A low power 50 Hz argon plasma for surface modification of polytetrafluoroethylene

Yen Theng LAU(劉恩婷),Wee Siong CHIU(趙偉祥),Hong Chun LEE(李鴻俊),Haw Jiunn WOO(胡浩俊),Oi Hoong CHIN(陳愛(ài)虹) and Teck Yong TOU(杜德?lián)P)

1 Department of Physics,Faculty of Science,University of Malaya,Kuala Lumpur 50603,Malaysia

2 Faculty of Engineering,Multimedia University,Persiaran Cyberjaya,Cyberjaya 63100,Malaysia

Abstract The characteristics of a low power 50 Hz argon plasma for surface treatment of polytetrafluoroethylene(PTFE)film is presented in this article.The current-voltage behavior of the discharge and time-varying intensity of the discharge showed that a DC glow discharge was generated in reversed polarity at every half-cycle.At discharge power between 0.5 and 1 W,the measured electron temperature and density were 2-3 eV and～108 cm-3,respectively.The optical emission spectrum of the argon plasma showed presence of some‘impurity species’such as OH,N2 and H,which presumably originated from the residual air in the discharge chamber.On exposure of PTFE films to the argon glow plasma at pressure 120 Pa and discharge power 0.5 to 1 W,the water contact angle reduced by 4% to 20% from the original 114° at pristine condition,which confirms improvement of its surface wettability.The increase in wettability was attributed to incorporation of oxygen-containing functional groups on the treated surface and concomitant reduction in fluorine as revealed by the XPS analysis and increase in surface roughness analyzed from the atomic force micrographs.Ageing upon storage in ambient air showed retention of the induced increase in surface wettability.

Keywords:50 Hz glow discharge,plasma surface treatment,PTFE,hydrophobic recovery,ageing effect

1.Introduction

Plasma is often used to modify both chemical and morphological characteristics at the polymer surface,in particular for improved adhesion[1-3],which enables applications in areas such as sensor fabrication[4],cell adhesion[5],and organic electronics[6].Improved adhesion necessitates the polymer surface to change from the hydrophobic to hydrophilic,hence raising research interest in plasma treatment.The plasma treatment may leverage on different gases such as oxygen,air,nitrogen and argon but oxygen-containing plasmas are more frequently used because it is more efficient in producing hydrophilic surfaces.The hydroxyl radical impurities from water vapour in residual air within the discharge chamber may render the surface more hydrophilic[7].In general,however,plasma treatment affects only the surface(up to few tens of nm)of the treated material while the bulk properties of the materials remain unchanged.

Among polymers,the fluoropolymers exhibit outstanding chemical and temperature resistance characteristics.One such fluoropolymer of interest is the polytetrafluoroethylene(PTFE)that possesses extremely low surface tension,and the contact angle with water is around 110°[8].The PTFE surface is hard to wet and non-sticky without surface modification or treatment.To modify the surface of PTFE to be hydrophilic,the C-C and C-F bonds on surface must be scissored off before being bonded to hydrophilic oxygencontaining functional groups.The C-C and C-F bonds are extremely strong bonds,and much energy is required to break these bonds[9].Comparing with non-fluorinated polymers such as polystyrene,it is more difficult to achieve the same level of hydrophilicity in PTFE when treated in the same plasma as additional energy is required to remove the fluorine on the surface.Besides the hydrophilic effect,the PTFE surface finish after treatment can attain the other wetting extreme,that is,super-hydrophobicity depending on the macroscopic or discharge parameters such as the discharge power,type of working gas,pressure,flow rate,and treatment time[10].It is also important to consider microscopic plasma properties such as degree of ionization,particle fluxes,ion energy,and etc.But measurement of these plasma parameters is a daunting task,and very few researchers reported on these.Hence,it is noted that in general the macroscopic properties are more frequently correlated to the change in surface finish properties.The most commonly used plasma system is based on the rf discharge at 13.56 MHz[8,11-15]with the discharge power ranging from tens to hundreds of watts.The microwave plasma[16-18]and dielectric barrier surface discharge[19-21]have also been used frequently for PTFE surface treatment.In comparison,other plasma systems which are less often used for PTFE,with different efficiencies,include the pulse width modulation plasma[22],DC plasma[23,24],bipolar plasma[25],AC and DC pulse plasma[26].It may be summarized that the plasma system and its characteristics influence the surface modification of PTFE,which can be easily observed from wettability behavior or water contact angle(WCA).Microscopic properties of the plasma are difficult to measure,hence,it is noted that in general the macroscopic properties are more frequently correlated to the change in surface hydrophilicity and hydrophobicity.

