Chiral bio-nanocomposites based on thermally stable poly(amide-imide) having phenylalanine linkages and reactive organoclay containing tyrosine amino acid

Article abstract of DOI:10.1007/s00726-012-1436-2

Montmorillonite clay modified with the bio-active trifunctional l-tyrosine amino acid salt was used as a reactive organoclay (OC) for the preparation of poly(amide-imide) (PAI)/OC hybrid films. One of the functional groups of the l-tyrosine as the swelling agent formed an ionic bond with the negatively charged silicates, whereas the remaining functional groups were available for further reaction with polymer matrix. The soluble PAI with amine end groups including phenylalanine amino acid was synthesised under green condition using molten tetra-butylammonium bromide by direct polymerization reaction of chiral diacid and 2-(3,5-diaminophenyl)benzimidazole. PAI/OC bio-nanocomposites films containing different contents of OC were prepared via solution intercalation method through blending of OC with the PAI solution. X-ray diffraction and transmission electron microscopy revealed that the dispersion of silicate layers in the PAI created an exfoliated structure as a result of using the trifunctional groups of the swelling agent. The structure and thermal behavior of the synthesised materials were characterized by a range of methods, including X-ray diffraction, Fourier transform infrared spectroscopy, ¹H-NMR, electron microscopy, elemental and thermogravimetric analysis techniques. Thermogravimetric analysis results indicated that the addition of OC into the PAI matrix was increased in the thermal decomposition temperatures of the resulted bio-nanocomposites.

Full text of DOI:10.1007/s00726-012-1436-2

Amino Acids (2013) 44:1021–1029
DOI 10.1007/s00726-012-1436-2
ORIGINAL ARTICLE
Chiral bio-nanocomposites based on thermally stable poly
(amide-imide) having phenylalanine linkages and reactive
organoclay containing tyrosine amino acid
•
Shadpour Mallakpour Mohammad Dinari
Received: 4 January 2012 / Accepted: 27 November 2012 / Published online: 11 December 2012
Ó Springer-Verlag Wien 2012
Abstract Montmorillonite clay modified with the bio-
active trifunctional L-tyrosine amino acid salt was used
as a reactive organoclay (OC) for the preparation of
poly(amide-imide) (PAI)/OC hybrid films. One of the
functional groups of the L-tyrosine as the swelling agent
formed an ionic bond with the negatively charged sili-
cates, whereas the remaining functional groups were
available for further reaction with polymer matrix. The
soluble PAI with amine end groups including phenylala-
nine amino acid was synthesised under green condition
using molten tetra-butylammonium bromide by direct
polymerization reaction of chiral diacid and 2-(3,5-diam-
inophenyl)benzimidazole. PAI/OC bio-nanocomposites
films containing different contents of OC were prepared
via solution intercalation method through blending of OC
with the PAI solution. X-ray diffraction and transmission
electron microscopy revealed that the dispersion of sili-
cate layers in the PAI created an exfoliated structure as a
result of using the trifunctional groups of the swelling
agent. The structure and thermal behavior of the syn-
thesised materials were characterized by a range of
methods, including X-ray diffraction, Fourier transform
infrared spectroscopy, 1H-NMR, electron microscopy,
elemental and thermogravimetric analysis techniques.
Thermogravimetric analysis results indicated that the
addition of OC into the PAI matrix was increased in the
thermal decomposition temperatures of the resulted bio-
nanocomposites.
Keywords Bio-nanocomposites ꢀ a-Amino acids ꢀ
Layered silicate ꢀ Poly(amide-imide) ꢀ L-Tyrosine ꢀ
Green chemistry
Introduction
In synthetic polymer chemistry, it seems that one of the
most challenging tasks is constructing functional poly-
meric systems that could be as effective as those in living
systems. In this respect, today, the optically active syn-
thetic macromolecules have received considerable atten-
tion. Due to their specific chiral structure, optically active
polymers have significant applications such as catalysts
for asymmetric synthesis and chiral stationary phases for
the direct optical resolution of enantiomers (Chen et al.
2007; Koyam et al. 1997; Mallakpour and Dinari 2011a;
Samui et al. 2002). A large number of studies on chiral
materials are devoted to amino acids and carbohydrates.
