Investigating the nanostructure and thermal properties of chiral poly(amide-imide)/Al2O3 compatibilized with 3-aminopropyltriethoxysilane

Article abstract of DOI:10.1016/j.materresbull.2013.05.095

Novel chiral poly(amide-imide) (PAI)/Al₂O₃ nanocomposites were prepared via incorporating surface modified Al ₂O₃ nanoparticles into polymer matrices for the first time. In the process of preparing the nanocomposites, severe aggregation of Al₂O₃ nanoparticles could be reduced by surface modification and γ-aminopropyltriethoxysilane. The optically active PAI chains were formed from the polycondensation reaction of N,N′- (pyromellitoyl)-bis-phenylalanine diacid with 2-(3,5-diaminophenyl)- benzimidazole in green condition. The obtained polymer and inorganic metal oxide nanoparticles were used to prepare chiral PAI/Al₂O₃ nanocomposites through ultrasonic irradiation. The resulting nanoparticle filled composites were also characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis (TGA) techniques. TGA thermographs confirmed that the heat stability of the prepared nanoparticle-reinforced composites was improved. Mechanical properties showed that the film containing 10 wt% of modified Al₂O₃ had a tensile strength of the order of 83.6 MPa, relative to the 64.3 MPa of the pure PAI.

Full text of DOI:10.1016/j.materresbull.2013.05.095

Materials Research Bulletin 48 (2013) 3865–3872
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Investigating the nanostructure and thermal properties of chiral
poly(amide-imide)/Al2O3 compatibilized with
3-aminopropyltriethoxysilane
a,b
Shadpour Mallakpour a,b, , Mohammad Dinari
*
a Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran
b Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Novel chiral poly(amide-imide) (PAI)/Al2O3 nanocomposites were prepared via incorporating surface
modified Al2O3 nanoparticles into polymer matrices for the first time. In the process of preparing the
nanocomposites, severe aggregation of Al2O3 nanoparticles could be reduced by surface modification
Received 13 January 2013
Received in revised form 30 April 2013
Accepted 30 May 2013
and g-aminopropyltriethoxysilane. The optically active PAI chains were formed from the polyconden-
Available online 6 June 2013
sation reaction of N,N0-(pyromellitoyl)-bis-phenylalanine diacid with 2-(3,5-diaminophenyl)-benz-
imidazole in green condition. The obtained polymer and inorganic metal oxide nanoparticles were used
to prepare chiral PAI/Al2O3 nanocomposites through ultrasonic irradiation. The resulting nanoparticle
filled composites were also characterized by Fourier transform infrared spectroscopy, X-ray diffraction,
field emission scanning electron microscopy, transmission electron microscopy and thermogravimetric
analysis (TGA) techniques. TGA thermographs confirmed that the heat stability of the prepared
nanoparticle-reinforced composites was improved. Mechanical properties showed that the film
containing 10 wt% of modified Al2O3 had a tensile strength of the order of 83.6 MPa, relative to the
64.3 MPa of the pure PAI.
Keywords:
A. Composites
A. Nanostructures
B. Chemical synthesis
C. Thermogravimetric analysis (TGA)
D. Mechanical properties
ß 2013 Elsevier Ltd. All rights reserved.
1. Introduction
From a materials science and engineering point of view,
aluminum oxide (Al2O3) is one of the most widely used ceramic
The field of nanocomposite (NC) materials has been widely
recognized as one of the most promising and rapidly emerging
research areas [1]. Nanostructured organic–inorganic composites,
mixed at the molecular level or near molecular level, are much
different from the conventional composites with the integration of
a variety of additives in the polymer matrices [2–4]. In inorganic–
organic NCs, strong chemical bonds (covalent or ionic) or
interactions such as van der Waals forces, hydrogen bondings,
or electrostatic forces, often exist between the organic and
inorganic components [5]. This usually leads to some novel NCs
with improved performance properties that may have large
potential applications in the fields of optics, electrical devices,
mechanics, photoconductors, etc. [6–8]. The challenge in this area
of organic–inorganic hybrid materials is to obtain significant
enhancements in the interfacial adhesion between the polymer
matrix and the reinforcing material since the organic matrix is
relatively incompatible with the inorganic phase [9].
