A mass spectrometric 'Dynamic Delta' method for the measurement of gas permeability of polymeric membranes has been developed. The method is universally applicable for measurement of the permeability of any gas through polymeric membrane materials. The usual large sample size of more than 100 square centimeters required for other methods is not necessary for this new method which requires a size less than one square centimeter. The new method should fulfill requirements and find applicability for industrial materials such as food packaging, contact lenses and other commercial materials where gas permeability or permselectivity properties are important

Brown, Kenneth G.

Burns, Karen S.

Upchurch, Billy T.

Wood, George M.

English

NASA Technical Reports Server

INSTRUMENTATION FOR MEASUREMENT OF GAS PEMEABILIW 
OF POLYhlERIC MEMB S# 
Billy T. Uphurch 
NASA Langley Research Center 
Hampton, VA 23681 
George M. Wood 
NASA Langley Research Center 
Wampton, VA 23681 
Kenneth 6. Brown 
Old Dominion University 
Norfolk, VA 23529 
Karen S. Burns 
Old Dominion University 
Norfolk, VA 23529 
ABSTRACT 
A mass spectrometric "Dynamic Delta" method for the measurement of gas permeabzty of polperic 
membranes has been developed. The method is universally applicable for measurement of the perrnebifiq of 
any gas through polymeric membrane materials. The usual large sample size of more than 100 square eentbeters 
required for other methods is not necessary for this new method which requires a size less than one squxe 
centimeter. The new method should fulfil requirements and find applicability for industrial materids such as 
food packaging, contact lenses and other commercial materials where gas permeability or permselectiiQ 
properties are important. 
INTRODUCTION 
The gas permeability of a polymeric material is an important physical property which helps determine 
whether or not a given material might be appropriate for a particular application. Some application exmples 
are food packaging, beverage containers, gas separation processes and oxygen permeability in contact lens 
materials. The permeability depends upon two physical processes, the solubility of the gas in the materid and 
the diffusivity (D) of the gas through the material. The solubility often follows Henry's Law behaeor and can 
be represented by the Henry's Law constant k. If the thickness of the membrane (L) is taken into acco~mt hen 
we can define the permeance or transmissibility by equation 1: 
where P is the gas permeation rate through a known area and thickness of material per unit t h e .  
Several analytical techniques have been applied to the problem of determining gas permealbilipy. A few 
'This research is a part of the Gas Permeable Polymeric Materials (GPPM) flight experbent, and is being 
carried out under the auspices of the Office of Commercial Programs, NASA Headquarters, Washington, DC. 
The GPPM experiment is manifested as part of the SPACEHAB payload on the STS-57 mission in April, 193, 
and includes materials from both NASA and an Industrial Guest Investigator, Paragon Optical Co., Mesa, AZ. 
https://ntrs.nasa.gov/search.jsp?R=19930016420 2020-03-24T07:27:09+00:00Z
are gas specific, such as polarography for oxygen determination1, the application of platinum and silver 
electrodes for measurement of oxygen in living tissues1, and the application of electron spin resonance (ESR) 
to oxgigen permeation in polyethylene2. Additional techniques involve the use of spectrophotometry but these 
bvo1ve the hcorporation of photosensitive dyes into the materials3&$. The American Society of Testing and 
Materids (ASTM) has adopted three standard test methods; a) a manometric technique, ASTM:D 1434-82~, 
b) a volumetmc method, ASTM:D 1434-82~, and c) a coulometric method, ASTM:D 3985-816. Each of these 
stmdwd t e c ~ q u e s  requires large sample test areas (50 - 100 cm2) and is not capable of differentiating between 
gases when gas mixtures are being measured. 
This paper describes a mass spectrometric technique that minimizes sample area and allows the 
determhation of permeation rates for individual gases in gas mixtures. This technique, termed the "Dynamic 
Delta" method7, is a sigmficant improvement upon standard mass spectrometric techniques. The technique is 
appEcabie for my gas and is easy to perform with good reproducibility. We shall demonstrate its appropriateness 
to be hcluded as a standard industrial test for gas permeability. 
METHOD 
The "dynmic delta" measurement, is not specific for any particular pressure measuring device or mass 
spectrometer as long as the response is linear over the range of interest. The basic setup is illustrated in figure 
1 with a standard X-Y plotter as the recording device. With this particular configuration we can simultaneously 
mornator the ion signal as a function of the inlet pressure. The ion signal IA is proportional to the partial pressure 