Amino acids as constitutional components of proteins
with a high degree of functionality and chirality play an
important role in the synthesis of bio-compatible and -
degradable polymers. Of these, macromolecules that
contain amino acid units are potentially useful in many
different medical applications such as bio-erodible
sutures, tissue engineering scaffolds and drug delivery
matrices (Wang et al. 2009; Mallakpour et al. 2011).
L-Tyrosine is the only major natural nutrient amino acid
containing an aromatic hydroxyl group. This kind of the
amino acid can be used to improve memory and mental
S. Mallakpour (&) ꢀ M. Dinari
Department of Chemistry, Organic Polymer Chemistry Research
Laboratory, Isfahan University of Technology, 84156,
83111 Isfahan, Islamic Republic of Iran
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, 84156,
83111 Isfahan, Islamic Republic of Iran
123

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S. Mallakpour, M. Dinari
alertness, act as an appetite suppressant, control depres-
sion and anxiety and enhance physical performance
(Eversloh et al. 2007; Garlick 2005).
antioxidant, antiulcer, antiamoebic, antihistaminic,
anthelmintic and antihypertensive ones (Koprowska-Ra-
tajska et al. 2009). The use of BI in PI structure combines
the characteristics and properties of six membered imides
and BIs and makes the polymer a good candidate for
high-performance thermoplastics. Consequently, polymers
modified with BI groups have shown high glass transition
temperatures, excellent thermal stability and good solu-
bility in common organic solvents (Abdalla et al. 2002;
Chung et al. 2001; Huang et al. 2009; Ivarez-Gallego
et al. 2007).
Montmorillonite (MMT), as one of the most commonly
used clays, is a type of layered silicate that has gained
considerable interest in recent years for the preparation of
nanocomposites. Anionically charged layers of magne-
sium/aluminum silicates and small cations such as sodium
or potassium those are located in silicate interlayer gal-
leries present chemical structures of MMT. The presence
of ion and ionic bonds makes MMT a hydrophilic subject
and prevents it from being compatible with organic
polymer matrix. Modification through replacing the small
cation existing in the interlayer regions with organic
cations, that are usually named cation exchange, is a way
to render the hydrophilic layer silicate organophilic and
also make long-drawn-out interlayer space which makes it
easy to come into polymers chain and therefore, leads to
the striking development of polymer–clay nanocomposite
(Chao et al. 2008; Delozier et al. 2003; Yin et al. 2009;
Wang et al. 2004). These excellent property improve-
ments result from the combination of organic polymers
and only a 2–5 % addition of the dispersed nanophase
(Liang et al. 2003).
Lately, a research subject that is related to well-dis-
persed clay platelets in various polymer matrices to prepare
polymer–clay nanocomposite (PCNs) materials has attrac-
ted increasing attention because PCNs exhibit great
potential for a wide range of commercial applications.
There are considerable publications associated with the
preparation and properties of PI–clay nanocomposite. PI–
silica hybrid materials show outstanding thermal stability,
mechanical strength and physical properties when applied
in various applications, including patterned electronic
devices, optical waveguide materials, high-refractive index
materials, high transparency materials, non-linear optical
materials, photovoltaic devices, and fuel cells (Ma et al.
2010; Yeh et al. 2005; Yu et al. 2004, 2010). In a proper
clay–polymer hybrid composite, engineering polymers are
encapsulated between smectite clay particles approxi-
mately 1 nm in thickness (Lan et al. 1994).
Among the heat-resistant and high-performance poly-
mers, polyimides (PIs) have received considerable scien-
tific and technological attention due to their outstanding
performance, dielectrical and mechanical properties and
thermal and chemical stability. During the past decade,
interest in aromatic PIs has been increased in response to
increasing technological applications in a variety of fields
such as aerospace, automobile and microelectronics (Ishii
et al. 2010; Li et al. 2008; Nguyen and Wang 2010; Zhai
et al. 2007). At the same time, aromatic PIs, like other
aromatic and heterocyclic polymers, are quite intractable
to process because they are usually infusible and insoluble
in organic solvents. The highly regular and rigid polymer
backbones are generally responsible for the difficulty of
processing. Recently, many efforts have been centered on
promoting the processability and solubility of aromatic
PIs through the change in monomer construction. These
structural modifications such as flexible bridging linkages
into the polymer backbone, incorporation of bulky sub-
stituents along the polymer or the synthesis of co-polyi-
mides such as poly(ester-imide)s and poly(amide-imide)s
(PAIs) by incorporating amide or ester functionality at
regular intervals in the PI chain could provide better
solubility and processability (Karatas et al. 2007; Li and
Li 2003; Li et al. 2005; Lina et al. 2004; Liang et al.
