Facile synthesis of nanocomposite materials by intercalating an optically active poly(amide-imide) enclosing (l)-isoleucine moieties and azobenzene side groups into a chiral layered double hydroxide

Article abstract of DOI:10.1016/j.polymer.2013.04.016

The use of layered inorganic solids as host materials for the creation of inorganic-organic host-guest supramolecular structures is of increasing interest. In the present investigation, the nanocomposites of chiral poly(amide-imide) (PAI) and different amount of modified layered double hydroxide (LDH) were prepared via solution intercalation method for the first time. Chiral organically-modified chiral LDH was prepared via ion exchange reaction of LDH in a solution of N,N′-(pyromellitoyl)-bis-l-isoleucine diacid in distilled water. Novel diamine with azo side group was synthesized from the reaction of 4-aminoazobenzene and 3,5-dinitrobenzoyl chloride followed by reduction of dinitro with iron oxide hydroxide catalyst and hydrazine monohydrate. An optically active PAI was prepared by direct polyamidation of the chiral diacid with azo diamine under green condition using molten tetra-n-butylammonium bromide with good yield and moderate inherent viscosity. The obtained polymer and organically-modified chiral LDH were used to prepare chiral and high performance hybrid materials. The effect of LDH contents on thermal, physicomechanical, and morphological properties of PAI was investigated by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy techniques. Thermogravimetric analysis results confirmed that the heat stability of the prepared nanoparticle-reinforced composites was improved.

Full text of DOI:10.1016/j.polymer.2013.04.016

Polymer 54 (2013) 2907e2916
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Facile synthesis of nanocomposite materials by intercalating an optically active
poly(amide-imide) enclosing (
L)-isoleucine moieties and azobenzene side groups
into a chiral layered double hydroxide
,
b
,
a
Shadpour Mallakpour a , Mohammad Dinari
*
a Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of layered inorganic solids as host materials for the creation of inorganiceorganic hosteguest
supramolecular structures is of increasing interest. In the present investigation, the nanocomposites of
chiral poly(amide-imide) (PAI) and different amount of modified layered double hydroxide (LDH) were
prepared via solution intercalation method for the first time. Chiral organically-modified chiral LDH was
Received 15 February 2013
Received in revised form
6 April 2013
Accepted 8 April 2013
Available online 15 April 2013
prepared via ion exchange reaction of LDH in a solution of N,N0-(pyromellitoyl)-bis-
L-isoleucine diacid in
distilled water. Novel diamine with azo side group was synthesized from the reaction of 4-
aminoazobenzene and 3,5-dinitrobenzoyl chloride followed by reduction of dinitro with iron oxide
hydroxide catalyst and hydrazine monohydrate. An optically active PAI was prepared by direct poly-
amidation of the chiral diacid with azo diamine under green condition using molten tetra-n-buty-
lammonium bromide with good yield and moderate inherent viscosity. The obtained polymer and
organically-modified chiral LDH were used to prepare chiral and high performance hybrid materials.
The effect of LDH contents on thermal, physicomechanical, and morphological properties of PAI was
investigated by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning
electron microscopy, and transmission electron microscopy techniques. Thermogravimetric analysis re-
sults confirmed that the heat stability of the prepared nanoparticle-reinforced composites was improved.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Modified layered double hydroxide
Chiral poly(amide-imide)
L-Isoleucine
1. Introduction
High performance polymer materials are used more and more
for engineering applications under hard working conditions [9].
Undoubtedly, one of the most significant advances of science is
the birth of nanohybrid technology. From the many important
research success in the past decade and the many potential prac-
tical applications, it is assumed that the prospects for nanomatrix
research are very bright indeed [1]. Recently, polymer/layered
crystal nanocomposites (NCs) have been identified as one of the
most promising research fields in material chemistry owing to their
attractive anisotropic behavior, their enormous variety in physical
properties and their usefulness for different applications [2,3]. Most
of the work has been focused on the area of layered silicate NCs and
where sharp improvements in the mechanical and thermal prop-
erties, reduction in the gas and vapor transmission and flamma-
bility have been reported [4e8].
The materials must provide unique mechanical and tribological
properties combined with a low specific weight and a high resis-
tance to degradation in order to ensure safety and economic effi-
ciency. Among them, aromatic polyimides (PIs) have exhibited
remarkable comprehensive characters, like: excellent thermal sta-
bility, balanced mechanical and electrical properties, as well as
good chemical and irradiation resistance [10e12]. One of the
problems with high-performance PIs is their poor solubility or
processability, and they do not react below their decomposition
temperature; this restricts their applications as engineering mate-
rials. Many efforts have been made to improve their processability
while maintaining their excellent thermal and mechanical prop-
erties. For example, bulky lateral substituents, flexible side chains,
noncoplanar biphenyl moieties, and flexible alkyl or aryl ether
spacers have been used to enhance the solubility and thus pro-
cessability [13e17].
* Corresponding author. Organic Polymer Chemistry Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,
Iran. Tel.: þ98 311 391 3267; fax: þ98 311 391 2350.
