Synthesis and characterization of novel nanostructured, organosoluble, thermally stable and wholly aromatic poly(amide-imide)s from a new diimide-diacid monomer by direct polycondensation

Abstract

In the present work, a new diimide-diacid monomer bearing pyridine rings was prepared. A series of novel nanostructured, organosoluble, thermally stable and wholly aromatic poly(amide-imide)s (PAIs) were synthesized from diimide-diacid, by direct polycondensation with various aromatic diamines in N-methyl-2-pyrrolidone using triphenyl phosphite and pyridine as a condensing agent. The nanostructured PAIs were achieved in good yields and possessed moderate inherent viscosities in the range of η_inh = 0.37–0.51 dL g⁻¹. The synthesized polymers were characterized with FT-IR, ¹H-NMR, UV–vis spectroscopy, elemental analysis, X-ray diffraction, field emission scanning electron microscopy (FE-SEM) and thermogravimetric analysis (TGA) techniques. All of the PAIs were amorphous in nature, were readily soluble in many organic solvents and could be solution cast into tough and flexible films. These polymers exhibited strong UV–vis absorption in the range of 294–311 nm in solution and in films. According to TGA data, these polymers exhibited initial decomposition temperature in the range of 396–433 °C, temperature of 10% weight loss from 446 to 537 °C, char yields more than 75% at 700 °C in nitrogen atmosphere and can be classified as self-extinguishing polymers. FE-SEM results show that, these nanostructured PAIs have spherical and cylindrical beads and the particle size is around 35–90 nm.

Keywords:

• synthesis
• poly(amide-imide)
• nanostructured
• thermally stable
• heteroaromatic
• direct polycondensation

1. Introduction

Wholly aromatic polyimides (PIs) or their copolymers are well known as high performance polymers due to their excellent thermal and thermooxidative stabilities, an extremely high purity, non-flammability, outstanding mechanical and electrical properties, good balance between chemical, physical and mechanical properties. Therefore, all kinds of PIs have been widely used in the fields of high temperature adhesives, composite matrices, fibers, films, foams, moldings, wire-coating enamels, membranes, as well as microelectronic materials.[1–8] However, wholly aromatic PIs, though thermally stable, do not always provide the optimal properties for many specialty applications because of deficiencies in solubility, processability and transparency.[9,10] Therefore, much effort has been made to increase the solubility and processability of PIs. Many monomers have been specially designed using different approaches, such as introducing of flexible linkages or bridging functional groups,[11,12] bulky pendant groups into the polymer backbones,[13–15] heterocyclic rings [16] and meta-catenated as a less symmetric aromatic units [17–19] to afford the success in preparation of organosoluble polyamides.

For the PIs materials, synthesis of newly heteroaromatic monomers and corresponding PIs that have both good processability and maintained thermal stability would be very interesting. In general, the heteroaromatic structures in the main chain of a polymer would impart certain properties to it, while pyridine with heteroaromatic structure would have excellent stabilities derived from its aromaticity and molecular symmetry, as well as polarizability resulting from nitrogen atom in pyridine ring. So new kinds of heteroaromatic diacide, diamine, dianhydride or other monomers holding pyridine unit should have contributions for the thermal stability, chemical stability, retention of mechanical property of the resulting polymer at elevated temperature and the resulting polymer holding pyridine ring should also exhibit a good solubility during processing.[20–22]

Replacement of PIs by copolyimides such as poly(amide-imide)s (PAIs) may be useful in modifying the intractable character of PIs.[23,24] Because of containing both imide and amide linkages in the main chain of polymer repeating units, PAIs have properties between PIs and polyamides; therefore, this kind of polymers offers a good compromise between high thermal property and processability. Aromatic PAIs possess desirable characteristic for a variety of applications as they retain exceptional thermomechanical properties at high temperature with good dimensional stability, excellent creep, chemical resistance as well as solvent resistance and have found extensive use in industries.[25–28]

The designing of new nanostructure materials from organic molecules has opened up new fundamental and practical frontiers to develop physical properties of materials significantly for different applications. Different nanostructure polymers have been developed for a wide range of advanced uses such as drug delivery devices, conducting wires and optoelectronic devices.[29–32]

