Preparation and Characterization of a Novel Hydrophilic ...download.xuebalib.com/xuebalib.com.52631.pdfPreparation and Characterization of a Novel Hydrophilic PVDF/PVA UF Membrane

Preparation and Characterization of a Novel HydrophilicPVDF/PVA UF Membrane Modified by CarboxylatedMultiwalled Carbon Nanotubes

Gui-E Chen,1 Sun-Jie Xu,1 Zhen-Liang Xu,2 Wei-Wei Zhu,1 Qiong Wu,1 Wei-Guang Sun1

1 School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road,Shanghai 201418, China2 State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, ChemicalEngineering Research Center, East China University of Science and Technology, 130 Meilong Road,Shanghai 200237, China

Polyvinylidene fluoride (PVDF)/polyvinyl alcohol (PVA) ultra-filtration (UF) membranes were prepared via a phase inver-sion method employing the modification of carboxylatedmultiwalled carbon nanotubes (MWCNTs-COOH). Variouscontents of MWCNTs-COOHs (0.00–0.15 wt%, weight ofcasting solution) were added into PVDF/PVA/dimethyl sulf-oxide systems for the fabrication of the plate UF mem-brane. Fourier transform infrared spectroscopy spectraidentified the successful introduction of carboxyl throughthe C@O peak at 1730 cm21. Scanning electron micros-copy images exhibited the external surface and the asym-metric morphology with the appearance of a sponge-likeinner structure. Atomic force microscopy analysis deter-mined the roughness values and rougher topography. Thehydrophilicity of the composite membrane containing 0.09wt% of MWCNTs-COOHs improved the most. This samplehas the highest pure water flux, approximately doubled(126.6 L m22 h21) compared to the PVDF/PVA membrane(68.6 L m22 h21), an enhanced bovine serum albumin fluxrecovery rate, showing an increase of 17%, and the bestfouling resistance ability. Meanwhile, the porosity anddynamic contactangle also indicate the enhancement ofmembrane hydrophilicity. Dextran (DEX) 600k rejectionreached 91.0%. Break strength, elongation at break, andYoung’s modulus also had improvements of 60%, 215.5%,and 56.7%, respectively, when the MWCNTs-COOH contentwas 0.12 wt%. POLYM. ENG. SCI., 56:955–967, 2016. VC 2016Society of Plastics Engineers

INTRODUCTION

Over the last dozen years, polymeric membranes have been

widespread in engineering applications and scientific usage [1],

such as the polyamide (PA) membrane for seawater treatment

and ethanol purification [2, 3], the polycarbonate (PC)

membrane for gas separation [4], the polyacrylonitrile (PAN)

membrane for polluted natural rubber effluent treatment [5], and

other polymeric membrane materials such as polytetrafluoroeth-

ylene (PTFE), polysulfone (PSF), and polyethersulfone (PES)

[6–8]. Polyvinylidene fluoride (PVDF) is regarded as a typical

example of a polymeric membrane material with outstanding

characteristics, such as being acid-proof, alkali-proof, corrosion-

resistant, and having excellent mechanical properties. Due to its

low molecular surface energy causing its strong hydrophobicity,

almost all studies have been carried out using either physical or

chemical modification methods to enlarge the pure water flux,

promote the surface anti-fouling capacity of the PVDF mem-

brane, and above all, enhance its hydrophilicity [9–12].

A simple, effective, and low-cost method is to blend quanti-

fied materials, which have hydrophilic functional groups such as

carboxyl, hydroxyl, and sulfonyl, into the casting solution to

prepare the composite membrane [13].

Polyvinyl alcohol (PVA) is a familiar polymer, which is non-

toxic, innocuous, and degradable, making it safe for the environ-

ment. Moreover, it has particular oil resistance, abrasive

resistance, solvent resistance, especially for hydrocarbon com-

pounds, and a remarkable membrane-forming ability [14, 15].

Since it was first discovered in 1924 by Hemalm, PVA has been

widely employed in food, pharmaceuticals, paper making, con-

struction, agriculture, and polymer materials [16, 17]. In the

structure of PVA, the molecular unit arrangement is linear with

a hydroxyl group on each molecular unit, so it can easily form

hydrogen bonds. Many researchers utilize PVA in diverse condi-

tions to prepare versatile membranes [18–21]. Zhang et al. [22]

used PVA as a hydrophilic additive in a PVDF/PES blend mem-

brane; their results indicated that when PVA content reached 0.3

wt% of the casting solution, the prepared membrane had the

best porosity, the lowest water contact angle of membrane sur-

face and improved pure water flux. Moreover, the extended Der-

jaguin–Landau–Verwey–Overbeek (XDLVO) theory also proved

the feasibility that adding PVA to the blend membrane system

can improve membrane surface hydrophilicity. Li et al. [23]

used a dip-coating method to immerse an alkali-treated PVDF

hollow fiber membrane into a self-prepared PVA-TiO2 disper-

sion and used glutaraldehyde (GA) as cross-linker to fabricate a

PVA/PVDF-TiO2 hollow fiber membrane. The results showed

that the presence of PVA induced no rejection effect for salts.

