A convenient strategy to functionalize carbon nanotubes with ascorbic acid and its effect on the physical and thermomechanical properties of poly(amide-imide) composites

Article abstract of DOI:10.1016/j.jssc.2013.12.021

Multi-walled carbon nanotubes (MWCNTs) were functionalized by ascorbic acid by a fast strategy under microwave irradiation to improve interfacial interactions and dispersion of CNTs in a poly(amide-imide) (PAI) matrix. This technique provides a rapid and

Full text of DOI:10.1016/j.jssc.2013.12.021

Journal of Solid State Chemistry 211 (2014) 136–145
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
A convenient strategy to functionalize carbon nanotubes with ascorbic
acid and its effect on the physical and thermomechanical properties of
poly(amide–imide) composites
a
Shadpour Mallakpour a,b, , Amin Zadehnazari
n
a Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, I.R. Iran
b Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, I.R. Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Multi-walled carbon nanotubes (MWCNTs) were functionalized by ascorbic acid by a fast strategy under
microwave irradiation to improve interfacial interactions and dispersion of CNTs in a poly(amide–imide)
(PAI) matrix. This technique provides a rapid and economically viable route to produce covalently
functionalized CNTs. The as-prepared, new type of functionalized CNTs were analyzed by several
techniques. The thermal stabilities and mechanical interfacial properties of CNT/PAI composites were
investigated using several techniques. The dispersion state of CNTs in the PAI matrix was observed by
field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The
mechanical interfacial property of the composites was significantly increased by the addition of ascorbic
acid treated CNTs. The FE-SEM and TEM results showed that the separation and uniform dispersion of
CNTs in the PAI matrix. The overview of these recent results is presented.
Received 29 October 2013
Received in revised form
13 December 2013
Accepted 19 December 2013
Available online 3 January 2014
Keywords:
Carbon nanotubes
Ascorbic acid
Electron microscopy
Mechanical properties
Thermal stability
& 2014 Elsevier Inc. All rights reserved.
1. Introduction
high stiffness-to-weight space mirror substrates [14]. In particular,
polymers with the incorporation of CNTs show great potential for
Carbon nanotubes (CNTs) have attracted much attention among
scientists in a variety of disciplines since their discovery in 1991
[1]. These materials have diverse arrangements at a nanometric
level that lead to different properties depending on the specific
kind of nanotubes. CNTs display different properties that suggest
potential for use in various advanced technological applications,
such as tips for Atomic Force Microscopy [2], cells for hydrogen
storage [3], nanotransistors [4], electrodes for electrochemical
applications [5], sensors of biological molecules [6], catalysts [7],
etc. Recently, much attention has been paid to the fabrication
of composites with the use of CNTs in polymer materials to
harness the exceptional intrinsic properties of CNTs [8–10].
Mechanical properties of CNTs suggest that they may be used as
reinforcing materials in high-toughness nanocomposites, where
stiffness, strength and low weight are important considera-
tions [11]. There are numerous possible applications; some exam-
ples are aerospace structural panels [12], sporting goods [13],
ultralightweight thin-walled space structures for use in space, and
electronic device applications, such as nonlinear optics include
protection of optical sensors from high intensity laser beams [15].
Additional applications involving the optical and electronic prop-
erties are organic field emitting displays, photovoltaic cells, highly
sensitive strain sensors, electromagnetic interference materials,
etc. [16–19]. Generally, there are two issues addressed in many
previous studies: dispersion of CNTs in polymer matrix and
interaction between CNTs and polymer. For the first issue, due to
the high surface-to-mass ratio of CNTs, molecular scale forces and
interactions should be considered among CNTs. Van der Waals
forces usually promote flocculation of CNTs, whilst electrostatic
charges or steric effects lead to a stabilization of the dispersion
through repulsive forces [20]. As a consequence, by considering
the nature of percolating network formed by very fine filler, e.g.,
CNTs, the balance of the two factors of reverse effects outlined
above should be taken into account. For the second issue, the fact,
that the nanotubes in the composites are coated or encapsulated
by a thin insulating polymer layer, was identified for CNTs [21].
This encapsulation acts as a barrier to the electrical charge transfer
between nanotubes. Actually, as-produced CNTs are almost inso-
luble in all media due to strong van der Waals interactions
between tubes. Many approaches were employed to introduce
functional groups covalently or noncovalently onto the surface of
CNTs to facilitate the processing of the material [22–24]. Among
n
Corresponding author at: Organic Polymer Chemistry Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,
I.R. Iran. Tel.: þ98 311 391 3267; fax: þ98 311 391 2350.
E-mail addresses: mallak@cc.iut.ac.ir, mallakpour84@alumni.ufl.edu,
mallak777@yahoo.com (S. Mallakpour).
