PERMEABILITY AND DIFFUSIONAL STUDIES ON SILICONE POLYMER NETWORKS WITH CONTROLLED DANGLING CHAINS

The permeability to oxygen of silicone polymer networks with controlle d high-molecular-weight side chains was measured using electrochemical techniques. Networks with controlled amounts of pendent chains were p repared by hydrosilation reactions involving the addition of hydrosila nes from crosslinking molecules to the vinyl end-groups of mono- and d ifunctional prepolymer molecules. Values of the permeability coefficie nt P were determined by measuring the steady electric current in an ex perimental set-up in which several layers of moistened paper were plac ed between the network and the electrodes. The curve representing the dependence of the permeability coefficient on the molecular weight of the dangling chains, M, shows that, for values of M < 130 000, P is ne arly a linear function of M; however, for M > 130 000, the permeabilit y coefficient does not show a noticeable dependence on the molecular w eight of the dangling chains. The diffusion coefficient increases with the molecular weight of the side chains, reaching a nearly asymptotic value for M approximate to 170 000.