If you write the following words into Google: polyethylene plasma treatment wetting

you will find a lot of papers, including introductory ones.

I am not familiar with 'oxidative' plasma but as you increase time do you know if you are etching away the polymer surface resulting in greater surface roughness? This could be the cause for the increase in contact angle. But this is just an initial hypothesis.

I dont think that roughness would induce the increase of WCA, because generally the increase of the surface roughness of hydrophilic surface decrease the WCA. Reverse will be for hydrophobic surface.

I think in your case the effect could be related to the particular set-up, e.g. you sputter the PE and change the gas composition of the chamber and instead of the plasma treatment you are depositing the hydrocarbon film. If you working at low pressure is can be possible. Specify what is the volume of reactor and fluxes of oxygen, pressure and so on.  Without this information it is difficult to say something more.

I recomend to check XPS complimentary to FTIR, cause WCA reflects the outer surafce properties, while IR probes bulk of material.

I recomend a AFM analysis of your surface in different times of exposition to plasma. I believe that a change in roughness may be the responsable for such behavior. Actually this is quite known. As additional characterization I also recomend XPS as previously mentioned. I attached a paper that might help you to understand this process.

Regards

Article Control of Nanotexture and Wetting Properties of Polydimethy...

Dear Fabio,

The roughness is enhancing the property : hydrophilic or hydrophobic, i.e. the increase of the roughness on hydrophilic surafce will decrease WCA and vice versa for hydrophobic - roughness goes up and WCA increase too. This is normal, this is well known. But I never heard that the hydrophilic surface exhibited an increase of the WCA with increase of roughness. If you have such example please send here a paper or link.

Dear Manakhov,

I think this subject is really interesting and intriguing, with a lot of questions to be answered. I have read some papers where researchers pointed out that transparency and hydrophobicity can be obtained with a roughness below ~100 nm as well as by lowering the surface energy. I attached some of these papers. Surfaces with higher roughness are expected to lose these characteristic, which could be what happened with Kikani's material after a long exposition to plasma, that is why I think AFM analysis performed on samples treated on different times could give some valuable information.

Regards

Article Superhydrophobic surface directly created by electrospinning...

Article Control of Nanotexture and Wetting Properties of Polydimethy...

Article ChemInform Abstract: Preparation of Transparent Superhydroph...

Article Hydrophobic and oleophobic anti-reflective polyacrylate coating

I am attaching my WCA plot here to make my question more clear. Increase in WCA is just after plasma treatment. I am not talking about aging. If I give 5 sec treatment it becomes hydrophillic, 10 sec more hydrophilic, 15 sec little rise in WCA still IR confirms presence of functional groups, 20 sec rise is as it is, 25 sec again starts reducing WCA. Please see the attached file.

FE-SEM analysis can help in this case ?

We have contact mode stylus profileometer which also measures roughness. I can get long 3D scans also in this. I can measure Ra and Rq by SPIP software. Will this measurement valid or I have to go for AFM only in tapping mode ?

I am giving air plasma treatment to Polyethylene film by atmospheric pressure plasma

I think ion bombardment plasma occurred when you increased treatment time. Therefore, it should be considered that the balance between functionalization and ion bombardment is important in determining the optimum parameters of the plasma treatment period.

Please see this reference: 

http://www.sciencedirect.com/science/article/pii/S0008622315002110

Article Maskless functionalization of a carbon nanotube dot array bi...

This increase/decrease in wettability is due to the preceeding surface etching and crosslinking at different treatment times as well as by changes in chemical composition of the surface layer. Enclosed please find some papers from our research done earlier.

http://link.springer.com/article/10.1007/s00396-003-0871-8/fulltext.html

http://www.sciencedirect.com/science/article/pii/S0927775703002425

I think that the surface roughness is not the main reason for what you see. According to my experience of PDMS layer, after long time plasma treating ,  the PDMS layer can't be bonded to silicon wafer. The wetting behavior is based on the silanol density on the polymer surface. And as the time increases,  H element density decrease. Because only oxygen is supplied to the chamber. Many papers show that,   the atom ratio of polymer surface between C and H increases with plasma treating time.

Basically, the contact angle is a function of hydrophobic/hydrophilic ratio. Your surface oxidation is likely to decrease contact angle. This ratio is varying depending on application. During continuously applying plasma at certain stage the polar groups might have strong interaction with substrate, allowing the non polar atoms/moiaties to be on surface making the contact angle rise.

