Recent developments in measuring permeation through barrier films and understanding of permeation processes H. Nörenberg, Technolox Ltd., Oxford, UK and V. M. Burlakov, Department of Materials, University of Oxford, Oxford, UK Key Words: Permeation barrier coatings Moisture permeation Gas permeation Modeling ABSTRACT A novel permeation measurement system with a mass spectrometer as detector has been set up. Test measurements on barrier layers gave a lower detection limit for WVTR of about 5x10-5 g/m 2 /day. The advantage of the mass spectrometric method is, that it can be used to study the permeation a wide range of gases or vapours. Proof of principle measurements have been carried out for position resolved permeation studies. The method could be used to measure the rate of permeation through samples other than films such as electronic components (batteries) and through the edges of samples. Theoretical studies of the pressure dependence of the rate of permeation of gases through polymer films (example: OPP) showed, that the pressure dependence of gas transport through these layers is caused by an effective decrease in the diffusion activation energy after gas sorption. The mechanism causing this decrease is related to saturation of deep traps so that fast-diffusion channels possessing lower activation energy are created. This confines mass transport to a network of shallow traps INTRODUCTION The implementation of experiments to measure rates of permeation at an extremely low level and the understanding of the permeation process itself are the key to the future progress in the area of barrier layers. OLED development and quality control, for instance, require a magic rate of water vapor transmission (WVTR) of about 10-6 g/m 2 /day or less [1]. The Calcium test is widely used to estimate the WVTR. However, the Calcium test is restricted to chemically reacting permeants such as water vapor and oxygen. Permeation measurements based on a mass spectrometer as detector have been reported for permeation of a variety of gases and water vapor through polymers [2-3]. Permeation of gases and vapors through barrier layers may be influenced by a number of parameters. Experimental observations showed, that even permeation of inert gases through simple polymers is pressuredependent [3-4]. EXPERIMENTS Figure 1 shows a schematic view of the experimental set up. The ultra-high vacuum (UHV) system consists of two chambers, which allows the conditioning of the sample S in the preparation chamber P before introducing it into the investigation chamber I. The investigation chamber I is equipped with a quadrupole mass spectrometer MS to detect the partial pressure and a xyz-stage for sample manipulation. A gas container is filled with water vapour similar to the procedure described in [1]. After introduction of the gas container with the sample into the preparation chamber P and evacuating to a sufficiently low vacuum level, the sample is then transferred into the investigation chamber I by means of a transfer arm and positioned to face the mass spectrometer MS. The partial pressure of water vapour of the unknown test sample is measured. With the known values for the partial pressure of water vapour and the WVTR of a calibration sample the WVTR of the test sample can be calculated. Figure 1. Schematic view of experimental set up with preparation chamber P and investigation chamber I, S: sample, MS: mass spectrometer RESULTS AND DISCUSSION Sensitive WVTR measurements Figure 2 illustrates the capability of the mass spectrometric method. The partial pressure of a calibration sample and a barrier sample is shown as 2004 Society of Vacuum Coaters 505/856-7188 1
function of time. The calibration sample was 0.1 mm thick foil of PEN with a WVTR of 1.25 g/m 2 /day (@40ºC). clear maximum at x=0 and decreases to both sides. The variation of partial pressure, which is the contrast of the permeation, is about 1x10-9 mbar or 30% between the center and 1 mm off center. This variation is the proof of principle for position resolved permeation measurements. Figure 2. Partial pressure as function of time for a barrier sample and a calibration sample (PEN) At about t=12 h (fig. 2) the sample mounted on the gas container was introduced into the investigation chamber. The water vapour coming from the gas container leads to an increase of the partial pressure of the water vapour signal. There is a pronounced increase of the water vapour partial pressure for the PEN sample. As figure 2 shows, the partial pressure settles around 2.5x10-9 mbar, which corresponds to a WVTR of 1.25 g/m 2 /day. For the barrier layer, the increase of the water vapour partial pressure is less pronounced. Figure 2 shows, that the increase in the partial pressure of the water vapour of the barrier layer is less than 10-4 g/m 2 /day. This means, that the current experimental set-up has a dynamic range of at least 6 orders of magnitude (see also ref. [3]) and a lower detection limit of about 5x10-5 g/m 2 /day is estimated. The lower detection limit may depend on the nature of the sample (i.e. whether outgasing occurs). In order to obtain the high dynamic range of sensitivity for water vapour H 18 2 O was used [2]. Figure 3. Partial pressure as function of lateral displacement (sample: PEN) Theoretical calculations A phenomenological model for diffusion of molecules through polymer substrates has been developed [5]. The diffusion is