In this work, a new kind of heteroaromatic diimide-diacid monomer containing imide ring and pyridine moiety, 2,6-Bis[3(1,3-dioxoisoindoline-5-carboxylic acid) benzoyl] pyridine, has been synthesized successfully, and a series of novel nanostructured, organosoluble, thermally stable and wholly aromatic PAI have been also prepared by the direct polycondensation in N-methyl-2-pyrrolidone (NMP), triphenyl phosphite/pyridine (TPP/Py), lithium chloride (LiCl) and calcium chloride (CaCl₂). This new diimide-diacid was characterized by Fourier transform infrared (FT-IR) and ¹H-NMR spectroscopy and elemental analysis. The obtained PAIs were fully characterized by FT-IR, ¹H-NMR and UV–vis spectroscopy, elemental analysis, viscosity measurement, quantitative solubility, X-ray diffraction (XRD), thermo gravimetric analysis (TGA) and field emission scanning electron microscopy (FE-SEM).

2. Experimental

2.1. Materials

All chemicals and solvents used were of analytical reagent grade purchased from Fluka Chemical Co. (Buchs, Switzerland), Aldrich Chemical Co. (Milwaukee, WI) and Merck (Darmstadt, Germany). 1-methylpyrrolidin-2-one (NMP) as solvent was distilled over barium oxide under reduced pressure. Other reagents were used without further purification.

2.2. Instrument and measurement

Furrier transform infrared (FT-IR) spectra of the samples were recorded with a Jasco-680 (Japan) spectrometer at a resolution of 4 cm⁻¹. The spectra of solids were obtained using KBr pellets. The pressed disk containing 2 mg of the sample and 200 mg of fine grade KBr was scanned at wavenumber range of 400–4000 cm⁻¹. Proton nuclear magnetic resonance (¹H-NMR, 500 MHz) spectra were recorded in DMSO-d₆ solution using a Bruker (Germany) Avance 500 instrument. Inherent viscosities (η_inh) were measured by a standard procedure using a Cannon-Fenske Routine Viscometer (Cannon, Mainz, Germany) at a concentration of 0.5 g/dL at 25 °C. Elemental analyses were performed by Leco, CHNS-932. Quantitative solubility of PAIs in various solvents was determined using 5 mg of the polymer in 1 mL of solvent. The XRD patterns were recorded by employing a Philips X’PERT MPD diffractometer (Cu Kα radiation: λ = 0.154056 nm at 40 kV and 30 mA) over the 2θ range of 10–80° at a scan rate of 0.05°/min. The samples for UV–visible measurements were prepared by cutting pieces of the cast films. These films have thicknesses 0.1–0.2 mm. UV–visible transmission spectra of PAI films were obtained using a JASCO V-750 UV/Vis/NIR spectrophotometer in the spectra range from 200 to 800 nm at a wavelength scan rate of 60 nm/min. Derivative thermal gravimetric (TGA/DTG) data for polymers were taken on Perkin Elmer in nitrogen atmosphere at a heating rate of 10 °C.min⁻¹ from room temperature to 800 °C. Surface morphology of polymers was characterized using FE-SEM [HITACHI: S-4160].

2.3. Synthesis of diimide-diacid (6)

For synthesis of heteroaromatic diimide-diacid (6), 2,6-Bis (3-aminobenzoyl) pyridine (4) was prepared and characterized based on literature.[33] A flask was charged with (1 g, 3.15 mmol) diamine (4), (1.21 g, 6.30 mmol) trimellitic anhydride (5), and 10 mL of acetic acid. The mixture was heated to reflux at 80 °C for about 12 h. The mixture was then cooled and poured into water. The white precipitate was filtered and dried in a vacuum oven at 70 °C overnight. The yield of the reaction was about 88% and the melting point was 243–245 °C. Elemental Analysis: Calculated. C, 66.77%; H, 2.88%; N, 6.31%; O, 24.04%, Found. C, 66.51%; H, 2.73%; N, 6.49%; O, 24.27%.