In this work,an argon glow discharge plasma in a 50 Hz AC parallel-electrode is characterised and applied to PTFE surface treatment at low discharge power≤1 W.The 50 Hz power source is simple to set up as it does not require matching network or rectification system.Although low frequency plasma does not produce a continuous discharge when compared to DC and RF plasmas,the low frequency plasma could produce relatively high electron temperature[34].To the best of our knowledge,the use of an AC glow plasma generated low frequency(50 Hz)and low discharge power for surface modification of PTFE has not been reported.The discharge parameters(macroscopic properties)varied are the discharge power(calculated from the discharge current and voltage waveforms)and treatment time.The plasma parameters(microscopic properties)such as the electron temperature and density were measured using the Langmuir probe,and the presence of active species that cause changes in surface chemistry was identified using the optical emission spectroscopy.The surface finish characteristics measured are surface wettability determined in terms of WCA of the untreated and treated PTFE samples,x-ray photoelectron spectra which confirm the functionalization of PTFE surface,and surface roughness determined through atomic force microscopy(AFM).The hydrophobic recovery of plasma treated PTFE was also studied to evaluate its ageing characteristics under ambient conditions.

2.Experimental setup

2.1.50 Hz glow discharge system

Figure 1 shows the schematic of the setup for plasma treatment of PTFE,which has been described in detail in our previous article[27].The plasma chamber was made of stainless steel with internal volume of 38.6 l.Two disc-shaped brass electrodes,each of radius 4.5 cm,were placed parallel to each other with a gap separation of 3 cm inside the chamber as shown in figure 1.The discharge chamber was evacuated by a dry scroll pump(XDS10C)to base pressure of 8 Pa,then argon gas was admitted through a mass flow controller(Dwyer GFC).The chamber was purged twice with argon gas before adjusting to the working pressure of 120 Pa at mass flowrate of 44 sccm.

Figure 1.Schematic of the glow discharge plasma chamber and PTFE samples which are placed on the bottom electrode.

Figure 2.Schematic of the simple discharge circuit based on a 50 Hz home-built power supply for argon plasma generation.

Figure 3.(a)Schematic of the Langmuir probe setup,and(b)typical semi-logarithmic plot of electron current versus probe voltage.

Figure 4.(a)Light collection by the PIN photodiode within a view angle θ confined by the alumina tube,(b)correlation of the current waveform to photodiode signals of the upper half,and(c)lower half of the interelectrode space.Inset:images of the glow plasma(time integrated)with the alumina tube pointed at the respective glowregions.

The argon glow discharge was produced with a simple home-built,50 Hz power supply(figure 2).The plasma condition was varied by a variable transformer which controlled the input voltage from the 50 Hz mains at 240 V.The step-up transformer provides high voltage to the upper electrode in the plasma chamber.The input current was limited by three series ballast resistors(900 kΩ).The applied voltage was measured across the output terminals of the step-up transformer,while the voltage across the parallel electrodes is the discharge voltage.The bottom electrode was grounded through a 1 kΩ resistor and the discharge current is determined from the voltage drop across this resistor.The discharge powerPdiswas calculated as the product of root mean square(rms)value of the discharge voltage and the rms of discharge current.Due to the power ratings of the components in the power supply circuit,the discharge current was capped at 5 mA.

2.2.Langmuir probe measurement

The electron temperatureTeand electron densityneof the plasma were measured with a single Langmuir probe,which consisted of a cylindrical tungsten probe tip of 9.5 mm in length and 0.2 mm in diameter.The probe was inserted from a side-port and placed at about 1.2 cm above the bottom electrode.The applied bias voltage to the Langmuir probe is a 700 Hz triangular wave,sweeping from-35 to 0 V after amplification by the bipolar operational amplifier(Kepco BOP 100-1M).The schematic of the probe setup is shown in figure 3(a).The probe currentIpwas measured as a voltage drop,VI,across the 1 kΩ resistor.As the probe current is small,an isolation amplifier(transistor AD215)with gain of 9.35 was used to amplifyVI.This arrangement is suited to work only around the negative half-cycle when the potential of the bottom electrode(acting as anode)is close to zero and the top electrode is at negative potential of several hundred volts.The plasma potential at the vicinity of the probe falls within the range of sweeping probe voltage.It is assumed that the probe is in the low field region of the‘positive column’with a potential slightly lower than that of the anode.Hence,bothTeandnevalues were obtained at the time when maximum negative current occurs.Maximum negative current here represents the point at which maximum power is coupled to the argon plasma.TheTeandneare determined from theIVcharacteristic curve of the probe by using the following equations assuming Maxwellian distribution for the electron energies[28]

