Plasma Chem Plasma Process (2008) 28:715–728
DOI 10.1007/s11090-008-9156-9
ORIGINAL PAPER
Surface Modification of Silicone Rubber by CF4 Radio
Frequency Plasma Immersion
Song-Hua Gao Æ Ke-Sheng Zhou Æ Ming-Kai Lei Æ Li-Shi Wen
Received: 30 July 2008 / Accepted: 29 October 2008 / Published online: 14 November 2008
Ó Springer Science+Business Media, LLC 2008
Abstract Silicone rubber samples were treated by CF4 capacitively coupled plasma at
radio frequency (RF) power of 60, 100 and 200 W for a treatment time up to 20 min under
CF4 flow rate of 20 sccm, respectively. Static contact angle, ATR-FTIR and XPS, and AFM
were employed to characterize the changes of surface on hydrophobicity, functional groups,
and topography. The results indicate the static contact angle is improved from 100.7 to
150.2°, and the super-hydrophobic surface, which corresponds to a static contact angle of
150.2°, appears at RF power of 200 W for a 5 min treatment time. It is suggested that the
formation of super-hydrophobic surface is ascribed to the co-action of the increase of surface
roughness created by the ablation reaction of CF4 plasma and the formation of
[–SiFx(CH3)2-x–O–]n (x = 1, 2) structure produced by the direct attachment of F atoms to Si.
Keywords
XPS
Silicone rubber Surface modification CF4 plasma Hydrophobicity Introduction
Pollution flashovers have become the major impediment to the uninterrupted supply of
electrical power. On traditional glass and porcelain insulator surfaces, water films easily
occur in wet atmospheric conditions, and if the contamination is heavy, salts in the
S.-H. Gao K.-S. Zhou L.-S. Wen
School of Physics Science and Technology, Central South University, Changsha 410083, China
S.-H. Gao (&) M.-K. Lei L.-S. Wen
Surface Engineering Laboratory, School of Materials Science and Engineering,
Dalian University of Technology, Dalian 116024, China
e-mail: [email protected]
L.-S. Wen
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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contaminants dissolved into the water films result in uncontrolled leakage currents and
flashovers. Silicone rubber (SIR) is used increasingly as outdoor high voltage insulators
because the SIR insulators are able to restore the surface hydrophobicity after building up a
pollution layer on the surface, which can suppress the development of leakage currents, dry
band arcing and flashover. However, SIR insulators are apt to suffer from loss of hydrophobicity, decreased tracking and erosion resistance, and degradation of their surface under
heavy wet atmospheric conditions such as heavy fog, drizzling rain and acid rain. If the
SIR’s surface has the properties of anti-adhesion, low surface energy and super-hydrophobicity, it can reduce the solid contaminations adhesion and get the ‘‘self-cleaning’’
surface on which some solid contaminations attached can be flushed off by the rain drops.
In this way, surface leakage currents and pollution flashovers due to the contaminations
deposited on the surface will be suppressed effectively. Up to now, many reports [1–7]
have described the employing of CF4 plasma treatments to make fluorinated polymer
surfaces exhibiting desirable properties of low surface energy, chemical inertness, and low
coefficient of friction. It is also found that the PDMS surfaces modified by other methods
such as laser induced surface, were performed for some specific purpose. For example,
Khorasani et al. [8, 9] made PDMS surfaces superhydrophobic and superhydrophilic to
serve as biomedical materials in vitro blood compatibility. Garra et al. [10] reported the
etching effects of O2 and CF4 mixture plasma on PDMS. Rangel et al. [11] applied sulfur
hexafluoride (SF6) plasmas and argon plasma immersion ion implantation (ArPIII) techniques to improve the hydrophobicity of polytetrafluoroethylene (PTFE), polyurethane and
silicone surfaces. However, there are no reports about the SIR surface modification because
researchers in the field of insulators still pay attention to the composition modification of
bulk materials by adding fillers to SIR and to the study of surface hydrophobicity transfer
[12, 13]. Although the fillers added into SIR, which are always alumina tri-hydrate
(Al2O33H2O) and silicon oxide (SiO2), can improve SIR tracking and erosion resistance,
and hardness, meanwhile they decrease the hydrophobicity of SIR and result in pollution
flashovers. Therefore, to further increase SIR’s resistance to pollution flashover and prolong the service life of its as outdoor high voltage insulators, surface modification is
required to improve the hydrophobicity of SIR surface while preserving its bulk properties.