2003; Xu et al. 2006; Mallakpour and Dinari 2011b;
Mallakpour and Khani 2010). Benzimidazole (BI) as one
bulky group has a wide variety of reported activities,
especially antimicrobial, antitumor, antiviral, antifungal,
The need to develop new cleaner chemical processes
makes ionic liquids (ILs) optional solvents for chemical
reaction. Non-volatility of ILs is an important property but
an interest in ILs also arises from other properties such as
non-flammability, high chemical/thermal stability, low
toxicity and favorable conductivity. It has been shown that
IL can also be advantageously used for polymer synthesis
(Absalan et al. 2010; Mallakpour and Dinari 2012a; Palacio
and Bhushan 2010; Zhao et al. 2008).
Generating optically active polymers based on amino
acids in our previous work was a guide to consider amino
acid for modification of MMT. Our interest in applying
amino acid as a swelling agent stems from the fact that
amine group of amino acid can provide cation (-?NH3)
which can form an ionic bond with the negatively charged
silicate layers of MMT and acid functional groups can react
with polymers chains. In this work, at first, diamine
monomer containing BI pendant group was synthesised and
then reacted with diacid containing natural amino acids in
IL environment to make PAI in which pendant BI groups
can improve solubility of polymer in organic solvent while
maintaining thermal stability. Then the synthesised PAI
was employed to prepare PAI/MMT bio-nanocomposite
(BNC) films in which MMT was modified with tyrosine
amino acid. This amino acid with three functional groups
123

Chiral bio-nanocomposites based on thermally stable poly(amide-imide)
1023
interacted with silicate layer and improved the compati-
bility of MMT with polymer matrix.
After that, the solution was stirred for 12 h at 120 °C. The
viscous solution was precipitated in 30 mL of methanol.
The white solid was filtered off and dried to give 0.14 g
25
(91 %) of white PAI. The specific rotation was ½aꢁD
=
Experimental
-78.27 (measured at a concentration of 0.5 g/dL in DMF
at 25 °C) (Mallakpour and Dinari 2012c).
Characterization
FT-IR peaks (KBr, cm-1): 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), 733 (s), 700 (s), 631 (m), 568 (s) and 492
(w).
Preparation of the modified clay
The modification of the clay was carried out as follows:
according to our previous work (Mallakpour and Dinari
2012b), 2 g of Cloisite Na? was dispersed separately in
150 mL of hot deionized water for 6 h at 80 °C. The
required amount of L-tyrosine was protonated with stoi-
chiometric amount of concentrated HCl in 50 mL of
deionized water and heated at 80 °C for 3 h. These two
solutions were mixed and the contents were vigorously
stirred for 6 h at 60 °C. The mixture was filtered through a
WhatmanÒ filter paper and then washed about five times
with deionized water to remove superficially protonated-
tyrosine sorbed on the surface.
An amine end-capped PAI chain was produced by
adding the excess amount (1 %) of diamine 1 near the end
of the polymerization reaction.
Preparation of the PAI/OC BNC films
The novel BNCs were prepared by mixing the appropriate
amounts of the PAI and OC 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 and then at
40 °C for 20 h for uniform dispersion of OC platelets in
the polymer matrix. Various compositions of OC (2, 5, 10
and 15 wt %) were prepared by mixing various amounts
of OC to the PAI solution. Thin BNC 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 to a
constant weight.
FT-IR peaks (KBr, cm-1) for BNC 2 %: 3,400 (br),
3,102 (w), 2,970 (w), 2,928 (m), 2,890 (w), 1,800 (w),
1,778 (m), 1,730 (s), 1,622 (m), 1,580 (m), 1,460 (w) 1,383
(s), 1,345 (m), 1,226 (m), 1,148 (m), 1,065 (m), 910 (m),
750 (s), 654 (m), 610 (m), 563 (w), 520 (m) and 465 (m).
