2011 - Fabrication New PES-Based Mixed Matrix Nanocomposite Membranes Using Polycaprolactone Modified Carbon Nanotubes as the Additive Property Changes and Morphological Studies

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Fabrication new PES-based mixed matrix nanocomposite membranes using

polycaprolactone modified carbon nanotubes as the additive: Property changes

and morphological studies

Y. Mansourpanah a,⁎, S.S. Madaeni b, A. Rahimpour c, M. Adeli a, M.Y. Hashemi d, M.R. Moradian e

a Department of Chemistry, Faculty of Science, Lorestan University, Khorramabad, Iranb Membrane Research Center, Department of Chemical Engineering, Razi University, Kermanshah, Iranc Nanobiotechnology Research Laboratory, Faculty of Chemical Engineering, Babol University of Technology, Babol, Irand Department of Chemistry, Faculty of Sciences, Islamic Azad University, Arak Branch, Irane Lorestan University, Khorramabad, Iran

a b s t r a c ta r t i c l e i n f o

Article history:

Received 25 October 2010

Received in revised form 5 April 2011

Accepted 6 April 2011

Available online 4 May 2011

Keywords:

Nanocomposite membranes

Polycaprolactone modified nanotubes

Antifouling

Hydrophilicity

In thisstudy, the effects of different concentrations (0.5, 1.5and 3% w/v) of polycaprolactone modified multiwall

carbon nanotubes (PCL-MWCNTs) as the additive in the casting solution were investigated. PCL-MWCNTs are

nanocomposite materials containing ―OH end groups and other functional groups (e.g., carbonyl (C=O)

groups) that can affectthe membrane properties.Some membrane characteristics such as surface hydrophilicity,

surface chemistry, thermal resistance, and surface and cross-section morphology were investigated by water

contactangle, ATR-IR,TGA, AFM, and SEMtests,respectively. These tests represent some outstanding changes in

the membrane properties due to the presence of PCL-MWCNTs. The membrane antifouling properties were

examinedby using the bovineserum albumin solution as the model system. The pure waterflux enhancedfrom

28 L/m2 h (theunmodified membrane) to 61 L/m2 h (the modified membrane including 3 w/v% PCL-MWCNTs).

SEM and AFM images show an even, porous and smooth surface along with the finger-like macrovoids in the

sub-layer of membranes composed of PCL-MWCNTs.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The membrane process is an attractive separation technology due

to the fast and energy ef ficient process without any phase change [1].

The application of membranes is growing in pharmaceutical,

chemical, paper, semiconductor, textile, water and wastewater

processes. The main goal in preparation of membranes is to control

the membrane structure, which affects the membrane performance.

Mixed-matrix membranes (MMMs) are a well-known way to

enhance the properties of polymeric membranes [2]. Their structure

consists of an inorganic material incorporated into a polymeric matrix.

In principle, theincorporation of theinorganic componentcan be seen

as a relatively easy modification of existing methods for fabricating

large-surface area polymeric membranes; therefore, MMMs possess

an economic advantage over inorganic membranes. In addition, they

may offer enhanced physical, thermal, and mechanical properties for

aggressive environments and could be a way to stabilize the polymer

membrane against change in permselectivity with temperature [3].

These membranes offer very interesting properties. The successful

development of MMMs depends on several factors such as the proper

selection of a polymeric matrix and inorganic filler and the elimi-

nation of interfacial defects between the two phases. Most commer-

cial membranes are fabricated from organic polymers. However, the

membranes composed of inorganic materials are developing due to

higher durability and performance in many separation applications

[4–6].

Polyethersulfone (PES) is the material of choice for numerous

membrane applications due to its outstanding mechanical strength,

thermal stability, and formability. However, the important disadvan-

tages of PES for membrane preparation are low permeability and high

fouling tendency which are due to the inadequate hydrophilic

property of PES compared to that of other polymers such as

polyacrylonitrile, cellulose acetate, polyamide, and polyimide. Many

approaches were applied to impart higher hydrophilicity to PES

membranes. One of these techniques includes addition of hydrophilic

additives to the membrane casting solution, leading to decreasing

membrane fouling.

