SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 28, 16–19 (1999)

Investigation of the Surface Structures andDynamics of Polyethylene Terephthalate (PET)Modified by Fluorocarbon Plasmas

L. Zhang,1* W. S. Chin,1 W. Huang2 and J. Q. Wang3

1 Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 1192602 Institute of Materials Research and Engineering, Lower Kent Ridge Road, Singapore 1192603 School of Chemical Engineering and Materials Science, Beijing Institute of Technology, Beijing 100081, People’s Republicof China

Polyethylene terephthalate (PET) surfaces were treated with CH4/CF4 plasmas and the surface structuresas well as the dynamic behaviour of the modified surfaces after water immersion were studied using angle-dependent x-ray photoelectron spectroscopy (XPS). It was found that the fluorocarbon plasma treatmentresulted in the formation of some fluorine-containing groups, which improve the surface hydrophobicity ofthe PET films. The extent of surface modification is dependent on the molar ratio of the CH4/CF4 feed gasesused in the plasma. The hydrophobicity of the modified PET films decreased after they were immersed inwater, partly as a result of the ‘turnover’ or movement of the fluorine-containing groups from the surfaceto the bulk. From XPS analysis, it was found that the movement of the fluorine-containing groups dependson the modified surface structures. The turnover rates can be described by a decay constantk of surfacedynamics; the larger the k value, the faster the movement of the fluorine-containing groups. Experimentsshowed that the degree of surface modification is higher and is maintained for a longer period for CH4/CF4

mixed plasmas as compared to that treated by pure CF4 plasma. The effect of the molar ratio of CH4/CF4

feed gases and its relationship with the movement of the fluorine-containing surface groups were discussed.Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: CH4/CF4 plasma; PET; angle-dependent XPS; surface dynamics; decay constant; hydrophobicity

INTRODUCTION

Owing to the many advantages of cold plasma processing,e.g. simple solid-phase reaction, less pollution etc., thetechnique has attracted much interest in recent years.1,2

Through plasma modification, the surface properties ofmaterials, such as hydrophilicity,3 hydrophobicity,1,2,4 – 6

biocompatibility,7 etc., can be improved without impairingthe other desired properties of the bulk.

Polyethylene terephthalate (PET) has gained widespreadapplications due to its good mechanical strength and hard-ness, good chemical resistance, high dielectricity, etc.However, it has poor hydrophobicity, poor biocompati-bility and is not very resistant to wear. The latter problemhas been suggested to be related to the easy movement ofthe surface groups and the flexible main chains of PET.8

Plasma treatment has been used to modify the surfaceproperties of PET in a number of studies.1,2,4,9

A very important subject on the surface modification ofpolymers is the dynamic behaviour of the modified prop-erties, i.e. the surface properties of the modified polymersmay change with the environment, temperature or time.For example, the modified hydrophobicity of PET mayalter with time when in contact with water.2,5,6,10,11 This is

* Correspondence to: L. Zhang, Department of Chemistry, NationalUniversity of Singapore, 10 Kent Ridge Crescent, Singapore 119260.E-mail: chmzl@nus.edu.sg

because, unlike inorganic materials, polymer surfaces con-tain many flexible segments and groups that may respondto the changing environment. It is thus important to inves-tigate the surface dynamic behaviour before any surfacetreatment of polymers for real applications.

In this paper, the behaviour of PET surfaces treated withdifferent CH4/CF4 plasmas is investigated. Fluorocarbonplasmas generated with mixtures of fluoro- and hydroflu-orocarbon are now widely used and it has been foundthat different molar ratios of the CH4/CF4 feed gases willresult in rather different properties of the treated surfaces.Angle-dependent XPS was employed to study the surfacecompositions and the dynamic behaviour of the surfacegroups after immersion in water.

