EUROPEAN

European Polymer Journal 41 (2005) 2343–2353

www.elsevier.com/locate/europolj

POLYMERJOURNAL

Expanded poly(tetrafluoroethylene) membranesurface modification using acetylene/nitrogen plasma treatment

Chen-Yuan Tu a, Yi-Chieh Wang b, Chi-Lan Li b, Kueir-Rarn Lee b,*,J. Huang c, Juin-Yih Lai a

a R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung Li 32023, Taiwanb Department of Chemical Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwanc Yeu Ming Tai Chemical Industrial Co., Ltd., Taichung Industrial Park, Taichung 407, Taiwan

Received 18 January 2005; received in revised form 16 April 2005; accepted 17 April 2005

Available online 28 June 2005

Abstract

In this article, expanded poly(tetrafluoroethylene) (e-PTFE) membrane surface modification was carried out using

acetylene/nitrogen plasma treatment (p-e-PTFE). The variation in surface morphology of the p-e-PTFE membranes

was confirmed by FTIR-ATR, scanning electron microscopy (SEM), and contact angle measurements. It was found

that the surface hydrophilicity increased with increasing nitrogen content in the feed gas mixture, RF power, and

plasma treatment time. The surface pore size decreased with increasing RF power and plasma treatment time. The

water contact angles of the modified e-PTFE membrane decreased from 125.8� to 34.1� through the acetylene/nitrogen

plasma treatment.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Expanded poly(tetrafluoroethylene); Plasma treatment; Hydrophilicity; Contact angle

1. Introduction

Polytetrafluoroethene (PTFE) displays many highly

desirable bulk and surface properties, e.g., high thermal

stability, chemical inertness, low dielectric constants,

and extreme frictional resistance and low surface energy

[1,2], based on the low polarizability of the CAF bond.

These properties are desired in many applications like

multilayer electronic packages, low friction films, sealing

or biomedical applications. The hydrophobic surface of

0014-3057/$ - see front matter � 2005 Elsevier Ltd. All rights reserv

doi:10.1016/j.eurpolymj.2005.04.022

* Corresponding author. Tel.: +886 3453 5521; fax: +886 3436

1747.

E-mail address: krlee@nanya.edu.tw (K.-R. Lee).

PTFE is a distinguishing property but a difficult prob-

lem in adhesion with other materials. Therefore, pre-

treatment is usually required to achieve satisfactory

hydrophilicity and adhesion. There have been many

studies on improving the surface properties of PTFE

using various surface modification methods, such as

chemical etching with sodium naphthalene [3], UV-

lasers, electron and ion beams irradiation [4,5], and

plasma modification [1,6–8]. All of these lead to large

increases in surface energy and surface hydrophilicity.

Among these methods, plasma modification is the most

notable technique.

Plasma surface modified polymer surfaces can achieve

the necessary properties, such as surface hydrophilicity,

adhesion, material selectivity, and biocompatibility [9].

ed.

Exhaust

Thermocouple

membrane

Mixing Box

Heater

MatchingNetwork

13.56 MHz

MFC

MFC

C2H2N2

PressureGauge

View port

Fig. 1. The schematic diagram of the plasma reactor system.

2344 C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353

Many kinds of gases can be used as a plasma medium

including noble and reactive gases or organic volatile

compounds. The former refers to polymer surface activa-

tion by plasma, while the latter refers to surface polymeri-

zation by plasma. Both methods can incorporate

hydrophilic functional groups, such as carbonyl, car-

boxyl, hydroxyl, hydroperoxide, and amino groups to

improve the polymer surface properties, such as surface

wettability and biocompatibility. Surface polymerization

by plasma enhances chemical vapor deposition

(PECVD) and can also reduce the surface pore size of

porous polymer membranes for reverse osmosis [10–

13], pervaporation [14] and gas separation [15].

