Reinforcement of poly(amide-imide) containing N-trimellitylimido-L-phenylalanine by using nano α-Al2O3surface-coupled with bromo-flame retardant under ultrasonic irradiation technique

Article abstract of DOI:10.1016/j.molstruc.2014.06.081

By the uniform dispersion of nanoparticles into a polymer matrix, a substantial improvement of physicochemical properties can be attained. In this study, a series of poly(amide-imide)/Al₂O₃nanocomposites (PANC)s based on various amou

Full text of DOI:10.1016/j.molstruc.2014.06.081

Journal of Molecular Structure 1075 (2014) 196–203
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Reinforcement of poly(amide–imide) containing N-trimellitylimido-L-
phenylalanine by using nano a-Al2O3 surface-coupled with bromo-flame
retardant under ultrasonic irradiation technique
a
Shadpour Mallakpour a,b,c, , Elham Khadem
⇑
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
c Center of Excellence in Sensors and Green Chemistry, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
A straightforward and fast method
was used for the surface modification
of ANPs.
ꢀ
ꢀ
ꢀ
Flame retardant THCB was utilized for
the modification of ANPs.
Modified ANP was improved the
thermal properties of PANCs.
TEM and FE-SEM images proved that
ANPs were dispersed in the PAI
matrix.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2014
Received in revised form 25 June 2014
Accepted 25 June 2014
Available online 2 July 2014
By the uniform dispersion of nanoparticles into a polymer matrix, a substantial improvement of physico-
chemical properties can be attained. In this study, a series of poly(amide–imide)/Al2O3 nanocomposites
(PANC)s based on various amounts of modified
a-Al2O3 nanoparticles (ANP)s were prepared using the
ultrasonic irradiation method. In the process of manufacturing the nanocomposites (NC)s, severe agglom-
eration of ANPs into the polymer matrix can be reduced using 2,3,4,5-tetrabromo-6-[(4-hydroxy-
phenyl)carbamoyl]benzoic acid as novel coupling agent. The effects of modified ANPs on the
morphology and properties of the polymer matrix were studied by means of Fourier transform infrared
spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron
microscopy and thermal gravimetric analysis (TGA). The results obtained by TGA showed that the ther-
mal stability of the NCs was improved with the addition of the small amounts of ANPs as effective ther-
mal degradation resistant reinforcement.
Keywords:
Nanocomposite
Surface modification
Flame retardant
Poly(amide–imide)
Thermogravimetric analysis (TGA)
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
Aromatic poly(amide–imide) (PAI) is one of the most class of
⇑
Corresponding author at: Center of Excellence in Sensors and Green Chemistry,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,
Islamic Republic of Iran. Tel.: +98 31 3391 3267; fax: +98 31 3391 2350.
E-mail addresses: mallak@cc.iut.ac.ir, mallak777@yahoo.com, mallakpour84@
alumni.ufl.edu (S. Mallakpour).
alternative polymers with high strength and good solubility in
polar amide-type solvents, which provide suitable accordance
between processability and high thermal stability. They bring
together both the advantages of polyamide and polyimide such
http://dx.doi.org/10.1016/j.molstruc.2014.06.081
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

