Synthesis and characterization of thermally stable poly(amide-imide)- montmorillonite nanocomposites based on bis(4-carboxyphenyl)-n,n'- pyromellitimide acid

Article abstract of DOI:10.4314/bcse.v27i1.10

Two new poly(amide-imide)-montmorillonite reinforced nanocomposites containing bis(4-carboxyphenyl)-N,N'-pyromellitimide acid moiety in the main chain were synthesized by a convenient solution intercalation technique. Poly(amide-imide) (PAI) as a source o

Full text of DOI:10.4314/bcse.v27i1.10

Bull. Chem. Soc. Ethiop. 2013, 27(1), 95-104.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v27i1.10
ISSN 1011-3924
 2013 Chemical Society of Ethiopia
SYNTHESIS AND CHARACTERIZATION OF THERMALLY STABLE
POLY(AMIDE-IMIDE)-MONTMORILLONITE NANOCOMPOSITES
BASED ON BIS(4-CARBOXYPHENYL)-N,N
'-PYROMELLITIMIDE ACID
1
*
2
Khalil Faghihi , Ali Asghari and Mohsen Hajibeygi
3
1
Department of Chemistry, Faculty of Science, Arak University, Arak, 38156-8-8349, Iran
2
Department of Chemistry, Arak Branch, Islamic Azad University, Arak, Iran
Department of Chemistry, Varamin Pishva Branch, Islamic Azad University, Varamin, Iran
3
(
Received May 10, 2011; revised November 28, 2012)
ABSTRACT. Two new poly(amide-imide)-montmorillonite reinforced nanocomposites containing
bis(4-carboxyphenyl)-N,N'-pyromellitimide acid moiety in the main chain were synthesized by a
convenient solution intercalation technique. Poly(amide-imide) (PAI) as a source of polymer
matrix was synthesized by the direct polycondensation reaction of bis(4-carboxyphenyl)-N,N'-
pyromellitimide acid with 4,4'-diamino diphenyl sulfone in the presence of triphenyl phosphite
(
2
TPP), CaCl , pyridine and N-methyl-2-pyrrolidone (NMP). Morphology and structure of the
resulting PAI-nanocomposite films with 10 and 20% silicate particles were characterized by FT-IR
spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of clay
dispersion and the interaction between clay and polymeric chains on the properties of
nanocomposites films were investigated by using UV-Vis spectroscopy, thermal gravimetry
analysis (TGA) and water uptake measurements.
KEYWORDS: Bis(4-carboxyphenyl)-N,N'-pyromellitimide acid moiety, Poly(amide-imide)-
montmorillonite nanocomposite, Thermal properties
INTRODUCTION
Polymer-clay nanocomposites typically exhibited mechanical, thermal and gas barrier
properties, which are superior to those of the corresponding pure polymers [1-9]. Unique
properties of the nanocomposites are usually observed when the ultra fine silicate layers are
homogenously dispersed throughout the polymer matrix at nanoscale. The uniform dispersion of
silicate layers is usually desirable for maximum reinforcement of the materials. Due to the
incompatibility of hydrophilic layered silicates and hydrophobic polymer matrix, the individual
nanolayers are not easily separated and dispersed in many polymers. For this purpose, silicate
layers are usually modified with an intercalating agent to obtain organically modified clay prior
to use in nanocomposite formation [10, 11]. Also aromatic polyimides are well recognized as a
class of high performance materials due to their remarkable thermal and oxidative stabilities and
excellent electrical and mechanical properties for long time periods of operation [12].
Unfortunately, strong interaction between polyimide chains and their rigid structure make them
intractable. Poor thermoplastic fluidity and solubility are the major problems for wide
application of polyimides. Thus, to overcome these processing problems various approaches
have been carried out by incorporating flexible units such as –NHCO–, –O–, and –SO –, and
2
some of which are commercialized [13-14]. Among them, poly(amide-imide) (PAI) is the most
successful material, which combines the advantages of high-temperature stability and
processability [15-22]. In this article two PAI-nanocomposite films with 10 and 20% silicate
particles containing chiral bis(4-carboxyphenyl)-N,N'-pyromellitimide acid moiety in the main
chain was prepared by using a convenient solution intercalation technique.
