Chinese Journal of Chemical Engineering, 20(1) 71—79 (2012)
Effects of Additives and Coagulant Temperature on Fabrication of
High Performance PVDF/Pluronic F127 Blend Hollow Fiber
Membranes via Nonsolvent Induced Phase Separation
Chun Heng Loh1,2 and Rong Wang1,2,*
1
2
School of Civil and Environmental Engineering, Nanyang Technological University, 639798 Singapore, Singapore
Singapore Membrane Technology Centre, Nanyang Technological University, 639798 Singapore, Singapore
Abstract Poly(vinylidene fluoride) (PVDF) has become one of the most popular materials for membrane preparation via nonsolvent induced phase separation (NIPS) process. In this study, an amphiphilic block copolymer, Pluronic F127, has been used as both a pore-former and a surface-modifier in the fabrication of PVDF hollow fiber
membranes to enhance the membrane permeability and hydrophilicity. The effects of 2nd additive and coagulant
temperature on the formation of PVDF/Pluronic F127 membranes have also been investigated. The as-spun hollow
fibers were characterized in terms of cross-sectional morphology, pure water permeation (PWP), relative molecular
mass cut-off (MWCO), membrane chemistry, and hydrophilicity. It was observed that the addition of Pluronic F127
significantly increased the PWP of as-spun fibers, while the membrane contact angle was reduced. However, the
size of macrovoids in the membranes was undesirably large. The addition of a 2nd additive, including lithium chloride (LiCl) and water, or an increase in coagulant temperature was found to effectively suppress the macrovoid formation in the Pluronic-containing membranes. In addition, the use of LiCl as a 2nd additive also further enhanced
the PWP and hydrophilicity of the membranes, while the surface pore size became smaller. PVDF hollow fiber with
a PWP as high as 2530 L·m−2·h−1·MPa−1, a MWCO of 53000 and a contact angle of 71° was successfully fabricated
with 3% (by mass) of Pluronic F127 and 3% (by mass) of LiCl at a coagulant temperature of 25 °C, which shows
better performance as compared with most of PVDF hollow fiber membranes made by NIPS method.
Keywords amphiphilic block copolymer, pore forming, surface modifying, additive, poly(vinylidene fluoride),
hollow fiber membrane
1
INTRODUCTION
The first asymmetric cellulose acetate membrane
via nonsolvent induced phase separation (NIPS) technique was developed by Loeb and Souriajan in 1960’s
[1]. Since then, much effort has been devoted to prepare
high-performance membrane by this method for various
applications. The use of NIPS for membrane preparation
has been extended from cellulose acetate to many other
polymer materials including poly(vinylidene fluoride)
(PVDF). PVDF is a semi-crystalline polymer with good
thermal stability, chemical resistance, and mechanical
properties [2-4]. In addition, among the common hydrophobic polymers, PVDF has a higher processing
ability compared to polytetrafluoroethylene (PTFE)
and polypropylene (PP) as the former can be easily
dissolved in common solvents [5]. All the above advantages make PVDF one of the most popular choices
as a membrane material for various applications including microfiltration [6], ultrafiltration [7], pervaporation [8], membrane distillation [9, 10], and gas-liquid
membrane contactor [11].
Due to the hydrophobic nature of PVDF material, however, it has been found that the penetration of
coagulant (water) into the polymer dope solution is
restricted during the phase inversion process [12]. As a
result, the slow coagulation rate of PVDF might cause
difficulty in the preparation of highly porous membranes. One efficient method to improve the porosity
of PVDF membranes is to introduce suitable hydrophilic additives as a pore former into the polymer
dope solution. As PVDF is usually thermodynamically
incompatible with the additives, the dope solution may
completely separates into two phases during demixing
process and large pores can be formed after the additive-rich phase leached out into the water due to their
hydrophilic nature [13]. A wide range of additives has
been used as pore-former in membrane fabrication,
which includes hydrophilic polymers, inorganic salts
or acids, weak co-solvents, and strong non-solvents [14].