S. Mallakpour, E. Khadem / Journal of Molecular Structure 1075 (2014) 196–203
197
as heat-resistant and good mechanical properties, easy processing
and solvent resistance characteristic. Thus, they can be widely used
in the numerous industries [1–3]. One of approaches to enhance-
ment of the mechanical property, the thermal and chemical stabil-
ity of PAI is the preparation of their composite ceramic materials [4].
The incorporation of an appropriate amount of nanosized mate-
rial into the polymer matrix has gained significant physicochemi-
cal properties, which is not possible for conventional composites
with micro or even macro scale fillers [5]. One well-known charac-
teristics of nanoparticles (NP)s is the high specific surface area
(total surface area of a material per unit of mass), which differen-
tiates them from microparticles, and is essential for the enhance-
ment of performance activity [6,7].
In nanocomposites (NC)s, strong chemical bonds (covalent or
ionic) or interactions such as the van der Waals force, hydrogen
bonding, or electrostatic force, often exist between the organic
and inorganic components [8]. These interactions, not only caused
the NPs fixated in the structure of NC, but also made large potential
applications in the polymer hybrid [9,10].
(DMF) was dried over BaO and then distilled under the reduced
pressure. Other chemicals used in this study were purchased from
Fluka Chemical Co. (Buchs, Switzerland), Aldrich Chemical Co.
(Milwaukee, WI), and Merck Chemical Co. (Germany).
Measurements
Melting points of the samples were measured on a melting
point apparatus (Gallenhamp, England) without correction. A Jas-
co-680 Fourier transform-infrared (FT-IR) spectroscopy (Japan)
was employed to examine the chemical bands on the polymer
and PANCs. The spectra of solids were obtained using KBr pellets.
The vibrational transition frequencies have been reported in wave
numbers (cmꢁ1). The band intensities were assigned as weak (w),
medium (m), strong (s), and broad (b). Thermal gravimetric analy-
sis (TGA) is performed with a STA503 (Bahr-Thermoanalyse GmbH,
Hüllhorst, Germany) at a heating rate of 20 °C minꢁ1 from 25 to
800 °C under argon atmosphere. The X-ray diffraction (XRD) pat-
terns were recorded using a Philips Xpert MPD diffractometer
Corundum aluminum oxide (
a
-Al2O3) is the most stable shape
equipped with a Cu Ka anode (k = 0.151418 nm) in 2h range of
between the other phases, which is formed at the high temperature
of about 1000 °C. This ceramic material, due to the superior proper-
ties such as high thermal conductivity, high hardness, high dielec-
tric constant, good wear resistance, and catalyst can be widely
used as reinforcer, in the polymer hybrids [11,12]. Nonetheless,
intrinsically poor compatibility of most inorganic nanofillers with
the organic polymer leads to poor dispersion and aggregation of
NPs in the polymer matrix. Since, most of the NPs surface was
covered with varying degrees of polar hydroxyl groups [13,14].
Hence, the pure NPs often discord in the hydrophobic matrix and
can attract each other by interactions between the hydroxyl groups,
and then form coagulum with size of several micrometers [15]. In
10–80° at the speed of 0.05° minꢁ1. The surface morphology of
the samples was observed by field emission scanning electron
microscopy (FE-SEM) (HITACHI S-4160, Japan). The dispersal of
nano-fillers within the host matrix has been controlled by the
transmission electron microscopy (TEM) (Philips CM 120) at an
accelerating voltage of 150 kV. Preparation of PANC was carried
out on a MISONIX ultrasonic XL-2000 SERIES (Raleigh, North Carolina,
USA) with frequency 2.25 ꢂ 104 Hz and the power of 100 W.
THCB synthesis procedure
The amount of 0.10 g (0.22 ꢂ 10ꢁ3 mol) tetrabromophthalic
anhydride (TBPA) and 0.02 g of (0.22 ꢂ 10ꢁ3 mol) p-aminophenol
(PAP) were dissolved in acetone. The mixture was stirred for 4 h
at room temperature. The resulting solution was poured in 20 mL
of distilled water, the obtained precipitate was filtered and dried
at 60 °C for 6 h under vacuum to give 0.103 g (83%) of white pow-
der 2,3,4,5-tetrabromo-6-[(4-hydroxyphenyl)carbamoyl]benzoic
acid (THCB). The purity of this compound was primarily checked
with thin-layer chromatography (TLC) in a mixture of 80/20 of
ethyl acetate/cyclohexane (m.p = 287–289 °C) [23].
order to alter the surface properties of the
a-Al2O3 NPs (ANPs)
and diminished intermolecular interaction, a number of functional
groups and organic compounds, such as silane coupling agent
[16,17], isocyanate group [18], stearic acid [19], dimethyl sulfoxide
[20], acetic acid [21], have been used as modifying agents. The exis-
tence of the halogen and aromatic ring in coupling agents can act as
flame retardant groups in NCs. However, the efficacy of halogenated
flame retardants depends on the sort of halogen. Bromine and chlo-
rine-based compounds, because of their weak bonding with carbon
atoms, can be readily released and participate in the combustion
process and so stopping the chain disintegration [22].
Surface treatment of ANPs
In the present study, we generally attempted to describe the syn-
thesis and characterization of a series of chiral poly(amide–imide)/
Al2O3 NCs (PANC) using a simple ultrasonically-assisted solution
blending procedure. Therefore, ANPs are etched and modified with
2,3,4,5-tetrabromo-6-[(4-hydroxyphenyl)carbamoyl]benzoic acid,
which is introduced the bulky substituent onto the surface of ANPs
and is prevented agglomeration of ANPs in the host polymer.
Organosoluble and thermally stable chiral PAI was synthesized dur-
ing direct polymerization reaction of N-trimellitylimido-L-phenyl-
alanine (DAPh) and 1,5-naphthalene diamine (NDA) under a
medium consisting of molten tetra-butylammonium bromide
(TBAB) and triphenyl phosphite (TPP). Then PANCs containing vari-
ous ratios of the modified ANPs were synthesized under ultrasoni-
cally-assisted technique.
Typical steps were carried out as follows: 15 wt.% of THCB was
dissolved in 20 mL of ethanol at room temperature and 0.10 g of
dried nano
a-Al2O3 were added into it. This solution was vigor-
ously stirred for 24 h and then, was ultrasonicated for 30 min.
Finally, solution containing the ANPs was filtrated off and washed
with ethanol three times. The solid was dried at 60 °C for 18 h.
ꢀ
m = 3500–3300 (m, b, OAH and NAH
FT-IR peaks (KBr, cmꢁ1):
stretching), 1700 (w, carboxylic acid’s C@O), 1170 (w), 1266 (w),
1127 (w, stretching vibration of the CAO), 1400 (w, CAOAH defor-
mation vibration).
Synthesis of diacid derived from amino acid
Optically active DAPh was prepared according to our previous
work [24].
Experimental
Reagents and materials
Polymer synthesis
ANP was purchased from Nanosabze Co. (Tehran, Iran), and size
of the nanoalumina was 80 nm. TBAB, trimellitic anhydride (TMA)
and NDA were purchased from Merck Co. and were used as
received without further purification. N,N0-dimethylformamide
PAI was synthesized by the direct polycondensation reaction of
an equimolecular mixture of DAPh and NDA in a system of TBAB/
TPP as condensing agent, which its procedure was reported in
our preceding work [25].

