Synthesis and properties of new clay-reinforced aromatic polyimide/nanocomposite-based 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 1,3-bis(4-aminophenoxy)propane

2.1 Materials

All chemicals were purchased from Fluka Chemical (Buchs, Switzerland), Aldrich Chemical (Milwaukee, WI), and Merck Chemical (Darmstadt, Germany). The organically modified clay (cloisite 20A) used in the preparation of nanocomposite films was modified with benzidine. This organoclay consists of a 95 mEq/100 g MMT/Na⁺ modified with a quaternary ammonium salt that contains a long-chain hydrocarbon tallow group.

2.2 Techniques

¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 300 MHz instrument (Bremen, Germany). FTIR spectra were recorded on Galaxy series FTIR 5000 spectrophotometer (England). Spectra of solid were performed by using KBr pellets. Vibration transition frequencies were reported in wave number (cm^-1). Band intensities were assigned as weak (w), medium (m), shoulder (sh), strong (s), and broad (br). Elemental analyses were performed by Vario EL equipment. Wide-angle XRD study of the samples was performed with a X’pert pw 3064 XRD with a copper target at a scanning rate of 4° min^-1. The morphologies of the fractured surfaces of the extrusion samples were investigated using a LEO 1455 VP SEM. TGA data for polymers were taken on a Mettler TA4000 System under N₂ atmosphere at rate of 10°C min^-1.

2.3 Synthesis of diamine monomer 4

The diamine was synthesized through the aromatic nucleophilic substitution of corresponding with 4-nitrophenol (1) and 1,3-dibromopropane (2) in the presence of potassium carbonate followed by catalytic reduction with hydrazine monohydrate 10% Pd/C. The procedure was as follows: a solution of 4-nitrophenol (1) (2 g, 14.40 mmol), 1,3-dibromopropane (2) (7.20 mmol), potassium carbonate (1 g, 7.20 mmol), and N,N-dimethylformamide (10 ml) was refluxed for 6 h at 120°C. The mixture was precipitated in an ice-water mixture, and the crude dinitro intermediate was recrystallized from ethanol (96%), affording 2.12 g (69%) of yellow whitish solid (3), m.p.: 133–134°C, FTIR (KBr, cm^-1): 3058 (w), 1609 (s), 1534 (s), 1411 (m), 1348 (s), 1181 (s), 1111 (s), 987 (s), 852 (s), 756 (m), 702 (s), 543 (m) cm^-1. H-NMR (300 MHz, DMSO-d₆, TMS): δ; 8.17–8.20 (t, 4H), 7.12–7.15 (t, 4H), 4.19 (t, 4H), 1.92 (s, 4H) ppm.

A solution of the obtained dinitro compound (1 g, 5.17 mmol), 0.5 g of 10% Pd on active carbon (10% Pd/C), and 200 ml ethanol was added dropwise to hydrazine monohydrate (15 ml) at 85°C and then refluxed for 5 h. The mixture was filtered to remove 10% Pd/C, and the filtrate was poured into water and dried to afford 1 g (83%) (4). m.p.: 109–111°C, FTIR (KBr, cm^-1): 3058 (w), 1609 (s), 1534 (s), 1411 (m), 1348 (s), 1181 (s), 1111 (s), 987 (s), 852 (s), 756 (m), 702 (s), 543 (m) cm^-1. ¹H-NMR (300 MHz, DMSO-d₆, TMS): δ; 8.17–8.20 (dd, 4H), 7.12–7.15 (dd, 4H), 4.19 (s, 4H), 1.92 (s, 4H) ppm.

The flowchart of the reaction is listed as Scheme 1.

Scheme 1

Synthesis of diamine 4.

2.4 Preparation of PI film through thermal imidization

A typical procedure to prepare PI film by thermal imidization was as follows: amounts of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (0.322 g, 1 mmol) (cup A) and 1,3-bis(4-aminophenoxy)propane (0.5 g, 1 mmol) (cup B) were dissolved separately in 3 ml DMAC stirring for 1 h under N₂ atmosphere at room temperature. Cup B was introduced into cup A followed by stirring at room temperature for 24 h to form the PAA. The resulting PAA solution was cast onto a glass and placed in an oven for a programmed heat treatment: 30 min at 80°C, 1 h at 110°C, 2 h at 170°C, and 3 h at 200°C.

2.5 Preparation of organically modified layered silicates

Organically modified layered silicates were synthesized by a cation exchange reaction between MMT/Na⁺ and the ammonium salt of benzidine. Then, 5 g MMT/Na⁺ was dispersed in 600 ml water. The amount of modifier was dissolved in water and a stoichiometric amount of concentrated HCl was added to the solution. Dissolved clay was added to the solution of the modifier and this mixture was agitated vigorously for 3 h. The blackish gray precipitates were isolated by suction filtration and washed with 400 ml hot water. This process was repeated thrice to ensure the removal of excess ammonium salt of benzidine diamine, thus ensuring complete removal of chloride ions. The final product obtained by filtration was dried in a vacuum oven for 24 h.

