Novel nanostructure amino acid-based poly(amide-imide)s enclosing benzimidazole pendant group in green medium: Fabrication and characterization

Article abstract of DOI:10.1007/s00726-012-1236-8

In the present work, several novel optically active nanostructure poly(amide-imide)s (PAI)s were synthesized via step-growth polymerization reaction of chiral diacids based on pyromellitic dianhydride-derived dicarboxylic acids containing different natura

Full text of DOI:10.1007/s00726-012-1236-8

Amino Acids (2012) 43:1605–1613
DOI 10.1007/s00726-012-1236-8
ORIGINAL ARTICLE
Novel nanostructure amino acid-based poly(amide–imide)s
enclosing benzimidazole pendant group in green medium:
fabrication and characterization
Shadpour Mallakpour Mohammad Dinari
•
Received: 10 November 2011 / Accepted: 29 January 2012 / Published online: 11 February 2012
Ó Springer-Verlag 2012
Abstract In the present work, several novel optically
active nanostructure poly(amide–imide)s (PAI)s were
synthesized via step-growth polymerization reaction of
chiral diacids based on pyromellitic dianhydride-derived
dicarboxylic acids containing different natural amino
acids such as L-alanine, S-valine, L-leucine, L-isoleucine,
L-methionine, and L-phenylalanine with 2-(3,5-diamin-
ophenyl)-benzimidazole under green conditions using
molten tetrabutylammonium bromide. The new optically
active PAIs were achieved in good yields and moderate
inherent viscosity up to 0.41 dL/g. The synthesized poly-
may well be classified under environmentally friendly
materials.
Keywords a-Amino acids ꢀ Chiral nanostructure
polymers ꢀ Benzimidazole ꢀ Poly(amide–imide) ꢀ
Green chemistry
Introduction
Polymers symbolize an imperative part of our daily life and
over several million tons of the synthetic polymers are
formed every year. They have many advantages like higher
energy efficiency, less weight, higher performance and
durability, and more flexibility in design and processing
over other conventional materials (Erdmenger et al. 2010;
Saito and Hearn 2008). For the synthesis of these important
materials, common organic solvents are used expensively,
which are volatile and most of them are flammable, toxic,
quite hazardous, and harmful (Azapagic et al. 2003). New
methods for polymer processing and synthesis could reduce
or eliminate the environmental problems associated with
polymer manufacturing as well as increase the energy
efficiency and decrease the waste generation. In this regard,
the most common explored green solvents for polymer
production are ionic liquids (IL)s, supercritical carbon
dioxide, and water (Kerton 2009; Nelson 2003; Mallakpour
1
mers were characterized with FT-IR, H-NMR, X-ray
diffraction, field emission scanning electron microscopy
(
(
FE-SEM), elemental and thermogravimetric analysis
TGA) techniques. These polymers show high solubility
in organic polar solvents due to the presence of amino
acid and benzimidazole pendant group at room tempera-
ture. FE-SEM results show that, these chiral nanostructured
PAIs have spherical shapes and the particle size is around
2
0–80 nm. On the basis of TGA data, such PAIs are
thermally stable and can be classified as self-extinguishing
polymers. In addition due to the existence of amino acids in
the polymer backbones, these macromolecules are not only
optically active but also could be biodegradable and thus
S. Mallakpour (&) ꢀ M. Dinari
Department of Chemistry, Organic Polymer Chemistry Research
Laboratory, Isfahan University of Technology,
2011).
The exceptional combination of non-volatility and
8
4156-83111 Isfahan, Islamic Republic of Iran
favorable solubility characteristics of ILs has motivated a
growing interest as a replacement for organic solvents, and
more generally of volatile organic compounds in many
industrial applications (Bideau et al. 2011; Parvulescu and
Hardacre 2007). These attractive features of ILs made them
fashionable candidates in a broad number of applications in
e-mail: mallak@cc.iut.ac.ir; mallak777@yahoo.com;
mallakpour84@alumni.ufl.edu
S. Mallakpour
Nanotechnology and Advanced Materials Institute,
Isfahan University of Technology,
8
4156-83111 Isfahan, Islamic Republic of Iran
123

1
606
S. Mallakpour, M. Dinari
various fields including organic synthesis, extraction/sep-
aration, electrochemical analysis, catalysis, and chemical
sensors (Betz et al. 2011; Chen et al. 2010; Moniruzzaman
et al. 2010; Olivier-Bourbigou et al. 2010; Quijano et al.
into PAI systems may play an important role in molecular
arrangement. It is also interesting to mention that macro-
molecules containing a high degree of amino acid func-
tionality can lead to the formation of polymers with
improved solubility, biodegradability, and biocompatibility
(Hsiao et al. 2010; Levesque et al. 2010; Mallakpour et al.