On the other hand, a membrane with hydrophobic
surface may also face severe organic fouling due to
the hydrophobic interaction between organic foulants
and membrane surface in water or wastewater treatments
[15, 16]. Therefore it is desirable to improve the hydrophilicity of PVDF membranes in order to enhance
the membrane performance and reduce the operating
cost. Among various techniques for hydrophilic modification of PVDF membranes, the blending of hydrophilic modifiers with PVDF contributes a more convenient method as it does not require additional processing steps [17]. Various hydrophilic polymers, amphiphilic block copolymers, and inorganic nanoparticles have been reported in the preparation of hydrophilic PVDF membranes via blending method [18, 21].
In the case of blending amphiphilic block copolymers
with PVDF, the copolymers tended to segregate to the
polymer/water interface due to the low interfacial
Received 2011-08-22, accepted 2011-11-16.
* To whom correspondence should be addressed. E-mail: [email protected]
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Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
energy between the hydrophilic segment and water,
while the hydrophobic, water-insoluble segment of the
copolymer firmly anchored in the polymer matrix
[17, 22]. Consequently, a hydrophilic layer was formed
on the membrane surfaces including the surface of
internal pores [22]. Due to the presence of hydrophobic chains in the molecules, amphiphilic copolymers
as the hydrophilic modifier are expected to have a
higher stability in membrane matrix compared to the
hydrophilic polymers.
It is interesting to note that the blending of amphiphilic block copolymers for the preparation of
PVDF membranes not only results in the improved
hydrophilicity, but also increases the membrane porosity due to the presence of hydrophilic segments in
the copolymers [23]. Hashim et al. [24] has also reported an improved hydrophilicity and a significant
flux enhancement of PVDF membranes prepared from
the dope containing PVDF grafted with poly(ethylene
glycol) methyl ether methacrylate (PEGMA) compared to the original PVDF membrane. These results
indicate that the amphiphilic block copolymers can act
as both surface-modifying and pore-forming agents in
the preparation of PVDF membranes.
To date, most of the amphiphilic block copolymers used as additives in PVDF membrane fabrication
via NIPS is self-synthesized while the report on the
use of commercially available ones is limited [17,
23-25]. Zhao et al. [23] has reported the preparation of
PVDF membrane using a commercial amphiphilic
block copolymer of poly(ethylene oxide) (PEO) and
poly(propylene oxide) (PPO). A series of similar commercial products with a trade name of Pluronic, which
are often denoted as PEO-PPO-PEO and PPO-PEOPPO, has also been successfully used to prepare hydrophilic polyethersulfone (PES) membranes [26-31].
Pluronic has been considered as a good alternative to
the self-synthesized amphiphilic copolymers as it is
less costly and does not require sophisticated techniques for polymerization. In our previous study, the
pore-forming ability of different types of Pluronic in
the fabrication of PES hollow fiber membranes was
also observed and the membranes prepared from Pluronic F127 and F108 exhibited the best performance
[32]. However, to the best of our knowledge, the preparation of hydrophilic PVDF hollow fiber membranes
using Pluronic via NIPS has not yet been reported.
In present study, an attempt was made to investigate the effects of Pluronic F127 as both a pore-former
and a surface modifier on the fabrication of PVDF
hollow fiber membranes via NIPS. To further improve
the membrane performance, lithium chloride and water as a second additive have been added into the Pluronic-containing PVDF dope, respectively. The effect
of coagulant temperature on the formation of PVDF
hollow fibers prepared using Pluronic F127 has also
been studied. It is anticipated that this study can provide guidance for the preparation of high performance
PVDF hollow fiber membranes via NIPS process using amphiphilic block copolymers as an additive.
2
2.1
EXPERIMENTAL
Membrane material and chemicals
PVDF (Kynar 761, relative molecular mass 444000,
Arkema) was dried at 50 °C under vacuum for at least
1 day before use. N-methyl-2-pyrrolidone (NMP) purchased from Merck was used as the solvent. Pluronic
F127 (relative molecular mass 12600, PEO100-PPO65PEO100, Sigma), lithium chloride (LiCl, anhydrous,
MP Biomed), and Mili-Q water were used as the
non-solvent additives in membrane preparation. Dextran with different relative molecular mass ranging
from 6000 to 500000 (Sigma) was used to determine
the relative molecular mass cut-off (MWCO) of the hollow fiber membranes. Hexane and 2-propanol (Merck)
were used for membrane drying by solvent-exchange
method. All the reagents were used as received.
