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Characterization of asymmetric polymeric membranes by gas permeation
Micron 38 (2007) 326–329 www.elsevier.com/locate/micron
Short communication
Characterization of asymmetric polymeric membranes by gas permeation Vaibhav Kulshrestha *, N.K. Acharya, Kamlendra Awasthi, Rashi Nathawat, M. Singh, Y.K. Vijay * Thin Films & Membrane Science Laboratory, Department of Physics, University of Rajasthan, Jaipur 302004, India Received 19 March 2006; received in revised form 13 June 2006; accepted 13 June 2006
Abstract Polycarbonate films (thickness 18, 25 and 38 mm) were irradiated by a beam of 100 MeV Ni7+ ion. The permeability for hydrogen and carbon dioxide was measured from both sides of membrane at increasing etching time. These membranes show larger permeability from the irradiation side, than the reverse side indicating the formation of conical tracks and asymmetrical membrane. The stopping range (Se) of 100 MeV Ni7+ ion beam in polycarbonate is 22 mm, for 18 mm thick membrane the etching time at which the permeability increases rapidly is less than that of 38 mm thick membrane, for both the gases. The difference in permeability from the two sides is attributed to the conical shape of the track generated by the ions. The controlled flow rate of the membrane leads to the design of a special type of gas filter. # 2006 Elsevier Ltd. All rights reserved. Keywords: Ion irradiation; Permeability; Track-etched asymmetric membranes; SEM
1. Introduction Polymeric membranes have been widely used for the gas separation for several decades (Koros and Mahajan, 2000). The effect of swift heavy ions on polymers has been studied in recent years. The production of porous membranes stands out among other applications of the track-etch technique as a nano filter (Fleischer et al., 1975). The track-etched membrane is applicable in a wide area of scientific and industrial research in particular for gas purification. Irradiation of a polymer membrane by heavy ions and subsequent chemical etching creates a membrane having an array of nano channels (Trautmann et al., 1996). An asymmetric membrane of poly(ethylene terephthalate) has been studied using I–V characteristics (Apel et al., 2001). The asymmetric membrane was produced by etching of an irradiated membrane with two different etchants placed on either side of the membrane (Shtanko et al., 1999; Apel et al., 2001). The distinctive properties of this track-etched membrane are a very narrow pore size distribution and low sorption ability, which is especially important for the filtration of gas mixtures.
The irradiation of swift heavy ions in polymers changes the physical and chemical properties (Avasthi et al., 1998). The passage of an energetic ion in polymers produces latent tracks of reduced density and molecular weight. To enable these tracks to pass gas molecules or liquids, it is necessary to preferentially etch to enlarge their size. Both track and bulk etching takes place in an irradiated membrane. The track-etch rate and bulk etch rate was fixed by controlling the temperature the etching temperature having an important role to determine the formation of permeating tracks (Kumar and Prasad, 1996; Kulshrestha et al., 2005). In the present work, we aimed to prepare large permeating track-etched membranes and characterize their symmetric and asymmetric behavior. Polycarbonate membranes of thickness of 18, 25 and 38 mm were irradiated by Ni7+ ions of 100 MeV energy at medium flux of 108 ion/cm2. The etching was done in 6N NaOH at 60 8C. These membranes were characterized by hydrogen and carbon dioxide gas permeation. SEM micrographs of etched pores are included in our discussion. 2. Experimental 2.1. Sample preparation
* Corresponding authors. Tel.: +91 141 3295402; fax: +91 141 2707728. E-mail addresses: [email protected] (V. Kulshrestha), [email protected] (Y.K. Vijay). 0968-4328/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2006.06.012
The lexan polycarbonate (PC) material was procured in granular form from National Chemical laboratory, Pune, India.
V. Kulshrestha et al. / Micron 38 (2007) 326–329
The polycarbonate membranes of thickness 18–38 mm were prepared by the solution-cast method. Details of this method have been given elsewhere (Kulshrestha et al., 2006a). The membranes were dried under a low vacuum of 10 3 torr at 50 8C (below the glass-transition temperature Tg = 147 8C) for 10 h to complete removal of the solvent.