Carbon nanotubes (CNTs) with unique properties such as physical,

chemical, mechanical, and electrical properties are interesting both in

academic and industrial aspects [7–9]. The outside wall modification

of CNTs is a well-known way to improve the CNTs properties and

obtain new materials with new and interesting properties. A well-

known procedure to increase the processability of CNTs is chemical

modification [10,11]. After modification, they are not only soluble in

Desalination 277 (2011) 171–177

⁎ Corresponding author. Tel.: +98 916 3611750; fax: +98 661 2202782.

E-mail address: [email protected](Y. Mansourpanah).

0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.desal.2011.04.022

Contents lists available at ScienceDirect

Desalination

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http://dx.doi.org/10.1016/j.desal.2011.04.022



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different solvents, depending on the modifier, but also contain

functional groups which turn them into multidisciplinary materials

in order to be used in different reactions and processes. Researchers

abundantly conjugated some molecules and macromolecules on the

sidewall of CNTs [12]. On the other hand, polymer-conjugated CNTs

lead to new nanocomposites with interesting properties [12–14].

In this research, first, polycaprolactone-modified multiwall carbon

nanotubes (PCL-MWCNTs) were prepared in laboratory and then

PES/PCL-MWNTS nanocompositemembraneswere fabricated.Theeffects

of polycaprolactone modified carbon nanotubes on the PES membrane

properties such as performance, morphology, hydrophilicity, thermal

resistance, increase of antifouling, and self-cleaning properties were

studied.

2. Experimental

2.1. Materials and apparatus

Polyethersulfone (PES Ultrason E6020P with MW=58,000 g/mol)

was supplied by the BASF Company (Germany). Polyvinylpyrrolidone

(PVP, 25,000 g/mol), dimethylacetamide (DMAc), and chloroform

from Merckwere used. Cd(NO3)2 salt (Merck) was used to investigate

the ion rejection capability of membranes. The bovine serum albumin

(BSA) powder [some properties are followed: assay: N96%, mol wt.:

66 kDa, pH≈7, and solubilityN40 mg/mL in H2O] was obtained from

Sigma. ε-caprolactone was purchased from Aldrich and purified by

using distillation under vacuum. Stannous-2-ethylhexanoate waspurchasedfrom Sigma. Distilled waterwas used throughout the study.

2.2. Preparation of PCL-modi fied carbon nanotubes

CNTs were opened according to reported procedures in the

literature [15]. Briefly, CNTs were milled and dispersed in a 3/1

mixture of H2SO4 andHNO3. Themixture wasrefluxed for10 h.Then,it

was cooled,filtered, andwashedby wateradjustedat pH 5. The opened

CNTs were dried at 120 °C for 6 h. One milliliter of Sn(Oct)2 in toluene

(1×10−3 M) was added to a polymerization ampoule equipped with a

magnetic stirrer and vacuum inlet. Toluene was evaporated under

vacuum at 60 °C for 30 min. One milliliter of ε-caprolactone and CNT

(the amount of CNTs for the reaction was 0.002 g) were added to the

polymerization ampoule. The polymerization ampoule was sonicated at25 °C for 10 min. Then, it was kept under vacuum for 1 h at 60 °C. The

polymerization ampoule wassealedand it wasthen stirred at 120 °C for

10 h. Then, it wascooled and itscontents were dissolved in chloroform.

The solution was filtered and the resultant product was precipitated in

diethylether. More detailsand explanations are present in the literature

from the current authors [16]. The modification procedure of CNTs is

shown in Scheme 1.

