Desalination 399 (2016) 178–184
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
Nanostructured PVDF membrane for MD application by an O2 and CF4
plasma treatment
Seongpil Jeong a, Bongsu Shin b,c, Wonjin Jo b, Ho-Young Kim c, Myoung-Woon Moon b, Seockheon Lee a,⁎
a
b
c
Water and Resource Recycle Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
Materials and Life Science Research Division, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826, Republic of Korea
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The PVDF membranes were modified
by plasma treatments with O2 and CF4
gases.
• The modified membrane have an enhanced flux and hydrophobicity after
the plasma treatment.
• The pore-size and porosity at the membrane surface increased after the plasma treatment.
• The hairy structure was formed at the
membrane surface by the HMDSO coating.
• The O2 plasma treatment with HMDSO
coating increased the receding contact
angle.
a r t i c l e
i n f o
Article history:
Received 26 April 2016
Received in revised form 2 August 2016
Accepted 5 September 2016
Available online xxxx
Keywords:
Membrane distillation
O2 plasma treatment
CF4 plasma treatment
Nanostructured membrane
PECVD
HMDSO
a b s t r a c t
Recently, various nanotechnologies have been utilized with regard to membrane modification due to their high
activities and the low cost of the nanomaterials involved. In order to enhance the hydrophobicity of the membrane surface for membrane distillation applications by decreasing the surface energy, a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process is suggested with surface nanostructuring and a
subsequent hydrophobic coating step. In this research, a commercial PVDF membrane was modified by plasma
treatments with the two different gases of O2 and CF4. The water contact angles of the active layers increased
from 73 to 117 and 101° and the fluxes of the treated membranes increased to 63 and 27.9% as compared to a
virgin PVDF membrane when the feed used was D.I. water by the O2 and CF4 plasma modifications, respectively.
Defluorination at the long exposure time (120 min) of the plasma treatment and increase of the overall hydrophobicity (the decrease of the contact angle hysteresis) by the HMDSO coating were the reasons of the flux variations for the plasma modified membranes.
© 2016 Published by Elsevier B.V.
1. Introduction
⁎ Corresponding author.
E-mail address: [email protected] (S. Lee).
http://dx.doi.org/10.1016/j.desal.2016.09.001
0011-9164/© 2016 Published by Elsevier B.V.
Membrane distillation (MD) is a thermally driven membrane process. The main mechanism of membrane distillation is the vapor pressure difference between the feed and the cooling sides. Water vapor
S. Jeong et al. / Desalination 399 (2016) 178–184
evaporated from the hot feed side moves through a porous hydrophobic
membrane and condenses as water drops on the cold cooling side. Because only vapor is transported across the membrane, the salt rejection
rate for non-volatile ions under the operating conditions of the MD process is theoretically 100%. In the desalination field, the MD process is
considered as a next-generation technology due to the small flux decrease, though the feed solution is highly saline. Recently, pilot plants
have been built worldwide in order to commercialize the MD system
in the desalination field or for other possible applications.
One of the major hurdles preventing the commercialization of MD
technology is the absence of a high-flux membrane. In order to develop
high-performance MD membranes, various approaches have been used.
Different polymeric and ceramic materials (PP, PVDF, PTFE, PE and zirconia) have been utilized as the active layer of MD membranes [1–3].
Flat-sheet and hollow-fiber membranes with a single layer or with a
bi-layer structure have been tested under various MD configurations
[4–6]. Novel methods also have been used to fabricate a nanofiber or a
mesh of carbon nanotubes [7–9].
Surface modification refers to a technique to enhance the required
performance of a membrane by adding a small amount of a functional
group onto the surface or by changing the surface characteristics by
means of physical or chemical methods. Due to the widespread application of surface modification methods, several practical technologies for
membranes have been developed. One method involves the attachment
of a functional group of ions (zwitterions and hydrophilic matter) to
provide anti-fouling characteristics on the membrane surface [10,11].
Membrane characteristics such as the hydrophobicity, surface charge
and roughness have also been shown to be feasible targets in the effort
to develop a high-performance membrane [12–14]. In order to change
the surface characteristics of the membrane, various techniques have
been suggested, such as surface polymerization, layer-by-layer
methods, chemical vapor deposition (CVD), or a plasma treatment
[14–19].