When the content of TiO2 was increased to 1 g L21, the modi-

fied membrane reached the highest performance, exhibiting an

outstanding antifouling property and thermal stability. In addi-

tion, it showed excellent separation efficiency in dye wastewater

Correspondence to: G.E. Chen; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China; con-

tract grant numbers: 51372153 and 51572176; contract grant sponsor: Shang-

hai Union Program; contract grant number: LM201249; contract grant

sponsor: Key Technology R&D Program of Shanghai Committee of Science

and Technology in China; contract grant number: 14231201503; contract

grant sponsor: the 2013 Year Special Project of the Development and Indus-

trialization of New Materials of National Development and Reform Commis-

sion in China; contract grant number: 20132548; contract grant sponsor: Key

Technology R&D Program of Jiangsu Committee of Science and Technol-

ogy in China; contract grant number: BE2013031.

DOI 10.1002/pen.24325

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2016 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2016

treatment. Li et al. [24] blended PVDF with various ratios of

PVA. The best ratio of PVDF:PVA is 80:20, for which the pre-

pared composite hollow fiber membrane showed a sharp

decrease of the water contact angle, and the hydrophilicity of

membrane surface increased extensively. Furthermore, they dis-

covered that the PVDF/PVA blends were incompatible, causing

the formation of interface microvoids in the membrane structure,

which benefit the membrane properties.

With the rapid development of nanomaterials, carbon nano-

materials have become a significant research focus in recent

years [25]. Carbon materials include a large family of different

species with various properties. Commonly, people are familiar

with graphene, graphene oxide (GO), carbon nanotubes, fuller-

enes and their derivatives, which are also classified into single-

walled or multiwalled or single-layer or multilayer in terms of

their structure [26]. Zhao et al. [27] added different contents of

GO and PVP into a PVDF membrane, followed by optimization

through the Taguchi design. This study determined that the best

content of PVDF is 12 wt%, GO is 3 wt%, and PVP is 5 wt%,

producing membranes showing the best properties. The intro-

duction of GO changed the characterization of the membrane

surface and improved the hydrophilicity, antifouling ability, and

mechanical properties. Wu et al. [28] used PSF as matrix mem-

brane, blended with a self-made “sandwich” structure SiO2-GO,

which was a derivative of graphene oxide. The SiO2-GO par-

ticles were well fixed into the membrane with a regular distribu-

tion. Analysis indicated that with an SiO2-GO addition of 0.3

wt%, the performance of the hybrid membrane surpassed that of

SiO2/PSF and GO/PSF membranes overall. The pure water flux

reached a peak value, which was nearly double compared with

the PSF membrane. Moreover, the hybrid membrane maintains

rejection above 98% to egg albumin. Carboxylated multiwalled

carbon nanotubes (MWCNTs-COOH) are the functionalized

derivative of carbon nanotubes. After some carbon atoms on the

nanotube wall have been carboxylated, properties of the carbox-

ylic MWCNTs are enhanced compared to pristine ones, includ-

ing mechanical performance, thermal stability, conductivity,

solubility, and hydrophilicity [29]. In addition, this process is

FIG. 1. (a) Particle size distribution and (b) SEM image of MWCNTs-COOH. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

956 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

simple to achieve [30]. Mahendra et al. [31] used multiblock

copolymers APAES and PAES with different contents of

MWCNTs-COOH to prepare a polymer ultrafiltration membrane

with the properties of porosity, hydrophilicity, a positive charge,

and an asymmetrical structure. They achieved a maximum pure

water flux and the best permeation performance when the

MWCNTs-COOH content was 2% of the polymer weight. Wang

et al. [32] modified polymethylmethacrylate (PMMA) and poly-

urethane (PU) matrix membranes with P-MWCNTs and

MWCNTs-NH2, respectively. The results implied that both pris-

tine MWCNTs and functionalized MWCNTs could improve

membrane performance effectively, but the functionalized

MWCNTs have better results than the pristine MWCNTs. Thus,

in this study, functionalized MWCNTs were adopted. While car-

bon nanomaterials are widely used in membrane modification,

the utilization of MWCNTs-COOH together with a hydrophilic

polymer to modify the membrane system is seldom reported.

In this study, a PVDF ultrafiltration (UF) membrane with

PVA and MWCNTs-COOH is prepared by the phase inversion

method [33–37]. The prepared plate UF membranes are ana-

lyzed through scanning electron microscopy (SEM), Fourier

transform infrared spectroscopy (FT-IR), pure water flux,

dynamic contact angle, porosity, polyethylene glycol (PEG) and

dextran (DEX) rejection, bovine serum albumin (BSA), flux

recovery rate (FRR%), and mechanical properties to illustrate

modified UF membrane capacity [38–40].

EXPERIMENTAL

Materials

Polyvinylidene fluoride (PVDF, SolefVR

6010) powder was

obtained from Solvay Advanced Polymers, L.L.C (Alpharetta

GA, USA). Polyvinyl alcohol (PVA, 99.8� moL/mol21, RG),

and dimethyl sulfoxide (DMSO, purity� 99.0%, AR) used as

the solvent were purchased from Shanghai Titan Scientific Co.,

Ltd. Polyethylene glycols in four molecular weights (PEG, MW

2 kg mol21, MW 4 kg mol21, MW 6 kg mol21, MW 10 kg

mol21) were purchased from Sinopharm Chemical Reagent Co.,

Ltd. Dextran (DEX, MW 600 kg mol21) was supplied by

Sigma-Aldrich Co., Ltd. The bovine serum albumin (BSA, MW

67 kg mol21) used in the flux recovery test was purchased from

Shanghai Lianguan Biochemical Reagent Company. All the

materials and reagents were without any chemical treatment

before use.