Plasma treatment has been proved to be a powerful approach for surface modification of
polymeric materials [14–16]. By using plasma treatment, interfacial properties can be
introduced to a polymer surface without affecting the desired bulk properties of a material,
such as strength, and toughness [3, 17, 18]. A number of methods are known to make
fluorinated polymer surface, for instance, direct exposure to F2 gas [19] or fluorine-containing electrical discharges [2], plasma polymerization of fluoromonomers [20], sputter
deposition from a polytetrafluorethylene (PTFE) target [21], and chemical derivatization
[22], etc. Among these methods, the employment of CF4 plasma treatments to introduce
fluorine groups onto polymer surface is an effectual way to lower surface adhesiveness,
surface energy and coefficient of friction, so that form super-hydrophobic surface.
In this paper, SIR samples used as outdoor insulators were exposed to CF4 radio
frequency (RF) capacitively coupled plasma at a RF power of 60–200 W and a CF4 flow
rate of 20 sccm for a treatment time up to 20 min, respectively. Surface chemical composition is analyzed via X-ray photoelectron spectroscopy (XPS) and attenuated total
reflection Fourier transform infrared spectrum (ATR-FTIR). Atomic force microscope
(AFM) was employed to observe the surface topography. To characterize the surface
hydrophobicity of modified SIR, static contact angle (SCA) measurement was performed.
The reasons for the improvement of SIR’s surface hydrophobicity are discussed detailedly
in the ‘‘Results and Discussion’’ section.
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Experimental
Materials Preparation
The SIR samples (1.0 cm 9 1.0 cm 9 0.2 cm) obtained from Dongguan Gaoneng
Industrial Co. Ltd, China consist of a polymer—Polydimethylsiloxane (PDMS), and fillers—alumina tri-hydrate (Al2O3 3H2O) and silicon oxide (SiO2). They were cleaned in
successive ultrasonic baths of acetone for 10 min, ethanol for 10 min, and deionized water
for 20 min, then dried in an oven at 60°C for 3 h, finally stored in a drying desiccator
before CF4 plasma modification.
Plasma Treatment
The plasma treatment system PEPP-300, designed by Surface Engineering Laboratory,
School of Materials Science and Engineering, Dalian University of Technology, is made
up of four parts: air supply and vacuum unit, etching/polymerization reactor, plasma
diagnosis unit, and composite RF power unit. With capacitively coupled or inductively
coupled method, large power and acreage, homogeneous plasma can be produced by the
SY RF power source and SP-I RF matching box, which were made in Institute of
Microelectronics of Chinese Academy of Sciences, and the employed gas flux was
controlled by D08-1D/ZM gas mass flow meter made in Beijing Sevenstar Electronics
Co., LTD. The schematic representation of plasma reactors are shown in Fig. 1. In this
study, the plasma treatments of the SIR samples were carried out in a radio frequency
(RF) capacitively coupled plasma reactor operating at 13.56 MHz. The reactor consists
of two cylindrical electrodes of 20 cm in diameter and 11 cm apart in a cylindrical
vacuum vessel. The upper electrode and the vessel wall are grounded. The SIR samples
were placed on the sample holder, where the SIR surfaces should be bombarded by
positively charged ions accelerated by the self-bias of plasma sheath adjacent to the
electrode. After the system was evacuated to 2 9 10-3 Pa, CF4 gas was introduced into
the vessel at a flow rate of 20 sccm, and the operating pressure was subsequently
maintained at a treatment pressure of about 10 Pa. The RF plasma was then generated at
the specified electric power of 60, 100, and 200 W, for a treatment time from 2 to
20 min, respectively.
