Defluorination of Polytetrafluoroethylene Surface By

Article Deﬂuorination of Polytetraﬂuoroethylene Surface by Hydrogen Plasma

Alenka Vesel 1,* , Dane Lojen 1,2, Rok Zaplotnik 1 , Gregor Primc 1 , Miran Mozetiˇc 1 , Jernej Ekar 1,2, Janez Kovaˇc 1, Marija Gorjanc 3 , Manja Kureˇciˇc 4 and Karin Stana-Kleinschek 5

1 Department of Surface Engineering, Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia; [email protected] (D.L.); [email protected] (R.Z.); [email protected] (G.P.); [email protected] (M.M.); [email protected] (J.E.); [email protected] (J.K.) 2 Jozef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia 3 Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerˇcevacesta 12, 1000 Ljubljana, Slovenia; [email protected] 4 Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia; [email protected] 5 Institute of Chemistry and Technology of Biobased Systems, Graz University of Technology, Rechbauerstraße 12, 8010 Graz, Austria; [email protected] * Correspondence: [email protected]

Received: 6 November 2020; Accepted: 27 November 2020; Published: 29 November 2020

Abstract: Defluorination of polytetrafluoroethylene (PTFE) surface film is a suitable technique for tailoring its surface properties. The influence of discharge parameters on the surface chemistry was investigated systematically using radio-frequency inductively coupled H2 plasma sustained in the E- and H-modes at various powers, pressures and treatment times. The surface finish was probed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The measurements of water contact angles (WCA) showed increased wettability of the pristine PTFE; however, they did not reveal remarkable modification in the surface chemistry of the samples treated at various discharge parameters. By contrast, the combination of XPS and ToF-SIMS, however, revealed important differences in the surface chemistry between the E- and H-modes. A well-expressed minimum in the fluorine to carbon ratio F/C as low as 0.2 was observed at the treatment time as short as 1 s when plasma was in the H-mode. More gradual surface chemistry was observed when plasma was in the E-mode, and the minimal achievable F/C ratio was about 0.6. The results were explained by the synergistic effects of hydrogen atoms and vacuum ultraviolet radiation.

Keywords: polytetrafluoroethylene; fluorine depletion; hydrogen plasma; VUV radiation; surface modification; hydrophilic

1. Introduction Fluorinated polymers are used in various applications [1]. They are renowned for their chemical inertness and thermal stability [2]. The chemical inertness does not allow for reasonable adhesion of any coating deposited by numerous techniques [3,4]. Whatever material is deposited, the surface energy of the coating is much larger than the energy of the substrate; therefore, thin films tend to form 3D particles spontaneously rather than a uniform film. Coatings from liquid solutions also do not adhere well on the surface, especially when the liquid is polar, i.e., water. In order to improve the adhesion of coatings, methods for increasing the surface energy of fluorinated polymers have been invented [5–10]. Fluorinated polymers are usually treated with aggressive chemicals which

Polymers 2020, 12, 2855; doi:10.3390/polym12122855 www.mdpi.com/journal/polymers Polymers 2020, 12, 2855 2 of 14 cause modification of the surface chemistry. Such techniques were invented in the 1960s, and some are still used nowadays. Common techniques include the application of sodium naphthalide [11], tetraalkylammonium radical anion salt, alkali metal vapours and amalgams [12], and electrochemical methods [13,14]. All these techniques represent an ecological hazard; therefore, researchers have been working on alternative techniques. A straightforward solution is irradiation of fluorinated polymers with beams of photons, electrons, or ions [15,16]. The insulating properties of fluorinated polymers make the techniques employing charged particles rather difficult, however. A medium that contains both electrons, positively charged ions, and photons is gaseous plasma. Gaseous plasma is a state of gas also consisting of neutral particles (thereafter: radicals), which are chemically extremely reactive. For example, diatomic molecules are partially dissociated upon plasma conditions, and the atoms typically interact with most of the polymers even at room temperature. The atoms may bind to the surface of a polymer, thus forming functional groups of different polarity than those presented in fluorinated polymers. For example, oxygen plasma of a high density of oxygen atoms will cause functionalization of most polymers with various oxygen-containing functional groups [17]. While such treatments with oxygen plasma perform well for most types of polymers, they fail in the case of polytetrafluoroethylene (PTFE) for one simple reason: the binding energy of fluorine to carbon atoms is much larger than those of oxygen; therefore, a simple substitution of fluorine atoms on the surface of PTFE with oxygen atoms is energetically unfavourable. Treatment of fluorinated polymers by oxygen plasma will cause gradual etching of the polymer material rather than surface activation, as shown recently by Primc et al. [18]. Because classical plasma techniques fail in the case of PTFE, researchers have invented a variety of alternative methods. As early as 1987, Clark et al. [14] used a high vacuum plasma reactor to treat PTFE samples with hydrogen plasma. A radiofrequency (RF) generator coupled in a capacity mode was used for plasma generation. The maximum power was 10 W, and the treatment times were up to approximately 250 s. Upon such conditions, the surface film of PTFE was depleted of fluorine, which was proven by X-ray photoelectron spectroscopy (XPS). The water contact angle (WCA) showed moderate hydrophilicity of the treated samples after prolonged treatment. The minimum WCA was 50◦. The authors explained the fluorine depletion by the interaction between hydrogen atoms and PTFE, causing the formation of HF molecules that were pumped away from the system. About a decade later, Badey et al. [19] performed similar experiments, except that they used a powerful microwave (MW) generator for sustaining a dense plasma in a narrow quartz tube. PTFE samples were placed downstream from the centre of the discharge. The best results were observed at large gas flows and moderate discharge powers. The F/C ratio dropped from an original 2.45 to about 0.8. The WCA of water dropped from 115◦ to approximately 85◦ and diiodomethane from 84◦ to approximately 50◦. The surface modifications were also monitored by secondary ion mass spectrometry (SIMS), and the authors found numerous CxHy peaks after accomplishing the remote plasma treatment. As by Clark et al. [14], Badey et al. [19] also explained the observed results by the interaction between the polymer surface and neutral hydrogen atoms, causing the formation of HF molecules. In the same year, Yamada et al. [20] also applied remote H2 plasma treatment for the modification of PTFE samples. The plasma source was inductively coupled RF discharge. As by Badey et al. [19], the authors used a quartz tube for the discharge chamber and placed samples in the afterglow region. The authors reported similar results as Badey at large discharge powers. They also performed XPS characterization and found similar functional groups as Badey. The same group also used remote hydrogen plasma treatment to study the adhesion of Cu film on PTFE samples [21]. They obtained the minimum WCA of approximately 75◦ at the treatment time of approximately 2 min. In another paper, Inagaki et al. [22], used pulsed plasma treatment. An capacitiveRF discharge in the power range between 75 and 100 W was applied at the pressure of 13 Pa. The results and WCAs were slightly larger than in the case of using continuous plasma treatment. The same applied to the F/C ratio. König et al. [23] used plasmas sustained in various gases for modification of a plasma-deposited fluorocarbon polymer with the structure close to PTFE. Plasma was sustained by a MW discharge in the Polymers 2020, 12, 2855 3 of 14 electron cyclotron resonance (ECR) mode. The working pressure was as low as 0.2 Pa. The maximum discharge power was 800 W. Samples were additionally biased using a capacitively coupled RF generator. The F/C ratio as deduced from the XPS survey spectra was 1.9 for the untreated material and dropped to 0.72 for plasma-treated samples. Simultaneously, the oxygen to carbon ratio O/C increased from 0.02 to 0.09. Such a surface finish enabled a slight decrease of WCA from the original 106◦ down to about 86◦. Tanaka et al. [24] investigated defluorination of PTFE by a combination of atmospheric pressure glow plasma treatment and a chemical transport method. Plasma was sustained in a mixture of He and H2. The lowest F/C ratio reported for such plasma treatment was 0.4. The addition of oxygen in the gas mixture caused a further drop of the F/C concentration and appearance of a few atomic % of oxygen. More recently, Hunke et al. [25] performed direct treatment of PTFE powders in a low-pressure hydrogen plasma. They used a MW discharge and obtained a moderate decrease in the F/C ratio. The authors adopted the explanation provided previously by Inagaki et al. [22]. The review of the early work can be summarized as follows: the defluorination of PTFE surface occurs upon treatment with hydrogen plasma or its flowing afterglow and is explained by the interaction of hydrogen atoms with pristine material causing the formation of a dangling bond on one C atom. The dangling bond is quickly occupied with another H atom, forming the CHF group. This group may decompose by desorption of the HF molecule, and the result is the formation of the CF=CF group. The abundance of H atoms causes the gradual transformation of the polymer surface. A typical treatment time needed for observing a rather low F/C ratio is of the order of a minute. In the present paper, we disclose experiments with hydrogen plasma performed in the same reactor at different conditions. Gaseous plasma was created in different discharge modes; therefore, some plasma parameters depended enormously on discharge conditions.