Carboxylated multiwalled carbon nanotubes, defined as

“MWCNTs-COOH” in this study, with 2.0 wt% of “ACOOH”

were purchased from Soochow Hengqiu Graphene Technology

Co., Ltd. The MWCNTs-COOH have an average surface area of

250–300 m2 g21 and an average length of 10–30 lm with outer

diameters and inner diameters in the ranges of approximately

20–30 and 5–10 nm, respectively. A laser particle size analyzer

(BT-9300Z) was used to measure the detailed information,

which includes the particle size distribution. As shown in Fig.

1a, the particle size of MWCNTs-COOH is under 30 lm, which

corresponds to the given length. Additionally, the median parti-

cle diameter obtained from the analyzer, which is 7.55 lm, sig-

nifies that 50% of the MWCNTs-COOH particles were larger

than 7.55 lm, and the other half were smaller than 7.55 lm. In

other words, this indicates significant dispersibility and less

agglomeration. Figure 1b shows the SEM image of MWCNTs-

COOH; the small size and long cylindrical morphology are ben-

eficial to the membrane performances [41, 42].

Preparation of Modified Composite Membranes

The PVDF/PVA/MWCNTs-COOH blend UF membrane was

prepared by the phase inversion method. The contents of the

membrane material and other reagents are listed in Table 1, in

which “MC” is the designation of this series of membrane,

“9010” represents the mass ratio of PVDF and PVA, and the

other three figures indicate the contents of MWCNTs-COOH in

the casting solution. For example, MC 9010 003 expresses the

composite membrane that contains 0.03 wt% MWCNTs-COOH

in the casting solution, and the mass ratio of PVDF to PVA is

90:10.

To briefly describe the preparation method, different amounts

of MWCNTs-COOHs and a constant 82 g DMSO were first

placed into a 250 mL conical flask. Shaking and ultrasonic treat-

ment for 10 min ensured that the MWCNTs-COOH were fully

dispersed. Then, polymer membrane materials were added.

PVDF powder and PVA grains were dried in the oven before

use for 12 h at 908C and 608C, respectively. The polymer con-

tent was set at 18 wt% in which PVDF:PVA is 90:10 by weight.

Then, strong mechanical stirring was carried out at 988C for

12 h so that the PVA could dissolve entirely, and a homogene-

ous casting solution was obtained. After complete degassing in

the oven at 608C for 12 h, a glass rod with fine copper wires

was used to transfer a film onto a glass plate. The glass plate

with a film of casting solution was immersed into deionized

water immediately. When peeled off from the glass plate, a

200-lm-thick membrane was successfully produced and then

stored in a container. The deionized water was changed twice a

day for 1 week to ensure the removal of residual solvent.

Finally, resultant membranes were stored in the container with

deionized water until needed.

TABLE 1. Compositions of different PVDF/PVA composite UF membranes.

Membrane no.

Membrane materials (wt%)MWCNTs-COOH

(wt%)

DMSO

(wt%) Coagulation bathPVDF PVA

MC 9010 000 16.2 1.8 0.00 82.0 Deionized water

MC 9010 003 16.2 1.8 0.03 82.0

MC 9010 006 16.2 1.8 0.06 82.0

MC 9010 009 16.2 1.8 0.09 82.0

MC 9010 012 16.2 1.8 0.12 82.0

MC 9010 015 16.2 1.8 0.15 82.0

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 957

Characterization of Modified Membranes

A scanning electron microscope (SEM, Hitachi S-3400N,

Japan) was used to observe the external surface and cross-

sectional structure of the modified membrane. Before observation,

the membrane was fractured in liquid nitrogen and fixed onto the

sample platform. Both surface and cross-sectional samples were

sprayed to form a thin platinum layer to make the samples con-

ductive. FT-IR (Thermo Nicolet iZ10, USA) was carried out to

investigate the functional groups and chemical composition of the

modified binary membrane, particularly to identify the introduc-

tion of carboxyl in the membrane. The wavenumber range of

4000–500 cm21 was observed. Atomic force microscopy (AFM,

Veeco, NanoScope IIIa Multimode AFM, USA) was carried out

to reveal the external surface topography and roughness of the

composite UF membranes using tapping mode. The membranes

were cut into small pieces and adhered to the microslide, and

then fixed onto the sample stage. The scanning area was selected

as 5 3 5 lm. Three-dimensional diagrams with average rough-

ness (Ra), root mean square roughness (Rms), and average maxi-

mum height (Rz) values were reported.

The hydrophilicity and permeation performance of the modi-

fied membranes were clarified though pure water flux, bovine

serum albumin (BSA, Mw 5 67,000) flux recovery rate (FRR%),

and the rejection of PEGs and DEX 600k. These events were

tested through self-made membrane permeation performance

equipment with an effective area of 38.5 cm2. All events were

tested at room temperature and under a pressure of 0.1 MPa.

After preloading the resultant modified membrane with

deionized water for 30 min until steady flux, the pure water flux

was obtained. In general, the pure water flux was defined as

Jw5V

A3t(1)

where Jw is the pure water flux (L m22 h21), V is the volume

of permeated water (L), A is the effective area of the equipment

(m2), and t is the time (h).

Subsequently, the BSA FRR% was tested. In other words,

this value reflects the cleaning effect of the membrane. The

higher the FRR% is, the higher the hydrophilicity of the mem-

brane is, which indicates a membrane with a low fouling degree

to a large extent. All the BSA solutions used in this study were

quantified BSA directly dissolved in deionized water to prepare

a 0.3 g L21 BSA aqueous solution (pH 5 7.0). The polluted flux

(Jp) was obtained by preloading the same modified membrane

used above with the 0.3 g L21 BSA aqueous solution for 30

min to make the membrane completely polluted. Then, the pol-

luted membrane was rinsed with sufficient deionized water and

immersed into the deionized water for 30 min, followed by

another wash. Then, the membrane was preloaded with deion-

ized water until the flux was steady. Recover flux (Jr) was

obtained, and the FRR% was calculated though the equation:

FRR%5Jr

Jw3100% (2)

where FRR% is the flux recovery rate (%) and Jr is the recover

flux (L).