Azobenzene group is an important class of organic compounds
which their main structural consist of a chromophore azo group
(N]N) connecting to one or more aromatic rings [18]. These
E-mail
addresses:
mallakpour84@alumni.ufl.edu,
mallak@cc.iut.ac.ir,
mallak777@yahoo.com (S. Mallakpour).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.016

2908
S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
compounds can exist in two different configurations, the trans-
isomeric form and cis-isomeric form in which the trans form is
more stable than the cis [19,20]. Azo compounds and its derivatives
have been used in various areas of industry and academic re-
searches as organic dyes, colored plastics, pH indicators, food ad-
ditives, liquid crystalline display as a catalyzer in photo-catalytic
reaction and optical switching applications [21e23]. Some of these
compounds have also been used in biological systems as inhibitors
of tumor-growth and for drug delivery [24,25]. Azo chromophore
groups can be placed in main or side chain of polymers as poly-
amides, polyesters, poly(amide-imide)s (PAI)s and so on [26e28].
High performance chiral polymers have attracted a great deal of
interest in the applications of fields such as dentistry, drug delivery
and tissue engineering due to their promising properties [29]. As an
important class of optically active polymers, PAI has received an
increasing amount of attention owing to their chemical and me-
chanical properties [30,31]. The uses of optically active monomers
by using 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 for
the pharmaceutical industry for enantioselective separation of
drugs [32,33].
bromide (TBAB) as molten ionic liquid (IL). Different amounts of the
novel modified LDH (2, 4, 6 and 8%) were used for the synthesis of
PAI/modified LDH NCs via solution intercalation technique. The
resulting materials were characterized by several different
techniques.
2. Experimental
2.1. Materials
All chemicals and solvents were purchased from Aldrich Chem-
ical Co. (Milwaukee, WI, USA), Riedel-deHaen AG (Seelze, Germany)
and Merck Chemical Co. (Germany). 4-Aminoazobenzene, 3,5-
dinitrobenzoyl chloride, pyromellitic dianhydride (PMDA), TBAB
and L-isoleucine were used as obtained without further purification.
Triethylamine (TEA), NaOH, iron chloride (FeCl3), hydrazine mono-
hydrate, triphenyl phosphite (TPP), and dichloromethane were
purchased from Merck. Co. (Germany). N,N-Dimethylformamide
(DMF) and N,N-dimethyacetamide (DMAc) were dried over barium
oxide and then were distilled under reduced pressure. LDH;
(Mg6Al2(CO3)(OH)16$4(H2O)) was purchased from SigmaeAldrich
Co. (USA).
Layered double hydroxides (LDHs), also known as hydrotalcite-
like compounds or anionic clays, have attracted much attention
because of their potential applications in catalysts, bio-active ma-
terials, flame retardants, ion exchangers, electrical functional ma-
terials and organiceinorganic NCs [34]. The novelty of eco-
compatible LDHs lies in their simple synthetic procedures, high
level of purity, ability to be organomodified with a variety of
organic ions, versatility in chemical composition and multiple in-
teractions with polymers [4,35]. The general chemical composition
2.2. Equipments
Proton and carbon nuclear magnetic resonance (1H NMR and 13
C
NMR) spectra were recorded in dimethylsulfoxide (DMSO-d6) so-
lution using a Bruker (Germany) Avance 400 MHz spectrometer.
Multiplicities of proton resonance were designated as singlet (s)
and multiplet (m). Fourier transform infrared (FT-IR) spectra for
pristine LDH and other samples were recorded for wave numbers
4000e400 cmꢀ1 using a Jasco-680 FT-IR spectrometer (Japan). The
powdered samples were mixed with KBr and pressed in the form of
pellets for the measurement. Band intensities are assigned as weak
(w), medium (m), shoulder (sh), strong (s), and broad (br).
Specific rotation was measured with a Jasco (Osaka, Japan) P-
1030 polarimeter at the concentration of 0.5 g dLꢀ1 at 25 ꢁC. The
inherent viscosities of the polymer were determined with a
Cannon-Fenske Routine Viscometer (Germany).
3þ
of LDH can be represented as [M2þ
M
x(OH)2][(Anꢀ
)x/n$mH2O],
1ꢀx
where M2þ and M3þ represent di- and tri-valent metal ions within
the brucite-like layers respectively, and Anꢀ is an interlayer anion
[35,36]. High charge density of the LDH layers associated with the
integrated hydrogen-bonding networks between the hydroxide
layers and intercalated anions/water molecules makes it difficult to
exfoliate or delaminate the LDH via conventional procedures,
especially when being used as reinforcing nanofillers in polymer
NCs. The LDH can be modified by the replacement of interlayer
2ꢀ
anions such as Clꢀ, NO3ꢀ, or CO3 with different organic anionic
X-ray diffraction analysis (XRD) over 2
q
¼ 1.8e40ꢁ, in steps of
surfactant [4,7,37].
0.02ꢁ was carried out using an X-ray diffractometer (Bruker, D8
In recent years, much attention has been paid to the intercala-
tion of biomolecules like amino acids [37,38] between LDH inter-
layer. Also LDHs containing interlayer carboxylate anions have
attracted considerable attention in recent years due to interesting
properties and potential applications, e.g., LDH modified with cit-
rate, malate, and tartrate ions are able to take up hazardous organic
materials and heavy metal ions from an aqueous solution [37].
