Polyimide-silver nanocomposite containing phosphine oxide moieties in the main chain: Synthesis and properties

Article abstract of DOI:10.1016/j.cclet.2010.05.033

New flame-retardant polyimide-silver nanocomposite containing phosphine oxide moiety in the main chain was synthesized by a convenient ultraviolet irradiation technique. A precursor such as AgNO₃ was used as the source of the silver particles.

Full text of DOI:10.1016/j.cclet.2010.05.033

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 1387–1390
www.elsevier.com/locate/cclet
Polyimide–silver nanocomposite containing phosphine oxide
moieties in the main chain: Synthesis and properties
*
Khalil Faghihi , Mohsen Hajibeygi, Meisam Shabanian
Organic Polymer Research Laboratory, Department of Chemistry, Faculty of Science, Arak University, Arak 38156, Iran
Received 25 January 2010
Abstract
New flame-retardant polyimide–silver nanocomposite containing phosphine oxide moiety in the main chain was synthesized by
a convenient ultraviolet irradiation technique. A precursor such as AgNO3 was used as the source of the silver particles. Polyimide 6
as a source of polymer was synthesized by polycondensation reaction of bis (3-aminophenyl) phenyl phosphine oxide 4 with
pyromellitic anhydride 5 in the presence of iso-quinoline as base and in m-Cresol solution. The resulting composite film was
characterized by FTIR spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry (TGA)
and differential scanning calorimetry (DSC). The average size of the nanometer Ag particles is about 10 nm. The temperature of 5
and 10% weight loss and also the char yield at 600 8C of polyimide–silver nanocomposite 6a were higher than the pure PI 6.
# 2010 Khalil Faghihi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Flame-retardant; Polyimide–silver nanocomposite; Phosphine oxide moiety
Polyimides (PIs), especially aromatic polyimides, are the most famous high-performance synthetic polymers with
versatile properties, such as high transition temperatures, good chemical resistance, excellent mechanical properties,
and outstanding thermal and thermo-oxidative stability. These properties have made polyimides widely used in many
fields including aerospace, electronics, and other modern industries [1]. With the development of nanotechnology, the
investigation on polyimide-based nanocomposites or hybrids with inorganic particles such as metals, metal oxides,
zeolites, organoclay and carbon nanotubes have been explored to further enhance the properties of PI materials [2–6].
Also incorporating a chemically reactive phosphorus-containing monomer into the polymer backbone is one of the
most efficient methods of improving the flame retardancy of polymers [7,8]. In this article a new polyimide (PI)–silver
nanocomposite containing phosphine oxide moiety in the main chain was prepared by using ultraviolet irradiation
technique at room temperature.
1. Experimental
Fourier transform infrared (FTIR) spectra were recorded on Galaxy Series FTIR 5000 spectrophotometer
(England). Thermal Gravimetric Analysis (TGA and DTG) data were taken on a Mettler TA4000 System under N2
* Corresponding author.
E-mail address: k-faghihi@araku.ac.ir (K. Faghihi).
1001-8417/$ – see front matter # 2010 Khalil Faghihi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.05.033

1[(Schme_1)TD$FIG] 388
K. Faghihi et al. / Chinese Chemical Letters 21 (2010) 1387–1390
Scheme 1. Synthetic route of diamine 4.
atmosphere at a rate of 10 8C/min. The morphology of nanocomposite film was investigated on a Hitachi H-800
transmission electron microscope with an accelerating voltage of 200 kV. Differential scanning calorimetric analysis
were performed on differential scanning calorimeter (Du Pont 910) at a heating rate of 10 8C/min. Limited oxygen
indexes (LOI) were measured on a Stanton Redcraft flame meter. X-ray diffraction (XRD) were performed on Philips
X-Pert (Cu-Ka radiation, l = 0.15405 nm).
Bis (3-aminophenyl) phenyl phosphine oxide 4 was prepared in a three-step reaction by using simple organic
compounds such as triphenyl phosphine according to a typical procedure that was shown in Scheme 1 [9].
A quantity of 5 mmol bis (3-aminophenyl) phenyl phosphine oxide 4, 5 mmol pyromellitic anhydride 5 and two
drops of iso-quinoline were dissolved in 25 mL m-cresol, consecutively. The mixture was stirred at room temperature
for 2 h and then at 180 8C for 3 h. The obtained PI solution was precipitated in 100 mL ethanol. The precipitate was
immersed in distilled water and ethanol for 2 h, and then washed with distilled water and ethanol several times to
remove the residual m-cresol. The inherent viscosity of this soluble PI is 0.65 dL/g.
A 250 W high-pressure mercury lamp was used as the ultraviolet irradiation source. The AgNO3 was introduced as
the source of Ag nanoparticles. First a solution such as 1.0 g soluble PI in 7.0 mL of 1-methyl-2-pyrrolidone (NMP)
and also a solution of 1.0 mmol AgNO3 and 1.2 mmol trifluoroacetic acid (TFA) in 5.0 mL NMP were prepared. Then
the two solutions were mixed and irradiated for 12 h to ensure the complete reduction of AgNO3 under ultraviolet
irradiation at room temperature. The product was precipitated quickly to distilled water and washed with distilled
water for several times. Then the sample was dried in vacuum, redissolved in chloroform by sonication and was cast on
a glass substrate and dried at 50 8C in vacuum for two days.
2. Results and discussion
Bis (3-aminophenyl) phenyl phosphine oxide 4 was prepared by using a three-step reaction from the simple organic
compounds such as triphenyl phosphine 1, as shown in Scheme 1. At first triphenyl phosphine 1 was oxidized to
triphenyl phosphine oxide 2, then compound 2 was converted to bis (3-nitrophenyl) phenyl phosphine oxide 3 by using
concentrated nitric acid in the presence of sulfuric acid. Bis (3-nitrophenyl) phenyl phosphine oxide 3 was reduced to
bis (3-aminophenyl) phenyl phosphine oxide 4 with SnCl2Á2H2O (Scheme 1).
Polyimide 6 was synthesized by reaction of an equimolar mixture of diamine 4 with pyromellitic anhydride 5 in m-
cresol solution and in the presence of iso-quinoline as a base (Scheme 2).
The FTIR spectrum of PI 6 exhibits characteristic absorption peaks for C O unsymmetrical stretching of imide
groups (at 1780 cmÀ1), C O symmetrical stretching of imide groups (at 1720 cmÀ1) and C–N stretching of imide
groups (at 1380 cmÀ1). These bands show PI 6 has been successfully synthesized. In addition, no obvious difference
between the infrared spectra of the pure PI 6 and the PI-silver nanocomposite 6a was observed.
[(Schme_2)TD$FIG]
Scheme 2. Synthetic route of PI 6.

