DSpace at VNU: Surface photochemical graft polymerization of acrylic acid onto polyamide thin film composite membranes

Surface photochemical graft polymerization of acrylic acid onto
polyamide thin film composite membranes
Thu Hong Anh Ngo,1 D. T. Tran,1 Cuong Hung Dinh2,3
1

Department of Chemical Technology, Faculty of Chemistry, Hanoi University of Science (HUS), Vietnam National University (VNU),

334 Nguyen Trai, Thanh Xuan District, Hanoi 10000, Vietnam
2
Laboratory for Materials and Engineering of Fibre Optics, Institute of Material Science (IMS), Vietnamese Academy of Science
and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay District, Hanoi 10000, Vietnam
3

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki,

Tsukuba 105-0044, Japan
Correspondence to: D. T. Tran (E-mail: )

ABSTRACT: Surface modification is an effective approach to enhance the properties of polymeric membranes. In this work, the UVphoto-induced graft polymerization of acrylic acid (AA) onto the surfaces of polyamide thin film composite (TFC-PA) membranes
was carried out using an immersion method performed under ambient conditions. The experimental results indicate that the membrane surface becomes more hydrophilic because of the appearance of new carboxylic groups on the surface after the modification.
This reduces the water contact angle and increases the water permeability compared with the unmodified membrane. The membrane
surface is relatively compact and smooth due to the formation of the polymeric AA-grafted layer. The separation performance of the
modified membrane is improved with enhancements of the permeate flux and the retention of humic acid from aqueous feed solutions compared with those of the unmodified membrane. The fouling resistance of the membrane is also improved because of the
higher maintained flux ratios and the lower irreversible fouling factors for the removal of various organic compounds from feed soluC 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44418.
tions. V

KEYWORDS: grafting; membranes; morphology; polyamide; radical polymerization

Received 28 March 2016; accepted 26 August 2016
DOI: 10.1002/app.44418
INTRODUCTION

Polymeric membranes have been widely used for many different
applications such as the production of pure water, treatment of
polluted water, filtration of beer and beverages, and separation
of proteins. However, one of the major problems with membrane processes is fouling, which is considered to be a severe
limitation for membrane applications. Fouling may result in a
significant decrease in the membrane separation capacity, shorten the membrane lifetime and increase the operational costs of
membrane processes. Therefore, improving the membrane antifouling property is one of the most important goals in membrane separation technologies. Surface modification is a very
useful method for developing fouling resistance in polymeric
membranes. Different techniques can be used for the surface
modification of thin film polymeric membranes such as UVinduced graft polymerization,1,2 plasma-induced polymerization,3,4 plasma treatment,5,6 chemical functionalization,7 physical coating of the polymer,8,9 and nanoparticle treatment.10
Using these methods, hydrophilic or charged functional groups

can be introduced onto polymeric membrane surfaces, changing
the surface chemistry, and topology, thus potentially reducing
the fouling factors and enhancing the membrane separation performance.11,12 For enhanced membrane resistance towards fouling, UV-photo-induced graft polymerization is a convenient
technique. Using this approach, various vinyl monomers have
been used for graft polymerization, and the reactions occur only
on the surfaces of the membranes. The first step involves the
absorption of UV light to generate free radicals that serve as
nucleation sites on the surface, which prepare the substrate for
the grafting of vinyl monomers and their subsequent polymerization. The attractive features of UV-induced grafting are the
easy and controllable introduction of graft chains with a high
density and an exact localization on the polymeric membrane
surface. Furthermore, the covalent attachment of the graft
chains onto a polymer surface is stable, which is in contrast to
physically coated polymeric layers.13 In this method, the UV
irradiation time and the monomer concentration are the most
important parameters that alter the degree of grafting. The

