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Available online at www.sciencedirect.com

Applied Surface Science 254 (2008) 2882–2888

Studies on surface modification of poly(tetrafluoroethylene)

film by remote and direct Ar plasma

Wang Chena,b, Chen Jie-ronga,*, Li Rub

a School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, Chinab College of Textile and Materials, Xi’an Polytechnic University, Xi’an 710048, China

Received 6 June 2007; received in revised form 13 October 2007; accepted 13 October 2007

Available online 22 October 2007

Abstract

Poly(tetrafluoroethylene) (PTFE) surfaces are modified with remote and direct Ar plasma, and the effects of the modification on the

hydrophilicity of PTFE are investigated. The surface microstructures and compositions of the PTFE film were characterized with the goniometer,

scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results show that the remote and direct plasma treatments

modify the PTFE surface in morphology and composition, and both modifications cause surface oxidation of PTFE films, in the forming of some

polar functional groups enhancing polymer wettability. When the remote and direct Ar plasma treats PTFE film, the contact angles decrease from

the untreated 108–588 and 65.28, respectively. The effect of the remote Ar plasma is more noticeable. The role of all kinds of active species, e.g.

electrons, ions and free radicals involved in plasma surface modification is further evaluated. This shows that remote Ar plasma can restrain the ion

and electron etching reaction and enhance radical reaction.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Remote and direct Ar plasma; Poly(tetrafluoroethylene); Surface modification

1. Introduction

Surface modification of polymers by plasma treatment is

industrially attractive, as the technique is simple, easy to

implement, reliable, no pollution, and cost effective [1–5].

Many studies [6,7] have been reported using various plasma-

based approaches and process gases, which about investigations

of plasma surface modification technologies, the effects of

plasma treatment, the nature of the plasma environment, and

the mechanisms that drive the plasma–surface interaction.

Plasma treatment affects the polymer surface to an extent of

several hundred to several thousand angstroms. The bulk

properties of polymers, therefore, remain unchanged. Apart

from being a surface-sensitive modification technique, plasma

treatment does not give rise to toxic waste problems as in the

case of chemical treatment. Plasma containing electrons, ions,

and radicals can interact with polymer surfaces and modify

their chemical and physical properties. The plasma is capable

* Corresponding author.

E-mail address: jrchen@mail.xjtu.edu.cn (J.-r. Chen).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.10.029

of exerting four major effects [1,8–10], that is, surface cleaning,

surface ablation or etching, surface cross-linking, and

modification of the surface chemical structure, both in situ

and on subsequent exposure to the atmosphere. These effects

depend on the presence of active species in plasma. However,

so far researches on plasma surface modification have merely

been limited to a mixed atmosphere constituted solely by active

species [11–15]. How great is the contribution of the different

active species to surface modification?

Plasma is a mixture of electrons, ions, and radicals. These

species disappear in processes of the electron–positive ion

recombination, the positive ion–negative ion recombination,

and the radical–radical recombination. The rate constant of

these reactions is in the order of 10�7 and 10�33 cm3/s,

respectively [16]. Therefore, radicals can possess extremely

longer lifetime than electrons and ions. Taking advantage of the

different lifetime of various active particles such as electrons,

ions and free radicals, these active particles are separated in a

special plasma field and the super pure and high free radical

concentration is attained at the position away from the plasma

discharge region. This is the concept of a remote plasma

treatment [9,10,17,18], that is, the concentration of the ion and

C. Wang et al. / Applied Surface Science 254 (2008) 2882–2888 2883

electron is relatively low and the concentration of free radicals

is relatively high in remote Ar plasma. The article studies the

effects of remote and direct Ar plasma surface treatment on

poly(tetrafluoroethylene) (PTFE) films in terms of changes in

surface wettability and surface chemistry. The surface proper-

ties are characterized by the water contact angle measurement,

scanning electron microscopy (SEM) and X-ray photoelectron

spectroscopy (XPS). Finally the mechanism is analyzed, and

the role of all kinds of active species, e.g. electrons, ions and

free radicals involved in plasma surface modification is further

evaluated.

