Degas PDMS in two minutes

Christopher LaFratta describes how to degas PDMS in two minutes.

Christopher N. LaFratta
Department of Chemistry, Tufts University, Medford MA, USA

Why is this useful?


Mixing the base and curing agent of PDMS inevitably incorporates air bubbles into the prepolymer.  These bubbles are usually removed after casting by degassing the sample under vacuum.  Vacuum degassing works well but requires about 30 minutes or more depending on the vacuum and amount of gas stirred in.  The technique described here uses a typical laboratory centrifuge to degas PDMS in two minutes.  The centrifuged PDMS will yield a bubble-free solid silicone rubber when cast on a smooth surface.

What do I need?


  • SYLGARD 184 silicone elastomer base & curing agent (Dow Corning)
  • 2 Centrifuge tubes (15 mL)
  • Clay Adams II compact centrifuge (3200 rpm or 1315 × g relative centrifugal force)

How do I do it?


  1. Weigh the PDMS base and curing agent (10:1) in a disposable centrifuge tube.
  2. Mix base and curing agent together with a wooden stick.
  3. Balance centrifuge with 2nd tube containing comparable amount of PDMS or water.  Place in centrifuge for 2 min.
  4. Cast centrifuged PDMS onto patterned wafer.
  5. Oven cure at 75°C for 1 h.

What else should I know?


If the PDMS is cast over high aspect ratio features, such as tall SU-8 photoresist lines, air bubbles may get folded in as the liquid PDMS flows over crevices.  These small isolated bubbles can usually be degassed under vacuum in about one minute.

A) Mixed PDMS in a centrifuge tube, note the air bubbles that have been mixed in. B) PDMS being degassed by centrifugation. C) Degassed PDMS after 2 minutes of centrifuging without any air bubbles.

Casting PDMS on a patterned silicon wafer

Cured PDMS cut from silicon wafer mold without air bubbles.

References


[1] C. N. LaFratta, T. Baldacchini, R. A. Farrer, J.T. Fourkas, M. C. Teich, B. E. A. Saleh, M. J. Naughton J. Phys. Chem. B, 2004, 108, 11256-11258.

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5 Responses to “Degas PDMS in two minutes”

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  3. Axel Tojo says:

    Great method, I tried this for a 5:1 PDMS mixture, spinning at 3200 rpm for two minutes (as suggested) and it worked perfectly. At least visibly there were no remaining bubbles at all.
    You might need to make a slightly larger volume than usual as the you will lose some pdms with every transfer and the centrifuge tubes add a few steps depending on your application.
    Thanks for sharing this method.

  4. Mike Shattock says:

    I have an even simpler method –

    1. Mix the Sylgard 184 elastomer as per manufacturers instructions (i.e. 10:1).
    2. Pour elastomer mix into required shape.
    3. Panic slightly because of all the tiny bubbles.
    4. Walk around your institute for 15 minutes looking for either a centrifuge or a vacuum/freeze drier unit.
    5. Give up looking.
    6. Return to pre-prepared elastomer.
    7. All the bubbles will have gone.

    Simple.

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