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modified membranes show more resistance to fouling and a
higher rejection than the unmodified ones, but the reduction of
permeability due to pore constriction or even blocking by grafting has been observed.14 The antifouling property of a membrane is highly influenced by the surface characteristics such as
hydrophilicity and surface electrical charge. Membranes with
hydrophilic surfaces are less susceptible to fouling than hydrophobic ones, while the ability to recover performance upon
washing is higher for the membranes with chemically neutral
surfaces than it is for charged membranes.15 The enhanced antifouling property of polymeric membranes has also been
obtained via the surface grafting of a zwitterionic copolymer via
UV-initiated polymerization.16
Among the polymeric membranes, thin film composite polyamide (TFC-PA) membranes have been widely used for water
treatments because of their superior water flux, good resistance
to pressure compaction, wide operating pH range and good stability to biological attack.17,18 However, TFC-PA membranes are
also sensitive to fouling because of the surface roughness and
electrical charge.19–21 In general, the fouling phenomenon can
be reduced when the membrane surface becomes more hydrophilic, smoother and/or has the same electrical charge as the
foulants.22–24 Recently, many research efforts have been devoted

to modifying the membrane surface to improve the TFC-PA
membrane filtration performance. Belfer et al.25,26 modified a
TFC-PA reverse osmosis (RO) membrane by a redox-initiated
radical grafting of hydrophilic polymers and the modified membranes exhibited improved fouling resistance. Van Wagner
et al.27 modified a TFC-PA RO membrane by grafting poly(ethylene glycol) diglycidyl ether onto the membrane surface and
found that the modified membranes showed an improved fouling resistance but a lower water flux. In another work, the modification of a TFC-PA membrane through in situ polymerization
for the purpose of coating with a layer of sorbitol polyglycidyl
ether was studied by Kwon et al.28 The modification resulted in
a more neutral, hydrophilic and smooth membrane surface, and
the formed membrane also showed an improved chlorine stability. Yu et al.29 modified the surface of TFC-PA RO membranes
by coating with N-isopropylamide-co-acrylic acid copolymers to
improve the membrane properties. Mondal and Wickramashinghe30 reported that polyamide TFC membrane surfaces
were successfully modified by the UV-photo-induced grafting of
N-isopropyl acrylamide. The grafted membranes had better
characteristics in terms of their separation properties, because
of their high salt rejections compared with that of the unmodified one. The modified membranes also showed good fouling
resistance for brackish water desalination. Mansourpanah and
Habili31 modified thin film polyamide nanofiltration membranes with acrylic acid (AA) and UV irradiation using two
methods (1) the finalization of the modification during the formation of the polyamide thin layer and (2) the finalization of
the modification after the formation of the polyamide thin layer. The first method was very effective due to the simultaneous
increases in the flux and rejection, as well as an improved antifouling property for desalting. In another work, Cheng et al.32
modified a commercial TFC-PA RO membrane via the redoxinitiated surface graft polymerization of N-isopropylacrylamide

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(NIPAm) followed by AA. The graft polymerization of NIPAm
reduced the membrane salt rejection but increased the water
permeability, while the following graft polymerization of AA
resulted in a decreased water flux and an increased salt rejection. The modified membrane possessed an improved fouling
resistance to bovine serum albumin (BSA).

In general, surface modifications via coating and grafting are
effective methods for improving the antifouling and/or chlorine
resistance of TFC-PA membranes. However, the modified membrane usually shows a lower water flux and/or a decreased salt
rejection compared with the unmodified one. Therefore, the
modification of TFC-PA membranes for the simultaneous
improvement of fouling resistance, flux and retention is still
very challenging for the development of membrane applications.
In this work, the UV-induced graft polymerization of AA onto
the surfaces of commercial TFC-PA membranes was studied.
The UV-induced grafting method was chosen due to the advantages of the technique such as mild reaction conditions at a low
temperature with high selectivity and easy incorporation into
the end stages of membrane manufacturing processes.33 The
influences of the surface modification conditions such as UV
irradiation time and AA monomer concentration on the membrane characteristics were investigated in terms of the membrane surface properties and the filtration performance.
EXPERIMENTAL