2011; Mallakpour and Dinari 2011b; Sanda and Endo
1999).
2
011). Actually, negligible vapour pressure allows easy
retrieval of the final products by distillation or using
biphasic chemical processes without degradation or loss of
solvent by evaporation and, consequently, an easy recy-
cling (Earle and Seddon 2000; Hubbard et al. 2011). Also,
they have found numerous applications not only as envi-
ronmentally benign reaction media, but also as catalysts for
many types of direct polycondensation due to the high
temperatures often employed in this type of reaction. Most
of these ILs are expensive and it would be cost effective to
use less expansive and safe molten ILs such as tetrabu-
tylammonium bromide (TBAB) (Mallakpour and Mirkar-
imi 2010). Molten TBAB is low-cost, readily accessible,
and has inherent properties like ecological compatibility,
operational simplicity, greater selectivity, non-corrosive
nature, and no difficulty of reusability (Kantevari et al.
The benzimidazole rings with heteroaromatic structure
have excellent stabilities derived from its molecular sym-
metry and aromaticity, so the incorporation of benzimid-
azole group into PI backbone would improve the solubility
without deterioration of their own excellent properties
(Ayala et al. 2005; Dubey et al. 2007; Wang and Wu 2003;
Wu et al. 2009).
The designing of new nanostructure materials from
organic molecules has opened up new fundamental and
practical frontiers to improve physical properties of mate-
rials significantly for different purposes (Livi et al. 2011).
Different nanostructure polymers have been developed for
a wide range of advanced uses such as drug delivery
devices, conducting wires, and optoelectronic devices (Dai
2004). In addition, amino acid-based systems have been
found to show nano-scale ordering into stable hierarchical
superstructures administered by the formation of secondary
structures in these segments (Klok and Lecommandoux
2001; Mallakpour and Dinari 2011a, b). Based on these
above considerations, in this study, we introduced natural
amino acid into PAIs backbone which not only classified
them as nanostructure materials and moreover induce chi-
rality in the obtained polymers, but also improve the sol-
ubility of the resulting macromolecules. To maintain the
thermal stability; benzimidazole groups were also intro-
duced into PAIs side chain. These polymers were prepared
with direct polycondensation reaction of different amino
acids-based diacid monomers and benzimidazole diamine
in green medium using molten TBAB. In this way, a good
balance of thermal resistance and solubility was obtained.
The properties of the obtained polymers, such as specific
rotation, viscosity, solubility, thermal stability and mor-
phology are discussed here.
2
008).
Among the heat resistant and high performance poly-
mers, polyimides (PI)s have received considerable scien-
tific and technological attention due to their outstanding
performance such as dielectrical and mechanical properties
as well as thermal and chemical stability (Yu et al. 2010).
Several potential applications of PIs have appeared in the
literature for advanced technology. Further, interest in their
functional construction and application has been increasing
to obtain PIs possessing certain specific properties (Chen
et al. 2009; Seckin et al. 2010; Yang et al. 2002). However,
wholly aromatic PIs do not always provide the optimum
properties for many specialty applications owing to defi-
ciencies in processability, solubility, and transparency as
well as their relatively high dielectric constant because of
their rigid backbones, and strong interactions between
chains (Eichstadt et al. 2002; Hsiao and Lin 2005; Wang
and Chen 2011). To overcome these problems, much
research effort has been focused on the synthesis of soluble
and processable PIs in fully imidized form without dete-
rioration of their own excellent properties. Several
approaches to fabricate soluble PIs including introduction
of flexible linkage and heteroaromatic rings into the poly-
mer backbone or preparation of co-PIs like poly(amide–
imide)s (PAI)s have been developed in the past decade
Experimental
(
Ding et al. 2002; Mallakpour and Dinari 2011a; Qiu et al.
007; Shockravi et al. 2009). PAI have better process-
Materials
2
ability than PI and excellent heat resistant properties than
polyamide. Among the PAIs, type of chiral is important
because the majority of bioorganic molecules are chiral.