Fig. 1 Schematic representation of plasma reactors: a Inductively coupled plasma; b Capacitively coupled
plasma
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Methods of Characterization
The surface hydrophobicity of SIR modified by CF4 RF capacitively plasma was tested
through SCA measurement in which the sessile drop technique is applied [23, 24]. SCA for
distilled water was analyzed on a drop shape analysis system—Kruss DSA100. The mean
values reported correspond to three measurement points located uniformly on the surface.
To observe the variation of the functional groups of the modified SIR, ATR-FTIR, which is
fit to investigate the materials which block the penetration of light, was employed. The
model of test equipment of ATR-FTIR is Nicolet-20DXB (made in America). X-ray
photoelectron spectroscopy (XPS) was employed to investigate the influence of hydrophobic property according to the variation of F content and composition of fluoric groups
of the SIR samples’ surface. An ESCALAB250 system was used with a monochromatic
Al Ka (hm = 1,486.6 eV) X-ray source operating at power of 300 W. X-ray beam with a
circular cross-section area 1 cm2 was irradiated onto the sample at an incident angle of 45°
with respect to the surface plane and the photoelectrons were detected at 90° takeoff angle.
Photoelectrons were analyzed with a concentric hemispherical analyzer at a pass energy of
50 eV. A step-scan interval of 1 eV was used for wide scan, and 0.05 eV for highresolution scans; acquisition times were 60 s at both resolutions. All binding energies were
referenced to the C1 s neutral carbon peak at 285.0 eV to compensate for surface-charging
effects. Element stoichiometries were determined by the high-resolution peak areas using
Shirley background subtraction. Atomic force microscope (AFM) was employed to detect
the surface topography of modified SIR samples, the model of AFM test equipment is
SOLVER-P47 (made in NT-MDT, Russia). In order to get the effective information of SIR
surface topography, the tapping mode of AFM was selected because SIR is an elastomer.
Results and Discussion
Static Contact Angle Analysis
Figure 2 shows SCA of SIR surface treated by CF4 capacitively coupled plasma at RF
power of 60, 100 and 200 W. At RF power of 200 W, the SCA of the modified SIR surface
increases rapidly to a maximum 150.2° at treatment time of 5 min, but decreases gradually
with the increase of treatment time. At RF power of 100 W, the SCA reaches a maximum,
and keeps the same state. However, at the lowest RF power of 60 W, the hydrophobicity of
modified SIR surface increases with the increase of treatment time up to 20 min. The
results indicate that the maximum of static contact angle is related with the RF power and
treatment time, and the plasma treatment time corresponding to the maximum of static
contact angle decreases with the increase of RF power. It is proposed that the competition
between CF4 plasma fluorination and ablation or etching reaches dynamic equilibrium
more quickly at the higher RF power because the self-bias on the SIR samples being
modified by capacitively coupled plasma treatment increases with the increase of the RF
power.
ATR-FTIR Analysis
Some typical characteristic IR absorption bands for PDMS are shown in Table 1, where the
wave numbers for the corresponding bonds are presented [24–27]. Absorption occurs at
around 2,962 cm-1 due to an aliphatic C–H stretch in CH3. The absorption at about
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160
H2O Contact Angle (degree)
Fig. 2 Static contact angle of
SIR surface treated by CF4
capacitively coupled plasma at
RF power of 60, 100, and 200 W
for different treatment time
719
150
140
130
200 W
100 W
60 W
120
110
100
90
0
5
10
15
20
Plasma Treatment Time (min)
Table 1 Characteristic IR
absorption bands in silicone
rubber
Wave number (cm-1)
Chemical bond
2,962–2,960
CH in CH3
1,440–1,390
CH3 in Si–CH3
1,280–1,240
Si–CH3
1,100–1,000
Si–O–Si
840–790
Si(CH3)2
1,260 cm-1 is caused by a symmetric –CH3 deformation of Si–CH3. The absorption at
1,000–1,100 cm-1 is characteristic peak for Si–O–Si, and the intense absorption around
1,015 cm-1 is due to asymmetric Si–O–Si stretching vibration, while the most intense
absorption around 794 cm-1 is characteristic for Si–(CH3)2. The absorption peaks at
3,300–3,900 cm-1 arise primarily from hydroxyl groups (OH) which exist in the fillers and
the ends in polymer backbone of Polydimethylsiloxane (PDMS) which is the main component of SIR.