2.6 In situ polymerization of PCNs by thermal imidization

A procedure for the preparation of PCNs by thermal imidization is shown in Scheme 2. An appropriate amount of organoclay (0.5, 1, 3, and 5 wt.%) was introduced into 3 ml DMAC with stirring for 24 h at room temperature (cup A). Then, 1 mmol dianhydride was dissolved in 1.5 ml DMAC with stirring for 20 min (cup B). Cup B was introduced into cup A and stirred for 24 h at room temperature. Then, 1 mmol diamine was dissolved in 1.5 ml DMAC with stirring for 10 min at room temperature under N₂ atmosphere (cup C). Cup C was introduced into cup (A+B) for 24 h at room temperature under N₂ atmosphere to form the PAA. The resulting solution was then cast onto a glass plate and heat treated in a high-temperature oven with the same program as that for the PI film.

Scheme 2

Synthesis of nanocomposite 8.

3.1 Characterization

¹H-NMR spectrum of diamine (4) that showed peaks at 2.15–2.20 ppm assigned to H (a) related to diamine protons. Also, a broad singlet peak at 3.41 ppm was assigned to the H (b) protons of the NH₂ groups. Peaks at 4.05–4.09 ppm were assigned to H (c) related to 4H of CH₂ near oxygen. Peaks at 6.74–6.78 and 6.62–6.65 ppm showed doublet of doublet, which was assigned to the H (d) and H (e) aromatic protons (Figure 1).

Figure 1

¹H-NMR spectrum of diamine 4.

3.2 XRD

Figure 2 illustrates the wide-angle powder XRD patterns of organoclay, PI, and a series of PCN materials. Raw clay was found to show a diffraction peak at 2θ=9.92° (d-spacing=0.89 nm). In this spectrum, the d-spacing of organoclay based on Bragg’s law (nλ=2dsinθ) at 2θ=6.78° was 1.30 nm. Also, the XRD curves of PCNs containing 0.5, 1, 3, and 5 wt.% of organoclay did not display any XRD peak at 2θ=2–10°, indicating that the d-spacing of silicate layers had been either intercalated or the silicate layers were exfoliated completely in PI matrix.

Figure 2

Wide-angle powder XRD patterns for PCN0.5, PCN1, PCN3, PCN5, organoclay, and MMT/Na⁺.

3.3 SEM

Figure 3(A–C) shows clay phases within the nanocomposite films with organoclay contents of 1%, 3%, and 5%. The PCN1, 3, and 5 was well dispersed in a continuous PI phase. A comparison of the micrographs reveals that the fractured surfaces of the hybrid films with higher clay contents were more deformed than those of the films with low clay contents.

Figure 3

SEM images shown in (A) PCN1%, (B) PCN3%, (C) PCN5%.

3.4 TGA

Thermal stability of the nanocomposites was determined by thermogravimetric technique under N₂ atmosphere at a heating rate of 10°C min^-1. Thermograms obtained for these materials are shown in Figure 4. The TGA results of PI, PCN1, and PCN5 are summarized in Table 1. The increase in thermal stability of these hybrids with an increasing clay loading was likely due to two factors [20]: (1) the significant effect of small amounts of dispersed clay layers on the free volume of the PI and (2) the confinement of intercalated polymer chains within the clay galleries, which prevented the segmental motions of the chains. Similar results have been obtained in other studies of polymer nanocomposites [21, 22].

Figure 4

Comparison between thermal gravimetric thermograms of PI, PCN1%, and PCN5%.

Thermal decomposition temperatures of the PI, PCN1, and PCN5 are between 400°C and 455°C. TGA results indicated that these materials were found thermally stable, which increases with the addition of clay content in the polymer matrix. The increase of PCN5 in the thermal stability may result from the high thermal stability of organoclay network and the physical crosslink points of the organoclay particles, which limited the movement of the molecular chain of polymer. PCN1 thermal stability decrease a little because of agglomeration of organoclay particles. The weight retained by these samples at 800°C was roughly proportional to the amount of organoclay in the nanocomposites. Organoclay was found to increase the thermal stability presumably due to superior insulating characteristics of the layered silicate acting as mass transport barrier to the volatile products generated during decomposition. The char yield can be a decisive factor to estimate the limited oxygen index (LOI) of PI, PCN1, and PCN5 according to the Van Krevelen-Hoftyzer’s equation [23]:

LOI=17.5+0.4 CR,

where CR is the char yield. PI, PCN1, and PCN5 had LOI values 30.3, 36.7, and 34.5, respectively, which were calculated from their char yield. Based on the LOI values, such macromolecules can be classified as self-extinguishing PI and PCNs.