EFFECT OF CARBON NANOTUBE SURFACE TREATMENT ON THE
MORPHOLOGY, ELECTRICAL AND MECHANICAL PROPERTIES OF THE
MICROFIBER REINFORCED POLYETHYLENE/POLY(ETHYLENE
TEREPHTHALATE)/CARBON NANOTUBE COMPOSITES
SUPPLEMENTARY MATERIALS
by
Sertan Yesil and Goknur Bayram
Carbon Nanotube Characterization:
Surface energy components (γsolid: total surface energy, γsolidd: dispersive component of total
surface energy, γsolidp: polar component of total surface energy, γsolidA: acidic component of
polar surface enery, γsolidB: basic component of polar surface energy) of the carbon nanotube
(CNT) samples were determined by measuring the contact angles of probe liquids on sample
surfaces. CNT particles were pressed as discs with 12 mm diameter under 150 bar oil pressure
and contact angles of probe liquids were determined from these pressed surfaces.
Diiodomethane, ethylene glycol and formamide were used as probe liquids. Fourier
Transformed Infrared Spectroscopy (FTIR) (Shimadzu IRPrestige 21) was used to check for
the presence of reactive groups on the CNT surface after purification and PEG treatment. The
infrared spectra of CNT which were pressed with KBr were recorded in the range of 400-4000
cm-1. X-ray diffraction (XRD) patterns of CNT samples were obtained with a 100 kV Philips
twin tube X-ray diffractometer (PW/1050) providing CuKα radiation (λ=0.15418 nm) at 40
kV and 40 mA. The morphological analyses of CNT samples were performed with a Scanning
Electron Microscope (Zeiss Supra 50V). Thermo gravimetric analyses (TGA) of the CNT
samples were investigated by using a Shimadzu DTG-60/DTG-60A thermal analyzer. Heating
rate of the samples was 25˚C/min.
Carbon Nanotube Characterization Results
Surface Energy and FTIR Analyses
Surface energy components of the CNT samples are shown in Table 1. When the surface
energy components of the CNT samples are analyzed, it is seen that the acidic component of
the polar surface energy (γsolidA) increased due to the formation of carboxylic acid groups on
CNT after purification (Figure 1) [1]. Moreover, PEG treatment causes a sharp increase in the
basic component of polar surface energy (γsolidB) due to the replacement of carboxylic acid
groups on CNT surface with hydroxyl end groups of PEG during the interactions between the
CNT with PEG. Figure 1 displays the FTIR spectra of ASCNT (as-received carbon nanotube),
pCNT (purified carbon nanotube) and mCNT (PEG treated carbon nanotube). The peak at
1500 cm-1 might be attributed to C=C stretching of the aromatic groups [2]. The peak at 1602
cm-1 are the characteristic stretching vibrations of C-C bonds related to the expected CNT
phonon modes. The presence of –OH (hydroxyl) groups on the surface are indicated by the
peaks at 1080 cm-1 and 3440 cm-1 and it is an evidence for the presence of functional groups
in CNT surface before any surface treatment [3]. After purification, the formation of carboxyl
(COOH) and quinone (C=O) functional groups on CNT surface are indicated by the presence
of peaks at around 1180, 1718 cm-1 [4, 5]. After grafting PEG on CNT surface the stretching
absorption peaks of C=O and C-O for the ester group at 1734 and 1056 cm−1 are clearly
observed in Figure 1 [3]. The appearance of the peak at around 2929 cm-1 which corresponds
to the C-H stretches of the alkyl group of PEG is also confirms the PEG functionalization.
XRD, SEM and TGA Analyses
X-Ray patterns of CNT samples are similar to each other and the characteristic peaks at 26°
and 43° are observed for all samples (Figure 2), which shows the surface treated CNT have
the same cylinder wall and crystalline structure with untreated CNT [6]. SEM micrographs of
ASCNT and pCNT show that carbon nanotubes are randomly entangled each other (Figure 3).
Chemically modified CNT structure is more compact, tubes are joined each other and PEG
residues can be observed on the CNT surfaces. No destruction of CNT is observed in XRD,
SEM analyses after purification and modification. TGA analyses are performed for all CNT
samples to show the existence and determine the amount of organic functional groups on
CNT surfaces [2]. The degradation of the amorphous carbon, metallic residues and mostly
graphitic structure results in about 87% weight loss between 600 and 1150 °C for ASCNT
(Figure 4 (a)) [7]. This graphitic decomposition is also observed for other two samples. The
absence of the disordered carbon structures and catalyst particles after purification causes a
lower total weight loss for pCNT and mCNT when compared with that of ASCNT. TGA
curve of pCNT (Figure 4 (c)) shows about 7% weight loss from 200 to 550°C because of the
decomposition of carboxyl and hydroxyl groups. As a result of the decomposition of PEG
residues on CNT surfaces, two decomposition regions are observable in the TGA curve of
mCNT (Figure 4 (b)). The decomposition temperature of the organic PEG chains is lower
than that of the graphite layers, in the range of 250 to 500°C. The concentration of PEG
molecules are estimated as 43 wt. % on carbon nanotube surface from TGA analyses.
References
[1] Park SJ, Seo MK, Nah C (2005) J Colloid Interface Sci 291:229-235
[2] Hsin YL, Lai JY, Hwang KC, Lo SC, Chen FR, Kai JJ (2006) Carbon 44:3328-3335
[3] Shanmugharaj AM, Bae JH, Lee KY, Noh WH, Lee SH, Ryu SH (2007) Compos Sci
Technol 67:1813-1822
[4] Hsu HL, Jehng JM, Sung Y, Wang LC, Yang SR (2008) Mater Chem Phys 109:148-155
[5] Chen XH, Chen CS, Xiao HN, Chen XH, Li WH, Xu LS (2005) Carbon 43:1800-1803
[6] Wang J, Fang Z, Gu A, Xu L, Liu F (2006) J Appl Polym Sci 100:97-104
[7] Avile´s F, Cauich-Rodrı´guez JV, Moo-Tah L, May-Pat A, Vargas-Coronado R (2009)
Carbon 47:2970-2975
Table 1. Surface energy components of the carbon nanotube samples (mN/m)
Sample
γsolid
γsolidd
γsolidp
γsolidA
γsolidB
ASCNT
46.14
35.67
10.47
2.17
12.62
pCNT
46.67
34.70
11.97
3.24
11.07
mCNT
52.28
40.10
12.18
1.78
20.87
Figure 1 FTIR spectra of the CNT samples
Figure 2 XRD patterns of the CNT samples
Figure 3 SEM micrographs of CNT samples; [(a), (a’)] ASCNT, [(b), (b’)] pCNT, [(c), (c’)]
mCNT
Figure 4 TGA graphs of CNT samples; (a) ASCNT, (b) mCNT, (c) pCNT