Mechanical properties of PET composites using multi-walled carbon nanotubes functionalized by inorganic and itaconic acids

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1. Introduction
Poly(ethylene terephthalate) (PET) is a semicrys-
talline polymer which has good mechanical proper-
ties, chemical resistance, thermal stability, melt vis-
cosity, and spinnability (ability to be spun, e.g. in
the form of fibers). PET has been used in diverse
fields such as food packaging, film technology,
automotive, electrical, beverages and containers,
and textile fiber industries, and even in the biomed-
ical field as Dacron [1, 2]. A way to further improve
the properties of this commodity polymer is through
the formation of a nanocomposite, which can be
achieved by the addition of nanoclays, carbon nan-
otubes or other nanostructures. Nanocomposite mate-
rials often possess a combination of physical prop-
erties that are not present in conventional polymer
matrix composites. Because of their high aspect
ratios, adding low concentrations of carbon nano -
tubes (CNTs) into a polymer matrix can improve
the mechanical, thermomechanical and electrical
properties of these polymer composite materials [3,
4]. Two key issues necessary to achieve superior
performance in CNT filled polymer composites are
an homogeneous distribution and dispersion of the
CNTs inside the polymer matrix and a strong inter-
action between the CNTs and matrix. One of the
most common methods used to disperse CNTs in a
thermoplastic polymer matrix is melt blending [5].
Melt blending is a convenient method to produce
CNT based composites given its cost effectiveness
96
Mechanical properties of PET composites using multi-
walled carbon nanotubes functionalized by inorganic and
itaconic acids
A. May-Pat1*, F. Avilés1, P. Toro2, M. Yazdani-Pedram3, J. V. Cauich-Rodríguez1
1Centro de Investigación Científica de Yucatán, A.C., Unidad de Materiales, Calle 43 # 130, Col. Chuburná de Hidalgo,
97200, Mérida, Yucatán, México
2Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
3Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Santiago, Chile
Received 13 June 2011; accepted in revised form 18 August 2011
Abstract. Multi-walled carbon nanotubes (MWCNTs) were oxidized by two different acid treatments and further function-
alized with itaconic acid (IA). The functionalized MWCNTs were used to fabricate Poly(ethylene terephthalate) (PET)
composites by melt mixing. The presence of functional groups on the surface of the treated MWCNTs was confirmed by
infrared spectroscopy and thermogravimetric analysis. The MWCNTs oxidized with a concentrated mixture of HNO3and
H2SO4exhibited more oxygen containing functional groups (OH, COOH) but also suffer larger structural degradation than
those oxidized by a mild treatment based on diluted HNO3followed by H2O2. PET composites were fabricated using the
oxidized-only and oxidized followed by functionalization with IA MWCNTs. PET composites fabricated with MWCNT
oxidized by mild conditions showed improved tensile strength and failure strain, while harsh MWCNT oxidation render
them overly brittle.