In the present paper, we report the preparation and properties
of novel PAI/LDH NCs using chiral dicarboxylic acid as a compati-
Advance, Germany) with Cu-K
a
radiation (
l
¼ 0.154 nm, mono-
chromatization by primary graphite crystal) generated at 100 mA
and 45 kV. XRD curves were interpreted with respect to the posi-
tion of the basal reflection (003), which depends on the distance
between two adjacent metal hydroxide sheets in the LDH lattice.
The higher order reflections of the same (hkl) series [(006), (009)
and so on] were also reported.
Thermogravimetric analysis (TGA) data for samples were taken
on a STA503 TA instrument (USA) in a nitrogen atmosphere at a rate
of 10 ꢁC/min. Elemental analyses were performed by LECO, CHNS-
932.
The morphology of the LDH and NCs was examined by field
emission scanning electron microscopy [(FE-SEM); HITACHI; S-
4160, Japan]. For FE-SEM sample preparations, at first the NC films
were converted to powder and then was dispersed in H2O, the
sediment was dried at room temperature before gold coating.
The dispersion of the nanoclay within the medium has been
controlled by transmission electron microscopy (TEM). The TEM
images were obtained from a Philips (CM120, Netherlands) using
an accelerator voltage of 100 kV. For TEM sample preparation, NC
was suspended in water and a small drop of it was deposited and
dried on a carbon coated copper grid. The inorganic components
appear black/grey colored on the micrographs.
bilizer. N,N0-(Pyromellitoyl)-bis-
sized in high yield from the reaction of pyromellitic dianhydride
with -isoleucine amino acid and then it was used for organo-
L-isoleucine diacid was synthe-
L
modification of chiral MgAleLDH as well as for preparation of
new PAI. Modified chiral LDH was synthesized from the ion ex-
change reaction of MgAleLDH and N,N0-(pyromellitoyl)-bis-
L-
isoleucine diacid in sodium hydroxide (NaOH) solution. For prep-
aration of PAI, first, azo diamine was synthesized by nucleophilic
substitution reaction between 4-aminoazobenzene and 3,5-
dinitrobenzoyl chloride and then reduction with iron oxide hy-
droxide catalysis. After that, step-growth polymerization reaction
between azo diamine and diacid based on isoleucine amino acid
was performed for synthesis of novel high performance and opti-
cally active PAI in green media by using tetra-n-butylammonium

S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
2909
2.3. Synthesis of diacid monomer
N,N-(Pyromellitoyl)-bis- -isoleucine diacid was prepared ac-
130.97 (1C), 136.43 (2C), 142.87 (1C), 147.42 (1C), 149.19 (2C), 152.03
(1C), 167.64 (1C).
Elemental analysis: calcd. for (C19H17N5O): C, 68.87%; H, 5.17%;
N, 21.23%. Found: C, 68.18%; H, 5.01%; N, 21.18.
L
cording to our published article [39].
2.7. Polymer synthesis
2.4. Preparation of chiral modified LDH
The PAI was prepared by the following procedure: a mixture of
0.25 g (5.61 ꢂ 10ꢀ4 mol) of diacid 3, 0.19 g (5.61 ꢂ 10ꢀ4 mol) of
diamine 8 and 0.50 g (2.25 ꢂ10ꢀ3 mol) of TBAB was transferred into
a 25 mL round-bottom flask then it was heated until a homoge-
neous solution was formed. TPP (0.40 mL, 2.25 ꢂ 10ꢀ3 mol) was
added to this mixture and the whole solution was stirred at 120 ꢁC
for 6 h. Then the viscous solution was precipitated in 30 mL of
methanol. The orange solid was filtered off and dried to give 0.37 g
(84%) of PAI. The specific rotation was ꢀ12.3ꢁ (measured at a con-
centration of 0.5 g dLꢀ1 in DMF at 25 ꢁC).
The synthesis of the chiral modified LDH is carried out via the
ion exchange method. LDH (2.00 g) was placed in 250 mL two ways
round neck flask with 100 mL de-ionized water. 0.85 g of sodium
salt of chiral anionic surfactant (N,N0-(pyromellitoyl)-bis-
L-isoleu-
cine diacid) was added with vigorous stirring under inert atmo-
sphere at 70 ꢁC for 48 h in an alkaline medium. After 48 h of ion-
exchange process the contents were filtered and washed several
times by de-ionized water to remove sodium carbonate formed
during the ion-exchange reaction. Finally, the precipitate was dried
at room temperature for 24 h under vacuum. The white solid thus
obtained was grounded and stored in a vial bottle.
PAI: FT-IR (KBr, cmꢀ1): 3340 (br), 3067 (m), 2965 (m), 2933 (w),
2876 (m), 1775 (m), 1723 (s), 1596 (s), 1529 (s), 1446 (m), 1404 (m),
1382 (m), 1342 (s), 1246 (m), 1153 (m), 1081 (m), 1001 (w), 943 (w),
724 (m), 689 (m), 552 (w).