2. Materials and Methods PTFE foils were purchased from Goodfellow Ltd. (Huntingdon, UK). The foils with a thickness of 0.5 mm were cut to pieces of 10 10 mm2 and cleaned with ethanol, followed by drying at × ambient conditions. PTFE was treated in a glass discharge chamber using an electrodeless radio-frequency (RF) discharge. A schematic of the discharge chamber is shown in Figure1. The discharge chamber was a long glass tube with a diameter of 4 cm. It was pumped on one side, and on the other side, H2 gas was introduced through the flow controller. The vacuum system was sealed with rubber gaskets. It was pumped with a two-stage rotary pump of a nominal pumping speed of 80 m3/h. The experimental conditions, therefore, enabled achieving the ultimate pressure just below 1 Pa after pumping for a reasonable time. Optical emission spectroscopy (OES) was used to check any gaseous impurities in the discharge chamber. Spectral features of any trace gases were below the detection limit of the spectrometer; therefore, the system was hermetically tight, and the residual atmosphere contained water vapour only. Plasma was sustained within the coil, as shown in Figure1 and di ffusing plasma expanded far away from the coil. The coil was connected to the RF generator via a matching network. The matching network allowed for coupling optimization to run the plasma mostly in the H-mode, depending on the power. The generator operated at the standard frequency of 13.56 MHz and adjustable output power up to 1000 W. The samples were treated at various conditions, i.e., various powers from 100 to 1000 W, hydrogen pressures from 10 to 60 Pa, and treatment times from 0.5 to 12 s. The surface wettability of samples was measured using a Drop Shape Analyser DSA-100 (Krüss GmbH, Hamburg, Germany). A static contact angle was measured using a sessile drop method. The volume of a drop was 1 µL. MiliQ water was used for determination of the wettability. Surface modifications were probed by XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The XPS characterization was performed using an XPS instrument (model TFA XPS from Physical Electronics, Münich, Germany). The samples were irradiated with monochromatic Al Kα1,2 radiation with the photon energy of 1486.6 eV. Spectra were measured at an electron take-off angle Polymers 2020, 12, 2855 4 of 14

(TOA) of 45◦. Selected samples were also measured at various TOAs to manipulate the detection depth of XPS. Survey spectra were acquired at a pass-energy of 187 eV using an energy step of 0.4 eV. High-resolution carbon C1s spectra were measured at a pass-energy of 23.5 eV using an energy step of 0.1 eV. An additional electron gun was used for compensation of the surface charge. Spectra were calibrated by adjusting the C1s peak corresponding to CF2 groups to 292 eV. The measured spectra were analyzed using MultiPak v8.1c software (Ulvac-Phi Inc., Kanagawa, Japan, 2006) from Physical Electronics, which was supplied with the spectrometer. Linear background subtraction was used. Polymers 2020, 12, 2855 4 of 14

FigureFigure 1. 1. SchematicSchematic of of the the plasma plasma set set-up.-up.

TheToF-SIMS surface analyseswettability were of performedsamples was using measured a ToF-SIMS using 5 instrumenta Drop Shape (ION-TOF, Analyser Münster, DSA-100 Germany) (Krüss GmbH,equipped Hamburg, with a bismuthGermany). liquid A static metal contact ion gun angle with was a measured kinetic energy using of a 30sessile keV. drop The analysesmethod. The were 7 volumeperformed of a indrop an ultra-highwas 1 µL. vacuumMiliQ water of approximately was used for 10determination− Pa. The ToF-SIMS of the wettability. spectra were measured by scanningSurface a Bimodifications3+ cluster ion were beam probed over the by surface XPS and spot time of approximately-of-flight secondary 100 100 ionµ mmass2. An spectrometry electron gun × (ToFwas-SIMS used) to. The allow XPS for characterization charge compensation was performed on the sample using an surfaces XPS instrument during the (model analysis. TFA Positive XPS from and Physicalnegative Electronics, ion spectra Münich, were measured Germany). The samples were irradiated with monochromatic Al Kα1,2 radiation with the photon energy of 1486.6 eV. Spectra were measured at an electron take-off angle (TOA)3. Results of 45°. and Selected Discussion samples were also measured at various TOAs to manipulate the detection depth of XPS. Survey spectra were acquired at a pass-energy of 187 eV using an energy step of 0.4 3.1. Modification of the Surface Wettability eV. High-resolution carbon C1s spectra were measured at a pass-energy of 23.5 eV using an energy step ofSamples 0.1 eV. An were additional placed at electron different gun positions was used along for the compensation discharge tube of to the measure surface the charge. gradient Spectra in the weresurface calibrated wettability. by adjusting Some of them the C1s were peak placed corresponding inside the RF to coil, CF and2 groups many to were 292 placed eV. The away measured from the spectracoil, in were the direction analyzed of using the pump MultiPak duct. v8.1c The software water contact (Ulvac angle-Phi wasInc., measuredKanagawa, for Japan, all these 2006) samples, from Physicaland the Electronics, result is presented which was in supplied Figure2. with Plasma the treatmentspectrometer. time Linear in all background cases was 1 subtraction s, the hydrogen was used.pressure was 25 Pa, and the discharge power was 400 W. The WCA for the untreated sample was approximatelyToF-SIMS analyses110◦. One were can observe performed a gradual using increase a ToF in-SIMS the WCA 5 instrument with the increasing (ION-TOF, distance Münster, from Germany)the RF coil. equipped The samples with treated a bismuth within liquid the coil metal assume ion gun the WCA with a of kinetic approximately energy of 83 ◦ 30, which keV. isThe the analysesvalue already were performed reported by in previouslyan ultra-high cited vacuum authors. of approximately This value is typical 10−7 Pa. for The oxygen-free ToF-SIMS polymers spectra weresuch measured as polyolefins. by sca Suchnning a rathera Bi3+ cluster low WCA ion beam extends over a few the cmsurface away spot from of the approximately coil, which is 100 explained × 100 µmby2. the An simple electron fact gun that was a dense used plasma to allow in for the charge H-mode compensation was not limited on tothe the sample coil only, surfaces but also during stretched the analysis.outside Positive of the coil and as negative shown schematicallyion spectra were in Figuremeasured1. Away from the coil, the WCA increases to approximately 100◦ within a distance of several cm. Thereafter, the WCA increases rather linearly 3.with Results increasing and Discussion distance, and at large distances approaches the value typical for the untreated sample. The measured points scatter somehow, but the trend is obvious and is presented by straight lines. 3.1.The Modification results summarized of the Surface in FigureWettability2, therefore, suggest almost complete defluorination of the PTFE samplesSamples upon were treatment placed with at different a dense, positions glowing hydrogenalong the plasma,discharge and tube a more to measure gradual the activation gradient upon in thetreatment surface withwettability. a diffusing Some plasma, of them which were in placed our case inside is a result the RF of coil, a weak and capacitive many were coupling placed between away fromthe coilthe andcoil, thein the metallic direction pump of duct.the pump duct. The water contact angle was measured for all these samples, and the result is presented in Figure 2. Plasma treatment time in all cases was 1 s, the hydrogen pressure was 25 Pa, and the discharge power was 400 W. The WCA for the untreated sample was approximately 110°. One can observe a gradual increase in the WCA with the increasing distance from the RF coil. The samples treated within the coil assume the WCA of approximately 83°, which is the value already reported by previously cited authors. This value is typical for oxygen-free polymers such as polyolefins. Such a rather low WCA extends a few cm away from the coil, which is explained by the simple fact that a dense plasma in the H-mode was not limited to the coil only, but also stretched outside of the coil as shown schematically in Figure 1. Away from the coil, the WCA increases to approximately 100° within a distance of several cm. Thereafter, the WCA increases rather linearly with increasing distance, and at large distances approaches the value typical for the untreated sample. The measured points scatter somehow, but the trend is obvious and is presented by straight lines. The results summarized in Figure 2, therefore, suggest almost complete defluorination of the