Figure 3 shows ATR-FTIR spectra of SIR samples treated by CF4 capacitively coupled plasma at RF power of 200 W for different treatment time. The intensity of –CH3
absorption peak decreases more than that of the absorption peaks of C–H and Si–O–Si
with the increase of treatment time, while the intensity of Si(CH3)2 peak stays at the
same state. Table 2 gives the ratio of the optical density of the three peaks and Si(CH3)2
peak for different treatment time at RF power of 200 W. The ratio of optical density of
–CH3 at 1,260 cm-1, C–H at 2,962 cm-1, and Si–O–Si at 1,015 cm-1 versus Si–(CH3)2
at 794 cm-1 decrease from the original 0.414, 0.081, and 0.914 to 0.131, 0.029, and
0.796, respectively. The results is ascribed to the fact that the chemical bond of Si–CH3
is more easily broken by the positive ions from the CF4 plasma than the chemical bonds
of Si–O and C–H because the chemical bond dissociation energy of Si–CH3 is weaker
than those of Si–O and C–H. Therefore, it is suggested that the replacement reaction of
CH3 in Si–CH3 predominates over the replacement reactions of H in C–H or O in Si–O–
Si. In addition, the absorption peaks at 3,300–3,900 cm-1 increase in modified films with
the increase of treatment time at the power of 200 W (Fig. 3). It is also suggested that
there are more hydroxyl groups (OH) produced in the course of plasma treatment, and
the reasonable ground is that the original polymer backbone of Si–O–Si is broken and
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Fig. 3 ATR-FTIR spectra of
SIR samples treated by CF4
capacitively coupled plasma at
RF power of 200 W for different
treatment time
CCP 200 W 20 min
CCP 200 W 5 min
CCP 200 W 2 min
Untreated SIR sample
500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm )
Table 2 The ratio of the optical density of other groups’ peaks and Si–(CH3)2 peak for the typical treatment
time
Wavenumber [cm-1]
Original
SIR sample
2 min
treatment
5 min
treatment
20 min
treatment
1,015 (Si–O–Si)
0.914
0.899
0.811
0.796
1,260 (CH3 in Si–CH3)
0.414
0.351
0.182
0.131
2,962 (C–H in CH3)
0.081
0.051
0.024
0.029
forms hydroxyl groups to serve as the end groups of the more and smaller polymer
backbone of Si–O–Si.
The main reason for not finding fluoric groups in the ATR-FTIR spectra, is that the ratio
between the modified thickness and the depth identified by ATR-FTIR is about parts per
thousand so that the signal of fluoric groups in the modification layer is much weaker than
that of the substrate because the radiation in the ATR technique penetrates a few microns
into the sample and then is reflected into the optical element, however, the depth of fluoric
layer on modified SIR is only several atoms distances.
In order to investigate whether fluoric groups were introduced onto the SIR samples
surface or not, XPS was performed to verify the result because it is one of useful tools to
detect chemical composition of the surface.
XPS Analysis
PDMS has a theoretical C/Si/O ratio of 2.0:1.0:1.0, while XPS analysis gives the elemental
C/Si/O ratio of 1.8:1.1:1.0 for the original SIR sample, which is different from the theoretical ratio because of the fillers existing in the SIR.