2. Experimental

2.1. Materials

The PTFE films used in this study are supplied by Fuxing

Fluorin Chemical Works Ltd. (China). Films of

25 mm � 50 mm are Soxhlet-extracted with acetone for 24 h

to remove any surface impurities. Clean films are dried under

vacuum at ambient temperature (22 8C) and stored in a

desiccator before use. Diiodomethane used is of analytical

grade. Deionized water is used in all experiments.

2.2. Remote plasma treatment

A self-designed reactor is used for the remote and direct Ar

plasma treatments of the PTFE samples, as shown in Fig. 1.

The reactor includes four parts—gas inlet, a reaction

chamber, a gas exhaust, a power supply and a matching

network (SY-500W power supply and SP-matcher made in

Micro-electronics Center, the Chinese Academy of Sciences).

The reaction chamber is a Pyrex glass tube (45 mm in

diameter, 1000 mm long), where inductance-coupling dis-

charge is applied. The Pyrex glass tube has a copper coil (nine

turns) for the energy input radio frequency (RF) power

(13.56-MHz frequency). The RF power is adjusted by a power

controller (SP-III model). The PTFE films are positioned at a

Fig. 1. Schematic structure of plasma reactor: (1) RF generator; (2) matching

system; (3) gas bottle; (4) valve; (5) mass flow meter; (6) inductance coil; (7)

reaction chamber; (8) sample; (9) vacuum gauge; (10) electromagnetic valve;

(11) vacuum pump; (12) ground protection.

constant distance of 0 (direct Ar plasma treatment) and 40 cm

(remote Ar plasma treatment) from the center of the copper

coil and exposed to the Ar plasma. First, the air in the reactor

is displaced with argon. Afterward, the reactor is evacuated to

approximately 1.3 � 10�2 Pa, and then the argon is intro-

duced into the Pyrex glass tube with a flow rate of 10–50 cm3/

min adjusted by a mass flow controller. The total pressure of

the plasma chamber is adjusted by the mass flow controller

and kept for 5 min. The argon flow of 10, 20, 30, 40 and

50 cm3/min corresponds to about the argon pressure of 13.3,

25.6, 34.5, 41.5 and 49.7 Pa, respectively. Then the plasma

was generated at a RF power of 30–180 W and the film is

exposed to the plasma for a time of 25–200 s. The purity of

argon is more than 99.99%.

2.3. Contact angle measurements

The static contact angles, characterizing surface wettability,

are measured immediately after finishing the plasma treatment

experiments to minimize the changes in the surface properties.

The contact angles of water on the PTFE film surface treated

with the remote and direct Ar plasmas are measured by the

sessile drop method using a contact angle meter with a

goniometer (Chengde, China; model JY-82). The readings are

stabilized and taken 50 s after dropping. To lessen the effect of

gravity, the volume of each drop is regulated to about 0.2 mL by

a micro-syringe. The measurement is carried out at a 20 8C and

humidity of 45% RH. The averaged value of the angles of both

sides of each drop is counted as one measurement. Each contact

angle is determined from an average of 10 measurements with a

standard deviation of 18.

2.4. Surface free energy measurement

The measurement of the contact angle between water and a

film surface is one of the easiest ways to characterize the

hydrophilicity of a film. When water is applied to the surface,

the outmost surface layers interact with the water. A

hydrophobic surface with low free energy gives a high contact

angle with water, whereas a wet high-energy surface allows the

drop to spread, that is, gives a low contact angle.

