From the Godin Lab, University of Ottawa, Canada:
To assure a strong bond between PDMS layers, oxygen plasma treatment is necessary. This can be done using the Godin:Protocols/Glow Research Auto Glow. This surface treatment is known to have a very short duration, that is to say that both surfaces must quickly be brought into contact after exposure. For this reason, it is best to place the wafer and PDMS in the plasma bonder before each bonding step. Though ethanol has been known to extend the effects, it caused a partial collapse in the valve area of the main channels, and as such is not recommended. A higher power setting does not create a stronger bond. A 30s 30W exposure works well.
Please refer to this link: