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Barrier properties of plasma-polymerized thin films
Surface and Coatings Technology 116–119 (1999) 996–1000 www.elsevier.nl/locate/surfcoat
Barrier properties of plasma-polymerized thin films M. Walker a, *, K.-M. Baumga¨rtner a, J. Feichtinger a, M. Kaiser a, E. Ra¨uchle a, J. Kerres b a Institut fu¨r Plasmaforschung der Universita¨t Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany b Institut fu¨r chemische Verfahrenstechnik der Universita¨t Stuttgart, Bo¨blinger Straße 72, 70199 Stuttgart, Germany
Abstract Thin hydrocarbon films are deposited on Nafion@ membranes in a low-pressure plasma excited by microwaves. Gas mixtures of hexane (C H ) with hydrogen (H ) were used as monomers. The permeability of methanol through the Nafion membranes 6 14 2 modified by plasma polymer films is investigated as a function of the C H /H ratio of the gas mixture. The methanol permeability 6 14 2 was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer film reduces the permeability of methanol by a factor of about 15. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Barrier layers; Methanol crossover; Nafion; Plasma polymerized films
1. Introduction Fuel cells that can operate directly on methanol are attractive candidates for portable power sources as well as for electric vehicle applications [1]. Direct methanol fuel cells (DMFC ) usually operate with perfluorosulfonate ionomer membranes, such as Nafion@. These membranes are composed of carbonMfluorine chains with perfluoro side chains containing sulfonic groups. The transport of protons through the membrane is made by the sulfonic groups. One major problem of DMFCs is the crossover of methanol through the Nafion membrane [2]. This methanol permeability not only reduces the fuel-utilization efficiency but also decreases the cathode performance. Both effects result in an overall loss of fuel-cell efficiency. The main components of a DMFC are shown in Fig. 1. It consist of: 1. a porous gas-diffusion electrode; 2. an electrocatalyst layer consisting of Pt–RuO at the x cathode side; 3. the proton-exchange Nafion membrane; 4. an electrocatalyst layer consisting of platinum at the anode side; and 5. a further porous gas-diffusion electrode. Additionally, Fig. 1 shows the barrier concept investigated to reduce the methanol crossover in DMFCs. In * Corresponding author. Tel.: +49-711-685-2302; fax: +49-711-685-3102.
this design, a thin hydrocarbon film is plasma-polymerized on the surface of the Nafion membrane. The films were deposited in a low-pressure plasma device, called Duo-Plasmaline@.
2. Experimental details 2.1. Plasma reactor In previous work, a new linearly extended low-pressure plasma source, called Duo-Plasmaline [3], was presented. The Plasmaline works like an inverted luminescent tube (inside atmospheric pressure and outside low pressure) supplied by microwave power. The DuoPlasmaline produces an axially homogeneous plasma with a length of up to several meters [4]. A schematic side view of the Plasmaline is shown in Fig. 2. In this work, a modified version of the Plasmaline is used, where two Plasmalines were arranged parallel to each other. A top view of this plasma array is shown in Fig. 3. The plasma source is installed in a vacuum chamber with a height of 80 cm and a diameter of 80 cm. Two quartz tubes (B3 cm), each with a copper rod as inner conductor (B0.8 cm), were mounted through the vacuum chamber. The distance between the two Plasmalines was 9 cm. The microwave generators are installed at both ends of the lines. The microwave power was splitted via a coaxial device to the
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 21 6 - 9
M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
997
Fig. 1. Concept of a direct methanol fuel cell. For details see text.
Fig. 2. Schematic side view of the Duo-Plasmaline.
