Recovery and Restructuring

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. 

Studies of this restructuring process have been carried out on PDMS with surfaces made hydrophilic by UV/ozone treatment, oxygen plasma, or corona discharge. In these cases, changes in surface composition were determined using chemical force microscopy,Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, or atomic force microscopy.  

Similar results have been obtained on PDMS elastomers. Examples are tetraethoxysilane-cured hydroxyl-terminated PDMS, methylhydrogensi-loxane-cured vinyl-terminated PDMS, and methylhydrogensiloxane-cured vinyl-terminated poly(dimethylsiloxane-co-diphenylsiloxane) chemical copolymer. Restructuring has also been observed in un-cross-linked copolymers, for example PDMS-co-polyst)u ene, using sum frequency generation vibrational spectroscopy. 

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