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IGOR SOBOLEV, J. A. MEYER, VIVIAN STANNETT, and MICHAEL SZWARC Department of Chemistry, State University of New York College of Forestry, Syracuse 10, N. Y.
Permeation, Diffusion, and Solubility of Methyl Bromide and Isobutene in Polyethylene
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permeability of polymer films to gases and vapors has been studied for a great many systems ( 2 ) . Most of these studies, however, have been concerned with the permeation of permanent gases or of water vapor. The permeability of polymer films to organic vapors on the other hand has received comparatively little attention. The permeability constants, P, of many systems are known over a range of temperatures, and all the observations indicate that P decreases as the temperature is lowered. On the whole, the temperature dependence of the permeability constant seems to be given by the Arrhenius relationship with a positive activation energy (2, 20). However, in some temperature-pressure regions the permeability constant sharply increases as the temperature decreases. This phenomenon was studied in some detail in these laboratories. This investigation is concerned with the permeability of polyethylene films to isobutene and to methyl bromide vapors. However, the phenomena observed in the present study may also occur in other systems, and therefore these conclusions are probably very general. T h e permeability constant, P, measures the flux of a gas or a vapor through a film under unit gradient of pressure, and it can be shown that P is a product of the diffusion constant, D, and the solubility coefficient, S-namely, P = DS. In view of the composite nature of the permeability constant the authors describe the observed phenomena in terms of changes in the diffusion constants and the solubility coefficients, as well as in terms of variations in permeability constants.
Experimental Polyethylene films used in this investigation were provided by the Du Pont Co. and were used in two thicknesses, 0.038 and 0.5 mm. Methyl bromide and isobutene were obtained from the Matheson, Co., their purities being 99.4 and 99.0%, respectively. The apparatus used for measuring the
Figure 1 .
permeability constants was of the high vacuum type, originally developed by Barrer (3, 4). The details of its construction were described in a previous article (7). The diffusion constants were calculated by the time lag technique, developed by Daynes (6) and by Barrer (7, 3). and described in further detail (7). The equilibrium sorption isotherms were measured by means of a
Permeability isobars for polyethylene and methyl bromide VOL. 49,
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Results The temperature dependence of the permeability constants for meth>-l bromide is shown in Figure 1 foi three different pressures. The isobars corresponding to higher pressures depart tremendously from the normally observed linear relationship, the permeability constants first decreasing and then increasing as the temperature is proyressively lowered. A similar behavior \vas observed in the permeation of isobntene through polyethylene films. The minima of these isobars are shifted toward lower temperatures as the pressure is decreased, and a t sufficiently low pressure-e.g., 100 mm. of mercury-normal linear plots are obtained across the whole temperature range covered in these experiments. The results discussed can be presented in the form of isotherms, as shown in Figures 2 and 3 , which exhibit the strange crossing over of the individual isotherms. However. the mutual interrelationship of these isotherms is greatly
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Figure 2. Pressure dependence of permeability of polyethylene to methyl bromide
quartz helis microbalance of the type described by Prager and Long (77).
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Figure 3. Pressure dependence permeability of polyethylene to isobutylene
simplified if? instead of I J ~ ~ S S U I T . the relative vapor pressures, p Po. are used as coordinates. I n this expression, / I , , denotes the saturation vapor pressure corresponding to the relevant zemperature. T h e results are shown in Figures 4 and 5. and extrapolation of these graphs to zero pressure leads to the values of \\.hat the authors propose 10 call intrinsic permeability constants. The log plots of intrinsic perrneabiht!, conslants versus reciprocals of absolute temperature give excellrnt straight lines (Figure 6) corresponding to activation energies uf 7.5 and 11.5 kcal. for meth!-I bromide and isobutene, respcctivel!.. .-Isimilar rstrapolation has been carricd out by Huckins and Kammermeyer ( 9 ) for the molecular flow of vapors through porous glass. T o gain a better insight into these phenomena. the diffusion consiant and the solubility coefficients of methyl bromide in po1yeth)-lene were determined independently. The time laq technique, applied in these studies, yields the integral diffusion constant, D Ixvhich is related to the conventiunal diffrrential diffusion constant, D. by the equaiion
INDUSTRIAL AND ENGINEERING CHEMISTRY
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'I'hr inlc:gral diffusion c o n s l a n t s \ \ C . I Y . drlerniiried in cxperiiiicnts iiivcjiviny