Keywords: polymer composites, nanocomposites, mechanical properties, MWCNTs, itaconic acid
eXPRESS Polymer Letters Vol.6, No.2 (2012) 96–106
Available online at www.expresspolymlett.com
DOI: 10.3144/expresspolymlett.2012.11
*Corresponding author, e-mail: amay@cicy.mx
© BME-PT

and ease of production, with the additional advan-
tage of being a solvent-free process [6, 7]. The use
of this polymer processing technique to fabricate
CNT-polymer composites has been reported for a
large variety of polymers, including PMMA, poly-
olefins (PE, PP), polyamides, polyesters (PET, PBT),
polyurethanes, and polystyrene [8–10]. Melt blend-
ing, however, may be limited to low CNT concen-
trations due to the high viscosity of the composites
at high nanotube loadings and problems of dispers-
ing the nanotubes for high CNT concentrations
[11]. Regarding the issue of promoting interfacial
interactions between the CNT and thermoplastic
polymer matrices, the most common procedures
used for covalent attachment of reactive groups to
the CNT surface are treatments based on inorganic
acids [5, 12]. Usually, the nanotubes are refluxed
with a nitric acid solution or a mixture of nitric and
sulfuric acid, sometimes concurrently with the appli-
cation of high power sonication [5, 13, 14]. These
oxidative treatments usually result in formation of
surface reactive groups, such as hydroxyl, carbonyl
and carboxylic acid, with the drawback of produc-
ing CNT structural damage and length shortening
[15]. Oxidation of multiwalled carbon nanotubes
(MWCNTs) starts at the tips and gradually moves
towards the central part of the tube, but, if the acid
concentration is too high or the exposure time is
long, some of the layers can be substantially rough-
ened or even removed successively [15–17]. Another
method which has been less explored to promote
chemical bonding between CNTs and thermoplastic
polymers is CNT functionalization with organic
acids [18, 19]. For the case of MWCNT/PET com-
posites, only scant research has been conducted on
the functionalization of carbon nanotubes with
organic acids and compounds [20–22]. Jin and
coworkers [21] functionalized MWCNTs with acetic
anhydride, which resulted in good dispersion of
MWCNTs in a PET matrix, as well as an increased
tensile strength and elastic modulus. Yoo et al. [22]
functionalized MWCNTs with benzyl and phenyl
isocyanates, and found that the resulting nanocom-
posites showed improved dispersion of nanotubes
in the PET matrix. Improved mechanical properties
were found in the nanocomposites fabricated with
MWCNT-phenyl isocyanate because of the favor-
able presence of !–! interaction. Regarding ita-
conic acid, this organic compound has been used as
compatibilizer for blends of thermoplastic polymers
[23, 24]. Yazdani-Pedram et al. [23] used PP func-
tionalized with itaconic acid as compatibilizer in
blends of PP/PET. They found that the presence of
modified PP improves the tensile mechanical prop-
erties of PP/PET blends. Sailaja and Seetharamu
[24] used grafted low density polyethylene (LDPE)
with itaconic acid in mixtures of LDPE/starch and
found that LDPE/starch blends compatibilized by
itaconic acid exhibited better mechanical properties
as compared to their uncompatibilized counterparts.
Research on functionalization of CNTs with ita-
conic acid for polymer composites was not found in
the literature search conducted by the authors.
Therefore, the aim of the present work is to modify
the surface of MWCNTs by the introduction of
hydroxyl groups through treatments based on nitric
and sulfuric acids, as well as itaconic acid (IA). In
particular, the influence of a mild and an aggressive
acid oxidation and the effect of a subsequent func-
tionalization of the MWCNTs with IA on the tensile
mechanical properties of MWCNT/PET composites
is investigated.
2. Materials and methods
2.1. Materials
The MWCNTs used in this work were purchased
from Bayer MaterialScience (Leverkusen, Ger-
many). The CNT product (Baytubes C150P®) con-
sist of MWCNTs synthesized by chemical vapor
deposition with high purity (~95%), an outer mean
diameter of 13–16 nm, inner diameter of ~4 nm and
average length of 1–4 µm [15]. Nitric acid (HNO3,
68% v/v), sulfuric acid (H2SO4, 98% v/v) and ita-
conic acid were purchased from Sigma Aldrich
Corporation (Milwaukee, USA). The polymer used
in this work is CLEARTUF 8006 Polyester Resin
(‘M & G Group’ Polymers USA, LLC). CLEARTUF
8006 is a high molecular weight polyethylene tereph-
thalate thermoplastic polymer with intrinsic viscos-
ity of 0.80 dl/g and a melting point of 250°C.
2.2. Oxidation of MWCNTs
Chemical oxidation was carried out using two acid
treatments, one (‘mild’) which is expected to be
gentle with the MWCNTs surface and cause mini-
mum structural damage and a second one which is
significantly more aggressive. The ‘mild’ treatment
consisted of oxidizing the nanotubes with nitric
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97

acid (3.0 M) followed by hydrogen peroxide [14].