Polymers 2020, 12, 2855 5 of 14

PTFE samples upon treatment with a dense, glowing hydrogen plasma, and a more gradual activation upon treatment with a diffusing plasma, which in our case is a result of a weak capacitive Polymers 2020, 12, 2855 5 of 14 coupling between the coil and the metallic pump duct.

FigureFigure 2.2. Variation ofof waterwater contact contact angle angle (WCA) (WCA on) on the the polytetraﬂuoroethylene polytetrafluoroethylene (PTFE) (PTFE surface) surface of the of samplesthe samples arranged arranged along along the dischargethe discharge tube. tube. The The treatment treatment time time was was 1 s, the1 s, hydrogenthe hydrogen pressure pressure was 25was Pa, 25 and Pa, theand discharge the discharge power power was 400was W. 400 W.

As already already reported reporte byd numerous by numerous authors, authors, the WCA the depends WCA depends on numerous on numeroustreatment parameters, treatment includingparameters, the including treatment time,the treatment the pressure, time, and the the pressure, power absorbed and the by power the gaseous absorbed plasma by [20 the– 22 gaseous,26,27]. Toplasma get additional [20–22,26,27 information]. To get additional about the evolutioninformation of theabout surface the evolution wettability, of wethe treatedsurface severalwettability, samples we insidetreated the several RF coil samples at various inside conditions. the RF coil Figure at various3 represents conditions. the WCA Figure versus 3 represents plasma the treatment WCA versus time. Weplasma adopted treatment the same time. parameters We adopted as the in same Figure parameters2, i.e., the pressureas in Figure of 252, i.e., Pa andthe pressure the power of of25 400Pa and W. Itthe seems power that of 400 the W. WCA It seems in Figure that 3the does WCA not in really Figure depend 3 does onnot treatmentreally depend time on at treatment these particular time at dischargethese particu conditions.lar discharge All values conditions. lay at approximatelyAll values lay at 90 ◦approximately, with the exception 90°, with of the the first exception measurement of the correspondingfirst measurement to thecorresponding lowest treatment to the lowest time of treatment 0.5 s where time the of standard0.5 s where deviation the standard is rather deviation large. Theis rather almost large. constant The almost WCA, constant as revealed WCA, from as Figurerevealed3, is from explained Figure by3, theis explained saturated by defluorination the saturated ofdefluorination the surface. of This the esurface.ffect will This be furthereffect will discussed be further later. discussed Figure 4later. is a Figure plot of 4 the is a WCA plot of versus the WCA the dischargeversus the power discharge for the power treatment for the time treatment of 1 s and time the of pressure 1 s and of the 25 Pa.pressure Again, of one 25 canPa. observeAgain, one a rather can constantobserve a WCA, rather except constant for WCA, the measurement except for the performed measurement at the lowestperformed power at ofthe 100 lowest W. The power WCA of also100 doesW. The not WCA depend also muchdoes not on depend the H2 pressure much on in the the H2 discharge pressure in tube, the asdischarge shown intube, Figure as shown5. Figures in Figure3–5, therefore5. Figures indicate 3–5, therefore that the indicate surface activationthat the surface is accomplished activation withinis accomplished the second with of plasmain the treatmentsecond of providingplasma treatment the discharge providing power the isdischarge reasonably power large, is reasonably and the pressure large, and is in the the pressure range between is in the 10 range and 60between Pa.Polymers 10 2020 and, 12 60, 2855 Pa. 6 of 14 The water contact angles, as observed from Figures 2–5, indicate a rather marginal increase of the PTFE wettability, i.e., the WCA remained above 80°. Such a WCA was reported already by Badey at al. [4] and Konig at al. [8]. Somehow lower WCA of 75° was reported by Yamada et al. [5], and the WCA of about 50° was found by Clark et al. [2]. The discrepancy was explained recently by Primc [3] who showed that the WCA on the surface of fluorinated polymers depends on the concentration of other elements. Even a small concentration of oxygen, for example, caused a decrease of the WCA below the values typical for polyolefins (about 80°). The results of the surface wettability as probed by the WCA do not show any obvious trend. Because this technique does not reveal the chemical modifications taking place on the sample surface upon plasma treatment, we additionally performed research on the composition and structure of the surface film as probed by XPS and ToF-SIMS to further elaborate details about the surface chemistry.

FigureFigure 3. Variation 3. Variation of surface of surfacewettability wettability with treatment with time. treatment The samples time. wereThe inside samples the radiofrequency were inside the (RF)radiofrequency coil. Discharge ( powerRF) coil. and Discharge hydrogen power pressure and were hydrogen constant pressure at 400 Wwere and constant 25 Pa, respectively. at 400 W and 25 Pa, respectively.

Figure 4. Variation of surface wettability with the forward discharge power. Treatment time and hydrogen pressure were constant at 1 s and 25 Pa, respectively.

Figure 5. Variation of surface wettability with hydrogen pressure. Treatment time and discharge power were constant at 1 s and 400 W, respectively.

3.2. Chemical Modifications as Determined by X-ray Photoelectron Spectroscopy (XPS) Figure 6 shows the XPS F/C ratio versus the treatment time when plasma was sustained at a large power of 400 W (lower curve) and at low power of 100 W (upper curve). See also supplementary Figures S1 and S2 showing individual survey spectra and elemental composition versus treatment time. The upper curve of Figure 6 corresponds to the same experimental conditions as the WCA measurement at the discharge power of 100 W in Figure 4, which reveals incomplete activation of the surface at 100 W after the treatment for 1 s. Examining Figure 6, it is obvious that incomplete surface activation is a consequence of the insufficient removal of fluorine from the surface of PTFE, because

PolymersPolymers 2020 2020, ,12 12, ,2855 2855 66 of of 14 14

FigureFigure 3. 3. VariationVariation of of surface surface wettability wettability with with treatment treatment time. time. TheThe samples samples were were inside inside the the Polymers radiofrequency2020radiofrequency, 12, 2855 ((RFRF)) coil.coil. DischargeDischarge powerpower andand hydrogenhydrogen pressurepressure werewere constant constant atat 400400 W W andand 2525 Pa,6Pa, of 14 respectively.respectively.