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S. Mallakpour, E. Khadem / Journal of Molecular Structure 1075 (2014) 196–203
Manufacture of PANCs
The 0.1 g of PAI was dispersed in 20 mL of absolute ethanol. A
uniform suspension was obtained after sonication for 15 min.
The appropriate amounts of modified ANPs (5, 10, 15 wt.%) were
added in the PAI suspension with simultaneous stirring and then
sonicated for 2 h. Finally, the solvent was removed and precipi-
tated solid was dried in vacuum at 80 °C for 10 h.
Results and discussions
Synthesis of THCB
Fig. 1 shows the reaction between PAP and TBPA in acetone. In
this reaction, the NH2 group of PAP with more nucleophilic charac-
teristic than phenolic OH attacked to the anhydride ring of TBPA
and led to the formation of amic acid with three final functional
groups, acid, amide and phenolic OH. The final product was precip-
itated in distilled water as a non-solvent. The presence of halogen
atoms in the molecule can improve the efficiency of the fire retar-
dant behavior. The FT-IR spectrum of THCB showed in Fig. 2c.
Absorption peaks at around of 3270 cmꢁ1 and 1550–1649 cmꢁ1
were related to the stretching vibration and bending vibration of
NAH group, respectively. Furthermore, the bands at around
3415 cmꢁ1 and 1218 cmꢁ1 could be attributed to the stretching
vibration of the OAH and CAO in the phenol group. On the other
hand, the absorption bands in the 1708 cmꢁ1 and 663–749 cmꢁ1
region were assigned to the stretching of the carboxylic acid C@O
group and the swing vibration of CABr in THCB.
Fig. 2. FT-IR spectra of (a) pure ANP, (b) ANP/THCB, (c) THCB.
and control the size distribution, we applied the ultrasonic device.
It is expected that the aggregation of ANPs can be broken down by
generating local conditions of extremely high temperatures and
pressures, which were produced by ultrasonic huge shock wave
in a solution.
The FT-IR spectra using KBr of the pure ANP and modified ANP
was shown in Fig. 2a and b. In the pristine alumina the broad
absorption band at 3300–3400 cmꢁ1 and weak band at
1628 cmꢁ1 can be caused by stretching vibration and bending
vibration of hydroxyl group peaks on the ANPs surface. Moreover,
peak appeared in the vicinity of 400–1000 cmꢁ1 is related to char-
Modification of alumina
The polar functional groups in the backbone of THCB can inter-
act with the hydrophilic surface of inorganic materials via hydro-
gen bonding and electrostatic interaction as a monolayer. These
chains, through of steric hindrance and interparticle electrostatic
repulsion were prevented further coalescence in ANPs (Fig. 1).
For getting good dispersion of the ANPs in the polymer matrix
acteristic of absorption band of a-Al2O3. After modification of ANP
with THCB (Fig. 2b), presence of a broad band in the 3500–
3300 cmꢁ1 region could be due to the stretching vibrations of
OAH or NAH groups with varying degrees of H-bonding. The
appearance of the new absorption peaks at 1170, 1266 cmꢁ1, and
Fig. 1. Synthesis of the THCB as a modifier and preparation of the modified Al2O3.