The optical activity of the polymer can be tuned by
choosing a suitable chiral initiator or by starting from a
chiral monomer. The introduction of chiral structural units
Solvents and chemicals were obtained from the Aldrich
Chemical Co. (Milwaukee, WI, USA), Riedel–deHaen AG
(Seelze, Germany), and Merck Chemical Co. (Germany).
Pyromellitic dianhydride (PMDA) and TBAB (mp =
100–103°C) were purchased from Merck Co. (Darmstadt,
Germany) and were used without further purification.
1
23

Novel nanostructure amino acid-based poly(amide–imide)s enclosing benzimidazole pendant group
1607
0
-1
N,N -Dimethylformamide (DMF) were dried over barium
FT-IR (KBr, cm ): 3,336 (m), 3,102 (W), 1,593 (w),
1,541 (s), 1,346 (s), 1,075 (m), 808 (m), 731 (s) and
789 (w).
oxide and then were distilled under reduced pressure.
L-Alanine, S-valine, L-methionine, L-leucine, L-isoleucine
and L-phenylalanine were used as obtained without further
purification. 3,5-Dinitrobenzoyl chloride, 1,2-phenylene-
diamine, hydrazine monohydrate, phosphorus pentoxide
2-(3,5-Diaminophenyl)-benzimidazole 8 was prepared
by reduction of dinitro precursor in refluxed ethanol using
palladium and hydrazine monohydrate (yield: 77%; m.p.
242–243°C) (Alvarez-Gallego et al. 2008; Ayala et al.
2005).
(
P O ), and methanesulfonic acid (MSA) were obtained
2 5
from commercial sources and used as received.
-
1
FT-IR (KBr, cm ): 3,409 (s, br), 3,063 (w), 1,607 (s),
Characterization
1,577 (s), 1,484 (w), 1,446 (m), 1,274 (w), 841 (m) and 743
1
(
s). H NMR (400 MHz, DMSO-d d, ppm): 4.91 (s, 4H,
6
,
1
Proton nuclear magnetic resonance ( H-NMR, 500 MHz)
NH ), 5.96 (s, 1H, Ar–H), 6.61 (s, 2H, Ar–H), 7.12–7.17
2
spectra were recorded in dimethylsulfoxide (DMSO-d6)
solution using a Bruker (Germany) Avance 500 instrument.
Multiplicities of proton resonance were designated as sin-
glet (s), doublet (d), and multiplet (m). Infrared spectra of
the samples were recorded at room temperature in the
(d, 2H, Ar–H, J = 8.56 Hz), 7.51–7.56 (m, 2H, Ar–H),
12.62 (s, 1H, NH benzimidazole). Elem. Anal. Calcd. for
C H N (224.23): C, 69.62%; H, 5.39%; N, 24.98.
1
3 12 4
Found. C, 69.60%; H, 5.36%; N, 24.88%.
-
1
range of 4,000–400 cm , on (Jasco-680, Japan) spectro-
photometer. Spectra of solids were obtained using KBr
pellets. Vibrational transition frequencies are reported in
Polymers synthesis
The PAIs were prepared by the following general proce-
dure: as an example for the preparation of PAI9a, a mix-
-
1
wavenumber (cm ). Inherent viscosities (linh) were
measured by a standard procedure using a Cannon–Fenske
-
ture of 0.10 g (2.36 9 10 mol) of diacid 4a and 0.30 g
4
2
5
-4
(9.42 9 10 mol) of TBAB was ground until a powder
Routine Viscometer (Germany). Specific rotation ([a] )
D
was measured with a Jasco (Osaka, Japan) P-1030 polar-
imeter at the concentration of 0.5 g/dL at 25°C. XRD
patterns were recorded using CuKa radiation on a Bruker,
D8 Advance, (Germany) diffractometer operating at cur-
rent of 100 mA and a voltage of 45 kV. The diffractograms
were measured for 2h, in the range of 58–808, using CuKa
was formed and then it was transferred into a 25-mL round-
-
4
bottom flask and 0.06 g (2.36 9 10 mol) of diamine 8
was added to the mixture. It was heated until homogeneous
solution was formed and then TPP (0.49 mL,
-
3
3.70 9 10 mol) was added. The solution was stirred for
12 h at 120°C, and the viscous solution was precipitated in
30 mL of methanol. The white solid was filtered off and
dried to give 0.13 g (87%) of PAI9a. The other PAIs were
prepared by a similar procedure.
