www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 4599–4606

Studies on surface graft polymerization of acrylic acid onto

PTFE film by remote argon plasma initiation

Chen Wang a,b, Jie-Rong Chen a,*a School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China

b College of Textile and Materials, Xi’an Polytechnic University, Xi’an 710048, China

Received 18 August 2006; received in revised form 8 October 2006; accepted 8 October 2006

Available online 11 December 2006

Abstract

The graft polymerization of acrylic acid (AAc) was carried out onto poly(tetrafluoroethylene) (PTFE) films that had been pretreated with

remote argon plasma and subsequently exposed to oxygen to create peroxides. Peroxides are known to be the species responsible for initiating the

graft polymerization when PTFE reacts with AAc. We chose different parameters of remote plasma treatment to get the optimum condition for

introducing maximum peroxides (2.87 � 10�11 mol/cm2) on the surface. The influence of grafted reaction conditions on the grafting degree was

investigated. The maximum grafting degree was 25.2 mg/cm2. The surface microstructures and compositions of the AAc grafted PTFE film were

characterized with the water contact angle meter, Fourier-transform infrared spectroscopy (ATR–FTIR) and X-ray photoelectron spectroscopy

(XPS). Contact angle measurements revealed that the water contact angle decreased from 1088 to 418 and the surface free energy increased from

22.1 � 10�5 to 62.1 � 10�5 N cm�1 by the grafting of the AAc chains. The hydrophilicity of the PTFE film surface was greatly enhanced. The

time-dependent activity of the grafted surface was better than that of the plasma treated film.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Poly(tetrafluoroethylene); Remote argon plasma; Acrylic acid; Graft polymerization

1. Introduction

Poly(tetrafluoroethylene) (PTFE) materials are applied

widely in many areas due to their excellent bulk properties,

such as low frictional coefficient, high stability against heat and

chemical reagents, and high electric resistance [1,2]. However

their applications are hampered in many cases because of its poor

wettability and adhesion property with other materials [3–5]. For

that reason, there is a strong demand to tailor their surface

properties for specific applications. Different surface modifica-

tion strategies are developed [6–10]. Recently, surface modifica-

tion of polymer material via plasma treatment has attracted more

and more attention. 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

* Corresponding author. Tel.: +86 29 82664818.

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

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.10.014

initiate the degradation reactions [11]. The former reaction

contributes to surface modification, but the latter reaction never

contributes to surface modification. As long as plasma is used to

develop reactive species for modificationof polymer surfaces, the

degradation process is unavoidable during the modification

reactions [12,13]. Therefore, to perform effective modification, a

key is how to accelerate the introduction reaction without the

degradation reaction.

In the remote plasma treatment, reactions of radicals with the

polymer surfaces will occur predominantly, but electron and ion

bombardment will occur scarcely [14]. What is the concept of a

remote plasma treatment? The polymer sample in the remote

plasma treatment is positioned away from the plasma zone, and

the polymer sample in the direct plasma treatment is positioned

just in the plasma zone. In our earlier reports [15,16], the remote

argon plasma treatment is distinguished from the direct argon

plasma treatment by the degradation occurring on the PTFE and

PVC film surface. The remote argon plasma treatment is not

effective in the surface modification, but it never injures the

polymer film surface. The direct argon plasma treatment is

effective, but it injures the film surface by degradation reactions.

Therefore, the remote plasma treatment is an adequate procedure

C. Wang, J.-R. Chen / Applied Surface Science 253 (2007) 4599–46064600

for surface modification. It causes less polymer degradation on

the film surface.

The surface modification of PTFE films by plasma treatment

lead to improvement in wettability and adhesion. However, the

wettability of plasma treated PTFE is gradually lost over time.