In this study, a radio-frequency plasma-enhanced chemical vapor
deposition (RF-PECVD) process was selected for use to improve the hydrophobicity of the MD membrane. In dry etching technologies such as
RF-PECVD, the membrane sample can be immersed in various reactive
gases (O2, CF4, etc.). The membrane samples can be modified by two
major reactions (physical bombardment by ions and a chemical reaction caused by radicals) in the RF-PECVD chamber. It is known that radical reactions cause greater modifications than physical ion
bombardment. The emitted ions are responsible for the physical (energetic) bombarding of the surface, which influences the chemical etching
process. The generated radicals react with the surface to produce volatile products (CO, CO2, etc.), which are responsible for the dry etching
and which become attached to the membrane surface by forming sites
for polymerization with other additives [20]. In earlier work, the plasma
polymerization of hexamethyldisiloxane (HMDSO), which has low surface energy of 24.2 mN/m, was shown to increase the degree of surface
hydrophobicity [21,22].
Other researchers have also utilized a plasma treatment to modify
the surfaces of MD membranes. Yang et al. modified an original hollow
fiber membrane using a P2i plasma enhancement machine [16]. In
Yang's research, a hydrophobic polymeric nano-coating with 1H, 1H,
2H, and 2H-perfluorodecyl acrylate was produced on the membrane
surface by a plasma treatment. The LEP (liquid entry pressure) was enhanced due to the increased contact angle and the decreased pore size
of the modified membrane. Wei et al. modified a hydrophilic polyethersulfone (PES) membrane with a CF4 plasma treatment [17]. The contact
angle of the plasma-treated membrane increased from 60° to 120° and
was maintained with excessive plasma power and exposure times. The
flux of the PES membrane modified with CF4 plasma was 66.7 kg/m2 h
when the feed and permeate temperatures were 73.8 and 20 °C, respectively. This represents one of the highest flux levels relative to those reported from the references in Wei's research when various
characteristics of the membranes and operating conditions are
179
considered. Yang et al. recently suggested that the flux of plasma-modified membranes is enhanced due to the increase in the effective evaporation caused by the change of the surface hydrophobicity [19].
However, research with respect to the relationship between membrane
characteristics and the membrane performance is still limited. In this research, we focused on the relationship between the hydrophobicity and
membrane flux. First, the static and dynamic hydrophobicities were
measured for the modified membrane surfaces in order to check the hydrophobicity uniformity. We also tried to compare the modified membrane surfaces and their fluxes according to the introducing gases of
the plasma treatment.
To investigate the performance enhancement caused by a plasma
treatment of a MD membrane, a commercial PVDF membrane was chosen and modified 1) by an O2 plasma treatment with a subsequent hydrophobic coating and 2) solely by CF4 plasma. The exposure time of
the each plasma treatment and the number of modified membrane surface (active layer only or active/support layer together) for each membrane was varied. Furthermore, to understand the relationship
between the modified membrane characteristics and the membrane
flux, SEM-EDX and contact angle analyses were conducted.
2. Experimental method
2.1. Membrane
A commercial PVDF membrane manufactured by a membrane company (S80306, Pall, U.S.A.) was selected as the candidate membrane for
the plasma treatment. The single-layer flat-sheet type of membrane
chosen for use here had a pore size of 0.45 μm and a membrane thickness of 147 μm.