The untreated films, the remote and direct plasma-treated

films are analyzed for their hydrophilic properties by carrying

out water and diiodomethane contact angle measurements. The

liquids used in measuring the contact angle of the film are

shown in Table 1. Wu [19] thought that the surface free energy

(g) could be separated into a dispersing parameter (gd) and a

polar parameter (gp). This procedure leads a harmonic mean

equation to the Young equation. g, gd, and gp can be calculated

by solving the system of equations as follows:

g1ð1þ cos u1Þ ¼4g

p1g

ps

gp1 þ g

ps

þ 4gd1g

ds

gd1 þ gd

s

;

g2ð1þ cos u2Þ ¼4g

p2g

ps

gp2 þ g

ps

þ 4gd2g

ds

gd2 þ gd

s

;

gs ¼ gds þ gp

s

Fig. 3. Effect of plasma treatment power on the contact angle of water for PTFE

film (plasma condition: 100 s and 20 cm3/min).

Table 1

Dispersion, and polar components of surface free energy of reference liquids at

20 8C (�10�5 N cm�1)

Liquids gd gp g

Water 29.1 43.7 72.8

Diiodomethane 46.8 4.0 50.8

C. Wang et al. / Applied Surface Science 254 (2008) 2882–28882884

2.5. SEM analysis

The surface morphology of the PTFE film is examined using

scanning electron microscopy (SEM) in a JEOL instrument

(Model JSM-6700F, Japan). A 200 A layer of gold is sputtered by

vacuum evaporation on the PTFE sample surface prior to the

SEM measurement. The samples are then scanned at magnifica-

tion of 5000 times.

2.6. XPS analysis

The XPS measurements are made on a PH-5400 ESCA

System (Perkin-Elmer, US) using a Mg Ka X-ray source with a

pass energy of 89.45 eV. The X-ray source power was set to

400 W. The pressure in the analysis chamber is maintained at

8 � 10�6 Pa. The take-off angle is 458 with respect to the

sample surface.

3. Results and discussion

3.1. Contact angle of water on PTFE film surfaces treated

with the remote and direct Ar plasma

Figs. 2–4 show results for the contact angle of water on

PTFE surfaces modified by remote and direct Ar plasma as

function of the plasma treatment time, the plasma treatment

power, and the Ar flow. Regardless of the treatment conditions,

the PTFE films treated with the Ar plasma have smaller contact

angles than the original PTFE film, their surfaces becoming,

and remote Ar plasma treatment leading to a higher hydro-

philicity than the direct Ar plasma treatment (Figs. 2–4).

Fig. 2. Effect of plasma treatment time on the contact angle of water for PTFE

film (plasma condition: 100 W and 20 cm3/min).

Because the ion and electron etching effects are intense and the

concentration of free radicals is relatively low in direct Ar

plasma, the etching reactions with electrons and ions scarcely

occur and reactions with radicals occur predominately in

remote Ar plasma. This is an essential difference in plasma

chemistry between remote and direct plasma treatments. So

more polar groups are introduced to the surface in remote Ar

plasma, enhancing polarity of the PTFE films.

Fig. 2 shows that with an increase in the plasma treatment

time, the contact angles of PTFE film decrease rapidly from the

untreated 1078 to the treated 588. After the plasma treatment

time of 100 s, the decrease becomes small. Clearly, the longer

plasma treatment time cannot help the hydrophilicity increases.

From Fig. 3, the water contact angle decreases with increasing

the plasma treatment power up to 100 W. Beyond 100 W the

change slows down. The curves in Fig. 3 imply that a quantity

of Ar molecules acquire more energy resulting from the

increase of plasma treatment power, thus enhancing the

ionization of Ar and the average energy of active particles,

and hence further raising the possibility of the reaction and the

intensity of active particles with PTFE. As a consequence,

the effect of surface modification is reinforced. After 100 W,

Fig. 4. Effect of Ar flow on the contact angle of water for PTFE film (plasma

condition: 100 s and 100 W).

C. Wang et al. / Applied Surface Science 254 (2008) 2882–2888 2885

the surface reaction reaches equilibrium in the plasma

treatment time of 100 s and the Ar flow of 20 cm3/min.