EXPERIMENTAL

Polyethylene terephthalate (PET) films of dimension6 cmð 6 cm and 0.1 mm thickness were soaked inchloroform followed by deionized water, respectively, for3–4 h at room temperature. After washing, XPS of thecleaned surfaces showed no other elements apart fromcarbon and oxygen.

The cleaned PET films were plasma-treated for 10 minin a home-made plasma processor. Plasma was gener-ated with parallel plate electrodes of 35 mm separationoperated at a frequency of 13.56 MHz and a power of60 W. The precursor gases, CF4 and CH4, were fed into

CCC 0142–2421/99/130016–04 $17.50 Received 30 November 1998Copyright 1999 John Wiley & Sons, Ltd. Revised 14 January 1999; Accepted 18 January 1999

PET MODIFIED BY FLUOROCARBON PLASMAS 17

the plasma processor with their flow rates controlled sep-arately by a mass flow-meter. The flow was monitoredin such a way that the composite parameterW/FM waskept at 0.1 GJ kg�1, as suggested by Yasudaet al.,2 whereW, F andM are, respectively, the discharge wattage ofr.f. power supply, the molar flow rate (in SCCM) and themolecular weight of the monomer.

Angle-dependent XPS (Al K̨) measurement wasrecorded on a Perkin-Elmer PHI 5300 ESCA spectrometer(12.5 kV, 20 mA). The vacuum system was maintainedat <1ð 10�9 Torr throughout all measurements. Spectrawere taken at take-off angles of 10°, 45°, and 90° withrespect to the surface. All binding energies (BEs) werecalibrated to adventitious carbon at 284.6 eV.

To study the dynamics of the surface fluorine groups,the plasma-treated PET films were immersed in deionizedwater for different durations and immediately transferredto the XPS vacuum chamber for measurement.

RESULTS AND DISCUSSION

Effect of different CH 4/CF4 molar ratios

Figure 1 shows two typical C 1s spectra of a PETsurface treated with CH4/CF4 plasma and an untreated

Figure 1. The XPS C 1s spectra measured at � D 45° for a PETsurface modified using plasma of: (a) CH4/CF4 D 1/20; (b) pureCF4; (c) The C 1s spectrum of the original PET film.

PET surface for comparison. Thus, from the shift to higherBE and a change in peak shape, it is clear that the treatedsurface has been modified with some electronegative com-ponents. This shift is expected to arise from fluorinebecause a main F 1s peak is detected in these cases. Theextent of surface modification may thus be discussed interms of the F/C ratio from XPS.

Different CH4/CF4 molar ratios in the plasma resultedin different extents of surface modification, as illustratedin Table 1. Thus, in general, the F/C ratio of the PETsurface increases with an increase in the amount of CF4

gas. The value reaches its highest at CH4/CF4 D 1/25and decreases for pure CF4 plasma.

The exceptional behaviour of the pure CF4 plasma-treated surface is also indicated by the angle-dependentresults shown in Table 1. Although the F/C ratios eitherdecrease gradually or remain constant with increasingtake-off angle for all the other cases, that of the pure CF4

plasma decreases drastically. This seems to suggest thatsurfaces treated with the various CH4/CF4 plasmas havea rather thick and homogeneous surface layer but thattreated with pure CF4 plasma is probably much thinner orheterogeneous. A similar observation has been reportedand was suggested to arise from the different reactionmechanisms of fluorocarbon species in the plasma.1,2,4

Thus, in the presence of hydrogen species, extraction ofHF from the surface may occur and lead to a higherdegree of cross-linking on the surface. In the case of pureCF4 plasma, the fluorine-containing species in the plasmacould only modify the PET surface through grafting orfluorine insertion.12

The degree of cross-linking on the modified surfacesmay be determined from the fitted C 1s components.Thus, five types of carbon components were consistentlyfitted with Gaussian–Lorentzian functions. These can beassigned, in order of decreasing BE, to –CF3, –CF2,–CF,>C< and C–C (or C–H), respectively, as shown inTable 2.4,13 Although these BEs fall within the range ofother PET functionals, such as –CF2O–, –CFO–,>C Oor –C–O–, the contribution of the latter is expected to