2.2. Hydrophobic coating by a plasma treatment
The PVDF membrane was modified using O2 and CF4 plasma via RFPECVD. The schematic diagram of the experimental set-up is shown in
Fig. 1. The voltage of the CVD machine was fixed at 400 V (62 W) and
the vacuum condition in the CVD chamber was 20 mTorr. The membranes were modified by O2 plasma and CF4 plasma in the following
three ways. First, a hydrophobic coating was deposited onto the membrane facing the feed side by means of an O2 plasma treatment (an
O2‐hydrophobic coating). The procedure of the O2 plasma coating had
two steps. In the first step, the target pristine PVDF membrane was
placed into the CVD chamber and was exposed to the plasma with O2
gas from 30 to 120 min according to the experimental conditions. Secondly, the plasma-treated membrane was coated with HMDSO
(hexamethyldisiloxane, Sigma Aldrich, U.S.A.) for 30 s. Because the O2
plasma-treated surface becomes hydrophilic, the low-surface-energy
material of HMDSO, with a surface energy level of 24.4 mJ/m2, i.e.,
lower than the surface energy (31.6 mJ/m2) of the PVDF membrane,
was used as a coating [23]. Second, with this method, a hydrophobic
and hydrophilic coating was deposited onto the membrane surfaces facing the feed and the permeate sides via the O2 plasma treatment (an O2hydrophobic/hydrophilic coating). One side of the target membrane
facing the feed solution was coated using the same procedure described
above, but for the other side of the target membrane facing the permeate solution, the HMDSO coating was omitted, leading to asymmetric
wettability. The third way, involving the CF4 plasma treatment, was
one in which the HMDSO coating was not conducted because the CF4treated surface became hydrophobic (a CF4‐hydrophobic coating). The
exposure time of the CF4 coating ranged from 60 to 120 m. Detailed information about the testing conditions is given in Table 1.
2.3. Module and DCMD operation
A plate and frame type of acryl module was used for the direct contact membrane distillation (DCMD) test conducted. The dimensions of
180
S. Jeong et al. / Desalination 399 (2016) 178–184
previous work [24]. The feed and permeate solutions were D.I. water.
For the integrity test, 0.6 M of a NaCl solution was used in the feed
with the O2-plasma-treated membrane (O2 plasma for 30 min and
HMDSO for 30 s). The permeate conductivity levels were maintained
at 1–2 μS/cm for an operation time of 90 min when the feed was the
0.6 M NaCl solution.
2.4. Instrumental analysis
In order to measure the variation of the hydrophobicity, surface
morphology and elemental composition of the modified membrane surfaces, the following instrumental analyses were conducted. Contact
angle (CA) observations of pristine and variously modified membranes
were conducted using the sessile drop method with D.I. water and a
micro-syringe on a static contact angle apparatus (DSA100, KRÜSS, Germany). The device measured the static contact angle 18–20 times per
each water drop and at least 2 water drops were observed for each
membrane sample. The contact angle hysteresis (CAH) was also measured with a dynamic contact angle analyzer (Sigma 700, Biolin Scientific, Finland). The surfaces of the pristine PVDF and plasma-modified
membranes were observed by FE-SEM (S-4200, Hitachi, Japan) and ESEM with Energy-Dispersive Spectroscopy (EDS) (XL-30, FEI, USA).
3. Results and discussion
3.1. Flux of the modified membrane after the plasma treatment
Fig. 1. Schematic diagram of rf-PECVD and DCMD systems.
the water channel for the feed and permeate sides are identical at
0.026 m × 0.077 m (the effective area of the membrane) × 0.003 m
(the height of the water channel). The inlet and outlet are connected
to the module perpendicularly to the water flow. The target membranes
were placed in a DCMD module and operated for 90 min under the
given experimental conditions. The operating conditions (i.e., feed and
permeate temperature, flow rate) are controlled using a water bath
with a stainless steel coil and by a flow-controllable gear pump
(75211-15, Cole-Parmer Instrument Company, U.S.A.). The feed and
permeate temperatures in all tests were 60 and 20 °C, respectively.
The cross-flow velocities of the feed and permeate were 0.34 m/s and
0.26 m/s, respectively. The experimental data (i.e., the increase in the
weight of the permeate solution, the EC in the feed and permeate) are
recorded by a balance and with an electronic conductivity meter
(HQ40d, Hach, U.S.A.) and are automatically stored on a laptop computer. The details and operation of the experiment unit were sourced from
Table 1
Detailed experimental conditions (plasma conditions and exposure times for the pristine
and plasma modified membranes).
