Siloxane surface restructuring

Possible mechanisms of dehydroxylation and the structural features left behind were simulated in some theoretical works. The mechanisms depend on the initial position and attachment of the pairs of hydroxyls. Some of them are obvious, like that upon heating the two vicinal hydroxyls, a siloxane group =Si-0-Si= is left behind. However, the geminal pairs of OH, which would seem to yield >Si=0 groups, were found to persist after drying the specimens at 600 °C [61] because silicon does not readily form > Si=0. Final dehydroxylation probably involves deep local restructuring of the surfaces and also produces the siloxane links. 

In some cases attempts to make PDMS surfaces more hydrophilic were only temporarily successful. Recovery or restructuring can occur, with methyl groups from the siloxane units migrating to the surface to lower the free energy of the system. Restructuring is facilitated by the high flexibility of the polysiloxane backbone. 

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