FT-IR peaks (KBr, cm-1) for BNC 5 %: 3,415 (br),
3,102 (w), 29,73 (m), 1,800 (w), 1,776 (m), 1,722 (s), 1,616
(m), 1,575 (w), 1,484 (m), 1,383 (s), 1,342 (m), 1,238 (m),
1,156 (w), 1,082 (m), 910 (m), 751 (m), 657 (m), 560 (w),
523 (w) and 464 (m).
Synthesis of 2-(3,5-diaminophenyl)-benzimidazole
For the preparation of 2-(3,5-diaminophenyl)-benzimid-
azole (1), at first, the corresponding dinito was synthesised
from the reaction of 1,2-phenylenediamine and 3,5-dini-
trobenzoyl chloride using methanesulfonic acid and P2O5
as condensation media. Then, diamine 1 was fabricated by
the reduction of the dinitro precursor using palladium on
activated carbon (Pd/C) and hydrazine monohydrate in
refluxed ethanol (yield: 77 %) (Ayala et al. 2005; Ivarez-
Gallego et al. 2007; Mallakpour and Dinari 2012c). Melt-
ing point: 242–243 °C, FT-IR (KBr, cm-1): 3,409 (s, br),
3,063 (w), 1,607 (s), 1,577 (s), 1,484 (w), 1,446 (m), 1,274
(w), 841 (m) and 743 (s).
Synthesis of PAI
FT-IR peaks (KBr, cm-1) for BNC 10 %: 3,420 (br),
3,108 (w), 2,971 (m), 2,931 (w), 2,876 (w), 1,805 (w),
1,777 (m), 1,722 (s), 1,618 (m), 1,565 (w), 1,488 (m),
1,462 (m) 1,383 (s), 1,345 (m), 1,217 (m), 1,154 (w), 1,082
(m), 910 (w), 750 (m), 650 (m), 619 (w), 523 (m) and 465
(s).
FT-IR peaks (KBr, cm-1) for BNC 15 %: 3419 (br),
3,103 (w), 2,960 (m), 2,937 (w), 1,775 (m), 1,720 (s), 1,620
(m), 1,580 (w), 1,488 (m), 1,463 (m) 1,380 (s), 1,342 (w),
1,223 (m), 1,154 (m), 1,080 (w), 1,050 (m), 917 (w), 754
(w), 654 (m), 578 (m), 525 (m) and 456 (s).
For the synthesis of the PAI, at first, chiral N,N’-(pyrom-
ellitoyl)-bis-L-phenylalanine diacid was prepared according
to our previous works (Mallakpour and Dinari 2011a).
Then, an optically active PAI was prepared by the fol-
lowing procedure: a mixture of 0.10 g (2.36 9 10-4 mol)
of diacid 2 and 0.30 g (9.42 9 10-4 mol) of TBAB was
ground until a powder was formed and then transferred into
a 25 mL round-bottom flask and 0.06 g (2.36 9 10-4 mol)
of diamine 1 was added to the mixture. It was heated until
homogeneous solution was formed and then triphenyl
phosphite (TPP) (0.49 mL, 3.70 9 10-3 mol) was added.
123

1024
S. Mallakpour, M. Dinari
Scheme 1 Preparation of organosoluble and thermally stable chiral PAI/OC BNCs
Results and discussion
modification of the MMT. In this study, positively charged
L-tyrosine amino acid swelling agent was chosen to replace
the chemically synthesised surfactant for the modification
of clay. This molecule has three functional groups of
amine, hydroxy and acid in which the amine is protonated
and is attached to the surface of the Cloisite Na? while the
other group is free to participate in the polymerization
reaction (Mallakpour and Dinari 2011c, 2012b ).
Preparation of monomers and neat PAI
Diamine 1 was prepared via the reduction of dinitro pre-
cursors by using hydrazine hydrate as the reducing agent
and Pd/C as the catalyst. By this procedure, pure diamine 1
was attained after recrystallization (Scheme 1).
Chiral diacid 2 was prepared by the condensation
reaction of one equimolar of PMDA and two equimolars of
L-phenylalanine amino acid in the refluxing acetic acid, as
shown in Scheme 1.
Preparation of the PAI/OC BNC films
Novel optically active PAI/OC BNC films containing 2, 5, 10
and 15 % of OC were successfully fabricated using func-
tionalized OC and amine end-capped PAI chains in dry
DMAc through solution intercalation technique (Scheme 1).
