I am giving a plasma treatment to polyethylene (PE) surface. Wetting time (time taken to completely wet the PE surface) on plasma treated polymer surface changes as a function of one of my operating parameter. Wetting time increases after plasma treatment. What does this indicate ?
Dear Purvi, I would suggest you to use contact angle goniometry to characterize your surface (advancing and receding angles), as well as XPS spectroscopy. The literature of plasma treated PE is very rich, you can find enormous number of papers. You have to tae into account, however, that the polymer surface is dynamic, i.e. its properties are changing upon storage. It gradually becomes more hydrophobic - the polar groups tur inward. If you put it in water for long time, it becomes again hydrophilic.
PE is a hydrophobic polymer in its properties so that water or everything else of polar liquid (hydrophilic) needs to very long time to wet the polymer, even it is impossible under normal condition. In contrast, a hydrophobic liquid will wet the polymer aforementioned much easier due to it has similar properties. Here, the principle of "like-dissolves-like" concept can be adopted to explain this phenomenon. Accordingly, in your case, molecular structure of PE might have changed became little bit polar (hydrophilic) due to the plasma irradiation so that a hydrophobic liquid needs more long time to wetting it compared to before the plasma irradiation.
What are you trying to wet onto the PE after plasma? The plasma should make the surface hydrophillic
What Kind of gas are you using for Plasma Treatment, O2? If so, you are introducing hydroxyl Groups chemically bonded on your PE surface which are hydrophlic and thus leading to an increase in wettability which can be easily followed by contact angle measurements and XPS Analysis. Additionally, a Change in wettalbility imply in a Change of your surface area, by development of a rougher surface which can be evaluated by AFM in tapping mode. Check changes in Ra and Rq before and after Plasma Treatment. For a Long term hydrophylic surface I advise the the use a silane based coating on Plasma treated PE.
Best regards!
If you're doing air plasma, then I'd say you're adding hydroxyl groups to the surface. This will make the surface more hydrophilic, so the wetting behaviour you're seeing is expected. I agree with the other comments, that you should be doing contact angle and XPS measurements at each treatment condition to examine the correlation between wetting and surface chemistry. If you do XPS, pay particular attention to the C-O and C=O groups in the C 1s high resolution XPS spectra as you should see these peaks increasing with increasing plasma treatments times, which should be accompanied by decreasing contact angles i.e. increases hydrophilicity.
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