_
*
_________
Corresponding author. E-mail: k-faghihi@araku.ac.ir

96
Khalil Faghihi et al.
EXPERIMENTAL
Materials
Pyromellitic dianhydride, p-amino benzoicacid, 4,4'-diamino diphenyl sulfone, acetic acid,
triphenyl phosphite (TPP), CaCl , pyridine and N-methyl-2-pyrrolidone (NMP), N,N-dimethyl
formamide (DMF) were purchased from Merck Chemical Company and used without previous
2
®
purification. Purified the organically-modified Cloisite 20A supplied by Southern Clay
Products (TX), were used as polymer nanoreinforcement.
Measurements
1
H-NMR spectra was recorded on a Bruker 300 MHz instrument (Germany). Fourier transform
infrared (FT-IR) spectra were recorded on Galaxy Series FT-IR 5000 spectrophotometer
o
England). UV-Vis spectra were recorded at 25 C in the 300-800 nm spectral regions with a
(
Perkin Elmer Lambda 15 spectrophotometer in NMP solution using cell lengths of 1 cm.
Thermogravimetric analysis and differential scanning calorimetry (TGA and DSC) data were
o
atmosphere at a rate of 10 C/min. The morphology
taken on a Mettler TA4000 System under N
2
and structure of nanocomposite film was investigated on Canbridge S260 scanning electron
microscope (SEM) and Rigaku D - max CIII X-ray diffraction (XRD). Elemental analyses were
taken on a Vario AV.
Monomer synthesis
A mixture of pyromellitic dianhydride 1 (3.00 g, 13.70 mmol), p-amino benzoic acid 2 (3.76 g,
27.50 mmol) and 130 mL acetic acid with appropriate amounts of pyridine was stirred and
mixture was refluxed at 120 ºC for 12 h until the precipitate was formed, the precipitate was
filtered off, washed thoroughly with water and dried to afford 4.52 g compound 3 (yield
7
1
2.1%). M.p.: 310 °C, FT-IR (KBr): 3500-2500, 3070, 1778, 1722, 1600-1580, 1422-1370,
-
1 1
277, 922, 709 cm . H-NMR (DMSO-d ) δ: 7.42-8.12 (10H, aromatic), 13.15 (2H, OH) ppm.
6
Polymer synthesis
Into a 100 mL round bottomed flask were placed a mixture of N-pyrromellitimido-4-amino
benzoic acid 3 (0.72 g, 1.57 mmol), 4,4'-diamino diphenyl sulfone 4 (0.39 g, 1.57 mmol),
calcium chloride (0.60 g), triphenyl phosphite (1.00 mL), pyridine (1.00 mL) and N-methyl-2-
pyrrolidone (4.00 mL). The mixture was heated for 1 h at 60 ºC, 2 h at 90 ºC and then refluxed
at 140 ºC for 9 h until a viscous solution was formed. Then it was cooled to room temperature
and 30 mL of methanol was added to reaction mixture. The precipitate was formed, filtered off
and washed with methanol. The resulting polymers 5 were dried under vacuum to leave 0.93 g
(
90%). FT-IR (KBr): 3415, 3190-3065, 1774, 1718, 1664, 1587, 1503, 1369, 1318, 1144, 1101,
-
08 cm . Elemental analysis for C36
1
7
6
20 4 8
H N O S, calculated; C, 64.7; H, 3.0; N, 8.4; found; C,
5.5, H, 3.0; N, 9.1.
PAI-nanocomposite synthesis 5a and 5b
PAI-nanocomposites 5a and 5b were produced by solution intercalation method, in two
different amounts of organoclay particles (10 and 20 wt.%) were mixed with appropriate
amounts of PAI (0.50 g, 0.74 mmol) solution in NMP (8.00 mL) to yield particular
nanocomposite concentrations. To control the dispersibility of organoclay in PAI matrix,
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Thermally stable poly(amide-imide)-montmorillonite nanocomposites
97
o
constant stirring was applied at 25 C for 24 h. Nanocomposite films were cast by pouring the
solutions for each concentration into Petri dishes placed on a labelled surface followed by the
o
evaporation of solvent at 70 C for 12 h. Films were dried at 80 C under vacuum to a constant
o
weight. FT-IR(KBr) for PAI-nanocomposites 5a: 3435, 3190, 1775, 1719, 1660, 1587, 1503,
-
369, 1318, 1144, 1100, 1041, 754, 511 cm . FT-IR (KBr) for PAI-nanocomposites 5b: 3440,
1
1
3
-
165-3045, 1775, 1720, 1661, 1587, 1503, 1369, 1318, 1145, 1101, 1042, 753, 458 cm .