The reversible fouling ratio (Rr%) and irreversible fouling

ratio (Rir%), which describe the concentration polarization

induced fouling and adsorption of proteins and organic pollutant

induced fouling, respectively, were also calculated from

Rr%5Jr2Jp

Jw

� �3100% (3)

Rir%5Jw2Jr

Jw

� �3100% (4)

where Rr% is the reversible fouling rate (%), Rir% is the irrevers-

ible fouling ratio (%), and Jp is the polluted flux (L m22 h21).

The sum of the reversible fouling rate and irreversible foul-

ing ratio described by the total fouling ratio (Rt%) was also cal-

culated to clarify the total flux loss through

Rt%5 12Jp

Jw

� �3100% (5)

Each test was carried out three times, and the average value was

recorded.

The same equipment was used to test the rejection of PEG in

different molecular weights and DEX 600k through another wet

membrane. PEG (1 g L21) or DEX (0.5 g L21) aqueous solution

were fed and permeated through the wet membrane under the

pressure of 0.1 MPa for 30 min; the permeation solution and

feed solution were collected, followed by a total organic carbon

(TOC) analyzer (TNM-1, Shimadzu, Japan) test. The concentra-

tions of the permeation solution and feed solution were obtained

and recorded as CP and CF, respectively. Thus, the rejection was

calculated as follows:

R%5CF2CP

CF3100% (6)

where R is the rejection (%), CF is the feed solution concentra-

tion (mg L21), and CP is the permeation solution concentration

(mg L21).

After that, the pore size distribution and molecular weight

cut-off (MWCO) were also obtained by a series of PEG rejec-

tion results though calculation on the log-normal distribution

function using Matlab software [43]. The log-normal distribution

function was defined as

f dð Þ51= ln r½ �dffiffiffiffiffiffi2pp� �

exp 2 1=2ð Þ ln d=l½ �=ln r½ �ð Þ2� �

(7)

After antifouling measurements, dynamic contact angle studies

were also carried out to clarify the membrane surface hydrophi-

licity. When a drop of pure water touched different membrane

surfaces, the rate of change in the contact angle was found to be

strongly related to hydrophilicity. Better hydrophilicity induces

faster decaying speeds [44]. Here, a contact angle analyzer

(JC2000D, Shanghai Zhongcheng Digital Technology Apparatus

Co., Ltd., China) with a sample injector was applied for this

test. A droplet, approximately 2 lL, was placed onto the modi-

fied membrane top surface, and images were collected with a

frequency of 4 s each until the droplet did not change. The

droplet was measured with the software, and the values were

recorded. To minimize the error, each membrane was tested five

times, and its average value was calculated.

Porosity is also an important parameter that can help explain-

ing some permeation and mechanical phenomena of the


composite membrane. The porosity was obtained by the gravi-

metric method. The samples were cut to fixed sizes and weighed

when both wet and dry. Calculations were made according to

Ref. 45:

e5Ww2Wd

qwAl(8)

where Ww is the weight of the wet membrane (g), Wd is the

weight of the dry membrane (g), qw is the density of water

(0.998 g cm23), A is the effective area of the binary membranes

(cm2), and l is the thickness of the composite membranes (cm).

Each membrane was measured five times, and the average value

was reported.

Break strength, elongation at a break, and Young’s Modulus

are the three most important parameters of membrane mechani-

cal properties. Tests were carried out by an electronic mechani-

cal property analyzer (QJ210A, Shanghai Qingji Instrument

Technology Co., Ltd. China) with two tongs in the vertical posi-

tion. The membranes were cut into strips and fixed onto the

tongs. The test started at a speed of 50 mm�min21; after the

series of tests, the reported values were averaged and recorded.

RESULTS AND DISCUSSION

FT-IR of PVDF/PVA Membrane and Modified PVDF/PVAMembranes

Figure 2 shows the spectra of the unmodified PVDF/PVA

membrane (MC 9010 000) and PVDF/PVA membranes modi-

fied with various MWCNTs-COOH contents (0.03 wt%, 0.06

wt%, 0.09 wt%, 0.12 wt%, 0.15 wt%, weight of casting solu-

tion). Obviously, the wide bands over 3100–3600 cm21 corre-

spond to the OAH band. This could be ascribed to the

introduction of PVA, but the absorbencies are very weak, which

may be because of the water solubility of PVA. During the

membrane forming process, a small amount of PVA attached to

PVDF precipitated through the phase inversion process, while

the majority of the PVA exchanged deionized water with

DMSO. In other words, the PVA in the casting solution worked

as the pore-forming agent. For the absorbance range of 2,920–

2,853 cm21, the ACH2A asymmetric stretching vibration is

present in both PVDF and PVA. In the comparison of those

modified membranes, all spectra show the stretching vibration

of C@O at 1,730 cm21, which becomes stronger. This phenom-

enon could be explained by observing that as the MWCNTs-

COOH content increases, the concentration of C@O in the

membrane also increases. This is strong evidence for the

enhancement of the surface hydrophilicity of modified mem-

branes. Meanwhile, the adsorption peak at 3,100–3,600 cm21

corresponding to OAH hardly changed, which implies that the

introduction of MWCNTs-COOH has no interaction with PVA

in the composite membranes. Next, the strong peak at

1,070 cm21 belongs to the CAF band, which is exclusive to

PVDF, and those peaks in the range of 840–1,400 cm21 demon-

strate that the polymeric crystal species is in the b-phase.