2.2 Spinning dope preparation and dope viscosity
measurements
A polymer dope solution for spinning was prepared by simultaneously dissolving desired amount of
pre-dried PVDF and the additives which consist of
Pluronic F127, LiCl, or water in NMP using a jacket
flask. The dope solution was mechanically stirred for
at least 2 d at 60 °C. The homogenous dope prepared
was then cooled down to room temperature and subsequently degassed under vacuum for overnight before
spinning.
Rheological characteristic of the spinning dope
solutions was measured by a rheometer (Physica MCR
101, Anton Paar). The measurements were carried out
using a 25 mm measuring plate (CP25-1) under
steady-state shear rate ranging from 0.01 to 1000 s−1 at
25 °C. The viscosity of dope solutions was recorded at
the shear rate of 10 s−1.
2.3 Spinning of PVDF hollow fiber membranes
and post-treatment
The PVDF hollow fiber membranes were fabricated via dry-jet wet spinning process. For the spinning of different dopes, the dope flow rate was kept
constant while different bore fluid flow rates were
employed in order to achieve a reasonable thickness of
as-spun membranes. The nascent hollow fiber was
collected at free-falling take-up speed and subsequently
stored in a water bath for at least 2 d to remove residual
solvent. The dope compositions and detailed spinning
conditions are listed in Tables 1 and 2, respectively.
For some characterizations which required the
hollow fibers to be dried, post-treatment was performed
to alleviate membrane shrinkage during drying process.
The membranes were immersed in 2-propanol followed by hexane for 4 h each. The membranes were
then air-dried at room temperature before use for
characterizations.
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Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
Table 1 Additive concentration in dope solutions and
viscosity of dopes containing 18% of PVDF
Dope
/membrane code
Pluronic
F127 mass
concentration/%
The 2nd
additive & mass
concentration/%
Dope
viscosity
/Pa·s
P0
0
—
3.6
P1
1
—
4.3
P3
3
—
5.0
P3L3
3
LiCl, 3
13.3
P3W3
3
water, 3
6.4
Table 2
Spinning parameters for PVDF hollow
fiber membranes
Spinning parameters
Value
spinneret dimension/mm
ID = 1.0, OD = 2.0
dope flow rate/g·min
−1
6.0
bore fluid (NMP/water mass ratio)
70%︰30%
bore fluid flow rate/ml·min−1
3-6
external coagulant
tap water
coagulant temperature/°C
25±1, 40±2
air gap/cm
1
take-up speed
free fall
room temperature/°C
20 ± 2
room humidity/%
70 ± 3
2.4 Pure water permeation and MWCO measurement of PVDF hollow fiber membranes
Hollow fiber membranes were kept wet in water
before use for pure water permeation (PWP) and
MWCO characterization. Lab-scale hollow fiber module was made by sealing 10 pieces of hollow fibers in
a glass tube. The effective length of the hollow fibers
was 26 cm and Mili-Q water as feed was circulated
through the shell side of the hollow fibers. Compaction on the membranes was done under the pressure of
at least 34 kPa for 1 h and the permeate was subsequently collected for 5 min under 34 kPa. The PWP of
the hollow fibers (L·m−2·h−1·MPa−1) was calculated
using the formula:
Q
J=
(1)
ΔP × A
where Q is the amount of permeate collected over a
duration of time for the hollow fibers; ΔP the pressure
difference between the feed and permeate side of hollow fibers; A the outer surface area of hollow fibers in
the module.
