whether it is porous or dense. However, both cases are idealized. So, for an imperfect nonporous membrane one would expect a contribution of some pore-like defects to the overall permeation, increasing the gas flux through the membrane. Following [16]: hA/B =
JA = JB
'
MB MA
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defects, aA/B decreases, thus changing disadvantageously for the separation of N2 and O2. The results in Fig. 4 demonstrate that there is an increase in gas flow with the test number leading to a higher permeability. The decrease of hA/B indicates that the rise in permeability is caused by pore-like defects. Therefore, one can deduce that by imposing pressure on the membrane, the number and/or the size of pore-like defects increases. Furthermore, there is a relatively great scatter of the permeability values for the tested membranes, and membranes consisting of only one layer all failed upon testing. Therefore, it is assumed that defects are already present in the membranes. That means, for multi-layered membranes, defects in one layer are covered by another layer. The results have to be interpreted such that in most cases it is sufficient to use three layers and that there is no further increase in selectivity with more layers. Experiments with only two layers have still to be carried out. Also, it is not yet proven whether the film thickness influences the number of defects in the layer.
3.2. Surface quality and orientation Fig. 5 presents a scanning electron micrograph of the surface of a polyether imide film on a PAN support produced at a drawing speed of 6 m/min.
(6)
for porous membranes, where Mi are the molecular masses for the components with MA BMB, for N2 and O2 one obtains hA/B $1. If hA/B,total is assumed to result from both porous and nonporous membrane regions: hA/B,total =xhA/B,nonporous +(1 −x)hA/B,porous
(7)
where hA/B,total is the separation factor of the whole membrane, aA/B,nonporous/porous are the separation factors of the nonporous and porous regions, respectively, and x is the area fraction of the nonporous membrane regions, it is obvious that for an increasing area fraction of pore-like
Fig. 5. Scanning electron micrograph of the surface of a polyether imide film on a PAN support. Film drawing velocity 6 m/min.
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4. Conclusions
Fig. 6. Birefringence of polysulfone films produced from a 5 wt.% solution in dependence on winding velocity.
A certain number of fine creases (A) are observed which are typical for the so produced films and probably result from vibrations induced by the rotating cylinder. Also, some defects (B) can be seen which are caused by the surface roughness of the support material. The number as well as the size of the creases depend on the production parameters of the film as it does the surface quality in general [12]. The tested polymers showed different abilities to form films spontaneously which also influence their appearance. As described above, the polymer solution spreads on the water surface without the application of an external force but due to a reduction of surface energy [11]. The geometrical conditions of the device allow spreading in only one direction. It is in this direction that the condensed film is pulled upon winding of the cylinder. Fig. 6 shows the birefringence of a PSU film produced from a 5 wt.% solution at winding speeds of 1, 3, 5, and 8 m/min. The thicknesses of the single layers are 45, 29, 16, and 11 nm. An increasing birefringence in the drawing direction was measured for specimens produced at higher speeds, even though the absolute values remain small. We assume that the molecules are already slightly oriented by the spreading process, while the evaporation of the solvent hinders the orientation relaxation of the macromolecules.
A new technique has been presented for the manufacture of thin polymeric membranes in the nanometer thickness range. It could be shown for membranes from polysulfone, polycarbonate and polyether imide, that their gas separation ability for O2/N2 was comparable to results reported in the literature. A decreasing separation ability of the membranes was found after repeated exposure to higher feed pressures. A molecular orientation in the films was observed caused by the solution spreading on the water surface and the drawing effect of the winding process. The quality of the film showed rather smooth surfaces in the case of films produced at medium drawing speeds. Creases as well as small defects were found which are suspected to arise from device vibrations and roughness of the support, respectively. Considering the nonideal laboratory conditions we are convinced that even more defect free films could be produced. To date, membranes can only be produced discontinuously because of the need for more than one layer. The process might be driven continuously either with defect free films on smooth membrane supports, where only one film layer might be sufficient to obtain a dense membrane, or by modifying the technique so that two cylinders are used for alternating film winding.
Acknowledgements The authors wish to thank the Deutsche Forschungsgemeinschaft for the financial support.
References [1] S. Singh, A. Kapoor, K.N. Tripathi, S. Misra, S. Chandra S, in: Proc. SPIE 2733, Narosa Publishing House, New Delhi, India, 1996, pp. 229– 231. [2] P. Story, D. Galipeau, R. Mileham, in: Proc. 27th Int. Symp. on Microelectronics, ISHM Microelectronic Society, Reston, 1994, pp. 548– 553. [3] L. Robeson, W. Buroyne, M. Langsam, A. Savoca, C. Tien, Polymer 35 (1994) 4970– 4978.
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[13] M. Hoffmann, H. Kro¨mer, R. Kuhn, Polymeranalytik II’, Georg Thieme, Stuttgart, 1977. [14] S. Kimura, T. Hirose, in: N. Toshima (Ed.), Polymers for Gas Separation, VCH, Weinheim, 1992. [15] H. Odani, T. Masuda, in: N. Toshima (Ed.), Polymers for Gas Separation, VCH, Weinheim, 1992. [16] E. Staude, Membranen und Membranprozesse, VCH, Weinheim, 1992. [17] S.T. Hwang, C.K. Choi, K. Kammermeyer, Sep. Sci. 9 (6) (1974) 461– 478. [18] S. Pauly, in: J. Brandrup (Ed.), Polymer Handbook, 3rd edn, Wiley, New York, 1989. [19] A. Tabe Mohammadi, T. Matsuura, S. Sourirajan, Gas Sep. Purif. 9 (3) (1995) 181– 187. [20] J. Hong, P. Anderson, J. Qian, C. Martin, Chem. Mater. 10 (1998) 1029– 1033. [21] J.D. Le Roux, O.G. Van Schalkwyk, J. Appl. Polym. Sci. 71 (1999) 163– 175.
[4] G. Eastmond, J. Paprotny, I. Webster, Macromol. Rep. A31 (Suppl. 6/7) (1994) 1177–1184. [5] K. Shuto, Y. Oishi, T. Kajiyama, C.C. Han, Macromolecules 26 (1993) 6589–6594. [6] P.V. Mercea, S.-T. Hwang, J. Membr. Sci. 88 (1994) 131– 144. [7] R. Enick, T. Gale, J. Klara, J. Jones, Env. Progr. 18 (1) (1999) 60 – 68. [8] J.T. Yeh, R.J. Demski, J.P. Strakey, J.I. Joubert, Env. Progr. 4 (4) (1985) 223–228. [9] K.A. Glass, K.M. Kaufman, A.L. Smith, E.A. Johnson, J.H. Chen, J. Hotchkiss, J. Food Prod. 62 (8) (1999) 872– 876. [10] N. Penney, R.G. Bell, S.M. Moorhead, Food Res. Int. 31 (6/7) (1998) 521– 527. [11] T. Schleeh, Thesis, TU Berlin, 1991. [12] A. Hoffmann, Thesis, TU Berlin, 1999.
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