The ‘aggressive’ treatment was conducted using a
concentrated combination of nitric (68% v/v) and
sulfuric (98% v/v) acids. Table 1 illustrates the
treatments carried out. Label ‘A’ is used for as-
received (untreated) MWCNTs, taken as a refer-
ence. Treatment ‘B’ is used for MWCNTs treated
with nitric acid (3.0 M) followed by H2O2. This
treatment was conducted by mechanically stirring
the CNTs in the nitric acid for 15 min, and then son-
icating the CNTs/HNO3solution for 2 h in a con-
ventional ultrasonic bath. After exhaustive washing
of the CNTs with distilled water, the nitric acid was
replaced with hydrogen peroxide (30% v/v) and the
process was repeated. For treatment ‘C’, the as-
received MWCNTs were mixed with sulfuric (20 ml)
and nitric (20 ml) acids, in concentrations of 98 and
68% v/v respectively, and initially sonicated for
10 min. The mixture was then refluxed for 1 h at
140°C. Acid oxidized MWCNTs were finally
obtained by washing and filtering the mixture with
distilled water and then drying in a vacuum oven at
60°C for 48 h.
2.3. Functionalization with itaconic acid
The oxidized MWCNTs (treatments B and C in
Table 1) were further functionalized with itaconic
acid as follows. First, oxidized MWCNTs were dis-
persed in 20 ml of acetone for 10 min in a conven-
tional ultrasonic bath. After 10 min, itaconic acid
(3.0 g), p-toluenesulfonic acid as catalyst (0.2 g)
and 80 ml of acetone were added to the initial
MWCNT/acetone mixture and stirred under reflux
for 3 h at 60°C. After this, the functionalized nan-
otubes were washed, filtered with acetone and then
dried at 60°C in a vacuum oven to constant weight.
An identical procedure with IA was conducted for
MWCNTs that were previously oxidized by treat-
ments B and C. The MWCNTs functionalized by IA
that had been previously oxidized by treatment B
were labeled as ‘D’, while those that were previ-
ously oxidized by treatment C were labeled as ‘E’
(see Table 1). A schematic of the proposed reaction
between the CNT, IA and PET is shown in Figure 1.
Overall, it is expected that the COOH groups gener-
ated on the CNT surface are more prone to form
strong bonding with the C=O groups of PET than
the OH ones, due to their ability to induce dimer
formation. However, ester formation is also possi-
ble.
2.4. Preparation of MWCNT/PET composites
MWCNT/PET composites were fabricated by melt
blending. Initially, PET powder was dried at 120°C
for 24 h in a convection oven. Nanocomposites of
0.5 wt% were prepared by melt mixing the MWCNTs
and PET powder in a Banbury mixing chamber
(Plasticorder PL330, G.W. Brabender, Hackensack,
NJ, USA.), with a volumetric capacity of 50 cm3at
250°C. The mixing process was performed in two
continuous steps. First, the MWCNT/PETs powder
material was placed inside the chamber and mixed
for 5 min at 20 rpm. The mixing speed was then
increased to 60 rpm for 5 min more. The PET/
MWCNTs composite material was then laminated
by compression molding into thin sheets of 110"
110 "1.0 mm. The lamination process was per-
formed at 270°C in a laboratory press with tempera-
ture control (Carver Laboratory Press) using 2 tons
of pressure for 20 min and then water-cooled down
to room temperature. After lamination, dog-bone-
shaped tensile samples were cut from the 1 mm
thick sheet with the geometry according to ASTM
D638 standard [25].
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98
Table 1. Identification of chemical treatments investigated
in this work
Label Treatment
A Untreated (as-received)
B HNO3(3.0 M) followed by H2O2(30% v/v)
C Mixture of HNO3(68% v/v) and H2SO4(98% v/v)
D Treatment B + Itaconic acid
E Treatment C + Itaconic acid
Figure 1. Proposed reaction between IA, MWCNTs and
PET

2.5. Characterization
2.5.1. Characterization of functionalized
MWCNTs
Infrared spectroscopy of as-received and chemi-
cally functionalized MWCNTs was conducted using
a Fourier transform infrared spectrometer (FT-IR)
Nicolet-Protege 460, in the spectral range from
4000 to 500 cm– 1. FT-IR spectra were obtained
using KBr discs containing a very small amount of
MWCNTs.