2.3. Preparation of PES/PCL-MWNTs membranes

The blend dope solution was prepared by dissolving 18 wt.% PES in

dimethylacetamide (DMAc) with 3 wt.% polyvinylpyrrolidone (PVP). On

the other hand, a solutionincluding different percentages of PCL-MWNTs

(0.5, 1.5 and 3% w/v) with the same amount of chloroform (0.7 mL) was

prepared and added to the prepared dope solution. The stirring was

carried outat 200 rpmfor 5 h at 50 °C.After formation of a homogeneous

solution, the dope solution was held at the ambient temperature for

around24 h to removethe airbubbles. Afterwards,the dope solution wascast on theglass support at 150μ m thickness by using afilm applicator at

the room temperature without evaporation. After coating, the support

was immersed into a distilled water bath for at least 20 h for removing

most of the solvent and water-soluble polymer. The composition of

obtained membranes is represented in Table 1.

2.4. Physical characterization methods

2.4.1. TEM test

The structure and feature of PCL-modified CNTs were observed by

the TEM apparatus. TEM photo was recorded by using Philips CH 200,

LaB6-Cathode 160 kV.

Scheme 1. The schematic procedure of synthesis of PCL-MWCNT nanocomposites.

Table 1

The composition of resultant membranes.

Membrane PES (%) PVP (%) PCL-MWNTs (%)

M0 18 3 –

M1 18 3 0.5

M2 18 3 1.5

M3 18 3 3.0

Fig. 1. FTIR-ATR spectra of M0 and M3 membranes.

Fig. 2. TEM image of a PCL-MWCNT nanocomposite.

172 Y. Mansourpanah et al. / Desalination 277 (2011) 171–177

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2.4.2. SEM test

Thesurface andcrosssection of membraneswereexaminedby using

a scanning electron microscope (SEM). The samples of the membranes

were frozen in liquid nitrogen and fractured. After sputtering with gold,

they were viewed with a Philips microscope at 25 kV.

2.4.3. AFM test

The atomic force microscopy (AFM, non contact mode) was used to

analyze the surface morphology and roughness of the membranes. TheAFM apparatus was a DualScope™ scanning probe-optical microscope

(DME model C-21, Denmark). Small squares of the prepared mem-

branes (approximately 1 cm2) were cut and glued on a glass substrate.

The membrane surfaces were analyzed in a scan size of 1 μ m× 1 μ m.

2.4.4. TGA test

TGA experiments were recorded by using an apparatus Model TGA

1500, England.

2.4.5. Water contact angle test

The static contact angles were measured with a contact angle

measuring instrument (G10, KRUSS, Germany). De-ionized water was

used as the probe liquid in all measurements and the contact angles

between water and the membrane surface were measured for the

evaluation of the membrane hydrophilicity. To minimize the

experimental error, the contact angle was measured at five random

locations for each sample and the average was reported.

2.5. Membrane performance evaluation

The performance of prepared membranes was analyzed by using a

batch cross flow system. The details of the experimental set up have

been described elsewhere [17]. The membrane surface area in the

filtration cell was 22 cm2. The flux of each membrane was determined

at 10 minute intervals under the 0.8 MPa transmembrane pressure.

Theexperiments were carried out at 25 °C. The crossflow velocity was

Fig. 3. SEM images from the cross-section structure of: (a) M0, (b) M1, (c) M2 and (d and e) M3.

173Y. Mansourpanah et al. / Desalination 277 (2011) 171–177




approximately 0.6 m/s for all tests. The permeation rate and salt

rejection were determined for all membranes using the Cd(NO3)2solution in the 500 ppm concentration. The rejection was obtained by

[18]:

R% = 1−λ p

λ f

" #× 100 ð1Þ

where λ p and λ f are the ion conductivity in the permeate and feed,respectively. The ion rejection was investigated by measuring the

permeate conductivity using a conductivity meter (Hanna 8733

Model, Italy).