whereIeis the electron current in the electron retardation region of theIp-Vpcurve,Iesis the electron saturation current,Vpis the probe voltage,Vsis the plasma potential,Ais the current collection area of the probe,eis the electronic charge,kis Boltzmann’s constant,andmis the mass of electron.A typicalIe-Vpcurve obtained for applied power of 28 W(corresponding to discharge power,0.8 W)is shown in figure 3(b).The gradient of the slope yieldedTe=2.4±0.3 eV,andne=(7.3±0.5)×108cm-3was determined by using equation(2).

2.3.Optical measurements

The optical emission spectra were measured using Ocean Optics USB2000 spectrometer with resolution of 1.4 nm(FWHM)and spectral range from 200 to 850 nm.The wavelengths were calibrated using the emission spectra from Ocean Optics AR-2 argon calibration source.No attempt was made to correct for the spectral sensitivity of the grating and the CCD detector in the intensity measured.A collimating lens(Model 74-UV)was mounted at the end of an optical fiber that collects the emitted light from the glow discharge after passing through a quartz viewport.Optical emission spectroscopy was used to identify the plasma species in the 50 Hz argon glow discharge that interact with the surface of PTFE contributing to the surface modification process.

The image of the glow plasma is captured by a Canon EOS 500D DSLR camera with a mounted Tamron SP 90 mm F/2.8 macro lens.The shutter speed was set at 1/200 s and light sensitivity at ISO3200.The time interval when the shutter is open is 5 ms which is halved that of the half-period of the current signal(10 ms).Although the opening of the shutter is not synchronized in time with the current signal,the discharge image was continually captured,and the image would correspond randomly within the half-period of either positive or negative half-cycle in some of the shots.The glow discharge of either the negative or positive half-cycle of the discharge current can be seen as a single disc of bright glow that is positioned either near the top or bottom electrode respectively.

On inspection with the naked eye(time integrated),either two visible glow discs occur,one near the top electrode and the other nearer the bottom or a single column of visible glow appear between the electrodes with a brighter part nearer one of the electrodes.To determine the position of the glow disc synchronized in time to the polarity of the current waveform,the setup in figure 4(a)is employed.An off-the-shelf light sensor module with a PIN diode(PD333-3B/H0/L)sensitive to spectral lines from 750 to 1150 nm(numerous atomic argon emission lines fall within this range)was used.An alumina tube with length 39.5 cm and internal diameter of 0.5 cm was used to limit the field of view such that light from either the upper half or the lower half of the interelectrode space could be viewed by the photodiode at the extreme end of the tube.From the photodiode signals that are time synchronized to the current waveforms shown in figures 4(b)and(c),and the respective inset pictures indicating the location of light collection,it is observed that at negative half-cycle a bright glow disc occurs near the top electrode(when this acts as cathode),and at positive half-cycle a bright glow occurs nearer the bottom electrode(when this acts as cathode).

2.4.Sample preparation and surface characterization

The PTFE film(Goodfellow FP301340)has thickness of 0.2 mm and mass density of 2.2 g cm-3.The cut samples,of size 1×2 cm2rectangle,were ultrasonically cleaned in acetone for 5 min followed another 5 min in deionized water,then were allowed to dry in a covered petri dish under ambient laboratory environment for at least 2 h prior to plasma treatment.

For the WCA measurement,a 2 μl droplet of deionized water was deposited onto the PTFE surface by using the Appendorf micropipette.The image of the droplet was captured using the same digital camera fitted with macro lens described in section 2.3.The droplet images were analyzed by using the Markus Bader’s triangular screen ruler(MB-ruler)in order to determine the WCA.The WCA for each sample was determined from six droplets deposited randomly on the surface,and the average and standard deviation values were computed from 12 measurements.