Thermogravimetric analysis (TGA) was conducted
in nitrogen atmosphere with a heating rate of
10°C/min using a Perkin–Elmer TGA7 equipment
within a temperature range of 50–750°C. The TGA
was repeated three times to yield reproducible
results.
The morphology of the acid-MWCNTs was observed
using an FEI-TITAN transmission electron micro-
scope (TEM) operated at 300 kV and registered
near the Scherzer focus. TEM samples were pre-
pared on lacey carbon grids using dispersion in an
ultrasonic bath for 30 minutes.
2.5.2. Characterization of nanocomposites
Tensile properties of MWCNT/PETs composites
were determined according to the ASTM D638
standard using type IV specimens [25]. Tensile tests
were conducted in a Shimadzu AGI-100 universal
testing machine equipped with a 500 N load cell
and using a cross-head speed of 5 mm/min. Ten
replicates for each nanocomposite material investi-
gated were tested.
The fracture surfaces specimens of the tested nano -
composites were examined by using a JEOL
6360LV scanning electron microscope (SEM), after
coating the surface with a thin layer of gold.
3. Results and discussion
3.1. Characterization of MWCNTs
3.1.1. FT-IR spectroscopy
FTIR spectroscopy was used to confirm the pres-
ence of hydroxyl, carboxyl as well as itaconic acid
units on the surface of MWCNTs. As seen in Fig-
ure 2A, the IR spectrum of as-received MWCNTs
shows characteristic bands due to O–H stretching
vibration at 3440 cm–1, C=C stretching at 1629 cm–1
as well as O–H bending (~1400 cm–1) and C–O
stretching (~1116 cm–1). The spectrum of oxidized
MWCNTs by using the mild treatment (Figure 2B)
shows additional absorption bands at 1736 cm–1
due to carbonyl stretching vibration of hydrogen
bonded –COOH groups as well as an increase of the
relative intensity of OH groups, confirming the
presence of different type of hydroxyl groups on the
surface of the MWCNTs. The spectrum of MWCNTs
oxidized by treatment C (Figure 2C) is similar to
that of the mild treatment (Figure 2B) but the band
corresponding to C–O stretching is more intense
and better resolved. This suggests that although the
same functional groups are generated for both oxi-
dation treatments (B and C), a larger number of
such functional groups should be present in the
MWCNTs oxidized by the method C. Functional-
ization of oxidized MWCNTs by itaconic acid (Fig-
ures 2D and 2E) show an increase in the relative
intensity of the band in the region 3130 cm–1 due to
–OH groups. The C=O absorption bands character-
istic of carboxyl functional groups (–COOH) of ita-
conic acid is now markedly observed around 1707–
1730 cm–1, which confirm the success of the
functionalization treatment with itaconic acid.
3.1.2. Thermogravimetric analysis
TGA in nitrogen atmosphere was conducted for all
MWCNTs examined in Table 1, as well as for IA.
The thermograms obtained are presented in Fig-
ure 3. It is observed that the most thermally stable
material is the as-received MWCNTs (A), which
loses only about 1% weight after being heated to
750°C. This high thermal stability is due to the low
content of amorphous carbon in the as-received
material. When a mild oxidation is conducted on the
MWCNTs (B), the amount and rate of weight loss is
very similar to that of the as-received material, indi-
cating that no or very few amorphous carbon has
been generated because of the acid treatment. The
harsh oxidation treatment (C), on the other hand,
significantly degrades the graphitic structure of the
MWCNTs by converting it to amorphous carbon,
which is evident by the pronounced weight loss
observed early in its TGA curve. Below 150°C evap-
oration of adsorbed water is expected. The pro-
nounced weight loss observed between ~150 and
350°C are attributed to elimination of hydroxyl and
carboxylic functionalities present on the surface of
the oxidized MWCNT probably due to dehydration
and decarboxilation [16, 17]. The fact that the weight
loss in this temperature range is markedly greater
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99

for sample C than B indicates that a larger number
of functionalities are present in the MWCNTs oxi-
dized by the aggressive treatment, with respect to
the ones oxidized under mild conditions. For tem-
peratures above 350°C, a large amount of weight is
lost by MWCNTs oxidized by treatment C, which
indicates that this treatment has converted part of
the CNT graphitic structure to amorphous carbon,
which is thermally oxidized slightly above 350°C.