In order to eliminate the time-dependent effects of plasma

treatment, one method is to further modify via surface grafting

copolymerization. The plasma-induced graft polymerization of

vinyl monomers has been found to be an extremely attractive

method of chemically modifying the surfaces of polymeric

materials [17–21]. Polymers may be treated with an inert-gas

plasma and then exposed to oxygen to generate peroxide active

species that may initiate the grafting of a desired monomer. An

interesting aspect of plasma-induced grafting is that the

changes are confined to the depth of a few nanometers at the

surface without influencing the bulk of the material. As a result,

the mechanical properties of the material remain unaffected.

This opens up vast possibilities for designing and developing

surfaces of scaffolds with tailored chemical functionality and

morphology suitable for protein and cell interaction. There

have been a number of studies on the graft polymerization of

monomers on various polymers by plasma exposure with the

objective of developing functional interfaces for the immobi-

lization of biomolecules. In one of these studies, Sun [22] has

pointed out that the PTFE film was grafted with poly(acrylic

acid) (PAAc), and this was followed by a immobilization of

64.5 mg/cm2 of chitosan. The chitosan-immobilized PTFE

films were as blood-compatible materials.

In this study, acrylic acid (AAc) was grafted by

copolymerization onto the remote plasma pretreated PTFE

film surface. The influence of the remote plasma treatment

conditions on the peroxide concentration and the grafting

reaction conditions on the grafting degree were discussed. The

surface microstructures and compositions of the AAc-grafted

PTFE film were characterized with the water contact angle

meter, Fourier-transform infrared spectroscopy (FTIR) and X-

ray photoelectron spectroscopy (XPS).

2. Experimental

2.1. Materials

The PTFE films used in this study were supplied by Fuxing

Fluorin Chemical Works Ltd. (China). Films of

25 mm � 50 mm were Soxhlet-extracted with acetone for

24 h to remove any surface impurities. Clean films were dried

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

desiccator before use. 1,1-diphenyl-2-picryhydrazyl (DPPH)

was purchased from Sigma–Aldrich. The monomer AAc was

supplied by Xi’an Chemical Works Ltd. (China). The AAc was

purified by distillation under vacuum to remove impurities and

stabilizers. Deionized water was used in all experiments.

2.2. Remote plasma graft polymerization

The remote plasma graft polymerization of AAc on the

PTFE film surface was carried out in two steps: the formation of

peroxides on the PTFE surface by the treatment of remote argon

plasma and the graft polymerization of AAc from the

peroxides. The PTFE film surface was treated for 10–300 s

with a remote argon plasma to form carbon radicals on the

PTFE surface and was subsequently exposed to air for 5 min to

modify them to peroxides, which were able to initiate the graft

polymerization. A self-designed reactor was used. We called

the ‘‘Remote plasma’’ reactive system, which was described in

our earlier articles [15,16]. The reactor includes four parts—gas

inlet, reaction chamber, gas exhaust, power supply and

matching network (SY-500 W power supply and SP-II matcher

which are made in Chinese Academy of Science Microelec-

tronics Center). The reaction chamber is Pyrex glass tube

(45 mm in diameter, 1000 mm long), where inductance-

coupling discharge is applied. The PTFE films were positioned

on the carrier to be treated by remote argon plasma. The purity

of argon was more than 99.99%. The conditions of the

treatment were at an RF (radio frequency) power of 30–180 W,

plasma treatment time of 0.5–3.5 min, and argon flux of

20 cm3/min.

The PTFE film treated with remote argon plasma and exposed

to air was placed in the three-necked round-bottomed flasks

containing an aqueous AAc solution of a required concentration.

(All AAc concentrations are volume percentage in water in this

article.) The flasks with stirring and cooling were then placed in a

constant-temperature water bath for a specified period. The graft

polymerization reactions were conducted under nitrogen atmo-

sphere. After the graft polymerization, the PTFE film was taken

out of the flaks and Soxhlet-extracted with water overnight to

remove any homopolymer adhering to the surface. The film was

finally dried under vacuum at 40 8C.

