Swift heavy ion (SHI) irradiated polymer blend ...

International Journal of Hydrogen Energy 32 (2007) 3105 – 3108 www.elsevier.com/locate/ijhydene

Swift heavy ion (SHI) irradiated polymer blend membranes for hydrogen permeation Vaibhav Kulshrestha ∗ , Kamlendra Awasthi, Y.K. Vijay Department of Physics, Thin Films & Membrane Science Laboratory, 10- Vigyan Bhawan, University of Rajasthan, Jaipur 302 004, India Received 28 November 2006; received in revised form 21 January 2007; accepted 21 January 2007 Available online 9 March 2007

Abstract The 25 m thick polymer blends of PSF and PC were prepared in different concentrations viz. (3:1), (1:1) and (0:1), by solution cast method. These membranes were then irradiated by Ni7+ heavy ion of 100 MeV at Inter University Accelerator Centre, New Delhi, India. The gas permeability for hydrogen and carbon dioxide of these membranes was measured as a function of etching time. The permeability was found to be increasing with the increase in PC concentration in blends. The permeability of irradiated membranes was measured from both the sides. It was observed that the permeability for the front side was larger than that for the backside, which shows asymmetric behavior of membranes. Besides these, permeability was also found to depend upon fluence of ions. Permeability of membranes irradiated by high fluence of ions was higher than the membrane irradiated with the low fluence of ions. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Gas separation; Polymer blend membrane; Ion irradiation; Track etched membrane; Critical etching time

1. Introduction Gas separation has become a major industrial application of membrane technology [1]. The major advantages of the membrane processes compared to other separation processes are low energy consumption, low capital cost, lightweight and ecofriendly. Blending is frequently used for improving the properties of polymeric membranes. Blending is less complicated than developing new polymerization or copolymerization. Blends of PAN/PSF, PVDF/PMMA and PES/PI [2,3], have been used in the preparation of ultra filtration membranes and for hydrogen purification. The PSF/PC blend membranes have been studied by Kulshrestha et al. [4] and it is reported that the permeability increases and permselectivity of H2 over CO2 decreases as the concentration of PC is increased in PSF. Track membrane (TM) technology is an example of industrial application of track etching technique. Track-etch membranes offer distinct advantages over conventional membranes due to their precisely determined structure. Their pore size, shape and density can be varied in ∗ Corresponding author. Tel.: +91 1413295402.

E-mail address: [email protected] (V. Kulshrestha).

a controllable manner so that a membrane with the required transport and retention characteristics could be produced. In ion irradiated polymeric membrane the trail of damaged material, left along the ion trajectories, can be preferentially etched to open and enlarge the tracks causing its wide use as membrane filters. Pore shape can be made cylindrical, conical, funnel-like, or cigar-like at will. A number of modification methods have been developed for creating TMs with special properties and functions [5]. When energetic particle passes through the polymeric membrane, it losses the energy via atomic collision and electronic excitation. In electron excitation, the energy lost from the irradiant is mostly transferred to the electrons. A passage of energetic projectile through polymeric membranes produces cylindrical zones of irreversible chemical and structural changes. These zones have a diameter of a few nanometers and known as latent tracks surrounded by a halo. The size of latent track is related with the electron energy loss [dE/dx]e . In polymers, the chemical changes are quite significant because of the macromolecular structure of these materials. The kind of changes, which predominate, depends on the polymer type and radiation dosage. The tracks create molecular chains breaking

0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.01.014

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V. Kulshrestha et al. / International Journal of Hydrogen Energy 32 (2007) 3105 – 3108

(scissioning), crosslinking, free radicals and other radiolytic processes [6]. Damaged zones can be converted in to useful ion track pore by controlled chemical etching. The size of ion tracks increases with etching time. These can be characterized by different techniques, out of which gas permeation is best for characterization of track-etched membranes. 2. Experimental Fig. 1. Schematic of irradiated and track etched membrane.

2.1. Membrane preparation

2.2. Irradiation The membranes were irradiated by Ni7+ ions of 100 MeV. Irradiation was performed in the General Purpose Scattering Chamber (GPSC) at IUAC, New Delhi. The fluence of ion beam was chosen 107 , 108 ions/cm2 . The fluence of ion was reduced by Rutherford scattering at different angles where samples were kept. The uniformity was achieved using rotating flywheel attachment, discussed elsewhere [8]. 2.3. Chemical etching The irradiated membranes were then etched chemically in 6N NaOH at 60 ◦ C [9,10]. The etching time was increased with the step of 1 min and after every etching the membranes were washed thoroughly with tap water. To keep the concentration of etchant same the etchant was changed periodically and the temperature of the etchant was kept constant throughout the process. 2.4. Permeability measurements The permeability of hydrogen and carbon dioxide has been measured from both the sides of the membrane i.e. ion incidence side and ion emergence side. The flow rate was measured using permeability cell and the permeability was calculated using Fick’s formula as discussed by different authors [11,12]. The penetration of gas takes place across the membrane due to the pressure gradient [13]. From one side of the membrane 30 psi pressure was applied. 3. Result and discussion Both polycarbonate and polysulphone are the glassy polymers having a common ring structure (bisphenol-A) in its repeating unit. The polysulphone has additional ring structure and −SO2 group in its repeating unit. The blend of these polymeric materials may form some new bonds. The permeability of polymers depends on the operating conditions such as tem-

2250 2000 Permeability (in barrer)

The polysulfone + polycarbonate membranes of thickness 25 m were prepared by solution cast method with different concentration of materials. The detail of solution cast method has been discussed elsewhere [7]. The membranes were dried in low vacuum at 60 ◦ C for 24 h to remove the solvent completely.