materials due to its superior properties such as high thermal
conductivity, high dielectric constant, high mechanical strength
and high thermal stability [10]. However, due to its huge surface
areas and large surface free energy, nano-Al2O3 particles aggregate
with each other easily [11]. To utilize the specific characteristics of
nanoparticles (NP)s more effectively, it is, therefore, necessary to
explore a new method to break down NP agglomerates or modify
the weak structures of NP agglomerates. In most cases, there are
two ways to modify the surface of inorganic particulates [12]. The
first one is the functionalization of the inorganic particles by
coupling agents to ensure, for instance, high compatibility with a
polymer matrix, or to modify the hydrophobic/hydrophilic balance
of the NPs [13,14]. The second method is based on grafting
polymeric molecules through covalent bonding to the hydroxyl
groups existing on the particles [15].
High performance polymer materials are used increasingly for
engineering applications under hard working conditions [16]. The
materials must provide unique mechanical and tribological
properties combined with a low specific weight and a high
resistance to degradation in order to ensure safety and economic
efficiency. Among them, aromatic polyimides (PI)s, as commer-
cialized heterocyclic polymers, have exhibited remarkable com-
prehensive characters, such as excellent thermal stability,
* Corresponding author at: Organic Polymer Chemistry Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,
Islamic Republic of Iran. Tel.: +98 311 391 3267; fax: +98 311 391 2350.
E-mail addresses: mallak@cc.iut.ac.ir, mallak777@yahoo.com,
mallakpour84@alumni.ufl.edu (S. Mallakpour).
0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2013.05.095

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S. Mallakpour, M. Dinari / Materials Research Bulletin 48 (2013) 3865–3872
balanced mechanical and electrical properties, as well as good
chemical and irradiation resistance [17–19]. Because of these
properties, they have been extensively used in aerospace as high-
performance engineering materials and in electronic industry as
interlayer dielectrics [20]. One of the problems with high-
performance PIs is their poor solubility or processability: they
do not react below their decomposition temperature; this restricts
their applications as engineering materials. Many efforts have been
made to improve their processability while maintaining their
excellent thermal and mechanical properties. For example, bulky
lateral substituents, flexible alkyl side chains, noncoplanar
biphenyl moieties, and flexible alkyl or aryl ether spacers have
been used to enhance the solubility and thus, processability [21–
25].
Polybenzimidazole has been widely used in aerospace industry
in recent years as high-performance polymers, because it has a
high limited oxygen index and possesses the outstanding ability to
withstand extreme condition without an extensive loss of
properties [26,27]. Apart from the other outstanding properties,
polybenzimidazole contains higher strength and modulus values
than PIs. It has been considered by many research groups to
develop a new promising system of high-performance materials by
combining the advantages of PIs and polybenzimidazoles. Many
efforts have been devoted to the preparation and development of
polymer blends. It should be noted that on heating above the blend
Tg, phase separation readily takes place. There are also some
studies on bonding benzimidazole in PIs, but they focus on taking
them as proton conductivity membrane [28,29]. The incorporation
of benzimidazole group into PI backbone is likely to improve the
solubility without deterioration of their own excellent properties
[29,30].
methanesulfonic acid were obtained from commercial sources
and used as-received. TBAB, -phenylalanine, and pyromellitic
dianhydride (PMDA) were purchased from Merck Co. and used
without further purification. N,N-dimethylformamide (DMF) and
N,N-dimethyacetamide (DMAc) were dried over barium oxide and
then distilled under reduced pressure. Nanosized a-Al2O3 powder
was purchased from Neutrino Co. (Iran) with an average particle
size of 10–25 nm.
Inherent viscosities were measured by a standard procedure
using a Cannon–Fenske routine viscometer (Germany) with the
concentration of 0.5 g/dL at 25 8C. Specific rotations were
measured by a Jasco Polarimeter (Japan).
FT-IR spectra of the hybrid films were recorded with a Jasco-680
(Japan) spectrometer at a resolution of 4 cmꢀ1 in the range of 400–
4000 cmꢀ1. Vibration bands were reported as wavenumber (cmꢀ1).