The chemical composition of PTFE surface was determined from the x-ray photoelectron spectra(XPS)obtained from ULTRAVAC-PHI Quantera IITMscanning XPS spectroscope.This machine produces monochromatized Al Kαx-rays with a take-off angle of 45°.The scanning tests were doneex situ(at a commercial facility),and hence,incurred a delay of approximately 3 h after the plasma treatment.The CF2peak of binding energy 292.0 eV was taken as a reference for the deconvolution of the narrow scan spectrum by using Casa XPS software,with a full width at half maximum of 1.7 eV.

AFM was used to examine the surface morphology of PTFE surface.The tests were conducted by using the Bruker MultiMode 8 Atomic Force Microscope.‘Tapping mode’was selected instead of‘Contact mode’to reduce damage to surfaces of the sample.It uses the ScanAsyst-Air Silicon Nitride probe with resonant frequency of 70 kHz,spring constant of 0.4 N m-1and the radius of curvature of the tip is 2 nm.The scans were obtained in ambient air at a rate of 0.5 Hz.The surfaces of PTFE were characterized by measuring the rms surface roughness over the test area of 5×5 μm2.The AFM images were analyzed and processed by mean plane subtraction,scanned rows are aligned by mean value and horizontal scars correction using Gwyddion software[29].

3.Results and discussion

3.1.Plasma characteristics

At 50 Hz,the time interval between field polarity reversals is 10 ms.Assuming a uniform electric fieldEof 90±1 V cm-1(from interelectrode distance of 3 cm and average breakdown voltage of 270±20 V from theV-Iplot of the discharge),the estimated critical distancexicthat ions can travel before polarity reversal of the voltage(half period interval)is approximately 438 cm calculated from the relation,xic=μiE/(πf)[30],which is much larger than the interelectrode spacing.Here,the ionic mobility μi=7.65×102cm2V-1s-1was computed from the relation μi=μ0[1+a(E/p)]-0.5[31]with μ0=1460 cm2V-1s-1,a=0.0264 Torr cm V-1,andp=0.9 Torr(120 Pa).The ions being able to reach the other electrode during the 10 ms half period do not accumulate within the gap,hence,the breakdown mechanism would be the same as that in a static DC field.The 50 Hz glow discharge plasma is then similar to a DC or steady glow discharge,except that it flips polarity every half-cycle.This has already been evidenced from figure 6 and the glow image captured during the half-cycle of positive current where the glow is observed near the bottom electrode,and at negative half-cycle the glow was observed near the upper electrode(figure 5).The measured Paschen minimum occurred atpd(min)=141 Pa cm,argon pressure in this experiment was 120 Pa and this corresponds topd=362 Pa cm.This means the discharge is operating on the right side of the Paschen minimum,and not an obstructed discharge that can generate high electron number density.Although the various glow and dark regions in the glow discharge were not clearly observed,the bright disc near the cathode that follows the polarity change was identified as the negative glow region.

The discharge voltage and current waveforms are observed to be in phase.When the applied power was increased,asymmetry in the waveforms occurred.The discharge powers(measured average power dissipated across the electrodes)that correspond to the applied powers of 5.61 and 20.6 W are 0.59±0.01 and 0.77±0.01 W,respectively.The asymmetry for the higher power discharge gave rise to hysteresis in the negative half-cycle of the discharge voltage as shown in theV-Iplot of a single AC cycle in figure 6(b).The hysteresis in glow discharges has been attributed to current related instability,a consequence of the onset of negative differential resistance,and it was usually seen when series resistors are present in the circuit in the case of hot cathode glow discharge[32].The observed hysteresis in a DC glow discharge using long discharge tube reported by Matthewet al[33]was attributed to power loss due to reduction of charged particles through recombination processes.In this work,the hysteresis at the negative half-cycle was observed to be accompanied by a rather intense glow region(negative glow)near the top electrode(the cathode in this half-period),and the glow is sustained at a higher voltage than that in the positive half-period.It can be noted that the negative glow observed in the positive half-cycle near the bottom electrode was always less intense in comparison.Hence,it is speculated that the hysteresis is due to power loss through heat dissipation to the cathode and its vicinity,and a higher voltage is needed to sustain the same current.The hysteresis during the negative half-cycle at higher power operation is not expected to be impactful to the surface modification of the polymer samples placed on the bottom electrode.