Morphology of the Modified PVDF/PVA Membrane

Figure 3 displays the SEM images of the PVDF/PVA mem-

brane and the modified PVDF/PVA membranes. The letter “A”

on the image panels refers to the external surface of each

composite membrane, while the letter “B” refers to the cross-

section of each composite membrane. Distinctly, all membranes

have a traditional asymmetric structure consisting of a compact

skin layer and a variational sublayer, which could be further

divided into the “upper part” finger-like structure and the “under

part” sponge-like structure. Compared to the unmodified PVDF/

PVA membrane (MC 9010 000), all modified PVDF/PVA mem-

branes show much denser “under part” structures, and with

increasing content of MWCNTs-COOH in the membrane

matrix, the “under part” becomes denser still. When the

MWCNTs-COOH content surpasses 0.12 wt%, the “under part”

is almost transformed into the sponge-like structure. This may

be because the presence of MWCNTs-COOH acted to densify

the “reticular-structure” induced by PVA in the membrane. This

effect endowed the modified membranes with greater mechani-

cal properties, which were also proven in later section. From the

external surface morphology, we can identify the appearance of

a bit macrovoid structure. This might result in the increase of

the pure water flux, which is attributed to the synergistic effect

of the MWCNTs-COOH and the different mutual solubilities

between PVDF and PVA. Quantified MWCNTs-COOH in the

casting solution can accelerate the exchange velocity between

the solvent DMSO and the nonsolvent deionized water during

the phase inversion process due to its hydrophilicity, which

might result in the formation of macrovoid structures. Other-

wise, the incompatibility between PVDF and PVA would cause

their phase separation, which is also conducive to this process.

Figure 3g Membrane “MC 9010 009 A” shows the most porous

structure, with no macrovoid structures. Moreover, the exchange

velocity was controlled at the most appropriate rate for this con-

tent level, and this kind of morphology provides the membrane

with the highest level of performance.

AFM Analysis of Different PVDF/PVA Composite UF Membranes

Figure 4 and Table 2 list the three-dimensional diagrams of

the top surface of the composite UF membrane and its rough-

ness values (Ra, Rms, and Rz). All modified PVDF/PVA compos-

ite membranes show rougher topography than the pristine

FIG. 2. FT-IR spectra of PVDF/PVA membrane and modified PVDF/PVA

membranes with various contents of MWCNTs-COOH. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]


PVDF/PVA membrane. In terms of the material properties,

rougher surface topography corresponds to greater hydrophilicity

[46]. On the other hand, average roughness (Ra), root mean

square roughness (Rms), and average maximum height (Rz) val-

ues were also larger than the values for the pristine PVDF/PVA

membrane. This is especially true for MC 9010 009, for which

the values of Ra, Rms, and Rz are the largest, reflecting the

lumpy membrane surface roughness with enlarged effective

filtration area and the best hydrophilicity of all the modified

PVDF/PVA membranes. Unfortunately, after the MWCNTs-

COOH content surpassed 0.12 wt%, the MWCNTs-COOH con-

tent in the casting solution could not move to the interface

between the solvent and nonsolvent adequately during the phase

inversion process due to the high concentration. Thus, the

hydrophilic functional groups fixed on the top surface of the

membranes “MC 9010 012” and “MC 9010 015” were

FIG. 3. SEM images of different PVDF/PVA composite UF membranes: (a) external surface; (b) cross-section.


marginally less than the membrane “MC 9010 009” and resulted

in the reduction of surface roughness. Furthermore, deeper

“valleys” and steeper “peaks” were observed from all of the

composite PVDF/PVA membranes containing MWCNTs-

COOH. MC 9010 009 is the most obvious one, which may due

to the concurrence of PVA and MWCNTs-COOH. This can be

explained by two reasons. First, PVA is a crystalline polymer;

during the demixing process, the “peaks” grow as the PVA crys-

tal grows. Second, the peculiarity of the strong hydrophilicity of

this material can accelerate the demixing speed, resulting in the

formation of “valleys” and “peaks” in which the morphology

may enhance the membrane hydrophilicity and permeability.