Abbreviation Plasma condition
Exposure
time
CF4 plasma
(min)
treatment
Hydrophobic Hydrophobic/hydrophilic
(hydrophobic
coating (H)
coating (HH)
coating)
O2 plasma treatment
O
O2-H-30
O2-H-120
O
O2-HH-30,30
O2-HH-80,60
CF4-H-60
CF4-H-90
CF4-H-120
a
O
O
O
O
O
30a
120a
30, 30b
80, 60b
60c
90c
120c
(T) O2 plasma (exposure time, A) + HMDSO coating (30 s).
(TF, TP) feed side: O2 plasma (exposure time, TF) + HMDSO coating (30 s)/permeate
side: O2 plasma (exposure time, TP).
c
(T) CF4 plasma (exposure time, A).
b
3.1.1. O2 hydrophobic coating and hydrophobic/hydrophilic coating
Fig. 2 shows the flux variations according to the exposure time to O2
plasma at 30 s for the HMDSO coating (the O2-hydrophobic coating).
The flux significantly increased when the exposure time of the O2 plasma was 120 min (shown as a black triangle in Fig. 2). The average fluxes
of the modified membranes (O2-H-30 (shown as an empty square in Fig.
2) and O2-H-120) were 21.9 and 33.2 kg/m2 h, respectively, while the
average flux of the virgin membrane (shown as a gray diamond in Fig.
2) was 20.4 kg/m2 h. The modified membranes treated with the O2 plasma with the HMDSO coating showed higher flux levels by 7.6 and 63.0%
compared to the pristine membrane depending on the O2 plasma treatment exposure time.
The flux behaviors according to the exposure time of the O2 plasma
treatment (both sides of the membrane) with the HMDSO coating (only
the feed side of the membrane) for the PVDF membrane are described in
Fig. 3 (O2-hydrophobic/hydrophilic coating). The average flux increased
from 23.0 to 28.0 kg/m2 h with longer exposure times of the O2 plasma
(O2-HH-30, 30 and O2-HH-80, 60). The flux (23.0 kg/m2 h) of the modified membrane (O2-HH-30, 30) with both sides of the membrane treated was slightly higher than that (21.9 kg/m2 h) of the modified
membrane with a single side of the membrane treated (O2-H-30) at
an identical O2 exposure time of 30 min. One possible reason for the increased flux of the modified membrane with both sides treated is the
decrease in the effective membrane thickness, as the membrane on
the permeate side became hydrophilic due to the grafting of hydrophilic
polar oxygenated species [25,26]. Consequently, after the O2 plasma
treatment with the HMDSO coating on the PVDF membrane, the flux
of the modified membrane increased with the O2 plasma exposure
time under the given experimental conditions.
3.1.2. CF4 hydrophobic coating
The flux behavior according to the exposure time of the CF4 plasma
for the target membrane is shown in Fig. 4. The average fluxes of the
modified membranes (CF4-H-60, CF4-H-90 and CF4-H-120) according
to the CF4 exposure times (60, 90 and 120 min) were 27.9, 24.6 and
25.5 kg/m2 h, respectively. All of the modified membranes showed enhanced flux compared to the virgin membrane. Unlike the O2 plasma
treatments results, the flux did not distinctly increase according to the
plasma exposure time under the given experimental conditions (CF4
S. Jeong et al. / Desalination 399 (2016) 178–184
181
Fig. 2. Flux behavior according to the exposure time of the O2 plasma treatment (single
side of the membrane) with the HMDSO coating on the PVDF membrane (feed water:
D.I. water, corresponding temperatures of the feed and permeate: 60 °C and 20 °C,
corresponding cross-flow velocities of the feed and permeate: 0.34 m/s and 0.26 m/s).
Fig. 4. Flux behavior according to the exposure time of the CF4 plasma treatment (single
side of the membrane) for the PVDF membrane (feed water: D.I. water, corresponding
temperatures of the feed and permeate: 60 °C and 20 °C, corresponding cross-flow
velocities of the feed and permeate: 0.34 m/s and 0.26 m/s).