FigureFigureFigure 4. 4.Variation 4. VariationVariation of of surfaceof surface surface wettability wettability wettability with with with the thethe forward forward forward discharge dischargedischarge power. power. power. Treatment Treatment Treatment time time time and and and hydrogenhydrogenhydrogen pressure pressurepressure were werewere constant constantconstant at 1 atat s 1 and1 ss andand 25 Pa,2525 Pa,Pa, respectively. respectivelyrespectively..

FigureFigureFigure 5. Variation 5. 5. VariationVariation of surface of of surface surface wettability wettability wettability with hydrogen with with hydrogen hydrogen pressure. pressure. pressure. Treatment Treatment Treatment time and time time discharge and and discharge discharge power werepowerpower constant werewere at constantconstant 1 s and at 400at 11 sW,s andand respectively. 400400 W,W, respectively.respectively.

3.2.3.2.The ChemicalChemical water contactModificationModification angles,ss asas as DeterminedDetermined observed by fromby XX--ray Figuresray PhotoelectronPhotoelectron2–5, indicate SpectroscopySpectroscopy a rather marginal ((XPSXPS)) increase of the PTFE wettability, i.e., the WCA remained above 80◦. Such a WCA was reported already by Badey at FigureFigure 66 showsshows thethe XXPSPS F/CF/C ratioratio versusversus thethe treatmenttreatment timetime whenwhen plasmaplasma waswas sustainedsustained atat aa al. [4] and Konig at al. [8]. Somehow lower WCA of 75◦ was reported by Yamada et al. [5], and the largelarge power power of of 400 400 W W (lower (lower curve) curve) and and at at low low power power of of 100 100 W W (upper (upper curve). curve). See See also also supplementary supplementary WCA of about 50◦ was found by Clark et al. [2]. The discrepancy was explained recently by Primc [3] whoFiguresFigures showed S1S1 thatandand theS2S2 showingshowing WCA on individualindividual the surface surveysurvey of fluorinated spectraspectra and polymersand elementalelemental depends compositioncomposition on the concentration versusversus trtreatmenteatment of othertime.time. elements. TheThe upperupper Even curvecurve a small ofof FigureFigure concentration 66 correspondscorresponds of oxygen, toto thethe for samesame example, experimentalexperimental caused a conditions decreaseconditions of asas the thethe WCA WCAWCA measurementmeasurement atat thethe dischargedischarge powerpower ofof 100100 WW inin FigureFigure 4,4, whichwhich revealsreveals incompleteincomplete activationactivation ofof thethe below the values typical for polyolefins (about 80◦). surfacesurfaceThe results atat 100100 W ofW after theafter surface thethe treatmenttreatment wettability forfor 11 as s.s. Examining probedExamining by Figure theFigure WCA 6,6, itit do isis obvious notobvious show thatthat any incompleteincomplete obvious trend. surfacesurface Becauseactivationactivation this techniqueisis aa consequenceconsequence does not ofof reveal thethe insufficientinsufficient the chemical removalremoval modifications ofof fluorinefluorine taking fromfrom place thethe surfacesurface on the sampleofof PTFE,PTFE, surface becausebecause upon plasma treatment, we additionally performed research on the composition and structure of the surface film as probed by XPS and ToF-SIMS to further elaborate details about the surface chemistry.

3.2. Chemical Modifications as Determined by X-ray Photoelectron Spectroscopy (XPS) Figure6 shows the XPS F /C ratio versus the treatment time when plasma was sustained at a large power of 400 W (lower curve) and at low power of 100 W (upper curve). See also supplementary Figures S1 and S2 showing individual survey spectra and elemental composition versus treatment time. The upper curve of Figure6 corresponds to the same experimental conditions as the WCA measurement at the discharge power of 100 W in Figure4, which reveals incomplete activation of the surface at 100 W after the treatment for 1 s. Examining Figure6, it is obvious that incomplete surface activation is a consequence of the insufficient removal of fluorine from the surface of PTFE, because the F/C concentration after the treatment for 1 is still about 1.25, thus far from complete defluorination. Polymers 2020, 12, 2855 7 of 14

the F/C concentration after the treatment for 1 is still about 1.25, thus far from complete defluorination. The upper curve in Figure 6 shows a gradual decrease of the F/C ratio with increasing treatment time. Gaseous plasma at the pressure of 25 Pa and RF power of 100 W is sustained in the E-mode. Because the matching network was optimized for coupling in the H-mode, a significant fraction of the RF power was reflected and thus not absorbed by the gaseous plasma. The power absorbed in plasma for the case of the upper curve of Figure 6 is, therefore, well below 100 W. Still, the surface film as probed by XPS is depleted of fluorine even after several seconds of plasma treatment and approaches a value of approximately 0.6. It seems that such a concentration of fluorine is about all one can achieve upon treatment of PTFE in hydrogen plasma sustained in the E-mode. The mechanism is completely different when plasma is sustained in the H-mode (lower curve). In this case, the surface film is depleted of fluorine even after approximately 0.2 s of plasma treatment. The F/C ratio further decreases with increasing treatment time until a minimum at approximately 1 s appears. Thereafter, a gradual but slow increase in the F/C ratio is observed. After approximately Polymers10 s, the2020 F/C, 12 ratio, 2855 assumes a value of approximately 0.4. This value is lower than what is achievable7 of 14 in the E-mode.

FigureFigure 6.6. Variation ofof thethe X-rayX-ray photoelectronphotoelectron spectroscopyspectroscopy (XPS)(XPS) FF/C/C ratioratio versusversus treatmenttreatment timetime forfor twotwo didifferentﬀerent powers.powers. HydrogenHydrogen pressurepressure waswas constantconstant atat 2525 Pa.Pa.