S. Mallakpour, E. Khadem / Journal of Molecular Structure 1075 (2014) 196–203
199
1127 cmꢁ1 (stretching vibration of the CAO group) and small peak
at 1400 cmꢁ1 (CAOAH deformation vibrations) are confirmed the
presence of THCB onto the ANPs surface.
Synthesis of monomer and polymer
DAPh monomer was obtained by the condensation reaction of
an equimolecular amount of TMA and L-phenylalanine amino acid
in refluxing acetic acid solution at 120 °C, as shown in Fig. 3 [24].
Phosphorylation polymerization technique has been success-
fully applied to synthesize high molecular weight organosoluble
PAI, which first was described by Yamazaki et al. [26]. In this paper,
we used a simple, safe, and efficient method, without any use of
carcinogenic organic solvents for the step-growth polymerization.
In synthetic strategy, an equimolar amount of the DAPh and NDA
monomers were mixed in the presence a molten ionic liquid
(TBAB), which acted both as solvent and catalyst in conjunction
with TPP. The inherent viscosity of the synthesized PAI under opti-
mized condensation was 0.18 dL gꢁ1 and the yield was 95%. The
optical specific rotation of this polymer was
(0.05 g in 10 mL of DMF) [25].
½
a 2D5
= + 18.82°
ꢃ
In the FT-IR spectrum of the nanostructure PAI (Fig. 4a), absorp-
tion bands in 3300–3400 cmꢁ1 (NAH stretching bands of the
amide group), 3059 cmꢁ1 (CAH aromatic), 2962 cmꢁ1 and
2931 cmꢁ1 (CAH aliphatic), 1777 cmꢁ1 and 1719 cmꢁ1 (imide
C@O asymmetric and symmetric stretching), and 1671 (amide
C@O stretching), are substantiated the polymer structure. More-
over, absorption peaks at 1381, 1171 and 729 cmꢁ1, shown the
presence of the imide heterocycle ring in the PAI.
Fig. 4. FT-IR spectra of (a) pure PAI, (b) PANC5%, (c) PANC10%, (d) PANC15%.
combined with the PAI matrix (Fig. 5). Surface modification of
ANPs improved successfully the adhesion of polymer matrix to
the surface of NPs. The THCB groups or remaining hydroxyl groups
on the surface of ANPs can interact with the functional groups of
PAI, like C@O and NAH via hydrogen bonding. Fig. 4b–d shows
the FT-IR spectra of PANCs with various ratios of ANPs. Compared
to the pure PAI, slight different positional displacement and new
absorption bands around 400–1000 cmꢁ1 in the FT-IR spectra of
PANCs can be related to the formation of new interaction and
hydrogen bonds between modified ANPs with the functional
groups of PAI. For example, absorption peaks of carbonyl group
Synthesis of PANC
The synthesis of PANCs was carried out by insertion of ANPs
into the polymer matrix via an ultrasonic irradiation process at
ambient temperature. Due to the hot spots (high temperatures
and pressures) which produce during sonication, the ANPs could
be dispersed and combined with the PAI matrix. Different percent-
ages of organically modified ANPs (5, 10, and 15 wt.%) were
from 1719 and 1671 cmꢁ1 shifted towards 1725 and 1660 cmꢁ1
respectively which are related to the partial transfer of the electron
pair in the functional groups of PAI.
,
Fig. 3. Synthesis of DAPh (monomer) and PAI.

