Synthesis and Characterization of Polyimide–Silver Nanocomposite Containing Chalcone Moieties in the Main Chain by UV radiation

Abstract

The soluble polyimide (PI)–silver nanocomposite (PISN) 6a containing chalcone moieties as a photosensitive group was synthesized successfully by a convenient ultraviolet irradiation technique. A precursor such as AgNO₃ was used as the source of the silver particles. PI 6 was synthesized by the one-step synthesis of PI from polycondensation reaction of 4,4′-diamino chalcone 4 with pyromellitic anhydride 5 in the presence of iso-quinoline solution. The resulting composite film was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, thermogravimetry analysis, and differential scanning calorimetry.

INTRODUCTION

Polyimides (PIs) are a class of high-performance polymers that have excellent thermal, electrical, and mechanical properties and are widely used in microelectronic devices, such as buffer coatings, interlayer insulation, and wafer-scale packages [1,2].

Recently, photosensitive PIs have attracted a great interest, because they simplify processing steps by avoiding the use of photoresists to obtain the desired patterns [3–5].

There is intense interest in the synthesis and properties of metal clusters and nanoparticles prepared in both aqueous and organic solutions and prepared in condensed state, for instance, polymers, zeolites, and glasses. Clusters, nanoparticles, and their containing materials are potentially useful in a wide range of applications, including highly active catalysts [6], magnetic materials, quantum dots, and miniaturization of electronic devices and nonlinear optical materials [7–11].

Various methods can be used to produce metal nanoparticles within a polymeric matrix [12,13]. Most of them are based on in situ reactions, that is, the particles are generated from the respective metal precursors in the presence of the matrix polymer. Different procedures for the synthesis range from chemical reduction, photoreduction, thermal decomposition to vapor deposition methods and sputtering [14]. Several basic routes can be employed, in the case of dispersions, the solution of the metal precursor and the polymer are combined, and the reduction is subsequently performed in solution. Another method is by deposition, where the metal precursor and the polymer are first sprayed onto a substrate, and the reduction to the metal colloids is then performed. The last method is the immersion, where the solid polymeric material is placed into a solution containing the metal precursor. The selection of the polymeric matrix is crucial for the optimization of the systems. Frequently, the polymers are employed not only as protective coatings but also as the dispersing and stabilizing media for the metal nanoparticles. In many cases, the polymers that surround the metal nanoparticles can also exert direct influence on the particles and material properties. Up to now, several types of stabilizing and protective agents for metal nanoparticles have been thoroughly investigated [15–17].

In this study, we investigated the preparation of new PI–silver nanocomposite (PISN) by convenient ultraviolet irradiation technique at room temperature. The silver nanoparticles were homogeneously dispersed in the PI matrix and the PISNs exhibited an ultraviolet–visible (UV–vis) absorption peak, corresponding to the characteristic surface Plasmon resonance (SPR) of silver particles.

EXPERIMENTAL

Materials

All chemicals were purchased from Fluka Chemical Co. (Switzerland), Aldrich Chemical Co. (Milwaukee), Merck Chemical Co. (Germany), and Acros Organics N.V/S.A (Belgium).

Techniques

Fourier transform infrared (FT-IR) spectra were recorded on Galaxy Series FTIR 5000 spectrophotometer (England). Thermogravimetry analysis (TGA) and differential thermal analysis (DTG) data were taken on a Mettler TA4000 System under N₂ atmosphere at a rate of 10°C/min. The morphologies of PISN 6a were investigated on Cambridge S260 scanning electron microscope (SEM). The film was cryogenically fractured in liquid nitrogen and then coated with Pt/Pd. Differential scanning calorimetric (DSC) analysis was performed on differential scanning calorimeter (Du Pont 910) at a heating rate of 10°C/min. X-ray diffraction (XRD) was performed on Philips X-Pert (Cu-Kα radiation, λ = 0.15405 nm).