It is seen in figure 6(a)that the voltage remains almost constant as discharge current was increased,this indicates that the discharge is operating in the normal glow discharge regime.In this regime,current density is constant as the discharge occurs over an increasing area of the electrode surface when current was increased.The breakdown voltages could also be determined from these plots.At 5.61 W,the average breakdown voltage was 280±10 V and-(300±10)V in the positive and negative half-cycle(each averaged from 4 to 5 half-cycles).At 20.6 W,the average breakdown voltage was 260±10 V and-(250±20)V in the positive and negative half-cycle.Overall,the magnitude of average breakdown voltage was|270±10|V.

Figure 7 shows plasma density(ne)to increase with applied powerPA.The range of applied power here corresponded to discharge powerPdisof 0.5-1 W.When the applied power is increased,the power transferred to the discharge(Pdis)increased,and more energy was transferred to the discharge.As a consequence,collision frequency among the particles increased,leading to more ionization collision and increase in the plasma density[34].The electron temperatureTeis between 2 and 3 eV at applied power from 5 to 35 W(corresponding to discharge power of 0.5-1 W).Although the electron temperature shows a slight decrement initially,the variation is insignificant as it is within the uncertainty range.

Figure 5.Discharge voltage and current waveforms,and plasma formation for two applied powers:(a)balanced plasma formation at applied power at 5.6 W,and(b)asymmetric plasma at applied power of 20.6 W.

Figure 8 shows the emission spectra of argon glow at discharge power of 0.8 W and 1.3 W.Other than the numerous atomic emission lines of argon that appear within the range of 680-850 nm,there are also molecular bands of nitrogen(C3Π→B3Π)and hydroxyl radical,OH(A2∑→X2Π).N2molecular band is contributed by the residual air in the vacuum chamber while OH species are due to the presence of moisture(argon feed with 99.999% purity contained＜3 ppm).Similar emission spectrum was reported in an earlier work at lower working pressure of 45 Pa[27].Mechanisms for the production of OH radicals involve collision of water vapour with electrons or dissociation by metastable argon[35]expressed in equations(3)and(4)below

Figure 6.V-I plots of the discharge at applied powers,(a)5.61 W and(b)20.6 W.Hysteresis is observed in the negative polarity at higher power.

Figure 7.Electron temperature Te and electron density ne at different applied powers.

Figure 8.Optical emission spectrum of 50 Hz argon glow discharge plasma at argon pressure of 120 Pa with discharge power of 0.8 and 1.3 W.

Figure 9.Dependence of WCA on discharge power for 5 min argon plasma treated PTFE surface at argon pressure of 120 Pa.

Figure 10.Comparison of low resolution XPS spectra for(a)untreated and(b)30 min treated at 0.56 W discharge power and(c)high resolution XPS scan of C 1s peak of untreated PTFE and(d)30 min treated at 0.56 W discharge power.

Figure 11.AFM images of(a)untreated PTFE,(b)plasma treated for 5 min and(c)30 min at argon pressure of 120 Pa and discharge power of 0.6 W.

Figure 12.Dependence of WCA and rms surface roughness of PTFE on treatment time at argon pressure of 120 Pa and 0.6 W discharge power.

Figure 13.WCA versus storage time,for PTFE samples treated at different treatment time and discharge power.

3.2.Surface modification and ageing effects

The static WCAs measured after 5 min of plasma treatment at different discharge powers are shown in figure 9.The discharge power is a more appropriate variable since it is the measured power dissipated across the electrodes that contributes effectively to the surface processes,while the applied power included large dissipative losses in the external components in the circuit.Initially,WCA of untreated PTFE surface was 114°±3° and it generally decreases with increasing discharge power of plasma treatment.The WCA dropped to 91°±4° at discharge power of 1.01±0.05 W(a 20% drop).At higher discharge power,more energy is transferred to the plasma to generate more active species(inferred from the increase in electron number densityneas shown in figure 7 as well as the increased intensity of the optical emission peaks as shown in figure 8).Their bombardment with the surface becomes more aggressive.This would lead to more C-F bonds being scissored off from the surface of PTFE,and hence,creating more active sites on the surface available for hydrophilic functional groups to attach to.Enhanced functionalization will lead to the drop in WCA(higher wettability).