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XRD
The XRD patterns of the PAI, PANCs and modified ANPs are
shown in Fig. 6. The obtained diffractogram of PAI, obviously
showed broad halo pattern without any obvious sharp peak dem-
onstrating that the polymer was essentially amorphous. This amor-
phous nature is related to the presence of pendent groups,
noncoplanar and twisted units, which reduce the backbone sym-
metry and regularity into the backbone of the obtained polymer.
According to Fig. 6a, the XRD pattern of the modified ANP with
THCB modifier indicated a good accordance with the reference
XRD pattern of
a-Al2O3 (42-1468 JCPDS file) and proved that the
capping agent do not have any effect on crystallinity of ANPs.
According to the XRD patterns of PANCs (Fig. 6b and c), after
embedment of ANPs into the polymer matrix, the phase of ANPs
has not been distorted during the sonication process. Of course,
the intensity of ANP peaks turned into weaker in the presence of
the host polymer, because of the ultrasonic irradiation by the pro-
duction of many localized hot spots in the solution may decreased
the crystallite size of a-Al2O3. The hexagonal structure of a-Al2O3
was confirmed by the diffraction peaks of (104), (113), and
(116). The average crystalline size of ANPs in the PANC was deter-
mined from the half width of the diffraction using the Debye–
Scherrer equation [27], which is approximately 17–22 nm.
Fig. 6. XRD curves of (a) modified ANP, (b) PANC10%, (c) PANC15% and (d) pure PAI.
FE-SEM
distribution histogram are shown in Fig. 7. The particle size distribu-
tion for modified ANPs showed a narrow Gaussian curve with mean
The FE-SEM images of modified alumina, neat nanostructure of
PAI and PANCs with magnification of 300 nm and particle size
Fig. 5. Preparation of PANC and possible interactions (hydrogen bonding) between PAI and modified Al2O3.