TheA huge upper difference curve in Figurein the 6surface shows composition a gradual decrease between of thethe F E/C- and ratio H with-modes, increasing as evide treatmentnt from time.Figure Gaseous 6, should plasma be explained at the pressure by different of 25 Pa mechanisms. and RF power Gaseous of 100 W plasma is sustained in the in H- themode E-mode. is an Becauseextensive the source matching of vacuum network ultraviolet was optimized (VUV for) radiation coupling [28 in] the. Recently, H-mode, Fantz a significant et al. investigated fraction of thethe RFdetails power regarding was reflected the radiation and thus not arising absorbed from by H2 the plasma gaseous sustained plasma. by The an power inductively absorbed coupled in plasma RF fordischarge the case in of the the H upper-mode curve[29]. The of Figuredischarge6 is, configuration therefore, well was below almost 100 identical W. Still, to the the surface one shown film as in probedFigure 1. by Fantz XPS iset depletedal. showed of that fluorine approximately even after several10% of secondsthe available of plasma discharge treatment power and is transformed approaches ainto value radiation of approximately in the VUV 0.6. range. It seems This thatradiation such acauses concentration bond scission of fluorine in the is surface aboutall film one ofcan a thickness achieve uponof the treatment order of of a PTFE penetration in hydrogen depth plasma for VUV sustained photons. in the The E-mode. penetration depth depends on the wavelengthThe mechanism (i.e., photon is completely energy) but di ffiserent definitely when larger plasma than is sustained the escape in depth the H-mode of photoelectrons. (lower curve). By Inconsidering this case, thethis surface fact, one film can is depleted assume ofa rather fluorine homogenous even after approximately treatment of the 0.2 ssurface of plasma film treatment. with the TheVUV. F/ CThe ratio bond further scission decreases enables with fu increasingrther reactions, treatment including time untilthe interaction a minimum with at approximately H atoms. The 1 H s appears.atoms attack Thereafter, the dangling a gradual bonds but as slow already increase reported in the by F/C Badey ratio iset observed. al. [19]. Furthermore, After approximately they interact 10 s, thewith F /C F atomsratio assumes forming a HF value molecules, of approximately which are 0.4. desorbed This value upon is vacuumlower than conditions what is achievable and pumped in theaway. E-mode. The combination of bond scission caused by absorption of VUV radiation and chemical interactionA huge with difference H atoms inthe should surface, therefore composition, ensure between an F-free the E-surface. and H-modes, Such an effectas evident cannot from be Figureconfirmed6, should from be XPS explained measurements by di fferent because mechanisms. of the final Gaseous escape plasma depth in of the photoelectrons. H-mode is an extensiveIt will be sourceshown oflater vacuum in this ultravioletpaper that the (VUV) best radiation technique [ 28to]. prove Recently, the F- Fantzfree surface et al. is investigated ToF-SIMS. the details regardingAfter the prolonged radiation treatment arising from of PTFE H2 plasma samples sustained in the H by-mode, an inductively the F/C ratiocoupled does RF not discharge remain inconstant the H-mode but increases [29]. The slowly discharge with configurationincreasing treatment was almost time. identicalSuch an increase to the one may shown be explained in Figure by1. Fantz et al. showed that approximately 10% of the available discharge power is transformed into radiation in the VUV range. This radiation causes bond scission in the surface film of a thickness of the order of a penetration depth for VUV photons. The penetration depth depends on the wavelength (i.e., photon energy) but is definitely larger than the escape depth of photoelectrons. By considering this fact, one can assume a rather homogenous treatment of the surface film with the VUV. The bond scission enables further reactions, including the interaction with H atoms. The H atoms attack the dangling bonds as already reported by Badey et al. [19]. Furthermore, they interact with F atoms forming HF molecules, which are desorbed upon vacuum conditions and pumped away. The combination of bond scission caused by absorption of VUV radiation and chemical interaction with H atoms should, therefore, ensure an F-free surface. Such an effect cannot be confirmed from XPS measurements because of the final escape depth of photoelectrons. It will be shown later in this paper that the best technique to prove the F-free surface is ToF-SIMS. After prolonged treatment of PTFE samples in the H-mode, the F/C ratio does not remain constant but increases slowly with increasing treatment time. Such an increase may be explained by a different thickness of the F-depleted surface film rather than by incomplete defluorination. This effect will be explained later by using angular-resolved XPS characterization (AR-XPS). Polymers 2020, 12, 2855 8 of 14

Polymersa different2020 , thickness12, 2855 of the F-depleted surface film rather than by incomplete defluorination.8 This of 14 effect will be explained later by using angular-resolved XPS characterization (AR-XPS). When plasma is in the the E E-mode,-mode, the the minimum minimum in in the the F/C F/C ratio ratio is is not not observed, observed, which which may may be be a aconsequence consequence of of the the fact fact that that such such plasma plasma is is not not a a significant significant source source of of VUV VUV radiation. The The luminosity of plasma plasma in in the the E-mode E-mode is typically is typically 3–4 orders3–4 orders of magnitude of magnitude lower lowerthan in than the H-mode in the H at-mode this pressure, at this i.e.,pressure, 25 Pa. i.e. The, 25 di Pa.fference The difference in plasma in luminosities plasma luminosities between thebetween E- and the H-modes E- and normallyH-modes increasesnormally withincreas increasinges with increasing pressure, aspressure, shown byas shown Fantz et by al. Fantz [29]. Theet al. F /[C29 ratio,]. The however, F/C ratio, does however, not deviate does not for ordersdeviate of for magnitude orders of butmagnitude is comparable but is forcomparable long treatment for long times. treatment The comparison times. The of comparison the two curves of the in Figuretwo curves6, therefore, in Figure indicate 6, therefore that the, intensive indicate VUV that radiation the intensive only accelerates VUV radiation defluorination only accelerates of the surfacedefluorinat film.ion A of rather the surface low F/C film. ratio A observed rather low after F/C the ratio treatment observed in after the E-mode the treatment should in be the because E-mode of othershould mechanisms. be because of It other was already mechanisms. mentioned It was that already H atoms mentioned attack the that polymer H atoms surface attack and the danglingpolymer bonds,surface causing and dangling the formation bonds, ofcausing the volatile the formation HF molecule. of the Badey volatile et al. HF [19 molecule.], as well asBadey Yamada et al. et [ al.19 []20, as], investigatedwell as Yamada the evolutionet al. [20], ofinvestigated the F/C ratio the in evolution the flowing of the afterglow F/C ratio where in the VUV flowing radiation afterglow is negligible, where butVUV the radiation density is of negligible, H atoms is but still the significant. density of They H atoms found is still moderate significant. F/C ratios They offound approximately moderate F/C 0.8. Somehow,ratios of app lowerroximately F/C ratio 0.8. was Somehow also reported, lower by F/C König ratio etwas al. also [23]. reported Unlike VUV by König radiation, et al. the[23] H. Unlike atoms doVUV not radiation, penetrate the into H the atoms solid dopolymer; not penetrate therefore, into the the chemical solid modification polymer; therefore, should thebe limited chemical to amodification thinner film should in the E-modebe limited as to compared a thinner to film the in H-mode. the E-mode Such as a compared difference to in the H thickness-mode. Such of the a well-adifferenceffected in filmthe thickness can explain of the well fact that-affected the achievable film can explain F/C ratio the in fact the that E-mode the achievable is lower than F/C in ratio the H-mode.in the E-mode A confirmation is lower than for in thethe statementH-mode. A about confirmation the F-free for surface the statement for the H-modeabout the is F- providedfree surface in Figurefor the7 H, which-mode showsis provided the F in/C Figure ratios as7, deducedwhich shows from the AR-XPS. F/C ratios The as lower deduced curve from is for AR the-XPS. sample The treatedlower curve for 1 sis in for the the H-mode, sample whereas treated for the 1 upper s in the for H the-mode, sample whereas treated the for upper the same for timethe sample in the E-mode. treated Thefor the F/C same ratio time in the in H-mode the E-mode. gradually The F/C increases ratio in with the increasingH-mode gradually take-off angleincreasesθ (TOA). with increasing Detection depthtake-off (d )angle is given θ (TOA). by the Detection following depth relation, (d) dis= given3λ sin by( theθ), wherefollowingλ is relation the inelastic, d  mean3 sin( free ) , path where of · photoelectrons.λ is the inelastic Themean detection free path depth of photoelectrons. thus increases The with detection increasing depth TOA. thus The increases gradual with increase increasing in the FTOA./C ratio The is gradual explained increase by the in formation the F/C ratio of the is almostexplained F-free by the surface formation as well of as the a subsurfacealmost F-free layer surface with aas thickness well as a subsurface close to the layer detection with a depth thickness of XPS. close Extrapolation to the detection of the depth lower of XPS. curve Extrapolation to the TOA θ of= the0◦ revealslower curve the F /toC ratiothe TOA is practically θ = 0° reveals zero onthe the F/C surface. ratio is Unfortunately,practically zero measurements on the surface. at Unfortunately, extremely low TOAmeasurements are not feasible. at extremely low TOA are not feasible.