Figure 4 gives the XPS survey spectra of SIR samples treated by CF4 capacitively
coupled plasma under RF power of 200 W for different treatment time, and it is clearly
shown that the percentage of oxygen keeps basically unchanged, the contents of Si and C
decrease, and the contents of F and Al increase inversely with the increase of treatment
time. Specifically, the percentage composition of oxygen maintains at about 25%, the
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721
Fig. 4 XPS survey spectra of
SIR samples treated by CF4
capacitively coupled plasma
under RF power of 200 W a
original sample; b treatment time
of 5 min; c treatment time of
20 min
percentage compositions of Si and C decrease from 28.26 to 45.59% of the original
sample to 21.11 and 32.62% of SIR sample treated for 20 min treatment time, and those
of F and Al increase from 0 and 0.37% to 14.01 and 7.19%, respectively. And the
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16
Atomic Concentration of Fluorine (%)
Fig. 5 Surface fluorine content
of SIR samples treated by CF4
capacitively coupled plasma at
RF power of 200 W for different
treatment time
Plasma Chem Plasma Process (2008) 28:715–728
14
12
10
8
6
4
2
0
0
5
10
15
20
Plasma Treatment Time (min)
decrease of Si and C contents is approximately equivalent to the increase of Al and F
contents, respectively. Therefore, it is suggested that the –CH3 groups are replaced by F
radicals in the CF4 plasma. As for the increase of Al content, it could be explained that
the decrease of percentage composition of Si, which exists in SiO2 and is etched by CF4
plasma, makes the percentage composition of Al increase relatively in the approximately
equal content.
Figure 5 shows that the F content increases monotonously and plateaus off with
treatment time from 2 to 20 min. The increase of fluorine observed in the XPS spectra is
attributed to introducing the fluoric groups onto the SIR surface, and the rule of F content
variation is fitted to the opinion that the CF4 plasma fluorination and ablation or etching
occurring on the polymer surface are parallel and competitive, and the competition
between fluorination and ablation or etching depends on RF power and treatment time [1,
28]. Under the constant RF power of 200 W, fluorination predominates over ablation or
etching initially, leading to the fluorine content increasing rapidly, but beyond a critical
treatment time, the two reactions reach a dynamic near-equilibrium with slowly varying F
content on the surface.
Figure 6 shows the carbon (C1 s) and fluorine (F1 s) high-resolution XPS spectra of
SIR sample treated by CF4 capacitively coupled plasma for 5 min. There are two peaks
in the C1 s spectrum, one presents at 284.8 eV corresponding to the [–Si(CH3)2–O–]n
species [29], and the other appears at 286.5 eV corresponding with C–CFn functional
groups [3, 17]. The reactive species in pure CF4 plasma are primarily fluorine atoms
with a small concentration of complex fluorocarbon species [30]. Fluorine atoms and Fsubstituted methyl species (–CFxH(3-x), 0 B x B 3) can be grafted onto a polymeric
surface via methyl or hydrogen replacement and opening of unsaturated bonds to
form the Si–F and C–F functional groups. The methyl replacement by fluorine atom is
highly probable, considering the lower chemical bond dissociation energy of C–Si in
Si–CH3 than that of C–H in CH3, which results in the F–Si component greatly
exceeding F–C. It is also proved by the fact that the increase of F content is
approximately equivalent to the decrease of C content, which is mentioned in the
explanation of Fig. 4 above.
The [F–C]/[F–Si] ratio as a function of CF4 plasma treatment time is shown in Fig. 7.
There is a dramatic decrease in the bond ratio from 0.2481 to 0.0897 with plasma treatment
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Fig. 6 XPS spectra of SIR
sample treated by CF4
capacitively coupled plasma at
RF power of 200 W for 5 min
treatment time. a C1 s; b F1 s
723
(a)
(-Si(CH3)2-O-)n
280
282
284
286
Binding Energy (eV)
C-CFn
288
290
(b)
F-Si
F-C
680
682
684
686
688
Binding Energy (eV)
690
692
time from 20 s to 5 min, and the bond ratio keeps the same state with the increase of
treatment time from 5 to 20 min. The result is attributed to the fact that F atoms added to
the Si atoms are more difficult to be ablated by energetic positive ions than the functional
groups of C–CFn added to the Si atoms, owing to the much stronger chemical bond
dissociation energy of Si–F than that of Si–C.