The MWCO of a hollow fiber membrane is defined as the relative molecular mass at which 90% of
the solute is retained by membranes. The 1-hour compacted hollow fibers were used for MWCO test and
1500 mg·L−1 of a dextran aqueous solution with a
relative molecular mass distribution ranging from 6000
to 500000 was used as the feed. The permeate was
collected after 30 min of feed circulation under 34 kPa
to make sure that the system was stabilized. Gel permeation chromatography (GPC) (PL-GPC50 Plus, A
Varian, Inc.) was used to characterize the dextran relative molecular mass distribution in the feed and permeate. Rejection of hollow fibers was then calculated
according to the formula expressed as
R (a ) =
Cf − Cp
Cf
(2)
where R is the rejection coefficient for dextran molecule
with a certain relative molecular mass a; Cf and Cp the
concentrations of dextran with a relative molecular mass
of a in the feed and permeate solutions, respectively.
2.5 Other characterizations of PVDF hollow fiber
membranes
The cross-sectional morphology of hollow fiber
membranes was observed using a scanning electron
microscope (SEM) (EVO 50, Carl Zeiss AG). The
hollow fibers were broken in liquid nitrogen and sputtered with gold prior to the test.
In order to examine the presence of Pluronic in
PVDF hollow fibers, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was
carried out by IRPrestige-21 spectrophotometer from
Shimadzu. The outer surface of dried membranes was
directly analyzed and the IR spectra were obtained by
45 scans at a resolution of 4 cm−1.
The hydrophilicity of PVDF hollow fibers was
examined by both dynamic contact angle and liquid
entry pressure of water (LEPw). Dynamic contact angle of PVDF hollow fibers was measured by a tensiometer (DCAT11 Dataphysics, Germany) according
to Wilhelmy method. Five immersion-emersion cycles
in Mili-Q water were carried out for each specimen
with a repetition of 5 times for all the samples. The
advancing contact angle of the fifth cycle was recorded. LEPw was measured using dead-end hollow
fiber modules containing a single fiber. Pressure applied to the lumen of the fibers was gradually increased until a continuous flow of water was observed
at the shell side of the fibers, and the pressure was
recorded as the membrane LEPw. The detailed methodology has been described elsewhere [33].
3
3.1
RESULTS AND DISCUSSION
Viscosity of PVDF dope solutions
The kinetics of phase inversion, which always
refers to the out-diffusion rate of the solvent and the
in-diffusion rate of the nonsolvent during membrane
formation, is highly related to the viscosity of the
polymer dope solution: a high viscosity hinders the
exchange of solvent/non-solvent during the phase inversion process and vice versa. The viscosity of the
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Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
dope prepared for spinning is presented in Table 1.
The results revealed that an increase in Pluronic F127
mass concentration from 0 to 3% in PVDF dopes
caused an increase in the dope viscosity from 3.6 to
5.0 Pa·s. The reason for this increase is that the entanglement between the polymer chains of PVDF and
Pluronic F127 becomes more significant with increasing Pluronic F127 concentration [34].
When comparing the dope with and without the
addition of 2nd additives in Table 1, it can be observed
that the dope viscosity increased with the addition of
2nd additives into the polymer dopes. This dope viscosity enhancement effect was more evident for LiCl
than that for water. For instance, the viscosity increased significantly from 5.0 Pa·s for dope P3 to 13.3
Pa·s for dope P3L3, which was the case with the addition of LiCl as a 2nd additive; whereas, a slight increase in viscosity from 5.0 Pa·s for dope P3 to 6.4
Pa·s for dope P3W3, which showed the case with the
addition of water as a 2nd additive, was observed. This
phenomenon has also been reported by other studies
[10, 14, 35]. The significant enhancement of viscosity
caused by the addition of LiCl can be attributed to the
formation of acid-base complexes between LiCl and
the solvent, as well as the interaction between Li+ cation
and electron donor in the polymer molecule [14].
3.2
Morphology of PVDF hollow fiber membranes
3.2.1 Effect of Pluronic as additive
Figure 1 (a-c) show the cross-sectional morphology of PVDF hollow fibers prepared with/without
the addition of Pluronic F127 at a coagulant temperature of 25 °C. Membrane P0, which was prepared
without any additive, exhibited an asymmetric structure
consisting of finger-like macrovoids developed underneath the outer surface of the fibers, and a sponge-like
structure near the lumen of the fibers. With the addition of 1% and 3% (by mass) of Pluronic F127 (membrane P1 and P3), the macrovoids almost fully devel-
oped across the cross-section and the size of macrovoids was significantly increased compared with the
morphology of membrane P0. It was also noticed that
irregular contour was formed at the lumen of the fibers with the addition of Pluronic F127.