1
The water absorption analysis
The water absorption of PAI-nanocomposite films was carried out using a procedure under
o
ASTM D570-81 [24]. The films were dried in a vacuum oven at 80 C to a constant weight and
o
then weighed to get the initial weight (W ). The dried films were immersed in deionized water
o
at 25 C. After 24 h, the films were removed from water and then they were quickly placed
between sheets of filter paper to remove the excess water and films were weighed immediately.
The films were again soaked in water. After another 24 h soaking period, the films were taken
out, dried and weighed for any weight gain. This process was repeated again and again till the
films almost attained the constant weight. The total soaking time was 168 h and the samples
were weighed at regular 24 h time intervals to get the final weight (W
f
). The percent increase in
weight of the samples was calculated by using the formula (W - W )/W .
f
o
o
RESULTS AND DISCUSSION
Monomer synthesis
bis(4-Carboxyphenyl)-N,N'-pyromellitimide acid 3 was synthesized by the condensation
reaction of one equimolar of pyromellitic dianhydride 1 with two equimolar of p-amino benzoic
acid 2 in a mixture of acetic acid solution and pyridine (Scheme 1).
O
O
O
O
3 2
CH CO H
O
+
2
HO C
2
NH2
HO
2
C
N
N
2
CO H
O
pyridine
O
O
O
O
(1)
(2)
(3)
Scheme 1. Synthetic route of bis(4-carboxyphenyl)-N,N
ꢀ-pyromellitimide acid 3.
1
The chemical structure of diacid 3 was confirmed by FT-IR and H-NMR spectroscopy. In
-
the FT-IR spectrum of diacid 3 peaks appearing at 3500-2500 cm (acid O-H stretching), 1778
1
-
cm (C=O asymmetric imide stretching), 1722 cm (C=O acid and symmetric imide stretching),
1
-1
-
1
422-1370 and 709 cm (imide characteristic ring vibration) confirmed the presence of imide
1
ring and carboxylic groups in this compound (Figure 1). The H-NMR spectrum of diacid 3
1
shows peaks between δ = 7.42-8.12 ppm related to aromatic protons and a broad peak at δ =
1
3.15 ppm was assigned for protons of the COOH groups (Figure 2).
Polymer synthesis
PAI 5 was synthesized by the direct solution polycondensation reaction of an equimolar mixture
of diacide 3, an equimolar mixture of diamine 4 with triphenyl phosphite, NMP, calcium
chloride and pyridine as condensing agents (Scheme 2).
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Khalil Faghihi et al.
The entire polycondensation readily proceeded in a homogeneous solution. Tough and
stringy precipitate formed when the viscous PAI solution was obtained in good yield (90%).
Figure 1. FT-IR spectrum of diacid 3.
1
Figure 2. H-NMR spectrum of diacid 3.
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Thermally stable poly(amide-imide)-montmorillonite nanocomposites
99
O
O
O
S
HO C
2
N
N
CO H
2
H N
2
NH2
+
O
O
O
(
4)
(3)
NMP
Py
CaCl2
TPP
O
O
O
O
O
O
S
H
N
C
N
N
C
NH
O
O
(
5)
Scheme 2. Synthetic route of poly(amide-imide) 5.
Polymer characterization
−1
This polymer had inherent viscositiy of 0.29 dL/g (Measured at a concentration of 0.5 g dL in
DMF at 25 ◦C) and was confirmed as PAI by using FT-IR spectroscopy and elemental analysis.
