PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Fabrication and characterization of carbon nanotubesimmobilized in porous polymeric membranes

Ornthida Sae-Khow and Somenath Mitra*

Received 23rd December 2008, Accepted 25th March 2009

First published as an Advance Article on the web 21st April 2009

DOI: 10.1039/b822879e

We demonstrate that the incorporation of carbon nanotubes (CNTs) in the pores of a membrane can

offer several advantages. A dispersion of CNTs in polyvinylidene fluoride was injected through

a porous membrane, which immobilized the nanotubes in the pore structure. The CNTs served as

a sorbent facilitating solute exchange between the two phases leading to enhancement of the enrichment

factor by as much as 93%. The presence of CNTs also developed a diffusion barrier by sorbing solvent

on its surface, which led to higher retention of the extractant within the membrane.

Introduction

There has been much interest in carbon nanotubes (CNTs), both

single-walled (SWNT) or multiple-walled (MWNT) because of

their excellent mechanical, electrical1 and more recently their

sorbent properties.2–4 They demonstrate excellent gas and liquid

phase retention of a variety of organic molecules and have been

self assembled as high resolution gas chromatography stationary

phases.5–8 The main advantage of CNTs is that their high aspect

ratios lead to large specific surface areas. At the same time, rapid

desorption is possible as they tend to be relatively nonporous.

The above factors lead to the desirable combination of rapid

mass transfer along with a high adsorption capacity.

CNTs have the potential to be the next generation of high

performance separation media with applications including those

in different types of membranes. Aligned MWNTs have facili-

tated the flow of small organic molecules,9 and have been

deposited on ceramic matrices by a combination of low-pressure

chemical vapor deposition, ion milling and reactive ion etching to

form open-ended membranes that exhibit high permeation

rates.10 In addition, theoretical studies have suggested that

permeabilities of certain liquids and gases through carbon

nanotubes far exceed what is expected from classical diffusion

models.9–12 This enhancement has been attributed to the smooth

CNT surface, frictionless rapid transport, and molecular

ordering.11

Selectivity and permeability are often compromising factors

and conventional membranes appear to be reaching their limits as

far as separation capability is concerned.13,14 The combination of

polymeric materials with inorganic fillers such as zeolites, silica,

carbon molecular sieves, and metal oxide nanoparticles15–19 have

been utilized to develop mixed matrix membranes which exhibit

greater permeation rates and selectivity in gases,20–22 liquids,23,24

as well as in bioseparations.25,26 The incorporation of CNTs in

a membrane system offers several advantages, and there can

be several alternate mechanisms of transport. One possibility

is that the CNTs can serve as a sorbent that enhances the

Department of Chemistry and Environmental Science, New Jersey Instituteof Technology, Newark, New Jersey 07102, USA. E-mail: Somenath.Mitra@njit.edu; Tel: (+1) 973 596 5611

This journal is ª The Royal Society of Chemistry 2009

partition coefficient. This activated process will be followed by

diffusion under a concentration gradient. Permeation can be

described by Fick’s law of diffusion:

J ¼ PAdC

dx(1)

where J is the total flux, P is the permeability, A is the surface

area, dC is the concentration gradient and dx is the diffusion

distance. Permeability is dependent on the thermodynamics and

kinetics of membrane–solute interactions, and can be expressed

as a product of solubility (or partition coefficient) in the

membrane and the diffusivity. Since CNTs are excellent

sorbents6–8 as well as molecular transporters,9–12 together these

properties can increase both selectivity and permeability. When

the two phases contact at the pores during liquid phase extrac-

tion, additional interactions can take place via rapid solute

exchange on the CNTs, thus increasing the effective rate of mass

transfer and flux. The high aspect ratio CNTs also dramatically

increase the active surface area, which may contribute to

enhanced permeation. However, the impregnation of the pores of

a membrane with CNTs can be challenging. The objective of this

study is to develop a carbon nanotube immobilized membrane

(CNIM) by incorporating CNTs in a porous polymeric

membrane substrate and to study their effectiveness in enhancing

mass transport.

Experimental section

Fabrication of CNIM

The base porous membrane used here was a polypropylene

hollow fiber membrane (Accurel Q3/2) with an average pore size

of 0.2 mm. The membrane had an I.D. of 600 mm and O.D. of

1000 mm. The CNTs were immobilized using a dispersion of

CNTs in a polymer solution. The polymer selected was poly-

vinylidene fluoride (PVDF). This was accomplished by first dis-

solving 0.1 mg of PVDF in 15 mL of acetone and dispersing

10 mg of MWNTs in the PVDF–acetone solution by sonication

for 3–4 h.