0022-4596/$ - see front matter & 2014 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2013.12.021

S. Mallakpour, A. Zadehnazari / Journal of Solid State Chemistry 211 (2014) 136–145
137
the various surface modification strategies, covalent functionaliza-
tion presents a possible strategy for the preparation of tubes with
multifunctional properties and improved dispersability in a sol-
vent. In the field of the nanocomposites, chemical functionaliza-
tion offers an important tool in order to improve the interface and
achieve the amazing properties that CNTs can provide. Different
strategies for the functionalization of CNTs have been proposed,
based on non-covalent and covalent bonding with many low-
dimensional structures to the surface of CNTs [25,26].
Ascorbic acid (vitamin C) is a familiar molecule because of its
dietary significance [27]. It is essential for the formation of
intercellular collagen and therefore is required for the mainte-
nance of tooth structures, cartilage, bone matrix the walls of
capillaries and wound healing [28]. Ascorbic acid influences the
formation of hemoglobin, erythrocyte maturation and certain
immunological and biochemical reactions in the body. Vitamin C
deficiency is rare in adults but may occur in infants, alcoholics or
the elderly [29]. Deficiency leads to the development of scurvy. It
has also played important roles in photosynthesis and photopro-
tection, in defense against ozone and other oxidative stresses and
speculations about its role in cell expansion and cell division will
be emphasized [30].
The specific objective of the present investigation is to provide
ascorbic acid-functionalized multi-walled CNTs (MWCNTs-AS) for
the first time under microwave irradiation and present evidences
for the attachment between MWCNTs and ascorbic acid molecule.
Multiple strategies, such as functionalization of MWCNTs and the
use of an ultrasonication solution blending technique were
adopted to disperse the MWCNTs-AS into a nanostructured poly
(amide–imide) (PAI) matrix. Due to the introduction of several
organic functionalities as well as aminophenol and amino acid
groups, It can be expected that the chain packing distances
increase and the intermolecular interactions decrease, leading to
better interaction of the PAI chains with MWCNTs-AS and better
dispersion of MWCNTs-AS in the PAI matrix. The influence of
various MWCNT-AS distributions on the thermal and mechanical
characteristics of PAI is also studied.
(Gallenhamp, Cambridge, United Kingdom) without correction.
1H nuclear magnetic resonance (NMR) spectrum was recorded
on a Bruker (Rheinstetten, Germany) Avance 500 instrument at
room temperature in dimethylsulfoxide-d6 (DMSO-d6). Multipli-
cities of proton resonance were designated as singlet (s), doublet
(d), and multiplet (m). Fourier transform infrared (FT-IR) spectra of
the samples were recorded with a Jasco-680 (Tokyo, Japan)
spectrometer [taken in potassium bromide (KBr)] at a resolution
of 4 cmꢁ1. They were scanned at wavenumber range of 400–
4000 cmꢁ1. Band intensities were assigned as weak (w), medium
(m), strong (s), and broad (br). Vibration bands were reported as
wavenumber (cmꢁ1). Elemental analysis was performed in an
Elementar Analysensysteme GmbH (Hanau, Germany). Inherent
viscosity was measured using a Cannon Fenske Routine Visc-
ometer (Mainz, Germany) at the concentration of 0.5 g/dL in
DMF at 25 1C. Optical specific rotation was measured at the
concentration of 0.5 g/dL in DMF at 25 1C using a quartz cell
(1.0 cm) with a Jasco Polarimeter (JASCO Co., Ltd., Tokyo, Japan).
Thermal stability of MWCNTs, PAI and MWCNT-AS/PAI nanocom-
posites was evaluated by recording thermogravimetric analysis
(TGA) traces (STA503 TGA, Bahr-Thermoanalyse GmbH, Hüllhorst,
Germany) in nitrogen atmosphere (flow rate 60 cm3/min).
A heating rate of 10 1C/min and a sample size of 1072 mg were
used in each experiment. The X-ray diffraction (XRD) was used to
characterize the crystalline structure of the samples. XRD patterns
were collected using a Bruker, D8AVANCE (Rheinstetten, Germany)
diffractometer with a copper target at the wave length of λ
CuKα¼1.54 Å, a tube voltage of 40 kV, and tube current of
35 mA. The samples were scanned at a rate of 0.051/min from
101 to 801 of 2θ. For XRD studies, rectangular pellets prepared by
compression molding were used. The morphology of the MWCNTs
and dispersion morphology of the MWCNTs-AS on the PAI matrix
were observed using field emission scanning electron microscopy
(FE-SEM). The images were taken at 15 kV using a HITACHI S-4160
instrument (Tokyo, Japan). Transmission electron microscopy
(TEM) images were obtained using a Philips CM 120 microscope
(Netherlands) with an accelerating voltage of 100 kV. For TEM
studies, ultra-thin sections (30–80 nm) of MWCNTs-AS and the
composites were prepared using Leica Ultramicrotome. Tensile
testing was performed at room temperature on a Testometric
Universal Testing Machine M350/500 (Mainz, Germany), according
to ASTM D 882 (standards). Tests were carried out with a cross-
head speed of 12.5 mm/min until reaching a deformation of 20%
and then, at a speed of 50 mm/min at break. The dimensions of the
test specimens were 35 ꢂ 2 ꢂ 0.04 mm3. Property values reported
here represented an average of the results for tests run on at least
five specimens. Tensile strength, tensile modulus, and strain were
obtained from these measurements. The composites were pre-
pared by using a MISONIX ultrasonic XL-2000 SERIES (Raleigh,
North Carolina, USA). Ultrasonic irradiation was performed with
the probe of the ultrasonic horn being immersed directly in to the
mixture solution system with the frequency of 2.25 ꢂ 104 Hz and
the power of 100 W.