plasma exposure times of 60–120 min). Plateaus of the membrane characteristics (surface porosity and pore size) and the performance (flux)
according to the exposure time of the plasma treatment were observed
in earlier studies when the exposure time was excessive [17,18]. However, in case of the O2 plasma treatment, the plateaus was not observed
under given experimental conditions (exposure time from 30 min to
120 min). Moreover, the flux (33.2 kg/m2 h) of the modified membrane
which underwent the O2 plasma treatment was higher than that
(25.5 kg/m2 h) of that of the CF4 plasma treatment with a long exposure
time (120 min). The fluxes of the plasma treated membranes by O2 and
CF4 plasma were enhanced up to 63 to 27.9%, respectively. With increase of the membrane flux, the required membrane area could be decreased. Therefore, the capital cost including the membrane
(purchasing and replacing), modulation, piping and land (foot print)
costs could decrease. Though the PTFE membrane has promising characteristics on hydrophobicity and flux, the plasma modified PVDF membrane also might have chance to be commercialized due to the difficulty
in fabrication and the limitation in the modification of the PTFE membrane. Many researchers have been modified a PVDF material for the
MD application (18–19, 27).
In order to understand the reasons for the flux variation according to
the exposure time of the plasma treatment and type of the plasma, the
membrane characteristics (i.e., the pore size, porosity and membrane
thickness) and the hydrophobicity (CA and CAH) were analyzed.
3.2. Effect of the plasma treatment on the physical and chemical characteristics of the membrane
Fig. 3. Flux behavior according to the exposure time of the O2 plasma treatment (both
sides of the membrane) with the HMDSO coating (feed side of the membrane only) on
the PVDF membrane (feed water: D.I. water, corresponding temperatures of the feed
and permeate: 60 °C and 20 °C, corresponding cross-flow velocities of the feed and
permeate: 0.34 m/s and 0.26 m/s).
3.2.1. Surface morphology and membrane thickness according to the type
and exposure time of the plasma
The surfaces of the virgin and modified membranes were observed
by SEM. The surface morphologies after the plasma treatments showed
distinct increases in the pore size and porosity compared to those of the
virgin membranes (Figs. 5 and 6). As shown in Fig. 5(a) (the virgin
membrane) and Fig. 5(b) and (c) (the O2-plasma-modified membranes), the pore size and porosity on the surface of the PVDF membrane increased significantly with an increase in the exposure time of
the O2 plasma up to 120 min. After the O2 plasma treatment (O2-H120), hairy structures were found to have formed on the modified
membranes (Fig. 5(c)). The hairy structures are related to the surface
wettability, as the surface had become more hydrophobic [28]. This result corresponded to the increased flux of the PVDF membrane with the
increased exposure time of the O2 plasma treatment (Fig. 2). For the CF4
plasma treatment of the PVDF membrane (Fig. 6), all of the modified
membranes had similar surface morphologies under the given the plasma exposure times after the plasma treatment. The results of observations of the SEM images correspond to the flux results, indicating that
the plasma treatment enhances the flux of the modified membrane by
changing its surface morphology. In summary, the increases in the
pore size and porosity may affect the increase in the flux of the PVDF
membrane when the membrane was modified by the plasma treatment. The plasma etched membrane surfaces were similar when the exposure time of the plasma treatment was higher than 30 min, however,
in the case of O2-H-120 experiment, the hairy structure was observed.
Additionally, the membrane thicknesses of the pristine and modified
membranes were investigated in cross-sectional images taken from the
SEM analysis. The membrane thicknesses of the tested membranes
ranged from 100 to 120 μm, and no significant change in the membrane
thickness occurred after the plasma treatment.
3.2.2. Hydrophobic characteristics of the modified membrane
The measured static contact angle (CA) and contact angle hysteresis
(CAH) for the virgin and modified membranes and the fluxes under
given experimental condition are listed in Table 2. The advancing and
receding contact angles are the maximum and minimum contact angles,
respectively. Additionally, the contact angle hysteresis can be calculated
from the difference between the advancing and receding contact angles.