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Micrographs of etched tracks from both sides of the membrane were recorded. The sample was cut to lengths of 5– 5 mm using a fresh razor blade and was snapped under liquid nitrogen, which gives a generally clean break. The sample was then mounted on sample stubs. These samples were then sputtered with gold from both the sides, before an investigation using the scanning electron microscope (SEM) KYKY-1000B.
2.2. SHI irradiation and chemical etching 3. Results and discussion The membranes were cut to the desired size to fit the permeability cell, then irradiated by Ni7+ions of 100 MeV. The irradiation was performed in the general purpose scattering chamber (GPSC) at the Inter University Accelerator Centre, New Delhi. The fluence of the ion beam was 108 ions/cm2. The ion fluence is reduced by Rutherford scattering at different angles. Uniformity is achieved using a rotating flywheel attachment, the details has been given elsewhere (Kulshrestha et al., 2006b). The irradiated membranes are etched chemically in 6N NaOH at 60 8C. The etching time is increased in steps of 30 s and after every etching the membrane was washed thoroughly. The etchant is changed periodically so that concentration of etchant remains the same during the experiment.
In this paper, efforts have been made to prepare and characterize the asymmetric membrane. In an irradiated membrane both track and bulk etch rates occur simultaneously. If the range of an energetic ion is less than the thickness of the membrane, then all the energy of the incident ion beam is lost in the material. Complete etching of tracks takes place in this case. However, in the case of transmitted tracks the track etching takes place from both sides of the membrane. The expected shapes for different thicknesses are shown in Fig. 1, where thickness is (a) lower (b) close to and (c) more than the stopping range (Se). The permeability for hydrogen and carbon dioxide was measured and the results can be described in the following way.
2.3. Permeability measurements and SEM
3.1. Membrane thickness is lower than Se
The permeability of hydrogen and carbon dioxide has been measured from both sides of the membrane, i.e. ion-incidence side and ion-emergence side. The flow rate was measured using permeability cell and calculated using Fick’s formula (Vijay et al., 2003). The penetration of gas takes place across the membrane due to the pressure gradient. The permselectivity is the ratio of permeabilities of one gas to the other.
As discussed above, the permeability of hydrogen and carbon dioxide increases with increasing etch time. After a particular etching time (critical etch time, tc) rapid changes in permeability are observed, which indicates the generation of permeating tracks as shown in Figs. 2 and 3. The stopping range of Ni7+ ion of 100 MeV in polycarbonate is 22 mm. For a 18 mm-thick membrane, tc is less than that of a 38 mm-thick
Fig. 1. Swift heavy ion irradiation before etching (a): (i) stopping range of ion is less than the thickness, (ii) stopping range is near the thickness and (iii) stopping range is greater than the thickness, and after etching (b): (i) stopping range of ion is less than the thickness, (ii) stopping range is near the thickness and (iii) stopping range is greater than the thickness.
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V. Kulshrestha et al. / Micron 38 (2007) 326–329
Fig. 2. Permeability vs. etching time for 18 mm-thick membrane at fluence of 108 ions/cm2.
membrane for both gases. In the 38 mm-thick membrane irradiated with 108 ions/cm2 tc is 20 min and for 25 mm-thick and the same fluence, tc is 3 min. This large difference can be attributed to the difference in the bulk and track-etch rates. At this stage the gas passes through conical tracks that are just meeting at their vertexes. In the case of an asymmetric membrane, the gas passes through the conical tracks formed at the ion-incidence side. The gas permeability from the ionincidence side (front) is more than that of ion-emergence side (back) due to the shape of the etched track, as shown in Figs. 2– 4. Therefore, the polarity of the membrane is important for production of an asymmetric membrane. The gas permeability of the same membrane is greater for hydrogen than for carbon dioxide due the difference of their molecular sizes but at tc the ratio of permeabilties of front to back is approximately 2.5 for both the gases. In a streamline flow, the product of area by which any fluid is passing and velocity of fluid is constant. It is expected that the area ratio of top to vertex should be the same 2.5, because all other parameters remain the same.