Materials
A commercial TFC-PA membrane (Filmtec BW30) was used as
the substrate material for the surface modification experiments.
The TFC-PA membrane consists of a topmost ultrathin polyamide active layer, which was synthesized in situ via interfacial
polymerization of m-phenylenediamine and trimesoyl chloride
on a reinforced polysulfone porous substrate. The commercial
TFC-PA membrane developed by Filmtec Corporation demonstrates up to 99.1% NaCl rejection with flux as high as 42.5 L/
m2 h at a pressure of 5.5 MPa.34 The membrane samples used
for the surface modification were cut (U 47 mm) and soaked in
a 25 v/v % aqueous solution of isopropanol (purity 99.9%, Sigma-Aldrich) for 60 min, carefully rinsed with pure water and
then kept wet until they were used for surface grafting. AA (liquid, purity 99.0%, Xilong Chemicals, China) was used as the
monomer for the graft polymerization without further purification. Deionized water with a conductivity of less than 5 lS/cm
was used to prepare the aqueous solutions and to rinse the
membrane samples. Other reagents, such as humic acid (HA)

(Wako, Japan), reactive red dye RR261 (China), and pure-grade
BSA (Wako, Japan), were used for the preparation of aqueous
feed solutions for membrane filtration experiments.
Modification of TFC Membrane
The membrane surface was modified using the UV-photoinduced graft polymerization method, under a UV light
(300 nm, 60 W). Aqueous solutions of the monomer (AA) were
prepared with different concentrations ranging from 3 to 50 g/
L. The membranes were immersed in the AA solutions in Petri
dishes under UV irradiation for different times ranging from 1
to 10 min. After grafting, the membranes were washed carefully

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spectroscopy (FTIR-ATR, Spectro 100 PerkinElmer). The measurements were performed at a nominal incident angle of 458
with 100 scans at a resolution of 4 cm21. The membrane samples used for the FTIR-ATR analysis were dried at 258C under
vacuum before characterization.
Contact Angle Measurements. The wettability of the membrane
was examined through the water contact angle (WCA) measurements using a goniometer (DMS012) equipped with a camera,
which captured images of deionized water drops on the dried
surfaces of the membranes at 258C, and the contact angles were
calculated. For each sample, three drops (3 lL) were placed at
different positions on the membrane surface, and the average
value of the contact angles was obtained.

Scheme 1. Potential mechanism of the surface graft polymerization of AA
onto the TFC-PA membrane.

with deionized water and then kept wet until they were used
for the filtration experiments.
The graft degree was determined based on the difference in the
membrane weight before and after grafting. The grafting yield
in percentage (G, %) was used for the calculation of the graft
degree over the surface area of the membrane sample (U
47 mm)



ðm1 –m0 Þ
:100 ð%Þ
G 5
m0
where m0 and m1 are the weights of the dried membrane samples before and after grafting, respectively.
A potential mechanism of the UV-photo-induced graft polymerization of AA onto the TFC-PA membrane surface is given in
Scheme 1. The first step is the absorption of UV light to
abstract hydrogen atoms from amide groups on the base of the
polymeric TFC-PA membrane. This produces the radical sites
required for grafting with AA monomer free radicals, which
then form the polymeric AA-grafted chains on the membrane
surface.
Characterization of the Membranes
FTIR-ATR Analysis. The surface chemical functionalities of the
unmodified and modified membranes were characterized using
attenuated total reflectance Fourier transform infrared

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SEM and AFM Images. The membrane surface morphology
was observed via scanning electron microscopy (SEM) using a
field-emission scanning electron microscope (FE-SEM, Hitachi
S-4800). The membrane samples were sputter coated with a
3 nm thick Pt layer before imaging. Quantitative surface roughness analysis of the membranes was measured by atomic force
microscopy (AFM), using a MultiMode scanning probe microscope. The samples were dried under vacuum before analysis.
Several different positions over a 5 lm 3 5 lm area were analyzed for each membrane sample to obtain an average value of
the root mean square (RMS) roughness. The AFM images were
obtained from different places on each membrane surface under
the same conditions, i.e., temperature, air medium, and scale.
The surface roughness was calculated using the data analysis
software provided by the equipment manufacturer (NanoScope
Analysis, Brucker).
Evaluation of the Membrane Filtration Properties. The membrane filtration experiments were performed in a dead-end
membrane filtration system consisting of a stainless steel cylindrical cell of 300 cm3 supplied by Osmonics (USA) and a stirrer
connected to a nitrogen gas cylinder, which provided a working
pressure in the cell of 15 bar. The experiments were carried out
at room temperature using a membrane area of 13.2 cm2. The
membrane was compacted by pure water at 15 bar for 15 min
before carrying out the filtration measurements. In all of the
experiments, the membrane cell was carefully rinsed with pure
water before and after using. The membrane pure water permeability was determined by


