polystyrene composite membranes

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 9 1 4 e3 9 2 1

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Study of gas transport properties of multi-walled carbon nanotubes/polystyrene composite membranes Sumit Kumar*, Subodh Srivastava, Y.K. Vijay Thin Film and Membrane Science Lab, University of Rajasthan, Jaipur, Rajasthan 302004, India

article info

abstract

Article history:

The multi-walled carbon nanotube (MWCNT) dispersed polystyrene (PS) composite

Received 14 March 2011

membranes have been prepared by solution cast method for hydrogen gas permeation

Received in revised form

application. The structural and morphological properties of these prepared composite

7 May 2011

membranes have been characterized by X-Ray diffraction (XRD), Atomic force microscopy

Accepted 10 May 2011

(AFM) and Scanning electron microscopy (SEM). The optical absorbance measurement of

Available online 12 June 2011

the composite membranes has been carried out by UVeVis Spectroscopy. The effect of electric field alignment of MWCNT in PS matrix on gas permeation has also been charac-

Keywords:

terized for hydrogen gas with different gas mixtures. The permeability measurements

Multi-walled carbon nanotube

indicated that the electrically aligned MWCNT in PS has shown higher permeability for

(MWCNT)

hydrogen gas as compare to randomly dispersed MWCNT in PS. The enhancement in

Polystyrene (PS)

permeability is explained on the basis of easy channels provided by electrically aligned

Composite membrane

MWCNT in PS matrix. The effect of thickness of membrane on the gas permeability has

Atomic force microscopy (AFM)

also been studied and thickness of about 60 mm was found to be optimum thickness for fast

Gas permeability

hydrogen gas permeations. The selectivity measurements show that MWCNT aligned PS

Scanning electron microscopy (SEM)

composite membrane are highly selective for hydrogen gas in presence of different gas mixtures therefore these composite membrane can be used for hydrogen purification in fuel cell technology. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, great attention has been paid to conductive composites consisting of intrinsically insulating polymeric matrices and conducting fillers which is being driven by their wide application possibilities and the growing market needs. Membrane science and polymer science have grown synergistically over the past thirty years. Clearly, today’s impressive set of membrane processes and products could not exist without the sophisticated array of polymers provided by synthetic chemists [1e6]. At the same time, the membrane field has provided a strong driving force and justification for

many fundamental studies of the polymer solid state and polymer solution thermodynamics [7e12]. Although, the theory still lags practice in membrane formation, and in tailoring the properties of selective layers responsible for the operation of membranes. The continued evolution of membrane and polymer science will undoubtedly lead to more sophisticated membranes enabling them to achieve increasingly difficult separations, someday possibly rivaling natural membranes. There is high level of interest in using filler particles having nano dimensional scale (nano filler) within polymeric matrix for making composite membrane with exceptional properties

* Corresponding author. Tel.: þ91 (0) 141 2702457; fax: þ91 (0) 141 2701149. E-mail address: [email protected] (S. Kumar). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.05.060

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Fig. 1 e Photographs showing (a) prepared pure PS membrane (b) randomly dispersed MWCNT/PS composite membrane and (c) electrically aligned MWCNT/PS composite membrane.

anisotropic structure and improved properties in the direction of alignment. Many authors have reported the alignment of CNTs in a polymer matrix using magnetic and electric field [48e50]. In this investigation, PS is selected as the matrix polymer while MWCNT as fillers. The free-standing MWCNT/PS composite membranes of different thickness varying from

(a) Pure PS (b) MWCNT dispersed PS (E=0)

A rb . u n i t

[13e16]. It is well-known that carbon nanotubes (CNTs) have outstanding mechanical and electrical properties, provide potential use in a wide range of nanotechnology applications in like gas sensors, gas permeation membranes etc [17e20]. CNT/ polymer composites have attracted considerable attention due to their unique mechanical, electrical, and thermal properties. CNTs usually have specific interactions with polymer matrices resulting from the nano-scale structure and extremely large interfacial area of CNTs [21e23]. In particular, CNT/polymer composites have been extensively studied to explore their unique electronic, thermal, optical, and mechanical properties [24e28], as summarized in recent review articles. Over the past two decades, gas separation using CNT/ polymeric composite membranes have drawn a great deal of interest for researchers due to its many advantages, such as simple processing, flexibility, low cost and high selectivity [29e41]. The polymeric membranes act as a filter to separate one or more gases from a feed mixture and generate a specific gas rich permeate [42]. Among the numerous synthetic polymers, polystyrene (PS) is featured as a cheap and easily accessible material with an aromatic group in each monomer unit. Zhang et al. reported in their experiments that melt mixing of PS with multi-walled carbon nanotubes (MWCNT) leads to enhanced interactions between PS and MWCNT and increases the homogeneous dispersion of MWCNT [43]. However, theoretical calculation and experimental observation both indicated that substantial interactions between PS and sidewalls of MWCNT [44e47] indeed exist. Aligned CNTs in polymer matrix have certain advantages due to their

(b)

(a)

10

20

30

40

50

60

2q

Fig. 2 e X-Ray diffraction pattern of pure PS and MWCNT/ PS composite membrane.