Plasmalines. To guide the microwave into the inner part of the reactor, a metallic shielding is applied. This shielding is located inside the quartz tube and prevents ignition of the plasma near to the feedthrough. The plasma ignites outside the tubes at the low-pressure side. Cooling of the device is obtained by an air flow between the inner conductor and the quartz tubes. Details regarding measured electron densities, electron temperatures and the homogenity of the depostion rate are given elsewhere [5]. The experiments were performed in gas mixtures of hexane (C H ) and hydrogen (H ). These gases were 6 14 2
supplied to the plasma chamber by means of a gas inlet system, consisting of electronic flow meters. To increase the deposition rate, a gas shower was installed between the Plasmalines ( Fig. 3). The Nafion membranes were placed 5 cm below the Plasmalines on a substrate holder movable perpendicular to the Plasmalines. The typical pressure for plasma deposition was 30 Pa. 2.2. Permeation-measuring device To determine the barrier properties of the plasmapolymerized films, the permeability of methanol was
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M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
2.3. Impedance measurements For an application in a DMFC, the ionic conductivity of the plasma-polymerized films is important. The ionic electric resistance of the plasma-treated and untreated Nafion membranes was determined on an impedance measurement system (IM6, Zahner Elektrik). The impedance for the case where the phase angle between the current and the voltage is zero, was taken as the ohmic ion-exchange membrane resistance. The measurements were performed in a 0.5 N HCl electrolyte solution. The area of the gold-plated copper electrodes was 0.25 cm2.
Fig. 3. Plasma array in operation.
measured for plasma-treated and untreated Nafion membranes. A 200 mm thick Nafion 117 membrane, supplied by Du Pont Chemicals, was used. The permeation measurements were performed in the device shown in Fig. 4. The upper part of the permeation cell is filled with methanol. The methanol molecules diffuse along the concentration gradient through the membrane into the opposite compartment of the permeation cell. There, a constant nitrogen flow, adjusted by a mass flow controller (MFC ), sweeps the molecules to a flame ionisation detector of a gas chromatograph. The permeation cell was placed in a temperature bath at a constant temperature of 40°C. The measuring area was 86 mm2. To determine absolute particle flux densities the device was calibrated via the injection port of the gas chromatograph. The calibration is described in Ref. [6 ].
3. Results and discussion The permeability of pure methanol through the coated Nafion membranes was investigated as a function of the C H /H ratio in the gas mixture. 6 14 2 Fig. 5 shows the methanol flux densities versus time for (a) an uncoated Nafion 117 membrane and for plasma polymer films deposited from the gas mixture with C H /H ratios of 10:10 (b), 10:20 (c) and 10:30 6 14 2 (d ). Here a gas mixture ratio of 10:10, for example, means a gas flow rate of 10 sccm (standard cm3 min−1) C H and 10 sccm H . Table 1 shows the 6 14 2 results of the permeation experiments as a function of the C H /H ratio of the gas mixture. In column 4 of 6 14 2 Table 1, the retention coefficient is given as the ratio of the stationary flux through the untreated Nafion membrane to the stationary flux through the coated Nafion membrane. The stationary permeation rate decreases from
Fig. 4. Experimental set-up of the permeation-measuring device.
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M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
a small amount. According to the diffusion theory, it follows that, if a solvent is not dissolved in the polymer, it will not permeate it easily [8]. The Nafion membrane coated with a hydrophobic layer like PE or PP causes a decrease in the solubility of methanol and therefore a corresponding decrease in permeation. The barrier films deposited from C H /H plasmas act as solvent bar6 14 2 rier layers. As shown in Fig. 5, the permeation plots first go through an unsteady-state portion and then, after a certain time, the permeation rate remains constant (stationary part of the permeation process). The permeation rate of an untreated Nafion membrane rises to a maximum within about 500 s. In comparison, a Nafion membrane coated with a plasma polymer film from a gas mixture with C H /H ratio of 10:30 attains steady 6 14 2 state within about 2000 s. The difference in the time behavior is due to the concentration dependence of the diffusion coefficient. For the membranes investigated here, the diffusion coefficient is a strong function of the sorbed methanol concentration and can be described by [8,9]:
Fig. 5. Methanol flux density as a function of time for (a) an uncoated Nafion membrane and (b)–(d ) different plasma-polymerized films.