2. Experimental
2.1. Materials
Carboxylated MWCNTs synthesized by chemical vapor deposition
(CVD) (the outer-diameter 8–15 nm, the inner-diameter 3–5 nm,
length ꢀ50 μm, carboxyl content 2.56 wt% and purity 495 wt%),
were obtained from Neutrino Co. (Iran). Other chemicals were
achieved commercially from Fluka Chemical Co. (Switzerland),
Aldrich Chemical Co. (Milwaukee, WI) and Merck Chemical Co.
(Germany). 4-Aminophenol, 3,5-dinitrobenzoylchloride, trimellitic
anhydride (TMA), S-valine amino acid, glacial acetic acid, propylene
oxide, tetrabutylammonium bromide (TBAB), and triphenyl phos-
phite (TPP) were used as received without further purification.
Propylene oxide was used as acid scavenger. Hydrazine monohydrate
and 10% palladium on activated carbon were used as obtained. N,N0-
dimethylformamide (DMF) (d¼0.94 g cmꢁ3 at 20 1C), and N,N0-
dimethylacetamide (DMAc) (d¼0.94 g cmꢁ3 at 20 1C) as solvents
were distilled over barium oxide under reduced pressure before use.
All other reagents were used as received from commercial sources.
2.3. Monomer synthesis
N-trimellitylimido-S-valine (3) was prepared according to our
previous work [31]. 3,5-Diamino-N-(4-hydroxyphenyl)benzamide
(7) as a diamine monomer was also prepared according to our
reported procedure (Scheme 1) [32].
2.2. Apparatus and measurments
2.4. Functionalization of MWCNT
The equipment used for the functionalization of MWCNTs
and synthesis of the polymer was a Samsung microwave oven
(2450 MHz, 900 W) (Seoul, South Korea). Melting points of
the monomers were measured on a melting-point equipment
The reaction sequence for the ascorbic acid-functionalized
MWCNTs is shown in Scheme 2. Ascorbic acid (200 mg, 1.13 mmol)
was dissolved in 15 mL of DMAc. After addition of 100 mg

138
S. Mallakpour, A. Zadehnazari / Journal of Solid State Chemistry 211 (2014) 136–145
Scheme 1. Synthetic route used to obtain the PAI.
Scheme 2. Schematic representation of the attachment of vitamin C with MWCNTs and preparation of their PAI-based composites.
carboxylated-MWCNT and 0.10 mL of conc. HCl, the solution was
reacted at 80 1C for 2 h. The suspension was then poured into a
porcelain dish (50 mL) and placed in a domestic microwave
chamber. The abovementioned MWCNTs suspension was heated
in microwave up to 120 1C for 15 min with output power of 700 W.
Then the resulting suspension was cooled to room temperature
and poured into 200 mL of water. The mixture was stirred for
30 min at room temperature and then decanted to remove the
solvent. When the amount of solvent became about 20 mL, the
mixture was subjected to irradiation with high-intensity ultra-
sound for 1 h. The obtained homogeneous suspension was placed
in an 80 1C oven overnight to evaporate most of the solvent.

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139
The filtration products were washed more than 7 times with water
for the removal of any unreacted residue of vitamin C, and dried
under vacuum to give the product of MWCNT-AS.
presence of hydrazine hydrate and a catalytic amount of palladium
on the activated carbon at 80 1C to produce yellow crystals of the
diamine 7. The structure of the dinitro 6 and diamine 7 was
identified by elemental analysis, FT-IR, 1H NMR, and 13C NMR
spectroscopy.