These new families of homogeneous transparent materials
are the combinations of organic polymers and layered sili-
cates blended with each other at ultrafine phase dimensions.
The uniform distribution of organically modified nanoclay in
polymer matrices can lead to enhancements of physical,
morphological, and mechanical properties in the resulting
BNCs. The transparency of BNC films was decreased
gradually by the addition of clay, and the films became
semitransparent and brittle at high loading of OC.
Because of the rapid emergence of ILs as green solvents
and replacement of a volatile and toxic organic solvent in the
polymerization with a nonvolatile solvent, in this study,
novel PAI was synthesised in green route by using molten
TBAB as an IL media. Phosphorylation polymerization
technique has been successfully applied to synthesize new
organosoluble PAI by the direct polycondensation of imide-
containing dicarboxylic acid 2 with aromatic diamine 1
(Scheme 1). The inherent viscosity of the PAI under opti-
mized condensations was 0.58 dL/g and the yield was 91 %.
25
The specific rotation of this polymer is ½aꢁD = -78.27,
confirming the optical activity of this polymer.
Characterization
Preparation of OC
1H-NMR and FT-IR spectra
The OC chosen for dispersion in high-performance poly-
mers needed enough thermal stability to withstand the high
temperature used to cure these polymers. This condition led
to the selection of thermally stable molecules for surface
The structures of new synthesised compounds were con-
firmed by FT-IR and 1H-NMR spectroscopy techniques. In
1
the H-NMR spectrum of PAI, the emergence of the N–H
123

Chiral bio-nanocomposites based on thermally stable poly(amide-imide)
1025
Fig. 1 1H-NMR (500 MHz) spectrum of 1H-NMR spectrum of
chiral PAI
protons of BI and amide group at 12.60–12.80 and
10.35–10.51 ppm indicates the presence of these groups in
the polymer chain (Fig. 1). The resonance of aromatic
protons appeared in the range of 6.85–8.25 ppm. The
proton of the chiral center appeared as multiplets in the
range of 5.23–5.37 ppm. The absorption of the diastrotopic
hydrogens that were bonded to the neighboring carbon of
chiral center appeared in the range of 3.20–3.47 ppm as
two discrete multiplets peaks. The aforementioned results
show that PAI was synthesised successfully.
Fig. 2 FT-IR spectra of the OC, neat PAI and different BNCs
468 cm-1 are for bending vibration of Si–O and Al–O,
respectively. Upon intercalation of L-tyrosine amino acids
into the clay gallery for modification of layer silicates, the
absorption peaks were observed with origins being from
amino acid molecules. For example, in OC spectrum,
shoulder in the range of 2,500–3,700 cm-1 is related to OH
stretching of the –COOH group (Fig. 2a). The peak at
2,960 is attributed to C–H stretching vibration aliphatic
hydrocarbon of phenylalanine and around 1,700 cm-1 it is
related to the carbonyl group of the amino acid (Mallak-
pour and Dinari 2012b).
In the FT-IR spectra of the diamine 1, the absorption of
amine NH2 and NH BI bonds appeared around 3,394 and
3,323 cm-1 and the peak at 1,627 cm-1 confirms that the
presence of NH deformation provided the successful for-
mation of this compound. Two absorption bands at 1,577
and 1,484 cm-1 were characteristic peaks for aromatic
rings. The BI group also gave bands at 1,484 cm-1 (in-
plane deformation of the BI ring), 1,590 cm-1 (ring
vibration of conjugation between fused benzene and
imidazole rings) and a shoulder at 1,630 cm-1 (C–N
stretching).
In the BNC 5, 10 and BNC 15 %, the presence of a peak
at 1,048 cm-1 is corresponding to the Si–O and peaks at
400–600 cm-1 for Al–O and Mg–O indicate the incorpo-
ration of OC in the PAI matrix (Fig. 2). By comparison of
these spectra, it can be concluded that the BNCs not only
have characteristic neat PAI bands, but also have charac-
teristic peak for OC. These data, in summary, confirm the
formation of the BNCs.