˚
incident beam (k = 1.51418 A). Thermal gravimetric
analysis (TGA) data were taken on STA503 WinTA
instrument in a nitrogen atmosphere at a heating rate of
2
5
1
0°C/min. Surface morphology of polymers were charac-
terized using field emission scanning electron microscopy
FE-SEM) [HITACHI: S-4160].
PAI9a: linh = 0.41 dL/g, [a]D = -67.60 (c = 0.5
-
1
g/dL, DMP). FT-IR peaks (KBr, cm ): 3,393 (br), 3,063
(w), 2,946 (m), 1,775 (s), 1,724 (s), 1,684 (m), 1,577 (m),
(
1
1
,487 (m), 1,455 (w), 1,375 (m), 1,347 (m), 1,222 (m),
,108 (w), 848 (w), 747 (m), 697 (m), 617 (w) and 558 (m).
Monomers synthesis
PAI9b: White solid, yield: 90%; l = 0.43 dL/g,
inh
2
5
[
a] = -80.41 (c = 0.5 g/dL, DMP). FT-IR peaks (KBr,
D
Synthesis of amino acid containing diacid monomers
-
cm ): 3,436 (br), 2,926 (m), 1,775 (m), 1,724 (s), 1,636
1
(
m), 1,575 (s), 1,416 (m), 1,380 (m), 1,188 (w), 1,114 (m),
1
26 (m), 877 (m),727 (m), 700 (m) and 562 (w). H-NMR
Optically active diacid monomers bearing different natural
amino acids (4a–4f) were prepared according to our pre-
vious works (Mallakpour and Dinari 2011a, b).
9
(
500 MHz, DMSO-d , d, ppm): 0.87–1.04 (m, 12H),
6
2
6
.77–2.79 (m, 2H), 4.86–4.87 (d, 2H, J = 8.15 Hz),
.91–6.92 (distorted d, 2H, Ar–H), 7.23–7.25 (distorted d,
Synthesis of dinitro and diamine
2H, Ar–H), 7.63 (s, 1H, Ar–H), 7.83 (s, 2H, Ar–H), 10.58
s, 2H, NH amide), 12.78–12.86 (br, 1H, NH benzimid-
azole). Elem. Anal. Calcd. for C H N O : C, 65.55%; H,
(
The pure dinitro intermediate, 2-(3,5-dinitrophenyl)-benz-
imidazole 7 was prepared from 3,5-dinitrobenzoyl chloride
and 1,2-phenylenediamine using MSA and P O (yield:
3
3 28 6 6
4.67%; N, 13.90. Found C, 65.26%; H, 4.52%; N, 13.78%.
2
5
PAI9c: Off white solid, yield: 94%; l = 0.51 dL/g,
inh
2
5
70%; m.p. 331–332°C) (Alvarez-Gallego et al. 2008; Ayala
et al. 2005).
[a] = -77.23 (c = 0.5 g/dL, DMP). FT-IR peaks (KBr,
D
-
cm ): 3,367 (br), 3,071 (w), 2,925 (m), 1,775 (m), 1,721
1
123

1
608
S. Mallakpour, M. Dinari
(
s), 1,617 (m), 1,586 (m), 1,487 (m), 1,382 (m), 1,355 (m),
,215 (m), 1,076 (w), 895 (m), 750 (m), 688 (m), 617
C H N O : C, 66.45%; H, 5.10%; N, 13.28. Found C,
35 32 6 6
1
65.89%; H, 5.03%; N, 13.17%.
1
(
w) and 555 (m). H-NMR (500 MHz, DMSO-d , d, ppm):
PAI9f: Off white solid, yield: 94%; l = 0.51 dL/g,
6
inh
2
5
1
.96–2.03 (m, 10H), 2.55–2.57 (m, 4H), 5.11-5.13 (m, 2H),
[a] = -77.23 (c = 0.5 g/dL, DMP). FT: FT-IR peaks
D
-
1
6
.74–6.76 (m, 2H, Ar–H), 7.34–7.36 (m, 2H, Ar–H), 7.78
(KBr, cm ): 3,479 (br), 3,165 (w), 3,038 (w), 2,937 (m),
1,771 (s), 1,720 (s), 1,634 (m), 1,602 (w), 1,498 (m), 1,456
(m), 1,385 (s), 1,366 (s), 1,258 (w), 1,226 (m), 1,170 (m),
1,079 (m), 1,045 (w), 941 (s), 918 (m), 881 (m), 828 (m),
(
s, 1H, Ar–H), 7.98–8.02 (m, 2H, Ar–H), 10.40 (s, 2H, NH
amide), 12.64–12.80 (br, 1H, NH benzimidazole). Elem. Anal.
