Indian Journal of Pure & Applied Physics Vol. 42, March 2004, pp 179-181

Study of temperature dependent gas permeability for polycarbonate membrane N K Acharya, P K Yadav & Y K Vijay*

Department of Physics, University of Rajasthan, Jaipur 302 004

Received 3 February 2003; revised 12 May 2003; accepted 20 January 2004

A polycarbonate membrane of thickness 30 (±2) μm was prepared by solution cast method for the present study. The gas permeability measured as a function of pressure as well as temperature for the membrane. The permeability measurements in the pressure range 30-100 psi at room temperature do not show any remarkable change for H2, but considerable changes in permeability in the temperature range 303-371 K at 30 psi has been observed for both the gases H2 and CO2.

Keywords: Polycarbonate membrane, gas permeabiulity, thermal properties

IPC Code: G 01L 13/04

1 Introduction

Polymeric membranes have wide applications because of their good mechanical and thermal properties especially in liquid and gas separation. The understanding of gas transport through non-porous polymeric membranes is the basis for technical application in the field of gas separation and as barrier films for packaging materials or for protective coating. For better understanding of transport phenomena it is essential to investigate the gas permeation properties, in particular the influence of pressure, temperature and the interaction between permeating molecules and the polymeric matrix. The permeability of permeate gases such as H2, He, N2 and O2 in rubbery or glassy polymeric membranes is not really affected by a variation of pressure as long it remains in a reasonable range1 (typically about 10 atm). Physically and chemically modified membranes exhibit difference in permeability than that of virgin membranes2,3. The modifications in membranes are either permanent or temporary depending upon the condition of use2-4.

The temperature can have a large effect on the transport of small penetrants in the polymeric media. The permeation process is described by the free volume molecular theory4. Transport of gas molecules is dependent upon the available free volume in the

polymer matrix, as well as sufficient energy of the molecules to overcome attractive forces between chains. Any change in the polymer structure induced by a heat treatment produces the thermal fluctuations in the polymer matrix leading to an emergence of excess space that then permits the passage of the gas molecules5.

2 Experimental Details 2.1 Membrane preparation

Bisphenol-A poycarbonate is a glassy polymer used for the present study. The 30(±2) μm thick polycarbonate membrane was prepared by solution cast method. The material was dissolved in dichloromethane (CH2Cl2) and a 5% solution was prepared. The solution was then put into flat-bottomed petrie-dishes floating on mercury. The solvent was allowed to evaporate slowly over a period of 10-12 hr. The films so obtained were peeled off and dried in vacuum at 333 K for 24 hr. 2.2 Permeability measurements

Gas permeation through polymeric membranes occurs by the solution-diffusion model6-8. The permeation process occurred in three stages, which are sorption/diffusion/desorption. The sorption of gas molecules occurs at the upstream side of the membranes. Because of the concentration gradient, the molecules diffuse across the membrane before they are finally desorbing on the downstream side of the membrane. The permeability is defined as the

__________ ∗ E.mail: yk_vijay@sancharnet.in

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product of solubility and diffusivity coefficients and obeys the Fick’s law9. Permeability was measured by permeability cell. The detail of the cell has been given elsewhere2.

Permeability is defined as:

P = D*S …(1)

where, D is the diffusion coefficient and depends on the gas polymer interaction and S is the solubility constant. Permeability is also determined by the formula:

differencepressuremembraneofthicknessFluxP ×

= … (2)

The flow rate meter connected with permeability cell estimated the flux. The pressure difference remains constant across the membrane for a single measurement. Permeability is expressed in barrer, 1 barrer is equal to 10-10 cm3@STP.cm/cm2.s.cmHg.

In present work we have measured the effect of temperature as well as pressure on the gas permeability of the membrane. The temperature is varied from 303 K to 373 K and it is well below its glass transition temperature1 (423 K), at constant pressure of 30 psi, whereas the pressure varies from 30-100 psi at room temperature. The permeability increases with temperature and can be represented by an Arrhenius-type relationship10:

P=P0 exp (-Ep / RT) …(3)

where, EP is the activation energy of permeation. For a good approximation the activation energy:

EP = De + Se …(4)

where De is the activation energy for diffusion and Se is the activation energy for solubility. The permeability measurements have been taken as a function of pressure as well as temperature. It was observed that permeability was not much affected by pressure in the given pressure ranget which is in agreement with Barabari et al1. For temperature dependence measurements, the permeability cell is dipped in constant temperature water bath to keep a constant temperature.

3 Results and Discussion

The mechanism for the permeation of gases through dense polymeric membranes is due to the solution-diffusion model11-12. The value of the permeability coefficient P depends on operating conditions, such as temperature, pressure, as well as on structural features of the polymeric material. The value of P is dominated by the diffusion coefficient, D since variations in D are greater than those in solubility values S as observed by Mohr et al.13-15.

Fig. (1) shows that the permeability increases with increasing temperature for both the gases. The permeability is four times larger at 370 K than room temperature for hydrogen, whereas in case of CO2 it is observed at 343 K. This difference may be due to the polymer-penetrant interaction. At room temperature the permeability of both the gases is different and agrees well with the values reported in literature8. The difference in permeability is explained on the basis of difference in the solubility and diffusivity of the gas. The result shows that the permeability depends on the molecular size of the penetrant. The size of hydrogen molecules is 2.3 Å, whereas size of carbon dioxide molecules is 4 Å. The difference in the permeability at room temperature is due to difference in their molecular size.

The permeability of membrane does not depend on pressure of gas in given range can be understood by solubility of penetrant in the polymer matrix. The solubility of both the gases in the polymer in the given pressure range is almost the same. But the temperature affects the diffusion process drastically. Fig. (2) shows that the permeability increases with

320 340 360 3800.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Re-normalisation curve of permeability for polycarbonate for H

2 for CO

2

P T/PR

T

Temperature (in K)

Fig. 1 ⎯ Renormalization curve of permeability

ACHARYA et al.: POLYCARBONATE MEMBRANE

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temperature due to the change in diffusivity of gas in the polymer matrix. The activation energy for permeation of hydrogen is found to be 5.5 kJ, whereas in case of carbon dioxide it is 11.5 kJ.

The solubility is found to decrease with temperature and the diffusivity increases with temperature. The combined effect is diffusion controlled by change in free volume and gas polymer interaction.

4 Conclusion It was observed from the graph that the activation

energy for permeation of H2 is less than that of CO2. The results are well supported by Eq. (3). The diffusion energy of carbon dioxide in the polymer matrix is larger because linear shape of molecules, whereas in case of hydrogen required activation energy for diffusion is less due to the spherical shape of molecules. Consequently the permeability of hydrogen is greater than that of carbon dioxide.

Acknowledgement The authors are thankful to Dr M Singh for

helpful discussion. The author NKA is thankful to Nuclear Science Centre, New Delhi, for providing fellowship during the work under the UF-UP Project.

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11 Koros W J, Fleming G K, Jordan S M, Kim T H & Hoehn H H, Prog. Polym. Sci. 4 (1988) 339.

12 LeBlanc O H, Ward W J & Matson S L, J Membr. Sci. 6 (1980) 339-343.

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14 Mohr J M, Paul D R, Tullos G L & Cassidy P E, Polymer, 32 13 (1991) 2387-2394

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2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.81.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Temperature dependence on gas permeability For H

2 For CO

2

(logP

)

1000/T (in K-1)

Fig. 2 ⎯ logP versus 103/T