2.5. Synthesis of dinitro compound with azo pendant group
1H NMR (500 MHz, DMSO-d6,
d, ppm): 0.60e0.62 (m, 6H), 0.91e
For the synthesis of dinitro compound 7, at first 0.86 g
(2.84 ꢂ 10ꢀ3 mol) of 4-aminoazobenzene 5 was dissꢀo3lved in 7 mL of
dichloromethane at 0 ꢁC. Then 1.0 g (4.34 ꢂ 10 mol) of 3,5-
dinitrobenzoyl chloride 6 was added gradually to the above solu-
tion. TEA was added to this mixture after 30 min and stirred at
room temperature for 12 h. Then a solid product was filtered, dried
and recrystallized in DMFewater mixture (3:1). Orange needles
were obtained; 1.45 g; yield 85.3%; melting point (m.p.) 241e
243 ꢁC.
1.02 (m, 6H), 1.41e1.52 (m, 4H), 2.54e2.68 (m, 2H), 4.69e4.74 (m,
2H), 6.83 (s, 1H), 7.10 (s, 2H), 7.25e7.31 (m, 1H), 7.88e7.94 (m, 2H),
8.09e8.14 (m, 2H), 8.22e8.24 (m, 2H), 8.37e8.42 (m, 2H), 9.04 (s,
2H), 10.27 (s, 2H), 10.82 (s, 1H).
Elemental analysis: calcd. for (C41H37N7O7): C, 66.57%; H, 5.04%;
N, 13.25%. Found: C, 66.25%; H, 4.98%; N, 13.05%.
2.8. Preparation of the PAI/modified LDH NC films
Dinitro 7: FT-IR peaks (KBr, cmꢀ1): 3417 (m), 3100 (m), 2923
(w), 1680 (s), 1625 (m), 1600 (m), 1533 (s), 1405 (m), 1346 (s), 1327
(s), 1232 (m), 1154 (m), 848 (m), 727 (s), 711 (m), 690 (m), 549 (m).
The novel NC films were prepared by mixing the appropriate
amounts of the optically active PAI and chiral modified LDH in
DMAc as a solvent in a flask for a particular concentration. The
solution was agitated to high speed stirring at 80 ꢁC for 2 h, at 60 ꢁC
for 2 h and then at 40 ꢁC for 20 h for uniform dispersion of modified
LDH in the polymer matrix. Several NCs were prepared by mixing
different amounts of modified LDH (2, 4, 6 and 8 wt.%) to the PAI
solution. Thin NC films of uniform thickness were obtained by
pouring the hybrid solutions into Petri dishes, followed by solvent
evaporation at high temperature. These films were further dried at
80 ꢁC under reduced pressure to a constant weight.
1H NMR: (400 MHz, DMSO-d6,
d, ppm): 7.56e7.62 (m, 3H), 7.87e
7.89 (d, 2H, J ¼ 8), 7.96e7.98 (d, 2H, J ¼ 8), 8.04e8.06 (d, 2H, J ¼ 8),
9.01 (s, 1H), 9.19 (s, 2H), 11.12 (s, 1H).
13C NMR: (100 MHz, DMSO-d6,
d, ppm): 120.89 (2C), 121.29 (2C),
122.39 (1C), 123.53 (2C), 128.13 (1C), 129.44 (2C), 131.26 (1C), 137.12
(2C), 141.33 (1C), 148.09 (1C), 148.25 (2C), 151.95 (1C), 161.64 (1C).
Elemental analysis: calcd. for (C19H13N5O5): C, 58.31%; H, 3.35%;
N, 17.90%. Found: C, 58.55%; H, 3.41%; N, 17.45.
2.6. Synthesis of diamine compound with azo pendant group
3. Results and discussion
Iron oxide hydroxide was used as a catalyst for the reduction of
the above dinitro compound. For the preparation of this catalyst, 3 g
of iron chloride was dissolved in 100 mL of distilled water. Then 2 M
sodium hydroxide was added to adjust the pH to 7e8. The mixture
was agitated at 60 ꢁC in 12 h. The final product obtained by filtra-
tion was dried at 40 ꢁC for 12 h [40].
3.1. Synthesis and characterizations of monomers
Optically active diimide dicarboxylic acid monomer 3 was pre-
pared by reacting of an equimolar amount of PMDA (1) with two
equimolar amounts of L-isoleucine amino acid (2) in refluxing
acetic acid solution, as shown in Scheme 1.
For the synthesis of diamine 8, a mixture of 0.5 g of dinitro
compound in 8 mL methanol is heated until 60 ꢁC, and then the
catalyst was added to the reaction mixture. Then hydrazine mon-
ohydrate was gradually added to the mixture and the solution was
stirred for 12 h at 60 ꢁC. After that, the solution was filtered to
remove catalyst. The yellow powder was recrystallized by acetonee
water mixture (3:1) (0.28 g; yield 67%; m.p. 212e214 ꢁC).
