Poly(tetrafluoroethylene) (PTFE) is a hydrophobic material due to its fully fluorinated backbone (-CF2-CF2-), with properties interesting for practical applications such as self-cleaning surfaces. It has been shown that super-hydrophobic surfaces can be obtained by the treatment of PTFE in a helium-oxygen plasma at atmospheric pressure [1]. Contrary to oxygen-containing plasmas, the modification of PTFE by a pure helium plasma at atmospheric pressure usually leads to a strong decrease in water contact angle, in addition to an oxygen incorporation [2].

The treatment of PTFE by the atmospheric post-discharge of an RF plasma torch supplied in He and He-O2 highlighted two totally different behaviors [3]. For this reason, the effect of the gas composition (ratio He/O2) on the polymer modifications has been investigated. The treated surfaces have been characterized by WCA, XPS and AFM. Moreover, the species responsible for the modifications have been identified by optical spectroscopies (OES/OAS).

In pure helium plasmas, no significant change of the surfaces regarding chemical composition (XPS), wettability (WCA) and morphology (AFM) has been observed and this, despite an important mass loss measured. According to these observations, we presume a layer-by-layer physical etching without any preferential orientation, where the highly energetic helium metastables should be the main species responsible for the scission of -(CF2)n- chains. The XPS analysis of an aluminum foil (known to be an efficient fluorine trap) placed close to the PTFE tends to confirm this assumption as fluorine atom and –CF2 fragments have been detected.

In He-O2 plasmas, the helium metastables species being consumed by oxygen, fewer of them are then available to etch the PTFE, reducing the mass losses and fluorine detection on aluminum for higher O2flow rates. However, as it was previously shown [2], WCA and AFM measurements indicate an increase in hydrophobicity and roughness of the PTFE surface. The alveolar structures observed by AFM are then assumed to come from the anisotropic etching where the atomic oxygen etches mainly the amorphous phase.

[1] T. Dufour, J. Hubert, P. Viville, C.Y. Duluard, S. Desbief, R. Lazzaroni, F. Reniers, PTFE surface etching in the post-discharge of a scanning RF plasma torch: evidence of ejected fluorinated species, Plasma Process. Polym.. 2012, DOI. 10.1002 /ppap.201100209.

[2] N. Zettsu, H. Itoh, K. Yamamura,Thin Solid Films, 2008, 516,6683.

[3] J. Hubert, T. Dufour, N. Vandencasteele, S. Desbief, R. Lazzaroni, F. Reniers, Etching processes operating on a PTFE surface exposed to He and He-O2 atmospheric post-discharges. (Accepted : Langmuir).