Raman spectroscopy of polystyrene nanofibers—Multiwalled carbon nanotubes composites

Applied Surface Science 275 (2013) 23–27

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Raman spectroscopy of polystyrene nanofibers—Multiwalled carbon nanotubes composites Dorina M. Chipara a,∗ , Javier Macossay b , Ana V.R. Ybarra b , A.C. Chipara c , Thomas M. Eubanks b , Mircea Chipara a a

The University of Texas Pan American, Department of Physics and Geology, Edinburg, TX-78539, USA The University of Texas Pan American, Department of Chemistry, Edinburg, TX-78539, USA c The University of Texas Pan American, Department of Mechanical Engineering, Edinburg, TX-78539, USA b

a r t i c l e

i n f o

Article history: Received 30 September 2012 Received in revised form 15 January 2013 Accepted 16 January 2013 Available online 26 January 2013 Keywords: Nanofibers Polystyrene Multiwalled carbon nanotubes Raman spectroscopy Line shape analysis Carbon nanotube–polymer interface TGA

a b s t r a c t Raman spectroscopy investigations of nanofibers of polystyrene loaded with various amounts of multiwalled carbon nanotubes are reported. The modifications of the main Raman bands (D and G) of multiwalled carbon nanotubes due to their dispersion in polystyrene demonstrates and quantifies the stress transfer from the polymeric nanofiber matrix (polystyrene) to multiwalled carbon nanotubes. TGA data show an increase of the thermal stability of polystyrene nanofibers upon the loading with multiwalled carbon nanotubes, conforming Raman data. Published by Elsevier B.V.

1. Introduction Electrospinning was patented in 1900 by Cooley [1] and in 1902 by Morton [2]. Starting with 1934, patents for textile production by electrospinning (from solution) have been filed by Formhals [3,4]. Melt electrospinning was patented by Norton in 1936 [5]. The electrospinning from solution consists of placing a polymer solution in a syringe. The syringe is connected to one electrode of a high voltage power supply, which generates a high voltage difference (5–30 kV) between the syringe needle and a grounded target. As the polymer is ejected, the electrical charges on the polymer solution promote solvent evaporation and fiber thinning, thus forming a dry polymer fiber that travels in a chaotic pattern (known as the Taylor cone) and finally deposits on the grounded target. The recent revolution in nanomaterials revived and amplified the interest in electrospinning as a technology capable to generate submicron

∗ Corresponding author at: The University of Texas Pan American, Department of Physics and Geology, 1201 W, University Drive, Edinburg, TX-78539, USA. Tel.: +1 956 607 4950. E-mail addresses: [email protected], [email protected] (D.M. Chipara). 0169-4332/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2013.01.116

one-dimensional materials with huge surface areas and enhanced mechanical properties. The potential uses of nanofibers produced by electrospinning includes tissue scaffolds, enzyme and catalyst support, filtration media, military protective clothing, sensors, electronic, optical devices, textiles and more recently reinforced nanocomposites. Multiwalled carbon nanotubes (MWCNTs) have outstanding mechanical properties (such as Young modulus in excess of 1 TPa and tensile strength up to 150 GPa [6,7]), which suggest that these materials could be used as additives to reinforce polymeric matrices [8,9] and eventually to add electrical [10,11] or thermal features [12,13]. This research focuses on carbon nanotubes embedded within electrospun polymer nanofibers with the objective of obtaining enhanced thermal, mechanical, and electrical properties. The loading of polymeric nanofibers with multiwalled carbon nanotubes (MWCNTs) is expected to result in one-dimensional nanocomposites with the nanofiller (MWCNTs) oriented along the direction of the polymeric nanocomposite. Previous studies on polymers filled with oriented nanofillers revealed important enhancement of the mechanical properties of the polymeric matrix (enhancement of the Young modulus for extensions along the direction of the nanocomposite) and suggested an important drop in the percolation at threshold.

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D.M. Chipara et al. / Applied Surface Science 275 (2013) 23–27

Fig. 1. Left panel: Raman spectra of PS-MWCNTs nanofibers. Right panel: Electron microscopy micrograph of PS-10% MWCNT nanofibers.