2.5. Polymer synthesis
About 0.10 g (3.43 ꢂ 10ꢁ4 mol) of diacid monomer 3, 0.08 g
(3.43 ꢂ 10ꢁ4 mol) of diamine 7, and 0.44 g of TBAB (1.37 ꢂ
10ꢁ3 mol) were placed in a porcelain dish and ground completely
for 5 min; then, 0.36 mL (1.37 ꢂ 10ꢁ3 mol) of TPP was added and
the mixture was ground for 3 min. The reaction mixture was
irradiated in the microwave oven for 8 min at 50% of power level
(900 W) (the temperature of the reaction was around 130 1C). The
resulting viscous solution was poured into 30 mL of methanol,
filtered, and dried at 80 1C for 6 h under vacuum to give 0.17 g
(96%) of yellow powder PAI. The optical specific rotation was
3.2. Polymer synthesis
A synthetic strategy, using molten TBAB and microwave irra-
diation collectively, was found to be a successful and efficient
method for the synthesis of polymer. The monomers (3 and 7)
mixtures were irradiated under microwave (optimized condition;
50% of power level, 130 1C) with TBAB in the presence of TPP for
8 min. The polymer precipitated in a powder form when slowly
pouring the resulting polymer solution under stirring into metha-
nol. It was observed that the PAI was obtained in 94% of isolated
yield. Thus, we found that molten TBAB salt was a highly polar
medium that was likely to be a strong microwave absorption. The
inherent viscosity of the synthesized PAI was 0.48 dL/g. The
resulting polymer showed optical rotation, which indicated that
the polymer was optically active and chirality was introduced into
the backbone of the polymer. The optical specific rotation of this
25
measured (½αꢃNa;589 ¼ ꢁ42.181) at a concentration of 0.5 g/dL in
DMF at 25 1C. The inherent viscosity was also measured (ηinh
0.32 dL/g) under the same conditions.
¼
FT-IR (KBr, cmꢁ1): 3302 (m, br, NH and OH stretching), 3101 (w,
C–H aromatic), 2964 (w, C–H aliphatic), 2927 (w, C–H aliphatic),
1777 (m, CQO imide, asymmetric stretching), 1719 (s, CQO
imide, symmetric stretching), 1665 (s, CQO amide, stertching),
1601 (s), 1544 (s), 1512 (s), 1446 (s), 1377 (s, C–N–C axial stretch-
ing), 1210 (m, C–N–C transverse stretching), 1071 (m), 865 (m),
835 (s), 757 (m), 727 (s, C–N–C out-of-plane bending), 687 (w),
526 (w). 1H NMR (400 MHz, DMSO-d6, ppm): 0.89 (d, 3H, CH3,
distorted), 1.08 (d, 3H, CH3, distorted), 2.84 (m, 1H, CH), 4.64–4.66
(d, 1H, CH, J¼7.60 Hz), 6.73 (s, 1H, Ar–H), 7.19 (s, 1H, Ar–H), 7.28–
7.30 (d, 2H, Ar–H, J¼8.00), 7.47–7.49 (d, 2H, Ar–H, J¼8.00 Hz), 7.75
(d, 1H, Ar–H, distorted), 7.99 (s, 1H, Ar–H), 8.09–8.11 (d, 1H, Ar–H,
J¼7.20 Hz), 8.45 (s, 1H, Ar–H), 9.26 (s, 1H, OH), 10.18 (s, 1H, NH),
10.80 (s, 2H, NH), 10.88 (s, 2H, NH).
25
polymer was ½αꢃNa;589 ¼ ꢁ35.11 (0.05 g in 10 mL of DMF). Elemental
analysis, FT-IR and 1H NMR spectroscopy were used to confirm the
formation of PAI [33].
3.3. Functionalization of MWCNTs
We have previously reported on the functionalization of
MWCNT with several organic functionalities [34–38]. Accordingly,
vitamin C was selected to react with MWCNTs. The reaction
sequence is depicted in Scheme 2. Functionalization was carried
out under microwave irradiation. The optimum microwave power
level and period of heating were selected according to our
previous work [34]. An esterification reaction occurred between
the ascorbic acid molecule and carboxylic acid group of the
MWCNT’s surface under microwave irradiation. The separation
and purification of products, is carried out easily and in a
convenient way by simple decanting. The resulting paste was
washed with DMAc and water, sonicated and dried.
Elemental analysis: calculated for (C27H22N4O6)n: C, 65.05%; H,
4.45%; N, 11.24%. Found: C, 64.90%; H, 4.41%; N, 11.07%.
2.6. Preparation of the MWCNT-AS/PAI composite films
Initially, PAI was dissolved and MWCNTs-AS was separately
dispersed in DMAc with stirring being for 1 day at 30–40 1C. Then,
two stock solutions were mixed to achieve the desired weight
percentages of MWCNTs-AS from 5 to 15 wt%. The MWCNT-AS/PAI
mixture was stirred for 1 day at 30–40 1C and then ultrasonicated
in a water bath for 1 h. To remove the DMAc solvent, MWCNT-AS/
PAI mixture was poured into uncovered preheated glass Petri
dishes and uniformly heated at 60 1C for 1 day; then the semidried
film was further dried in vacuum at 160 1C for 8 h in order to
remove the residue solvent and a solid film was formed. Compo-
site films formed after the evaporation of DMAc could be easily
lifted from the glass Petri dishes. Freestanding polymer films with
50 μm thickness were then peeled from the glass plate and
subjected to different tests.