C2H2/N2 plasma systems were used in this work to

systematically investigate the role of plasma treatment

in e-PTFE membrane surface modification. C2H2/N2

plasma easily polymerizes the thin hydrophilic film on

the e-PTFE membrane surface. The addition of nitrogen

to the hydrocarbon plasma reduces the degree of cross-

linking in the polymeric material and forms polar or

hydrophilic functional groups such as amines on the sub-

strate polymer surface [16]. The plasma condition effects

such as the power, monomer flow rate, and plasma treat-

ment time on the surface hydrophilicity and morphology

of the e-PTFE membranes are discussed in details.

2. Experimental

2.1. Materials

The expanded PTFE membranes used were commer-

cially available from YMT Co., Ltd., Taiwan, with a

melting point of about 327 �C. The mean pore size of

the e-PTFE membranes was about 0.4 lm, with a thick-

ness of 150 lm. The expanded PTFE membranes were

cut to 5 cm · 5 cm dimensions and provided as speci-

mens for the surface modification experiments. The sam-

ple sheets were prepared by Soxhlet extraction. The

nitrogen and acetylene used for the plasma treatments

were pure grade (99.5% purity).

2.2. Plasma treatment by C2H2/N2 mixture gas

Plasma treatment was carried out in a plasma en-

hanced chemical vapor deposition (PECVD) reactor sys-

tem with two internal electrodes. This device was fitted

with a monomer inlet, pressure gauge, vacuum system,

and matching network for 13.56 MHz radio frequency

(RF) source capacity coupling. A schematic diagram

of the plasma reactor system is shown in Fig. 1. The vac-

uum reactor system was evacuated to 20 mTorr with a

rotary pump before plasma treatment. The monomers

with controlled flow rates using a mass flow controller

(MFC) system were then introduced into the reactor

through a showerhead. The operating pressure and

power were fixed at 300 mTorr and 50–300 W, respec-

tively. The acetylene/nitrogen gas flow rate ratios were

varied. The total gas mass flow rates were fixed at

15 sccm (standard cm3/min).

2.3. Surface analysis

2.3.1. FTIR-ATR spectrum

The spectrum of the pristine e-PTFE and the

p-e-PTFE membranes were detected using a Fourier

transform infrared spectrum analyzer with a attenuated

total reflection element (ATR), collected at 4 cm�1 reso-

lutions, and analyzed with a built-in standard software

package (Perkin–Elmer Spectrum One, Perkin–Elmer

Co., Norwalk, CT, USA).

2.4. Surface and cross-section morphology studies

The morphologies of pristine e-PTFE and the p-e-

PTFE membranes were analyzed with scanning electron

microscopy (SEM, Hitachi Model S4700). The mem-

branes were immersed in liquid nitrogen to fracture

and then sputtered wit Pt before analysis.

2.5. Surface hydrophilicity characterization

The surface hydrophilicity of the pristine e-PTFE

and the p-e-PTFE membranes was characterized using

the surface contact angle of water. The contact angles

of water on the membranes were measured in air at

room temperature using the sessile drop method using

an anglemeter (Automatic Contact Angle Meter, Model

CA-VP; Kyowa Interface Science Co., Ltd., Japan).

Increasing and decreasing the volume of the water drop

until the three-phase boundary moves on the membrane

surface measured the advancing and receding contact

angles. The distilled water were dropped at least ten dif-

ferent sites on each sample, and the measured values of

the contact angle were averaged.

2.6. Water–alcohol mixtures dehydration using vapor

permeation measurement

A traditional vapor permeation process was used [17].

The feed vapor is in direct contact with the membrane

C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353 2345

that effective area was 3.3 cm2. In this experimental, the

permeation rate was measured the weight of permeate.

The permeant compositions were measured using gas

chromatography (G.C. China chromatography 8700T).