FT-IR spectra of PAI and NCs were also collected by making their
pellets in KBr as a medium. The band intensities were assigned as
weak (w), medium (m), shoulder (sh), strong (s), and broad (br).
The diffraction pattern of related materials was recorded in the
reflection mode using a Bruker, D8 Advance diffractometer. Nickel
L
filtered CuKa radiation (radiation wavelength,
l = 0.154 nm) was
produced at an operating voltage of 45 kV and a current of 100 mA.
All the measurements were carried out between 2u = 5–1008 with
a step size of 0.028.
TGA was performed with a STA503 TA instrument at a heating
rate of 10 8C/min from 25 8C to 800 8C under nitrogen atmosphere.
The dispersion morphology of the NPs on PAI matrix was
observed using FE-SEM (HITACHI S-4160) and TEM (Philips CM
120) techniques.
Tensile properties of the NC films were measured according to
DIN procedure 53455 with a crosshead speed of 5 mm minꢀ1 using
Zwick 1446-60.
High performance chiral polymers have attracted a great deal of
interest in fields such as dentistry, drug delivery and tissue
engineering due to their promising properties [31,32]. As an
important class of optically active polymers, PAI has received an
increasing amount of attention owing to their chemical and
mechanical properties [33,34]. The uses of optically active
monomers by employing natural amino acids have been of great
interest. The presence of amino acid in the polymer chains as a
chiral group capable of inducing optical activity into polymer is
very useful in the pharmaceutical industry for enantioselective
separation of drugs [35,36].
2.2. Surface treatment of Al2O3 NPs
The silane coupling agent (KH550) was pre-hydrolyzed before
using it by adding concentrated hydrochloric acid (12 M) to the
ethanol solution of KH550 gradually until the pH value of the
solution became between 4 and 5. Nano Al2O3 was dried at 120 8C
in an oven for 24 h to remove the adsorbed water. These particles
were dispersed in absolute ethanol and heated at 70–75 8C in a
water bath for 2 h. Then an appropriate amount (15 wt%) of pre-
hydrolyzed KH550 was added to the Al2O3 suspension under
ultrasonic agitation. The mixture was then stirred mechanically for
another 4 h, followed by heating to 100 8C for 16 h to remove the
ethanol.
The aim of the present study was to use the modified a-Al2O3 to
prepare the novel NC of PAI/Al2O3. The Al2O3 NPs were treated with
coupling agent of 3-aminopropyltriethoxylsilane to introduce
organic functional groups on the surface of Al2O3-NPs and prevent
their aggregation. The soluble and chiral PAI enclosing phenylala-
nine amino acid was synthesized during direct polymerization
reaction of chiral diacid and 2-(3,5-diaminophenyl)benzimidazole
under green condition using molten tetra-butylammonium
bromide (TBAB). Novel NCs of PAI/Al2O3 were synthesized under
ultrasonic irradiation conditions and characterized by Fourier
transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD),
field emission scanning electron microscopy (FE-SEM), transmis-
sion electron microscopy (TEM), thermogravimetric analysis (TGA)
and mechanical testing. The results showed that the NPs were
uniformly dispersed on the surface of PAI matrix.
2.3. Synthesis of chiral diacid monomer
Chiral N,N0-(pyromellitoyl)-bis-
L-phenylalanine diacid (1) was
prepared according to our previous works [37].
2.4. Synthesis of 2-(3,5-diaminophenyl)-benzimidazole
At first, the corresponding dinito was synthesized from the
reaction of 1,2-phenylenediamine and 3,5-dinitrobenzoyl chloride
using methanesulfonic acid and P2O5 as condensation media. Then,
2-(3,5-diaminophenyl)-benzimidazole (2) was fabricated by the
reduction of dinitro precursor using palladium on activated carbon
(Pd/C) and hydrazine monohydrate in the refluxed ethanol (yield:
77%), melting point: 242–243 8C [37,38].