2.6. Antifouling properties and flux recovery

Fouling can be quantified by the resistance appearing during the

filtration, and cleaning can be specified by the removal of this

resistance. The resistance is due to the formation of a cake or gel layer

on the membrane surface. The flux (J) through the cake and the

membrane may be described by the following equation:

J =m

AΔ

t ð2Þ

where m is the mass of the permeated water, A the membrane area,

and Δt the permeation time.

After water flux measurement (Jwi), the solution reservoir was

refilled with a 0.1 g/L BSA solution and the flux was obtained (Jp).

After 2 h of filtration, the membrane was washed with deionized

water for 10 min and the water flux of cleaned membranes was

measured (Jwc). In order to evaluate the fouling-resistant capability of

the membrane, the flux recovery ratio (FRR) was calculated using the

following expression:

FRR =Jwc

Jwi

× 100 ð3Þ

R r and R ir, described by Eqs. (4) and (5) show reversible deposition

and irreversible fouling [19]:

R r %ð Þ =Jwc− Jp

Jwi

× 100 ð4Þ

R ir %ð Þ =Jwi− Jwc

Jwi

× 100 ð5Þ

3. Results and discussion

3.1. ATR-IR studies

The chemical structure of unmodified and modified membranes is

shown in Fig. 1. It is obviously clear that the membrane composed of

PCL modified carbon nanotubes (Fig. 1b) shows a strong peak at

1720 cm−1 which is assigned to C O functional groups in polycapro-

lactone. This band, which confirms the presence of C O functional

groups, is not observed in the spectrum of the original PES membrane

(Fig. 1a). Hence, the obtained ATR-IR spectra demonstrate the

presence of PCL-MWCNTs in the matrix of modified membranes and

confirm the production of PES-based mixed matrix membranes.

Fig. 4. SEM images from the membrane surface of: (a) M0, (b) M1, (c) M2 and (d) M3.





3.2. TEM, SEM and AFM images

Fig. 2 shows the TEM image of polycaprolactonemodified MWCNTs.

This image demonstrates clearly the settling and attachment of

polycaprolactone segments on the outside wall of the nanotubes.

Carbonnanotubesincludingpolycaprolactonecan be betterdispersedin

the polymeric dope solution and form a homogeneous casting solution

for preparation of mixed matrix membranes composed of modified

carbon nanotubes.The membrane morphology including cross-section and surface

structures was visualized by using SEM. As shown in Fig. 3, all mem-

branes had an asymmetric structure consisting of a thin selective barrier

and a much thicker poroussub-structure. The reason forthe formationof

the asymmetric structure can be found in the literature [20]. As clearly

seen in Fig. 3, the variation in the membrane structure occurred in the

presence of PCL-MWCNTs in the casting solution.As it canbe observed in

the images, the sponge-like pores in the sub-layer of the PES membrane

were turned into finger-like pores in PES/PCL-MWCNT blend mem-

branes. The M0 membrane (Fig. 3a) exhibits a typical asymmetric

structure and fully developed macro-pores.

The membrane structure comprises a dense thin top layer and a

porous sub-layerwhich isfilled up by closed cells within the membrane

matrix. The increase of PCL-MWCNTs leads to the decrease of the skin

layer thickness and changes the sub-layer structure so that finger-like

macro-voids (large elongated pores) are formed. Longermacro-voids in

the sub-layer appear by increasing PCL-MWCNTs, leading to higher

porosity in the membrane structure which results in higher permeabil-ity of the membrane. By increasing PCL-MWCNT loading, a membrane