Therefore, One or two F atoms can be added to the one Si atom to replace one or two
methyl groups linked to Si and form the [–SiFx(CH3)2-x–O–]n (x = 1, 2) structure. The
Si–F or F–Si–F in the [–SiFx(CH3)2-x–O–]n (x = 1, 2) structure is more stable than
Si–CH3 in the [–Si (CH3)2–O–]n structure due to its higher chemical bond dissociation
energy, and the lower surface energy can be acquired via Si–F or F–Si–F rotating and
arranging in the [–SiFx(CH3)2-x–O–]n (x = 1, 2) structure. Thus it is proposed that the
formation of [–SiFx(CH3)2-x–O–]n (x = 1, 2) structure is one of the factors for the superhydrophobic surface of modified SIR.
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Fig. 7 The [F–C]/[F–Si] ratio as
a function of CF4 plasma
treatment time at RF power of
200 W
0.28
[F-C] : [F-Si] Ratio
0.24
0.20
0.16
0.12
0.08
0.04
0
5
10
15
20
Plasma Treatment Time (min)
AFM Analysis
Figure 8 selectively shows the effects of CF4 plasma treatment time on surface topography
of the SIR samples. The untreated SIR sample is not as smooth as expected owing to the
fillers such as SiO2 and Al2O3 particles added to the SIR, and the separate peaks distribute
on the surface uniformly. After treatment, the separate peaks on the surface are linked
together and not as uniform as the untreated sample. Moreover, it seems that the peaks or
mountains are shifted from the upper side to the lower side of horizontal plan with the
increase of treatment time. In Fig. 9, the surface root-mean-square (RMS) data are plotted
as a function of CF4 plasma treatment time at RF power of 200 W. The untreated SIR
sample exhibits a rough surface with RMS roughness value of 42.46 nm. With short
treatment time (B5 min), the surface RMS roughness increases remarkably to the maximum 134.425 nm of 5 min-treated SIR sample. Within 5–10 min of treatment, the surface
RMS roughness decreases markedly from the maximum to 87.618 nm. Between 10 and
20 min of treatment, the surface RMS roughness does not change much. It is very clear that
the maximum of surface RMS roughness appears at the 5 min treatment time, corresponding to the maximal value of static contact angle of distilled water (Fig. 2).
The results indicate that ablation or etching increases more quickly than fluorination
with the increase of treatment time and the two reactions reach a dynamic near-equilibrium
under the constant RF power of 200 W. With short plasma treatment time (B5 min), it is
suggested that the sharply increased roughness and the separate peaks of original SIR
sample linked together are mainly attributed to selective etching of the filler of SiO2
particles on the SIR surface caused by F chemical radicals in CF4 plasma, and the fluoric
groups introduced onto the SIR surface is also one minor factor for the increased roughness
and the change of surface micro-topography because the fluorine content of treated SIR
increases rapidly from 0 to 9.41% (Fig. 5). Within 5–10 min of treatment, fluorination
predominates over ablation or etching because the new fluoride film weakens or lowers
etching or ablation, leading to the decrease of surface RMS roughness (Fig. 9) and the
sharply increase in the fluorine content of modified SIR from 9.41 to 13.57% (Fig. 5).
Between 10 and 20 min of treatment, it seems that the rates of fluorination and ablation or
etching are approximately equal, resulting in almost constant RMS roughness (Fig. 9) and
slight increase in fluorine content of modified SIR from 13.57 to 14.01% (Fig. 5).
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Fig. 8 AFM images of SIR samples treated by CF4 capacitively coupled plasma at RF power of 200 W a
untreated sample; b 5 min treatment; c 20 min treatment
Moreover, the surface RMS roughness curve is similar to the static contact angle curve at
the RF power of 200 W, and testing liquid on the modified SIR surface, which corresponds
to a static contact angle of 150.2°, almost does not move when the SIR sample plate is
circumgyrated slowly from 0 to 90°, even to 180°. Therefore, the contact form between the
liquid drop and the solid surface conforms to the wetting contact model as reported in the
previous studies [31–33].