Monomer Synthesis

SYNTHESIS OF 4,4′-DINITROCHALCONE 3

To a solution of 4-nitroacetophenone 1 (1.65 g, 10 mmol) and 4-nitrobenzaldehyde 2 (1.51 g, 10 mmol) in mixture of H₂O/EtOH (3:1, 100 mL), Na₃PO₄·12H₂O (0.038 g, 0.1 mmol) was added. The mixture was stirred under reflux conditions for 2 h and then the mixture was cooled to room temperature. The precipitate was filtered off, washed thoroughly with water, and dried to afford 2.72 g 3 (yield 91%). mp: 215–216°C; FT-IR (KBr): 3113 (m), 3086 (w), 3049 (w), 1670 (s), 1599 (s), 1518 (s), 1415 (w, sh), 1338 (s, br), 1288 (s), 1209 (s), 1107 (m), 1028 (s), 987 (m, sh), 839 (s, sh), 787 (m), 742 (m), 682 (m), and 499 (w) cm⁻¹.

SYNTHESIS OF 4,4′-DIAMINOCHALCONE 4

To a solution of Na₂S (1.56 g, 20 mmol) and NaHCO₃ (1.26 g, 15 mmol) in water (10 mL), methanol (10 mL) was added. The mixture was stirred for 30 min at room temperature. The precipitate was filtered, the filtrate was added to a mixture of 4,4′-dinitrochalcone 3 (1.19 g, 4 mmol) and methanol (100 mL), and stirred for 3 h under reflux conditions. Then, the mixture was concentrated using rotary evaporator, and the residue poured into water, an orange crude product was formed and collected by filtration, washed thoroughly with water, and dried to afford 0.69 g (7) (yield 72%). mp: 177–179°C; FT-IR (KBr): 3485 (w), 3367 (m), 3329 (s), 3213 (w), 1630 (s), 1595 (s, sh), 1512 (s), 1439 (m), 1346 (m), 1300 (m), 1226 (s), 1170 (s), 1024 (m), 983 (w), 815 (m), 611 (w), and 513 (w) cm⁻¹. ¹H-NMR (DMSO-d₆), δ: 5.84 (br, 4H), 6.58–6.63 (m, 4H), 7.50–7.53 (m, 4H), and 7.88 (d, J = 8.6 Hz, 2H) ppm.

Anal. Calcd. for C₁₅H₁₄N₂O: C, 75.6; H, 5.9; and N, 11.8; Found: C, 75.4; H, 6.0; and N, 11.6.

Preparation of PI

A quantity of 5 mmol 4,4′-diamino chalcone 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°C 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 0.74 dL/g.

Preparation of PISN 6a

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 N-methyl-2-pyrrolidinone (NMP) and also a solution of 1.0 mmol AgNO₃, and 1.2 mmol trifluoroacetic acid 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 several times. Then, the sample was dried in vacuum, redissolved in chloroform by sonication, cast on a glass substrate, and dried at 50°C in vacuum for 2 days.

RESULTS AND DISCUSSION

Then, 4,4′-dinitrochalcone 3 was reduced using Na₂S/NaHCO₃ to produce 4,4′-diaminochalcone 4 (Scheme 1) [18]. The chemical structure and purity of compound 7 were determined by elemental analysis, FT-IR and ¹H-NMR spectroscopic techniques. The ¹H-NMR spectrum of compound 7 showed a broad peak at δ = 5.75–5.97 ppm, which was assigned to the H_g and H_h protons of the NH₂ groups. Peaks at δ = 6.58–6.63, δ = 7.50–7.53, and δ = 7.87–7.90 ppm were assigned to the H_f, H_e, H_b, H_c, H_d, and H_a protons of the phenyl rings and vinyl group (Figure 1).

Scheme 1.

Synthetic route of diamine 4 .

Figure 1.

¹H-NMR spectrum of diamine 4.

Polymer Synthesis

PI 6 as a source of polymer was synthesized by the one-step synthesis of PI from polycondensation reaction of 4,4′-diamino chalcone 4 with pyromellitic anhydride 5 in m-cresol solution and in the presence of iso-quinoline as a base (Scheme 2).

Scheme 2.

Synthetic route of PI 6.

Preparation of PISN

The soluble PISN was prepared using ultraviolet irradiation is presented in this section. A precursor of the silver (AgNO₃) particles was used.