2. Experimental
2.1. Materials and measurements
Chemicals and solvents were obtained from the Aldrich
Chemical Co. (Milwaukee, WI, USA) and Merck Chemical Co.
(Germany). 3,5-Dinitrobenzoyl chloride, 1,2-phenylenediamine,
hydrazine monohydrate, phosphorus pentoxide (P2O5), and
2.5. Synthesis of chiral PAI
PAI was prepared using triphenyl phosphite (TPP)/TBAB as an
activator by the following procedure:
a mixture of 0.10 g

S. Mallakpour, M. Dinari / Materials Research Bulletin 48 (2013) 3865–3872
3867
(2.36 ꢁ 10ꢀ4 mol) of diacid 1, 0.06 g (2.36 ꢁ 10ꢀ4 mol) of diamine
3. Results and discussion
2, and 0.30 g (9.42 ꢁ 10ꢀ4 mol) of TBAB was ground until a
powder was formed. Then it was transferred into a 25-mL flask
fitted with a mechanical stirrer. TPP (0.49 mL, 3.70 ꢁ 10ꢀ3 mol)
was added and then, the reaction temperature was raised to
120 8C and held for 12 h. On cooling to room temperature, the
viscous polymer solution was dropped into the 20 mL of
methanol, giving rise to 0.14 g of PAI (91% yield). The inherent
viscosity of the resulting PAI was obtained to be 0.58 dL/g
(measured at a concentration of 0.5 g/dL in DMF at 25 8C). The
3.1. Modification of Al2O3-NPs
Silane coupling agents are usual ligands for surface modifica-
tion of oxide NPs. These bifunctional molecules include a trialkoxy
group and organic head group functionality (e.g., amino, glycidyl,
mercapto, vinyl). The trialkoxy group can bind covalently to the
free –OH groups at the surface of the oxide NPs, while the organic
head group determines the final chemical character of the
functionalized surface. In this work, the surface of Al2O3-NPs
was treated by KH550 as a silane coupling agent by using the
sonochemical reaction that supplied suitable reaction temperature
specific rotation was [
tion of 0.5 g/dL in DMF at 25 8C).
a
]
25 = ꢀ78.27 (measured at a concentra-
D
FT-IR peaks (KBr, cmꢀ1): 3479 (br), 3165 (w), 3038 (w), 2937
(m), 1771 (s), 1720 (s), 1634 (m), 1602 (w), 1498 (m), 1456 (m),
1385 (s), 1366 (s), 1258 (w), 1226 (m), 1170 (m), 1079 (m), 1045
(w), 941 (s), 918 (m), 881 (m), 828 (m), 733 (s), 700 (s), 631 (m),
568 (s) and 492 (w).
to accelerate the hydrolysis of coupling agent. Scheme
demonstrates the reaction mechanism.
1
3.2. Preparation of monomers, neat PAI and PAI/Al2O3 NCs
2.6. Preparation of the PAI/Al2O3 NCs
Chiral diacid 1 was prepared by the condensation reaction of
one equimolar of PMDA and two equimolars of -phenylalanine
L
The PAI/Al2O3 NCs were synthesized via mixing the PAI with
different amounts of modified Al2O3-NPs (5, 10 and 15 wt%) in
20 mL of absolute ethanol followed by irradiation with high-
intensity ultrasonic wave for 4 h. The solvent was removed and the
obtained solid was dried in vacuum at 80 8C for 2 h. Also, in order to
investigate the mechanical properties of the hybrid materials, the
above solid NCs were dissolved in DMAc and then thin films with
uniform thickness of these materials were prepared by pouring the
hybrid solutions into Petri dishes, followed by solvent evaporation
at high temperature. These films were further dried at 80 8C under
the reduced pressure to a constant weight.
amino acid in refluxing acetic acid, as shown in Scheme 1.
Diamine 2 was prepared via the reduction of dinitro precursors
by using hydrazine hydrate as the reducing agent and palladium on
the activated carbon as the catalyst. By this procedure, pure
diamine 2 was attained after recrystallization (Scheme 1).