Calcd. for C H N O S : C, 59.55%; H, 4.67%; N, 12.90; S,
33 28 6 6 2
1
9
.59. Found C, 58.89%; H, 4.06%; N, 12.44%; S, 9.47.
733 (s), 700 (s), 631 (m), 568 (s) and 492 (w). H-NMR
PAI9d: White solid, yield: 83%; l = 0.46 dL/g,
(500 MHz, DMSO-d , d, ppm): 3.37–3.39 (m, 4H), 5.31 (d,
inh
6
2
5
[
a] = -84.72 (c = 0.5 g/dL, DMP). FT-IR peaks (KBr,
2H, J = 7.65 Hz), 7.14–6.16 (m, 14H, Ar–H), 7.67 (s, 1H,
Ar–H), 7.98 (s, 2H, Ar–H), 8.21 (s, 2H, Ar–H), 10.34
(s, 2H, NH amide), 12.67-12.71 (br, 1H, NH benzimid-
azole). Elem. Anal. Calcd. for C H N O : C, 70.26%; H,
D
-
1
cm ): 3,434 (br), 3,066 (w), 2,967 (m), 2,934 (m), 1,777
m), 1,722 (s), 1,617 (m), 1,577 (s), 1,488 (m), 1,418 (w),
,385 (m), 1,220 (m), 1,164 (w), 1,087 (m), 970 (w), 752
(
1
4
1 30 6 6
1
(
m), 690 (m), 620 (m) and 558 (m). H-NMR (500 MHz,
4.00%; N, 11.99. Found C, 69.94%; H, 3.97%; N, 11.76%.
DMSO-d , d, ppm): 0.85–0.92 (m, 12H), 1.52–1.54 (m,
6
2
H), 1.98–2.02 (m, 4H), 4.81–4.83 (m, 2H), 6.92–6.93
distorted d, 4H, Ar–H), 7.19-7.21 (d, 2H, Ar–H,
J = 9.45 Hz), 7.37 (s, 1H, Ar–H), 7.70 (s, 2H, Ar–H), 8.31
s, 2H, Ar–H), 10.37 (s, 2H, NH amide), 12.80–12.84 (br,
H, NH benzimidazole). Elem. Anal. Calcd. for
C H N O : C, 66.45%; H, 5.10%; N, 13.28. Found C
(
Result and discussion
(
Synthesis of chiral diacids
1
Chiral diacid monomers 4a–4f were prepared by the con-
densation reaction of one equimolar of dianhydride 1 and
one equimolar of different natural amino acids (L-alanine,
S-valine, L-methionine, L-leucine, L-isoleucine, and
L-phenylalanine) in refluxing acetic acid, as shown in
Scheme 1.
3
5 32 6 6
6
6.17%; H, 5.00%; N, 13.08%.
PAI9e: White solid, yield: 89%; l = 0.48 dL/g,
inh
2
D
-
5
[
a] = –78.45 (c = 0.5 g/dL, DMP). FT-IR peaks (KBr,
1
cm ): 3,423 (br), 3,074 (w), 2,987 (m), 2,917 (w), 1,774
m), 1,718 (s), 1,628 (m), 1,576 (w), 1,439 (w), 1,385 (s),
,364 (m), 1,288 (m), 1,158 (w), 1,108 (m), 1,011 (w), 898
(
1
1
(
m), 833 (m), 730 (m), 627 (m) and 562 (w). H-NMR
Synthesis of diamine
(
500 MHz, DMSO-d , d, ppm): 0.83–1.03 (m, 12H),
6
1
.48–1.50 (m, 4H), 1.94–1.96 (m, 2H), 4.77–4.79 (m, 2H),
Diamine 8 was prepared with a two-step procedure out-
lined in Scheme 2. In the first step, the dinitro compound
was obtained by the direct condensation of 1,2-phenyl-
enediamine and 3,5-dinitrobenzoyl chloride with Eaton’s
reagent (1:10 P O /MSA) (Eaton et al. 1973; Leykin et al.