Characterization of PISN

The FT-IR spectrum of PI 6 exhibits characteristic absorption peaks for C=O unsymmetrical stretching of imide groups (at 1776 cm⁻¹), C=O symmetrical stretching of imide groups (at 1710 cm⁻¹), and C–N stretching of imide groups (at 1380 cm⁻¹). These bands show that PI 6 has been successfully synthesized. In addition, no obvious difference between the infrared spectra of the pure PI 6 and the PISN 6a was observed.

Figure 2 shows the XRD pattern of the soluble PISN 6a. The five diffraction peaks in the XRD patterns of samples 6a widen greatly, indicating the formation of the nanometer scale of silver particles in the PISN. Figure 2 containing diffraction signals at 2 h values of 38.2°, 45.3°, 66.1°, 75.5°, and 83.7° attributed to the diffraction planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) of fcc silver nanoparticles confirming the presence of silver nanoparticles in the nanocomposites [19].

Figure 2.

XRD of PISN 6a.

The SEM micrograph of the PISN 6a in Figure 3 shows that the silver nanoparticles were homogeneously dispersed in PI matrix.

Figure 3.

SEM image of PISN 6a.

UV–Vis Absorption Characteristics

The photosensitive property of the new polymers 6 and 6a in the DMF solution was studied by a UV spectrophotometer. The polymer 6 solution exhibits two positions of absorption maximum in UV–vis spectrum around 380 and 270 nm.

The absorption maximum at around 270 nm corresponds to π → π* transition of the olefinic double bond present in the chalcone moiety and carbon double bonds in aromatic rings in the polymer backbone. Also, the absorption maximum at around 380 nm corresponds to n → π* transition of the nonbonding electrons present in nitrogen and oxygen atoms in the polymer backbone. The PI shows no obvious absorption at wavelength above 400 nm. The PISN 6a, however, shows a broadened peak at 418 nm. Overall, the formation of silver nanoparticles was confirmed using SPR phenomenon. This is in agreement with most AgNPs having kmax value in the visible range 400–500 nm [20].

Thermal Properties

The thermal properties of pure PI 6 and PISN 6a were investigated by TGA (in a nitrogen atmosphere at a heating rate of 10°C/min) and DSC experiments (Table 1). The initial decomposition temperatures of 5% and 10% weight losses (T₅ and T₁₀) and the char yield at 800°C for these samples are summarized in Table 1. The temperatures of 5% and 10% weight losses and also the char yield at 800°C of PISN 6a were higher than the pure PI 6 (Figure 4). The higher thermal stability of nanocomposite 6a can be attributed to the presence of inorganic silver nanoparticles in the PI matrix.

Figure 4.

TGA curves of PI 6 and PISN 6a.

Table 1.

Thermal behaviors of PI 6 and PISN 6a.

Polymer	Tg  a	T5 (°C) b	T10 (°C) b	Char yield c
6	149	370	410	53.89
6a	176	410	420	56.19

a

Glass transition temperature was recorded at a heating rate of 10°C/min in a nitrogen atmosphere.

b

Temperature at which 5% or 10% weight losses were recorded by TGA at a heating rate of 10°C/min under N₂.

c

Weight percentage of material left after TGA analysis at a maximum temperature of 800°C under N₂.

Also, the T_g of nanocomposite 6a was higher than pure PI 6 (Figure 5). The increase in the T_g can be attributed to the strong interaction between the silver nanoparticles and polar imide chain.

Figure 5.

DSC thermograms of PI 6 and PISN 6a.

CONCLUSION

In this study, a PISN containing chalcone moieties was successfully prepared by a convenient reduction of silver by ultraviolet irradiation technique. PI 6 as a source of polymer was synthesized by the one-step synthesis of PI from polycondensation reaction of 4,4′-diamino chalcone 4 with pyromellitic anhydride 5 in the presence of iso-quinoline solution. From the scanning electron microscopy (SEM) and XRD investigations, the silver nanoparticles homogeneously dispersed in the PI matrix. In the UV–vis absorption spectra of the PISN, the absorption peak due to the SPR of silver particles was observed at 418 nm. Due to the presence chalcone moieties in polymer backbone and good thermal properties, these silver/PI nanocomposites can be photosensitive and have the potential for the use in microfabrication of conductive components in microelectronic industry.