Figure 7. Variation of the AR-XPSAR-XPS FF/C/C ratio versus the photoelectron take-otake-offﬀ angle for two didifferentﬀerent powers andand thethe samesame pressurespressures ofof 2525 PaPa andand treatmenttreatment timestimes ofof 11 s.s.

As shown before in Figure Figure 66,, a defluorinationdefluorination is incomplete when when plasma plasma is is in in the the E E-mode-mode for for 1 1s. s.The The upper upper curve curve in in Figure Figure 77 confirmsconfirms this this.. However However,, it it is is interesting that the FF/C/C ratio in Figure 7 7 increasesincreases byby aa factorfactor ofof approximatelyapproximately 1.51.5 whenwhen thethe TOATOA isis increasedincreased fromfrom θθ == 15 to 7575°.◦. Obviously, Obviously, the surface surface film film contains contains much much less less fluorine fluorine than than the subsurface the subsurface one, and one, there and is there a significant is a significant gradient ingradient F concentration. in F concentration. Any extrapolation Any extrapolation of the curve of towards the curve the towards TOA θ the= 0 ◦ TOAwould θ be= 0° speculation; would be therefore,speculation; it istherefore, not shown it is in not Figure shown7. in Figure 7. Figures6 and7 indicate large di fferences in surface chemistry, depending on the type of discharge. The differences can be further elaborated by performing measurements at a fixed treatment time and

Polymers 2020, 12, 2855 9 of 14

Figures 6 and 7 indicate large differences in surface chemistry, depending on the type of discharge. The differences can be further elaborated by performing measurements at a fixed treatment time and pressure, but different RF powers. The result is plotted in Figure 8. Here, the F/C Polymers 2020, 12, 2855 9 of 14 ratio reaches the minimum at 400 W. At larger powers, the F/C is somehow slightly larger. The behaviour of the curve for powers between 400 and 900 W is similar to the lower curve in Figure 6. Inpressure, both cases, but dithefferent fluence RF powers.of VUV radiation The result increases is plotted with in Figure increasing8. Here, value the at F /theC ratio abscissa. reaches More the interestingminimum atis 400the W.behaviour At larger at powers, lower powers. the F/C One is somehow can observe slightly a gradual larger. Theand behaviourrather linear of thedecrease curve offor the powers F/C ratio between with 400 increasing and 900 Wpower. is similar The todischarge the lower power curve of in Figure300 W6 results. In both in cases, the F/C the ratio fluence of approximatelyof VUV radiation 0.5, increases similar to with wha increasingt is observed value in Figure at the 6 abscissa. for the upper More curve interesting after prolonged is the behaviour plasma at treatment.lower powers. Comparison One can of observe Figures a gradual6 and 8, andtherefore, rather indicates linear decrease that the of key the parameter F/C ratio with governing increasing the surfacepower. chemistry The discharge is the power fluence of of 300 reactive W results species in the and/or F/C ratioVUV of radiation approximately rather than 0.5, similardischarge to whatpower is orobserved treatment in Figuretimes. 6Nonetheless for the upper, it curveis important after prolonged that the surface plasma finish treatment. in the E Comparison-mode as deduced of Figures from6 XPSand 8is, therefore,different to indicates that in the that H- themode. key In parameter addition to governing Figure 8, thesee also surface supplementary chemistry is Figures the fluence S3 and of S4reactive showing species individual and/or survey VUV radiation spectra and rather elemental than discharge composition power versus or treatment the discharge times. power. Nonetheless, it is importantThe variation that of the the surface F/C ratio finish versus in thethe E-modedischarge as power deduced (Figure from 8) XPS is different is different to the to behaviour that in the ofH-mode. WCA (Figure In addition 4). Comparison to Figure8, seeof Figures also supplementary 4 and 8 indicates Figures that S3 even and S4an showingincomplete individual defluorinati surveyon causesspectra the and drop elemental of the water composition contact versus angle to the values discharge typical power. for polyolefins.

FigureFigure 8. 8. VariationVariation of of the the F/C F/C ratio ratio versus versus the the discharge discharge power power.. Treatment Treatment time and H H22 pressurepressure were were constantconstant at at 1 1 s s and and 25 25 Pa, Pa, respectively. respectively.

FigureThe variation 9 reveals of the the high F/C- ratioresolution versus C1s the spectra discharge for powerthe untreated (Figure sample8) is di ffanderent samples to the behaviourtreated in theofWCA H-mode (Figure for 14 and). Comparison 12 s. There ofis a Figures huge difference4 and8 indicates between that the even untreated an incomplete and treated defluorination samples. The causes the drop of the water contact angle to values typical for polyolefins. untreated sample contains only carbon bonded in CF2 groups. The C1s peak, therefore, appears at a bindingFigure energy9 reveals of approximately the high-resolution 292 eV. C1sThere spectra is also for a small the untreated peak at approximately sample and samples 285 eV, treated which correspondsin the H-mode to forsurface 1 and impurities. 12 s. There The is a treated huge di samplesfference betweenexhibit the the opposite untreated behaviour and treated—the samples. major peakThe untreated is at abou samplet 285 contains eV, whereas only carbon some bonded featuresin also CF 2 persistgroups. at The higher C1s peak, binding therefore, energies appears up to at approximatelya binding energy 294 eV. of approximatelyTaking into account 292 eV.Figure There 7, and is alsothe discussion a small peak thereafter, at approximately it can be concluded 285 eV, thatwhich the corresponds features correspond to surface impurities. to degraded Thetreated PTFE- sampleslike film, exhibit whereas the opposite the main behaviour—the peak at 285 major eV correspondspeak is at about to 285F-depleted eV,whereas surface some film. features Therefore, also persist we can at higher conclude binding that energiesthe surface up tofilm approximately of samples 294 eV. Taking into account Figure7, and the discussion thereafter, it can be concluded that the features treated in H2 plasma in the H-mode contains olefin-like carbon. It is interesting that the intensity in thecorrespond range of tobinding degraded energies PTFE-like from abo film,ut whereas287 and the294 maineV is slightly peak at 285larger eV for corresponds the case of to 12 F-depleted s than for 1ssurface of plasma film. treatment.Therefore, weThe can reasons conclude for this that have the surface already film been of elaborated samples treated when in explaining H2 plasma the in behaviourthe H-mode of the contains lower olefin-likecurve in Figures carbon. 6 and It is 7. interesting that the intensity in the range of binding energiesBy considering from about 287the andupper 294 discussion, eV is slightly the larger C1s forpeak the of case samples of 12 s treated than for in 1s the of plasma E-mode treatment. should differThe reasons from those for this in have the H already-mode. been Figure elaborated 10 represents when explaining solid proof the of behaviour the evolution of the of lower the curve surface in chemistryFigures6 and upon7. treatment of PTFE samples in the E- and H-modes. The behaviour of the two curves at 400By and considering 800 W was the already upper explained. discussion, Interesting the C1s peak, and of samplessound with treated the inprevious the E-mode discussion, should is di theffer behaviourfrom those of in the the curves H-mode. acquired Figure after 10 represents the treatment solid at proof discharge of the powers evolution of of100 the and surface 200 W. chemistry In both cases,upon plasma treatment was of in PTFE the E samples-mode. The in the curve E- and at 100 H-modes. W indicates The that behaviour the rather of the intact two PTFE curves still at persists, 400 and 800 W was already explained. Interesting, and sound with the previous discussion, is the behaviour of the curves acquired after the treatment at discharge powers of 100 and 200 W. In both cases, plasma was in the E-mode. The curve at 100 W indicates that the rather intact PTFE still persists, but a fraction is Polymers 2020, 12, 2855 10 of 14 Polymers 2020, 12, 2855 10 of 14 Polymers 2020, 12, 2855 10 of 14 but a fraction is already modified enough to form various carbon chemistries including intermediate but a fraction is already modified enough to form various carbon chemistries including intermediate ones, as already reported by Badey et al. [19]. A well-expressed peak is observed at approximately ones, as already reported by Badey et al. [19]. A well-expressed peak is observed at approximately already292 eV, modifiedas well as enougha broad tofeature form between various carbon292 and chemistries 285 eV. This including indicates intermediate that CH groups ones, have as alreadyalready 292 eV, as well as a broad feature between 292 and 285 eV. This indicates that CH groups have already reportedappeared by on Badey the surface, et al. [19 but]. Athe well-expressed thin surface film peak also is observedcontains other at approximately groups with 292binding eV, as energies well as appeared on the surface, but the thin surface film also contains other groups with binding energies abetween broad feature 292 and between 285 eV. 292 The and spectrum 285 eV. This corresponding indicates that to CH treatment groups at have 200 already W shows appeared a gradual on between 292 and 285 eV. The spectrum corresponding to treatment at 200 W shows a gradual thedeviation surface, from but the untreated thin surface PTFE film to also a surface contains film other of polyolefin groups with-like binding structure energies. As discussed between above 292 deviation from the untreated PTFE to a surface film of polyolefin-like structure. As discussed above andand 285in keeping eV. The with spectrum the observations corresponding in Figures to treatment 6 and 7, at the 200 thickness W shows of a the gradual F-free deviation surface film from in thethe and in keeping with the observations in Figures 6 and 7, the thickness of the F-free surface film in the untreatedE-mode is PTFE much to lower a surface than film the of escape polyolefin-like depth of structure.photoelectrons, As discussed so the abovesignificant and incontribution keeping with of E-mode is much lower than the escape depth of photoelectrons, so the significant contribution of thephotoelectrons observations with in Figures binding6 andenerg7,ies the of thickness 292 eV still of thepersists. F-free In surface between film the in two the well E-mode-defined is much peaks photoelectrons with binding energies of 292 eV still persists. In between the two well-defined peaks lowerat 292 thanand the285 escapeeV, there depth is a of rich photoelectrons, structure, which so the could significant be deconvoluted contribution almost of photoelectrons arbitrarily taking with at 292 and 285 eV, there is a rich structure, which could be deconvoluted almost arbitrarily taking bindinginto account energies a variety of 292 of eVfunctional still persists. groups In as between well as peculiarities the two well-defined of XPS regarding peaks at th 292e interpretation and 285 eV, into account a variety of functional groups as well as peculiarities of XPS regarding the interpretation thereof F-containing is a rich structure, functional which groups could [30 be]. deconvolutedFor this reason, almost we made arbitrarily no attempt taking to into deconvolute account a C1s variety peaks of of F-containing functional groups [30]. For this reason, we made no attempt to deconvolute C1s peaks functionalshown in Figure groups 10. as well as peculiarities of XPS regarding the interpretation of F-containing functional groupsshown [in30 Figure]. For this10. reason, we made no attempt to deconvolute C1s peaks shown in Figure 10.