The above finding is consistent with the XPS wide scan results presented in table 1,and the representative wide scan and narrow scan spectra for untreated and treated PTFE surface are shown in figure 10.The F/C ratio for untreated PTFE is close to the theoretical value of 2,which is based on the chemical structure of the polymer(CF2-CF2)n.At higher discharge power of 0.65 W,F/C ratio is lower than that of the untreated PTFE as well as the sample treated at lower discharge power of 0.52 W.With higher energy,more C-F bonds were scissored off from the surface,resulting in lower hydrophobicity of the surface[13,36].The 1 at.%of O 1s in the untreated PTFE sample is likely incorporated by the oxygencontaining liquid(acetone(C3H6O)and deionized water(H2O))that were used in the ultrasonic cleaning during the material preparation step.Treatment at low discharge power and shorter exposure time might have removed these initial oxygen(e.g.the dip in O/C ratio at 0.52 W,5 min treatment),and at higher power or longer exposure,oxygen-containing groups(e.g.OH radical evident from its emission line of figure 8)from water vapour in residual air could likely be incorporated(rise in O/C ratio presented in table 1).Presence of hydrogen-and oxygencontaining residual gases,in particular water vapour in argon plasma has been known to contribute to the surface chemistry in treatment of polymers[7,27,37,38].

Plasma treatment with longer time also increased the wettability of the PTFE surface,WCA is reduced further after being treated for 30 min consistent with the observed reduction of fluorine and increment of oxygen in the XPS results.Similar trend was reported by Carboneet al[17]for PTFE treated by remote cold atmospheric pressure argon plasma and Morraet al[37]in capacitively coupled radio-frequency(rf)plasma at 100 W for treatment time up to 2 min.For longer treatment time,accumulatively more bombardment of active species in plasma on the PTFE surface created more active sites on surface which allowed oxygen-containing functional groups to bond to the surface and thus increased the wettability.

The breaking of C-F and backbone C-C bonds is possibly activated by electrons,UV photons or argon metastables in the plasma through the following reactions[17]:

Table 1.Chemical compositions(in at.%)of untreated and treated PTFE in argon glow discharge at different discharge powers and treatment durations from XPS wide and narrow scan spectra.

Breaking of C-F bonds at low discharge power condition is negligible as the change in F/C ratio is not significant.However,high discharge power treatment of 0.65 W for 5 min,there is a 6% reduction of F/C ratio.On the other hand,a reduction of 4% is observed for the longer treatment time of 30 min with discharge power of 0.56 W.

These results are further affirmed from the deconvolution of the narrow scan spectra of C 1s peak(figures 10(c)and(d))that provides detailed information on the various fluorocarbon,hydrocarbon,and carbonyl/carboxyl groups.For the same duration of treatment of 5 min,the increment of CF3groups by 15%(from 5.2 at.%to 6.0 at.%)at discharge power 0.65 W as compared to 0.52 W,is presumably due to the chain scission of PTFE macromolecule and C-F bonds[39],followed by bonding of floating F atoms by the-CF2*(produced via equations(6)-(8))at the chain end such as:-CF2-CF2*+F*→-CF2-CF3.On the other hand,CF2group decreases by 3%(from 87.0 at.%to 84.0 at.%)for the higher discharge power of 0.65 W when compared to 0.52 W,while C-F,C=O and C-OH groups increase by 30%(from 4.4 at.%to 5.7 at.%),26%(from 2.3 at.%to 2.9 at.%)and 25%(from 1.2 at.% to 1.5 at.%)respectively.Fluorine-based polymers like PTFE are sensitive to electron attack.Also,from figure 7,at higher power,electron densityneis higher[40].So,at higher discharge power,more energetic discharge species will scissor C-F and C-C bonds in the PTFE polymer chains and create more active sites on the PTFE main polymer chains[35].The active sites will then combine with active oxygen species,which are available in the discharge plasma,to form oxygen-containing functional groups.

Comparing the longer treatment time of 30 min(0.56 W)to the 5 min treatment(0.52 W),the groups of CF,C=O and C-O increase by 50%(from 4.4 at.%to 6.6 at.%),43%(from 2.3 at.% to 3.3 at.%)and 8%(from 1.2 at.% to 1.3 at.%)respectively.When treatment time is extended,chain scission continues and creates a higher number of oligomeric segments,which contribute to increment of CF group[35].At the same time,more active sites are produced on the PTFE surface,followed by incorporation of oxygen species in discharge plasma to produce more oxygen-containing functional groups.These hydrophilic functional groups on PTFE surface are responsible for the enhancement of wettability of PTFE.