The formation of finger-like macrovoids developed from the outer surface of the fibers was caused
by the instantaneous demixing promoted by the rapid
solvent/nonsolvent exchange since water was used as
the external coagulant. On the other hand, due to the
presence of large amount of NMP [70% (by mass)] in
the bore fluid, the solvent/nonsolvent exchange rate
was reduced. Consequently, delayed demixing occurred which was accompanied with the formation of
sponge-like structure and the elimination of skin layer
near the lumen side [36, 37].
The observation on the formation of larger
macrovoids in Figs. 1 (b) and 1 (c) is believed to be
associated with thermodynamic and kinetic effects
caused by the addition of Pluronic F127 into the dope
system. It has been reported in our previous paper that
the addition of Pluronic into polymer dopes resulted in
both the reduction in thermodynamic stability and
increase in non-solvent in-diffusion rate due to the
nonsolvent nature and hydrophilic properties of the
Pluronic F127, respectively [32]. Both factors can
promote instantaneous demixing during the phase inversion process and in turn enhance macrovoid formation. On the other hand, the slight enhancement in
dope viscosity with the addition of Pluronic F127 may
cause a slower solvent-nonsolvent exchange in the
nascent fibers and suppress macrovoid formation, but
it is likely that this kinetic effect is not significant
compared to the thermodynamic effect induced.
These big macrovoids formed in membrane P1
and P3 are not desirable as the mechanical strength of
the membranes will be strongly deteriorated. Therefore, an attempt was made to suppress the macrovoid
formation by the addition of 2nd additives, including
LiCl and water, into the dope containing 3% (by mass)
Figure 1 Cross-sectional morphology of hollow fiber membranes P0 (a, A) , P1 (b, B), P3 (c, C) spun with different concentration of Pluronic F127 as additives at coagulant temperature of 25 °C (a to c) and 40 °C (A to C)
Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
75
Figure 2 Cross-sectional morphology of hollow fiber membranes P3 (a, A) , P3L3 (b, B) , P3W3 (c, C) spun with/without
2nd additives at coagulant temperature of 25 °C (a to c) and 40 °C (A to C)
Pluronic F127, as discussed in the following section.
3.2.2 Effect of second additives
Figures 2 (a-c) present the cross-sectional morphology of PVDF hollow fiber membranes prepared
with/without the addition of 2nd additives at a coagulant temperature of 25 °C, while the concentration of
Pluronic F127 was kept constant at 3% (by mass). The
morphology was observed to change from big macrovoids to narrower finger-like structure when LiCl and
water were added into the Pluronic-containing dopes,
respectively. Similar macrovoid suppression during
membrane formation has also been observed with the
addition of LiCl into a poly(ethylene glycol)
(PEG)-containing PVDF dope [38] and the addition of
water into PEG-containing PES dope [39]. With the
presence of LiCl in the dope, the dope viscosity enhances significantly, as mentioned in Section 3.1. The
high viscosity in turn hindered the exchange of solvent/nonsolvent during the phase inversion process
and hence the size of macrovoids was reduced due to
the delayed precipitation [38]. On the other hand, the
macrovoid suppression reported by Liu et al. [39] was
attributed to the presence of high water content in the
dope but the mechanism was unclear.
3.2.3 Effect of coagulant temperature
Comparing Figs. 1 (a-c) with (A-C) as well as
Figs. 2 (a-c) with (A-C) which shows the crosssectional morphology of fibers spun at a coagulant
temperature of 25 °C and 40 °C, respectively, the effect of coagulant temperature on macrovoid formation
can be observed. The size of macrovoids developed
underneath the outer surface of the fibers became
smaller when a higher coagulant temperature was employed during the spinning process. In addition, the
area of sponge-like structure near the lumen of the
fibers was increased at a coagulant temperature of
40 °C compared to that at 25 °C.
