The sorption and diffusion of water in silicone rubbers. Part II. Filled rubbers

Abstract

Equilibrium sorption, permeation, and sorption kinetic measurements have been made for water in polydimethylsiloxane rubbers containing various amounts of either sodium chloride or silica filler. In general the addition of filler leads to an increase in the uptake of water by the rubber. On the other hand, the permeability P which is independent of the relative humidity is not affected to the same extent suggesting that the water sorbed by the fillers does riot contribute significantly to the steady-state flux. For the salt-filled polymers the diffusion coefficient D is initially constant and then decreases with the concentration C of sorbed water. Compared with the unfilled rubbers the decrease in D can be orders of magnitude larger. The D vs. C curves for the silica-filled samples exhibit maxima. The results are interpreted in terms of a model in which a fraction of the water sorbed is immobilized by the filler or by the formation of pockets of salt solution.