D=D exp(cc). 0
4.7×1017 methanol molecules cm−2 s−1 (MeOH/cm2 s1) for an uncoated Nafion membrane to 3.6×1016 MeOH/cm2 s1 for a plasma polymer obtained from a C H /H plasma deposited with a mixture ratio of 6 14 2 10:30. One result is that a 0.2 mm thick layer plasmapolymerized on a Nafion membrane with a thickness of 200 mm reduces the permeability by an order of magnitude. In previous work, the chemical composition of such plasma-polymerized films was investigated [7]. It was shown that the plasma films consist mainly of MCH M and MCH bonds. The chemical composition 2 3 is similar to the polymers polyethylene (PE) or polypropylene (PP). It is well known that these polymers have good barrier properties to methanol and other polar organics. PE and PP are hydrophobic substances, whereas Nafion is one of the most hydrophilic. Nafion absorbs methanol, water and some other organics rapidly, whereas PE or PP absorbs these liquids only in
(1)
Here, c is a characteristic parameter of the polymer– permeant system, c(x, t) is the sorbed concentration and D is the diffusion coefficient in the limit of low 0 concentrations. As a result of Eq. (1), a hydrophobic barrier layer causes a decrease of the sorbed concentration c in the Nafion membrane related with a decrease of the diffusion coefficient. This results in a decrease of the permeation flux in the unsteady-state phase as well as in the stationary phase of the permeation process. The ohmic proton resistance RH+ of the untreated and treated Nafion membranes is shown in Table 1. The resistance increases from 7.7 V cm for an untreated Nafion membrane to 55.3 V cm for a membrane coated with a barrier film plasma-polymerized from a gas mixture with C H /H ratio of 10:30. 6 14 2 For the application in a DMFC, the reduced permeability would lead to an improvement in the fuel-cell efficiency. However, the improved efficiency is partly
Table 1 Permeation rate of methanol, film thickness, retention coefficient and specific proton resistance of plasma-polymerized films produced with different C H /H ratios on Nafion membranes 6 14 2 C H /H ratio 6 14 2 in gas mixture
Stationary methanol flux (MeOH/cm2 s1)
Film thickness (mm)
Retention coefficient
Proton resistance, RH+ (V cm)
Nafion 117 untreated 10:10 10:20 10:30
4.7×1017 9.9×1016 7.2×1016 3.6×1016
– 0.2 0.2 0.2
– 4.7 6.5 13.1
7.7 70.5 –a 55.3
a Not measured.
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reduced by the increased resistance. In further work, the resistance of the barrier films is to be improved by addition of another gas, e.g., SO , which forms ionic 2 conducting groups in the layer.
Acknowledgement
4. Conclusions
References
This investigation contributes to the problem of methanol permeability through Nafion membranes. Such membranes are used as polymeric electrolytes in direct methanol fuel cells (DMFCs). We demonstrate a barrier concept to reduce the methanol crossover in DMFCs. The barrier films are plasma-polymerized in a new plasma source, called a Duo-Plasmaline. This linearly extended plasma source is well suited for industrial applications. It is shown that a thin hydrocarbon layer deposited from C H /H plasmas is an effective method to reduce 6 14 2 the methanol permeability of Nafion. For example, a barrier film with a thickness of 0.2 mm, plasma polymerized on a 200 mm thick Nafion membrane reduces the permeability of methanol by a factor of about 15.
[1] J.T. Wang, S. Wasmus, R.F. Savinell, J. Electrochem. Soc. 143 (4) (1996) 1233. [2] C. Pu, W. Huang, K.L. Ley, E.S. Smotkin, J. Electrochem. Soc. 142 (7) (1995) 119. [3] German Patent, DE 19503205 C1, 2 February 1995. [4] W. Petasch, E. Ra¨uchle, H. Muegge, K. Muegge, Surf. Coat. Technol. 93 (1997) 112. [5] M. Kaiser, K.-M. Baumga¨rtner, M. Walker, E. Ra¨uchle, Surf. Coat. Technol. 116–119 (1999) 552. [6 ] M. Walker, Permeation von Alkanen durch plasmapolymerisierte Barriereschichten auf Polyethylen hoher Dichte. Doctoral Thesis, Universita¨t Stuttgart, 1996. [7] M. Walker, K.-M. Baumga¨rtner, M. Ruckh, M. Kaiser, H.W. Schock, E. Ra¨uchle, J. Appl. Polym. Sci. 64 (1997) 717. [8] J. Comyn, Polymer Permeability, Elsevier Applied Science, Kidlington, UK, 1988. [9] R.F. Baddour, A.S. Michaels, H.J. Bixler, R.P. de Filippi, J.A. Barrie, J. Appl. Polym. Sci. 8 (1964) 897.
The authors wish to thank the Stiftung Energieforschung Baden Wu¨rttemberg (Contract No. A 00009696) for funding this research.