2.3. Peroxide measurement

For remote plasma treated film, the amount of peroxide

formed on the PTFE surface was determined by using a DPPH

method [23]. The remote plasma pretreated PTFE films were

immersed in a 1.0 � 10�4 M benzene solution of 1,1-diphenyl-

2-picryhydrazyl (DPPH) and kept at temperature 70 8C for 24 h

to decompose the peroxide formed on or near the film surface.

The DPPH consumed was measured by the change of

absorbance at 520 nm of UV–vis spectra (model 754N,

Shanghai, China) between the untreated and the treated films.

2.4. Determination of grafting degree

The AAc grafting degree was determined as follow: each

AAc-grafted PTFE film was reacted for 2 h, at 60 8C, with

10 ml of 0.01 M NaOH, then 5 ml of the supernatant were back

titrated with 0.01 M HCl using a titrator.

2.5. Contact angle measurements

The static contact angle was measured by a contact angle

meter (JY-82) made in Chengde, China. To lessen the effect of

gravity, the volume of each drop was regulated to about

0.2 ml by a micro syringe. The measurement was carried out

Fig. 1. Schematic representation of the surface modification of PTFE films.

C. Wang, J.-R. Chen / Applied Surface Science 253 (2007) 4599–4606 4601

at a 20 8C and humidity of 45% RH. The averaged value of

the angles of the both sides of each drop was counted as

one measurement. Each contact angle was determined

from an average of 10 measurements with a standard

deviation of 18.

2.6. 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 plasma treated films and the

grafted films were analyzed for their hydrophilic properties by

carrying out water and diiodomethane contact angle measure-

ments. The liquids used in measuring the contact angle of the

film are shown in Table 1. Wu [24] 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

2.7. ATR–FTIR measurements

ATR–FTIR measurements were carried out using a Nicolet

Nexus 870. The samples were analyzed in the reflectance mode

in the range of 675–4000 cm�1.

2.8. XPS analysis

The XPS measurements were 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 was

maintained at 8 � 10�6 Pa. The take-off angle was 458with

respect to the sample surface. The binding energies were

calibrated against a value of the C1s hydrocarbon component

centered at 284.6 eV.

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

3. Results and discussions

When PTFE was treated with remote argon plasma, three

types of radicals – midchain radicals, �CF2�C�F�CF2� ,

end-chain radicals, �CF2�C�F2, and dangling-bond sites

(structureless radicals) – are easily formed on the PTFE

surface. The radicals were rapidly modified into corresponding

peroxides by contacting with air, and the formed peroxides are

stable at room temperature for a long time [25]. The peroxides

may be preferable initiators to graft polymerize AAc, because

the radicals exist mainly on the PTFE surface. A schematic

presentation of the modification of the surface is depicted in

Fig. 1.

3.1. Optimum plasma treated conditions for maximum

peroxide concentration

Fig. 2 shows the effect of remote argon plasma treated time

and power on the peroxide concentration of PTFE by using

DPPH method measurement. From Fig. 2, the peroxide

concentration increases with increasing the plasma treated

time up to100 s as well as the plasma treated power. After the

Fig. 2. Peroxide concentration on PTFE surface as functions of plasma treated

power and time.

Fig. 4. Effect of reaction time on grafting degree at different temperatures

(plasma treated condition: 100 W, 100 s; grafting condition: 10% AAc).

C. Wang, J.-R. Chen / Applied Surface Science 253 (2007) 4599–46064602

plasma treated time of 100 s, the increase becomes small.