PA throu& an experimentally determined proportionality constant SA. The magnitude of IA is determined by 
the sum of the partial pressure of gaseous constituent A introduced into the ion source, the background pressure 
of A wKch may already exist in the ion source, and the contribution from an unresolved ion equivalent in mass 
to that of constituent A. 
In general practice, the background signal is experimentally determined prior to introducing a sample 
into the mass spectrometer. This signal is then subtracted from the total signal prior to calculating the 
concentration. W e n  the concentration of the sample is large with respect to the background, the quantitative 
analysis can be carried out with high accuracy. When analyzing at trace levels however, the signal from the 
sample may approximate that from the background and the accuracy is significantly reduced. 
The effects of background on accuracy can be reduced if the measurements are carried out with several 
samples introduced into the ion source at incrementally increasing pressures and the concentration calculated 
from the differences in the resulting signals. Since the ion source background signal should not change, its effect 
will be reduced along with a reduced requirement for making highly accurate measurement of total pressure. 
Studies in the NASA Langley Instrument Research Division Mass Spectrometry Laboratory determined that the 
accuracy w u l d  increase with the number of difference calculations. If the ion source pressure were allowed to 
increase in a linear manner and the ion current from a single mass is simultaneously measured, this would be 
eqgkivaiiene to obtaining an essentially infinite number of difference measurements and the concentration could 
be directly determined from a determination of the slope of the resulting line. 
When the ion signal, representative of the gas species of interest, is provided as input to the Y axis of 
the recorder and the inlet pressure is provided as a signal to the X axis the resultant plot should be linear with 
any devsatiows from linearity indicating either experimental error or additional phenomena taking place elsewhere 
in the system. Typical experimental data, using oxygen as an illustration with argon as an internal reference gas 
are shorn in figure 2. These data may be used to calculate the concentration of a particular species from the 
foPollo~ng relationship: 
100. tanel- Q2 . p1- a 
Vol % = 
tane2 . Q1- p2 
where: 
tanel = the slope of the line for the analyte gas 
tane2 = the slope of the line for the reference gas 
Q1 = the ion current amplifier resistance used for the reference gas 
QZ = the ion current amplifier resistance used for the analyte gas 
pl = the recorder attenuation used for the reference gas 
p2 = the recorder attenuation used for the analyte gas 
a = the relative gas sensitivity for the analyte and reference gases: identical gases a = 1. 
In the above expression everything but the angle measurements represents properties of the system and equation 
2 may be rewritten as: 
tanel 
Vol % = mol% = 100 -' K 
tme, 
where K is the experimentally determined proportionality constant for the analysis. 
EXPERIMENTAL 
Specimen holders, shown in figure 3, were designed to enable the study of both flat and curved sueface 
specimens and were fabricated from brass. Samples were held tightly in place using an O-ring seal to prevent 
leakage around the polymer specimen. As shown in figure 4, the analysis system was designed to assure ildential 
gas pressure and flow rates on both sides of the membrane. In this design the permeating mdFe gas was 
allowed to flow by the upstream side of the membrane while the carrier gas, in this case argon, flowed by the 
membrane on the mass spectrometer side. Permeant gas then enters the argon gas stream and is carrried to the 
capillary inlet of the mass spectrometer for analysis as discussed above. 
All of the mass spectrometric measurements were made on a 180°, 12.7 cm radius magnetic sector mass 
spectrometer of the Dempster type. The instrument is equipped with a capillary inlet desiped to permit 
continuous sampling from atmospheric pressure and to provide control of the pressure in the ion solwce. A 
representative sample of the gas is introduced directly into the ion source through a gold foil molecular leak md 
quartz tube. The entire assembly can be maintained at elevated temperature to prevent condernsation of volatile 
gaseous constituents. The exit slit was adjusted to provide a flat top peak. 
The gases used were argon, nitrogen, carbon dioxide with stated purities of 99.998% and research grade 
oxygen. The gravimetric primary standard calibration gas mixture was certified with the following concentrations: 
0.4609% 02;  0.5122% N2; 0.8077% C02 and 98.23% Ar. 
The standard reference material was purchased from the National Bureau of Standards (Pda3S). F i e e n  