A representative FT-IR spectrum of PAI 5 is shown in (Figure 3). PAI 5 had absorption bands
-
1
1
between 1774 cm and 1664 cm due to imide and amide carbonyl groups. Absorption bands
-
around 1369-1318 cm and 732 cm demonstrated the presence of the imide heterocyclic ring in
1
-1
-
1
this polymer. Also absorption band of amide group appeared at 3415 cm (N-H stretching). The
elemental analysis value of the resulting polymer was in good agreement with the calculated
values for the proposed structure.
The solubility of PAI 5 was investigated with 0.01 g polymeric sample in 2 mL of solvent.
PAI (5) was dissolved in organic solvent such as DMF, DMSO and NMP at room temperature
and it was insoluble in solvents such as chloroform, methylene chloride, methanol, ethanol and
water.
PAI-nanocomposite films
PAI-nanocomposite films were transparent and yellowish brown in color. The incorporation of
organoclay changed the color of films to dark yellowish brown. Moreover, a decrease in the
transparency was observed at higher clay contents. (Scheme 3) show the flow sheet diagram and
synthetic scheme for PAI-nanocomposite films 5a and 5b.
The chemical structure of PAI-nanocomposite films 5a and 5b were determined by using
FT-IR, X-ray diffraction (XRD) and Scanning electron microscopy (SEM) techniques.
FT-IR spectra of PAI-nanocomposite films 5a and 5b showed the characteristic absorption
-
1
bands of the Si-O and Mg-O moieties at 1041, 511 and 465 cm , respectively. The
incorporation of organic groups in PAI-nanocomposite films were confirmed by the presence of
peaks at 1775, 1719, 1369, 754 (imide rings) and 1660, 1318 (amide groups) (Figure 4).
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Khalil Faghihi et al.
Figure 3. FT-IR spectrum of PAI 5.
O
H
2
N
S
NH2
O
O
O
O
O
o
stirring at 25 C
HO
2
C
N
N
2
CO H
Polymerization
Aramid Solution
Modified Organoclay
o
Stirring at 25 C
Composite Material
Film Casting
o
Baking at 70 for 12 h
o
Drying underVacuum at 80 C for 12 h
Scheme 3. Flow sheet diagram for the synthesis of PAI-nanocomposites film 5a and 5b.
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Thermally stable poly(amide-imide)-montmorillonite nanocomposites
101
Figure 4. FTIR spectra of PAI-nanocomposite films 5a (a) and 5b (b).
Figure 5 shows the XRD patterns of PAI-nanocomposite films 5a and 5b containing 10 and
o
0 wt.% of silicate particles. The result reveals an increased d-spacing from 1.00 nm (8.93 ) of
2
o
o
Na-MMT to 1.49 nm (5.15 ) of PAI-nanocomposite film (10 wt.%) and (4.23 ) of PAI-
nanocomposite film (20 wt.%). These results indicated significant expansion of the silicate layer
after insertion to PAI chains. The shift in the diffraction peaks PAI-nanocomposite films
confirms that intercalation has been taken place. This is direct evidence that PAI-
nanocomposites have been formed as the nature of intercalating agent also affects the
organoclay dispersion in the polymer matrix. Usually there are two types of nanocomposites
depending upon the dispersion of clay particles. The first type is an intercalated polymer clay
nanocomposite, which consists of well ordered multi layers of polymer chain and silicate layers
a few nanometers thick. The second type is an exfoliated polymer-clay nanocomposite, in which
is there is a loss of ordered structures due to the extensive penetration of polymer chain into the
layer silicate. Such part would not produce distinct peaks in the XRD pattern [25]. In our PAI-
o
o
nanocomposite films there are coherent XRD signal at 5.15 and 4.23 related to 10 and 20
wt.% nanocomposite films, respectively.
Figure 5. X-ray diffraction patterns of organoclay (a), PAI-nanocomposite films 5a (b) and 5b
(
c).
The surface morphology of the PAI-nanocomposite films prepared by solution intercalation
technique is compared by SEM analyses. (Figure 6) show the morphological images of 10 and
2
0 wt.% nanocomposite films, respectively. The SEM images show that PAI matrix has a
smooth morphology, where as the PAI matrix has an amorphous morphology. Also SEM
micrographs of PAI-nanocomposite containing 10 and 20 wt.% clay platelets were uniformly
distributed without agglomeration.
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Khalil Faghihi et al.
