SORPTION OF LOW MOLAR-MASS SILICONES IN SILICONE ELASTOMERS

Elastomers based on polydimethylsiloxane (PDMS) are used as insulating material in outdoor electrical power applications. It is believed tha t migration of small molecule PDMS species plays an important role in the recovery of hydrophobicity of oxidized or polluted PDMS elastomer surfaces. This paper reports data on diffusivity and solubility of low molar mass PDMS liquids in PDMS rubbers (8000<<(M)over bar (c)><16,00 0 g/mol) obtained by sorption measurements. It was found that the diff usivity (D) of linear PDMS liquids was approximately independent of th e concentration of penetrant and that in the molar mass range 400<<(M) over bar (n)><18,000 g/mol it decreased with molar mass (<(M)over bar (n)>) of the diffusing liquid according to D proportional to<(M)over b ar (n)>(-0.8). Theory and previous data for other oligomers and elasto mers predict that D is proportional to M(-1). Linear PDMS liquids of l ower molar mass exhibited a stronger molar mass dependence. The diffus ivity of a given PDMS liquid increased with increasing elas tomer cros slink density. The activation energy of the diffusivity was constant a t 15.5+/-2 kJ/mol for linear PDMS liquids of <(M)over bar (n)> larger than 1000 g/mol(-1) with only a negligible influence of network densit y and filler content. The activation energy of the lowest molar mass p enetrant was considerably lower, 6 to 7 kJ/mol. The solubility increas ed markedly with decreasing molar mass of the penetrant and with decre asing elastomer crosslink density.