2.4. Synthesis of PAIs

The PAIs were prepared by the following general procedure: in a 50 mL two necked round bottomed flask equipped with a reflux condenser, a magnetic stirrer bar and a nitrogen gas inlet tube, a mixture of 0.50 mmol diamine, diacid (6) (0.33 g, 0.50 mmol), TPP (1.20 mL, 5 mmol), LiCl (0.60 g, 14.15 mmol), CaCl₂ (0.60 g, 6.21 mmol), 1 mL pyridine and 5.0 mL NMP was refluxed at 100 °C for 8 h under nitrogen atmosphere. After cooling, the reaction mixture was poured into a large volume of methanol and the precipitate was collected and washed thoroughly with hot water, then dried in a vacuum oven at 80 °C for 24 h. The yield of the polymerization reaction after extraction of polymers in hot methanol and removing low molecular weight oligomers were in the range of 74–91%. The inherent viscosity of the polymers were in the range of η_inh = 0.37–0.51 dL/g.

2.5. PAIs film preparation

The PAI films were fabricated by casting from solution. A solution of polymer was made by dissolving about 0.5 g of the PAI sample in 5 mL of NMP to afford an approximately 10 wt% solution. The homogeneous solution cast in glass Petri dishes. (bubbles were removed by shaking and blowing air), which was placed in a 90 °C oven overnight to evaporate most of the solvent; then the semidried film was further dried in vacuum at 160 °C for 5 h, for the removal of residue solvent and form solid films. The thickness of the films was controlled by using the same amount of total materials and same sized glass Petri dish. Freestanding PAI films were then peeled out from the glass plate and were used for transmission UV–vis absorption, XRD measurements, solubility tests and thermal analyses.

3. Result and discussion

3.1. Synthesis and characterization of monomer (6)

A new hetroaromatic diimide-diacid compound, 2,6-Bis[3(1,3-dioxoisoindoline-5-carboxylic acid) benzoyl] pyridine, was synthesized by a four step procedure, respectively, as shown in Scheme 1. Firstly, 2,6-dibenzoylpyridine was formed by Friedel–Crafts acylation of benzene with 2,6-pyridinedicarbonyl chloride using anhydrous aluminum chloride as a catalyst, then dinitro compound, 2,6-bis(3-nitrobenzoyl) pyridine, was synthesized by the nitrification of 2,6-dibenzoylpyridine with nitric acid (99%), and 2,6-bis(3-nitrobenzoyl) pyridine was converted to 2,6-Bis (3-aminobenzoyl) pyridine by the reduction of the nitro groups into NH₂ groups by reaction with SnCl₂ and HCl in ethanol. Finally, the hetroaromatic diacid (6) was prepared by the condensation of 2,6-Bis (3-aminobenzoyl) pyridine with two mole equivalents of trimellitic anhydride (5) in acetic acid.

The structure of diimide-diacid monomer (6) was confirmed by FT-IR spectrum, ¹H-NMR spectroscopy and elemental analysis. The FT-IR spectrum (Figure 1) of monomer (6) showed absorption bands around 2900–3500 cm⁻¹ (COOH group), 1750 and 1732 cm⁻¹ (typical of imide carbonyl asymmetric and symmetric stretching) confirming the presence of carboxylic acid groups and imide ring in the structure. As shown in Figure 2, in the ¹H-NMR spectrum of diimide-diacid (6), the resonance signal at 7.52–8.46 ppm is ascribed to the aromatic protons. The signal at 12.82 ppm is related to the proton of carboxylic acid. These results clearly confirmed that diacid prepared herein is consistent with the proposed structure.

Figure 1. FT-IR spectrum of diimide-diacid (6).

Figure 2. ¹H-NMR spectrum of the diimide-diacid (6).

Figure 3. FT-IR spectra of the PAI8d (a) and PAI8f (b).

Figure 4. ¹H NMR spectra of the PAI8d (a) and PAI8f (b).

Figure 5. XRD patterns of PAI8a, PAI8c and PAI8f.

Figure 6. FE-SEM images of PAIs.