In the FT-IR spectrum of the chiral PAI, the absorption
bands for the imide ring were observed around
1,722 cm-1, which is indicative of the asymmetrical and
symmetrical C=O stretching vibration, and at 1,375, 1,064,
and 731 cm-1 due to imide ring deformation. The NH
stretching band of the amide and BI groups was observed
around 3,320 cm-1, and the C=O stretching band of amide
group at 1,669 cm-1 (Fig. 2b).
In the FT-IR spectrum of Cloisite Na?, the absorption
bands of about 3,100–3,700 cm-1 are due to O–H
stretching and stretching band of hydrogen-bonded water.
A weak broad peak at 1639 cm-1 is related to H–O–H
bending region. The characteristic peaks at 1,115 and
1,042 cm-1 were due to the out-of-plane Si–O stretching
and in-plane Si–O stretching for layered silicates, respec-
tively. Other characteristic vibration peaks at 524 and
X-ray diffraction
The most common technique used to analyze the OC and
polymer/clay nanocomposites is XRD, which measures the
interlayer d-spacing. The XRD patterns of the OC and PAI/
OC BNCs containing different amounts of OC (2, 5, 10 and
15 wt %.) are shown in Fig. 3. Pristine Cloisite Na? has an
XRD peak at 2h = 7.56° that was caused by the diffraction
of the (001) crystal surface of layered silicates, equaling a
d-spacing of 1.17 nm. In the XRD pattern of OC, the d-
spacing of OC was 13.8 nm based on Bragg’s Law
(nk = 2d sin h). The presence of 2, 5, 10 and 15 wt % of
OC in the PAI matrix resulted in a shift of this diffraction
123

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S. Mallakpour, M. Dinari
peak toward smaller angle. The peak that was observed in
OC at 2h = 6.40° corresponded to the basal spacing of OC
and disappeared in the BNCs 2 and 5 %, suggesting the
disorder and loss of structure regularity of the clay layers.
Thus, the clay tactoids are considered to be exfoliated and
the 0.96-nm thick clay layers are dispersed at the molecular
level into PAI. When the OC content was increased to 10
and 15 wt %, the samples showed a small diffraction peak
at 2h = 2.20°, equaling a d-spacing of 4.01 nm, respec-
tively, as opposed to the diffraction peak at 2h = 6.40° (d =
1.38 nm) for the OC. This implies that there is a small
amount of OC that cannot be exfoliated in PAI and exists in
the form of an intercalated layer structure. The peak value
indicates the possible presence of PAI molecules outside
the MMT layers that squeezed the clay layers during the
solvent removal process and caused a reduction in the
spacing between the layers (Wang et al. 2004). These XRD
results imply that the OCs are well dispersed or exfoliated
in the PAI polymer matrix. However, further evidence
should be found using TEM to reveal the exact micro-
structures of the BNCs.
dispersed in the polymer matrix without appreciable
agglomeration of particles (Fig. 4).
TEM
More direct evidence for the formation of a true nanoscaled
composite was provided by using TEM analysis. Figure 5a, b
shows the TEM micrographs of the PAI/OC BNC 5 and 15 %
in which the brighter region represents the polymer matrix
while the dark narrow stripes represent the stacked and
intercalated stacked nanoparticles. The micrograph 5a
exhibits well-dispersed individual OC layers, which are
apparent as dark lines, for the BNC 5 %. Compared to this
result, the 15 wt % OC hybrid shows more banded or
aggregated lines in Fig. 5b. Owing to the potential for inter-
action between the free carboxylic acid and hydroxyl groups
of the tyrosine modified OC with amide, imide and BI groups
of PAI, the TEM micrographs of BNC 5 % reveal well-
exfoliated structures. This suggests that a greater interaction
force between PAI and organic modifiers is required for the
formation of exfoliated structures. Both exfoliated silicate
lamellae and intercalated stacks are visible, consistent with
the XRD and FE-SEM results. The majority of the particles
observed were exfoliated silicate-lamella stacks.