Clearly, the longer plasma treated time cannot help generating

more peroxides. The produced peroxides are partly converted to

inactive species, which cannot yield radicals, after prolonged

plasma treated time. A plasma treated power increase means an

increased rate of excitation or an increased rate of initiation of

growth reactions. The change is most drastic when the plasma

treated power becomes insufficient. With sufficient plasma

treated power, the radical formation is not greatly influenced by

the plasma treated power. We consider that a longer treatment

time than 100 s with argon plasma of 100 W causes heavy

degradation of the PTFE surface [15]. Therefore, we find that

the optimum plasma treatment conditions of 100 W and 100 s

for the maximum concentration (2.87 � 10�11 mol/cm2) of

reactive peroxides on the surface of PTFE film. For the

subsequent graft polymerization of AAc, we fix the plasma

treated conditions for sample treatment as the optimum plasma

treated conditions.

3.2. Effect of monomer concentration

Fig. 3 shows that the grafting degree of AAc on the PTFE

surface increases with the monomer concentration. The

grafting degree shows a characteristic behavior with increases

in the monomer concentration. When the concentration is over

40%, the monomer tends to form homopolymerization PAAc

during polymerization reaction. This phenomenon can be

explained as a Trommsdorff effect which is called either auto

acceleration or gel effects. As polymerization proceeds, the

viscosity increases to cause the chains mobility decrease, and

the chains termination becomes difficult. As a result, the rate of

polymerization eventually increases. In AAc solution, the

surface graft polymerization usually competes with homo-

polymerization. When AAc concentration is low, the surface

graft polymerization has more chance to occur than homo-

polymerization. However, at high AAc concentration, homo-

polymerization becomes more dominant than the surface

polymerization occurred only in a confined region. Thus, over a

certain concentration, the grafting degree rather decreases due

to fast consumption of monomers by homopolymerization.

Fig. 3. Effect of AAc concentration on grafting degree (plasma treated con-

dition: 100 W, 100 s; grafting condition: 50 8C, 6 h).

3.3. Effect of reaction time and temperature

Fig. 4 shows the effect of reaction time on the grafting

degree at various temperatures. The grafting was carried out

from 30 to 70 8C at a 10% monomer concentration. All curves

show an increase in the grafting degree with the reaction time.

Graft polymerization of AAc is initiated from the peroxide

groups. With the reaction time increase, the peroxide groups

have more time for reaction, and as the result, the grafting

degree increase. After 6 h, all peroxide groups may be used up

and then the grafting degree levels off with an increase in

reaction time. Fig. 4 also shows that the grafting degree

increases with the reaction temperature. The result assumes that

elevated temperature can accelerate the monomer diffuse and

the decomposition of the peroxides produced by plasma

treatment on the PTFE film surface into radicals. As a

consequence, a higher grafting degree is achieved. It is because

the AAc homopolymerization occurs easily as the reaction

temperature is high. The homopolymerization will cause the

viscosity of AAc solution to increase. The high viscosity further

inhibits AAc monomer moving to the surface of PTFE. For this

reason, the grafting degree of AAc is reduced.

3.4. Effect of inhibitor

It is known that inhibitors are usually added during grafting

to minimize homopolymerization formation. The grafting of

AAc onto PTFE films is generally accompanied by the

formation of PAAc homopolymerization in the reaction

medium. This leads to large-scale monomer waste. It was

reported that the metal salt effectively consumed reactive

hydroxides in solution and normally worked as an inhibitor

against the formation of homopolymerization. A similar

inhibitory effect of Mohr’s salt on homopolymerization has

been observed for the radiation-induced graft polymerization of

AAc onto polyethylene films and polyaniline powders [9,26].

The results for the variation of the grafting degree with and

without Mohr’s salt are listed in Table 2. Since, however,

Table 2

Effect of Mohr’s salt on the grafting degree

Mohr’s salt (mol/l) Grafting degree (mg/cm2)

0 12.56

4 � 10�5 7.12

Plasma treated condition: 100 W, 100 s; grafting condition: 60 8C, 6 h, 10%

AAc.

Fig. 5. Variation of the contact angle with grafting degree in PTFE films.