Figure 6. Scanning electron micrographs of PAI-nanocomposite films 5a (a) and 5b (b).
Comparing of thermal gravimetry analysis, optical clarity and water absorption between PAI
and PAI-nanocomposite films
Optical clarity
Optical clarity of PAI-nanocomposite films containing 10 and 20 wt.% clay platelets and neat
PAI were compared by UV-Vis spectroscopy in the region of 300-800 nm. (Figure 7) show the
UV-Vis transmission spectrum of pure PAI and PAI-nanocomposite films containing 10 and 20
wt.% clay platelets. This spectrum show that the UV-Vis region (300-800 nm) is affected by the
presence of the clay particles and exhibiting low transparency reflected to the primarily
intercalated composites. Results show that the optical clarity of PAI-nanocomposite films
system is significantly lower than the neat PAI system.
Thermal gravimetry analysis
The thermal properties of PAI-nanocomposite films containing 10 and 20 wt.% clay platelets
and neat PAI were investigated by using TGA and DSC in nitrogen atmosphere at a rate of
o
heating of 10 C/min, and thermal data are summarized in (Table 2). These samples exhibited
o
good resistance to thermal decomposition, up to 145-180 C in nitrogen, and began to
o
for these samples ranged from 145-180 C and
decompose gradually above this temperature. T
T
5
o
10 for them ranged from 175-220 C, and residual weights at 600 C ranged from 50.07-
o
55.72% in nitrogen. Incorporation of organoclay into the PAI matrix also enhanced the thermal
stability of the nanocomposites. Figure 8 shows the TGA thermograms of PAI-nanocomposites
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Thermally stable poly(amide-imide)-montmorillonite nanocomposites
103
under nitrogen atmosphere. Thus, we can speculate that interacting PAI chains between the clay
layers serve to improve the thermal stability of nanocomposites. The addition of organoclay in
polymeric matrix can significantly improve the thermal stability of PAI.
Figure 7. UV-Vis spectrum of PAI 5
, PAI-nanocomposite films 5a
and 5b
.
Figure 8. TGA spectrum of PAI 5
, PAI-nanocomposite films 5a
and 5b
.
Water absorption
The PAI under investigation contains polar and cyclic imide rings and also polar amide groups
in the backbone that have the tendency to uptake water through hydrogen bonding. Thus water
absorption measurements become necessary for neat PAI 5 and PAI-nanocomposite films 5a
and 5b and data are shown in (Table 1). In the water permeability studies, we found that the
incorporation of clay platelets into PAI matrix results in a decrease of water uptake relative to
pure PAI by forming the tortuous path of water permeant. Water permeability depends on
length, orientation and degree of delamination of layered silicate. It should be noted that a
further increase in clay concentration resulted in an enhanced barrier property of
nanocomposites which may be attributed to the plate-like clays that effectively increase the
length of the diffusion pathways, as well as decrease the water permeability.
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Khalil Faghihi et al.
Table 1. Thermal behaviors and water uptake of neat PAI (5) and PAI-nanocomposite films 5a and 5b.
a
o
( C)
o
b
c
d
Sample
T
5
T
10 ( C)
Char Yield
50.0
Water uptake (%)
5
144
160
179
174
198
220
4.17
1.81
5a
51.3
5b
55.7
0.22
o
a,b
Temperature at which 5% and 10% weight loss was recorded by TGA at heating rate of 10 C/min in N
.
respectively Percentage weight of material left undecomposed after TGA analysis 800 C.
2
,
d
o
CONCLUSION
The PAI-nanocomposites were successfully prepared using solution intercalation method. The
structure and the uniform dispersion of organoclay throughout the PAI matrix were confirmed
by FT-IR, XRD and SEM analyses. The optical clarity and water absorption property of PAI-
nanocomposites were decreased significantly with increasing the organoclay contents in PAI
matrix. On the contrary the thermal stability of PAI-nanocomposites were increased
significantly with increasing the organoclay contents in PAI matrix. The enhancements in the
thermal stability of the nanocomposites films 5a and 5b by introducing organoclay may be due
to the strong interactions between polymeric matrix and organoclay, as well as intercalation and
dispersion of clay platelets in the PAI matrix.
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