CH3 groups are the one that stick out on the surface of PDMS. Oxygen plasma
will replace CH3 with OH. You want to completely replace CH3 with OH, but
overdone it will result in the formation of a glass layer on top of the PDMS
surface because of the presence of OH and Si. This glass layer on the surface of
PDMS will break and prevents the bond. You should try your minimum power, the
power that allows you to barely see the plasma and do it for about 15 seconds.

  Good luck,

  H