C. Wang, J.-R. Chen / Applied Surface Science 253 (2007) 4599–4606 4603

Mohr’s salt can also consume surface reactive sites and let to

faster chain termination, the grafting degree can also decrease

with the addition of Mohr’s salt. The phenomenon can be

explained by assuming that the Fe2+ ions present in the grafted

layer of the copolymer take part in chain termination reactions

as shown below [26]:

3.5. Variation in contact angle

Fig. 5 shows that the plasma treatment and grafting AAc

lead to a considerable decrease in the contact angle of the PTFE

films. The untreated film has contact angle of 1088 which

decreases to 588 for the plasma treated film for 100 s, 100 W.

This value is selected for zero grafting degree in Fig. 5. The

Table 3

Contact angle for water, surface energy, and grafting degree results of treated PTF

Sample Contact angle

(8)Surface energy (�10�5 N cm�1)

Bs As gd gp g

Bs As Bs As Bs

Untreated 108.0 108.0 19.8 19.8 2.3 2.3 22.1

Plasma treated 58.0 71.8 22.5 20.9 24.7 13.4 47.2

AAc-grafted 41.1 44.7 29.4 29.3 32.7 30.1 62.1

Before storage: Bs, After storage: As.

contact angle decreases further with the increase in the grafting

degree up to 5.2 mg/cm2 and thereafter reaches the asymptotic

value of approximately 418.These observations indicate the magnitude of the surface

changes on PTFE films due to the AAc graft chains. This

reveals that the surfaces become more hydrophilic after surface

grafted AAc on PTFE films. A decrease in the contact angle by

air and N2 plasma-induced graft polymerization of AAc onto

PTFE films has also been reported by Sun et al. [22].

According to the literature, the wettability of plasma treated

PTFE is gradually lost with aging time [27]. A similar

phenomenon was observed in our case. The storage of films

under ambient conditions leads to a significant increase in the

contact angle for plasma treatment (zero grafting degree) and at

low grafting degree. This is attributed to the surface contamina-

tion and the rearrangement of the modified layer on the surface.

At low grafting degree, although the grafting AAc density is low,

the hydrophilicity at the surface is enhanced. The interfacial

tension at the surface increases to such an extent that the intensive

migration of the polar segments from the surface towards the bulk

of the film follows. As a result, the surface hydrophilicity

diminishes, as reflected by the relatively large increase in the

contact angle. At higher grafting degree, the grafting AAc chains

are much denser to form independent domain at the surface. The

migration of such bulky structure from the surface towards the

bulk would be hindered, consistent with the relatively small

changes in the surface hydrophilicity, as reflected by the

relatively small changes in the contact angle on storage at high

grafting degree as shown in Fig. 5. It is possible that the

crosslinking at the surface also contributes to the observed low

levels of rearrangement. From these observations it may be stated

that the grafted surface is dynamic in nature and that any post-

grafting application has to take into account the time-dependent

activity of the surface. However the time-dependent activity of

the higher grafting degree surface is better than that of the plasma

treated film (zero grafting degree) in Fig. 5. The permanent AAc

grafting layer is formed on the PTFE surface. There are

probabilities for enhancing surface hydrophilicity, improving

adhesion, and immobilizing biological materials.

3.6. Effect of surface modification on the surface free

energy of PTFE film

Table 3 shows the surface free energy and its components of

the PTFE film, which are before and after storage under

E samples

gd/g (%) (Bs) gp/g (%) (Bs) Grafting degree (mg/cm2)

As

22.1 89.6 10.4 0

34.3 47.7 52.3 0

59.4 47.3 52.7 5.6

C. Wang, J.-R. Chen / Applied Surface Science 253 (2007) 4599–46064604

ambient conditions. It shows that surface free energy of the

PTFE film is increased after modification by plasma treatment

and grafting AAc. Although the grafting degree is not high, the

surface properties of the films change drastically. Compared

with untreated film, surface free energy (g) increases more than

two-fold for plasma treated and grafted PTFE film before

storage. The surface free energy’s polar (gp) component

increases from 10.4% to about 52.3%, and 52.7%, respectively.