Fig. 4 shows the effect of Ar flow on the water contact angle of

PTFE film. Since when the plasma treatment power is constant,

with an increase in the flow, the discharge intensity of gas is

weakened and the ionicity of Ar and the average energy of

active particles decreases, and the reaction chance and intensity

of active particles with PTFE are weakened. So the contact

angle enlarges.

3.2. Etching effects

The PTFE films are treated with the remote and direct Ar

plasma at 100 W and 20 cm3/min as a function of treatment

times of 25–200 s. The PTFE films, treated with the remote and

direct Ar plasma, are divided into two groups for the contact

angle measurements, respectively. Group A was the plasma-

treated PTFE films, and group B is the PTFE films treated with

the Ar plasma and then rinsed with acetone using an ultrasonic

washer for 5 min. Fig. 5 shows the contact angle change of

water on the two categorized PTFE film surfaces (groups A and

B) as functions of the treatment time. The contact angle change

is a difference between groups B and groups A for the contact

angle. Regardless of the remote and direct Ar plasma treatment,

the contact angle change increases with increasing plasma

treatment time. The PTFE films treated with the remote Ar

plasma have smaller contact angle change than with the direct

Ar plasma. A comparison of two curves in Fig. 5 shows a large

difference in the contact angle change between the remote and

direct Ar plasma treatment. The contact angles change is

strongly related to the remote and direct Ar plasma treatment.

These changes indicate that some products are formed on the

PTFE film surfaces by the Ar plasma treatment. Products are

removed from the PTFE film surfaces by the acetone rinsing.

As a result, a large difference in the contact angle results

between the PTFE film surfaces before and after the acetone

rinsing. We believe that the products may be small molecules

that are formed by the degradation reactions of the PTFE

polymers. When electrons and ions with high energy bombard

Fig. 5. The contact angle change of water with plasma treatment time in PTFE

films (plasma condition: 100 W and 20 cm3/min).

PTFE film surfaces during the plasma treatment, C–F bond

scission and defluoration occur on the PTFE polymer chains

and surface. As a result, etching reactions occur and

degradation products, which are pieces of the PTFE polymer

chains and of low molecular weight, are deposited on the PTFE

film surfaces. From this viewpoint, the difference in the contact

angle between the PTFE film surfaces before and after the

acetone rinsing may be evaluated as an indicator of the extent of

the etching reactions occurring during the Ar plasma treatment.

In this sense, the direct Ar plasma treatment leads to

hydrophilic surface modification but with heavy etching

reactions occurring. On the other hand, the remote Ar plasma

treatment is more effective in hydrophilic surface modification

than the direct Ar plasma treatment, and more limited etching

reactions occur in the remote plasma treatment process.

From these results we can conclude that the direct Ar

plasma treatment is effective in hydrophilic surface mod-

ification, but heavy etching reactions occur during the plasma

treatment process. The directly plasma-treated PTFE films

have damaged surfaces, which are etched and contain many

degradation products that are easily removed by acetone

rinsing. On the other hand, the remote Ar plasma treatment

makes PTFE film surfaces hydrophilic without heavy etching

reactions. Deposition of degradation products on the surface

is small.

3.3. Surface free energy

Table 2 shows the contact angle for water, the surface free

energy and its components of the PTFE film, which are remote

and direct Ar plasma treatment. It shows that surface free

energy of the PTFE film is increased after modification by

plasma treatment. Compared with untreated film, surface free

energy (g) increases more than twofold for remote and direct

plasma-treated PTFE film. The surface free energy’s polar (gp)

component increases from 10.4% to about 47.9 and 52.3%,

respectively. Dispersion (gd) decreases from 89.6% to about

52.1 and 47.7%, respectively. Accordingly, it can be concluded

that the improvement of surface wettability largely lies in the

increase of the gp.

3.4. Surface morphology of remote and direct Ar plasma-

treated PTFE film surface

When a polymer surface is exposed to plasma containing

electrons, ions, and radicals, two main reactions occur

simultaneously on the polymer surface: One is the introduction

of functional groups such as carbonyl and hydroxyl. Radicals in

plasma contribute mainly to the formation of functional groups.