The CNT dispersion in PVDF was injected into the lumen of

a 15 cm long hollow fiber clamped on one end. The pristine

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MWNT obtained from CheapTubes, Inc. was used to fabricate

the CNIM. The PVDF–MWNT dispersion was forced under

pressure into the pore structure of the polypropylene membrane.

Immobilization was achieved during this step as the PVDF

served as a glue that held the CNTs in place. The membrane was

flushed with decane to remove any extra CNTs.

Scanning electron microscopy (SEM) images were taken using

a LEO 1530 VP instrument. Confocal Raman imaging and

Raman spectra were measured using a Thermo Electron Nicolet

Almega XR Dispersive Raman Spectrometer with an Olympus

BX51 research-grade microscope. Thermal gravimetric analysis

(TGA) was carried out using a Perkin-Elmer Pyris 7 TGA

system. Measurements were carried out by heating in air from

30 to 900 �C at a rate of 10 �C min�1.

Evaluation of CNIM performance

Microscale membrane extraction was used to evaluate the

performance of the membrane.27,28 The experimental system is

shown in Fig. 1. To study the effectiveness of CNIM, micro-scale

membrane extraction (m-ME) was carried out from an aqueous

phase into an organic solvent held in the membrane lumen. The

compounds studied were atrazine, 4-chloro-3-nitro-

benzophenone, naphthalene, biphenyl, and phenanthrene. The

first two are polar compounds while the latter are non-polar.

Decane was used as the extractant. The solutes were extracted

from an aqueous solution into the organic extractant at the CNT

immobilized pores. The hollow fiber membrane was filled with 50

mL of organic solvent. The extraction was carried out with stir-

ring at 80 rpm for an hour (A Corning PC-353 Stirrer). The

extract was withdrawn and analyzed by high performance liquid

chromatography (Hewlett-Packard 1050 equipped with a Waters

486 tunable absorbance UV detector, and a 4.6 � 150 mm, 5 mm

Zorbax column). Peak Simple ver. 3.29 (SRI Instruments, Tor-

rance, CA) was used for the chromatographic data acquisition

and analysis.

During the membrane extraction, the solvents tend to diffuse

out, and their retention is an important issue. It was anticipated

that the CNTs could either restrict or enhance the permeation of

Fig. 1 Schematic of micro-scale membrane extraction.

3714 | J. Mater. Chem., 2009, 19, 3713–3718

the solvents. To study this phenomenon, 50 mL of solvent was

placed in the membrane fiber and lowered into the water. After

a predetermined period, the fiber was withdrawn and the amount

of solvent lost through the membrane was measured. Dichloro-

methane, acetone, hexane, n-nonane, n-decane, n-undecane,

1-octanol, dihexyl ether, benzyl alcohol, n-butanol, isopropyl

alcohol, ethanol, and methanol were evaluated as candidate

solvents.

Results and discussion

Soaking a porous membrane in a liquid is known to immobilize

the latter in the micro-pores via capillary forces. This is widely

used in supported liquid membrane extraction.29 A somewhat

similar approach was taken to achieve the incorporation of the

CNTs into the membrane using a polymer dispersion. PVDF was

selected because the CNTs dispersed well in it, as shown in Fig. 2,

and the polymer itself did not alter the membrane properties

significantly. The polymer served as the binder that immobilized

the CNTs in the membrane pores.

The CNIM was characterized using SEM, confocal Raman

imaging, and TGA. The SEM images of plain polypropylene, the

membrane exposed to the PVDF solution, and the CNT immo-

bilized membrane surface are shown in Fig. 3. No visible change

on the membrane surface was detected when just the dilute

solution of PVDF was used [Fig. 3 (a) and (b)]. On the other

hand, when the PVDF–CNTs dispersion was used, the CNTs

were detected all over the CNIM. This is shown in Fig. 3 (c).

The confocal Raman images are shown in Fig. 4. The presence

of the CNTs led to the formation of a dark color on the

membrane surface. Fig. 4 (a) and (b) show the images of the pure

polypropylene and one treated with a PVDF solution. No visible

change in morphology was seen. However, Fig. 4 (c) shows the

CNIM, where CNTs were present in abundance.

The Raman spectra of the polypropylene, PVDF treated

membrane, and the CNIM are also shown in Fig. 4. The Raman

bands of the polypropylene membrane were observed at

frequencies of 2881 and 2719 cm�1. In the CNIM, they are shifted

slightly to 2879 and 2717 cm�1 respectively [Fig. 4 (b)]. In

Fig. 2 The dispersion of CNTs in a PVDF–acetone solution before (left)

and after (right) sonication.