3.4. Composite films preparation
The dispersion of the MWCNTs-AS in 5, 10, and 15 wt% solu-
tions of PAI in DMAc was attained by a vigorous stirring with a
speed of 15,000 rpm for 1 day, using a homogenizer, which was
followed by utrasonication process for 1 h to form a new series of
MWCNT/PAI composites. The reaction pathway for preparing
MWCNT-AS/PAI composites is shown in Scheme 2. The effective
use of CNTs in composite applications depends on the ability to
disperse the CNTs homogeneously throughout the matrix without
reducing their aspect ratio. Due to the van der Waals attraction,
CNTs are held together as bundles and ropes. Therefore, they have
a very low solubility in solvents and tend to remain as entangled
agglomerates. To employ CNTs as an effective reinforcement in
polymer composites and ensure proper dispersion and appropriate
interfacial adhesion between the CNTs and polymer matrix,
several mechanical/physical methods were used [39,40]. So, in
this study, at first, the MWCNTs were functionalized with ascorbic
acid. Several researches have recently investigated the properties
of CNT/polymer composites with CNT content from 1 to 15 wt%
and good results have been obtained and reported [41–43]. From
this point of view, we selected 5%, 10%, and 15% of CNT content in
the composites. The lower level of aggregation in the modified
CNTs can be attributed not only to the presence of functional
groups such as ascorbic acid groups but to their high aspect ratio.
3. Results and discussion
3.1. Synthesis and structural characterization of monomers
Diacid monomer 3 was synthesized by the condensation
reaction of an equimolar amount of TMA and S-valine in refluxing
acetic acid solution (Scheme 1) [31]. Diamine monomer 7 was
synthesized using a two-step process (Scheme 1) [32]. In the first
step, aromatic nucleophilic displacement of 3,5-dinitroben-
zoylchloride with 4-aminophenol in the presence of propylene
oxide in DMAc solvent resulted in dinitro compound 6 as a yellow
solid. In the second step, this dinitro was reduced in ethanol in the

140
S. Mallakpour, A. Zadehnazari / Journal of Solid State Chemistry 211 (2014) 136–145
Fig. 1. Presentation of possible interactions of hydrogen bonding between the
MWCNT-AS and the PAI chains.
This transformation should contribute positively to the good
dispersion of CNTs in the PAI matrix. Moreover, the introduction
of several functional groups into the backbone of aromatic poly-
mer performs a hydrogen bonding with modified CNTs and a
composite based on hydrogen bond by which PAI chains are
tightly attached to the surface of MWCNT-AS. Possible interactions
between PAI chains and MWCNT-AS are described in Fig. 1.
3.5. Characterization of MWCNTs and the composites
3.5.1. Structural data
The spectral analysis data indicated the successful incorpora-
tion of ascorbic acid onto MWCNT surfaces through esterification
reaction. Fig. 2 shows FT-IR spectra of the MWCNT-COOH and the
MWCNT-AS. The organic moieties of MWCNT-AS was character-
ized with the absorptions of the –OH groups at 3425 cmꢁ1, –CH
groups at 2926 cmꢁ1, –COO– groups at 1735 cmꢁ1, and –C–O–
groups at 1118 cmꢁ1 in its FT-IR spectrum. The spectrum of a
carboxylated MWCNTs/KBr pellet showed a strong, broad absorp-
tion band centered at 3433 cmꢁ1, which could be related to the
stretching vibration of O–H bands of carboxylic acid moieties from
the surface of MWCNTs. The small peak around 2923 cmꢁ1 was
ascribed to aliphatic sp3 C–H of MWCNTs [44]. Strong band of
absorption characteristic for the acid linkage appeared at
1629 cmꢁ1 assigned to CQO stretching vibration. The structure
of PAI was also confirmed by using FT-IR spectroscopy. Strong
absorption bands were observed at 1777 cmꢁ1. This can be related
to the asymmetric and symmetric stretching vibrations of the
imide carbonyl groups. The bands of C–N bond stretching and ring
deformation appeared at 1377 and 727 cmꢁ1. Strong bands of
absorption were characteristic of the hydroxyl groups and the
newly formed amide linkage appeared at around 3302 cmꢁ1. They
Fig. 2. FT-IR spectra of MWCNT-COOH, MWCNT-AS, PAI, and the composite
containing 15 wt% of MWCNT-AS.