Vw
ðL=m2 h barÞ
Jw 5

ðA:t:PÞ
where Vw is the water volume obtained through a membrane
area of A within a time of t at a determined pressure of P. The
normalized pure water permeability ratio (Jw/Jwo), where Jw and
Jwo are the average pure water fluxes of the modified and
unmodified membranes, respectively, was used to evaluate the
changes in the pure water permeability of the membranes
resulting from the surface graft polymerization.
The retention (R) for the removal of a certain object in the feed
solution was determined by

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RESULTS AND DISCUSSION

Membrane Surface Characteristics
FTIR-ATR Spectra. The functionality of the membrane surface
was studied by FTIR spectroscopy using the FTIR-ATR technique to verify the successful graft polymerization of AA onto
the surfaces of the membranes. The results (Figure 1) show a
comparison of the FTIR-ATR spectra of the unmodified membrane and the modified membranes grafted with AA under different grafting polymerization conditions.

Figure 1. FTIR-ATR spectra of unmodified and modified membrane surfaces: (a) 10 g/L AA - UV 1 min (10AA-1), (b) 10AA-5, (c) 50AA-5.




C0 2C
:100 ð%Þ
R 5
C0

where C0 and C are the concentrations of the object in the feed
and filtrate, respectively.
The permeate flux (J) was evaluated by


V
ðL=m2 hÞ
J 5
ðA:tÞ
where V is a filtrate volume obtained through a membrane area
of A within a separation time of t at the determined pressure
driving force. The normalized permeate flux ratio (J/Jo) was
used to evaluate the changes in the membrane flux caused by
the surface modification, where J and Jo are the average permeate fluxes of the modified and unmodified membranes,
respectively.

For the raw TFC-PA membrane surface, the absorptions of the
polyamide active layer are characterized by NAH (1550–
1640 cm21), C@O (1640–1690 cm21), C@C (1400–1600 cm21),
and CAN (1080–1360 cm21) signals. For the modified membranes, the spectra show new peaks at approximately
1730 cm21, which are ascribed to the carboxylic group of the
grafted AA. In addition, the absorption intensity of the carboxylic group increases with prolonged graft polymerization times

and/or a higher AA monomer concentration. This could be due
to the differences in the grafting degree on the membrane surfaces modified under the different conditions. As shown in Figure
2, the surface modification conditions such as UV irradiation
time and AA monomer concentration could greatly influence
the graft degree. For the same UV irradiation time, the grafting
yield clearly increases when the concentration of AA was varied
from 10 to 50 g/L. Meanwhile, for the same AA concentration,
the grafting yield increases more slowly as the UV irradiation
time was varied from 1 to 10 min. The experimental results
regarding the grafting yield under the different graft polymerization conditions are also in good agreement with the FTIRATR spectral data.
Wettability. Changes in the membrane wettability that resulted
from the graft polymerization of AA were investigated through
WCA measurements, which revealed the changes in hydrophilicity that occurred in the outermost layer of the membrane surface after modification.
Figure 3 shows a comparison of WCAs of the unmodified membrane and modified membrane surfaces formed under the different graft polymerization conditions. The obtained results

Evaluation of Membrane Antifouling Property. The antifouling property of the membranes was estimated through the
maintained flux ratio (%) during the filtration of different feed
solutions containing high fouling tendency compounds such as
HA, dyes or proteins. An irreversible fouling factor (FRw) of the
membranes was calculated and compared through



Jw1 2Jw2
:100 ð%Þ
FRw 5
Jw1
where Jw1 and Jw2 are the pure water fluxes of the membranes
before and after using them for the filtration of feed solutions,
respectively. The higher the maintained flux ratio and the lower

the irreversible fouling factors, the better the antifouling property of the membranes.