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Fig. 3 e SEM micrograph images of (a) randomly dispersed MWCNT/PS composite membrane (b) cross section view of randomly dispersed MWCNT/PS composite membrane (c) electrically aligned MWCNT/PS composite membrane and (d) cross section view of electrically aligned MWCNT/PS composite membrane.

55 to 75 mm were fabricated using solution casting method. The effect of electric field on the alignment of MWCNT in PS matrix has been studied on gas permeation for hydrogen gas. These composite membranes have also been characterized by atomic force microscopy (AFM), X-ray diffractometer (XRD), Scanning electron microscopy (SEM) and UVeVis spectroscopy.

2.

Experimental

The PS and MWCNT (1e2 mm length and 10e30 nm diameters) were purchased from Good fellow, Metals, U.K. and Helix Material Solution Richardson, Texas respectively. In this work we employed a simple solution mixing method assisted by high-energy ultrasonication to prepare uniform MWCNT/PS composite samples. First, the PS granules were dissolved in 20 ml benzene using magnetic starrier for a duration of 5 h. The MWCNT were dispersed separately in 10 ml benzene by high-energy ultrasonication (220W, 20 kHz) for 1hr. The PS solution and MWCNT suspension were subsequently mixed by further ultrasonication for 30 min. This homogeneous solution was then transferred into flat bottom petrie dish floated over mercury and then benzene was allowed to evaporate completely. During this process, a transverse electric field applied to align the suspended MWCNT in PS matrix as reported earlier [51]. The prepared composite membrane is shown in Fig. 1.

X-ray diffraction measurements have been performed using PANalytical system having CuKa as a radiation source of ˚ within 2q ¼ 10e60 . The AFM images wavelength l ¼ 1.546 A were recorded using VICCO made CP-II SPM unit. The SEM image of composite membrane has been recorded by MIRA/ TESCAN-FESEM scanning electron microscope. The optical absorption studies were recorded at room temperature in wavelength range 200e800 nm using the UVeVIS (Model U2900) spectrophotometer. Permeability is defined as the product of solubility and diffusivity coefficient and obeys the Fick’s law. The flow rate is measured using permeability cell as reported elsewhere [52]. The permeability is calculated using Fick’s formula given as P¼

Flux  Thickness of Membrane Pressure Difference

The detail description of gas permeability measurement has been reported elsewhere [51].

3.

Results and discussion

3.1.

X-ray diffraction (XRD)

Fig. 2 shows the X-ray diffraction pattern of pure PS and MWCNT/PS composite membrane. In case of pure PS, XRD spectra show a broad amorphous hump around 2q ¼ 15 ,

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 9 1 4 e3 9 2 1

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Fig. 4 e AFM images of (a) Pure PS (b) randomly dispersed MWCNT in PS composite membrane (c) electrically aligned MWCNT in PS composite membrane.

indicating the disordered chain structure and spacing between polymer chains [53]. The XRD pattern for MWCNT/PS composite membrane show a small peak about 2q ¼ 26.3 , corresponding to the graphite like diffraction which confirms the presence of MWCNT in PS matrix.

3.2.

Scanning electron microscopy (SEM)

To gain more information on the organization of MWCNT in the PS matrix, we have applied a microscopy technique recently described in the literature [54], which is based on charge contrast imaging using a SEM. Fig. 3 show the SEM images of the randomly dispersed MWCNT/PS composite and electrically

aligned MWCNT/PS composite membranes. Several features can be notice as follows: Fig. 3a shows a SEM image of the surface of randomly dispersed MWCNT/PS composite membrane. The MWCNTs are represented by the bright tiny dots. Because of the different capability for charge transport of the conductive MWCNT and the insulating PS matrix, the secondary electron yield is enriched at the location of the MWCNT, which results in the contrast between the MWCNT networks appearing in form of small dots and the polymer matrix (PS). The MWCNTs are homogenously distributed in the polymer matrix and no large aggregations of MWCNTs are visible. In case of the electrically aligned MWCNT/PS composite membranes (Fig. 3c), a large amount of MWCNT bundles are

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1.2

(a) Pure PS (b) MWCNT/PS (E = 0) (c) MWCNT/PS (E > 0)

300

(a) Pure PS (b) MWCNT/PS

(c)

Permeability (barrer)

Intensity (Arb.)