9.26 ppm as a singlet peak, respectively, indicated the presence of
amide groups in the polymer’s side chain as well as main chain
and hydroxyl groups in the polymer’s side chain. The resonance of
aromatic protons appeared in the range of 6.73–8.45 ppm. The
proton of the chiral center appeared as doublet at 4.64–4.66 ppm.
3.5.2. XRD
To further study the structure of the MWCNT/PAI nanocompo-
sites, the XRD characterizations of MWCNTs, the pure PAI, and the
MWCNT/PAI nanocomposites with 5, 10, and 15 wt% of MWCNTs
content were conducted (see Fig. 3). In the case of MWCNTs, three
peaks appeared at 2θ¼261 and 441, which are typically associated
with diffraction metal impurities (2θ¼261 corresponds to the
(0 0 2) diffraction plane of the impurity graphite, and 2θ¼441 to
α-Fe (1 1 0) and/or Ni (1 1 1) diffractions [46]). MWCNTs-AS
showed very few changes in the XRD pattern. It could be seen
that XRD pattern is very similar to the carboxylated MWCNTs.
MWCNTs-AS still had the same cylinder wall structure as raw
MWCNTs and inter-planner spacing of all samples remained the
same. As for the pure PAI, an obvious broad peak centered at 201
indicates that PAI is amorphous. The XRD patterns of the MWCNT/
PAI composites appeared the both the characteristic peaks of the
pure PAI and MWCNTs. Moreover, the intensity of the peaks
assigned to the MWCNTs in the composite increased with the
increase of the content of MWCNTs. However, the position of the
peaks corresponding to the two constituents of the composite was
same to the individual of PAI and MWCNTs, which illustrated that
either the orientation of the PAI chains or the structure of
were assigned to O–H and N–H stretching vibrations at 1665 cmꢁ1
,
which can be attributed to amide CQO stretching vibration, and at
1544 cmꢁ1, they were due to N–H bending vibration. The absorption
band at around 3101 cmꢁ1 was attributed to QCH aromatic linkage.
The aliphatic C–H stretching peak was also appeared at around
2964 cmꢁ1 [45]. The presence of MWCNTs-AS in the polymer matrix
showed very few changes in the FT-IR spectrum, presumably due to
the low MWCNT-AS composition and the weak vibration signals of
MWCNTs. Fig. 2 shows the representative FT-IR spectrum of MWCNT-
AS/PAI 15 wt%.
The structure of neat PAI was also studied by 1H NMR spectro-
scopy. The presence of the –NH protons of amide groups at 10.18,
10.80, and 10.88 ppm as three singlet peaks, and –OH group at

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141
MWCNTs was not affected by each other along the process of
fabrication.
MWCNTs became a rough and debundled structure, as can be seen
in Fig. 4. FE-SEM images of neat PAI and composites were also
examined to characterize the morphology and dispersity of
MWCNTs-AS in the composites, as displayed in Fig. 5. The neat
PAI copolymer showed spherical particles which were self-
organized into a nanoscale size. The average diameter of polymeric
particles was about 36 nm. For MWCNT-AS/PAI composites,
MWCNTs-AS were well dispersed and embedded in the PAI matrix
without showing noticeable MWCNT-AS aggregation.
3.5.3. FE-SEM
The typical microstructure of the unmodified and modified
MWCNTs investigated by FE-SEM is presented in Fig. 4. The FE-
SEM image of the MWCNTs-COOH surface is approximately
smooth. After functionalization with ascorbic acid, the surface of
3.5.4. TEM
The TEM images of the MWCNT-AS are presented in Fig. 6. The
images show a high surface roughness which implies the partial
damage of graphitic carbon. This phenomenon could have resulted
from the severe functionalization or oxidation processes [47].
Although TEM could not distinguish minute functional groups, it
could represent surface deterioration of the CNTs that occurred as
a result of functionalization. As mentioned above, the functiona-
lization reaction disrupted the sp2 carbon network of graphitic
CNTs, so it may be responsible for the roughness of CNTs’ surfaces.
In fact, since the microwave radiation was harsh enough to
produce highly disordered carbon (Fig. 6), it is likely that it could
also damage graphitic MWCNTs [48]. Fig. 7 is the TEM observation
of the dispersion of the modified MWCNTs in the PAI matrix (mass
fraction of MWCNTs is 10 wt.%). It can be seen that the MWCNTs
were almost attached to the polymeric particles, resulting from the
increased polarity by the functional groups formed on the surfaces
of MWCNTs as well as carbonyl groups in the PAI structure.
Possible interactions between the carboxyl groups at the
MWCNT-AS with the oxygen atom at the carbonyl groups of the
PAI are shown in Fig. 1.