Because of the rapid emergence of ILs as green solvents and
replacement of
a volatile and toxic organic solvent in the
polymerization with a nonvolatile solvent, in this study, chiral
PAI3 was synthesized in green route by using molten TBAB as an IL
media. Phosphorylation polymerization technique has been suc-
cessfully applied to synthesize new high molecular weight
Scheme 1. Modification of Al2O3 NPs, preparation of chiral PAI3 and the interaction between the polymers and the modified Al2O3 NPs.

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S. Mallakpour, M. Dinari / Materials Research Bulletin 48 (2013) 3865–3872
organosoluble PAI by the direct polycondensation of imide-contain-
ing dicarboxylic acid 1 with aromatic diamine 2 (Scheme 1). The
inherent viscosity of the PAI under optimized condensations was
0.58 dL/g, and the yield was 91%. The specific rotation was
[
a D
]
25 = ꢀ78.27 (measured at a concentration of 0.5 g/dL in DMF
at 25 8C). But for PAI based NCs, because of the suspension of the
inorganic particles of the Al2O3, it is not possible to measure the
optical activity of the NCs, which was induced by chiral amino acids.
Thin NC films of uniform thickness were obtained by pouring the
hybrid solutions into Petri dishes, which was followed by solvent
evaporation at a high temperature. These films were further dried at
80 8Cunder the reduced pressuretoa constantweight. Scheme 1also
displays the details of the fabrication mechanism for PAI/Al2O3 NCs
under ultrasonic irradiation.
3.3. Structure characterizations
3.3.1. FT-IR spectra
The structure of chiral PAI was confirmed by FT-IR, 1H NMR
spectroscopy and elemental analysis techniques as mentioned in
our previous paper [39]. The results of elemental analyses were
also in good conformity with calculated percentages of carbon,
hydrogen and nitrogen contents in the polymer repeating unit.
The FT-IR spectra of pure Al2O3 NPs and KH550-modified-Al2O3
Fig. 2. FT-IR spectra of (a) PAI, (b) PAI/Al2O3 NC (5%), (c) PAI/Al2O3 NC (10%) and (d)
PAI/Al2O3 NC (15 wt%).
band consisting of the amidic C55O stretching and the amide II band
consisting of the N–H bending appeared at 1684 and 1599 cmꢀ1
,
are presented in Fig. 1. In addition to the band at 300–900 cmꢀ1
which is due to the Al–O bond, the FT-IR spectrum of the
unmodified Al2O3-NPs displayed absorption band at 1640 cmꢀ1
,
respectively, and the absorption band at 3384 cmꢀ1 corresponded
to the stretching vibration of amidic N–H. The presence of bands at
1776 (asymmetry C55O), 1727 (symmetry C55O), 1366, 1062, and
727 cmꢀ1 in the FT-IR spectra confirmed the presence of the imide
heterocyclic ring in this polymer. These FT-IR data confirm the
formation of the aromatic PAI. Fig. 2 also shows the FT-IR spectra of
NC5%, NC10% and NC15%, respectively. The incorporation of Al2O3-
NPs in PAI caused the slight changes in the intensities of absorption
bands. The formation of new absorption bands in the range of 300–
900 cmꢀ1 could be attributed to the Al–O stretching. This
confirmed the existence of Al2O3-NPs in the polymer matrix.
While the loading of the Al2O3-NPs was increased, the intensities of
Al–O bands became more obvious. From these spectra, it can be
concluded that the NCs not only have characteristic neat PAI bands,
but also they have characteristic peaks for Al2O3-NPs.
.
An extraordinarily broad pea observed at 3430 cmꢀ1 was assigned
to surface hydroxyl groups. In the spectrum of KH550-function-
alized Al2O3-NPs, the bands at 800–1100 and 2800–3000 cmꢀ1
belonged to the stretching vibration of the Si–O–Al bond and the
C–H bond, respectively. However, the asymmetric and symmetric
stretching vibrations of the N–H bond, located at wavenumbers
3377 and 3292 cmꢀ1 on the spectrum of KH550, were not seen,
possibly owing to the overlap with the stretching vibration of the
hydroxyl group. Such results indicate that the covalent bonds were
formed by the chemical reaction between the hydroxyl groups on
the surface of Al2O3-NPs and KH550. In general, KH550 molecules
form a chemical bond to the metal oxide surface, which is thought
to progress via hydrolysis of one or more alkoxy groups, followed
by condensation with the surface hydroxyl groups.