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Figure 2. Graft degrees on the surfaces of the modified membranes.

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Figure 3. Water contact angles of the unmodified and modified membrane surfaces.

indicate that the modified membrane surface becomes more
hydrophilic due to a significant decrease of WCA, from 518 for
an unmodified membrane to 23–258 for the modified ones. This
is due to the formation of the hydrophilic polymeric AA-grafted
layer on the membrane surface after modification. The enhancement of the surface hydrophilicity is desirable because it could

reduce foulants adsorbed on membrane surface during filtration.
In addition, as shown in figure, the WCA is clearly decreased and
nearly stable for a prolonged UV irradiation time of up to 10
min. This is also observed for the AA concentrations ranging
from 10 to 40 g/L. The almost stable WCA values could be due
to the stability of the poly(acrylic acid) (PAA) layer formed on
the modified membrane surfaces. Although the graft degree has

been gradually increased for the prolonger grafting time and/or
with the higher AA concentration in graft solution, the chemical
functionality of the grafted PAA layer is maintained for the modified membranes prepared at the different graft polymerization
conditions, as shown in the FTIR-ATR spectra (Figure 1). The
formation of PAA-grafted layer leads to the increased hydrophilicity of the membranes, and it is the reason for the reduced
WCA; thus, the wettability of the modified membranes improved.
The improvement in the hydrophilicity of the modified membranes can result not only in enhanced water flux but also in
reduced fouling because hydrophilic surfaces are preferentially
adsorbed by water and lead to the lower tendency for adsorption
of foulants.35,36 In addition, it is also known from the literature
that the grafting of acrylic monomers with carboxylic groups can
enhance the negative surface charge of the TFC-PA membrane,
thereby changing the surface energy of the membrane,25,32 which
also affects the membrane antifouling property.

Figure 4. SEM images of the cross-section and surface of the unmodified membrane (a) and the modified membranes: (b) 10AA-7 and (c) 50AA-7.

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Figure 5. AFM images of the unmodified membrane (a) and the modified membranes: (b) 10AA-7 and (c) 50AA-7. [Color figure can be viewed at
wileyonlinelibrary.com]

Membrane Surface Morphology. The surface morphological
structure of the membrane was characterized by SEM and AFM
images. Figure 4 shows the SEM images of the unmodified
membrane and the modified ones, which were grafted with AA
concentrations of 10 g/L and 50 g/L under a UV irradiation
time of 7 min (10AA-7 and 50AA-7). The cross-section images
demonstrate the formation of the polymeric AA-grafted layer
on the top, and the grafting polymerization mainly occurred on
the surface of the membrane. As shown in the figure, the surface of the modified membrane becomes more compact than
the unmodified one.
The AFM images (Figure 5) show the changes in the TFC-PA
membrane surface morphology after the grafting polymerization
of AA. The values of the average and RMS roughnesses (Ra and
Rms) are given in Table I and clearly demonstrate that the
modification highly influences the surface roughness of the
composite polyamide membranes. The membrane surface
becomes smoother with reduced roughness values of Ra and
Rms compared with the unmodified one. The lower surface
roughness is also desirable because it improves the antifouling
properties for membranes. Kang and Cao18 and Sagle et al.37
suggested that a smoother surface is commonly expected to
experience less fouling, presumably because foulant particles are
more likely to be entrained by rougher topologies than by
smoother membrane surfaces. Consequently, the decrease of
surface roughness can improve antifouling property of RO
membranes. Kochkodan et al.38 suggested also that there is a
strong correlation between the fouling and the surface roughness for RO and NF membrane. Van der Bruggen et al.19 suggested that surface roughness may also increase membrane
fouling by increasing the rate of attachment onto the membrane
surface and it is accepted that membranes with a rough surface