It has been reported that the thermodynamic
stability of a polymer dope system is enhanced at a
higher temperature, which in turn reduced the polymer
precipitation rate [38, 40, 41]. When the nascent fibers
are immersed into the coagulant bath with a temperature of 40 °C, the temperature of the dope in contact
with the water is increased. This higher temperature
results in a delayed demixing process due to the enhanced thermodynamic stability of the dope. Therefore, smaller and shorter macrovoids has been developed from the shell of fibers compared to the case
with a lower coagulant temperature.
3.3 Effect of additives and coagulant temperatures on membrane performance
The PWP and MWCO of hollow fiber membranes prepared with different conditions, including
Pluronic F127 concentration, types of 2nd additive,
and coagulant temperature, are presented in Fig. 3 and
Table 3, respectively. It was observed that membrane
P0, which was prepared without any additive, exhibited a very low PWP of 60 L·m−2·h−1·MPa−1 when a
coagulant temperature of 25 °C was used. With the
Figure 3 PWP of hollow fibers spun with different additives and different coagulant temperature
■ 25 °C; □ 40 °C
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Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
Table 3 MWCO of hollow fibers spun with different
additives and different coagulant temperatures
MWCO
Membrane code
25 °C
40 °C
P0
11000
17000
P1
>500000
>500000
>500000
P3
485000
P3L3
53000
398000
P3W3
>500000
>500000
addition of 1% and 3% (by mass) of Pluronic F127,
the PWP of membrane P1 and P3 increased sharply to
760 and 780 L·m−2·h−1·MPa−1, respectively while their
MWCO was close to or larger than 500000. Similar
trend has been observed for the case with a coagulant
temperature of 40 °C. The significant improvement of
PWP might be attributed to the enhanced surface porosity while the large MWCO indicates the formation
of large surface pores when Pluronic F127 was added
as an additive to the polymer dopes. These phenomena
provide an evidence of the good pore-forming ability
of Pluronic F127 in the formation of PVDF hollow
fiber membranes.
The effect of addition of 2nd additives into the
Pluronic-containing dope on the membrane performance can be examined by comparing the experimental
results (at coagulant temperature of 25 °C) of P3,
P3L3, and P3W3 in Figs. 3 (a-c) and Table 3. With
the addition of LiCl as a 2nd additive into the dope
containing 3% (by mass) of Pluronic F127, the PWP
of membrane P3L3 increased largely to 2530
L·m−2·h−1·MPa−1 compared to membrane P3 with a
PWP of 780 L·m−2·h−1·MPa−1. Surprisingly, a significant reduction of MWCO from 485000 to 53000 was
also observed with the addition of LiCl. On the other
hand, an even sharp increase in PWP to more than
6000 L·m−2·h−1·MPa−1 was observed on membrane
P3W3 with the addition of water as a 2nd additive
Figure 4
while its MWCO was larger than 500000.
The large enhancement of PWP might be attributed to the higher membrane surface porosity or thinner skin layer formed due to the change of precipitation path, since the addition of LiCl and water brings
the dope system closer to the cloud point due to the
reduction of thermodynamic stability. Similar enhancement in surface porosity has also been reported
when LiCl was added into a PEG-containing dope [10,
42]. Although the PWP of both membrane P3L3 and
P3W3 was enhanced significantly, the results of
MWCO reveal that the addition of LiCl causes a reduction in surface pore size while this effect cannot be
observed with the addition of water. This indicates
that LiCl not only acts as a flux enhancer but also effectively controls the pore size.
As shown in Fig. 3, the PWP of PVDF hollow
fibers spun with the addition of Pluronic F127, including membrane P1, P3, P3L3, and P3W3, was significantly increased when a higher coagulant temperature was employed. The hotter external coagulant
also caused an increase in the MWCO of membrane
P3L3, indicating that the surface pore size was enlarged.
This phenomenon might be related to the temperaturedependent aggregation number of Pluronic F127 micelle. The aggregation number of Pluronic F127, indicating the number of molecules present in the Pluronic F127 micelle, has been reported to be increased
with increasing temperature [43, 44]. It is believed that
the higher aggregation number of Pluronic F127,
which means the larger size of micelles, leads to the
formation of bigger pore size near the outer surface of
the nascent fibers when it is in direct contact with the
external coagulant with a temperature of 40 °C [30, 45].