Change in surface morphology with treatment time is illustrated in the atomic force micrographs shown in figure 11.The rms surface roughness increased from 56(untreated)to 69 nm(5 min treatment)and 73 nm(30 min treatment)for samples treated in the argon plasma at discharge power 0.6 W(figure 12).While the surface roughness increased with treatment time,a corresponding reduction in WCA from 114°(untreated)to 106°(5 min treatment)and 93°(30 min treatment)was observed;implying that longer treatment time resulted in increased surface roughness and improved wetting.Argon ions/atoms are relatively heavy/large particles that are inert.Since no additional external biasing field was applied to the electrode,the ionic bombardment on the surface(during the positive half-cycle of the discharge voltage)would be of low energy and light etching of the surface could have occurred.With longer treatment time,more bombardment occurs leading to rougher surface.In the plasma etching process,the chain segments on the outer layers of the PTFE surface get ablated,presumably the C-C bonds in the polymer backbone are scissored off and subsequently-[CF2]n-fragments are released[38].The reduction of the F atoms on the PTFE surface would lead to increase in wettability of the surface.Similar result was reported by Pelagadeet al[25],of which an argon plasma generated by a bipolar pulsed power source was used to treat PTFE;the average surface roughness was reported to increase from 8.5 to 22.8 nm after 10 min of the plasma treatment corresponding to a decrease in contact angle from 76°to 60°.Saraniet al[35]also reported an increase in surface roughness of PTFE from 25 to 50 nm after 20 s treatment in an atmospheric pressure argon plasma jet;and a corresponding decrease in WCA from 110° to 89° was observed.

Ageing effect up to 50 d storage is shown in figure 13.The untreated and treated samples were stored in a sterilized disposable petri dish with cover in the laboratory at 27°C-28°C and relative humidity of～40%.The WCA did not revert to pristine untreated state.For the 5 min treated sample at lower discharge power 0.55 W,there was an initial slight increase in WCA up to 30 d(720 h)storage,after which it decreases.The slight hydrophobic recovery may be due to reorientation of the polar groups resulting from surface reconstruction that was produced during the plasma treatment towards the polymer bulk[41].It is speculated that at the lower discharge power,fraction of surface area covered by‘mobile’polar surface groups that could reorientate towards the polymer bulk exceeded the area covered by‘immobile’polar groups[42].Chatelieret alclassified the‘mobile’polar surface groups as polar surface groups attached to‘mobile’surface chain segments that can migrate to deeper than 1 nm of the surface,whereas the‘immobile’ones are attached to nearby cross-links that could not take part in polymer chain motions.On the other hand,Kolskáet al[24]reported hydrophobic recovery but it was not accompanied by decrease in oxygen content on the surface and suggested that it was not due to the migration of oxidized groups toward the polymer bulk.From their zeta potential analysis,the ageing was attributed to loss of low mass degradation products.

WCA for the other two samples(5 min,0.64 W and 30 min,0.59 W)decreased(became less hydrophobic)progressively up to about 20-30 d before recovering slightly in hydrophobicity but did not revert back to the measured WCA immediately after plasma treatment.The decrease in WCA of the treated surface as it ages during storage under ambient condition is likely to be caused by chemical reactions with component gases in the air storage medium.This is rather surprising as hydrophobic recovery in argon plasma treated PTFE surface usually ensued upon storage in air,as reported by other researchers who used different plasma treatment sources,for example,Valerio,Nakajima and Vasquez Jr[18]used a 700 W microwave plasma treatment source,Wilsonet al[38]used a 3.4 W capacitively coupled rf plasma,and Kolskáet al[24]an 8.3 W DC argon discharge.

In general,it can be said that the treated PTFE surface maintained its wettability over a storage period up to 50 d.This stable wettability effect after plasma treatment was also observed by Wojcieszaket al[22]in an argon plasma that was excited by pulse width modulation at 50 W,17 Pa for 1 min.Further tests are needed to elucidate the cause of retention of the hydrophilic effect upon storage of the low power plasma treated PTFE surface in this work as compared to those treated in plasma systems at much higher power.

3.3.Comparison:plasma systems and wettability change

The changes in WCA resulting from surface modification of PTFE using various plasma sources are listed in table 2 and compared to the results obtained in this work.