3. Results and discussion

3.1. Plasma condition effects on the surface

hydrophilicity of the e-PTFE membranes

The C2H2/N2 plasma treatment effect on the surface

hydrophilicity of the e-PTFE membranes was investi-

gated using water contact angle measurement, as shown

in Table 1. It shows that the contact angle of the pristine

e-PTFE membrane is 125.8�, higher than that of the

plasma treated p-e-PTFE membranes with different

C2H2/N2 gas flow rate ratio. These phenomena might

be due to the fact that the amine groups introduced onto

the e-PTFE surface via plasma treatment resulted in the

p-e-PTFE surface becoming more hydrophilic. More-

over, the surface hydrophilicity increased with increas-

ing nitrogen content in the C2H2/N2 mixture gas.

However, the hydrophilicity of the p-e-PTFE mem-

branes did not increase using pure nitrogen plasma

treatment. The chemical composition on the pristine e-

PTFE and C2H2/N2 plasma modified e-PTFE surfaces

was analyzed with XPS. The XPS spectra for the above

membrane surfaces are shown in Fig. 2. The C1s core

level spectrum of pristine e-PTFE membrane (Fig. 2(a))

presented only a peak at 292.7 eV attributing to CF2

species. After C2H2/N2 plasma treatment, a new peak

at 285 eV, which was contributed from CAH group.

The other two peaks found for plasma treated PTFE

membranes (Fig. 2(b)–(d)) at 288.6 eV and 287.5 eV

were due to the existence of CAN, C@O and CAO

groups, respectively. The N1s core level spectrum of

the C2H2/N2 plasma treated e-PTFE membranes (Fig.

2(e)–(g)) exhibit N1s component at the bonding energy

of 399.7 eV and 400.7 eV, which attributable to the

amide and imide groups, respectively. These results con-

firmed that through the C2H2/N2 plasma treatment suc-

Table 1

Effect of C2H2/N2 gas flow rate ratio on the surface contact

angle of the p-e-PTFE membrane

C2H2/N2 gas flow rate ratio Surface contact angle of water (�)

Pristine membrane 125.8 ± 2.0

15/0a 90.6 ± 2.9

10/5a 58.7 ± 1.6

7.5/7.5a 56.3 ± 1.9

5/10a 53.6 ± 1.4

0/15a 115.0 ± 5.5

a RF power: 100 W, 300 mTorr of system pressure, and 180 s

plasma treatment time.

cessfully induce the hydrophilic functional groups onto

the e-PTFE membrane surface, resulting in the p-e-

PTFE membranes with lower water contact angle than

that of the pristine e-PTFE membrane. These results

correspond well with the results from the surface water

contact angle study, as indicated in Table 1.

The p-e-PTFE membrane surface structure charac-

terization using FTIR-ATR measurement can further

explain the above phenomenon. Fig. 3 shows the FTIR-

ATR spectrum of pristine e-PTFE and p-e-PTFE

membranes using different C2H2/N2 mixture gas flow

rate ratios. For the pristine e-PTFE (Fig. 3(a)), the

two strong absorbance peaks appeared at 1200 cm�1

and 1148 cm�1 were due to the CAF asymmetric and

symmetrical stretching vibrations in the e-PTFE. When

treated with pure C2H2 gas plasma (Fig. 3(b)), a thin

hydrocarbon film was deposited onto the e-PTFE sur-

face. The absorbance peaks appeared at 2920 cm�1

and 1450 cm�1 were due to the CAH stretching vibra-

tions; 1740 cm�1 and 3300 cm�1 due to the carbonyl

and hydroxyl groups, respectively. Those oxygen-con-

taining functional groups were incorporated by the

residual free radicals that reacted readily with oxygen

and water in the air. However, the e-PTFE treated with

C2H2/N2 mixture gas plasma, introduced different func-

tional groups onto the e-PTFE surface. The new absor-

bance peaks appeared at 2820 cm�1 due to the stretching

vibration of the CAH bond of the NACAH group, and

1640 cm�1 were due to the NAH bending of primary

amine. When the C2H2/N2 mixture gas were excited to

a plasma phase, the C2H2 and N2 gas molecules might

be excited to form hydrocarbon and nitrogen atoms or

free radicals from electrons. These reactive atoms and

free radicals could be recombined to form another mol-

ecule. Amine groups were thus formed on the e-PTFE

membrane surface. Increasing the nitrogen content in

the mixture gas provided more nitrogen atoms to com-

bine with the hydrocarbon radicals to increase the amine

group concentration on the p-e-PTFE surface, as shown

in Fig. 3. These amine groups introduced onto the e-

PTFE surface resulted in the membrane surface becom-

ing more hydrophilic. This observation corresponds well

with the results shown in Table 1.