3.4
ATR-FTIR analysis
To assess the presence of Pluronic F127 in the
as-spun hollow fibers, ATR-FTIR analysis has been
carried out. Figs. 4 and 5 present the ATR-FTIR spectra
ATR-FTIR spectra of PVDF hollow fibers spun with different additives at a coagulant temperature of 25 °C
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Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
Figure 5 Enlarged ATR-FTIR spectra (1050-1150 cm−1) of PVDF hollow fibers spun with different additives at a coagulant
temperature of 25 °C
and its enlarged spectra at 1050-1150 cm−1, respectively, for PVDF hollow fibers prepared with different
additives at a coagulant temperature of 25 °C. No significant difference can be observed between the spectra for different membranes in Fig. 4. When comparing the enlarged spectra in Fig. 5, small new peaks at
around 1115 cm−1 were observed for membranes prepared with the addition of Pluronic F127, regardless of
the addition of 2nd additives. Similarly, the new peak
was also observed from the spectra of membranes
prepared with the addition of Pluronic F127 at a coagulant temperature of 40 °C (figure not shown). This
absorbance peak represents the characteristic band for
C O C stretching corresponding to the ether group,
which confirms the presence of Pluronic F127 molecules in the membrane matrix [24]. The results suggest
that a portion of Pluronic F127 might have washed out
by water or 2-propanol/hexane during the membrane
drying process, while some portions of Pluronic F127
still stay in the membrane matrix as the PPO block in
the Pluronic F127 molecules provides an anchorage in
the membrane despite the highly water-soluble nature
of Pluronic F127 [27, 46].
3.5
Membrane contact angle and LEPw
The hydrophilicity of PVDF hollow fiber membranes can be examined by dynamic contact angle as
shown in Fig. 6. At a coagulant temperature of 25 °C,
the advancing contact angle was reduced from 89° to
81° with increasing Pluronic mass concentration from
0 to 3% (membrane P0 to P3). Similar trend was also
observed for the case of 40 °C coagulant. This can be
attributed to the presence of the hydrophilic PEO
chains on the membrane surface due to the surface
segregation of Pluronic F127 molecules during the
phase inversion process. The results are in accordance
with the ATR-FTIR analysis in the previous section,
which indicates the presence of ether group in the membranes prepared with the addition of Pluronic F127.
At a coagulant temperature of 25 °C, the addition
of LiCl and water as a 2nd additive resulted in a further decrease in advancing contact angle of membrane
P3L3 and P3W3 to 71° and 73°, respectively. When a
higher coagulant temperature was employed, the significant decrease in contact angle was also observed
for membrane P3L3 while the contact angle of membrane P3W3 did not differ much from that of membrane P3. The decrease in advancing contact angle has
also been reported on poly(vinylidene-fluoride-cohexafluoropropylene) hollow fiber membranes prepared with the addition of LiCl as an additive [14].
This might be because of the presence of LiCl in the
membrane matrix as it does not completely diffuse out
during the membrane post-treatment. On the other
hand, the significant decrease in contact angle caused
by the addition of water has not been reported in literature to the best of our knowledge, and more investigation is required to carry out in the future to understand this phenomenon.
The advancing contact angle of membrane P0 to
P3 prepared at a coagulant temperature of 40 °C was
Table 4
LEPw of PVDF hollow fiber membranes spun
with different additives at coagulant
temperatures of 25 and 40 °C
LEPw/kPa
Membrane code
Figure 6 Advancing contact angle of PVDF hollow fibers
spun with different additives at coagulant temperatures of
25 and 40 °C
■ 25 °C; □ 40 °C
25 °C
40 °C
P0
138-172
172-207
P1
<28
14-41
P3
<28
14-41
P3L3
28-41
28-41
P3W3
28-41
28-41
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Chin. J. Chem. Eng., Vol. 20, No. 1, February 2012
Table 5
Comparison of various PVDF hollow fiber membranes prepared via NIPS
Dope mass content/%
PWP/L·m−2·h−1·MPa−1
①
Contact angle
Reference
PVDF/PVP/DMAc (20/7/73)
1790
(39.5%)
N.A.