are more prone to fouling than membranes with a smoother
surface. Vrijenhoek et al.22 indicated that particles preferentially
accumulate in the “valleys” of rough membranes, resulting in
“valley clogging” which causes more severe flux decline than in
smooth membranes. Ishigami et al.39 investigated the antifouling property of polyelectrolyte multilayered RO membranes.
The results illustrated that the antifouling capacity increased
with increasing layer number due to enhanced hydrophilicity
and smoothed surface morphology.
The changes in the membrane surface characteristics after modification confirm the successful grafting polymerization of AA
onto the surface of the TFC-PA membrane. The changes in the
membrane surface functionality and morphological structure
could lead to changes in the membrane filtration performance,
especially on the antifouling property, because the hydrophilicity
and brush-like form of the grafted layer, as well as the lower
surface roughness, may reduce the adsorption of foulants on
the surface of the modified membrane during filtration.
Membrane Filtration Performance
Pure Water Permeability. The difference in the pure water permeability between the unmodified and modified membranes
was obtained using the normalized pure water permeability (Jw/

Table I. Membrane Surface Roughness
Membranes

Ra (nm)

Rms (nm)

Unmodified

93.0

121.0

Modified 10AA-7

24.6

33.3

Modified 50AA-7

39.1

51.1

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Figure 6. Normalized pure water permeability of the membranes.

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25–30% compared with the unmodified one. The membrane

retention is expressed through the values of R1 for UV254 and
R2 for the TOC measurements. The results indicate that R1 is
higher than R2, and for all of the modified membranes, the values of R1 are nearly maintained (95%); meanwhile, the values
of R2 increased from 63% for the unmodified membrane to
higher than 80% for the modified ones. The improvement in
the membrane retention is due to the formation of a polymeric
layer grafted on the membrane surface after modification, leading to the relative compactness of the membrane surfaces.
Meanwhile, the increase in the membrane surface hydrophilicity
is the reason for the enhancement of the membrane flux. The
results concerning membrane retention and flux are in good
agreement with the changes in the membrane surface wettability
and morphology.

Figure 7. Retention and flux of the membranes.

Jwo). The results given in Figure 6 indicate the increased water
permeability of the modified membranes, which is 20 to 24 %
higher than that of the unmodified one. The changes in water
permeability caused by the graft polymerization of AA lead to
an enhancement in the membrane wettability. The obtained
experimental results are also in good agreement with the WCA
results.
Retention and Flux. The membrane separation property was
determined through the possibility for the removal of HA in a
neutral feed solution with 50 ppm HA. The concentration of
HA was evaluated using ultraviolet (UV254) absorption spectroscopy and a total organic carbon (TOC) analyzer. The TOC analyzer has a higher oxidation efficiency for smaller and more
aliphatic compounds, and UV254 is better for large aromatic
compounds. The smaller aliphatic compounds would be able to
pass more easily through the membrane than the larger aromatic compounds.40 The experimental results (Figure 7) demonstrate that all of the modified membranes have improved
separation performance with flux enhancements of more than

Antifouling Property. The membrane fouling resistance was
evaluated through filtration experiments using different aqueous
feed solutions containing 50 ppm HA, 50 ppm dye (RR261), or
50 ppm BSA. The modified 10AA-7 membrane was selected to
perform the fouling experiments and to compare with the base
TFC-PA membrane. Figure 8 shows a comparison of the
decrease in flux based on the maintained flux ratio between the
unmodified and the modified membranes. The surface characteristics highly impact the membrane antifouling property,
which could be improved if the surfaces have a higher hydrophilicity, lower roughness, and/or the same charge as the foulants.11,12 When the filtration time increases, the fluxes of both
the unmodified and modified membranes gradually decrease as
a result of membrane fouling. However, the degree of fouling
differs between the two membranes. The experimental results
illustrate that the flux decrease for all of the modified membranes is less than that of the unmodified one, resulting in a
higher flux maintenance during filtration. For example, after 60
min of filtration, the maintained flux ratios of the unmodified
membrane for RR261, HA, and BSA feed solutions are 70, 80,
and 70%, while they are 88, 94, and 85% for the modified one,
respectively. After 300 min of filtration, these values for the
unmodified membrane are reduced to 65, 70, and 60%, while
the values for the modified one are 82, 83, and 72%. After 600

Figure 8. Filtration performance of the unmodified and modified membranes.