Figure 7. UV–vis transmission spectra of PAIs film.

Figure 8. The TGA (a) and DTG (b) curves of PAIs.

Scheme 1. Synthesis of diimide-diacid (6).

3.2. Polymers synthesis

According to the phosphorylation polyamidation technique first described by Yamazaki and co-workers,[34] a series of novel PAIs containing imide ring and heteroaromatic pyridine group was prepared by the direct polycondensation reaction between the diimide-diacid compound (6) and the commercially available aromatic diamines (7a–7f), as shown in Scheme 2. The polymerization was carried out via solution polycondensation using TPP and Py as condensing agents. The reaction readily proceeded within a homogeneous solution in NMP and in the presence of LiCl and CaCl₂. The synthesis and some physical properties of these new PAIs (8a–8f) are given in Table 1. All the polymers were obtained in high yields (83–91%) and the inherent viscosities were in the range of η_inh = 0.37–0.51 dL/g, indicative of the formation of moderate molecular weights polymers.

Scheme 2. Synthesis of PAIs.

Table 1. Synthesis and some physical properties of PAIs.

Diamine	Polymer	Yield (%)^a	ηinh^b	Color
7a	PAI8a	83	0.42	Light yellow
7b	PAI8b	87	0.39	Light yellow
7c	PAI8c	91	0.48	Light yellow
7d	PAI8d	89	o.45	Yellow
7e	PAI8e	86	0.51	Yellow
7f	PAI8f	87	0.37	Brown

^a Polymers were precipitated in methanol.

^b Measured at a concentration of 0.5 g dL⁻¹ in NMP at 25 °C.

3.3. PAIs structural identification

The polymer structures were confirmed by FT-IR, ¹H-NMR spectroscopy and elemental analysis techniques. In general, in the FT-IR spectrum of PAIs, the characteristic absorption bands for the five membered imide ring appears at 1773–1779 (C=O asymmetrical stretching), 1714–1720 cm⁻¹ (C=O symmetrical stretching), 1377–1389 cm⁻¹ (C–N stretching), 721–728 cm⁻¹ (imide ring deformation). The absorptions of amide groups appeared at 3333–3367 cm⁻¹ (N–H stretching) and 1663–1674 cm⁻¹ (amide C=O stretching). The FT-IR spectra of PAI8d and PAI8f are shown in Figure 3.

¹H-NMR spectra of the PAI8d and PAI8f are illustrated in Figure 4. All the protons in these spectra are in good agreement with the proposed structures. The resonance peaks appearing in the region of 10.09–10.94 ppm also support the formation of amide linkages. The signals in the range of 7.25–8.21 ppm are attributed to the aromatic groups in the polymer’s main chain. The spectral data of PAIs are listed in Table 2 and elemental analysis values for the obtained PAIs are also listed in Table 3. The elemental analysis values generally agreed with the calculated values for the proposed structures of PAIs.

Table 2. FT-IR and ¹H NMR data of the PAIs.

Polymer	Spectral data
PAI8a	FT-IR (KBr, cm^−1): 3381 (N–H stretching), 2990 (Ar–H stretching), 1773 (C=O imide, asymmetric stretching), 1715 (C=O imide, symmetric stretching), 1661 (C=O amide, stertching), 1604 (C=C aromatic), 1517 (N–H bending), 1387 (CNC axial stretching), 1187 (CNC transverse stretching), 721 (CNC out-of-plane bending). ^1H NMR (400 MHz, DMSO-d6, ppm): 7.39–8.21 (Ar–H), 10.68 (N–H).
PAI8b	FT-IR (KBr, cm^−1): 3376 (N–H stretching), 3083 (Ar–H stretching), 1776 (C=O imide, asymmetric stretching), 1714 (C=O imide, symmetric stretching), 1658 (C=O amide, stertching), 1607 (C=C aromatic), 1510 (N–H bending), 1377 (CNC axial stretching), 1196 (CNC transverse stretching), 723 (CNC out-of-plane bending).
PAI8c	FT-IR (KBr, cm^−1): 3379 (N–H stretching), 3100 (Ar–H stretching), 1775 (C=O imide, asymmetric stretching), 1718 (C=O imide, symmetric stretching), 1658 (C=O amide, stertching), 1603 (C=C aromatic), 1510 (N–H bending), 1377 (CNC axial stretching), 1192 (CNC transverse stretching), 1059 (C–O stretching), 726 (CNC out-of-plane bending). ^1H NMR (400 MHz, DMSO-d6, ppm): 7.27–8.57 (Ar–H), 10.68 (N–H).
PAI8e	FT-IR (KBr, cm^−1): 3379 (N–H stretching), 3108 (Ar–H stretching), 2983 (C–H stretching, aliphatic) 1773 (C=O imide, asymmetric stretching), 1719 (C=O imide, symmetric stretching), 1657 (C=O amide, stertching), 1600 (C=C aromatic), 1512 (N–H bending), 1379 (CNC axial stretching), 1190 (CNC transverse stretching), 728 (CNC out-of-plane bending).