2. Material and methods 2.1. Sample preparation Fibers of polymer nanocomposites have been obtained by electrospinning. The chemical reagents were obtained from Sigma–Aldrich and the multiwalled carbon nanotubes (MWCNTs) from CheapTubes. All materials were used as received. To obtain one-dimensional nanocomposites, the polymeric matrix polystyrene (PS) with MW = 100,000 was dissolved into a mixture of Tetrahydrofuran 99+% (THF) and N,N-dimethylformamide 99.8% (DMF) to which the nanofiller was added. To achieve a uniform dispersion of nanotubes the mixture was sonicated for 45 min and electrospinned at a potential difference of 15 kV. The flow rate was 0.05 ml/min and the ground target was placed at 24.5 cm. Nanofibers containing various amounts of MWCNTs (0%, 0.1%, 0.5%, 1.0%, 5% and 10%) have been obtained. 2.2. Testing Scanning electron microscopy (SEM) investigations provided details about the morphology of the nanofibers. Raman measurements have been performed by using a Bruker Senterra microRaman, operating in the confocal mode and equipped with a laser diode emitting at 785 nm. Raman spectroscopy was used to assess the presence of multiwalled carbon nanotubes within the nanofibers and their effect on the morphology and physical properties of the nanofibers. In order to avoid the overheating of the samples, the power of the laser diode has been set at 1 mW and the number of accumulations has been increased to 10. The effect of the polymeric matrix on the typical limes assigned to MWCNTs is analyzed in detail. Thermogravimetric investigations have been performed by using a TA Instrument equipment (TGA Q500), operating in nitrogen atmosphere, at a heating rate of 10 ◦ C/min. 3. Results and discussions Experimental studies on the same electrospun nanofibers of PS filled with MWCNTs, as obtained by FTIR, TGA, and electron microscopy have been reported elsewhere [14]. This report focuses on the Raman spectroscopy of PS-MWCNTs nanofibers and

provides a more detailed analysis of TGA data. Raman spectroscopy is a powerful technique in the investigation of the molecular vibrations in materials. In the Raman spectroscopy the sample is irradiated by an electromagnetic radiation from a laser source. The reflected light is analyzed at any wavelength except the wavelength of the incoming laser beam. The reflected light shows some peaks corresponding to various molecular motions excited by the incoming laser beam. The position of Raman peaks is typically assigned to specific motion(s) of certain chemical groups. The width of Raman line vibrations are reflecting interactions between these vibrations and the integral of the Raman line is proportional to the number of excited vibrations. As may be observed from the left panel of Fig. 1, the Raman spectra of PS-MWCNTs nanofibers are complex revealing the superposition between the lines due to the polymeric matrix (PS) and the filler (MWCNTs). Fig. 1 shows that the Raman lines of the polymeric matrix are rapidly broadened and weaker as the concentration of the filler is increased, a behavior observed in other polymer-carbon nanotube composites and assigned [15] to the dephasing of the local motions of macromolecular chains due to the interactions with the carbon nanotubes. The right panel of Fig. 1 shows a micrograph of the as obtained nanofiber containing a large fraction of MWCNTs (about 10 wt.%). It is noticed that the nanofibers are thin and very uniform. In contrast with this result, the PS nanofibers with a small amount of MWCNTs are thicker and exhibits large and numerous beads. The left panel of Fig. 2 exemplifies this situation for the sample PS-1% MWCNTs. The right panel of Fig. 2 focuses on the main components of the Raman lines of carbon nanotubes, namely the D and G bands. The D band characterizes the disorder in carbon nanotubes and the G band is assigned to graphitic structure. The D band is a dispersive band, i.e. the position of this peak depends on the wavelength of the incoming laser beam. Typically the ratio between the area of the D and G peaks is quantifying the defects (disorder) of carbon nanotubes. It is noticed that in the frequency range 1250–1750 cm−1 the Raman spectra are showing solely the lines corresponding to the nanofiller (MWCNTs). Pristine MWCNTs exhibit an intense D band localized at 1311 ± 0.5 cm−1 (see the right panel of Fig. 2), assigned to disorder originating from sp3 carbons [16]. As the concentration of MWCNTs dispersed within the nanofibers is increased, the position of this line moves gently toward lower values (see Fig. 3), as

D.M. Chipara et al. / Applied Surface Science 275 (2013) 23–27

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Fig. 2. Left: Electron microscopy micrograph of PS-1% MWCNT nanofibers. Right: Raman spectra of PS-MWCNT nanofibers (detail in the domain 1200–1750 cm−1 ).

expected for carbon nanotubes subjected to mechanical stresses [17]. The position of the D line is affected by stresses. The local stress exerted on MWCNTs is very small for the fibers containing 0.5 wt.% MWNTs and increases with the concentration of MWCNTs (see Fig. 3). For the sample containing 10% MWCNTs the local stresses exerted on MWCNTs are of the order of 2 GPa. The G band is split with the components at 1584 and 1614 cm−1 , respectively. This is a “graphite-like” line. The G− Raman line is typically assigned to a plasma resonance in carbon nanotubes and it is associated with displacement along the circumference of nanotubes [18] while the G+ line is associated with defected structures [19,20] and with displacements of carbon atoms along the axis of the nanotube [18]. The spacing between these two peaks (G+ –G− ) is proportional to the diameter of the nanotubes and suggests a high electrical conductivity for pristine MWCNTs [16,21]. G+ − G− =