Tables 2 and 3 show the RF power and plasma

treatment time influence on the p-e-PTFE surface hydro-

philicity, respectively. The p-e-PTFE surface hydrophi-

licity increased with increasing RF power and plasma

treatment time. These results show that the active species

from the acetylene and nitrogen gas mixture dissociation

increases when the RF power increases. Thus, the amine

group on the p-e-PTFE membrane surface increased,

resulting from the recombination of active species. In

addition, the higher RF power supplied to the C2H2/N2

mixture gas also increased the deposition rate in the

plasma treatment system. For example, RF power in-

creased to 300 W induced the hydrophobic pristine

Fig. 2. C1s core-level XPS spectra of (a) pristine e-PTFE, (b) C2H2/N2 = 10/5, (c) C2H2/N2 = 7.5/7.5, (d) C2H2/N2 = 5/10 plasma

treated e-PTFE membrane, and N1s core-level XPS spectra of (e) C2H2/N2 = 10/5, (f) C2H2/N2 = 7.5/7.5, (g) C2H2/N2 = 5/10 plasma

treated e-PTFE membrane.

2346 C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353

e-PTFE surface (contact angle with water = 125.8� ±2.0�) to become a hydrophilic surface (contact angle with

water = 34.1� ± 0.9�). Fig. 4 shows that the intensity of

the 1640 cm�1 (due to the NAH bending of primary

amine) increases with increasing the RF power. This re-

sult confirms that the p-e-PTFE surface hydrophilicity

increased with increasing the RF power.

3.2. Aging time influence on the surface hydrophilicity

of p-e-PTFE membrane

To investigate the effect of storage time on the surface

hydrophilicity of p-e-PTFE membrane, the plasma trea-

ted membrane was stored in the environment keep in

25 �C and relative humidity of 70%, as shown in Fig.

5. After plasma treatment (C2H2/N2 = 5/10 sccm,

100 W, 300 mTorr, 180 s), the p-e-PTFE surface became

a hydrophilic surface (water contact angle = 53.6� ±1.4�). However, the surface hydrophilicity decreased

rapidly and leveled off after 2 weeks storage in atmo-

sphere. These phenomena might be due to polymers

modified using plasma caused a reversible conforma-

tional transformation. The plasma modified layer might

be submerged to change the surface free energy of a

polymer stored in air [18]. Thus, the hydrophilicity of

the p-e-PTFE membrane decreases in the initial aging

4000 3500 3000 2500 2000 1500 1000

tran

smitt

ance

wave number (cm-1)

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3. FTIR-ATR spectrum of the plasma treated e-PTFE membranes by different C2H2/N2 gas flow rate ratio (100 W, 180 s).

(a) Pristine, (b) 15/0, (c) 10/5, (d) 7.5/7.5, (e) 5/10, (f) 0/15 (sccm).

Table 2

Effect of RF power on the surface contact angle of p-e-PTFE

membrane

RF power Surface contact angle of water (�)

Pristine membrane 125.8 ± 2.0

50 Wa 69.0 ± 2.1

100 Wa 53.6 ± 1.4

150 Wa 48.4 ± 1.0

200 Wa 44.4 ± 2.9

250 Wa 39.3 ± 1.3

300 Wa 34.1 ± 0.9

a C2H2/N2 gas flow rate ratio of 5/10 sccm, 300 mTorr of

system pressure, and 180 s plasma treatment time.