Fig. 4. Permeability vs. etching time for 38 mm-thick membrane at fluence of 108 ions/cm2.
track-etch rate and the cone angle was calculated as 0.578. In the present study the cone angle is calculated by assuming that the vertex of conical tracks is of the order of 0.3 nm, which represents the size of hydrogen molecule passing through rapidly when the tracks are etched through and through. The thickness of the membrane can be taken as the
3.2. Analysis The cone angle of tracks for Mackrofol-KL polycarbonate irradiated by 136Xe is reported by Kumar and Prasad (1996), using an optical research microscope for measuring the
Fig. 3. Permeability vs. etching time for 25 mm-thick membrane at fluence of 108 ions/cm2.
Fig. 5. Scanning electron micrographs of: (a) front and (b) back side view of a fully etched track.
V. Kulshrestha et al. / Micron 38 (2007) 326–329
length of the cone 25 mm, the area of the base of cone is estimated to be 2.5 times the vertex from the gas permeation measurements. The cone angle is also found 0.578 from above assumptions. This supports the assumption that the opening of the tracks is of the order 3 nm. Such a tracketched membrane can be used for filtration of gases. The asymmetry in the membrane is also shown in Fig. 5(a and b). Fig. 5(a and b) shows scanning electron micrographs of the front and back side of a fully etched track. The diameter at the front and back side is 2 and 1.75 mm respectively, which confirms the asymmetric nature of the membrane. 4. Conclusion It is concluded from the above study that the asymmetric track-etched membrane can be produced by selection of an appropriate projectile ion, ion energy and thickness. These membranes can be used for controlled gas-release devices, drug delivery and the fabrication of nanofilters. Acknowledgements The authors are indebted to Inter University Accelerator Centre, New Delhi for ion irradiation and Ministry of NonConventional Energy Sources, New Delhi and UGC, New Delhi for providing financial assistance for the work.
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References Apel, P.Yu., Korchev, Yu.E., Siwy, Z., Spohr, R., Yoshida, M., 2001. Diode-like single-ion track membrane prepared by electro-stopping. Nucl. Instrum. Methods Phys. Res. B 184, 337–346. Avasthi, D.K., Singh, J.P., Biswas, A., Bose, S.K., 1998. Study on evolution of gases from Mylar under ion irradiation. Nucl. Instrum. Methods Phys. Res. B 146, 504–508. Fleischer, R.L., Price, P.B., Walker, R.M., 1975. Nuclear Tracks in Solids: Principal & Applications. University of California Press, Berkeley. Koros, W.J., Mahajan, R., 2000. Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 175, 181–196. Kumar, A., Prasad, R., 1996. Study of track registration response of makrofol KL to 40Ar and 136Xe ions. Nucl. Instrum. Methods Phys. Res. B 119, 515– 518. Kulshrestha, V., Acharya, N.K., Awasthi, K., Singh, M., Vijay, Y.K., 2005. Effect of temperature and alpha irradiation on gas permeability for polymeric membrane. Bull. Mater. Sci. 28 (7), 643–646. Kulshrestha, V., Awasthi, K., Acharya, N.K., Singh, M., Avasthi, D.K., Vijay, Y.K., 2006a. Gas and ion transport through a track-etched large-area polymer film. Desalination 195, 273–280. Kulshrestha, V., Acharya, N.K., Awasthi, K., Singh, M., Avasthi, D.K., Vijay, Y.K., 2006b. Study of gas permeation for asymmetric track etched polymer blends. Int. J. Hydrogen Energy 31, 1266–1270. Shtanko, N.I., Kabanov, V.Ya., Apel, P.Yu., Yoshida, M., 1999. The use of radiation-induced graft polymerization for modification of polymer track membranes. Nucl. Instrum. Methods Phys. Res. B 151, 416–422. Trautmann, C., Bouffard, S., Spohr, R., 1996. Etching threshold for ion tracks in polyimide. Nucl. Instrum. Methods Phys. Res. B 116, 429–433. Vijay, Y.K., Acharya, N.K., Wate, S., Avasthi, D.K., 2003. Nanofilter for hydrogen purification. Int. J. Hydrogen Energy 28, 1015–1018.