The static contact angle of the O2-plasma-modified membrane increased with the plasma exposure time (Table 2). Camacho et al.
found that the overall flux of the membrane distillation was dependent
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S. Jeong et al. / Desalination 399 (2016) 178–184
(a)
(a)
5 µm
5 µm
(b)
(b)
5 µm
5 µm
(c)
(c)
5 µm
5 µm
Fig. 5. Surface images of the virgin and modified membranes according to the O2 plasma
exposure time ((a) virgin PVDF membrane, (b) O2 plasma (30 min) + HMDSO (30 s),
(c) O2 plasma (120 min) + HMDSO (30 s)).
with surface contact angle because hydrophobic structures has lower
thermal conductivity according to their review paper [29]. Fan et al. fabricated and tested the various hydrophobic PVDF membranes and the
flux of the membrane which poses higher hydrophobicity had a higher
flux [27]. However, the static contact angle of the modified membranes
did not increase with an increase in the plasma treatment time in the
case of the CH4 plasma treatment. These results correspond to the surface morphology variations (Fig. 6).
The CAH of the modified membrane decreased after the O2 and CF4
plasma treatment (120 min) and the advancing and receding contact
angles were increased. In detail, the O2-H-120 membrane showed a
Fig. 6. Surface images of the membranes according to the CF4 plasma exposure time ((a)
CF4 plasma (60 min), (b) CF4 plasma (90 min), (c) CF4 plasma (120 min)).
higher receding contact angle (109.4°) and a lower CAH (36.4°) than
the CF4-H-120 membrane even though the advancing contact angles
of both membranes are similar. This suggests that the O2-H-120 membrane has a more evenly distributed hydrophobic surface than the
CF4-H-120 membrane. Moreover, when we compared the performance
result (flux) between the O2-H-120 membrane and the CF4-H-120
membrane, it could be concluded that the low adhesive hydrophobic
surface having low CAH results in high flux during the membrane distillation process. The suggested reason for the flux increase with an increased hydrophobicity of the active layer of the MD membrane was
the increase of the effective evaporation area [19]. However, the flux increase according to the CAH decrease has not been suggested. The water
vapor pressure near the relatively adhesive surface (CF4 plasma modified membrane than O2 plasma modified membrane) could be decreased due to the decrease of the adhesion force between the water
vapor and the membrane.
S. Jeong et al. / Desalination 399 (2016) 178–184
183
Table 2
Static contact angle, contact angle hysteresis and flux of the pristine and plasma modified membranes.
Static contact angle (°)
Dynamic contact angle (°)
Advancing contact angle
Receding contact angle
Contact angle hysteresis
Flux (kg/m2h)
Virgin membrane
O2-H-30
O2-H-120
O2-HH-80,60
CF4-H-60
CF4-H-90
CF4-H-120
73.4 ± 4.9
111.5
28.5
83.0
20.4
99.5 ± 3.3
–
–
–
21.9
117.2 ± 14.5
145.8
109.4
36.4
33.2
103.8 ± 1.5
–
–
–
28.0
101.3 ± 3.6
–
–
–
27.9
101.4 ± 16.2
–
–
–
24.6
99.6 ± 3.4
145.6
81.7
63.9
25.5
3.2.3. EDX analysis of the plasma-modified membranes
The chemical compositions of the pristine and modified membranes
were investigated in an EDX analysis, as shown in Table 3. Carbon (68%)
and fluoride (32%) were the major components of the virgin PVDF
membrane. The measured fluorine-to-carbon (F/C) atom ratio of the
virgin membrane was 0.5, similar to the F/C atom ratio (0.48) of the
PVDF membrane from the research of Masuelli et al. [30]. For the modified membranes, the F/C atom ratios increased after the shorter O2 and
CF4 plasma treatments (30 min and 60 min, respectively) and decreased
after the longer plasma treatment (120 min).
The increase in the F/C ratio at an early stage of the O2 and CF4 plasma treatments may be due to the strong etching effect of the plasma in
these cases [31]. In case of the O2 plasma treatment, the produced oxygen radical in the plasma chamber liberates carbon and hydrogen from
the PVDF membrane by forming carbon dioxide (CO2) [32]. Additionally, after the O2 plasma treatment, oxygen attaches onto the modified
membrane surface and the HMDSO containing the oxygen and silica
also attach onto the O2-plasma-treated area. The attached HMDSO molecules can boost the surface hydrophobicity on the membrane surface
[21,22]. As shown in Table 3, the oxygen and silica contents of the O2plasma-treated membrane occupied b5.9 and 1.8 atomic percentage,
respectively, due to the HMDSO coating. The increased silica contents
with the increased exposure time can be explained by the enhanced adsorption sites for the HMDSO coatings formed by the increased exposure time of the O2 plasma. The decomposed HMDSO and its
hydrophobic functional group could be coated as SiOx–C:H film [21].