[(ig._1)TD$FIG]
K. Faghihi et al. / Chinese Chemical Letters 21 (2010) 1387–1390
1389
Fig. 1. XRD of polyimide–silver nanocomposite 6a.
[(ig._2)TD$FIG]
Fig. 2. TEM of polyimide–silver nanocomposite film 6a.
The XRD patterns of polyimide–Ag nanocomposite 6a further confirmed the incorporation of silver nanoparticles
in the composites as shown in Fig. 1. The weak reflection centered at a 2u value around 208 was characteristic of the
amorphous polyimide [10]. Another five sharp diffraction peaks at 2u values of 38.28, 44.48, 64.58, 77.38 and 81.58
corresponded to Bragg’s reflections from the (1 1 1), (2 0 0) (2 2 0), (3 1 1) and (2 2 2) planes of Ag and were in good
agreement with the reported data [11]. These data which are close to the reported data in the JCPDS file No. 4-783.
The TEM micrograph of the PI-silver nanocomposite 6a in Fig. 2 shows that the silver nanoparticles were
homogeneously dispersed in polyimide matrix. The average size of the nanometer Ag particles is about 10 nm
calculated from the half-peak width using the Scherrer formula. Also the content of Ag nanoparticles is around 10% of
polyimide matrix of nanocomposite film 6a.
The thermal properties of pure polyimide 6 and polyimide–silver nanocomposite 6a were investigated by TGA (in a
nitrogen atmosphere at a heating rate of 10 8C/min) and DSC experiments and the results are summarized in Table 1.
The initial decomposition temperatures of 5 and 10% weight losses (T5 and T10) and the char yield at 600 8C for these
samples are summarized in Table 1. The temperature of 5 and 10% weight loss and also the char yield at 600 8C of
polyimide–silver nanocomposite 6a were higher than the pure PI 6. The content of Ag nanoparticles the char yield at
Table 1
Thermal and flame-retardant behaviors of polyimide 6 and polyimide–silver nanocomposite 6a.
T5 (8C)b
T10 (8C)c
Char yield (%)d
LOIe
a
Polyimide
Tg
6
6a
153
160
325
375
375
400
53
62
30
32
a
The glass-transition temperatures (Tg) were measured by DSC.
b
c
d
e
Temperature at which 5% weight loss was recorded by TGA at heating rate of 10 8C/min in N2.
Temperature at which 10% weight loss was recorded by TGA at heating rate of 10 8C/min in N2.
Percentage weight of material left undecomposed after TGA analysis 600 8C.
Limited oxygen index (LOI).

1[(ig._3)TD$FIG] 390
K. Faghihi et al. / Chinese Chemical Letters 21 (2010) 1387–1390
Fig. 3. TGA thermograms of PI 6 and polyimide–silver nanocomposite film 6a.
[(ig._4)TD$FIG]
Fig. 4. DSC thermograms of PI 6 and polyimide-silver nanocomposite film 6a.
600 8C of polyimide–silver nanocomposite 6a is around 20%. The higher thermal stability of nanocomposite 6a can be
attributed to the presence of inorganic silver nanoparticles into the polyimide matrix (Fig. 3).
Also the Tg of nanocomposite 6a was higher than pure polyimide 6 (Fig. 4). The increase in the Tg can be attributed
to the strong interaction between the silver nanoparticles and polar imide chain.
The flame-retardant behavior of these samples was examined by measuring the limiting oxygen index (LOI). As
can be seen from Table 1, the pure PI 6 had LOI value of about 30, the LOI value of the PI-silver nanocomposite 6a was
higher than that of pure PI 6, and an increase in LOI value can be attributed to the presence of inorganic silver
nanoparticles in polyimide matrix (Table 1).
3. Conclusion
In this work, a polyimide–silver nanocomposite film was successfully prepared by a convenient reduction of silver
by ultraviolet irradiation technique. The high char yield and good LOI data of this nanocomposite film can be related to
the presence of inorganic silver nanoparticles in the polyimide matrix. These properties can make this nanocomposite
attractive for practical applications such as processable high-performance engineering plastics.
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