Table 1. The F/C ratio estimated from XPS for differentmolar ratios of CH4/CF4 feed gases and take-offangles

Take-off angle Molar ratio of CH4/CF4 feed gases(degrees) 1 : 5 1 : 10 1 : 15 1 : 20 1 : 25 Pure CF4

10° 0.20 1.39 1.51 1.63 1.70 1.3745° 0.20 1.37 1.45 1.53 1.57 1.1190° 0.20 1.34 1.40 1.50 1.58 0.83

Table 2. Curve-fitting of C 1s XPS (q = 45°) for PET surfaces modified using plasma of differentCH4/CF4 molar ratio

CH4/CF4

molar ratio CF3 CF2 CF >C< C C/C H

1/5 0 292.0(5.4%) 289.7(9.4%) 287.2(20.6%) 284.6(64.6%)1/10 293.6(21.1%) 291.5(25.2%) 289.2(22.8%) 287.1(25.7%) 284.6(5.2%)1/15 293.8(21.4%) 291.6(29.2%) 289.4(22.3%) 287.3(23.6%) 284.6(3.5%)1/20 293.9(22.9%) 291.9(31.0%) 289.7(22.4%) 287.5(20.3%) 284.6(3.4%)1/25 293.8(23.8%) 291.8(32.0%) 289.7(21.4%) 287.3(20.0%) 284.6(2.8%)Pure CF4 293.4(13.0%) 291.7(24.6%) 289.8(23.1%) 287.7(10.1%) 284.6(29.2%)

Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 28, 16–19 (1999)

18 L. ZHANG ET AL.

Figure 2. Amount of ( CF C >C<) vs, take-off angle:(1) CH4/CF4 D 1/5; (2) CH4/CF4 D 1/10; (3) CH4/CF4 D 1/15;(4) CH4/CF4 D 1/20; (5) CH4/CF4 D 1/25; (6) pure CF4 plasma.

be minor because the XPS O/C ratios were consistentlyfound to be<0.1 at take-off angle of 45°.

The appearance of –CF and>C< components indi-cates the presence of cross-linking on the surface of thepolymer.4,14 In order to estimate the degree of cross-linking, the total XPS area of these two components isplotted against the take-off angle as shown in Fig. 2.It can be seen that the slope of these plots varies withthe molar ratio of CH4/CF4 used in the plasma. Thus,within 1/25< CH4/CF4 < 1/10, the percentage area of(–CFC>C<) is large and increases with increasing take-off angle. For a CH4/CF4 molar ratio of 1/5, the amount of.–CFC >C</ is relatively small and stays almost con-stant. In a reverse trend, the amount of.–CFC >C</decreases with increasing take-off angle for pure CF4

plasma. The above observation illustrates that the opti-mum CH4/CF4 molar ratio should be selected from therange 1/25–1/10 in order to produce a higher degree ofcross-linking. At a CH4/CF4 ratio of<1/5, dual functionsplayed by hydrogen species result in a low degree of cross-linking.4 Although pure CF4 plasma may still producesome degree of cross-linking, the extent of such an effectis probably very small beyond the topmost surface layer.

Dynamic behaviour of the modified PET surfacesin contact with water

In order to study the dynamic behaviour of the modifiedPET surface, XPS measurement is performed after thefilms have been immersed in deionized water for differentdurations. Some typical changes in the C 1s spectraare shown in Fig. 3. It is apparent that although theenvelope of peaks at high BE attenuates with the lengthof immersion time, the peak at¾285 eV reappears justafter 10 s of contact with water and its intensity increasesfurther with the length of immersion time. After 35 minof contact with water, the peaks at high BE have almostvanished.