3.5.5. TGA
Fig. 8 gives the TGA curves of the nanocomposites, pure PAI,
and MWCNTs. The results obtained from these curves are com-
pletely tabulated in Tables 1 and 2. Single step degradation was
observed in all samples. The curve of pristine CNT showed a small
mass loss in a temperature range of 0–550 1C. The weight loss of
Fig. 3. XRD pattern for MWCNT-COOH, MWCNT-AS, and MWCNT-AS/PAI compo-
sites samples with different MWCNT-AS content.
Fig. 4. FE-SEM micrograph of MWCNT-COOH (top) and MWCNT-AS (bottom).

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Fig. 5. FE-SEM micrographs of: (a) neat PAI and composites containing (b) 5 wt%, (c) 10 wt%, and (d) 15 wt% of MWCNTs-AS.
Fig. 6. TEM images of MWCNT-AS at different magnifications.
MWCNT-AS resulted from the decomposition of the functionalized
organic moieties attached to the surface of MWCNTs. TGA data of
the pristine and the purified MWCNTs suggested that the MWCNTs
were able to withstand oxidation temperatures up to 700 1C. This
could be due to the incorporation of carboxyl groups in the
defective sites and tips. The initial decomposing temperature of
the MWCNT-COOH was 550 1C, indicating that the MWCNTs-
COOH were thermally stable up to this temperature, as shown in
Table 1. The modified MWCNTs decomposed slowly from 220 1C
because of the losing organic groups on the surface of MWCNTs. By
comparing the MWCNTs-AS with MWCNTs-COOH, it can be con-
cluded that the thermal stability of MWCNTs-COOH was destroyed
upon functionalization with ascorbic acid. The thermal behavior
data for the PAI and the composites are summarized in Table 2.
The thermal stability of the MWCNT/PAI composites increased
linearly with increasing MWCNT-AS content. This could be related
to the higher thermal conductivity of CNTs that facilitated heat
dissipation within the composites, hence preventing the accumu-
lation of heat at certain points for degradation [49]. The end
temperature of decomposition was also retarded with increasing
MWCNT-AS content. The weight percent remaining after major
degradation at 800 1C was higher for composites rather than neat
PAI. This indicated that MWCNT reduced the degradation of PAI at
high temperature as the effect was clearly seen in the curves.

S. Mallakpour, A. Zadehnazari / Journal of Solid State Chemistry 211 (2014) 136–145
143
Fig. 7. TEM micrographs of MWCNTs-AS/PAI composites containing 10 wt% of MWCNT-AS.
Table 2
Thermal properties of PAI and the composites.a
MWCNT-AS content (%) Td5 (1C) Td10 (1C) CR (%)b LOI (%)c ΔHcomb (kJ/g)d
a
a
0
5
10
15
315
340
374
403
354
368
405
437
43
47
52
58
34.7
36.3
39.1
41.5
23.0
22.0
20.4
19.3
a
Temperature at which 5 and 10 wt% loss was recorded by TGA at a heating
rate of 10 1C minꢁ1 in a nitrogen atmosphere.
b
Percentage weight of material left undecomposed after TGA analysis at
maximum temperature 800 1C in a nitrogen atmosphere.
c
Limiting oxygen index (LOI) evaluating at char yield at 800 1C.
Heat of combustion, calculated from LOI.
d
oxygen, so a material with an LOI of less than 21% would burn
easily in air. That being said, a material with LOI of greater than
21% but less than 28% would be considered “slow burning”.
A material with LOI of greater than 28% would be considered
“self-extinguising”. A self-extinguishing material is one that would
stop burning after the removal of the fire or ignition source [50].
Theoretically, two interesting relationships have been found
between the LOI and the parameters of the combustion process:
char yield or char residue (CR) and heat of combustion.
Fig. 8. TGA curves of the composites with different MWCNT-AS loading.
Table 1
Thermal stability of CNT samples obtained from TGA thermograms.a
According to Van Krevelen [51]. There is a linear relationship
between LOI and CR only for halogen-free polymers (Eq. (1)):
Sample
Td5 (1C)
Weight residue at 800 1C
LOI ¼ 17:5þ0:4CR
ð1Þ
MWCNT-COOH
MWCNT-AS
550
274
90
60
From this equation, a higher char yield will improve flame
retardance. PAI and composites, containing 5, 10, and 15 wt%, had
LOI values of 34.7, 36.3, 39.1, and 41.5, respectively, as calculated
from their CR. On the basis of the LOI values, such materials can be
classified as self-extinguishing materials.
a
Temperature at which 5 wt% loss was recorded by TGA at a heating rate of
10 1C minꢁ1 in a nitrogen atmosphere.
Therefore, it could be verified that a small amount of MWCNT acted as
effective thermal degradation resistant reinforcement in the PAI
matrix, increasing the thermal stability of the MWCNT/PAI composites.