250

0.8

(b) 0.4

(a)

200

(b) 150

(a)

100

0.0

50

300

400

500

600

700

800

0

2

4

wavelength (nm)

Fig. 5 e Absorption spectra of Pure PS and MWCNT dispersed PS composite membrane.

A

10

(a) Pure PS (b) MWCNT/PS E=0 (c) MWCNT/PS E>0

The cross sectional investigation of the surface of electrically aligned MWCNT/PS composite membrane (Fig. 3d) revealed that MWCNT/PS composite exhibited a porous open structure and high surface area, which is suitable for gas permeation applications.

B

300

(c)

750

(a) Pure PS (b) MWCNT/PS E=0 (c) MWCNT/PS E>0

250 Permeability(barrer)

Permeability(barrer)

8

Fig. 6 e Gas permeation measurements (60 mm thick) for (a) Pure PS and (b) randomly dispersed MWCNT/PS composite membrane and (c) electrically aligned MWCNT/PS composite membrane.

observed and appeared as big spherical dots on the membrane surface. Because of the local charging of the polymer, matrix around the MWCNT has rendered the average diameter of the MWCNT to be larger than expected, which might be an indication for the presence of small MWCNT bundles.

900

6

No. of cycle

600

450

(b)

300

(a)

(c)

200 (b)

150

(a)

100

150

50 0

100

4

6 No. of cycle

8

0

10

22

(a) Pure PS (b) MWCNT/PS E=0 (c) MWCNT/PS E>0

80

2

4

6

8

10

No. of cycle

C

90

D (a) Pure PS (b) MWCNT/PS E=0 (c) MWCNT/PS E>0

20

(c)

(c)

18

70 (b) 60 (a)

50 40

Permeability(barrer)

Permeability(barrer)

2

16 14 (b) 12

30

10

20

8

(a)

10 0

2

4

6 No. of cycle

8

10

0

2

4

6

8

10

No. of cycle

Fig. 7 e Hydrogen gas permeation measurement for (A) 55 mm thick, (B) 60 mm thick, (C) 65 mm thick and (D) 70 mm thick MWCNT/PS composite membranes with and without field alignment of MWCNT in PS.

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The AFM images of electrically aligned MWCNT/PS composite membrane show regularly spaced dot pits which have been grown in perpendicular direction to the surface as shown in Fig. 4c. It may be influence of a transverse high DC electric field due to which the alignment of MWCNT takes place within the polymer solution during solution casting. During this process some of the polymer was coated over the aligned MWCNT which resulted in formation of pits.

1000 (c) (a) Pristine PS (b) randomly dispersed MWCNT/PS (c) Electrically aligned MWCNT/PS

P e r m e a b i l i t y ( b a rr e r)

800

600

400

(b) (a)

3.4.

UVeVIS measurements

200

0 55

60

65

70

75

Thickness( m)

Fig. 8 e Thickness dependence on gas permeability for (a) pristine PS (b) randomly dispersed MWCNT in PS composite membrane and (c) aligned MWCNT in PS composite membrane.

Fig. 5 represents the UVeVIS spectra of pure and MWCNT dispersed PS composite membrane. This figure indicates a sharp absorption edge at z274 nm for pure PS whereas for MWCNT dispersed PS composite membrane the absorption edge is shifted towards higher wavelength at z313 nm. There appeared no additional peak, except a slight increase in absorption which may be due to very low concentration of MWCNT in polymer.

3.5. 3.3.

Atomic force microscopy (AFM)

Fig. 4 Shows the AFM images of the randomly dispersed MWCNT/PS composite and electrically aligned MWCNT/PS composite membrane. As can be seen from Fig. 4a that pure PS membrane exhibits the smooth and uniform surface, While in case of randomly dispersed MWCNT in PS composite the surface appears rough with uneven dots and lumps due to the uneven distribution of MWCNT in the matrix as shown in Fig. 4b.

The Fig. 6 shows the permeability of H2 gas through pure PS membrane and MWCNT/PS composite membranes at room temperature. This figure clearly revealed the increase in permeability with number of H2 pressurized cycles and saturated after 4 cycles in pure PS (Fig. 6a). It has been observed that after dispersion of MWCNT in PS the permeability of H2 gas is slightly increased upto 160  6 barrer [1 barrer ¼ 1010 cm3 (STP) cm/cm2 s cm Hg] as shown in Fig. 6b. The effect of electric field alignment of MWCNT in PS matrix on the gas permeation measurements has been

(a) P(H2 )

300

(a)

(b) P(N2 ) (c) P(CO2 )

250

Permeabilit y (b arrer)

Gas permeation

200

150

(b) 100

(c)

50

0 0

2

4

6

8

10

No. of cycle Fig. 9 e Permeability versus no. of cycle graph for H2, N2 and CO2 through aligned MWCNT/PS composite membrane.