S. Mallakpour, E. Khadem / Journal of Molecular Structure 1075 (2014) 196–203
201
size 23 5 nm. Fracture surface micrograph of PAI (Fig. 7b) indi-
cated that the average diameter of polymeric particles was about
25 3 nm. In addition, means of particle size in PANCs 5, 10, and
15 wt.% with the narrow size distribution curves were established
to be about 19, 21, and 22 4 nm, respectively. These images and
curves demonstrated that the ANPs were uniformly dispersed into
the PAI matrix, also slightly agglomeration can be related to inter-
molecular interaction. The organic chains grafted on the surface of
ANPs bring mutual exclusion and steric hindrance effect, thus the
surface free energy has been reduced correspondingly and the
agglomeration is controlled.
TEM
The formation of
a true nanoscaled composite is further
evidenced by using TEM analysis. This analysis, unlike of the
FE-SEM analysis that is provided images from the samples surface,
is employed to insights into the internal structure of different
components. The TEM image, and the size distributions of the pure
ANPs, modified ANP and PANCs are shown in Figs. 8 and 9. TEM
micrograph of the pure ANPs (Fig. 8) exhibited particles with
polyhedral morphology in the range of 60–90 nm [28]. Images of
modified ANPs (Fig. 9a and b) have shown a broad size distribution,
with minimum size NPs of about 10 nm and largest NPs with irreg-
ular shape and pumpkin in the size more than 50 nm. This particle
size distribution probably is due to aggregation or clustering of NPs
via hydrogen bondings (or could be due to arrangement of a few
particles by hydrogen bonding which cannot resist the attraction
with THCB). The representative TEM images of the PANC 10 wt.%
with magnification of 30 and 40 nm are shown in Fig. 9c and d.
It can be seen that the ANPs with spherical shapes and average
particle diameter 18 4 nm were dispersed in the polymer matrix.
In PANCs, the particle sizes were believed to have been reduced;
because of capping agents onto the NP surface could inhibit growth
and agglomeration of NPs. Above and beyond, ultrasonic irradia-
tion is a well-established method for particle size reduction in
suspensions.
Fig. 8. TEM micrograph of pure ANPs [28].
Thermal properties investigation
TGA curves of the modified ANP, PAI and the PANCs are shown
in Fig. 10. The thermal stability of the polymer and NCs were inves-
tigated by comparing the 5% and 10% weight losses (T5 and T10
,
respectively) of the samples and percent char yield at 800 °C. The
curve of pristine ANP shows a small mass loss 3% in temperature
range of 0–800 °C, which is related to the removal of the physically
absorbed water [28]. In the thermogram of the modified ANP
weight loss at around 270–800 °C is 18%, which is attributed the
decomposition of the chemical combined water and functionalized
organic moieties attached on the surface of ANPs. The thermal
behavior data for the PAI and the PANCs are summarized in Table 1.
The T10 of pure PAI and the PANCs appeared at 381, 427, 437, and
441 °C, respectively. The results showed that decomposition
Fig. 7. FE-SEM micrographs and particle size distribution histograms of (a) modified ANPs, (b) pure PAI, (c) PANC5%, (d) PANC10% and (e) PANC15%.

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S. Mallakpour, E. Khadem / Journal of Molecular Structure 1075 (2014) 196–203
Fig. 9. TEM micrographs of modified ANP (a and b), PANC10% (c and d) and particle size distribution histogram of PANC10%.
temperature of PANCs shifted towards higher temperatures as the
percentage of ANP was increased. The concentration of carbonized
residue (char yield) of these composites in an argon atmosphere
was more than 50% at 800 °C. The high char yield can be ascribed
to their aromatic content and high heat resistance supplied by
modified ANPs with flame retardant compound.
The limiting oxygen index (LOI) is a measure of the percentage
of oxygen present to support the combustion of the material and
can be used to assess the flame-retardancy of them.
Table 1
Thermal properties of the PAI and different PANC materials.
Sample
Decomposition temperature (°C)
Char yield (%)b
LOIc
a
a
T5
T10
PAI
352
355
369
372
381
427
437
441
49
53
54
57
37
38
39
40
PANC 5 wt.%
PANC 10 wt.%
PANC 15 wt.%
a
Temperature at which 5% and 10% weight loss was recorded by TGA at the
heating rate of 20 °C minꢁ1 under argon atmosphere.
LOI ¼ 17:5 þ 0:4CR
b
Weight percentage of material left undecomposed after TGA analysis at a
temperature of 800 °C under argon atmosphere.
c
where CR = char yield.
Limiting oxygen index (LOI) evaluated at char yield at 800 °C.
According to Van Krevelen and Hoftyzer [29] there is a linear
relationship between LOI and CR. From this equation, a higher char
yield will increase flame retardancy. PAI and composites contain-
ing 5, 10, and 15 wt.% had LOI values 37, 38, 39, and 40, respec-
tively, which were calculated from their char yield. On the basis
of the LOI values, such materials were classified as self-extinguish-
ing polymers and tend not to be burned.
Conclusions
In this work, PANCs containing various ratios of the modified
nano-ANPs with THCB were successfully synthesized using ultra-
sonic assistance method. Grafting of THCB on the surface of ANPs
not only improved compatibility of ANPs in the polymer matrix,
but also increased flame retardant property in the obtained PANCs.
Fig. 10. TGA thermograms of modified ANPs, neat PAI and different PANC materials.

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203
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Acknowledgments
This study was financially supported by the Research Affairs
Division Isfahan University of Technology (IUT). Further financial
support from National Elite Foundation (NEF), Center of Excellence
in Sensors and Green Chemistry Research (IUT) is gratefully
acknowledged.
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