The sorption and diffusion of water in silicone rubbers: Part I. Unfilled rubbers

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Abstract
The sorption, permeation and diffusion of water in polydimethyl-siloxane (DMS), polydimethylmethylphenylsiloxane (PMS), and poly-3,3,3-trifluoropropylmethylsiloxane (FMS), and of methanol in FMS have been measured at a number of temperatures in the range 20°-50°C. For water, satisfactory agreement is obtained between steady-and transient-state results. At high relative vapor pressures the diffusion coefficient (D) and the activation energy for diffusion (ED) for each system are found to decrease and increase, respectively, with increasing sorbed penetrant concentration (C). This behavior is interpreted in terms of sorbed molecules undergoing rapid association such that an increasing fraction becomes relatively immobile. At low relative vapor pressures, D and ED are constant and equilibrium sorption isotherms are linear. Possible reasons for this behavior are discussed. Isotherms and D vs. C curves have been compared with those expected on the basis of a model which treats the polymer as an inert medium and considers association of penetrant to take place by a polycondensation process involving hydrogen bond formation. The model predicts the qualitative features of the results although quantitative agreement is not found.

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