Barrier properties of plasma-polymerized thin films M. Walker a, *, K.-M. Baumga¨rtner a, J. Feichtinger a, M. Kaiser a, E. Ra¨uchle a, J. Kerres b a Institut fu¨r Plasmaforschung der Universita¨t Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany b Institut fu¨r chemische Verfahrenstechnik der Universita¨t Stuttgart, Bo¨blinger Straße 72, 70199 Stuttgart, Germany
Abstract Thin hydrocarbon films are deposited on Nafion@ membranes in a low-pressure plasma excited by microwaves. Gas mixtures of hexane (C H ) with hydrogen (H ) were used as monomers. The permeability of methanol through the Nafion membranes 6 14 2 modified by plasma polymer films is investigated as a function of the C H /H ratio of the gas mixture. The methanol permeability 6 14 2 was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer film reduces the permeability of methanol by a factor of about 15. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Barrier layers; Methanol crossover; Nafion; Plasma polymerized films
1. Introduction Fuel cells that can operate directly on methanol are attractive candidates for portable power sources as well as for electric vehicle applications [1]. Direct methanol fuel cells (DMFC ) usually operate with perfluorosulfonate ionomer membranes, such as Nafion@. These membranes are composed of carbonMfluorine chains with perfluoro side chains containing sulfonic groups. The transport of protons through the membrane is made by the sulfonic groups. One major problem of DMFCs is the crossover of methanol through the Nafion membrane [2]. This methanol permeability not only reduces the fuel-utilization efficiency but also decreases the cathode performance. Both effects result in an overall loss of fuel-cell efficiency. The main components of a DMFC are shown in Fig. 1. It consist of: 1. a porous gas-diffusion electrode; 2. an electrocatalyst layer consisting of Pt–RuO at the x cathode side; 3. the proton-exchange Nafion membrane; 4. an electrocatalyst layer consisting of platinum at the anode side; and 5. a further porous gas-diffusion electrode. Additionally, Fig. 1 shows the barrier concept investigated to reduce the methanol crossover in DMFCs. In * Corresponding author. Tel.: +49-711-685-2302; fax: +49-711-685-3102.
this design, a thin hydrocarbon film is plasma-polymerized on the surface of the Nafion membrane. The films were deposited in a low-pressure plasma device, called Duo-Plasmaline@.
2. Experimental details 2.1. Plasma reactor In previous work, a new linearly extended low-pressure plasma source, called Duo-Plasmaline [3], was presented. The Plasmaline works like an inverted luminescent tube (inside atmospheric pressure and outside low pressure) supplied by microwave power. The DuoPlasmaline produces an axially homogeneous plasma with a length of up to several meters [4]. A schematic side view of the Plasmaline is shown in Fig. 2. In this work, a modified version of the Plasmaline is used, where two Plasmalines were arranged parallel to each other. A top view of this plasma array is shown in Fig. 3. The plasma source is installed in a vacuum chamber with a height of 80 cm and a diameter of 80 cm. Two quartz tubes (B3 cm), each with a copper rod as inner conductor (B0.8 cm), were mounted through the vacuum chamber. The distance between the two Plasmalines was 9 cm. The microwave generators are installed at both ends of the lines. The microwave power was splitted via a coaxial device to the
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 21 6 - 9
M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
997
Fig. 1. Concept of a direct methanol fuel cell. For details see text.
Fig. 2. Schematic side view of the Duo-Plasmaline.
Plasmalines. To guide the microwave into the inner part of the reactor, a metallic shielding is applied. This shielding is located inside the quartz tube and prevents ignition of the plasma near to the feedthrough. The plasma ignites outside the tubes at the low-pressure side. Cooling of the device is obtained by an air flow between the inner conductor and the quartz tubes. Details regarding measured electron densities, electron temperatures and the homogenity of the depostion rate are given elsewhere [5]. The experiments were performed in gas mixtures of hexane (C H ) and hydrogen (H ). These gases were 6 14 2
supplied to the plasma chamber by means of a gas inlet system, consisting of electronic flow meters. To increase the deposition rate, a gas shower was installed between the Plasmalines ( Fig. 3). The Nafion membranes were placed 5 cm below the Plasmalines on a substrate holder movable perpendicular to the Plasmalines. The typical pressure for plasma deposition was 30 Pa. 2.2. Permeation-measuring device To determine the barrier properties of the plasmapolymerized films, the permeability of methanol was
998
M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
2.3. Impedance measurements For an application in a DMFC, the ionic conductivity of the plasma-polymerized films is important. The ionic electric resistance of the plasma-treated and untreated Nafion membranes was determined on an impedance measurement system (IM6, Zahner Elektrik). The impedance for the case where the phase angle between the current and the voltage is zero, was taken as the ohmic ion-exchange membrane resistance. The measurements were performed in a 0.5 N HCl electrolyte solution. The area of the gold-plated copper electrodes was 0.25 cm2.