The limiting oxygen index (LOI) is the minimum concentration
of oxygen, expressed as a percentage, which will support combus-
tion of a polymer. It can be used to evaluate the flame retardancy
of the polymers. Normal atmospheric air is approximately 21%
According to Johnson [52], the LOI values of many common
materials can be logically well predicted by the expression as follows:
LOI ¼ 8000=ΔHcomb
ð2Þ
where ΔHcomb is the specific heat of combustion in J/g. So, in the case
of PAI and MWCNT/PAI composites (5, 10, and 15 wt%), ΔHcomb is
23.0, 22.0, 20.4, and 19.3 kJ/g, respectively.

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S. Mallakpour, A. Zadehnazari / Journal of Solid State Chemistry 211 (2014) 136–145
the composites. This phenomenon has also been observed in other
CNTs reinforced polymer systems [53,54].
4. Conclusions
In an efficient, simple and low cost strategy involving the use of
microwave technology, covalent functionalization of MWCNTs
with ascorbic acid was used as a means of improving the state of
dispersion of MWCNTs in the polymer matrix. Functionalization
was followed by reaction with a carboxylic acid moiety allows
direct attachment by an ester bond. The occurrence of ascorbic
acid-functionalization of the nanotubes was confirmed by the use
of several methods usually used in materials science such as FT-IR,
XRD, FE-SEM, TEM, and TGA. The as-prepared MWCNTs-AS were
dispersed throughout a aminophenol and amino acid containing
PAI. Through ultrasonically-assisted solution casting of these
dispersions, MWCNT-AS/PAI composite films were successfully
fabricated on substrates. The attached ascorbic acid molecules
were expected to improve the interaction between the modified
MWCNTs and polymer molecule chains. The composites prepared
with combined functionalization or ultrasonic treatment showed
substantially increased mechanical and thermal properties with
increasing MWCNT content, indicative of good dispersion of
MWCNTs. The results of the tensile mechanical tests showed that
the tensile strength remarkably increased from 75 to 102.2 MPa,
which was about 36% higher than that of neat PAI, with the
addition of MWCNT-AS contents within 5 wt%. The decomposition
temperatures (Td5 and Td10) was also delayed with the increasing
MWCNT-AS content in the PAI matrix. The significant improve-
ments effects are due to the reinforcement of finely dispersed
MWCNTs fillers throughout the matrix as well as the strong
hydrogen interaction between carbonyl and hydroxyl groups on
the surface of MWCNTs-AS and functional groups in the PAI
structure, thus being favorable to stress transfer from polymer to
CNTs. The obtained results from TEM showed that the MWCNTs-
AS were wrapped around PAI particles and MWCNTs-AS were also
mixed well with the PAI matrix, leading to the fine dispersal of
MWCNTs-AS in the PAI matrix.
Fig. 9. Typical stress–strain curves of the composite films.
Table 3
Mechanical properties of MWCNT-AS/PAI composite films.
MWCNT-AS
content (%)
Tensile strength
(MPa)
Young’s modulus Elongation at
(GPa)
break (%)
0
5
10
15
75.070.2
102.270.2
127.5 70.2
143.170.3
2.170.1
2.970.1
3.570.1
4.070.1
7.370.4
5.970.3
5.070.5
3.670.3
Acknowledgments
3.5.6. Mechanical properties
Fig. 9 shows the tensile properties of the MWCNT/PAI compo-
sites. The tensile strength and the strain at tensile strength varied
depending on the amount of MWCNTs present (Table 3). The
physical interpretation for such an occurrence is that the MWCNTs
were homogenously oriented in the composite, and the fraction of
MWCNTs oriented in the direction of the tensile force applied was
not sufficient to have an impact on Young’s modulus. However, as
the sample was stretched, the CNTs are gradually aligned in the
direction under tensile testing and thus reinforce the material
against the fracture. The more MWCNTs were added to the
composite, the bigger was the reinforcement and the higher was
the strain. Furthermore, the incorporation of the ascorbic acid
molecules on the surface of MWCNTs and several functional
groups into the PAI matrix resulted in the good interfacial adhe-
sion between the MWCNT-AS and the PAI matrix. Therefore, the
improvement of the mechanical properties of the MWCNT/PAI
composites could be attributed to the better interfacial interaction
between the MWCNTs-AS and the PAI matrix as well as the better
dispersion of the MWCNTs-AS in the PAI matrix. The value in
102.2 MPa for the sample with 5 wt% MWCNTs-AS loading was
36.3% higher than that of the pure PAI. The elongation at the break
of composite films was decreased with the introduction of
MWCNT, indicating that the composites became somewhat brittle
when compared with pure PAI because of the increased stiffness of
We are grateful to the Research Affairs Division of Isfahan
University of Technology (IUT), National Elite Foundation (NEF),
and Center of Excellency in Sensors and Green Chemistry
Research (IUT).
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