The MWCNTs that were functionalized by IA after
oxidation present a distinctive behavior. For the
MWCNTs previously oxidized by treatment B and
then functionalized by IA (D), a marked weight loss
occurs around 200°C, which matches with the
decomposition of the IA (see curve for IA in Fig-
ure 3). After thermal decomposition of the IA
bonded to the CNT, the curve of sample D follows a
slope (rate of weight loss) similar to that of the
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100
Figure 2. FT-IR spectra of MWCNTs. Treatment A, B, C, D and E refer to Table 1.

curve corresponding to the sample that was only
oxidized with the same treatment (B). The behavior
of the MWCNTs that were functionalized with IA
after oxidation with an aggressive acid treatment
(samples E) was different. In this case, a larger
weight loss was observed with respect to samples
oxidized by treatment D for all temperature ranges
examined. The rate of weight loss (slope of the
curve) is markedly smaller for samples E than for C
before 350°C, but after 350°C both slopes were
similar. The smaller mass loss of sample E com-
pared to sample C may suggest that some of the
amorphous carbon generated during the harsh CNT
oxidation by method C is physically removed dur-
ing the subsequent treatment with IA. This will be
further examined in connection to the mechanical
properties of the composites in section 3.2.1.
Therefore, from the TGA analysis it is concluded
that an aggressive acid oxidation treatment (such as
C) generates a larger concentration of functional
groups on the surface of the MWCNTs with respect
to a milder oxidation treatment (B), but also destroys
a great deal of the graphitic structure converting it
to amorphous carbon. The TGA curves of MWCNTs
that were functionalized with IA confirm the pres-
ence of IA on the CNT, which is indicative of an
adequate functionalization process.
3.1.3. Transmission electron microscopy
TEM analysis of the as-received (A) and oxidized
samples (B and C) is shown in Figure 4. No images
are presented for treatments with IA since no new
particular features are expected after functionaliza-
tion with IA. Figure 4a reveals the morphology of
an individual as-received multi-walled carbon nan-
otube. The as-received material is featured by the
characteristic graphitic layers arranged in a coaxial
cylindrical fashion. Some amorphous carbon is
observed at the outermost layers and structural
defects are also present in the as-received material.
The morphology of the MWCNTs oxidized by a
sequential treatment based on diluted HNO3fol-
lowed by H2O2(treatment B, Figure 4b) is similar
to that of the as-received MWCNTs, indicating that
structural damage is either absent or small. For the
MWCNTs oxidized by the concentrated mixture of
HNO3and H2SO4(treatment C), the CNTs exhibit
severe structural damage, as shown in Figure 4c.
This concentrated mixture of acids causes conver-
sion of the original graphitic structure to amorphous
carbon with the consequent increase in CNT rough-
ness and waviness as observed in Figure 4c. In this
case, the damage reaches even the inner nanotube
walls. Treating the MWCNTs with a concentrated
mixture of HNO3and H2SO4causes severe etching
of the graphitic surface of the CNT, leading to defec-
tive tubes with a large population of disordered
sites and amorphous carbon. These TEM observa-
tions are consistent with the thermal degradation
evidenced by the thermogram of treatment C in Fig-
ure 3. On the other hand, the MWCNT oxidation
with a mild treatment such as B yielded no or few
structural damage, which is also consistent with the
TGA findings. However, FT-IR analysis (Figure 2)
suggests that more functional groups are present on
MWCNTs oxidized by treatment C than for B. For
the improvement of the composite mechanical prop-
erties, structurally pristine CNTs with a large den-
sity of functionalities are desirable. In our case,
MWCNTs oxidized by treatment C have a large
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101
Figure 3. TGA curves of MWCNTs samples. Labels refer to Table 1.