6
.85–6.86 (distorted d, 4H, Ar–H), 7.13–7.15 (d, 2H, Ar–H,
J = 8.76 Hz), 7.34 (s, 1H, Ar–H), 7.66 (s, 2H, Ar–H), 8.25
(
(
s, 2H, Ar–H), 10.45 (s, 2H, NH amide), 12.86–12.92
br, 1H, NH benzimidazole). Elem. Anal. Calcd. for
2
5
Scheme 1 Preparation of
amino acid containing diacid
monomers
O
O
O
O
O
O
O
NH2
*
AcOH
HO
NH
HOOC
HN
OH
O
O
+
HO
H
R
H
H R
R
COOH
O
O
1
3a-3f
2a-2f
,
,
CH3
CH
CH2
H2C
H C
3
CH3
O
N
O
S
O
O
2a
2b
2c
CH3
N
Reflux HO
OH
,
H
R
,
R
H R
O
H2C
CH
CH2
CH3
CH2
O
CH
H C
3
4
a-4f
H C
CH3
3
2d
2e
2f
1
23

Novel nanostructure amino acid-based poly(amide–imide)s enclosing benzimidazole pendant group
1609
O2N
NO2
O2N
NO2
Characterized techniques
H2N
NH2
1
FT-IR and H-NMR study
MSA/ P O
2
5
Pd/ C
N H
O
N
NH
Cl
5
+
N
NH
The FT-IR spectrum of dinitro 7 revealed a strong peak at
3
2
4
-
,336 cm , which was assigned to the NH stretching
1
H2N
NH2
-
1
group, and two absorption bands at 1,541 and 1,346 cm
,
7
which were characteristic peaks for NO asymmetric and
2
8
symmetric, respectively (Fig. 1). In the FT-IR spectra of
synthesized diamine 8, the characteristic peaks for NH2
functions and the absence of the original peaks arising from
6
Scheme 2 Preparation of 5-(2-benzimidazole)-1,3-phenylenediamine
the NO groups in the corresponding dinitro intermediate
2
provided that this compound was successfully prepared
(
Fig. 1). Absorption of amine NH and NH benzimidazole
2
2
010) as the condensing agent and solvent. In the second
-1
bonds appeared around 3,394 and 3,323 cm and the peak
step, nitro groups were reduced to the corresponding amino
groups with hydrazine hydrate as the reducing agent and
palladium as the catalyst. By this procedure, pure diamine
-1
at 1,627 cm confirms the presence of NH deformation.
-1
Two absorption bands at 1,577 and 1,484 cm
were
characteristic peaks for aromatic rings. The benzimidazole
-1
8
was attained after recrystallization.
groups gave bands at 1,484 cm (in plane deformation of
-
1
the benzimidazole ring), 1,590 cm
(ring vibration of
Preparation of the organosoluble chiral PAIs
conjugation between fused benzene and imidazole rings)
-
and a shoulder at 1,630 cm (C–N stretching).
1
In the current investigation, in order to extend the utili-
zation of ILs in polymers synthesis, molten TBAB was
used as solvent and catalyst for the formation of several
novel optically active, organo-solube and thermally stable
nanostructure PAIs by the direct polymerization reaction
of compound 8 with different chiral monomers 4a–4f
1
In the H-NMR of diamine 8, appearances of the N–H
protons of benzimidazole group at 12.62 ppm as broad
singlet peaks indicate the presence of this group in the
diamine side chain. The absorption of aromatic protons
appeared in the range of 5.96–7.50 ppm. The proton of the
amine groups appeared as broad singlet peaks at 4.91 ppm.
The structures of new PAIs were confirmed by FT-IR
(
Scheme 3). In this method, the polymerization reaction
was performed under oil-bath heating in the presence of
molten TBAB and TPP. It is interesting to mention that
the above polyamidation in the absence of either TPP or
TBAB will not occur. Therefore, both TPP and TBAB are
required which will act both as catalyst and solvent
1
and H-NMR spectroscopy and elemental analysis tech-
niques. The N–H stretching bands of the amide and benz-
imidazole groups could be observed by FT-IR spectroscopy
-1
for all the polymers around 3,400–3,300 cm . FT-IR
spectra of all polymers also show the characteristic
absorption peaks for the imide ring at 1,775 and
(
Mallakpour and Mirkarimi 2010). The inherent viscosi-
ties of the resulting polymers under optimized conden-
sations were in the range of 0.41–0.58 dL/g and yields
were 87–94%. The incorporation of a chiral unit into the
polymer backbone was obtained by measuring the specific
rotations of polymers that represent all PAIs are optically
active.