Diamine 8: FT-IR peaks (KBr, cmꢀ1): 3422 (m), 3342 (m), 3049
(w), 2922 (w), 1648 (m), 1594 (s), 1525 (s), 1405 (m), 1360 (m), 1320
(m), 1249 (m), 843 (m), 754 (m), 688 (m).
Diamine 8 was prepared by a two-step procedure: in the first
step, the dinitro compound 7 was obtained in a high yield by the
nucleophilic substitution reaction of 4-aminoazobenzene (5) and
3,5-dinitrobenzoyl chloride (6) in dichloromethane as the solvent.
In the second step, the catalytic hydrogenation of the nitro groups
to the amine groups was prepared by using hydrazine monohydrate
as the reducing agent and iron oxide hydroxide as the catalyst
(Scheme 1).
Structures of the dinitro 7 and diamine 8 were investigated by
elemental analysis, FT-IR, 1H NMR and 13C NMR techniques. Fig. 1a
shows the FT-IR spectrum of the synthesized dinitro 7. The eNH
stretching bands of the amide group could be observed by FT-IR
spectroscopy at 3417 cmꢀ1. Absorption bands have been found for
eNO2 groups at 1533 and 1346 cmꢀ1 and absorption peak at
1H NMR: (400 MHz, DMSO-d6,
d, ppm): 4.92 (s, 4H), 6.04 (s, 1H),
6.35 (s, 2H), 7.52e7.62 (m, 3H), 7.86e7.92 (m, 4H), 8.00e8.02 (d, 2H,
J ¼ 8), 10.25 (s, 1H). 13C NMR: (100 MHz, DMSO-d6,
d, ppm): 102.29
(2C), 119.51 (2C), 119.97 (1C), 122.29 (2C), 123.46 (1C), 129.40 (2C),

2910
S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
O
O
O
N
O
O
O
O
H
O
O
O
O
AcOH
Reflux
NaOH
NH2
N
N
-
-
N
O
HO
O
O
O
OH
+ HO
H
H
H
H
O
O
O
O
O
O
Chiral Diacid 3
2
1
Chiral Dicarboxylate 4
NO2
O2N
NH2
H2N
NH2
O
O
NO2
O2N
CH2Cl2, TEA
HN
N
HN
N
Iron oxide hydroxide
N2H4
+
N
N
O
Cl
N
N
6
7
5
8
Scheme 1. Synthesis of chiral dicarboxylate 4, azo containing diamine monomer 8.
1680 cmꢀ1 is characteristic for carbonyl of the amide group. The
bond of the N]N linkage appeared around 1405 cmꢀ1. The ab-
sorptions at 1625 and 1463 cmꢀ1 are attributed to the aromatic
rings. The FT-IR spectrum of azo diamine compound (Fig. 1b) shows
characteristic absorption bands for the eNH amide and amine e
NH2 at around 3422e3342 cmꢀ1, respectively.
related to aromatic protons of azobenzene group. The peaks at 6.04
and 6.35 ppm are attributed to protons of H2 and H3, respectively.
The 13C NMR technique helps to further understanding of
monomer structure. The 13C NMR spectrum of dinitro compound 7
is shown in Fig. 3a. It shows the carbon of carbonyl group (C13) at
161.64 ppm. Aromatic carbons of azobenzene group appeared at
region 121e152 ppm due to the difference effect of electronega-
tivity. Aromatic carbons 1 and 5 appear at 120.89 and 128.13 ppm
due to the presence nitro groups on aromatic ring. The 13C NMR of
diamine compound 8 showed peak for C1 and C5 at 102.29 and
123.46 ppm, respectively (Fig. 3b). Also, the presence of eight
different aromatic carbons of azobenzene group and carbon for
carbonyl groups in this spectrum confirms the structure of this
compound.
The 1H NMR spectrum of dinitro 7 is shown in Fig. 2a. In
accordance with chemical structure, it shows the characteristic
absorption of aromatic protons H6, H7 and eNH amidic group at
9.01, 9.19 and 11.12 ppm, respectively. The aromatic protons of
azobenzene group were observed in the range of 7.56e8.06 ppm.
Fig. 2b shows the 1H NMR spectrum of the synthesized diamine 8.
This spectrum illustrates a new singlet peak at 4.92 ppm (H1) due
to amine protons. The peaks in the region of 7.52e8.03 ppm are
3.2. Synthesis and characterizations of chiral polymer with azo
group
Molten TBAB was used as a solvent for the formation of novel
optically active, organo-soluble and thermally stable PAI by the
direct polymerization reaction of chiral diacid 3 with diamine 8
(Scheme 2). In this method, the polymerization reaction was per-
formed under conventional heating in the presence of TBAB and
TPP, and it took 12 h at 120 ꢁC for the completion of the reaction.
The inherent viscosity of the PAI with azo pendant group under
optimized condensations was 0.43 dL/g and yield was 84%. The
incorporation of a chiral unit into the polymer backbone was ob-
tained by measuring the specific rotations of polymer that repre-
sents this polymer is optically active. The specific rotation of this
polymer is ꢀ12.3ꢁ (0.05 g in 10 mL of DMF).