Table 3

Effect of plasma treatment time on the surface contact angle of

p-e-PTFE membrane

Plasma treatment time Surface contact angle of water (�)

Pristine membrane 125.8 ± 2.0

30 sa 79.1 ± 1.3

180 sa 34.1 ± 0.9

300 sa 33.7 ± 2.1

600 sa 33.6 ± 2.4

a C2H2/N2 gas flow rate ratio of 5/10 sccm, RF power of

300 W, and 300 mTorr of system pressure.

C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353 2347

test period. The surface contact angle of water remained

at about 63� until 2 weeks storage in air. The hydropho-

bic recovery phenomenon is due to the migration of

hydrophilic groups that are form by the plasma treat-

ment from the surface layer into the bulk layer because

of the thermal motion of polymer chain. If we can re-

strain the migration of polymer chain, the hydrophobic

recovery rate might be suppressed. With increasing the

plasma treatment time, the amount of hydrophilic func-

tional groups can be introduce onto the e-PTFE mem-

brane surface (as shown in Fig. 6). Thus the long

hydrophilic polymer chain is difficult to migrate into

the bulk layer, and the modified polymer surface will

keep hydrophilic.

3.3. Post-treatment influence on p-e-PTFE surface

hydrophilicity

The polymer treated with plasma could produce

many free radicals on the polymer surface even in atmo-

sphere. These highly reactive radicals might react rapidly

with oxygen and water in air to form oxygen-containing

species such as hydroperoxides, alcohols, acids, ketones,

etc. When the plasma treated p-e-PTFE membrane was

4000 3500 3000 2500 2000 1500 1000

tran

smitt

ance

wave number (cm-1)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 4. FTIR-ATR spectrum of the plasma treated e-PTFE membranes by different RF power (C2H2/N2 = 5/10 sccm, 180 s).

(a) Pristine, (b) 50 W, (c) 100 W, (d) 150 W, (e) 200 W, (f) 250 W, (g) 300 W.

0 10 20 30 40 50 6030

40

50

60

70

Con

tact

ang

le w

ith w

ater

(de

gree

)

Time (day)

Fig. 5. Effect of aging time on the surface hydrophilicity of the

p-e-PTFE membrane (C2H2/N2 = 5/10 sccm, 100 W, 300 mTorr,

180 s).

2348 C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353

immediately immersed into distilled water (w-p-e-

PTFE), the residual radicals could react directly with

the water to form hydrophilic species. Table 4 shows

the post-treatment effect on p-e-PTFE membrane sur-

face contact angle with water after 2 days storage in

air. The water contact angle of the w-p-e-PTFE mem-

brane was obviously smaller than that of the p-e-PTFE

membrane. In addition, increasing the acetylene content

in the C2H2/N2 mixture gas followed by post-treatment

resulted in increased hydrophilicity in the p-e-PTFE

membrane. The acetylene plasma excited a great amount

of free radicals because of the unsaturated acetylene

molecule bond.

3.4. Scanning electron microscopy studies

Scanning electron micrographs of p-e-PTFE mem-

branes treated under different plasma conditions are

shown in Figs. 7–9. The C2H2/N2 mixture gas flow rate

ratio effects on the surface morphology of p-e-PTFE

membranes are shown in Fig. 7. Compared with the

pristine e-PTFE membrane (Fig. 7(a)), the surface pore

size of the p-e-PTFE membrane was reduced slightly

with increased acetylene content in the gas mixture.

When acetylene content in the mixture gas was in-

creased, thus increasing the amount of polymerized spe-

cies and deposition rate, resulting in reduced p-e-PTFE

membrane surface pore size. In addition, the RF power

and plasma treatment time effects on the p-e-PTFE

membrane surface morphology are shown in Figs. 8

and 9, respectively. The surface pore size decreased with

4000 3500 3000 2500 2000 1500 1000

tran

smitt

ance

wave number (cm-1)

(a)

(b)

(c)

(d)

(e)

Fig. 6. FTIR-ATR spectrum of the plasma treated e-PTFE membranes by different treatment time (C2H2/N2 = 5/10 sccm, 300 W).