Different plasma systems of different electrical excitation scheme resulted in wettability change of varying degree.This could be observed even when comparing only the argon plasma systems but sustained at different excitation frequency,power and pressure with the treatment carried out under different powers and time durations.The largest drop in WCA was reported in a 3.16 W atmospheric pressure dielectric barrier discharge jet[21](72% reduction in WCA)after a short exposure of 90 s and the 8.3 W DC plasma[24](97%reduction in WCA)but at a much longer exposure time of 600 s.Both these plasma systems produced strong hydrophilic surface(WCA＜90°).The 25 W microwave excited plasma torch(WCA reduced by 18% in 600 s)[17]and 1 W capacitively coupled rf plasma(WCA reduced by 18% in 60 s)[41]produced comparable degree of improved wetting with PTFE samples treated in the 50 Hz AC plasma system(1 W discharge power)in this work(WCA reduced by 20%in 300 s).The WCA of the plasma treated PTFE in these three systems lie between 85° and 91° which is close to the value that defines whether it is good(WCA＜90°)or poor(WCA＞90°)wetting.A much poorer performance was reported in the 100 W inductively coupled plasma[15](WCA reduced by 7% to 110° in 300 s)whereby the surface finish remained hydrophobic(WCA＞90°).Although there is no consistent dependence of wettability change on power when comparing different plasma systems,increasing the power in the same plasma system did result in decrease in WCA[21,22]and was observed also in this work.In the same plasma system,increasing treatment time also resulted in decrease in WCA[21,24]as was observed in this work.Decrease in WCA was generally attributed to defluorination and grafting of hydrophilic oxygen-containing groups on the treated surface as well as increase in surface roughness[21,41],and this is consistent with the results presented using the 50 Hz AC plasma.

Adding water vapour to an argon rf plasma[15]promoted slightly improved wetting by 7%,while adding ammonia(Ar/NH3-H2O plasma)resulted in a strongly hydrophilic surface(improved by 80%).In this work,the OH radicals arising from residual air in the discharge chamber could have contributed to the increase in wetting of the treated PTFE samples.On the other hand,adding small amount of oxygen(0.1%)to argon plasma promoted hydrophobic effect as was reported in a plasma torch system[17].In general,oxygen plasma treatment[8,13]was shown to produce hydrophilic effect at low power and short treatment time.At higher plasma power and longer treatment time,hydrophobic effect was produced instead.However,an exception case was found,where hydrophilic effect was instead obtained in the pulsed oxygen plasma system(AC and DC mode)of Zhou,Sun and He[26],with stronger hydrophilic effect in the AC excitation mode.Both nitrogen[13]and air plasma treatment[19,20,23]generally produced improved wetting on PTFE surface,surface became more hydrophilic at longer treatment time.The hydrophilization process was attributed to defluorination and grafting of oxygen-containing polar species.

On the ageing effect of treated surface upon storage,most of the treated surfaces did not revert completely to the pristine surface state,sustaining improved wetting(albeit with some degree of recovery)within the storage period investigated[19,21,22,24],and this is also seen in this work.However,complete hydrophobic recovery after a much longer period of 3 months was reported by Karolyet al[20].The penetration depth of plasma modification of PTFE is speculated to be in the range of tens of nm to hundreds of nm.This is inferred from the reported morphological changes analyzed from AFM[8,15,35,41].Also,from XPS analysis,the depth of chemical composition changes could be probed to few nm scale range,while the ATR/FTIR analysis could probe up to the order of a μm[8].

Table 2.Comparison of various plasma sources and resulting water contact angle change after plasma treatment of PTFE surface.

Table 2.(Continued.)

4.Conclusions

The discharge behaviour of the 50 Hz argon glow discharge plasma is similar to that of a DC glow discharge except that it was sustained in a reversed polarity at each half period(10 m s).For discharge power of 0.5-1 W,the electron temperatureTeand densitynewere 2-3 eV and～108cm-3respectively.neincreased with discharge power and with increasing charge particle fluence,the wetting effect increased.Despite the low discharge power,a 20% drop in the static WCA was observed at 1 W for the treated PTFE surface.The resulting surface finish lies at the borderline hydrophobic/hydrophilic state.From XPS analysis,the increased wetting was attributed to incorporation of hydrophilic functional groups on the active sites on PTFE surface created after scission of C-F bonds by active species in the plasma,in agreement with those reported in other argon plasma systems.Increase in surface roughness was reflected in increase in wettability with treatment time,and this trend is consistent with that of PTFE treated in other argon plasma systems.The ageing study over a storage period of 50 d showed that the induced hydrophilic effect was retained.

Acknowledgments

This work was supported by the University of Malaya Postgraduate Research(PPP)(No.PG062-2016A)and RU Grant-Faculty Program(No.GPF042B-2018).

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