Figure 9. Comparison of XPS high-resolution C1s spectra of the untreated PTFE, and PTFE exposed FigureFigure 9.9. ComparisonComparison ofof XPSXPS high-resolutionhigh-resolution C1sC1s spectraspectra ofof thethe untreateduntreated PTFE,PTFE, andand PTFEPTFE exposedexposed to H2 plasma for 1 and 12 s. The discharge power and the pressure were constant at 400 W and 25 Pa, toto HH22 plasma for for 1 1 and and 12 12 s s.. The The discharge discharge power power and and the the pressure pressure were were constant constant at 400 at 400 W and W and 25 Pa, 25 respectively. Pa,respectively. respectively.

FigureFigure 10.10. ComparisonComparison of of the the selected selected XPS XPS high-resolution high-resolution C1s C1s spectra spectra of theof the PTFE PTFE samples samples treated treated for Figure 10. Comparison of the selected XPS high-resolution C1s spectra of the PTFE samples treated variousfor various powers. powers. The The treatment treatment time time and and the pressurethe pressure were were constant constant at 1 sat and 1 s and 25 Pa, 25 respectively.Pa, respectively. for various powers. The treatment time and the pressure were constant at 1 s and 25 Pa, respectively. 3.3. Chemical Modifications as Determined by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) 3.3. Chemical Modifications as Determined by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) 3.3. Chemical Modifications as Determined by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Deconvolution of C1s XPS spectra of fluorinated samples definitely represents a scientific challenge. Deconvolution of C1s XPS spectra of fluorinated samples definitely represents a scientific Therefore,Deconvolution it is useful of to C1s characterize XPS spectra selected of fluorinated samples also samples with an definitely alternative represents technique, a scientific such as challenge. Therefore, it is useful to characterize selected samples also with an alternative technique, ToF-SIMS.challenge. TheTherefore, evolution it is ofuseful the ToF-SIMSto characterize spectral selected features samples was investigatedalso with an systematicallyalternative technique, for the such as ToF-SIMS. The evolution of the ToF-SIMS spectral features was investigated systematically treatmentsuch as ToF time-SIMS. of 1 sThe and evolution hydrogen of pressure the ToF of-SIMS 25 Pa. spectral We varied feat theures discharge was investigated power to systematically obtain further for the treatment time of 1 s and hydrogen pressure of 25 Pa. We varied the discharge power to obtain insightfor the treatment into the surface time of chemistry. 1 s and hydrogen Some examples pressure ofof the25 Pa. selected We varied positive the discharge in negative power ion spectra to obtain of further insight into the surface chemistry. Some examples of the selected positive in negative ion thefurther samples insight are into shown the in surface Supplementary chemistry. Figures Some examples S5–S8. Figure of the 11 selectedshows the positive selective in negativenegative ionion