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140
RMS Roughness (nm)
Fig. 9 Surface RMS roughness
of SIR samples treated by CF4
capacitively coupled plasma at
RF power of 200 W for different
treatment time
120
100
80
60
40
0
5
10
15
20
Plasma Treatment Time (min)
Surface Energy Analysis
The surface energy c is calculated by Owens–Wendt geometric mean equation [34, 35].
cL ð1 þ coshÞ ¼ 2ðcdS cdL Þ1=2 þ 2ðcPS cPL Þ1=2
ð1Þ
cdS
ð2Þ
cS ¼
þ
cPS
where cL is surface energy of testing liquid, cS is solid surface energy, cdL is dispersion
force of testing liquid, cdS is dispersion force of solid, cPL is non-dispersion force of liquid,
and cPS is non-dispersion force of solid. If static contact angles of two types of testing
liquids are measured by the SCA method, solid surface energy could be calculated via the
two equations. The deionized water and diiodomethane are chosen as the testing liquids.
By using measured contact angles, surface energy of original SIR sample is calculated
to be 27.37 mJ/m2, which is higher than the reported value of 21.6 mJ/m2 for pure SIR due
to the inclusion of fillers. Figure 10 shows the surface energy of SIR modified by CF4
plasma treatment at RF power of 200 W for the different treatment time. The surface
30
Surface Energy (mJ/m2)
Fig. 10 Surface energy of SIR
samples treated by CF4
capacitively coupled plasma at
RF power of 200 W for different
treatment time
25
20
15
10
5
0
123
0
5
10
15
Plasma Treatment Time (min)
20
Plasma Chem Plasma Process (2008) 28:715–728
727
energy decreases significantly from 27.37 mJ/m2 of original sample to 2.94 mJ/m2 of
modified SIR sample for a short treatment time of 5 min. As for treatment time longer than
5 min, surface energy keeps relatively stable.
According to Wenzel’s Law [36], the roughening of an already hydrophobic surface can
further decrease its wettability. Therefore, ablation-induced surface roughening can reduce
the surface energy by increasing the contact area between the liquid drop and solid surface,
which can be proved by the comparison of Figs. 9 and 10. Although the fluorine content of
SIR samples treated at RF power of 200 W does not reach the maximum at 5 min treatment time, the increase of F content on the surface is fit to the decrease of surface energy.
Conclusions
1. The static contact angle of SIR surface is improved from 100.7 to 150.2° via the CF4
plasma modification, and the super-hydrophobic surface of modified SIR with the
corresponding static contact angle 150.2°, appears at RF power of 200 W for treatment
time of 5 min.
2. When the SIR samples are modified by CF4 capacitively coupled plasma at RF power
of 200 W for treatment up to 20 min, the fluorine replacement –CH3 predominates
over the fluorine replacement H because the ratio of optical density of –CH3 at
1,260 cm-1 and Si–(CH3)2 at 794 cm-1 decreases more from the original 0.414–0.131
than that of C–H at 2,962 cm-1 from the original 0.081–0.029.
3. The fluorine mainly exists in the [–SiFx(CH3)2-x–O–]n (x = 1, 2) structure formed by
the F replacement –CH3 because the ratio of [F–C] and [F–Si] on the modified SIR
surface is very small and it changes in the range of 0.0673–0.2481.
4. The wide-range improvement on hydrophobicity of modified SIR surface is attributed
to the combined action between the increase of roughness created by the ablation or
etching action and the formation of [–SiFx(CH3)2-x–O–]n (x = 1, 2) structure
produced by F atoms replacement methyl groups reaction.
Acknowledgments We acknowledge contributory discussions and technical assistant of X. P. Zhu, X. G.
Han, C. Liu, H. Wang, P. Li.
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