Fig. 3. Plasma array in operation.
measured for plasma-treated and untreated Nafion membranes. A 200 mm thick Nafion 117 membrane, supplied by Du Pont Chemicals, was used. The permeation measurements were performed in the device shown in Fig. 4. The upper part of the permeation cell is filled with methanol. The methanol molecules diffuse along the concentration gradient through the membrane into the opposite compartment of the permeation cell. There, a constant nitrogen flow, adjusted by a mass flow controller (MFC ), sweeps the molecules to a flame ionisation detector of a gas chromatograph. The permeation cell was placed in a temperature bath at a constant temperature of 40°C. The measuring area was 86 mm2. To determine absolute particle flux densities the device was calibrated via the injection port of the gas chromatograph. The calibration is described in Ref. [6 ].
3. Results and discussion The permeability of pure methanol through the coated Nafion membranes was investigated as a function of the C H /H ratio in the gas mixture. 6 14 2 Fig. 5 shows the methanol flux densities versus time for (a) an uncoated Nafion 117 membrane and for plasma polymer films deposited from the gas mixture with C H /H ratios of 10:10 (b), 10:20 (c) and 10:30 6 14 2 (d ). Here a gas mixture ratio of 10:10, for example, means a gas flow rate of 10 sccm (standard cm3 min−1) C H and 10 sccm H . Table 1 shows the 6 14 2 results of the permeation experiments as a function of the C H /H ratio of the gas mixture. In column 4 of 6 14 2 Table 1, the retention coefficient is given as the ratio of the stationary flux through the untreated Nafion membrane to the stationary flux through the coated Nafion membrane. The stationary permeation rate decreases from
Fig. 4. Experimental set-up of the permeation-measuring device.
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M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
a small amount. According to the diffusion theory, it follows that, if a solvent is not dissolved in the polymer, it will not permeate it easily [8]. The Nafion membrane coated with a hydrophobic layer like PE or PP causes a decrease in the solubility of methanol and therefore a corresponding decrease in permeation. The barrier films deposited from C H /H plasmas act as solvent bar6 14 2 rier layers. As shown in Fig. 5, the permeation plots first go through an unsteady-state portion and then, after a certain time, the permeation rate remains constant (stationary part of the permeation process). The permeation rate of an untreated Nafion membrane rises to a maximum within about 500 s. In comparison, a Nafion membrane coated with a plasma polymer film from a gas mixture with C H /H ratio of 10:30 attains steady 6 14 2 state within about 2000 s. The difference in the time behavior is due to the concentration dependence of the diffusion coefficient. For the membranes investigated here, the diffusion coefficient is a strong function of the sorbed methanol concentration and can be described by [8,9]:
Fig. 5. Methanol flux density as a function of time for (a) an uncoated Nafion membrane and (b)–(d ) different plasma-polymerized films.