number of OH and COOH functionalities, but also
are severely damaged. On the other hand, the
MWCNTs oxidized by treatment B were not severely
damaged by the oxidizing treatment, but they also
have less density of functional groups than those
oxidized by treatment C. The effect of these com-
peting factors on the mechanical properties of the
composites fabricated with these MWCNTs will be
examined in the next section.
3.2. Characterization of nanocomposites
3.2.1. Tensile properties
The tensile stress-strain curves of pure PET and the
examined MWCNT/PET composites at 0.5 wt%
loading are presented in Figure 5. Table 2 presents a
summary of the measured mechanical properties,
where average and standard deviations are reported.
The stress-strain curves and measured mechanical
properties (Table 2) are similar for MWCNT/PET
composites which used as-received MWCNTs
(MWCNT/PETs-A) to those of PET, motivating the
need of CNT functionalization. The absence of sig-
nificant improvements in the mechanical properties
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Figure 4. TEM of MWCNTs. a) As-received (A), b) oxi-
dized by HNO3followed by H2O2(B), c) oxi-
dized by a concentrated mixture of HNO3and
H2SO4(C).
Table 2. Mechanical properties of tested MWCNT/PET
composites
– = Not possible to measure
Sample
Tensile
strength
[MPa]
Elastic
modulus
[GPa]
Failure
strain [%]
PET 33.9±7.06 1.54±0.04 2.22±0.49
MWCNT/PETs-A 34.1±6.48 1.47±0.09 2.35±0.43
MWCNT/PETs-B 50.0±5.30 1.58±0.06 3.45±0.45
MWCNT/PETs-C – – –
MWCNT/PETs-D 23.4±6.32 1.39±0.15 2.40±0.93
MWCNT/PETs-E 26.9±5.16 1.48±0.14 2.43±0.72
Figure 5. Representatives stress-strain curves for PET and
PET nanocomposites. Labels A to E refer to
Table 1.

of the as-received CNT composites may be the
result of aggregation, as well as poor interfacial
interaction between the as-received MWCNTs and
PET. Quite different properties are obtained for the
composites whose CNTs were only oxidized by
HNO3followed by H2O2(MWCNT/PETs-B). With
respect to neat PET, these composites showed an
increase in the strength and failure strain of 47 and
55% (based on averages) respectively, although the
improvement in elastic modulus may not be statisti-
cally significant. The large improvement in the ten-
sile properties of the MWCNT/PETs-B composites
is attributed to the improved dispersion of MWCNTs
inside the matrix (which will be further examined
by SEM) and improved interactions between the
MWCNTs and PET mostly by hydrogen bonding,
enhancing the interfacial bonding. The PET-com-
posites that utilized MWCNTs treated by the con-
centrated mixture of nitric and sulphuric acids
(treatment C) were not possible to manufacture in
the form of tensile coupons (indicated by dash lines
in Table 2). These composites were very brittle and
cracked in many pieces immediately after releasing
from the press used for compression molding. This
behavior, which was very distinctive from the rest
of the composites, confirms that the MWCNTs
treated by treatment C contain a large number of
(COOH) functionalities and that such functionali-
ties are interacting with the PET matrix (through
C=O) to render the composites increased brittleness
and affecting its processability. These composites
may also be more thermally conductive (which
influence the processing parameters), since it has
been shown that MWCNTs treated by strong mix-
tures of nitric and sulfuric acids generate a large
number of hydroxyl and carboxyl functional groups
which increases the thermal conductivity of the
CNTs and their composites [26]. The nucleating
effect of the functionalized nanotubes during the
PET melt compounding [27] may also be a con-
tributing factor for its brittleness.
The composites prepared with IA-functionalized
MWCNTs (D and E) did not show improvements in
their mechanical properties with respect to the neat
PET. With respect to PET, the tensile strength of
both composites (D and E) reduced, and the elastic
modulus and failure strain remained practically
unchanged, see Table 2. The decrease in tensile
strength (and decreasing trend in elastic modulus)
of these composites could be explained by reduced
interactions between the functionalized CNTs and
the PET matrix and the presence of agglomerations.