O
O
O
O
TBAB/ TPP
H
H
4a-4f
+
8
N
N
N
N
1
20 oC, 12h
R
H
R
H
O
O
n
N
NH
PAI9a-PAI9f
Scheme 3 Polycondensation reactions of diamine 8 with chiral
diacids 4a–4f in molten TBAB
Fig. 1 FT-IR (KBr) spectrum of dinitro 7, diamine 8 and PAI9a
123

1
610
S. Mallakpour, M. Dinari
-
1
1
,714 cm
due to the symmetrical and asymmetrical
chiral center appeared as multiplets in the range of
4.72–5.34 ppm. The resonance of the diastrotopic hydro-
gens bonded to neighbor carbon of chiral center appeared
in the range of 1.97–3.37 ppm as two discrete multiplets
carbonyl stretching vibrations. All of them exhibited
-
1
medium absorptions at 1384 and 718–720 cm that show
the presence of the imide heterocycle ring in these poly-
mers. The N–H bending and C–N stretching bands at
peaks. The resonance of the CH protons groups of the
3
-
1
1
,550 cm could also be observed. For example, the FT-
IR spectrum of PAI9a (Fig. 1) displays characteristic
amino acids appeared as a broad multiplet peak at
0.86–1.37 ppm.
-
1
absorption bands for the imide ring at around 1,722 cm
,
which is indicative of the asymmetrical and symmetrical
C=O stretching vibration, and at 1,375, 1,064, and
Morphology investigations
-
1
7
31 cm
due to imide ring deformation. The N–H
Morphology of the PAIs was recorded by X-ray diffraction
which is the most commonly used methods to study the
structure of nanostructure materials. For PAI9a, PAI9b
and PAI9c, it could be observed that except one crystalline
peak, there is a lack of any diffraction peak in the range of
2h angle (Fig. 2). The diffraction patterns reveal the pres-
ence of a little percentage of crystalline phases compared
to an amorphous one, primarily because of the bulky
pendant groups. Furthermore, the amorphous nature of
these polymers is also reflected in their good solubility,
which is in agreement with the general rule that the solu-
bility decreases with increasing crystallinity.
stretching band of the amide and benzimidazole groups
-
observed around 3,320 cm , and the C=O stretching band
1
-
of amide group at 1,669 cm . Other PAIs had similar
1
functional groups.
1
In the H-NMR spectrum of these PAIs, appearances of
the N–H protons of benzimidazole and amide groups at
1
2.60–12.80 and 10.35–10.51 ppm indicate the presence of
these groups in the polymers side chain, as well as main
chain, respectively. The resonance of aromatic protons
appeared in the range of 6.73–8.35 ppm. The proton of the
In this study, it is worth mentioning that the FE-SEM
pictures of the synthesized PAIs showed that the obtained
macromolecules have nanostructured morphology (Fig. 3).
Also, in order to study the effect of ultrasound wave on
morphology of these polymers, synthesized PAIs were
irradiated for 30 min in cold ethanol mixture under ultra-
sonic waves. By comparing FE-SEM images before
0 0
(
Fig. 3a–d) and after (Fig. 3a –d ) ultrasonic irradiation, the
results showed that after sonication process, the size of PAI
particles was decreased. According to these data, before
sonication process, PAIs showed the spherical shapes and
the particle size is around 40–80 nm, while after this
Fig. 2 XRD pattern of PAI9a (a), PAI9b (b) and PAI9c (c)
0
0
Fig. 3 FE-SEM micrographs of PAI9a–PAI9d before ultrasound (a–d) and after ultrasound (a –d )
1
23

Novel nanostructure amino acid-based poly(amide–imide)s enclosing benzimidazole pendant group
1611
Table 1 Thermal properties of the optically active PAIs
b
c
PAI code
Decomposition
o
temperature ( C)
Char yield (%)
LOI
a
5
a
T
T
10
9
9
9
9
9
9
a
a
b
c
d
e
f
397
401
404
411
416
425
429
431
440
462
465
471
49
51
53
54
55
58
37.1
37.9
38.7
39.1
39.5
40.7
Temperature at which 5 and 10% weight loss was recorded by TGA
Fig. 4 TGA thermograms of PAI9a–PAI9f under
atmosphere
a
nitrogen
-1
at heating rate of 10°C min in N
2
atmosphere
Weight percent of the material left undecomposed after TGA at
maximum temperature 800°C in N atmosphere
b
2
c
process, the size of PAIs decreased and showed smaller
particle in the range of 20–65 nm.