modelled as random hopping over a potential barrier of the diffusion activation energy E act between sites. These sites have random sorption site energies. At t=0 the polymer is free of the permeant gas. The gas then enters and diffuses through the polymer. Figure 4 shows the situation at t>0. Some gas atoms are already inside the polymer occupying sorption sites that had a low enough energy to trap these atoms (black spots in fig. 1, unoccupied sorption sites are shown as circles). Once the site is occupied, the pathway through this site is blocked. The gas atoms take pathways, which do not contain such deep trap sites and can therefore diffuse faster. Position resolved permeation measurements To study the permeation position-resolved, the gas container with the sample was positioned very close to the mass spectrometer. The sample was of a piece of PEN film of about 1mm exposed diameter. Outside this exposed area the sample was covered with impermeable material. The sample was moved across the mass spectrometer by means of the xyz-stage (see figure 1), so that different areas of the sample were exposed to the ionizing region of the mass spectrometer at each point of measurement. Figure 3 shows the partial pressure as function of lateral replacement. As the sample moves across the mass spectrometer, the partial pressure shows a 2004 Society of Vacuum Coaters 505/856-7188 2
Figure 4. Model of diffusion of small molecules through a polymer. The sample is divided into N slices perpendicular to the concentration gradient. Molecules enter from the left hand side. Formally the dependence of permeation upon the gas ambient pressure is taken into account via the dependence of the activation energy for gas diffusion E act upon the gas concentration n(x,t) 0 E (,) xt = E 1 α nxt (,) (1) act act ( ) 0 where Eact and α are constants. Solving numerically the diffusion equation with a space- and time-dependent activation energy given by Eq. (1) nxt (,) Eact (,) xt = D0 exp t kt 2 2 (2) nxt (,) 0 nxt (,) + α 2 Eact, x x and taking into account the gas solubility through the boundary conditions, we simulated the permeation process as a function of time. The consequence of having such fast channels is, that at a high pressure on the feeding side a higher rate of permeation is achieved. At a low pressure, there is a higher probability that gas atoms become trapped at unoccupied deep trap sites, which slows down mass transport. The model is used to study the transient behaviour of the rate of permeation. Figure 5. Comparison of experimental data with calculations Figure 5 compares experimental data and the results from the calculations. Experimental data were used from Xenon permeation through an OPP sample (for details see [3]). Input parameters are the solubility, the diameter of the permeating gas atoms and the activation energy for diffusion. Figure 5 shows, that the model is capable of reproducing experimental results with good accuracy. Hence, the random distribution of sorption site energies is indeed responsible for the pressure dependence of the rate of permeation. A further advantage of the method is, that the results obtained for one gas can be used as input to predict the transient behaviour P=P(t) of other gases through the polymer. CONCLUSION A method to measure WVTR with a mass spectrometer as detector was set up. Test measurements gave a lower detection limit of about 5x10-5 g/m 2 /day WVTR. As the mass spectrometer can detect a wide range of gases and vapours, the application the method is not restricted to measure WVTR. It has potential for the study of permeation in other applications such as vacuum insulating panels, where the permeation of all gases from the environment is of great interest. Proof of principle for position resolved permeation measurements has been obtained. In this mode, the sample is moved with respect to the mass spectrometer giving the rate of permeation along a line or as a twodimensional pattern on the sample. A phenomenological model has been developed to describe the transient permeation of gas through a 2004 Society of Vacuum Coaters 505/856-7188 3
polymer membrane. The distribution of the trap site energies accounts for the observed pressure-dependent permeation of heavy noble gases through OPP. REFERENCES 1. H.-C. Langowski, A. Melzer and D. Schubert, Ultra High Barrier Layers for Technical Applications SVC 45 th Annual Technical Conference Proceedings, 45, 471, 2002. 2. H. Nörenberg, T. Miyamoto, G. D. W. Smith, G. A. D. Briggs and Y. Tsukahara, Mass spectrometric estimation of gas permeation coefficients for thin polymer membranes Rev. Sci. Instruments, 70, 2414, 1999. 3. H. Nörenberg, V. M. Burlakov, H.-J. Kosmella, G. D. W. Smith, G. A. D. Briggs and Y. Tsukahara, Permeation of noble gases (He, Ne, Ar, Kr, Xe) through thin membranes of OPP studied by mass spectroscopy Polymer 42, 10021, 2001. 4. Y. Naito, K. Mizoguchi, K. Terada and Y. Kamiya, The Effect of Pressure on Gas Permeation Through Semicrystalline Polymers above the Glass Transition Temperature Journal of Polymer Science: Part B: Polymer Physics, 29, 457, 1991. 5. V. M. Burlakov, H. Nörenberg, G.A.D. Briggs and Y. Tsukahara, Model for the Pressure-Dependent Permeation of Small Molecules through a Polymer Membrane Journal of Polymer Science: Part B: Polymer Physics, 41, 1308, 2003. 2004 Society of Vacuum Coaters 505/856-7188 4
2004 Society of Vacuum Coaters 505/856-7188 5