Short communication
Characterization of asymmetric polymeric membranes by gas permeation Vaibhav Kulshrestha *, N.K. Acharya, Kamlendra Awasthi, Rashi Nathawat, M. Singh, Y.K. Vijay * Thin Films & Membrane Science Laboratory, Department of Physics, University of Rajasthan, Jaipur 302004, India Received 19 March 2006; received in revised form 13 June 2006; accepted 13 June 2006
Abstract Polycarbonate films (thickness 18, 25 and 38 mm) were irradiated by a beam of 100 MeV Ni7+ ion. The permeability for hydrogen and carbon dioxide was measured from both sides of membrane at increasing etching time. These membranes show larger permeability from the irradiation side, than the reverse side indicating the formation of conical tracks and asymmetrical membrane. The stopping range (Se) of 100 MeV Ni7+ ion beam in polycarbonate is 22 mm, for 18 mm thick membrane the etching time at which the permeability increases rapidly is less than that of 38 mm thick membrane, for both the gases. The difference in permeability from the two sides is attributed to the conical shape of the track generated by the ions. The controlled flow rate of the membrane leads to the design of a special type of gas filter. # 2006 Elsevier Ltd. All rights reserved. Keywords: Ion irradiation; Permeability; Track-etched asymmetric membranes; SEM
1. Introduction Polymeric membranes have been widely used for the gas separation for several decades (Koros and Mahajan, 2000). The effect of swift heavy ions on polymers has been studied in recent years. The production of porous membranes stands out among other applications of the track-etch technique as a nano filter (Fleischer et al., 1975). The track-etched membrane is applicable in a wide area of scientific and industrial research in particular for gas purification. Irradiation of a polymer membrane by heavy ions and subsequent chemical etching creates a membrane having an array of nano channels (Trautmann et al., 1996). An asymmetric membrane of poly(ethylene terephthalate) has been studied using I–V characteristics (Apel et al., 2001). The asymmetric membrane was produced by etching of an irradiated membrane with two different etchants placed on either side of the membrane (Shtanko et al., 1999; Apel et al., 2001). The distinctive properties of this track-etched membrane are a very narrow pore size distribution and low sorption ability, which is especially important for the filtration of gas mixtures.
The irradiation of swift heavy ions in polymers changes the physical and chemical properties (Avasthi et al., 1998). The passage of an energetic ion in polymers produces latent tracks of reduced density and molecular weight. To enable these tracks to pass gas molecules or liquids, it is necessary to preferentially etch to enlarge their size. Both track and bulk etching takes place in an irradiated membrane. The track-etch rate and bulk etch rate was fixed by controlling the temperature the etching temperature having an important role to determine the formation of permeating tracks (Kumar and Prasad, 1996; Kulshrestha et al., 2005). In the present work, we aimed to prepare large permeating track-etched membranes and characterize their symmetric and asymmetric behavior. Polycarbonate membranes of thickness of 18, 25 and 38 mm were irradiated by Ni7+ ions of 100 MeV energy at medium flux of 108 ion/cm2. The etching was done in 6N NaOH at 60 8C. These membranes were characterized by hydrogen and carbon dioxide gas permeation. SEM micrographs of etched pores are included in our discussion. 2. Experimental 2.1. Sample preparation
* Corresponding authors. Tel.: +91 141 3295402; fax: +91 141 2707728. E-mail addresses: [email protected] (V. Kulshrestha), [email protected] (Y.K. Vijay). 0968-4328/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2006.06.012
The lexan polycarbonate (PC) material was procured in granular form from National Chemical laboratory, Pune, India.
V. Kulshrestha et al. / Micron 38 (2007) 326–329
The polycarbonate membranes of thickness 18–38 mm were prepared by the solution-cast method. Details of this method have been given elsewhere (Kulshrestha et al., 2006a). The membranes were dried under a low vacuum of 10 3 torr at 50 8C (below the glass-transition temperature Tg = 147 8C) for 10 h to complete removal of the solvent.