Polymers 2020, 12, 2855 11 of 14 Polymers 2020, 12, 2855 11 of 14 spectra of the samples are shown in Supplementary Figures S5–S8. Figure 11 shows the selective spectranegative of ion the intensities samples areand shown Figure in12 Supplementarypositive ion intensities. Figures OneS5–S8. can Figure observe 11 ashows gradual the decreasing selective negativeof the F2 - ionintensity intensities versus and the Figure discharge 12 positive power. ion This intensities. behaviour One indicates can observe depletion a gradual of the surfacedecreasing film ofas the probed F2- intensity by ToF versus-SIMS. theIt should discharge be notedpower. that This the behaviour surface sensitivityindicates dep of letionToF-SIMS of the is surface superior film in ascomparison probed by to ToF XPS.-SIMS. Simultaneous It should beto decreasingnoted that theof the surface F2- intensity, sensitivity a number of ToF -ofSIMS fluorine is superior-free ions in Polymerscomparisonappear2020 in ,the12 to, 2855ToF XPS.-SIMS Simultaneous negative ion to decreasingspectra. All ofof thethem F2 -keep intensity, increasing a number with ofincreasing fluorine -dischargefree11 ofions 14 appearpower inup the to theToF power-SIMS negativeof 400 W. ion Thereafter, spectra. Allthe ofintensity them keep of fluorine increasing-free with ions increasing in ToF-SIMS discharge spectra powerremains up constant. to the power This observationof 400 W. Thereafter, is in keeping the intensitywith results of fluorine presented-free in ions Figure in ToF 11, -wheSIMSre spectraexactly 2 intensitiesremainsthe same constant. effect and Figure was This observed. 12 observation positive Unfortunately, ion is intensities.in keeping ToF with One-SIMS results can does observe presentednot allow a gradual forin Figurereliable decreasing 11, quantification whe ofre theexactly F −of intensity versus the discharge power. This behaviour indicates depletion of the surface film as probed thethe samemeasured effect spectra. was observed. The drop Unfortunately, of the intensity ToF of -FSIMS2- for doesmore not than allow a factor for ofreliable 50 indicates quantification practically of bytheF-free ToF-SIMS. measured surface. Itspectra. should The be drop noted of that the theintensity surface of sensitivityF2- for more of than ToF-SIMS a factor is of superior 50 indicates in comparison practically 2 toF-free XPS.Figure surface. Simultaneous 12 shows tothe decreasing behaviour of of the positive F − intensity, ion fragments. a number The of fluorine-freeintensities of ions F-containing appear in ions the ToF-SIMSdecreasesFigure negative monotonously 12 shows ion the spectra. be withhaviour All increasing of themof positive keep discharge increasing ion fragments. power, with and increasingThe the intensities minimum discharge of isF - power observedcontaining up to at ions the the powerdecreasespower of of 400 400 monotonously W.W. Thereafter, By contrast with the, the intensityincreasing ions containing of discharge fluorine-free hydrogen power, ions increase andin ToF-SIMS the gradually minimum spectra up isto remains observedthe power constant. atof the400 ThispowerW and observation of remain 400 W. fairlyBy is in contrast keeping intact, thereafter.the with ions results containing The presented ToF hydrogen-SIMS in resultsFigure increase 11are, where, graduallytherefore exactly, upin the keepingto the same power with effect of those was 400 observed.Wobtained and remain by Unfortunately, XPS. fairly intact ToF-SIMS thereafter. does The not allowToF-SIMS for reliable results quantificationare, therefore of, in the keeping measured with spectra. those 2 Theobtained drop ofby theXPS. intensity of F − for more than a factor of 50 indicates practically F-free surface.

Figure 11. Variation of time-of-flight secondary ion mass spectrometry (ToF-SIMS) intensities of Figure 11. Variation of time-of-ﬂight secondary ion mass spectrometry (ToF-SIMS) intensities of selected Figureselected 11. negative Variation ions. of The time pressure-of-flight was secondary 25 Pa and ion treatment mass spectrometry time 1 s. (ToF-SIMS) intensities of negative ions. The pressure was 25 Pa and treatment time 1 s. selected negative ions. The pressure was 25 Pa and treatment time 1 s.

Figure 12. Variation of ToF-SIMS intensities of selected positive ions. The pressure was 25 Pa and Figure 12. Variation of ToF-SIMS intensities of selected positive ions. The pressure was 25 Pa and treatment time 1 s. Figuretreatment 12. timeVariation 1 s. of ToF-SIMS intensities of selected positive ions. The pressure was 25 Pa and Figuretreatment 12 timeshows 1 s. the behaviour of positive ion fragments. The intensities of F-containing ions decreases4. Conclusions monotonously with increasing discharge power, and the minimum is observed at the power 4. Conclusions ofSystematic 400 W. By characterization contrast, the ionsof PT containingFE was performed hydrogen to reveal increase the kinetics gradually of fluorine up to depletion the power of ofthe 400 surface W and film remainSystematicof this polymerfairly characterization intact upon thereafter. treatment of PTFE withThe was ToF-SIMS hydrogenperformed plasma resultsto reveal at are, the various therefore,kinetics conditions. of fluorine in keeping Unlike depletion with previous of those the surface authors, obtained film we byof thisXPS. polymer upon treatment with hydrogen plasma at various conditions. Unlike previous authors, we

4. Conclusions Systematic characterization of PTFE was performed to reveal the kinetics of ﬂuorine depletion of the surface ﬁlm of this polymer upon treatment with hydrogen plasma at various conditions. Unlike previous authors, we concentrated on rather short treatment times of the order of a second. Polymers 2020, 12, 2855 12 of 14

Such short times are attractive for any application of gaseous plasma technology for surface processing of products made from fluorinated polymers. A broad range of parameters was found useful for the depletion of the surface film. Nominal discharge powers of the RF generator of as low as 100 W are capable of depletion of the surface film within several seconds of plasma treatment. The intensity of surface chemical reactions increases with increasing discharge power, and the reactions become almost instant once the discharge is in the H-mode. A significant difference in the surface finish between the discharge modes was observed. In the case the discharge is in the E-mode, the F concentration decreases monotonously with increasing treatment time, and the minimal achievable F/C ratio is just above 0.5. By contrast, when the plasma is in the H-mode, a well-defined minimum in the F concentration occurs at rather short treatment times. The minimal F/C ratio as deduced from the XPS spectra acquired at standard take-off angle is about 0.2. The surface layer of the polymer, however, is almost free from fluorine, which is proved by ARXPS as well as by the behaviour of specific ion fragments acquired by ToF-SIMS. Experiments performed at different discharge conditions qualitatively indicate that the key parameter governing the surface finish is the fluence of reactive plasma species. Unfortunately, our experimental set-up did not allow for reliable determination of the radiation in the VUV range. The surface finish versus the fluence of VUV radiation upon treatment of fluorinated polymers with hydrogen plasma, therefore, remains a scientific challenge.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/12/2855/s1, Figure S1: XPS survey spectra of PTFE treated in hydrogen plasma for various treatment times. Hydrogen pressure was constant at 25 Pa and the discharge power was: (a) 100 W and (b) 400 W., Figure S2: XPS surface composition of PTFE treated in hydrogen plasma at various treatment times: (a) for discharge power of 100 W and (b) for discharge power of 400 W. Hydrogen pressure was 25 Pa., Figure S3: XPS survey spectra of PTFE treated in hydrogen plasma at various forward powers. Hydrogen pressure was constant at 25 Pa and treatment time was 1 s., Figure S4: XPS surface composition of PTFE treated in hydrogen plasma at various forward powers. Hydrogen pressure was 25 Pa and treatment time was 1 s., Figure S5: ToF-SIMS spectra of the untreated PTFE: (a) positive ion spectra and (b) negative ion spectra. Figure S6: ToF-SIMS spectra of the PTFE treated in H2 plasma at the power of 150 W: (a) positive ion spectra and (b) negative ion spectra. The pressure was 25 Pa and treatment time was 1 s., Figure S7: ToF-SIMS spectra of the PTFE treated in H2 plasma at the power of 400 W: (a) positive ion spectra and (b) negative ion spectra. The pressure was 25 Pa and treatment time was 1 s., Figure S8: ToF-SIMS spectra of the PTFE treated in H2 plasma at the power of 800 W: (a) positive ion spectra and (b) negative ion spectra. The pressure was 25 Pa and treatment time was 1 s. Author Contributions: Conceptualization, A.V. and R.Z.; methodology, A.V. and R.Z.; validation, A.V., D.L. and R.Z.; formal analysis, D.L., A.V., J.E. and M.G.; investigation, D.L., A.V. and R.Z.; resources, G.P.; data curation, D.L., A.V., J.E. and J.K.; writing—original draft preparation, A.V.; writing—review and editing, M.M.; supervision, R.Z. and A.V.; project administration, M.K. and G.P.; funding acquisition, M.K. and K.S.-K. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the financial support from the Slovenian Research Agency—project No. J2-1728 (Initial stages in surface functionalization of polymers by plasma radicals) and P2-0082 (Thin-film structures and plasma surface engineering). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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