including a thin skin layer and large elongated finger-like pores were

formed. Visualization of cross-section morphology shows that the M3

membrane (Fig. 3d and e) with 3 w/v% of PCL-MWCNTs possesses a

porous structure. An increment in the surface porosity especially in the

pore walls and membrane matrix is clearly observed. Probably, the

presence of ―OH and ―COO― functional groups in PCL-MWCNTs

affects the hydrophilicity property of the casting solution and changes

the membrane characteristics. Authors believe that the increase of

PCL-MWCNTs affects the rates of non-solvent inflow and solvent

outflow. The rapid demixing in the PES/PCL-MWCNT blend and water is

greater than that of PES and water due to the higher af finity of water

with the PES/PCL-MWCNTs. This leads to the quick formation of a skin

layer, creating an additional resistance to mass transfer, and results in a

longer time for the entire exchange between the non-solvent bath and

the polymer casting film. Experimentally, the required time for the

formation of thefilm made of PES/PCL-MWCNTs was much longer than

that of the PES membrane. Increased exchange time between the

solvent and non-solvent results in a more development of the growth

and coalescence of the polymer-leanphase.Therefore, the largerfinger-

like pores are formed. It is well known that membranes with large

macro-voids usually have a thin skin layer. Fig. 4, which was recorded

from the top surfaces of the membranes, indicates that the top sur-

faces of obtained membranes are strongly influenced by addition of

PCL-MWCNTs. As shown in Fig. 4a, the M0 membrane shows a dense

and compressed surface while by addition of PCL-MWCNTs in the

casting solution a porous structure containing a smooth shape was

observed (Figs. 3e and 4d).

Observation of the AFM images (Fig. 5) confirms the obtainedresults from Fig. 4. The M0 membrane shows a dense and rough

surface (Fig. 5a). The nodules on the surface of the M0 membrane

were removed by addition of PCL-MWCNTs in the casting solution,

and theheight of the surface convexes waschanged from 80.4 nm (for

M0) to26.3 nm (forM3).As seenin Fig. 5b, the M3 membrane with the

highest amount of PCL-MWCNTs exhibits an even and smooth surface

with a porous structure. The brightest areas indicate the highest

pointsof themembrane surface andthe dark areas show thevalleys or

pores on the surface. These results support the alterations in the

membrane surface shape. Table 2 shows some surface properties

which were measured from the height profile of two-dimensional

AFM images by using the SPM software and averages were reported.

Considering the morphology and data for M0 and M3 membranes

(Fig. 5 and Table 2) indicate that the M0 membrane possesses a lowporosity. On theotherhand, theexistence of many numbers of smaller

pores on the M3 membrane surface causes the further porosity and

the flux increasing. Fig. 5 clearly shows that thesurface porosity of the

M3 membrane was increased by adding PCL-MWCNTs. By using the

Fig. 5. AFM topographic images of: (a) M0 and (b) M3.

Table 2

The mean pore size and the convex height of membrane surfaces.

Membrane Mean pore size of surface (nm) Surface convex height (nm)

M0 85 (±16) 80.4

M3 35 (±19) 26.3

Table 3

The water contact angle of obtained and modified membranes.

Membrane Water contact angle (°)

M0 66.7 (±2.0)

M1 52.8 (±1.1)

M2 51.3 (±1.5)

M3 57.0 (±1.8)

175Y. Mansourpanah et al. / Desalination 277 (2011) 171–177




SPM software the mean pore sizes for M0 and M3 membranes were

obtained about 85 and 35 nm, respectively.

The static water contact angle test was conducted to measure

the hydrophilicity changes of membrane surfaces by introducing

PCL-MWCNTs in the casting solution. As seen in Table 3, the

unmodified membrane(M0) without PCL-MWCNTs possessesa higher

contact angle (66.7°) compared to PCL-MWCNT modified membranes.

By increasing PCL-MWCNT loading, the water contact angle decreased

(52.8° for M1 and 51.3° for M2) due to the presence of ―OH and

―COO― functional groups. On theotherhand, thewatercontactangle

of the M3 membrane with 3 w/v% polycaprolactone increased slightly

(57°). Authors suppose that the increaseof carbon nanotubesamounts

and alkyl chains in polycaprolactone may increase slightly the

hydrophobicity. Generally, the hydrophilic property of obtained

membranes was influenced by introducing PCL-MWCNT additives,

resulting in the remarkable changes in the membrane characteristics

such as flux, rejection, and anti fouling properties.