327
Micrographs of etched tracks from both sides of the membrane were recorded. The sample was cut to lengths of 5– 5 mm using a fresh razor blade and was snapped under liquid nitrogen, which gives a generally clean break. The sample was then mounted on sample stubs. These samples were then sputtered with gold from both the sides, before an investigation using the scanning electron microscope (SEM) KYKY-1000B.
2.2. SHI irradiation and chemical etching 3. Results and discussion The membranes were cut to the desired size to fit the permeability cell, then irradiated by Ni7+ions of 100 MeV. The irradiation was performed in the general purpose scattering chamber (GPSC) at the Inter University Accelerator Centre, New Delhi. The fluence of the ion beam was 108 ions/cm2. The ion fluence is reduced by Rutherford scattering at different angles. Uniformity is achieved using a rotating flywheel attachment, the details has been given elsewhere (Kulshrestha et al., 2006b). The irradiated membranes are etched chemically in 6N NaOH at 60 8C. The etching time is increased in steps of 30 s and after every etching the membrane was washed thoroughly. The etchant is changed periodically so that concentration of etchant remains the same during the experiment.
In this paper, efforts have been made to prepare and characterize the asymmetric membrane. In an irradiated membrane both track and bulk etch rates occur simultaneously. If the range of an energetic ion is less than the thickness of the membrane, then all the energy of the incident ion beam is lost in the material. Complete etching of tracks takes place in this case. However, in the case of transmitted tracks the track etching takes place from both sides of the membrane. The expected shapes for different thicknesses are shown in Fig. 1, where thickness is (a) lower (b) close to and (c) more than the stopping range (Se). The permeability for hydrogen and carbon dioxide was measured and the results can be described in the following way.
2.3. Permeability measurements and SEM
3.1. Membrane thickness is lower than Se
The permeability of hydrogen and carbon dioxide has been measured from both sides of the membrane, i.e. ion-incidence side and ion-emergence side. The flow rate was measured using permeability cell and calculated using Fick’s formula (Vijay et al., 2003). The penetration of gas takes place across the membrane due to the pressure gradient. The permselectivity is the ratio of permeabilities of one gas to the other.
As discussed above, the permeability of hydrogen and carbon dioxide increases with increasing etch time. After a particular etching time (critical etch time, tc) rapid changes in permeability are observed, which indicates the generation of permeating tracks as shown in Figs. 2 and 3. The stopping range of Ni7+ ion of 100 MeV in polycarbonate is 22 mm. For a 18 mm-thick membrane, tc is less than that of a 38 mm-thick
Fig. 1. Swift heavy ion irradiation before etching (a): (i) stopping range of ion is less than the thickness, (ii) stopping range is near the thickness and (iii) stopping range is greater than the thickness, and after etching (b): (i) stopping range of ion is less than the thickness, (ii) stopping range is near the thickness and (iii) stopping range is greater than the thickness.
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V. Kulshrestha et al. / Micron 38 (2007) 326–329
Fig. 2. Permeability vs. etching time for 18 mm-thick membrane at fluence of 108 ions/cm2.
membrane for both gases. In the 38 mm-thick membrane irradiated with 108 ions/cm2 tc is 20 min and for 25 mm-thick and the same fluence, tc is 3 min. This large difference can be attributed to the difference in the bulk and track-etch rates. At this stage the gas passes through conical tracks that are just meeting at their vertexes. In the case of an asymmetric membrane, the gas passes through the conical tracks formed at the ion-incidence side. The gas permeability from the ionincidence side (front) is more than that of ion-emergence side (back) due to the shape of the etched track, as shown in Figs. 2– 4. Therefore, the polarity of the membrane is important for production of an asymmetric membrane. The gas permeability of the same membrane is greater for hydrogen than for carbon dioxide due the difference of their molecular sizes but at tc the ratio of permeabilties of front to back is approximately 2.5 for both the gases. In a streamline flow, the product of area by which any fluid is passing and velocity of fluid is constant. It is expected that the area ratio of top to vertex should be the same 2.5, because all other parameters remain the same.