This would mean that a second scenario to that ini-
tially proposed in Figure 1, is also possible, which
is depicted in Figure 6. Accordingly, the itaconic
acid may react with the hydroxyl groups present on
the acid oxidized surface of the CNT by forming
ester bonds. This reduces the effective number of
hydroxyl functionalities available for bonding the
CNT to the polymer, leaving a hydrophobic CNT
which is difficult to disperse in the hydrophilic
PET.
3.2.2. Scanning electron microscopy
Figure 7 shows SEM fracture surfaces of MWCNT/
PET composites whose CNTs were functionalized
by treatments A, B, D and E. The CNTs seem mod-
erately well dispersed in all composites in Figure 7.
Although clear differences are difficult to discern
among the figures, some particular features were
observed depending on the treatment. For the non-
treated composites (MWCNT/PETs-A), Fig ure 7a,
a few small aggregates of MWCNTs appeared sys-
tematically during the SEM analysis. For this com-
posite, MWCNTs are also seen protruding the frac-
ture plane (plane of the SEM image) suggesting a
large occurrence of CNT pullout, which is consis-
tent with a weak interfacial bonding between the
nanotubes and PET. The scenario was different for
MWCNT/PETs-B composites, Figure 7b. For these
composites, imaging the MWCNTs in the compos-
ite was markedly more difficult than for the rest of
the samples, and the few CNTs that could be imaged
appeared covered by the polymer, as seen in Fig-
ure 7b. We believe that this particular feature relates
to a better dispersion into the PET matrix, as a con-
sequence of the oxidative treatment used. MWCNTs
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103
Figure 6. Possible ester formation on IA functionalized
MWCNTs

oxidized by treatment B also appeared wrapped by
the PET matrix, which suggests good interfacial
bonding. This finding is consistent with the results
of the tensile tests, where enhanced mechanical
properties were obtained for this treatment. Fig-
ures 7c and 7d show fracture surfaces of MWCNT/
PET composites which used MWCNTs functional-
ized by IA after acid oxidation by mild (Figure 7c)
and harsh (Figure 7d) conditions (treatments D and E
in Table 2). For both cases, MWCNTs are seen
clearly and moderately well dispersed into the PET
matrix, but not covered by the polymer as in the
case of Figure 7b.
4. Conclusions
The influence of two acid oxidation methods and
further functionalization with itaconic acid to func-
tionalize the surface of MWCNTs and improve the
mechanical properties of PET composites has been
examined. Two routes for acid oxidations employ-
ing inorganic acids were examined, a mild one based
on a sequential treatment of diluted nitric acid and
hydrogen peroxide, and an aggressive one based on
a concentrated mixture of nitric and sulfuric acids.
Oxidizing by the mild experimental conditions pro-
posed herein generate hydroxyl and carboxyl func-
tional groups on the CNT surface with minimum
CNT damage, which produces enhanced mechani-
cal properties of MWCNT/PET composites. When
the oxidation uses concentrated acids, a larger den-
sity of functional groups is generated, with the con-
comitant damage of the CNT structure. The com-
posites manufactured using the MWCNTs oxidized
by this aggressive route are overly brittle, to the
extent that compromises its practical use.
For PET composites, sequential functionalization
of the MWCNTs with IA after CNT oxidation is not
recommended, since the OH groups of the IA may
bond to similar acidic groups existent on the oxi-
dized MWCNTs, reducing the density of function-
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Figure 7. SEM images of fractured nanocomposites. (a) MWCNT/PETs-A, (b) MWCNT/PETs-B, (c) MWCNT/PETs-D,
(d) MWCNT/PETs-E. Labels refer to Table 2.

alities available on the MWCNT surface for hydro-
gen bonding with the ester groups of PET. A simple
oxidation of the MWCNTs using nitric acid and
hydrogen peroxide in mild experimental conditions
is enough to significantly improve the tensile prop-
erties of the composite.
Acknowledgements
This work is part of an international project between Mex-
ico and Chile (CIAM) sponsored by CONACYT-Mexico
(projects No. 105567 and 79609) and CONICYT-Chile
(project FONDECYT No.1090260). The authors also wish
to thank Santiago Duarte and Dr. Arturo Ponce for the SEM
and TEM analyses.
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