sheets of a poly(ethyleneterephtha1ate) film were supplied. Three samples, 1.35 cm in diameter, were cut for the 
mass spectrometry measurements. Contact lens samples were provided by Paragona Optical with a concave radius 
of 8.00 mm and a chord diameter of 10.00 mm. The center thickness was nominally 0.20 mm and was measured 
to rt 0.002 mm. 
RESULTS AND DISCUSSION 
This method is very amenable to computerization utilizing one channel of an A-D converter for the 
pressure signal and another channel for the ion signal. Computer programs have been written to process data 
of this type with highly precise results. Raw data is shown tn table I to illustrate measurement precision obtained 
from duplicate runs on two samples of one contact lens material. 
Table I: Raw data for oxygen from the Dynamic Delta method 
for contact lens s~ecimen 5 at 35 "C. 
The results of the mass spectrometric analyses of some general polymer materials are presented in Table 
II. The measured value for the SRM 1470 represents the current detection limit of the system using this grade 
of Ar gas and the small sample size. This detection limit is due to the presence of oxygen in the argon carrier 
gas. This level of impurity was a problem in trying to determine the permeabiity of the standard reference 
material (SRM 1470). We determined the permeabiity at 23 "C to be 452a 0.01 x 10-l2 mL 02/ansaun Hg 
, which corresponds to the level of oxygen in the argon gas, and is an order of magnitude greater than the NBS 
certified value of 2.67 x mL 02/ansmm Hg. It should be noted that the SRM is a barrier material 
meant as a standard for the testing of barrier materials. 
Run 
Peak (ratio) 
%Ar reference 
320 impurity 
" o / ~ A ~  impurity 
%A, reference 
320 total 
320/36~ total 
32~/36Ar corrected 
The SRM 1470 is a highly crystalline film, biaxially cold-drawn when it is made, and has a glass transition 
temperature T close to 90 "C or well above room temperature.8 These first two characteristics lead to 
constraints of %e polymer chains to reorient?*1° When a glassy polymer is below its glass transition, it is not 
in a state of true thermodynamic equilibrium, and the permeabiity and solubility coefficients are more dependent 
on gas pressure or concentration in polymers and on temperature.10 These types of materials generally exhibit 
dual-mode sorption behavior unless their "excess" free or void volume below Tg is smalL'O Dual mode or type 
I1 isotherm can result when these glassy polymers absorb gases into pre-existing voids and then behaves as a true 
solution. With the exception of the silicone membrane, the other materials in Table 11 are known to have 
crystalline or semi-crystalline forms and a Tg well above room temperature. Subsequent literature review did not 
find oxygen permeability data on these materials for comparison. The oxygen permeabiity of the silicone 
membrane data is comparable to the product literature1' and other data12. 
The gas permeabiities for the contact lens materials are summarized in tables III and IV for the 
temperatures 22 "C and 35 "C respectively. With the exception of the two materials of lowest permeabiity, 6 
and 8, the permeabilities at 35 "C for oxygen, nitrogen, and carbon dioxide were significantly greater than 
permeation rates determined at room temperature. 
Sample 5a (L = 0.0186m) 
Slope or Slope Ratio 
0.7236 2 0.0039 
0.1377 + 0.0099 
0.1903 2 0.0137 
0.7318 2 0.0019 
0.4677 & 0.0052 
0.6391 0.0073 
0.4488 2 0.0155 
Sample 5b (L = 0.0192 an) 
Slope or Slope Ratio 
0.7257 0.0202 
0.1347 -C 0.0141 
0.1856 -C 0.0201 
0.7321 a 0.0083 
0.4380 2 0.0066 
0.5983 0.0115 
0.4127 + 0.0232 
Our measured carbon dioxide permeation rates are much greater than oxygen or nitrogen permeation 
rates which is consistent with trends reported in the literature. On average C02 permeabilities were &re times 
greater than oxygen permeabilities and ten times greater than nitrogen permeabiities. Carbon &ogde permeddon 
rates are least affected by increased temperature. The Nitrogen and Oxygen permeabilities for materids 2 m d  
3 are affected the most by increased temperature. At 35 "C these specimens have over a two fold increase hi 
permeation rates. The remaining materials have nitrogen permeation rates only about 50% greater at 35 "C, 
while the oxygen permeation rates for the corresponding materials have a somewhat smaller bcrease, This can 
be explained by the fact that the Henry's law solubility constant of gases in liquids decreases with inazreasing 
temperature while the diffusion constant (D) increases with increasing temperature. Consequentlgi, those 
materials affected least by temperature increases have lower temperature coefficients for a speciiic gas 
permeation rate. 
TABLE 
Material 
Polyamide 
(Kevlar) 
Silicone 
Polysulfone 
Polyester 7d 
(SRM 1470) 
7a 
Polyimidee 
(ODPA/p-PDA) 
2% offset 
: Oxygen Permeability Coefficients for Selected Polymer Materials 