The structure of novel optically active PAI was confirmed by FT-
IR spectroscopy method (Fig. 1c). The eNH stretching bands of the
amide groups exist in side and main chain of PAI could be observed
by FT-IR spectroscopy around 3400e3300 cmꢀ1. Absorption peak at
around 1600 cmꢀ1 is attributed to C]C vibration of the benzene
rings. FT-IR spectrum of this polymer also shows the peaks for the
imide ring at 1774 and 1723 cmꢀ1 due to the symmetrical and
asymmetrical carbonyl stretching vibrations. Absorptions at
1382 and 724 cmꢀ1 corresponding to the imide heterocycle ring in
these polymers.
In the 1H NMR spectrum of PAI, appearances of the NeH protons
of amide groups in the main chain as well as side chain at 10.30e
Fig. 1. FT-IR spectra of (a) dinitro, (b) diamine, and (c) neat PAI.

S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
2911
Fig. 2. 1H NMR spectra of dinitro (a) and diamine (b).
10.80 indicate the presence of these groups in the polymer chain.
Protons of aromatic rings of main chain of the polymer and azo
group side chain were observed in the region of 6.80e9.03 ppm.
The proton of the two different chiral centers appeared as multi-
plets in the range of 2.54e2.68 and 4.69e4.74 ppm. The absorption
of the diastereotopic hydrogens bonded to the neighbor carbon of
the chiral center (CH2) appears as two discrete multiplets peaks
from 1.41 to 1.52 ppm. The peaks of aliphatic protons of CH3
appeared at 0.60e1.02 ppm. These results support the presence
of PAI.
study natural L-isoleucine amino acid was used as building blocks
for efficient synthesis of novel potentially biodegradable and chiral
nanohybrid material of LDH/dicarboxylate via simple an ion ex-
change reaction. An optically active diacid 3 was synthesized from
the reaction of PMDA and L-isoleucine amino acid and then it was
converted to dicarboxylate 4 by using NaOH solution (Scheme 1).
Obtained diacid exhibited specific rotations (¼ꢀ88.5ꢁ; measured at
a concentration of 0.5 g dLꢀ1 in DMF at 25 ꢁC). This compound was
used for the preparation of novel modified chiral LDH. Because the
L-isoleucine amino acid is chiral, the nanohybrid is created a chiral
The elemental analysis results for the synthesized new PAI were
in good agreement with calculated percentages for carbon,
hydrogen and nitrogen contents in dinitro, diamine and PAI.
environment in the modified LDH. The spatial orientation of the
chiral dicarboxylate/LDH is shown in Scheme 3. As a possible
model, chiral dicarboxylate was considered to be arranged verti-
cally or horizontally to the LDH basal layer. During the ion-
exchange reaction, the LDH changes from hydrophilic to organo-
philic and the interlayer space of this compound was increased.
These phenomena cause enlarge in the d-spacing which facilitates
the entry of the host polymer molecules between the layered of
modified LDH for further applications. This scheme only serves to
emphasize the process of adsorption and does not provide infor-
mation on the order in which the adsorbent molecule is present
along with carboxylate ion ionically bound on the surface
3.3. Preparation and characterization of modified chiral LDH
In recent times, LDHs are employed as the host material to
synthesize a new organiceinorganic nanohybrid material and have
received significant attention. The organic/LDHs nanohybrid have
been studied because the resulting intercalation compounds are
expected to possess a novel nanostructure and new function
[37,38,41e43]. A synthesis of biomolecule/LDH nanohybrid mate-
rials in particular is of great interest. Actually, the intercalation of
the biomolecule such as amino acid, nucleotide, deoxyribonucleic
acid, and polypeptide into LDHs was described in order to prepare
the biomolecule/LDH nanohybrid materials [37,44e47]. In this
(Scheme 3).
2ꢀ
LDH containing CO3
anions has characteristic bands for
various modes of infrared sensitive vibrations of the anion [48]. The
broad band in the range 3200e3700 cmꢀ1 is due to the OeH
Fig. 3. 13C NMR spectra of dinitro (a) and diamine (b).

2912
S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
O
O
N
O
O
O
H
O
O
NH2
O
H2N
H
H
TBAB
N
N
N
N
N
HO
OH
+
12h, 120 oC
H
H
H
O
O
O
O
O
O
n
R
R
Chiral Diacid 3
8
N
N
R:
NH
PAI with azobenzene side chain
Scheme 2. Preparation of chiral PAI.
stretching vibration of the metal hydroxide layer and interlayer
water molecules (Fig. 4a). The bending vibration of the interlayer
H2O is also reflected in the broad bands around 1590 cmꢀ1. The
band characteristic to metaleoxygen bond stretching appears
below 700 cmꢀ1. The sharp bands around 780 cmꢀ1, 554 cmꢀ1 and
440e450 cmꢀ1 are caused by various lattice vibrations associated
with metal hydroxide sheets (Fig. 4a) [48].
dimensions. The uniform distribution of modified LDH in polymer
matrices can lead to physical, morphological, and thermal property
enhancements in the resulting NC materials.