Fig. 2a–d presents the FT-IR spectra of pristine PAI, NC5%,
NC10% and NC10%. Neat PAI showed absorptions around
3.3.2. XRD patterns
The crystallinity of the PAI/Al2O3 BNC films was studied via the
XRD test. The XRD patterns of neat PAI, pure a-Al2O3, modified a-
3360 cmꢀ1
,
3100 cmꢀ1
,
2930 cmꢀ1 for N–H of amide and
benzimidazole, aromatic C–H stretching and CH2 stretching,
respectively. This PAI contained amidic groups. So the amide I
Al2O3 with silane coupling agent KH550, NC5%, NC10% and NC15%
are shown in Fig. 3a–f. In the neat PAI, it could be observed that
there is a lack of any diffraction peak in the range of 2u angle for
PAI. XRD results showed an amorphous diffraction pattern. The
XRD pattern of the modified Al2O3 NPs with silane coupling agent
KH550 showed a good agreement with the reference XRD pattern
(42-1468 JCPDS file). According to Fig. 3b and c, there is no
difference between XRD patterns of the pure Al2O3 and the
modified Al2O3 and the silane coupling agent do not have any effect
on crystallinity of Al2O3 NPs. Meanwhile, in the PAI/Al2O3 NCs, the
diffraction peaks of
diffraction patterns similar to those of modified
indicates that the -Al2O3 remains stable after being doped into
a
-Al2O3 were unchanged at 2u, showing
a
-Al2O3. This
a
the polymer matrix. However, all the diffraction peaks were
broadened. These characteristic peaks are assigned in Fig. 3d–f.
The average crystalline size of Al2O3 NPs, determined from the
half width of the diffraction using the Debye–Scherrer equation
(D = K
size,
the full width at half maximum. According to the XRD patterns of
NCs, the peak intensity at 2 = 19.58 for PAI is reduced when
surface Al2O3-NPs are added. This indicates that the crystallinity of
l
/
b
cos
u
), is approximately 27 nm, where D is the crystallite
l
is wavelength of the radiation,
u
is the Bragg’s angle and is
b
u
Fig. 1. FT-IR spectra of (a) Al2O3 and (b) KH550-modified-Al2O3 NPs.

S. Mallakpour, M. Dinari / Materials Research Bulletin 48 (2013) 3865–3872
3869
PAI matrix was reduced by the addition of the modified Al2O3
nanofillers.
3.3.3. FE-SEM study
The morphology of the PAI/Al2O3 NCs loaded with different
weight fractions of Al2O3-NPs was investigated with FE-SEM
micrographs. The FE-SEM images of pure PAI, PAI/Al2O3 NC (5 wt%)
and PAI/Al2O3 (15 wt%) are exhibited in Fig. 4. According to these
photographs, it was observed that Al2O3 NPs were homogenously
dispersed in the PAI matrix, because the modified Al2O3 nanofiller
had excellent adhesion and strong interfacial bonding to the PAI
beads. In addition, the results demonstrated that the structure of
the prepared hybrid films of NC5% was more compact and uniform
than that of NC15%.
3.3.4. TEM study
TEM is a supplementary employed as an effective means of
developing insights into the internal structure and spatial
distribution of the various components through direct visualiza-
tion. Fig. 5 shows the morphology obtained by TEM micrographs of
the neat Al2O3, KH550-modified Al2O3, PAI/Al2O3 NC (5 wt%) and
PAI/Al2O3 NC (15 wt%). According to Fig. 5a, because of high surface
energy, neat Al2O3 shows some aggregations. In order to prevent
the NPs aggregations, in this study, KH550 was used to modify the
surface of Al2O3 NPs via ultrasonic energy. In this way, the hydroxyl
groups on the surface of Al2O3 were replaced or reacted with
Fig. 3. XRD patterns of (a) pure PAI, (b) neat
a-Al2O3, (c) modified a-Al2O3, (d) PAI/
Al2O3 NC (5 wt%), (e) PAI/Al2O3 NC (10 wt%) and (f) PAI/Al2O3 NC (15 wt%).