as Measured by Mass Spectrometry 
a.The units are (cm2/s)(mL ~ ~ / c m ~ r n m ~ ~ )  
b. Correction to STP assumed pressure differences from one atmosphere or 760 mm Hg were negligble. The 
temperature was corrected using To = 273 K and Tmeasured(K) by multiplying Dk by the factor 
T~ITmepsured(~ 4 The units are arrer units mL 02(STP)/cms-cm Hg 
c.From GE product literature after converting from room temperature of 25 "C to STP. 
d.Rogers, C.E., In Polvmer Permeability, Comyn, J., Ed.; Elsevier Applied Science Publishers, London, 1985: 
Chapter 2. 
e.ODPA/p-PDA are acronyms for oxydiphthalic anhydride/ p-phenylenediamine. 
TABLE 111: Gas Permeability Coefficientsa for Contact Lens 
Specimens at 22 "C 
TABLE IV: Gas Permeability Coefficientsa for Contact Lens 
Specimens at 35 "C 
O m e n  I Nitrogen I Carbon Dioxide 
a. The units are (cm2/s)x(m~ ~ ~ / c m ~ m m ~ ~ )  x 10-l1 
CONCLUSIONS 
We believe that the method reported herein is the most accurate method for determining true or 
htrhsic gaseous permeabilities of any polymeric system. The "Dynamic Delta" method offers significant 
advmtages over standard mass spectrometric techniques including, but not limited to, speed of measurement. 
By the desip of the test cell that we have employed the total pressure gradient is eliminated. The technique 
permits the use of a very small sample size, 0.5 cm2, is selective for specific gases or isotopes, and is sensitivity 
limited only by the impurity of the carrier gas and the sealing ability of the gasket material. We have been able 
to demonstsa,te a leak-free system by using a metal blank. It was determined that flat Teflon gaskets on both 
sides of the flat membrane specimens sealed better than the Buna-N or Viton materials. The Bma-N O-ring, 
however, has the better sealing ability for the contact lens samples. 
The ASTM methods have traditionally been done with large sample sizes in order to p r s ~ d e  test. areas, 
of 50 - 100 cm2. Such large sizes meant that the sample had to placed upon a support. As a result the gas of 
interest had to permeate through both the sample and the support. In the "Dynamic Delta" methd desmihd 
in this paper the sample size is less than one square centimeter eliminating the need for the supprt.  The 
permeability of the material can be measured directly. The new method should fulfill requirements amd fmd 
applicability for industrial materials such as food packaging, contact lenses and other commercid materids where 
gas permeability or permselectivity properties are important. 
REFERENCES 
Clark, L. C., in Polaroma~hic Oxygen Sensors, ed. Fatt, I, CRC Press, Cleveland, Ohio 1976. 
Hori, Y, Shimada, S., and Kashiwabara, N., ESR Studies on Oxidation Processes in Imdiated 
Po eth lene: 1. Diffitsion of Oxygen into Amorphous Parts at Low Temperatures, 
15?(1&7). 
Val. 18, 151- 
Shaw, G., Quenching by Oxygen Phosphorescence Emission of A r m a ~ c  Molecukes in 
Polymethylmethacrylate, Trans. 01. 63, 2181-2189, (1%7). 
Petrak, K., Permeability of O en Throu h Polymers. I. A Novel Spectrophotochemicd MetJz~bd, J.f 
Appl. Polym. Sci., Vol. 23, 28-2371, (19%9). 
Rooney, M. L., and Holland, R. V., Measuring Oxygen Permeability of Polymer Films by a New Singlet 
Oxygen Technique, Angew. Makromol. Chem., Vol. 88, 209-221, (1980). 
Annual Book of ASTM Standards. 15.09, ASTM: Philadelphia, 1992. 
Hughes, D. B. and Nowlin, T. D., Trace Gas Analysis, resented at the 19th Annual Conference on Mass 
Spectrometry and Allied Topics, Atlanta, Georga, 1971. 
Standard Re erence Materials: SRM 1470; Pol ester Film for Oxygen Gas Transmission Memuresnents* # NBS Spec. ubl., (U.S.) No. 260-58 June, 19 7 9. 
Stern,. S. A., and Trohalaki, S., in Barrier Polymers and Structures, Koros, W. J., ed., h e r i c a n  
Chemical Society Symposium Series No. 423, Washington, D. C., Chapter 2, (1990). 
Weinkauf, D. H., and Paul, D. R., in Barrier Polymers and Structures, Koros, W. J., ed., Annericm 
Chemical Society Symposium Series No. 423, Washington, D. C., Chapter 3, (1990). 
General Electric Corporation Product Literature, Permselective Membranes, Medial Development 
Operation, Chemical and Medical Division, Membrane Products, Schenectady, N. Y. 
Ro ers, C. E., in Polymer Permeability, Comyn, J. ed., Elsevier Applied Science, London, chapter 2, 
(1985). 
MICROMETER 
VALVE 
ION SOURCE 
Figure 1.  Schematic diagram of mass spectrometer analysis system. 
Figure 2. Typical "Dynamic Delta" for oxygen permealion system. 
458 
SPECIMEN 1IOI.DER 
SCREW CRP (Brass)  
30 Tlireads / Inctt 
0 - r l n q  S e a l s  
Cor~tact  I.cns Spcciman 
and 0 - r l t ~ q  Sral 
Figure 3. Mass spectrometer sample holder accommodating flat Specimens and conbch lens 
specimens. 
Exhaust 
Figure 4. Gas flow schematic for mass spectmmetric permeation measurement. 


Instrumentation for Measurement of Gas Permeability of Polymeric Membranes

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