K D

where K is a constant and D is the diameter of the nanotube [17]. As the concentration of MWCNTs is decreased, the two peaks are converging slowly each to other probably due to the decrease of the constant K because of the drop in the electrical conductivity of the 1312

nanofiber (see Fig. 4). This collapse is noticed as the G− component has a weaker dependence on the loading with MWCNTs than the G+ component. The resonance spectrum of all samples in the 1200–1800 cm−1 range has been fitted by using a Breit–Wigner–Fano-like function [17,21]:



y = A1

1 + D1 ((x − P1 )/W1 ) (1 + ((x − P1 )/W1 )

 + A3

2



2

1 + D3 ((x − P3 )/W3 ) (1 + ((x − P3 )/W3 )



+ A2

 2

1 + D2 ((x − P2 )/W2 ) (1 + ((x − P2 )/W2 )

2



2

+ B + Sx + Qx2

2

(1)

where A1 , A2 , and A3 are the amplitudes of the 3 Raman lines, P1 , P2 , and P3 defines the position of Raman lines, W1 , W2 , and W3 characterizes the width of Raman peaks, B, S, and K are constants associated with the base line correction, D1 , D2 , and D3 describe the deviation from a Lorentz line (correspond to the parameter 1/q). The fitting was consistent with a Raman G− peak with a width of 29 ± 0.5 cm−1 while for the G+ Raman component the estimated width was about 9.5 ± 0.5 cm−1 (for the nanofiber containing 5% MWCNTs). Similar results were reported elsewhere [20,18], confirming the proposed interpretation of G bands. As expected, the

2.5

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0.5

1308

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0

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-1

Position of G lines [cm ]

-1

1309

36

G+ G-

1585

34

32

30

1580

-1

Position of the D line [cm ]

1.0

1615

Splitting of the G line [cm ]

1.5 1310

Stress Exerted on MWCNTs [GPa]

2.0 1311

28

110

CNTcontent (%)

0

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50

60

70

80

90

100

MWCNT content [%] Fig. 3. The dependence of the D line position on the loading with MWCNTs. The internal stress exerted on MWCNTs, as estimated from the position of the D line.

Fig. 4. The dependence of the G lines’ components on the loading with MWCNTs.

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Line Intensity [Arb. Units]

150

125

100

75

50 1200

1400

1600

Derivative of Mass Loss versus Temperature [g/K]

26

3

2

1

0

200

300

400

Fig. 5. The Raman spectrum of pristine MWCNTs (gray) and the simulated spectrum (black).

calculated deviations from a Lorentzian line (toward a BWF line) were greater for the G− component than for the G+ component [18,20,22]. Over this range of frequencies, the fitting involved about 1000 points. As noticed from Fig. 5, the fit was in excellent agreement with the as recorded spectra. The parameter D for the G lines was found to be zero for MWCNTs concentration smaller or equal to 1%. This reflects again the decrease of the interactions between MWCNTs due to their dispersion within PS. An additional Raman line, found at about 2612 cm−1 has been tentatively assigned as a G band (or the D*-band or 2D band [16]), This is an overtone (2 phonon process) of the D-band located between 2450 and 2950 cm−1 , and characterized by a width of about 33 cm−1 . The line was also fitted by a Breit–Wigner–Fano lineshape, but it was concluded that for all samples the Raman line is almost Lorentzian. The agreement between the measured spectra and the fitted ones is excellent (see Fig. 6). As expected this line is absent in pristine PS. The Raman spectra of PS-MWCNTs contain also lines due to the polymeric matrix. Most of these lines are located below 1200 cm−1 . As expected, the lines due to the polymeric matrix are slowly disappearing and replaced by the lines of MWCNTs as the loading with

500

600

o

Temperature [ C]

-1

Raman Shift [cm ]

Fig. 7. The derivative of mass loss versus temperature as a function of temperature for pristine polystyrene. Black line shows the best fit and empty circles are representing experimental data.