(a) Pristine, (b) 30 s, (c) 180 s, (d) 300 s, (e) 600 s.

C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353 2349

increasing RF power and plasma treatment time, form-

ing a thin film on the e-PTFE surface. Yasuda stated

that, for the power-deficient, the deposition rate is line-

arly dependent [19]. The higher supplied RF power in-

creased the deposition rate to reduce the p-e-PTFE

membrane surface pore size. Fig. 8(c) shows that many

spherical particles were deposited onto the e-PTFE

surface. As the higher RF power supplied to excite

the monomer gas formed too many active species, the

Table 4

Effect of post-treatment on the surface contact angle of p-e-

PTFE membranes

C2H2/N2 gas flow

rate ratio

Surface contact angle of water (�)

e-p-PTFE (2 day) w-p-e-PTFEa (2 day)

Pristine membrane – –

15/0b 92.2 ± 1.5 60.9 ± 5.0

10/5b 79.1 ± 1.1 62.7 ± 4.3

7.5/7.5b 75.5 ± 5.2 65.7 ± 3.4

5/10b 70.2 ± 2.6 73.2 ± 3.4

0/15b 116.2 ± 2.0 110.2 ± 5.4

a w-p-e-PTFE: immersed the plasma treated p-e-PTFE

membrane into distill water immediately.b RF power: 300 W, 300 mTorr of system pressure, and 180 s

plasma treatment time.

plasma polymerization of C2H2/N2 might be dominant

in the gas phase reaction. The polymerization took place

in the gas phase and deposited onto the e-PTFE surface.

Fig. 10 shows a cross-sectional view of pristine e-PTFE

and plasma treated p-e-PTFE membranes. It shows that

the plasma polymerization is located on the surface also

in the bulk e-PTFE membrane. This demonstrates that

for a porous polymer membrane, plasma could pass

through the pores and modify the inner membrane

layers.

3.5. Water contact angle hysteresis of p-e-PTFE

membranes

The water contact angle of a material surface is a

function of the surface topography, surface heterogene-

ity, swelling and superficial reorganization. Wenzel [20]

developed the first equation relating the surface water

contact angle and surface roughness:

cos hw ¼ r cos h

where hw is the contact angle on a rough surface, h is the

contact angle observed on a smooth surface, and r is the

surface roughness or the average ratio of the actual to

the apparent areas. The surface roughness can therefore

increase or decrease the water contact angle depending

Fig. 7. Surface scanning electron micrographs of the plasma modified e-PTFE membranes by C2H2/N2 gas flow rate ratio (100 W,

180 s). (a) Pristine, (b) 10/5, (c) 7.5/7.5, (d) 5/10, (e) 0/15 sccm.

2350 C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353

on the equilibrium contact angle value on a smooth sur-

face. Thus, the water contact angle hysteresis data of the

pristine e-PTFE and p-e-PTFE membranes were shown

in Table 5. It shows that the water contact angle hyster-

esis decreased with increasing the C2H2/N2 plasma treat-

ment time. These phenomena might be due to the fact

that the C2H2/N2 plasma treatment time increased

results the surface pore size and/or surface roughness

of the p-e-PTFE membrane decreased.

3.6. Water–ethanol mixture dehydration using vapor

permeation

The plasma treatment effects on the vapor perme-

ation performance of 90 wt% aqueous ethanol solution

through the e-PTFE and p-e-PTFE membranes are

shown in Table 6. The permeation rate of the pristine

e-PTFE membrane was higher than that for the C2H2/

N2 plasma treated p-e-PTFE membrane. An opposite

trend for the water concentration in the permeate was

also observed. These phenomena might be because the

C2H2/N2 plasma deposited a thin film onto the e-PTFE

surface resulting in decreased pore size and porosity.