The representative FT-IR spectra of the bulk PAI and PAI/modi-
fied LDH NC materials are shown in Fig. 4. Thin films of PAI and NCs
were made by evaporating solvent at 80 ꢁC and used for FT-IR
analysis. Fig. 4c and d shows the FT-IR spectra of PAI/LDH NCs
containing 2 and 8% of LDH, respectively. From these figures, it can
be concluded that the resulting NC materials not only have char-
acteristic neat PAI bands, but also have characteristic pick for
modified LDH.
The FT-IR spectrum of modified LDH shows two types of bands:
one corresponding to the anionic species intercalated and other
corresponding to the host LDH materials (Fig. 4b). The band around
447 cmꢀ1 results from the lattice vibration of the hydroxide sheet
and the broad band in the range 3200e3700 cmꢀ1 mainly from Oe
H groups of the hydroxide layers. The appearance of characteristic
3.5. Characterization of the PAI/modified LDH NCs
2ꢀ
2ꢀ
bands for CO3 indicates that still some CO3 exist in the inter-
layer region. In addition to carbonate ions retained during the
calcination process, carbonate ions could be adsorbed during the
regeneration process from the atmospheric CO2 dissolved in the
dispersion medium. However, a broad band or shoulder is observed
in all modified samples in the range 1600e1640 cmꢀ1, which may
indicate the presence of H2O molecules as a bending vibration
appears in this region [48]. The absorption bands at 2930e
3100 cmꢀ1 correspond to the nasym and nsym (CeH) modes of CH2
group in the chiral dicarboxylate molecules (Fig. 4b). The other
dicarboxylate bands originated from various functional groups are
also found at 1740e1700 cmꢀ1 (C]O) and 1300e1000 cmꢀ1 (CeO).
The band broadening by intercalation results from the electrostatic
interaction between chiral diacid molecules and hydroxide sheets
to suggest their safe stabilization in the interlayer space of LDH.
3.5.1. X-ray diffraction
The XRD analysis is used as a very significant technique to
explain the extent of intercalation and exfoliation of the nanofiller
having layered structure. The complete or high degree of exfolia-
tion of layered crystalline filler in polymer matrix certainly means
disappearance of corresponding peaks from the XRD spectrum. The
XRD patterns of the MgAleLDH, modified LDH, neat PAI, NC4% and
NC8% are shown in Fig. 5. The main diffraction peaks of the MgAle
LDH (d003 ¼ 0.76 and d006 ¼ 0.38 nm) were observed in Fig. 5. After
modification of MgAleLDH with chiral dicarboxylate, these
diffraction peaks were shifted to lower angles. The position of the
basal reflections of modified sample is shifted to higher d value
indicating the expansion in the interlayer distance. The modified
3.4. Preparation of the PAI/modified LDH NCs
The affinity between polymer matrix and LDH is one of the most
important factors in achieving good exfoliation; to a certain extent
the affinity can be enhanced by optimizing the structure of the LDH
for a given polymer matrix. Novel optically active PAI/LDH NCs
containing 2, 4, 6 and 8% of LDH were successfully fabricated using
modified LDH with chiral isoleucine containing dicarboxylate and
PAI chains with same group in dry DMAc through solution inter-
calation technique (Scheme 3). These new families of homogeneous
transparent materials are the combinations of organic polymers
and modified LDH blended with each other at ultrafine phase
Fig. 4. FT-IR spectra of (a) LDH, (b) modified LDH, (c) NC2% and (d) NC8%.
Scheme 3. Preparation of modified chiral LDH and NCs of PAI and modified LDH.

S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
2913
corresponding to the LDH irrespective of the variation in LDH
content. This complete disappearance of LDH peaks may be due to
the partial exfoliated structure, in which the gallery height of
intercalated layers is large enough and the layer correlation is not
detected by XRD (Fig. 5). In addition, the disappearance of the
higher order peaks in the XRD patterns of the NCs could be due to
the loss of crystalline symmetry in the stacking direction of the
hydroxide layers, reducing the number of hydroxide layers and
insertion of polymer chains into the intergallery space of the
modified LDH [49]. Although XRD provides a partial picture about
distribution of nanofiller and disappearance of peak corresponding
to d-spacing does not always confirm the exfoliated NCs, a com-
plete characterization of BNC morphology requires microscopic
investigation.
3.5.2. FE-SEM
The FE-SEM micrograph of pristine LDH and modified LDH is
shown in Fig. 6aed. The FE-SEM image of pure LDH reveals the
nature of LDH particles, which roughly consists of plate-like shape
stacked on top of each other with lateral dimensions ranging over
few micrometer and thickness over few hundred nanometer
(Fig. 6a and b). The morphological feature of the modified LDH is
similar to the unmodified LDH. It appears more floppy compared to
unmodified LDH (Fig. 6c and d).
The FE-SEM micrograph of pure PAI and PAI/modified LDH NC
containing 4 and 8 wt.% of modified LDH nanoparticles is shown in
Fig. 7. Neat polymer exhibits relatively a spherical shape (Fig. 6a and
b). The FE-SEM images of PAI/modified LDH NC4% and NC8% show
that the morphology is changed (Figs. 6f and 7c). It seems that the
particles are distributed uniformly in the polymer matrix.
