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 that 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)><16,000 g/mol) obtained by sorption measurements. It was found that the diffusivity (D) of linear PDMS liquids was approximately independent of the 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 bar (n)>(-0.8). Theory and previous data for other oligomers and elastomers predict that D is proportional to M(-1). Linear PDMS liquids of lower molar mass exhibited a stronger molar mass dependence. The diffusivity of a given PDMS liquid increased with increasing elas tomer crosslink density. The activation energy of the diffusivity was constant at 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 density and filler content. The activation energy of the lowest molar mass penetrant was considerably lower, 6 to 7 kJ/mol. The solubility increased markedly with decreasing molar mass of the penetrant and with decreasing elastomer crosslink density.