3.3. TGA studies

Fig. 6 shows the TGA diagrams of unmodified PES (M0) and PES/

PCL-MWCNT nanocomposite membranes (M3). The rate of decom-

position of the M3 nanocomposite membrane decreased slightly.

According to this result, the thermal stability of PES membranes

containing PCL-MWCNTs was enhanced. At the highest point of

temperature (600 °C), the weight loss of the M0 membrane is around

44%, while that for the M3 membrane is around 46%.

3.4. Fouling behavior of obtained membranes

The flux decline properties of M0, M1, M2 and M3 membranes,

during 2 h of filtration by the BSA solution, were shown in Fig. 7.

Comparingthe data from theBSA solutionfiltration demonstrates that

M0, M1 and M2 membranes have a similar flux decline, but the M3

membrane shows a higher BSAflux duringfiltration compared to that

of other membranes. Table 4 compares the flux recovery ratios (FRR),

reversible resistances (R r), and irreversible resistances (R ir) of

obtained membranes. The flux recovery ratio (FRR) of the M1

membrane is higher compared to that of other membranes, but the

total flux losses are similar to M0 and M2 membranes (see Table 4 and

Fig. 9). The irreversible resistance of the M1 membrane is lower

(83.8%), while the flux recovery ratio of this membrane is higher

(16.2%) compared to that of other membranes. This may be attributed

to the presence of hydrophile―OH and ―COO― functional groups in

the membrane structure which prevent from settlement of BSA

molecules on the membrane surface. M1 and M2 membranes possess

lower tendency to fouling due to the further hydrophilicity. By

increasing PCL-MWCNT loading, the FRR decreased (11.1%), and the

R ir enhanced (88.9%) for the M3 membrane. Although the PCL-

MWCNT amount was increased but there was a decrease in the FRR

and an increasing R ir which may be attributed to the increase of the

surface porosity. Probably, the blockage of surface pores by BSA

molecules results in a lower FRR and higher R ir. Figs. 3d and 5b show

clearly a good formedporous surface. On the other hand, the presence

of ―OH and ―COO― functional groups in the membrane structure

results in a higher R r (6.1%). Actually, hydrophile functional groups

prevent from further settlement of BSA molecules on the membranesurface.

3.5. Water permeation and rejection capability

The presence of PCL-MWCNTs in the casting solution increases the

water permeability and changes the rejection capability of the

obtained membranes. As shown in Fig. 8, the M0 membrane (without

modified nanotubes) had the pure water flux around 28 L/m2 h. On

the other hand, the pure water flux of the M3 membrane with the

highest amount of PCL-MWCNTs reached about 61 L/m2 h. Comparing

M0 and M3 membrane performances indicates a remarkable flux

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60 70 80 90 1 00 110 120 130

Time (min)

B S A f l u x ( L / m 2 h )

M0

M1

M2

M3

Fig. 7. Time-dependent flux of BSA solution during the filtration.

Table 4

Flux recovery, irreversible and reversible deposition on membrane surfaces.

Membrane FRR (%) R ir (%) R r (%)

M0 13.3 86.6 1.9

M1 16.1 83.8 2.1

M2 15.3 84.7 3.0

M3 11.1 88.9 6.1

0

10

20

30

40

50

60

70

M0 M1 M2 M3

Membranes

P u r e w a t e r f l u x ( L / m 2 h

)

Fig. 8. The pure water flux of membranes.

40

50

60

70

80

90

100

110

0 100 200 300 400 500 600 700

Temperature (oC)

W t . ( % )

M0

M3

Fig. 6. TGA diagrams of M0 and M3 membranes.




































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2011 - Fabrication New PES-Based Mixed Matrix Nanocomposite Membranes Using Polycaprolactone Modified Carbon Nanotubes as the Additive Property Changes and Morphological Studies