This journal is ª The Royal Society of Chemistry 2009

Fig. 3 SEM images of the polypropylene membrane: (a) the plain

membrane, (b) the membrane treated with PVDF, and (c) the CNIM

(CNTs are identified by arrows).

Fig. 4 Confocal Raman images and Raman spectra of (a) the plain

membrane, (b) the membrane treated with PVDF, and (c), the CNIM.

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addition, the intensity of the higher frequency line relative to the

lower frequency line was reduced by 30%. These effects indicate an

interaction between the polypropylene and the PVDF. Fig. 4 (c)

shows the Raman spectrum of the CNIM. It showed four peaks at

2878, 2669, 1562, and 1332 cm�1. The first peak at 2878 cm�1 was

attributed the polypropylene. The rest were identified as G0, G,

and D bands of the CNTs. The PVDF here served as binder that

immobilizes the CNT in the membrane pores. This led to the

frequency shift and intensity attenuation of the peak at 2878 cm�1

in the CNIM compared to 2881 cm�1 in the plain membrane.

The enhancement in the thermal stability and the overall

composition of the CNIM were determined by TGA. As shown in

Fig. 5, the degradation of the plain polypropylene occurred

This journal is ª The Royal Society of Chemistry 2009

between 180–380 �C, while the degradation of CNIM took place

in the range 250–420 �C. While thermal stability was not an

important factor for this application, it is clear that the CNTs

were effective in altering the material’s characteristics. The overall

concentration of CNTs in CNIM was estimated to be 1.4 wt%.

Solvent retention in CNIM

During membrane extraction, while the solutes flow into the

extractant, the extracting solvents also have a tendency to

J. Mater. Chem., 2009, 19, 3713–3718 | 3715

Fig. 5 TGA of the plain polypropylene membrane and the CNIM.

Fig. 6 Schematic representation of solvent retention and solute trans-

port in (a) the plain membrane and (b) the CNIM. The triangles represent

the path of the analyte molecules, and the dashed lines mark the solvent

barriers.

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diffuse out. Retention of the extractant in the membrane is an

important issue.28 The solvent is lost through the membrane

by diffusion and by dissolution in water. The outflow of the

solvent is undesirable because some solutes are lost along with

it, thus reducing membrane performance. The retention of

a variety of solvents were tested by enclosing a few microlitres

of the solvent in the membrane lumen and following their

out-migration over time. In both the plain membrane and the

CNIM, hexane, acetone, and dichloromethane were

completely lost within 5 min. These small molecules had high

permeability through the membrane pores and were difficult

to retain.

The presence of the CNTs showed significantly higher levels

of solvent retention compared with the plain membrane. The

change in solvent retention was affected by the decrease in

porosity because some of the empty space was occupied by

CNTs. The CNIM helped retain both polar and non-polar

solvents. As shown in Table 1, the presence of CNTs reduced

the loss of all the solvents. Being a strong adsorbent, the CNTs

retained solvent via adsorption, thus decreasing the overall

concentration gradient across the solvent–water interface. The

net solvent outflow decreased. This is shown in Fig. 6. The

effect was more pronounced for the highly polar, water

miscible compounds such as the alcohols, where retention

increased by 10–26%. For example, in the case of methanol, all

the solvent would be lost without the CNTs. As expected, the

Table 1 Solvent retention in the presence of CNTsa

Solvents log KowbSolubility in waterat 20 �C

Molecularmass/g mol�1

n-Nonane 4.76 0.007 g mL�1 128.20n-Decane 5.01 Immiscible 142.30n-Undecane 5.74 Immiscible 156.311-Octanol 3.00 Immiscible 130.23Dihexyl ether 4.98 Immiscible 186.33Benzyl alcohol 1.10 0.04 g mL�1 108.14n-Butanol 0.88 Miscible 74.12Isopropyl alcohol 0.05 Miscible 60.10Ethanol �0.31 Miscible 46.07Methanol �0.77 Miscible 32.04

a Notes: experimental conditions were as follows: initial solvent volume,(* not significant at 95% confidence interval). b Kow is octanol–water coeffic

3716 | J. Mater. Chem., 2009, 19, 3713–3718

non-polar solvents were not as dramatically affected and the

retention increased only by 1–7%. The percentage solvent lost

was calculated from three replicate measurements, whose

relative standard deviations were between 2–6%. The

enhancement of solvent retention was significant at a 95%

confidence interval for all the solvents except decane and

dihexyl ether. It should be noted that the amount of CNTs in

these membranes is relatively low, and much of the contact

Boiling point/�C

Solvent loss (%)Retention Enhancementin CNIM (%)Plain CNIM

150.8 79 76 3.80*174.2 68 63 7.35196.0 64 60 6.25194.5 63 56 11.11226.6 58 57 1.72*205 57 42 26.32117.7 66 58 12.1282.5 75 65 13.3378.2 77 65 15.5864.7 100 75 25.00

50 mL; sample solution 200 mL; retention time, 60 min.; no stirringient.