Fig. 4. FE-SEM images of (a and b) pure PAI, (c and d) PAI/Al2O3 NC (5 wt%) and (e and f) PAI/Al2O3 NC (15 wt%).

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S. Mallakpour, M. Dinari / Materials Research Bulletin 48 (2013) 3865–3872
Fig. 5. TEM images of (a) neat
a-Al2O3, (b) modified
a-Al2O3, (c) PAI/Al2O3 NC (5 wt%) and, (d) PAI/Al2O3 NC (15 wt%).
OCH2CH3 of the KH550 to bond to it. By this method, the organic
chains of KH550 caused steric hindrance between Al2O3 NPs and
prevented their aggregation (Fig. 5b). TEM images of NC5% and
NC15% demonstrated a homogeneous nanostructure and con-
firmed that PAI/Al2O3 NCs were in nanoscale size (Fig. 5c and d).
The obtained results indicated that the coupling agent played an
important role in the dispersion of the NPs. The relatively strong
interactions between PAI matrix and Al2O3-NPs were responsible
for observing NPs with almost spherical shapes. For coupling agent
KH550, functional group that provided the interaction with PAI
matrix was the amino group that could interact with the amide
group (C55O), and –NH of benzimidazole group in the PAI via
hydrogen bonding. The NPs size ranged from 20 to 35 nm, showing
a rather spherical shape.
plays a very important role in the materials’ ability in transferring
stresses and elastic deformation from the matrix to the fillers. This
is especially true for NCs, because they impart a high portion of
interface. If filler matrix interaction is poor, the particles are unable
to carry any part of the external load; instead, if the bonding
between fillers and matrix is strong enough, the yield strength of a
particulate composite can be higher than that of the matrix
polymer [40]. Hence, the gradual increase in stiffness and flexural
strength, as observed for the NCs, reveals that stresses are
efficiently transferred via the interface. This is because NPs have
high specific surface areas and high surface energy because of their
3.4. Mechanical properties
The stress–strain curves of pure PAI and NC materials with 5, 10
and 15 wt% of Al2O3 NPs are shown in Fig. 6. The NCs were found to
be stiffer than the neat polymer and the elastic modulus was
increased with increasing the amount of Al2O3 NPs. The maximum
modulus value was observed with 10 wt% Al2O3 in the polymer
matrix, providing more stiffness to the materials, since stiffness is a
function of the aspect ratio. The maximum stress at break (ultimate
strength) was found to increase initially with the increase in Al2O3
NPs content, and at 10 wt% of Al2O3 NPs, it showed a maximum
value of 85.6 MPa (relative to the 61.8 MPa of the neat PAI),
representing
a significant enhancement in tensile strength.
Nevertheless, if the fillers exceed 15 vol.%, the failure strain
undergoes a slight decay. Some important characteristics of
composites have to be considered in order to explain this
phenomenon. The quality of the interface in composites, i.e. the
static adhesion strength as well as the interfacial stiffness, usually
Fig. 6. Stress versus strain curves of (a) pure PAI, (b) PAI/Al2O3 NC (5 wt%), (c) PAI/
Al2O3 NC (10 wt%), and, (d) PAI/Al2O3 NC (15 wt%).

S. Mallakpour, M. Dinari / Materials Research Bulletin 48 (2013) 3865–3872
3871
reacted with the PAI principal chain, forming some coordinate
bonds, such as hydrogen bonds. These coordinate bonds limited
the thermal motion of PAI molecular, prevented the breakdown of
the polymer molecular chain, enhanced the breaking energy
during the heating process, and improved the thermal stability of
PAI/Al2O3 films.