Pure Water Flux

Pure water flux is measured to evaluate the permeation ability

of the modified composite membranes. As shown in Fig. 5 and

Table 3, the unmodified PVDF/PVA membrane was also tested as

a comparison. The hydrophobic characteristics of PVDF restrict

the pure water flux at a low level. After blending, the pure water

flux increased evidently. With an increase of the MWCNTs-

COOH content, the pure water flux also grows. When the

MWCNTs-COOH content reaches 0.09 wt%, the pure water flux

of the modified membranes is at its maximum (126.6 L m22 h21),

which is almost double compared to the unmodified membranes

(68.6 L m22 h21). Schematic diagram of introduction of

MWCNTs-COOH in the PVDF/PVA UF membrane is placed in

Fig. 6. The credible explanation for this is the synergistic effect of

MWCNTs-COOH and PVA. On one hand, with the addition of

PVA, macrovoid structure exists due to its hydrophilicity, and the

pore channels of the inner structure become wider, which can

increase the opportunity for water molecule inflow. These results

are in accordance with the SEM analysis (Fig. 3). On the other

hand, due to the immobilization of MWCNTs-COOH into the

membrane matrix, the abundant carboxyls disperse on the surface

of the modified membranes uniformly, resulting in the enhance-

ment of membrane surface hydrophilicity. Additionally, the cylin-

drical structure of MWCNTs-COOH may supply tunnels for water

molecules, making it easier to pass through. Otherwise, from the

AFM analysis, the largest effective filtration area also contributes

significantly to the flux. However, the pure water flux did not keep

the trend of monotonically increasing after the content of

MWCNTs-COOH exceeded 0.09 wt%; it exhibited a small decline

but still retained at a satisfactory value. Because of the solubility

of MWCNTs-COOH in the solvent DMSO, when the concentra-

tion of MWCNTs-COOH in the casting solution is high, some

small portion of the MWCNTs-COOH may agglomerate due to

the intermolecular interactions, which may somewhat influence the

performances of these membranes. Additionally, the high value of

porosity (82.4%) measured through the gravimetric method is

shown in Fig. 5. The variation tendencies are almost the same as

the pure water flux, which can also illustrate the effects of the

usage of MWCNTs-COOH.

Antifouling Ability

The antifouling ability of modified PVDF/PVA membranes

was estimated through the BSA FRR%. The BSA aqueous solu-

tion was prepared by directly dissolving quantified BSA in

deionized water to prepare the 0.3 g L21 BSA aqueous solution

at a pH value of 7.0. It is the rate of recovery flux (Jr) and pure

water flux (Jw) in which a higher FRR% corresponds to a better

cleaning effect of the membrane. The results are presented in

Fig. 5. The modified composite membrane, which has an

FIG. 3. Continued.


optimal antifouling ability when the MWCNTs-COOH content

in the casting solution is 0.09 wt%, has an FRR% of 81.2%.

Compared to MC 9010 000, it has an improvement of approxi-

mately 17%. This may be attributed to the introduction of

MWCNTs-COOH with sufficient carboxyls; the membrane sur-

face with plenty of carboxyls can obstruct most pollutants out-

side the compact skin layer. In other aspects, with the

enhancement of surface hydrophilicity, the pollutant is easy to

FIG. 4. AFM three-dimensional diagrams of different PVDF/PVA composite UF membranes. [Color figure can be


TABLE 2. Roughness values of different PVDF/PVA composite UF membranes.

Membrane no. MC 9010 000 MC 9010 003 MC 9010 006 MC 9010 009 MC 9010 012 MC 9010 015

Ra (nm) 6.8 7.9 13.9 15.2 14.0 14.3

Rms (nm) 9.0 11.5 17.7 19.4 19.1 18.4

Rz (nm) 39.7 41.8 62.0 64.9 60.6 60.6


wash out using deionized water from the membrane surface,

especially for a protein pollutant. For further investigation, as

shown in Fig. 7, the fouling resistance ratios change identically

with the FRR%. MC 9010 009 has the highest reversible fouling

ratio and the lowest total fouling ratio, representing the best

membrane surface antifouling ability among the modified com-

posite membranes [47]. Moreover, the MWCNTs-COOH modi-

fied composite membranes have higher reversible ratios and

lower irreversible fouling ratios than the pure membrane, except

MC 9010 012, for which the reversible ratio is 1.7% lower, but

its total fouling ratio was decreased by 5.8%. This may be

induced by the increase of membrane surface roughness; an

enhanced hydrophilic surface can defend against a majority of

pollutants, but a small part of them can be trapped in the

“valleys” and wrapped on the “peaks” of the membrane surface.

Therefore, the PVDF/PVA membrane modified with 0.09 wt%

of MWCNTs-COOH must represent the optimal loading. Addi-

tionally, the variation tendency of the FRR% and fouling resist-

ance ratios are consistent with the phenomenon reflected by the

membrane surface dynamic contact angle in the next section.

Dynamic Contact Angle

The dynamic contact angle is an important parameter for

assessing the surface hydrophilicity of a membrane. Generally,

researchers consider a lower contact angle value desirable.

The dynamic contact angle variation trend in 120 s is listed in

Fig. 8. An unmodified PVDF/PVA membrane has the highest

contact angle value, next to it is the modified PVDF/PVA

FIG. 5. The relationship between MWCNTs-COOH content, pure water

flux, recover flux, flux recovery rate, and porosity. [Color figure can be


TABLE 3. Permeability performances of different UF membranes.

Membrane no.

Pure water flux

(L�m22�h21)

BSA Polluted

flux (L�m22�h21)

Recovery flux

(L�m22�h21)

Flux recover

rate (%)

Reversible

fouling

rate (%)

Irreversible

fouling

ratio (%)

Total

fouling

ratio (%)