The results discussed in the preceding paragraph were

further confirmed by SEM analysis. The SEM study in

Fig. 11 shows that the surface structure of the pristine

e-PTFE membrane has high porosity. However, the sur-

face of the C2H2/N2 plasma treated p-e-PTFE mem-

brane was smooth, tight, and with fewer voids. Thus,

the permeation rate of the p-e-PTFE membrane was

Fig. 8. Surface scanning electron micrographs the plasma treated e-PTFE membranes by different RF power (C2H2/N2 = 5/10 sccm,

180 s). (a) Pristine, (b) 50 W, (c) 100 W, (d) 150 W, (e) 200 W, (f) 250 W, (g) 300 W.

Fig. 9. Surface scanning electron micrographs of the plasma treated e-PTFE membranes by different plasma treatment time (300 W,

C2H2/N2 = 5/10 sccm). (a) 30 s, (b) 180 s, (c) 300 s, (d) 600 s.

C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353 2351

Fig. 10. Cross-section view of scanning electron micrographs of the plasma modified e-PTFE membranes (C2H2/N2 = 5/10 sccm,

300 s). (a) Pristine and (b) 300 W.

Table 5

Water contact angle hysteresis of e-PTFE and p-e-PTFE

membranea

Item/treatment time (s) 0 30 180 300 600

Advancing angle (�) 152.3 93.5 65.8 57.1 37.8

Receding angle (�) 101.8 44.9 27.1 23.9 14.2

Contact angle hysteresis (�) 50.5 48.6 38.7 33.2 23.6

a C2H2/N2 plasma condition: C2H2/N2 gas flow rate ratio of

5/10 sccm, RF power of 300 W, and 300 mTorr of system

pressure.

Table 6

Effect of C2H2/N2 plasma treatment on the vapor permeation of

90 wt% aqueous ethanol solutions through e-PTFE membrane

Membrane Permeation rate

(g/m2 h)

H2O in permeate

(wt%)

Pristine e-PTFE 31,078 10

p-e-PTFEa 666 72

a C2H2/N2 mixture gas flow rate ratio: 5/10; power: 300 W;

pressure: 300 mTorr; plasma deposition time: 30 min.

Fig. 11. Surface scanning electron micrographs of the plasma m

(a) Pristine e-PTFE and (b) 300 W.

2352 C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353

lower and the water concentration in the permeate was

higher than that from the e-PTFE membrane. These

results correspond well with the results from the vapor

permeation study, as indicated in Table 6.

4. Conclusions

With the goal of improving the surface hydrophilicity

of e-PTFE membranes, an investigation was performed

to test the effects of C2H2/N2 mixture gas plasma on

the surface morphology of e-PTFE membrane. It was

found that the surface hydrophilicity increased with

increasing nitrogen content in the mixture gas, RF

power, and plasma polymerization time. The water con-

tact angles of the pristine e-PTFE and acetylene/nitro-

gen plasma treated p-e-PTFE membrane were 125.8�and 34.1�, respectively. However, the surface hydrophi-

licity decreased with increasing aging time in an atmo-

sphere environment within 2 weeks. The surface pore

size could be controlled using different plasma parame-

ters. As the plasma treatment time was increased, a thin

odified e-PTFE membranes (C2H2/N2 = 5/10 sccm, 30 min).

C.-Y. Tu et al. / European Polymer Journal 41 (2005) 2343–2353 2353

hydrophilic layer was deposited onto the e-PTFE

membrane surface. The vapor permeation results were

obtained using a p-e-PTFE membrane with 30 min.

C2H2/N2 plasma deposition, giving a permeation rate

of 666 g/m2 h and water concentration in permeate of

72 wt% for a 90 wt% aqueous ethanol mixtures.

Acknowledgements

The authors wish to sincerely thank the Ministry of

Economic Affairs and National Science Council of Tai-

wan, ROC, for the financial support of this work.

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