Two possible mechanisms may account for attenuationof the C 1s high-BE component—thus, the fluorine-containing groups could have dislodged themselves

Figure 3. The XPS C 1s spectra measured at � D 10° fora PET surface modified by plasma of CH4/CF4 D 1/20 vs.immersion time.

Figure 4. The F/C ratio calculated at different take-off anglesfor a PET surface modified by plasma of CH4/CF4 D 1/25 vs.immersion time.

in some way and diffused into water,5 or thefluorine-containing groups on the surface may move or‘turnover’ into the bulk during immersion due to theirhydrophobicity.6 Evidence for the latter mechanism canbe obtained using angle-dependent XPS studies. In Fig. 4,the F/C ratios recorded at different take-off angle areplotted against the duration of immersion for CH4/CF4 D1/25. In general, the F/C ratios decrease with immersionperiod. It is noted, however, that although the F/C ratioobtained at a 90° take-off angle is initially lower thanthose obtained at glancing angles (� D 10° and 45°), the

Surf. Interface Anal. 28, 16–19 (1999) Copyright 1999 John Wiley & Sons, Ltd.

PET MODIFIED BY FLUOROCARBON PLASMAS 19

values become higher after 60 s of immersion in water.This observation clearly illustrates that the ‘turnover’mechanism is operative to some extent.

Similar plots and observations are obtained for theother plasma-treated PET surfaces. In general, the F/Cratio for a surface modified by pure CF4 plasma is foundto decrease at a much faster rate compared to thosemodified by CH4/CF4 mixed plasmas. This is in agreementwith the higher degree of cross-linking produced in thelatter cases.

The dynamic behaviour of the polymer surface groupsmay be described by first-order kinetics from the relax-ation theory of polymers

At D A0e�kt

In order to describe the rate of ‘turnover’ for the fluorine-containing groups in our case, the decay constantk isestimated from the initial rate of decrease of the F/C ratios.The estimated decay constants for two typical CH4/CF4

plasmas and for the pure CF4 plasma are presented inTable 3. First, it is found thatk is always larger forsurfaces treated with pure CF4 plasma. Furthermore, thedecay constant for such surfaces is reducing more drasti-cally with increasing take-off angle. These results indicatethat fluorine-containing groups on the surface treated bypure CF4 plasma ‘turnover’ faster than those treated bythe CH4/CF4 mixed plasmas. This will be expected if themodified surface of the former is less homogeneous andcontains a lower degree of cross-linking structures.

When the molar ratio of CH4/CF4 is 1/25, the compet-ing actions of surface polymerization and surface etchingwill be almost balanced.4 The amount of HF extrac-tion is so large in this situation that the degree ofcross-linking becomes a maximum. The movement of

Table 3. Decay constantk estimated fromthe rate of change in F/C ratiofor different modified PET surfaces(×10−2)

Take-off angle Molar ratio of CF4/CH4

(degrees) 1/10 1/25 Pure CF4

10 1.7 1.4 3.445 1.2 1.1 2.090 1.1 1.1 1.3

fluorine-containing groups will thus be a minimum in thiscase andk will be the lowest, as indicated in Table 3.

CONCLUSION

It is shown that surface modification by fluorocarbonplasmas can improve the surface hydrophobicity of PETand the modified surface structures depend on the molarratio of the CH4/CF4 feed gases. Although mixed CH4/CF4

plasmas produce a homogeneous and highly cross-linkedsurface, pure CF4 plasma is found to give a shallowerand more heterogeneous surface. When the modified PETsurfaces are in contact with water, the fluorine-containinggroups seemed to ‘turnover’ from the surface into thebulk. The surface groups have the least movement ona surface treated with a plasma of CH4/CF4 D 1/25,suggesting the maximum degree of cross-linking andhydrophobicity of the surface. Finally, it is noted thatalthough CH4/CF4 plasma is effective in improving thehydrophobicity of a PET film, the surface dynamics arerather high and the surface properties may change easilywith the environment.

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Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 28, 16–19 (1999)