[2]
PVDF/water/NMP (17/3/80)
300
40000
N.A.
[3]
PVDF/phosphorus acid/NMP (17/3/80)
890
290000
N.A.
[11]
②
PVDF/LiCl/water/DMAc (15/5.22/1.95/77.83)
2920
(3.2%)
N.A.
[12]
PVDF/PFSA/PEG/ethanol/DMAc (19/1/5/75)
200
15000
60º
[18]
PVDF/PVA/PEG/DMSO (14/6/6/74)
~1400
(~56%)
~70º
[19]
PVDF/Pluronic/LiCl/NMP (18/3/3/76)
2530
53000
71º
this work
①
③
Rejection for dextran, 500000; ② Rejection for dextran, 110000; ③ Rejection for egg albumen.
observed to be similar with their counterparts prepared
at a lower temperature, indicating that the change in
coagulant temperature exhibits no obvious influence
on the hydrophilicity of the membranes prepared
with/without Pluronic F127 as a single additive. This
observation is also valid for membrane P3L3 which
was prepared with the addition of LiCl as a 2nd additive.
The LEPw of PVDF hollow fibers prepared
with/without the addition of Pluronic F127 and 2nd
additives at different coagulant temperature is listed in
Table 4. It was observed that all the membranes prepared
with Pluronic F127 exhibited LEPw of less than 50
kPa, which is much lower than that of membrane P0
with a LEPw of more than 0.13 MPa. The coagulation
temperature seems to have no much influence on the
LEPw of membranes prepared with the addition of
Pluronic F127. The lower LEPw reveals that the
membranes become wetted easily with the addition of
Pluronic F127 and with/without the 2nd additives, due
to the formation of bigger pores and the enhanced hydrophilicity as discussed previously. This feature is
favorable for the membrane used in water or wastewater treatments.
Table 5 lists the PWP, MWCO, and contact angle
of PVDF hollow fiber membranes previously made by
blending of pore-former or surface modifier via NIPS.
It can be seen that the membrane prepared in this
study has a relatively high PWP compared to most
other self-fabricated membranes while the MWCO is
reasonably low, which means smaller pore sizes.
However, the membrane prepared with Pluronic F127
is not as hydrophilic as that prepared with the addition
of Perfluorosulfonic acid (PFSA) [18]. This might be
attributed to the higher stability of PFSA in the PVDF
membrane matrix due to the presence of fluorocarbons.
Nevertheless, Pluronic as an additive is still competitive considering its lower cost and higher permeability
of resultant membrane compared to other additives.
4
MWCO (rejection)
CONCLUSIONS
The amphiphilic block copolymer, Pluronic F127,
was used as an additive in the PVDF hollow fiber
membranes fabrication in an attempt to increase the
membrane permeability and enhance the membrane
hydrophilicity. With the addition of Pluronic F127 into
the polymer dope, the as-spun membranes exhibited
higher PWP and slightly enhanced hydrophilicity. It
was found that the macrovoid size, surface pore size,
water permeability, and hydrophilicity of the membranes was affected when LiCl or water were added as
a 2nd additive into the dopes containing Pluronic
F127. A higher coagulant temperature was also found
to suppress the macrovoids formation, enhance the
water permeability, and increase the surface pore size.
From this study, it can be concluded that PVDF
hollow fiber membrane with high performance can be
prepared with Pluronic F127 as both the pore-former
and the surface modifier. By using a coagulant temperature of 25 °C and a combination of Pluronic F127
and LiCl as the additive, PVDF hollow fiber with
PWP as high as 2530 L·m−2·h−1·MPa−1, MWCO of
53000, and contact angle of 71° was successfully fabricated, which is better than most of PVDF hollow
fiber membranes made by NIPS method.
ACKNOWLEDGEMENTS
We would like to thank the Environment and Water Industrial Program Office of Singapore for funding
support under the project #0901-IRIS-02-03. We are
also grateful to Singapore Economic Development
Board for funding Singapore Membrane Technology
Centre. Ms. Kuah Hui Mun is acknowledged for her
kind assistance.
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