Dispersion (gd) decreases from 89.6% to about 47.7%, and

47.3%, respectively. Accordingly, it can be concluded that the

improvement of surface wettability largely lies in the increase

of the gp.

Compared with plasma treated film before storage, surface

free energy (g) decreases from 47.2 to 34.3 � 10�5 N cm�1

after storage. The surface free energy’s polar (gp) component

decreases from 24.7 to 13.4 � 10�5 N cm�1. Dispersion (gd)

decreases from 22.5 to 20.9 � 10�5 N cm�1. Therefore, it can

be concluded that the decrease of surface energy and the

deterioration of surface wettability largely lies in the decrease

of the gp for the plasma treated film after storage. Compared

with grafted film before storage, surface free energy (g)

decreases small from 62.1 to 59.4 � 10�5 N cm�1 after storage.

The surface free energy’s polar (gp) component decreases small

from 32.7 to 30.1 � 10�5 N cm�1. There was no obvious

change on the dispersion (gd). Hence, it can be concluded that

the decrease of surface energy and the deterioration of surface

wettability is very small for the grafted film after storage. From

surface energy it also may be stated that the plasma treated and

grafted surface is dynamic in nature and that any post-grafting

application has to take into account the time-dependent activity

of the surface. However the time-dependent activity of the

grafted surface is better than that of the plasma treated film

(zero grafting degree) in Table 3.

3.7. ATR–FTIR studies

Fig. 6 shows the ATR–FTIR results of the PTFE films. For

untreated PTFE film, Fig. 6(a) indicates the expected

absorbance of F2 stretching at 1147 and 1205 cm�1, and there

are no other absorbance peaks in the experimental wave number

scope. Compared to untreated films, there is no obvious change

Fig. 6. ATR–FTIR spectra of (a) untreated PTFE films; (b) plasma treated

PTFE films; (c) AAc-grafted PTFE films.

on the film after plasma treatment (Fig. 6(b)). This step only

attribute to the formation of activated radicals. Fig. 6(c) shows

that after 6 h of AAc graft polymerization, stretching vibration

of C O at 1695 cm�1 is well observed, as well as asymmetric

stretching vibration of COO� at 1548 cm�1 and symmetric

stretching vibration of COO� at 1445 cm�1, which imply a

thicker layer of grafted AAc on the surface [28].

Morrison and Boyd [29] had described carboxylate ion in

their organic chemistry textbook as follows. According to the

resonance theory, then, a carboxylate ion is a hybrid of two

Fig. 7. C1s spectra of (a) the untreated PTFE; (b) the treated PTFE; (c) the

grafted PTFE.

C. Wang, J.-R. Chen / Applied Surface Science 253 (2007) 4599–4606 4605

structures which, being of equal stability, contribute equally.

Carbon is joined to each oxygen by a ‘one-and-one-half’ bond

and the negative charge is evenly distributed over both oxygen

atoms. Carboxyl carbon is joined to three other atoms by s

bonds; since these bonds utilize sp2 orbitals, they lie in a plane

and are 1208 apart. The remaining p orbital of the carbon

overlaps equally well p orbitals from both of the oxygens, to

form hybrid bonds. In this way the electrons are bonded not just

to one or two nuclei but to three nuclei (one carbon and two

oxygens); they are therefore held more tightly, the bonds are

stronger, and the anion is more stable. It means that carboxylate

ion is of overlapped p orbitals in both directions: delocalization

of p electrons, and dispersal of charge. Furthermore, Fig. 6(c)

shows stretching vibration of COO� at 1548 and 1445 cm�1, as

well as stretching vibration of C O at 1695 cm�1. These results

suggest that AAc was successfully grafted on the PTFE surface.