Fig. 5. XRD patterns of different compounds.
LDH don’t show a distinct reflection at d ¼ 0.76 nm. The XRD pat-
terns of the modified LDH showed the expanding LDH structure
with a sharp (003) spacing of 1.53 nm in Fig. 5.
In the XRD pattern of the neat PAI, no sharp diffraction peaks
were observed and this indicated that this polymer is amorphous
(Fig. 5). The XRD patterns of the PAI/modified LDH NCs (4 and 8%)
are characterized by the disappearance of the diffraction peaks
3.5.3. TEM
Typical TEM images of modified LDH, NC4% and NC8% are shown
in Fig. 8. For modified LDH, the platelets have a hexagonal shape
Fig. 6. FE-SEM micrographs of (a, b) LDH and (c, d) modified LDH.

2914
S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
Fig. 7. FE-SEM images of (a, b) pure PAI, (c, d) NC4% and (e, f) NC8%.
with rounded corners. There are no signs of aggregation visible in
the micrographs (Fig. 8a and b). Extensive TEM observations of
NC4% and NC8% reveal a coexistence of modified LDH in the
intercalated and partially the exfoliated states. TEM micrograph
shows two-dimensional objects which are oriented largely parallel
to the grid surface and thin sheet like object with similar lateral
dimensions. The sheets have homogeneous contrast, reflecting
their ultrathin nature and uniform thickness. This TEM photograph
demonstrates that most of the modified LDH layers were interca-
lated and dispersed homogeneously into the PAI matrix (Fig. 8cef).
temperature decomposition step in the unmodified LDH lies below
230 ꢁC. The high temperature decomposition of the unmodified
LDH takes place in two distinct steps with decomposition peaks
around 300 ꢁC and 430 ꢁC (Fig. 9). The organic modification of the
LDH changes its the thermal decomposition behavior in compari-
son to the unmodified sample.
The TGA curves of PAI and PAI/modified LDH NCs are shown in
Fig. 10 and the resulting TGA data are summarized in Table 1 that
including temperatures at which 5% (T5), 10% (T10) degradation
occur and the residue at 800 ꢁC. TGA has been used under nitrogen
atmosphere at a heating rate of 10 ꢁC minꢀ1. The initial decompo-
sition temperatures of the NCs with different modified LDH (2, 4
and 8%) are above 360 ꢁC. The residues at 800 ꢁC of the NCs with
different modified LDH content are higher than that of pure PAI. The
observed increases in residue for NCs with increasing modified LDH
might be due to the charring of the polymer consisting of carbo-
naceous char along with increasing level of decomposition prod-
ucts of LDH, i.e. MgO and Al2O3 residue [52].
3.5.4. Thermal studied
LDHs contain large amount of bound water due to the presence
of eOH group on the metal hydroxide sheets and some free water
molecules in the interlayer region. The comparison of the TGA plots
of the modified LDH with that of the unmodified one gives an
indication how the interlayer surfactants anions influence the
decomposition of the host material. The most widely reported
proposition suggests a two-stage decomposition process: a low
temperature dehydration stage due to the loss of interlayer water
and a high temperature decomposition stage due to the loss of
interlayer carbonate and dehydroxylation of the metal hydroxide
layer [50,51]. The thermal decomposition analysis of the unmodi-
fied LDH and its modified forms is presented in Fig. 9. The low
3.5.5. Limited oxygen index-self extinguishing material
The limiting oxygen index (LOI) is a measure of the percentage
of oxygen to be present to support the combustion of the materials
and it can be used to evaluate the flame-retardancy of them. Two
interesting relationships have been found between the LOI and the

S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
2915
Fig. 8. TEM micrograph of (a, b) modified LDH, (c, d) NC4% and (e, f) NC8%.
parameters of the combustion process: char yield or char residue
(CR) and heat of combustion. The percentage of oxygen in the air is
around 21%. It is clear that all materials with an LOI lower than this
level will burn easily, while those with a higher LOI will tend not to
burn. Theoretically, CR can also be used as a criterion for evaluating
LOI of materials.
According to Van Krevelen [53], there is a linear relationship
between LOI and CR only for halogen-free polymers (Eq. (1)):
LOI ¼ ð17:5 þ 0:4 CRÞ=100
(1)
Fig. 9. TGA thermograms of (a) LDH and (b) modified LDH.
Fig. 10. TGA thermograms of the neat PAI and different NC materials.

2916
S. Mallakpour, M. Dinari / Polymer 54 (2013) 2907e2916
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Decomposition temperature (ꢁC)
Residue at
800 ꢁC (wt.%)
a
a
T5
T10
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New organically modified chiral LDH was prepared via simple
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the presence of these carboxylate anions in the LDH interlayer. A
new aromatic diamine containing azobenzene group was success-
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Acknowledgments
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We wish to express our gratitude to the Research Affairs Divi-
sion Isfahan University of Technology (IUT), Isfahan, for partial
financial support. Further financial support from National Elite
Foundation (NEF), Iran Nanotechnology Initiative Council (INIC)
and Center of Excellency in Sensors and Green Chemistry Research
(IUT) is gratefully acknowledged.
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