MWCNTs is increased (see Figs. 1 and 2). For the pristine polymer, in the region 0–1200 cm−1 was observed the Bose peak (below 125 cm−1 ), an intense doublet was noticed at 1003 and 1030 cm−1 , respectively, weak and broad resonances were noticed at 225, 405, 620, 795, and 1193 cm−1 . A strong line located at 3055 cm−1 and assigned to PS was noticed. The line at 3055 cm−1 was also observed by FTIR and assigned to aromatic CH stretch. The Raman line observed at 1030 cm−1 was noticed also by FTIR spectroscopy and assigned to in-plane CH deformations. The derivative of mass loss versus temperature was recorded for each nanocomposite in nitrogen, at a heating rate of 10 ◦ C/min. The experimental data showed a skewed Lorentzian like dependence of the mass loss versus temperature. The experimental data were fitted by using Eq. (2) where I is the intensity of the mass loss derivative, P1 is the position of the peak (i.e. the temperature at which the mass loss is maximum), P3 is the amplitude of the TGA derivative, P4 , P5 , P6 , and P8 describes the deviations from a pure Lorentzian line (characterized by P4 = P5 = P6 = P8 = 0). P9 and P10 are parameters that describe the base line correction.

I(T ) = P3

1 + P4



(T −P1 ) P2

1 + P7





+ P5



(T −P1 ) P2

(T −P1 ) P2

2

2

+ P8



+ P6



(T −P1 ) P2

(T −P1 ) P2

4

3 + P9 + P10 T (2)

Fig. 6. The Raman spectrum of the G line for pristine MWCNTs (open circles) and the simulated one (black line).

As seen in Fig. 7, Eq. (2) (see the black line) describes accurately the experimental data. The temperature at which the mass loss is maximum shows a sudden decrease at low concentration of nanotubes (less than 0.05 wt.%) and increases as the concentration of the filler is increased above 0.5 wt.% (see Fig. 8). This suggests that a low concentration of nanofibers shuffles the orientation of the macromolecular chain, providing a reduced thermal stability by promoting an enhanced porosity that enhances the diffusion step involved in the thermal degradation. This is consistent with the dominant beads formation at low loading with MWCNTs. As the concentration of nanofibers is increased, the weight of the polymer confined within the interface is increased. This indicates that more and more macromolecular chains will interact with the nanotubes and explains the enhancement of the thermal stability of the polymer. As seen from Fig. 8 the width of the degradation process is increased by increasing the loading with carbon nanotubes, supporting the enhancement of the thermal stability.

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confirmed by TGA data, which confirm the enhancement of the thermal stability of PS by the loading with MWCNTs.

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o

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Mass Loss Rate Width [ C ]

15

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426 0

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Temperature at Maxim Mass Loss Rate [ C]

D.M. Chipara et al. / Applied Surface Science 275 (2013) 23–27

Partial financial support for this work from NIH-NIGMS-NIA grant no. 1SC2AG03 6825-01 for Javier Macossay is gratefully acknowledged. References [1] [2] [3] [4] [5] [6]

10

CMWNT [wt. %] Fig. 8. The dependence of the temperature at which the mass loss is maximum (left side) and of the mass loss rate width on the loading with MWCNTs.

[7]

[8] [9]

4. Conclusions [10]

The synthesis of polystyrene-multiwalled carbon nanotubes is reported. Raman spectroscopy confirmed the presence of MWCNTs in the as obtained nanofibers and revealed the interactions between the macromolecular chains and MWCNTs. The presence of D and G bands has been confirmed. The data are consistent with a mixture of semiconducting and conducting MWCNTs. The ratio between these two components is not affected by the dissolution, sonication, and electrospinning steps (within the experimental errors). The width and shape of the two G bands support this interpretation while the slight shift of their position reflects the pressure exerted by macromolecular chains on MWCNTs. The M band has not been observed; however a weak shoulder noticed in the range 1600–2000 cm−1 may originate for such vibrations [23]. The shoulder is too weak to allow for a reasonable estimation of its parameters. The lack of polarization capabilities prevented the investigation of the orientation of carbon nanotubes within these nanofibers. A complex spectrum was noticed in the radial breathing mode. Typically, the Raman lines in this spectral range are analyzed/simulated solely for single and double walled carbon nanotubes, the spectrum of MWCNTs being too complex for an accurate and unique deconvolution. In the polymer nanofiber-MWCNTs composites, the Raman spectra below 500 cm−1 are complicated by the competition between the acoustic modes in polymers and the radial breathing mode in MWCNTs. It was shown that the position and the splitting of the G band depend on the loading with MWCNTs. The Raman lines of the polymeric matrix are broadened quickly by the addition of carbon nanotubes, up to their complete disappearance for samples loading with 5 wt.% MWCNTs or more, suggesting strong vibrational interactions between macromolecular chains and MWCNTs. The formation of an interface between MWCNTs and PS is additionally

[11]

[12]

[13]

[14]

[15]

[16] [17] [18]

[19]

[20]

[21]

[22] [23]

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