MC 9010 000 68.6 6 2.9 28.2 6 1.2 47.5 6 1.7 69.4 6 4.8 28.2 6 4.6 30.6 6 4.8 58.8 6 2.5

MC 9010 003 91.6 6 9.2 42.7 6 4.7 70.1 6 9.0 76.4 6 4.5 29.7 6 8.4 23.6 6 4.5 53.3 6 3.8

MC 9010 006 105.0 6 7.3 51.2 6 2.8 84.3 6 9.0 80.2 6 4.6 31.4 6 6.2 19.8 6 4.6 51.2 6 1.7

MC 9010 009 127.0 6 7.4 62.1 6 2.9 103.0 6 2.7 81.2 6 3.8 32.2 6 1.4 18.8 6 3.8 51.0 6 3.0

MC 9010 012 103.0 6 7.5 48.2 6 3.3 75.6 6 4.6 73.5 6 2.6 26.5 6 6.1 26.5 6 2.6 53.0 6 5.0

MC 9010 015 97.5 6 8.1 45.5 6 4.5 74.0 6 8.0 75.8 6 2.1 29.1 6 2.8 24.2 6 2.1 53.3 6 1.8

FIG. 6. Schematic diagram of introduction of MWCNTs-COOH in the PVDF/PVA UF membrane. [Color figure

can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


membrane with only 0.03 wt% additive, which is designated

MC 9010 003. Membrane MC 9010 006 and MC 9010 015 are

superior to MC 9010 003 but inferior to MC 9010 009, which

has the smallest contact angle value. From these curves, we can

find that for the contact angles of the PVDF/PVA membrane

modified with different MWCNTs-COOH contents, the contact

angle decreased in varying degrees. This indicates that the intro-

duction of MWCNTs-COOH could improve the surface hydro-

philicity of composite membranes. Furthermore, with the

increase of MWCNTs-COOH content, the contact angle value

decreases at first, followed by a small increase, similar to the

other performance parameters. When the MWCNTs-COOH con-

tent is 0.09 wt%, the contact angle of the unmodified PVDF/

PVA membrane decreases from 60.18 to 38.98 at the period of

the 30th second, reaches a minimum and then becomes steady,

which reflects the optimal situation of surface hydrophilicity.

The mechanism could be interpreted as follows: MWCNTs-

COOH is hydrophilic, and during the membrane preparation

process, the abundant carboxyl was fixed into the membrane

and surface. At the same time, as the MWCNTs-COOH content

increases, it leads to an increase of surface roughness, and the

higher surface roughness facilitates the enhancement of mem-

brane surface hydrophilicity.

FIG. 7. Fouling resistance ratio of PVDF/PVA membrane and modified

PVDF/PVA membranes with various contents of MWCNTs-COOH. [Color

figure can be viewed in the online issue, which is available at wileyonlineli-

brary.com.]

FIG. 8. Dynamic contact angles of PVDF/PVA membrane and modified

PVDF/PVA membranes with various contents of MWCNTs-COOH. [Color

figure can be viewed in the online issue, which is available at wileyonlineli-

brary.com.]

FIG. 9. The relationship between MWCNTs-COOH content, pure water

flux, and DEX 600k rejection.

FIG. 10. (a) Cumulative pore size distribution curves and (b) probability

density function curves of various modified membranes. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com.]


Pore Size, Pore Size Distribution, and MWCO of VariousMembranes

The liquid–liquid displacement porosimetry (LLDP) method

was used by many researchers to acquire pore size distributions

and molecular weight cut-off (MWCO) values due to its precise,

accurate, and reliable nature [48–50]. However, in this study, a

simple and efficient method was used to investigate the pores of

the composite membranes [51–53]. Figure 9 clarifies the rela-

tionship between MWCNTs-COOH content, pure water flux,

and DEX 600k rejection. Figure 10 displays the cumulative pore

size distribution curves and probability density function curves

of various binary membranes. The rejection data of PEGs,

which are used for the curve fitting, and other parameters are

shown in Table 4. From the figures, we can clearly determine

the larger mean effective pore size of membrane MC 9010 000,

which was not modified by MWCNTs-COOH. Compared to the

unmodified PVDF/PVA membrane, all of those modified PVDF/

PVA membranes possess smaller pore sizes and narrow pore

size distributions. These phenomena may be attributed to the

introduction of MWCNTs-COOH into the membrane matrix.

With the increase of the MWCNTs-COOH content, the mean

effective pore size first decreases then increases. This trend is

similar to porosity and the rejection of PEGs and DEX 600k,

but it is a reverse relationship. Unusually, in terms of the rejec-

tion of DEX 600k and pure water flux, membrane MC 9010

009 has the highest values. This can be explained in terms of

the smallest mean effective pore size at the highest quantities

and the auxiliary hydrophilic MWCNTs-COOH fixed on the

membrane contributing a great deal to the high pure water flux

and rejection values. Otherwise, according to the DEX rejection,

the modified membranes exhibit differences in MWCO between

measured values and computation values. This discrepancy can

be explained as follows: the introduction of MWCNTs-COOH

into the casting solution influenced the membrane formation

process, and it accelerated the exchange velocity between the

solvent and nonsolvent. Moreover, the strong solvent DMSO

also can maintain the speed at a high level, which may result in

the production of large-sized pores on the membrane surface,

but it makes little contribution to the flux at extremely small

amount. Therefore, the emerging pores offer a favorable channel

for the small solute to pass through, but it is just the size to pre-

vent the passage of the DEX 600k solute.