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For blend membrane (PSF+PC) 3:1 1:1 0:1 (pure PC)

1500 1250 1000 750 500 250 0 0

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Etching time (in minutes) Fig. 2. Permeability of H2 versus etching time for different blend membranes for 107 dose.

perature, pressure, composition and on structural features of the materials as well. The permeability increased with etching time and at a particular etching time a large enhancement in permeability was observed. The time at which the permeability increases rapidly is known as the critical etching time. It depends on irradiant parameters like mass, energy and dose of irradiant. Permeability also depends on thickness of the membrane and molecular size of the gas used. SHI (swift heavy ion) produces tracks along its path when it passes through the polymer. The modified polymer chain structure in blends creates an additional space to pass the permeant and the etching of these blends depends on the characteristics of polymers. The measurements have been performed for Ni+7 ion irradiated and track etched membranes. The energy of ion was 100 MeV and range calculated by SRIM-2003 software was approximately 22 m. The thickness of the membranes was 25 m; for this thickness the ion cannot pass through the membrane and create a narrow zone which has been modified by the etching into a conical shape shown in Fig. 1. It has been observed that as the concentration of polycarbonate increases in the polysulphone, the permeability for hydrogen increases with etching time. This shows that the polycarbonate is affected by etching whereas the polysulphone restricts the etching as shown in Fig. 2. The permeability of membrane increases rapidly at the critical etching time, which also depends upon the ion dose. The critical etching time is 3 min for 108 ions/cm2 dose and 5 min


Permeability (in barrer)

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For Ni+7 (100MeV) ion irradiated poly (sulfone+carbonate, 3:1) of 25µm thick at flunce 107 ions/cm2

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Fig. 3. Permeability versus etching time for blend membrane for 107 dose.

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Etching time in (minutes)

Permeability (in barrer)

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For Ni+7(100MeV) ion irradiated poly (sulfone+carbonate, 3:1) of 25µm thick at flunce 108 ions/cm2

Fig. 5. Permselectivity of H2 over CO2 versus etching time for blend membrane for 108 dose.

for H2 (front) for H2 (back) for CO2 (front) for CO2 (back)

1500

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0 0

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Etching time (in minutes) Fig. 4. Permeability versus etching time for blend membrane for 108 dose.

for 107 ions/cm2 given in Figs. 3 and 4. Here the permeability for the front side and back side is different for both gases, which confirms the asymmetrical behavior of the membranes. In order to study the effect of molecular size dependence of the permeability, measurements for H2 and CO2 gases have been carried out at the same conditions of temperature and pressure given in Figs. 3 and 4. The CO2 molecules are larger than H2 molecules, so less flow rate and correspondingly lower permeability were observed. By increasing the etching time, the permeability of both gases was found to increase. The permselectivity of a polymeric membrane for one gas over another is given by the ratio of their permeabilities. The permselectivity also increases with etching time and was found to reach a maximum at the critical etching time as shows in Fig. 5. At this time, the membrane allows to pass a larger amount of H2 than the CO2 . Further with increasing etching time the permeability for both gases increases, but the selectivity decreases. The AFM images of the chemical etched membranes are shown in Figs. 6(a), (b) and 7, where the large scan represents the irradiation dose of the order of 107 ions/cm2 . The etched pits that are of 300–500 nm size at the surface are observable.

Fig. 6. AFM (a) 2D (b) 3D view of chemical etched polymeric membranes.

3.1. Conclusions It was observed that as the concentration of polycarbonate is increased in the polysulphone, the permeability for hydrogen

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Fig. 7. Sectional analysis curve of chemical etched polymeric membranes.

and CO2 increases with the increase in the etching time. The permeability for the gases from the front and back sides of the membrane are found different, higher permeability for the front side than the back side confirms the asymmetric behavior of the membrane. The dose of ions also plays an important role. At the higher dose critical etching time is less as compared to lower dose i.e. at higher dose permeability is high. Acknowledgments The authors are indebted to Inter University Accelerator Centre, New Delhi for ion irradiation and providing financial assistance for the work. References [1] Ismail AF, Ridzuan N, Rahman SA. Songklanakarin J Sci Technol 2002;24:1025. [2] Kapantaidakis GC, Koops GH. J Membr Sci 2002;204:153.

[3] Yang M-C, Liu T-Y. J Membr Sci 2003;226:119. [4] Kulshrestha V, Acharya NK, Awasthi K, Singh M, Avasthi DK, Vijay YK. Int J Hydrogen Energy 2006;31:1266–70. [5] Avasthi DK, Singh JP, Biswas A, Bose SK. Nucl Instrum Methods B 1998;146:504–9. [6] Fleischer RL, Price PB, Walker RM. Nuclear tracks in solids: principals & applications. Berkeley: University of California Press; 1975. [7] Vijay YK, Wate S, Acharya NK, Garg JC. Int J Hydrogen Energy 2002;27:905–9. [8] Vijay YK, Wate S, Acharya NK, Bhahada K, Kothari A, Barua P, et al. Annual Report Nuclear Science Center; New Delhi: 2000–2001. p. 55. [9] Spohr R. (GSI Darmstadt, Planckstr, 1, D-64291 Darmstadt), European Research Training Network EuNITT Report. 2.1, 2001. p.1. [10] Awasthi K, Kulshrestha V, Acharya NK, Singh M, Vijay YK, 2006;42:883–7. [11] Acharya NK, Yadav PK, Wate S, Vijay YK, Singh F, Avasthi DK. Bull Mater Sci 2004;27:417–20. [12] Kulshrestha V, Acharya NK, Awasthi K, Singh M, Avasthi DK, Vijay YK. Bull Mater Sci 2005;28:643–6. [13] Vijay YK, Acharya NK, Wate S, Avasthi DK. Int J Hydrogen Energy 2003;28:1015–8.