This journal is ª The Royal Society of Chemistry 2009

Fig. 7 Outflow of n-decane as a function of time.

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probably occurred via direct interaction between the two

phases. However, the role of the CNTs is quite clear and these

results provide base line data for the elucidation of this

phenomenon.

The retention of n-decane was investigated in detail as

a function of time. This is shown in Fig. 7. The solvent reten-

tion was higher in the CNIM than in the conventional

membrane, The differences were significant at 95% confidence

intervals. For example, 30% of decane was still retained in the

CNIM after 90 min, while only 20% was retained in the plain

membrane.

Performance enhancement

The enrichment factor (EF) is defined as the ratio of the solute

concentration in the final extract to that in the original water

sample:

EF ¼ Cs

Cw

(2)

where Cs is the analyte concentration in the final extract, and Cw

is the analyte concentration in the original water sample.

Extraction is usually quantified as extraction efficiency (EE),

which is the fraction of analyte removed by the acceptor from the

original water sample.30 EE is defined as:

EE ¼ ns

nw

¼ CsVs

CwVw

¼ Vs

Vw

(3)

where ns and nw are the analyte masses in the final extract and

in the original water sample, and Vs and Vw are the volumes

Table 2 Enrichment factors and extraction efficiencies in the plain membran

Analytes

Plain membrane PVDF

EF EE (%) EF

Polar CompoundsAtrazine 2.19 0.02 2.174-Chloro-3-nitrobenzophenone 151.34 1.21 139.21Non-polar CompoundsNaphthalene 203.38 1.53 223.74Biphenyl 205.21 1.54 196.19Phenanthrene 209.72 1.57 191.80

a Notes: experimental conditions were as follows: initial decane volume 50 m

This journal is ª The Royal Society of Chemistry 2009

of the concentrated extract and the original water sample,

respectively.

The effectiveness of the CNIM was evaluated by microscale

membrane extraction (m-ME) via direct solvent enrichment or

liquid–liquid extraction. The solutes were extracted into the

membrane lumen, while both the solvent and nanotubes acted as

the extractants in the case of CNIM. The solutes first adsorbed

on the CNTs and then extracted into the solvent in the lumen.

The proposed mechanism is shown in Fig. 6.

Atrazine and 4-chloro-3-nitrobenzophenone represent the

polar compounds and naphthalene, biphenyl, and phenanthrene

the non-polar. The extractions were carried out with plain

membrane, PVDF treated membrane and the CNIM. The

results are presented in Table 2. The relative standard devia-

tions for all the measurements were between 1–6%. The PVDF

modification did not affect membrane performance signifi-

cantly. This demonstrated that the pore structure and chemical

nature of the polypropylene membrane were not greatly

affected by PVDF. In the case of the CNIM, the EF improved

significantly for the non-polar compounds (30–93%) but

decreased for 4-chloro-3-nitrobenzophenone (�36%). Atrazine

showed a relatively smaller 16% enhancement of the EF. These

differences were statistically significant at 95% confidence

interval. As shown in Fig. 6, the CNTs served as a strong

sorbent. First, the solutes adsorbed on the CNT surface and

then extracted into the solvent in the lumen. The chemical and

surface properties of CNTs are relevant here. The CNT makes

the membrane more non-polar and this may reduce its wetta-

bility. It also alters the partition coefficient, and it appears that

the polar compounds were affected negatively.

Conclusions

A dispersion of CNTs in PVDF was effective in immobilizing

nanotubes in the pore structure of a membrane. The PVDF

served as a binder for the CNTs. The presence of CNTs provided

some major advantages. First, the permeability of the non-polar

solutes increased quite dramatically, while there was a decrease

for polar compounds. This was expected because the CNTs are

known to be non-polar in nature. The presence of CNTs also

decreased the outflow of solvent from the membrane lumen. The

results were quite dramatic for methanol, which could not be

retained by the original membrane but the CNTs helped its

retention.

e and CNIMa

modification CNIM

Enhancement (%)EE (%) EF EE (%)

0.02 2.55 0.04 16.531.11 96.56 1.45 �36.20

1.68 267.89 4.02 31.721.47 329.63 4.94 60.631.44 404.69 6.07 92.97

L; sample volume 200 mL; extraction time 60 min; stirring rate 80 rpm.

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Acknowledgements

Dr Dachuan Yang of Ethicon Corp. is acknowledged for the

Raman measurements.

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