Mechanical Properties

The data of the break strength, elongation at the break, and

Young’s modulus are shown in Table 5. The obtained data indi-

cate that with the addition of MWCNTs-COOH, the three

mechanical property parameters of the modified PVDF/PVA

membranes all improved. As the MWCNTs-COOH content in

the casting solution increases, the three parameters increase until

the loadings reach to 0.12 wt%. This phenomenon disagreed

with the speculation in terms of permeation properties. The fol-

lowing reasons may account for this. In one aspect, with the

introduction of one-dimensional MWCNTs-COOH with long

and rigid structures, the PVDF chain wrapped with the well-

dispersed MWCNTs-COOH induced the enhancement of

mechanical properties. With the increase of the MWCNTs-

COOH content, the PVDF chain wrapped with more MWCNTs-

COOH continuously enhances the mechanical properties until

0.12 wt%. In addition, the mechanical properties lost by the

membrane containing 0.15 wt% was caused by the agglomera-

tion of superfluous MWCNTs-COOH. Before the turning point,

the cross-sectional structure of membrane becomes much more

compact with the presence of increasing MWCNTs-COOH con-

tent, which induces the membrane sublayer morphology to

transform gradually from finger-like into sponge-like, accompa-

nied by the disappearance of the net-like structure resulting

from the presence of PVA. Thus, the superior mechanical prop-

erties of the modified composite membrane including break

strength, elongation at break, and Young’s modulus have

enhancements of 60%, 215.5%, and 56.7%, respectively, com-

pared to membrane MC 9010 000. Nevertheless, membrane MC

9010 009 still reveals considerable mechanical properties versus

membrane MC 9010 012.

CONCLUSIONS

PVDF/PVA/MWCNTs-COOH modified UF membranes with

various contents of MWCNTs-COOH (0.00 wt%, 0.03 wt%,

0.06 wt%, 0.09 wt%, 0.12 wt%, 0.15 wt%, weight of casting

solution) were prepared via the phase inversion method.

TABLE 4. Porosity, pore size, molecular weight cut-off (MWCO), and rejections values.

Membrane no. MC 9010 000 MC 9010 003 MC 9010 006 MC 9010 009 MC 9010 012 MC 9010 015

Porosity (%) 80.0 6 1.6 80.6 6 3.9 81.6 6 3.1 82.4 6 1.4 79.4 6 3.8 80.4 6 3.1

l (nm) 9.7 5.4 5.0 3.2 3.6 4.3

r 1.9 1.5 1.8 2.0 2.1 1.8

MWCO/Da 116,000 22,700 32,000 17,300 25,300 25,400

PEG 2k rejection (%) 15.2 6 0.9 15.7 6 1.7 24.6 6 1.1 37.5 6 1.3 36.3 6 2.5 29.6 6 1.7

PEG 4k rejection (%) 23.1 6 1.7 27.5 6 2.1 41.1 6 1.5 56.8 6 0.7 50.5 6 1.1 39.4 6 0.7

PEG 6k rejection (%) 23.6 6 0.6 43.9 6 1.3 45.1 6 0.3 65.6 6 0.9 59.0 6 1.0 53.0 6 0.6

PEG 10k rejection (%) 30.4 6 0.6 43.0 6 0.9 47.7 6 0.9 68.9 6 0.8 64.4 6 0.4 60.2 6 0.4

DEX 600k rejection (%) 66.8 6 0.6 86.2 6 0.6 85.6 6 1.0 91.0 6 0.6 86.1 6 0.7 89.7 6 0.7

TABLE 5. Mechanical properties of various membranes with different con-

tents of MWCNTs-COOH.

Membrane no.

Break

strength (MPa)

Elongation at

break (%)

Young’s

modulus (MPa)

MC 9010 000 1.30 6 0.07 9.7 6 0.8 30.7 6 2.9

MC 9010 003 1.58 6 0.03 12.8 6 0.7 34.7 6 1.5

MC 9010 006 1.78 6 0.08 14.2 6 2.3 36.5 6 3.2

MC 9010 009 1.86 6 0.09 17.3 6 2.7 48.0 6 2.0

MC 9010 012 2.08 6 0.07 30.6 6 3.9 48.1 6 4.0

MC 9010 015 2.06 6 0.12 27.9 6 8.3 45.2 6 6.7


FT-IR spectra show the absorbency of C@O at 1,730 cm21

and prove the successful introduction of carboxyl groups. The

weak peaks of the OAH band over 3,100–3,600 cm21 are

ascribed to the introduction of PVA, implying the pore-forming

agent function of PVA in the membrane forming process, which

has contributions to the hydrophilicity and high flux. SEM

images demonstrate that all composite membranes have a tradi-

tional asymmetric structure constituted by a compact skin-layer

and a variational sublayer, which could be divided into the

“upper part” finger-like structure and the “under part” sponge-

like structure. MC 9010 009 displayed a perfect porous struc-

ture, and MC 9010 012 displayed the best sponge-like structure.

AFM three-dimensional diagrams and roughness values identify

that the roughest and most hydrophilic external surface belongs

to MC 9010 009, which exhibits the best surface hydrophilicity

and permeation performances.

MC 9010 009 displays the best permeation performances and

surface structure. The pure water flux was approximately

doubled (126.6 L m22 h21) compared to MC 9010 000 (68.6

L m22 h21), which can be identified by high porosity (82.4%).

Otherwise, the rejection tests of PEGs and DEX illuminate the

smallest mean effective pore size and MWCO of 3.2 nm and

17300 Da, respectively, which can obstruct DEX 600k up to

91.0%. The BSA flux recovery rate and fouling resistance tests

clarify the remarkable effect of the introduction of PVA and

MWCNTs-COOH. The FRR% of MC 9010 009 increased by

17% compared to MC 9010 000 and exhibited the highest

reversible fouling ratio and the lowest total fouling ratio. In

addition, the dynamic contact angle showed a decreasing trend,

with that of MC 9010 009 being the most obvious.

MC 9010 012 exhibits impressive mechanical properties of

which break strength, elongation at break, and Young’s modulus

have improved by 60%, 215.5%, and 56.7%, respectively, com-

pared to MC 9010 000. These properties benefited from the denser

structure formed by the addition of PVA and MWCNTs-COOH.

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