3.8. XPS studies

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

C1s spectra of untreated PTFE consists of a main component

with a binding energy of 291.7 eV due to the CF2 species on the

sample surface [30], as showed in Fig. 7(a). The C1s spectra of

the plasma treated PTFE contains two peaks as shown in

Fig. 7(b). It can be seen that a small amount of oxygen moieties

is incorporated (Table 4) after argon plasma treatment and

subsequent exposure to air. It is reasonable to assign the peak

lying at the binding energy about 292.4 eV mainly to the CF2

species corresponding to the unmodified PTFE underneath the

treated surface, and the peak lying at the binding energy around

285.2 eV to the C O (288.5), C–O (286.2), C C and CH2

(284.6) corresponding to the modified surface PTFE [30]. It is

seen that the chemical environment of the carbon on the PTFE

surface changes significantly with the defluoration and the

introduction of oxygen during the plasma treatment, which

contributes to the improvement of the PTFE surface properties.

After AAc graft polymerization, the amount of oxygen and

carbon increased further and the amount of fluorine decreased

(Fig. 7(c)). From Table 4 it can be seen that the F/C atomic ratio

after each step of the modifications subsequently decreases

from 1.97 to 1.10, while the O/C atomic ratio increases from 0

to 0.14. The N atomic percentage comes from the contamina-

tion from the nitrogen gas in air.

From the XPS survey spectra it can be seen that argon

plasma treatment and subsequent exposure to air lead not only

to the oxidation of PTFE, but also the substantial surface

defluoration. Taking into account that the decrease in the F/C

Table 4

Surface composition of treated 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

Plasma treated 56.9 39.5 3.4 0.2 1.44 0.09

AAc-grafted 48.9 44.7 6.1 0.3 1.10 0.14

ratio exceeds the increase in the O/C ratio, it can be concluded

that the C–F bonds on the film surface are broken by the argon

plasma treatment and the C–C, C–O and C O are introduced

effectively.

4. Conclusion

In this study, we successfully introduced carboxyl functional

groups on the surface of PTFE film through the remote plasma-

induced graft polymerization of AAc. The PTFE surface was

modified by the graft polymerization of AAc in two steps: the

formation of peroxides on the PTFE surface by treatment of

remote argon plasma and the graft polymerization of AAc from

the peroxide groups. The main results are summarized as

follows:

1. W

e found the optimum remote plasma treated conditions of

100 W and 100 s for the maximum concentration

(2.87 � 10�11 mol/cm2) of reactive peroxides on the surface

of PTFE film. Peroxides formed on the surface worked as

initiators to synthesize AAc grafted PTFE.

2. W

e also known the optimum reaction conditions of 60 8C,

6 h and 40% AAc for the maximum grafting degree

(25.2 mg/cm2) on the surface of PTFE. Successful synthesis

of AAc grafted PTFE was identified using ATR–FTIR and

XPS. It was concluded that the F/C atomic ratio after each

step of the modifications subsequently decreased from 1.97

to 1.10, while the O/C atomic ratio increased from 0 to 0.14.

3. T

he grafting led to a considerable decrease from 1088 to 418in the contact angle of the PTFE films. Similarly, the grafting

led to a considerable increase from 22.1 � 10�5 to

62.1 � 10�5 N cm�1 in the surface energy of the PTFE

films. It can be concluded that the improvement of surface

wettability largely lies in the increase of the gp. Studied on

the time-dependent by the contact angle and the surface

energy, it is important to visualize the dynamic nature of the

grafted surface. The time-dependent rearrangement of

surface should be taken into account in any post-grafting

application. However, the time-dependent activity of the

grafted surface is better than that of the plasma treated film.

Further work is in progress to improve the wettability and

apply the immobilized biomolecules onto PTFE films as blood-

compatible materials.

Acknowledgements

The authors thank the financial support of the National

Natural and Science Foundation Council of China 20174030

and 30571636, the specialized research Fund for the Doctoral

Program of Higher Education 20010698007, and the key

Scientific Technique item of Shaanxi province 2003K10-G61.

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