Permeability, solubility, and diffusion coefficients have been determined for halothane (CF3CHClBr) and methoxyflurane (CHCl2CF2OCH3) in silicone rubber at temperatures from 17 to 60°C and at relative pressures from 0.05 to 0.96. The solubility of both penetrants in silicone rubber is a strong function of penetrant concentration (or relative pressure), and can be represented satisfactorily by the Flory‐Huggins relation with single values of the interaction parameter χ. The solubility coefficients decrease with increasing temperature at constant pressure. Mutual diffusion coefficients exhibit maxima when plotted against penetrant concentration; these maxima are attributed to the mass flow of polymer together with dissolved penetrant. Intrinsic diffusion coefficients increase linearly with increasing concentration. The energies of activation for diffusion are low, probably because of the ease of segmental motion about the SiO linkage. The diffusivity data are examined in terms of Fujita's “free volume” model and of transition‐state theory. Permeability coefficients for the two penetrants are large, of the order of 10−6–10−5 cm3(STP)‐cm/(sec‐cm2‐cm Hg), and increase markedly with increasing concentration or decreasing temperature. This behavior is regarded as a consequence of the low energies of activation for diffusion.

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SynopsisPermeability, solubility, and diffusion coefficients have been determined for cyclopropane (c-C,H,) in silicone rubber at temperatures between -8 and 70°C at relative pressures from 0.04 to 0.30. The permeability coefficients, p, are of the order of cm3 (STP) . cm/(s . cm2 . cmHg). p increases slightly with increasing penetrant pressure and decreases with increasing temperature, the energy of activation for permeation being -1.27 kcal/gmol at zero pressure. The solubility of cyclopropane in silicone rubber can be represented over the experimental concentration range by the Flory-Huggins equation. The solubility decreases with increasing temperature and the partial molar heat of solution is -4.95 kcal/gmol. The solubility coefficient in the Henry's law limit, S(O), for cyclopropane and many other gases and vapors can be correlated with (T,/T)*, where T and T, are the experimental and critical temperatures, respectively. The mutual diffusion coefficients, D, increase with increasing concentration and temperature, the energy of activation for diffusion being 3.68 kcal/gmol. The pressure dependence of p is described satisfactorily by a free-volume model proposed by Fujita and extended by Stern, Frisch, and coworkers. The permeability, diffusion, and solubility behavior of cyclopropane in silicone rubber is similar to that of propane (C,H,).

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