Hi all,
I am trying to fabricate PDMS based microfluidic devices, after plasma treatment the PDMS does not bond to glass for some reason. I have tried to drop a water droplet to see if it changes the contact angle and I have seen increased hydrophilicity on PDMS. I cannot control the plasma power nor the pressure since the device I am using has no pressure gauge or power control.
Briefly, after 2 min of vacuum I apply 2 min of plasma ( I can see the plasma forming). I have tried to silanize the glass and PDMS, wash the glass with ethanol, heat treatment after plasma and PDMS-PDMS bonding. None of which worked out. Can anyone comment on what is the issue here?
Thanks in advance.
Check for oxygen concentration as you proceed with plasma treatment. Humid conditions are not recommended in the above fabrication method.
When you can see the (air?-)plasma developing, your plasma should be okay I guess. Have you tried shorter or longer activation times of the PDMS? Make sure to do the annealing step after the bonding to create a permanent bond. In our lab we usually do 40°C for 4h or 80°C for 1h in a oven for the annealing.
Short addition to my answer above: Try following Ron's bonding procedure described here: https://www.researchgate.net/post/How_can_get_a_permanent_bonding_between_PDMS_SiO2_coated_glass
I have tried from 2 min to 15 min, after 2 min treatment I can clearly see the contact angle of water changes. However, there is no bonding happening in any treatment time even after annealing.
@Ibrahim : The availability of silanol groups also determines the bonding, apart from surface roughness and hydrophilicity.
Maybe even 2 minutes is too long? We use corona discharge to activate our surface (normally around 20 s for PDMS) and when we use the corona stick for too long, the bonding will fail. So maybe lowering the activation time could help.
Plasma bonding is a delicate balance of "activation" and "destruction" of the PDMS surface. You want to destroy either the C-H bonds or the S-C bonds such that they are replaced with C-OH or S-OH groups. But if you go too far with the plasma, you'll either create too many -OH groups to the point that they start interacting with each other or you start breaking Si-O bonds in the back-bone of the elastomer. In the former case, you are left with too few -OH groups to make a sufficient bond and the surface will be mostly closely packed islands of SiO2 with very few -OH groups available at the surface. In the latter, you "ash" the surface to the point that you can't get any covalent bonds to form that are still covalently bonded to the bulk.
As Jasper Giesler suggested, you should try significantly lower treatment times and possibly lower your power. My lab typically uses around 20W for 20-40 seconds.
One additional detail is in the timing of the contact after plasma treatment. You need to be relatively quick getting you sample out of the chamber and placing it on the surface to be bonded. Much more than 60 seconds after the plasma is turned off and your bond strength will begin to decrease rapidly. One way aound this is to use a drop or two of methanol on the bonding surface. This also allows you to move the PDMS around a bit to get the two surfaces aligned.
Best of luck!
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