Morphology studies
FE-SEM
Thermal properties
FE-SEM micrographs of fracture surfaces of PAI with
different OC contents (0, 5, 10 and 15 wt %) are shown in
Fig. 4. The micrographs of the PAI hybrids from 5 and
10 wt % OC contents showed dispersed platelet orientation
morphology and the wave patterns appeared to be uni-
formly distributed over the fracture surface. When the OC
content was increased to 15 wt %, besides intercalation
and exfoliation, partially agglomerated OC particles were
also observed in the PAI matrix. Therefore, at low OC
contents (below 10 wt %), the OC particles may be
The TGA curves for thermal degradation of neat PAI and
PAI/OC BNCs are shown in Fig. 6. It shows that a loss of
mass in the BNC materials occurred above 450 °C. The 5
and 10 % weight loss temperatures of the BNCs were
higher than those of the pristine PAI (Table 1). The higher
thermal stability of BNCs was attributed to the formation
of char-like material as a result of the decomposition of
OC. This material acted both as a barrier to mass transport
of the decomposed products to the surface of the decom-
posing polymer and a thermal barrier to prevent additional
exposure of the polymer to the flame.
The percent char yield at 800 °C was increased with an
increase of OC content in the BNCs, which indicated that
the dispersion of OC in the PAI matrix was uniform. Char
yield can be applied as a decisive factor for estimating
limiting oxygen index (LOI) of the polymers based on Van
Krevelen and Hoftyzer equation (1976).
LOI ¼ 17:5 þ 0:4 CR
Where CR = char yieldThe BNC 5, 10 and 15 % have LOI
values in the range of 42–46 %, as calculated from their
char yield at 800 °C. On the basis of LOI values, all BNCs
can be classified as self-extinguishing materials (Table 1).
From these data, it is clear that neat PAI and its BNCs are
thermally stable owing to the existence of various linkages
such as imide and BI groups in polymer backbones.
Fig. 3 XRD of OC and different BNCs
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Chiral bio-nanocomposites based on thermally stable poly(amide-imide)
1027
Fig. 4 FE-SEM micrographs of pure PAI (a), BNC 5 % (b), BNC 10 % (c) and BNC 15 % (d)
Fig. 5 TEM images of BNC 5 % (a) and BNC 15 % (b)
Table 1 Thermal properties of the PAI and different BNCs
Samples
Decomposition temperature
(oC)
Char yield (%)b LOIc
T5a
Ta10
PAI
418
449
451
478
492
506
54
58
61
63
39.1
40.7
41.9
42.7
BNC 5 %
BNC 10 % 472
BNC 15 % 484
a
Temperature at which 5 and 10 % weight loss was recorded by
TGA at heating rate of 10 °C min-1 in a N2 atmosphere
b
Weight percent of the material left undecomposed after TGA at
maximum temperature 800 °C in a N2 atmosphere
c
Limiting oxygen index (LOI) evaluating at char yield at 800 °C
Fig. 6 TGA thermograms of PAI and different BNCs
123

1028
S. Mallakpour, M. Dinari
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Conclusions
In this study, a method for achieving a homogeneous dis-
persion of exfoliated layered silicates as nanometer-thick
layers in a polymer matrix was developed. This involved
the use of an organic molecule with different polar groups
as the matrix to aid in the dispersion of the layered silicate
inclusions. A specially designed reactive swelling agent
(protonated L-tyrosine amino acid) containing three func-
tional groups was used for the modification of Cloisite Na?
during ion exchange reaction. A novel end-capped or-
ganosoluble PAI containing L-phenylalanine and BI pen-
dant group was synthesised via direct polycondensation in
green media. Near complete dispersion of exfoliated lay-
ered silicates was observed in FE-SEM and TEM micro-
graphs, as confirmed by XRD. Dispersed OC platelets into
PAI matrix were found to increase the thermal stability, as
in the enhancement of thermal decomposition temperature
and char yield of PAI/OC BNCs based on the TGA studies.
Overall, these results show that the addition of a small
amount of reactive OC is sufficient to improve the prop-
erties of a chiral PAI. The presence of amino acids as a bio-
degradable segment in both novel organo-modified nano-
clay and optically active PAI makes the obtained BNCs
materials more susceptible for better bio-degradation pro-
cess. These findings are important and relevant to the
preparation of low-cost and bio-degradable chiral OCs and
related BNCs for industrial applications.
Acknowledgments The authors wish to sincerely thank the
Research Affairs Division 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. We also thank
Dr. H. Jalili for the English editing.
Liang ZM, Yin J, Xu HJ (2003) Polyimide/montmorillonite nano-
composites based on thermally stable, rigid-rod aromatic amine
modifiers. Polymer 44:1391–1399
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