Therefore, additional hydrophobicity could be achieved. For the CF4
plasma treatment, the etching, atomic insertion, deposition and polymerization could be happened [19]. The results (Table 3) showed that
the fluorination happened at the modified membrane surface possibly
by etching and F atom insertion. Therefore, it can be expected that
thin F layer may be formed at the modified hydrophobic membrane
surface.
The decrease in the F/C ratio (1.1–N0.6) after 120 min of exposure to
the O2 and CF4 plasma can be explained as defluorination [33]. The O/C
ratio also decrease at 120 min of exposure time to the O2 plasma, which
represents that the decrease of the O content (deoxygenation). In case
of the O2 plasma modified membrane, the major contributor of the hydrophobicity could be the HMDSO coating because the more exposure
time of the O2 plasma resulted in more Si content at the membrane surface (Table 3). The one of the possible candidates of the hydrophobic
functional group is [-SiF2-O-](n). The formation of [-SiF2-O-](n) structure
was suggested as the major factor for super-hydrophobic surface [34]. In
this research, the occurred defluorination for O2-H-120 membrane
Table 3
EDX analysis results for the pristine and plasma modified membranes.
Types of the
plasma
Exposure time
(min)
C
(%)
F
(%)
O
(%)
Si
(%)
F/C
(%/%)
O/C
(%/%)
Non-treated
O2
–
30
120
60
90
120
68
47.7
58.6
50.2
50.3
61.8
32
48.4
36.8
49.8
49.7
38.2
0
3.5
2.8
0
0
0
0
0.4
1.8
0
0
0
0.5
1.0
0.6
1.0
1.0
0.6
0
0.1
0.05
0.0
0.0
0.0
CF4
supported that the formation of the SiFxOy complex. The other candidate functional groups formed at the membrane surface can be combined hydrophilic polar oxygenated species [25,26] with fluoride, O–
CH2 or O–CH2–CF2 [35]. And the possible reason of the deoxygenation
for O2-H-120 membrane could be the decrease of those functional
groups at the membrane surface (Table 3).
In case of the CF4 plasma treatment, the fluorinated layer is the
major contributor for the hydrophobicity of the modified membrane.
The site for the F attachment can be limited due to the number of the hydrogen at the membrane surface is limited. Therefore, when the
defluorination happened (CF4-H-120), the hydrophobicity and the flux
were slightly decreased (Table 2).
4. Conclusion
A plasma treatment was utilized as a surface modification tool for a
PVDF membrane. The flux of a hydrophilic PVDF membrane increased
63% with the exposure time of an O2 plasma treatment. The contact
angle of the treated membrane was also increased from 73 to 117°
with the exposure time of the O2 plasma treatment with a subsequent
HMDSO coating. The flux of the modified membrane was also increased
27.9% by a CF4 treatment; however, no significant enhancement was observed when the exposure time ranged from 60 to 120 min. Both plasma
(O2 and CF4) treatments enhanced the hydrophobicity of the PVDF
membrane surface from 73 to 117 and 101°, respectively. The flux of
the O2 plasma-modified membrane after a long plasma exposure time
(120 min) was higher than that of the CF4 plasma-modified membrane
because the HMDSO coating enhanced the overall hydrophobicity by
decreasing CAH of the modified membrane and defluorination
happened.
Acknowledgements
We greatly appreciate the analytical support from Mr. Younggil
Yoon (E-SEM) and Ms. Sohee Kim (FE-SEM) at the Korea Institute of Science and Technology. This research was supported by the Korean Ministry of the Environment as an “Eco-Innovation Program”
(Environmental Research Laboratory) (414-111-011) and a grant
[MPSS-CG-2016-02] through the Disaster and Safety Management Institute funded by Ministry of Public Safety and Security of Korean
government.
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