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Table II. Irreversible Fouling Factors of the Membranes
FRW (%)
Feed solutions

Unmodified

Modified

30 ppm HA

5.1

4.5

100 ppm HA

8.3

5.1

30 ppm RR261

7.5

5.5

200 ppm RR261

17.9

10.9

1000 ppm BSA

18.1

15.1

5000 ppm BSA

19.5

17.4

min of filtration, the maintained flux ratios of the membranes
are reduced further; however, the values for the modified one
are still higher, indicating the improved fouling resistance of the
membrane after the surface grafting of AA. The separation performance of the modified membrane was evaluated through the
normalized flux (J/Jo) and retention (R) for RR261, HA, and
BSA. The experimental results reveal that the separation performance of the AA-grafted membrane remains well kept after prolonged usage. After 10 h of filtration, the retentions for RR261
and BSA are still maintained at 99.8 and 99.9%, respectively;
the retention for HA remains at 99.8% (RUV) and 88.0%
(RTOC). Furthermore, the fluxes of the modified membranes are
greatly enhanced compared with the base, with normalized flux
values of 1.31, 1.27, and 1.28 for the filtration of RR261, HA,

and BSA feed solutions, respectively.
A comparison of the irreversible fouling factors between the
unmodified and modified membranes is given in Table II,
which indicates that all of the modified membranes have irreversible fouling factors that are lower than the unmodified one.
The experimental results also point out that the irreversible
fouling factor of the membrane increased with the concentration of fouling objects in the feed solution.
The experimental results illustrate that the antifouling property
of the TFC-PA membranes is clearly improved by the graft
polymerization of AA onto the membrane surface. The
improvement in the membrane fouling resistance is mainly due
to the improved surface hydrophilicity and the reduced surface
roughness of the modified TFC-PA membrane, leading to less
fouling and thereby higher maintained flux ratios, as well as
lower irreversible fouling factors.
CONCLUSIONS

A commercial TFC-PA membrane surface was successfully modified through the UV-photo-induced graft polymerization of AA
using an immersion technique carried out under ambient conditions. The experimental results demonstrate that the grafting
polymerization led to changes in the membrane surface characteristics and membrane filtration performance. The FTIR-ATR
spectra illustrate the appearance of new carbonyl groups on the
surface after the modification, and the membrane’s surface
becomes more hydrophilic with a highly reduced WCA. The
morphological characterization of the modified membranes
observed through SEM and AFM analysis demonstrate the formation of a polymeric AA-grafted layer on the membrane

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surface, which is relatively compact with a low surface roughness compared with the unmodified one. The AA-grafted membranes possess an improved separation performance with an
enhancement of both the membrane flux and the retention for
the removal of HA in an aqueous feed solution. The antifouling

of the membrane is also clearly improved after the surface modification, which is due to the higher maintained flux ratios and
the lower irreversible fouling factors during the filtration of feed
solutions containing strongly fouling objects such as HA, dye
and BSA. Thus, the surface modification of the TFC-PA membrane via the UV-photo-induced graft polymerization of AA
results in an enhancement of both the membrane antifouling
property and filtration performance, in terms of simultaneously
increasing the membrane retention and flux.
ACKNOWLEDGMENTS

The authors would like to thank the National Foundation for Science and Technology Development (NAFOSTED) for financial
support under grant number 104.02-2013.42. They are grateful to
the Vietnamese Ministry of Education and Training for support
through the Program No. 911. Substantial contributions to
research design by Dung Thi Tran. Acquisition of the data by Thu
Hong Anh Ngo, Dung Thi Tran, and Cuong Hung Dinh. Interpretation of the data and drafting the paper by Dung Thi Tran and
Thu Hong Anh Ngo.

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