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Fig. 10 e Selectivity of gases (H2, N2, CO2) for MWCNT/PS composite membranes.

studied as shown in Fig. 6c. This Figure clearly reveal that hydrogen gas permeability is higher for aligned MWCNT/PS composite membrane under higher electric field by applied DC voltage of about 5 kV (E ¼ 1250 V/cm). The alignment mechanism can be understood on the basis of electrophoretic effect purposed [49,51]. The Fig. 7A to 7D show the gas permeation measurements for all three types of composite membranes (pure PS, MWCNT/PS and electrically aligned MWCNT/PS) for the thickness varying from 55 mm to 75 mm. All the permeation measurements have been taken for constant H2 pressure (40 psi) at room temperature. It was observed that initially the permeability of H2 gas (upto thickness 75 mm) increased exponentially and then saturated after 4 cycles. In case of randomly dispersed MWCNT/PS composite membrane, permeability versus number of cycle graph follow almost same trend as that of pure PS membrane, in which permeability is increasing exponentially and then saturates at higher value of permeability. The higher value of permeability on dispersed sample may be due to easy passes of H2 through MWCNT in comparison to pure PS sample. For electrically aligned sample, there is huge enhancement of permeability as compare to pure PS and randomly dispersed MWCNT sample along with this it saturates at higher value of permeability. The possible reason for higher permeability on aligned of MWCNT by electric field is that it provide well aligned easy channel of MWCNT to permeate hydrogen. The effect of membrane thickness on permeability is combinedly shown in Fig. 8. This figure clearly shows that the permeability of H2 gas continuously decreases as thickness of the composite membranes increases. When membrane is thinner (t z 60 mm) then effect of alignment is well separated. It has also been observed that as thickness is increased, this separation starts decreasing and at the thickness of about 75 mm, effect of electric alignment vanishes. The aim of these measurements is to collect an idea about thickness of membrane for which the effect of aligned MWCNT is significant and highly reproducible. Therefore, we conclude that 55  2 mm thickness is optimum thickness

to see the effect of electrical field on alignment of MWCNT in PS for gas permeation. In addition to this electrically aligned MWCNT in PS is good filter for hydrogen gas permeate which have good selectivity and fast response time. Fig. 9 shows the permeability versus no. of cycle graph for H2, N2 and CO2 gases through aligned MWCNT/PS composite membrane. It has been observed that gas permeability has found to be higher for the H2 gas (z290  5 barrer) in comparison to the N2 (z127  5 barrer) and CO2 (z47  5 barrer) gases. It may be attributed to the selective transport of H2 gas due to the presence of the aligned MWCNT in polymer matrix. It has been reported by many research groups that MWCNT act as good H2 storage media which can absorb H2 by either physical or chemical process at the room temperature. Therefore MWCNT can be used as conductive fillers which promote the selective permeation of H2 gas in comparison to the N2 and CO2 gases. Further hydrogen gas selectivity has also been studied by calculating the permeability ratio of H2 gas in different gases like N2 and CO2 as shown in Fig. 10. These values clearly indicate that the maximum permeability ratio has been observed 6.17 for H2/CO2 mixer.

4.

Conclusions

MWCNT/PS composite membranes of different thickness were prepared by the solution cast technique by dispersion of MWCNT in PS solutions. It was observed that by applying high DC electric field, MWCNT distributes uniformly in comparison to the randomly dispersed MWCNT in composite membrane. The XRD measurement confirms the presence of CNT in PS matrix. The optical micrograph and AFM images clearly reveal the effect of electric field alignment of MWCNT on the surface morphology of composite membranes. Surface morphological study indicated that aligned MWCNT in PS is quite visible and perpendicular to the base PS and show a dot pits like hillocks type structure. It was observed that the permeability for H2 gas increased after dispersion of MWCNT in PS, which is further enhanced after the alignment of MWCNT within polymer matrix. It is also observed that the permeation of gas through composite membranes depends on their thickness. It has also been observed that all the composite membranes show an unstable behavior and decrease in gas permeability with increasing temperature. The selectivity permeation results reveal that MWCNT/PS composite membranes can be used as nanofilter for selective permeation of H2 gas.

Acknowledgment The authors are thankful to Council of Scientific and Industrial Research (CSIR), New Delhi for providing financial support. The authors are also thankful to DSA, Department of Physics, University of Rajasthan, Jaipur (India) for providing experimental facilities and personally Dr. Vidhyadhar Singh and Arpita Saxena for cooperation during the research work.

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