The other is the degradation of polymer chains to products with

low molecular weight. Ions and electrons in plasma mainly

initiate the degradation reactions. The former reaction

contributes to surface modification, but the latter reaction

fewer contributes to surface modification. As long as plasma is

used to develop reactive species for modification of polymer

surfaces, the degradation process is unavoidable during the

modification reactions. Therefore, to perform effective mod-

Table 2

Contact angle for water and surface energy results of treated PTFE samples

Sample Surface energy (�10�5 N cm�1)

Contact angle (8) gd gp g gd/g (%) gp/g (%)

Untreated 108.0 19.8 2.3 22.1 89.6 10.4

Direct plasma treated 65.2 21.4 19.7 41.7 52.1 47.9

Remote plasma treated 58.0 22.5 24.7 47.2 47.7 52.3

Plasma condition: 100 s, 100 W and 20 cm3/min.

C. Wang et al. / Applied Surface Science 254 (2008) 2882–28882886

ification, a key is how to accelerate the introduction reaction

without the degradation reaction.

The contact angle measurement shows that remote Ar

plasma treatment produces a higher hydrophilicity than the

direct Ar plasma treatment. The contact angle change

measurement shows that remote Ar plasma treatment produces

a lighter etching reaction than the direct Ar plasma treatment.

One of the main differences between the remote and direct Ar

plasma is the relative concentration between free radicals and

charged species such as ions and electrons. The free radicals in

the remote Ar plasma have higher concentration than the

charged species. On the other hand, both charged species and

radicals in the direct Ar plasma have high concentration.

Therefore, we expect that the surface modification to the remote

Ar plasma will prevent the PTFE film surface from degrading

initiated by heavy collision of the charged species. Surface

morphology of the PTFE film treated with the remote and direct

Ar plasma is examined by scanning electron microscopy (SEM)

in order to investigate how the PTFE film surface is damaged by

the plasma treatment.

Fig. 6 shows typical SEM pictures for the remote and direct

Ar plasma-treated PTFE film surfaces at 100 W and 20 cm3/

min for 100 s. In a comparison of their surface morphology, the

remote Ar plasma produces less damage on the PTFE film

surface; its surface morphology is similar to that of the original

PTFE film. On the other hand, the direct Ar plasma injures the

PTFE film surface, and the surface morphology is apparently

different from that of the original PTFE, but is somewhat

rougher. This comparison shows that less of an etching reaction

occurs in the remote Ar plasma treatment, and heavy etching

reactions occur in the direct Ar plasma treatment. These results

coincide with the contact angle change of water. Therefore, we

conclude that the remote Ar plasma does not initiate remarkable

Fig. 6. SEM of (a) the untreated PTFE; (b) the direct plasma-treated PTFE; (c) the r

etching effects on the PTFE film surface with hydrophilic

modification. On the other hand, the direct Ar plasma initiates

heavy etching effects on the PTFE film surface with hydrophilic

modification.

3.5. Chemical composition of remote and direct Ar plasma-

treated PTFE film surface

The XPS spectra for various samples are shown in Fig. 7.

The samples for the analysis are the PTFE films treated with the

remote and direct Ar plasma at 100 W and 20 cm3/min for

100 s. As can be seen in the XPS survey spectra in Fig. 7(a), the

untreated PTFE surface shows a C1s peak at 291.7 eV and a F1s

peak at 689.6 eV due to CF2 units [20]. In the spectra of the

remote and direct Ar plasma-treated PTFE, an O1s peak at

532.1 eV is observed. It can be seen that a small amount of

oxygen moieties is incorporated (Table 3) after Ar plasma

treatment and subsequent exposure to air. At the same time, the

amount of carbon increases and the amount of fluorine

decreases. From Table 3 it can be seen that the F/C atomic ratio

after remote and direct Ar plasma treatment subsequently

decreases from 1.97 to 1.44 and 1.35, while the O/C atomic

ratio increases from 0 to 0.086 and 0.059, respectively. The N

atomic percentage comes from the contamination from the

nitrogen gas in air. This result shows a bigger O/C atom ratio

with remote Ar plasma treatment (the O/C atom ratio is 0.086)

than direct Ar plasma treatment (the O/C atom ratio is 0.059).