4. Conclusions
In this work, for the first time, a series of novel PAI/Al2O3
hybrids were prepared through ultrasonic-assistant method. By
effective surface modification of Al2O3-NPs using silane coupling
agent (KH550), the Al2O3-NPs were uniformly dispersed into the
PAI matrices without aggregation. The use of coupling agent could
effectively improve the compatibility and the homogenous
dispersion of Al2O3-NPs in PAI matrix. Organosoluble and
thermally stable chiral PAI was successfully synthesized by a
simple efficient and green approach using oil bath heating in
conjunction with molten TBAB as a green solvent. The reaction
proceeded efficiently with IL/TPP as the condensing agent without
the need for any additional promoters, which are necessary upon
utilizing traditional organic solvents. TEM and FE-SEM analysis
showed dispersion of Al2O3-NPs with average particle sizes of
about 20–35 nm in the polymer matrix. The TGA curves showed
that the addition of Al2O3-NPs into the NCs could enhance the
thermal stability of samples. The presence of amino acid in PAI
architecture not only caused them to be used as optically active
materials, but also made them turn in to a potentially biocompati-
ble and biodegradable compound. In addition, the resulting NCs
showed good thermal stability due to the presence of the amide
and heterocyclic imide groups in the main chain of the polymer
matrix. They may have potential to be used as a chiral medium in
asymmetric synthesis and also as chiral stationary phase in
chromatographic technique for the separation of racemic mixtures.
According to the mechanical properties and the TEM results, by
homogeneous distribution of NPs within the matrix, the more
Al2O3-NPs are in contact with the polymer, the higher is the
probability of interactions. Large contact areas benefit an efficient
stress transfer between both parts, resulting in improved flexural
stiffness, strength and failure strain.
Fig. 7. TGA thermograms of (a) PAI/Al2O3 NC (15 wt%), (b) PAI/Al2O3 NC (10 wt%), (c)
PAI/Al2O3 NC (5 wt%) and (d) pure PAI.
small scales. They can react with macromolecular chains to
enhance the interactions between macromolecular chains when
added to polymer. Besides, the presence of NPs inhibits the
movement of macromolecular chains, enhancing the stiffness of
macromolecular chains [41].
According to Fig. 6, the strain at break usually declines with
rising filler content. Low filler loadings can already cause a
dramatic drop in the fracture strain. Particles may induce matrix
yielding under certain conditions, further acting as stoppers to
crack growth by pinning the cracks [42]. In sum, as the Al2O3
content in the polymer increases, the stress level gradually
increases but at the same time, the strain of the NCs decreases.
3.5. Thermal properties investigation
The thermal properties of neat PAI and hybrid materials were
studied by means of TGA at a heating rate of 10 8C/min under a
nitrogen atmosphere. Fig. 7 and Table 1 exhibit the respective TGA
profiles and the corresponding thermoanalysis data, including the
temperatures at which 5% (T5) and 10% (T10) degradation occurs
and char yield at 800 8C. The pure PAI gave the decomposition
temperatures of 415 8C at 5% weight loss under a nitrogen
atmosphere. However, with increasing Al2O3 composition, the T5%
was enhanced to 453, 476, 493 8C, corresponding to Al2O3 content
of 5, 10 and 15%, respectively. The char yields at 800 8C of the NCs
with different Al2O3 contents are higher than those of pure PAI. As
can be observed from Table 1, PAI showed 54% residue at 800 8C
while the NCs showed 58–67% residue at this temperature. It is
worth pointing out that the thermal stability of PAI was enhanced
with increasing modified Al2O3 contents. This enhancement in the
char formation can be ascribed to the high heat resistance exerted
by the Al2O3, because the Al2O3-NPs have high thermal stability. So
the incorporation of Al2O3-NPs can improve the thermal stability of
the NCs. In addition, the Al2O3 is a nanoscale particle, offering a
larger surface area and improving the effect of thermal cover. It is
based on this fact that the coupling agents in the Al2O3-NPs may be
Acknowledgements
We gratefully acknowledge the partial financial support from
the Research Affairs Division of Isfahan University of Technology
(IUT), Isfahan. The partial support of Iran Nanotechnology
Initiative Council (INIC), National Elite Foundation (NEF) and
Center of Excellency in Sensors and Green Chemistry (IUT) is
gratefully appreciated. We also thank Dr. H. Jalali for the English
editing.
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