Gareth,

Those Silanol groups difffuses through the cracks
generated in PDMS ,when overexposed.,so you will not
get good bonding. This is being discussed in lot of
papares .

The best one among them is..

"Studies on Surface Wettability of Poly(Dimethyl)
Siloxane (PDMS) and Glass Under Oxygen-Plasma
Treatment and Correlation With Bond Strength"
JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14,
NO. 3, JUNE 2005

Bala.