As shown by the O1s spectra in Fig. 7(b), compared with

untreated film, the remote and direct Ar plasma-treated film

contains an O1s peak at 532.1 eV. The O1s spectra show a

stronger and wider peak with remote Ar plasma treatment than

direct Ar plasma treatment. As can be seen in the O1s separated

spectra in Fig. 8, the remote and direct argon plasma-treated

emote plasma-treated PTFE (plasma condition: 100 s, 100 W and 20 cm3/min).

Fig. 7. The XPS spectra of PTFE samples: (a) plasma-untreated; (b) direct argon plasma; (c) remote argon plasma (plasma condition: 100 s, 100 W and 20 cm3/min).

Fig. 8. The separated XPS spectra O1s of (a) the direct plasma-treated PTFE; (b) the remote plasma-treated PTFE (plasma condition: 100 s, 100 W and 20 cm3/min).

C. Wang et al. / Applied Surface Science 254 (2008) 2882–2888 2887

PTFE films show good O1s spectra for the decomposition. The

spectra are decomposed into two components, which appear at

532.3 and 533.6 eV. The first component at 532.3 eV is

assigned the C O groups, and the other is assigned to the C–O

groups. The remote and direct argon plasma treatments lead to

distribution of oxygen functionalities, increase in C O groups

and C–O groups. As shown also in Fig. 8, the C O and C–O

peak show a stronger and wider peak with remote Ar plasma

treatment than direct Ar plasma treatment. Some polar

functional groups (oxygen-containing functional groups) are

introduced onto the PTFE surface. The surface wettability of

PTFE film is improved by remote and direct Ar plasma, and the

remote Ar plasma yielded higher hydrophilicity than the direct

Ar plasma.

Table 3

Surface composition of PTFE samples measured by XPS

Treatment Atomic percentage (%) Elemental

ratios

F C O N F/C O/C

Untreated 66.3 33.7 – – 1.97

Direct plasma treated 55.9 41.5 2.4 0.2 1.35 0.059

Remote plasma treated 56.9 39.5 3.4 0.2 1.44 0.086

Plasma condition: 100 s, 100 W and 20 cm3/min.

4. Conclusions

PTFE surfaces are modified with remote and direct Ar plasma,

and the effects of the modification on the hydrophilicity of PTFE

are investigated. The remote and direct Ar plasma treatment

produces a noticeable decrease in the contact angle, which is

mainly due to the introduction of some polar functional groups

(oxygen-containing functional groups) into the PTFE surface.

The remote Ar plasma gives rise to higher hydrophilicity than the

direct Ar plasma. The hydrophilicity depends on the plasma

treatment time, the plasma treatment power, and the Ar flow. The

contact angles of PTFE film decrease from the untreated 108–588when the remote Ar plasma treatment time is set at 100 s, the

plasma treatment power at 100 W, and the Ar flow at 20 cm3/min.

The remote Ar plasma does not cause notable etching effects on

the PTFE film surface with hydrophilic modification. In contrast,

the direct Ar plasma causes heavy etching effects on the PTFE

film surface with hydrophilic modification. Remote plasma

treatment can enhance radical reaction and restrain electron and

ion etching effects.

Acknowledgements

The authors thank the financial support of the National

Natural and Science Foundation Council of China 30571636

C. Wang et al. / Applied Surface Science 254 (2008) 2882–28882888

and 20174030, the specialized research Fund for the Doctoral

Program of Higher Education 20060698002, the key Scientific

Technique item of Shaanxi province 2003K10-G61, and the key

Scientific Technique item of Xi’an city GG06049.

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