Limiting oxygen index (LOI) evaluated at char yield at 800°C
Solubility of polymers
on Van Krevelen and Hoftyzer equation (Van Krevelen and
Hoftyzer 1976).
The solubility of these thermally stable polymers was
measured qualitatively (0.01 g of polymeric sample in
LOI ¼ 17:5 þ 0:4 CR
where CR = char yield
2
mL of solvent) in various solvents and the results dem-
onstrate that the synthesized PAIs were well dissolved in
aprotic polar solvents such as DMF, N-methyl-2-pyrrolid-
inone, N,N-dimethylacetamide and dimethylsulfoxide at
room temperature. But these polymers were insoluble in
general organic solvents such as xylenes, dioxane, CHCl3
and acetone. In THF, only PAI9f based on L-phenylalanine
as a chiral moiety diacid monomer was soluble. The
enhanced solubility of the resulting PAIs is attributed not
only to the bulkiness of the benzimidazole and amino acid
pendant groups but also to the amide and imide groups in
polymer backbone.
All polymers have LOI values around 37–40 which were
calculated from their char yield at 800°C. On the basis of
LOI values, all macromolecules can be classified as self-
extinguishing polymers (Table 1). According to Table 1, it
is clear that the PAI9f (based on L-phenylalanine) has
higher thermal stability than others (Fig. 4). It could be
pertained to aromatic, rigid structure of phenylalanine for
PAI9f compare to aliphatic, flexible structure of other
macromolecules.
Conclusions
Thermal properties
From the results achieved in this investigation, it can reach
to the following conclusions: the chiral dicarboxylic acids,
containing a rigid pyromellitoyl and flexible amino acid
groups and diamine with benzimidazoles pendant group
were used for the preparation of novel organosoluble
optically active aromatic PAIs. From the chemical point of
view, the incorporation of benzimidazole diamine into the
backbone of diverse polymer systems results in versatile
polymers with interesting properties such as thermal sta-
bility and good solubility. Furthermore, in this work molten
TBAB was used as a solvent and catalyst for the poly-
merization that makes the polycondensation reactions with
safe operation, low pollution and rapid access to products
which make it as an environmentally friendly and green
method, and also decreases the cost of polymerization
reaction. FE-SEM micrographs of these amino acid con-
taining fabricated polymers showed that, all of them have
Thermogravimetric analysis was applied to evaluate
the thermal properties of the PAIs at a heating rate of
1
0°C/min, under a nitrogen atmosphere. The results of
TGA given in Fig. 4 show that the PAIs as a class possess
outstanding thermal stability. The polymers exhibited a
one-step pattern of decomposition with no significant
weight loss below 400°C. From these data, it is clear that
all of the PAIs are thermally stable owing to existence of
various linkages such as imide bond and benzimidazoles
groups in polymer backbones. The PAIs with an aliphatic
chain as part of the side chain are not as thermally stable as
the aromatic polymer. At 800°C, the residue is up to 50%
for all PAIs. These values are remarkably high and they
vary according to the structure of the polymers.
Char yield can be applied as decisive factor for esti-
mated limiting oxygen index (LOI) of the polymers based
123

1
612
S. Mallakpour, M. Dinari
nanostructure that nanosize particle is at range of
0–80 nm. Also, the effect of ultrasonic irradiation on the
Eichstadt AE, Ward TC, Bagwell MD, Farr IV, Dunson DL, McGrath
JE (2002) Synthesis and characterization of amorphous partially
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2
size and shape of polymer nanoparticles was studied and
the results show that the sizes of polymer particles were
decreased after sonication process. In addition, due to the
existence of amino acids in the macromolecules backbone,
these polymers are anticipated to be biodegradable and are
therefore could be classified under environmental benign
materials. These materials are excellent candidates for the
utilization in different systems such as the chiral medium in
asymmetric synthesis and chiral stationary phases in high-
performance liquid chromatography (HPLC) for resolution
of racemic mixtures.
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Affairs Division, Isfahan University of Technology (IUT), Isfahan is
gratefully acknowledged. Also, thankfully acknowledged of Iran
Nanotechnology Initiative Council (INIC), National Elite Foundation
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