The pressure dependence of diffusion of vapors In polymer films arises not only from the concentration dependence of the diffusion coefficient, D, but also because D can vary with the mean pressure within the film. For sufficiently large organic molecule vapors for which a free volume theory of diffusion is applicable one expects that this latter pressure dependence can be obtained also from the pressure dependence of the free volume of the polymer-vapor system.
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References
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J. Crank and G.S. Park, "Diffusion in Polymers", Academic Press, New York, 1968.
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R.M. Barrer, "Diffusion in and through Solids", Cambridge University Press, London, 1941.
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Without this assumption, taken together with the condition of mechanical equilibrium, one would have to compute the local pressure at any point within the membrane from the appropriate equations of continuum mechanics and boundary conditions which apply to the film.
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C.E. Rogers, "Physics and Chemistry of the Organic Solid State", ed. by D. Fox, M. M. Labes and A. Weissberger, Interscience Publishers, New York, 1965.
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H. Fujita, A. Kishimoto and K. Matsumoto, Trans. Faraday Soc.56, 424 (1960).
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