Thank you all ... I will carry out AFM, SEM  and Surface Energy analysis to understand it more deeply.

 Hi, in our group, we've observed the same phenomena and just submitted the paper where we discuss it. Hopefully it will be accepted soon so I'll send you the link. Maybe thechniques used in the study below can be helpful "Oxygen plasma functionalization of parylene C coating for implants surface: Nanotopography and active sites for drug anchoring"

Article Oxygen plasma functionalization of parylene C coating for im...

Thank you Monika mam, Fabio sir, Tomy sir, Lubomir sir for sending links, it is very useful. unfortunately we do not have XPS inhouse, so that analysis I am not able to do. Can do FTIR, HR-SEM, AFM, Surface Energy etc. Will be back if still have doubts.

Just an idea: Plasma also causes UV irradiation , which is known to alter the material properties (especially of polymers) at high dosage. Maybe your PDMS changes its morphology during long treatment times...

Did you measure all of your samples immediately after plasma treatment and at same storage times, resp.? PDMS has a very low glass transition (-190 °C) leading to very fast hydrophobic recovery. This possibly can't be detected by FT-IR due to its "high" information depth when compared to XPS. Surface energy measurements are more concentrated on measuring the outermost surface. This might explain the discrepancy between FT-IR results and contact angle determination.

Yes we are taking contact angle measurements just after plasma treatment and our substrate is Polyethylene not PDMA.

I've read somewhere in the answers PDMS and thought you investigated it. Sorry for this. In the meantime I found in our old investigations on PP an influence of treatment time during exposition in an oxygen plasma. After prelongation of exposition time in the plasma (30 -> 60 sec.) we also detected an increased contact angle. The corresponding XPS-data indicate a similar trend in oxygen content of the surface which was higher at lower exposition times. This is possibly due to ablation of oxygen-bearing groups with extended treatment time and may be an explanation for the increased contact angle. Possibly this is of help for you.

thank you Helga Thomas mam. My FTIR results shows that with plasma treatment time oxygen content increases. So it may be possible that polymer chains containing oxygen content might have turned towards bulk and that is why contact angle shows increase and FTIR shows presence of functional groups. But still trying to confirm that by some indirect method. Measurement of wetting time vs. plasma exposure time can help ?

During plasma treatment oxidised water extractable materials are formed (amount depends on the plasma exposition time) which are removed from the surface during measuring contact angle in water and thus, influence the results. Possibly this is a reason for the phenomenon you observed. I would suggest to wash your plasma treated materials with water (also the non-treated reference) and measure contact angle as well as chemical composition (FT-IR) of the surface before and after extraction. For this you can devide your sample into two pieces, one for direct measurement and one for washing. You can also dry the extracted matters and analyse them via FTIR. You will detect a lot of carboyxylic groups (extract has acidic pH).

What is your initial contact angle (CA) on polymer surface and minimum and maximum after plasma treatment? Oxygen plasma treatment reduces/remove the C-H bonds (hydrophobic) and incorporate (C-O) bonds (hydrophilic), therefore, reduction of CA is obvious. Now, increasing plasma process time might increase the roughness of the surface (without changing the surface composition). Hence, Wenzel model can explain the further increase of CA with the increase of oxygen plasma process time.

Well, can anyone please explain, why after plasma processing hydrophobic aging take place after 4-5 days later? Though after plasma processing it creates C-O bond And so far oxide bond is hydrophilic.

Hi Mohamed, I think you wanted to ask why hydrophilic and hydrophobic recovery is happening on plasma treated surfaces. This is due to the thermodynamic factors that facilitate the recovery of non-treated surface. Hydrophilic/phobic recovery depends on the treated plasma, material and the sample stored conditions. Hope the following 2 articles will give you some idea.

http://www.tandfonline.com/doi/abs/10.1080/00218469508014379

http://www.sciencedirect.com/science/article/pii/S0257897207003374

One hypothesis is the grafting/etchcing balance. At low treatment times, WCA decrease due to polar functions grafting, without modifying surface roughness. Then, o2 starts to etch the polymer (or remove low molecular weight fragments), increasing roughness leading to WCA decrease . For very long times of treatment, the hydrophilicity is recovered and should move towards a plateau (as the O/C ratio).

I would recommend some AFM measurements (Rms vs time of treatment) and XPS (FTIR  tell you more about the global O/C content, not on the different oxydation states: C-O, C=O, O-C=O...).