D=D exp(cc). 0
4.7×1017 methanol molecules cm−2 s−1 (MeOH/cm2 s1) for an uncoated Nafion membrane to 3.6×1016 MeOH/cm2 s1 for a plasma polymer obtained from a C H /H plasma deposited with a mixture ratio of 6 14 2 10:30. One result is that a 0.2 mm thick layer plasmapolymerized on a Nafion membrane with a thickness of 200 mm reduces the permeability by an order of magnitude. In previous work, the chemical composition of such plasma-polymerized films was investigated [7]. It was shown that the plasma films consist mainly of MCH M and MCH bonds. The chemical composition 2 3 is similar to the polymers polyethylene (PE) or polypropylene (PP). It is well known that these polymers have good barrier properties to methanol and other polar organics. PE and PP are hydrophobic substances, whereas Nafion is one of the most hydrophilic. Nafion absorbs methanol, water and some other organics rapidly, whereas PE or PP absorbs these liquids only in
(1)
Here, c is a characteristic parameter of the polymer– permeant system, c(x, t) is the sorbed concentration and D is the diffusion coefficient in the limit of low 0 concentrations. As a result of Eq. (1), a hydrophobic barrier layer causes a decrease of the sorbed concentration c in the Nafion membrane related with a decrease of the diffusion coefficient. This results in a decrease of the permeation flux in the unsteady-state phase as well as in the stationary phase of the permeation process. The ohmic proton resistance RH+ of the untreated and treated Nafion membranes is shown in Table 1. The resistance increases from 7.7 V cm for an untreated Nafion membrane to 55.3 V cm for a membrane coated with a barrier film plasma-polymerized from a gas mixture with C H /H ratio of 10:30. 6 14 2 For the application in a DMFC, the reduced permeability would lead to an improvement in the fuel-cell efficiency. However, the improved efficiency is partly
Table 1 Permeation rate of methanol, film thickness, retention coefficient and specific proton resistance of plasma-polymerized films produced with different C H /H ratios on Nafion membranes 6 14 2 C H /H ratio 6 14 2 in gas mixture
Stationary methanol flux (MeOH/cm2 s1)
Film thickness (mm)
Retention coefficient
Proton resistance, RH+ (V cm)
Nafion 117 untreated 10:10 10:20 10:30
4.7×1017 9.9×1016 7.2×1016 3.6×1016
– 0.2 0.2 0.2
– 4.7 6.5 13.1
7.7 70.5 –a 55.3
a Not measured.
1000
M. Walker et al. / Surface and Coatings Technology 116–119 (1999) 996–1000
reduced by the increased resistance. In further work, the resistance of the barrier films is to be improved by addition of another gas, e.g., SO , which forms ionic 2 conducting groups in the layer.
Acknowledgement
4. Conclusions
References
This investigation contributes to the problem of methanol permeability through Nafion membranes. Such membranes are used as polymeric electrolytes in direct methanol fuel cells (DMFCs). We demonstrate a barrier concept to reduce the methanol crossover in DMFCs. The barrier films are plasma-polymerized in a new plasma source, called a Duo-Plasmaline. This linearly extended plasma source is well suited for industrial applications. It is shown that a thin hydrocarbon layer deposited from C H /H plasmas is an effective method to reduce 6 14 2 the methanol permeability of Nafion. For example, a barrier film with a thickness of 0.2 mm, plasma polymerized on a 200 mm thick Nafion membrane reduces the permeability of methanol by a factor of about 15.
[1] J.T. Wang, S. Wasmus, R.F. Savinell, J. Electrochem. Soc. 143 (4) (1996) 1233. [2] C. Pu, W. Huang, K.L. Ley, E.S. Smotkin, J. Electrochem. Soc. 142 (7) (1995) 119. [3] German Patent, DE 19503205 C1, 2 February 1995. [4] W. Petasch, E. Ra¨uchle, H. Muegge, K. Muegge, Surf. Coat. Technol. 93 (1997) 112. [5] M. Kaiser, K.-M. Baumga¨rtner, M. Walker, E. Ra¨uchle, Surf. Coat. Technol. 116–119 (1999) 552. [6 ] M. Walker, Permeation von Alkanen durch plasmapolymerisierte Barriereschichten auf Polyethylen hoher Dichte. Doctoral Thesis, Universita¨t Stuttgart, 1996. [7] M. Walker, K.-M. Baumga¨rtner, M. Ruckh, M. Kaiser, H.W. Schock, E. Ra¨uchle, J. Appl. Polym. Sci. 64 (1997) 717. [8] J. Comyn, Polymer Permeability, Elsevier Applied Science, Kidlington, UK, 1988. [9] R.F. Baddour, A.S. Michaels, H.J. Bixler, R.P. de Filippi, J.A. Barrie, J. Appl. Polym. Sci. 8 (1964) 897.
The authors wish to thank the Stiftung Energieforschung Baden Wu¨rttemberg (Contract No. A 00009696) for funding this research.