Fig. 4. Permeability vs. etching time for 38 mm-thick membrane at fluence of 108 ions/cm2.
track-etch rate and the cone angle was calculated as 0.578. In the present study the cone angle is calculated by assuming that the vertex of conical tracks is of the order of 0.3 nm, which represents the size of hydrogen molecule passing through rapidly when the tracks are etched through and through. The thickness of the membrane can be taken as the
3.2. Analysis The cone angle of tracks for Mackrofol-KL polycarbonate irradiated by 136Xe is reported by Kumar and Prasad (1996), using an optical research microscope for measuring the
Fig. 3. Permeability vs. etching time for 25 mm-thick membrane at fluence of 108 ions/cm2.
Fig. 5. Scanning electron micrographs of: (a) front and (b) back side view of a fully etched track.
V. Kulshrestha et al. / Micron 38 (2007) 326–329
length of the cone 25 mm, the area of the base of cone is estimated to be 2.5 times the vertex from the gas permeation measurements. The cone angle is also found 0.578 from above assumptions. This supports the assumption that the opening of the tracks is of the order 3 nm. Such a tracketched membrane can be used for filtration of gases. The asymmetry in the membrane is also shown in Fig. 5(a and b). Fig. 5(a and b) shows scanning electron micrographs of the front and back side of a fully etched track. The diameter at the front and back side is 2 and 1.75 mm respectively, which confirms the asymmetric nature of the membrane. 4. Conclusion It is concluded from the above study that the asymmetric track-etched membrane can be produced by selection of an appropriate projectile ion, ion energy and thickness. These membranes can be used for controlled gas-release devices, drug delivery and the fabrication of nanofilters. Acknowledgements The authors are indebted to Inter University Accelerator Centre, New Delhi for ion irradiation and Ministry of NonConventional Energy Sources, New Delhi and UGC, New Delhi for providing financial assistance for the work.
329
References Apel, P.Yu., Korchev, Yu.E., Siwy, Z., Spohr, R., Yoshida, M., 2001. Diode-like single-ion track membrane prepared by electro-stopping. Nucl. Instrum. Methods Phys. Res. B 184, 337–346. Avasthi, D.K., Singh, J.P., Biswas, A., Bose, S.K., 1998. Study on evolution of gases from Mylar under ion irradiation. Nucl. Instrum. Methods Phys. Res. B 146, 504–508. Fleischer, R.L., Price, P.B., Walker, R.M., 1975. Nuclear Tracks in Solids: Principal & Applications. University of California Press, Berkeley. Koros, W.J., Mahajan, R., 2000. Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 175, 181–196. Kumar, A., Prasad, R., 1996. Study of track registration response of makrofol KL to 40Ar and 136Xe ions. Nucl. Instrum. Methods Phys. Res. B 119, 515– 518. Kulshrestha, V., Acharya, N.K., Awasthi, K., Singh, M., Vijay, Y.K., 2005. Effect of temperature and alpha irradiation on gas permeability for polymeric membrane. Bull. Mater. Sci. 28 (7), 643–646. Kulshrestha, V., Awasthi, K., Acharya, N.K., Singh, M., Avasthi, D.K., Vijay, Y.K., 2006a. Gas and ion transport through a track-etched large-area polymer film. Desalination 195, 273–280. Kulshrestha, V., Acharya, N.K., Awasthi, K., Singh, M., Avasthi, D.K., Vijay, Y.K., 2006b. Study of gas permeation for asymmetric track etched polymer blends. Int. J. Hydrogen Energy 31, 1266–1270. Shtanko, N.I., Kabanov, V.Ya., Apel, P.Yu., Yoshida, M., 1999. The use of radiation-induced graft polymerization for modification of polymer track membranes. Nucl. Instrum. Methods Phys. Res. B 151, 416–422. Trautmann, C., Bouffard, S., Spohr, R., 1996. Etching threshold for ion tracks in polyimide. Nucl. Instrum. Methods Phys. Res. B 116, 429–433. Vijay, Y.K., Acharya, N.K., Wate, S., Avasthi, D.K., 2003. Nanofilter for hydrogen purification. Int. J. Hydrogen Energy 28, 1015–1018.