Table 3. Elemental analysis of PAIs.

Polymer	Formula (formula weight)^a		Elemental analysis (%)
			C	H	N
PAI8a	(C43H23N5O8)n	Calcd.	69.94	3.11	9.49
	(737.74)n	Found.	69.38	3.27	9.21
PAI8c	(C49H27N5O9)n	Calcd.	70.85	3.25	8.43
	(829.84)n	Found.	70.18	3.47	8.59
PAI8e	(C50H29N5O8)n	Calcd.	72.47	3.50	8.45
	(827.86)n	Found.	71.92	3.79	8.11
PAI8f	(C62H35N5O8)n	Calcd.	76.07	3.57	7.15
	(978.04)n	Found.	75.23	3.66	7.39

^a The polymer sample was dried in vacuum at 80 °C for 5 h.

3.4. PAIs solubility

It is known that conventional wholly aromatic PIs are insoluble in common organic solvents. One of the major objectives of this study was to produce nano structure PAIs containing imide ring and hetroaromatic pyridine group with improved solubility. The solubility behavior of the new resulting PAIs was investigated quantitatively in different organic solvents at 10% (w/v), and the results were summarized in Table 4. These polymers exhibited excellent solubility in the polar aprotic solvents such as NMP, N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) at room temperature, and even in less polar solvents like m-cresol and pyridine by heating, but showed poorer solubility in dioxane (DO) and chloroform (CHCl₃) due to the low value of dielectric constant of these solvents. The good solubility of these PAIs might be attributed to their nanostructure, the presence of the polar amide linkage in the main chain and the polarizability resulting from the nitrogen atom in pyridine ring. The PAI8a derived from the more rigid p-phenylenediamine toward PAI8b containing m-phenylenediamine showed a decreased solubility in m-cresol and pyridine.[35] Also the unique enhanced solubility of the meta-related polymer (PAI8b) might be attributed to the higher entropy state of the polymer chains resulting from the meta orientation. The greater disorder should facilitate solvation of the polymer chains. For PAI8c and PAI8d the presence of sulfone and ether linkages can also contribute effectively in the solubility of these polymers by interacting with the polar molecules of solvents. The presence of methylene groups in the backbone of PAI8e increased flexibility and also disturbed the planarity of aromatic units, resulting in a reduction of the close packing and increased the solubility in a less polar solvent like pyridine. The solubility of PAI8f should be the result of the introduction of the bulky pendent group in PAI backbone which disturbed dense packing of the polymer chains and led to the increased chain distances and decreased chain interactions; finally, the solvent molecules were able to penetrate more easily to solubilize the polymer chains. Therefore, the good solubility makes these PAIs potential candidates for practical applications in spin or dip coating and ink jet printing processes. All the polymers could afford flexible and tough films by solvent casting, and these films are essentially amorphous as evidenced by the XRD patterns.

Table 4. Quantitative solubility^a of the PAIs.

Polymer	Solvent
	NMP	DMAc	DMF	DMSO	m-Cresol	Py	DO	CHCl3
PAI8a	++	+	+	+	±	±	–	–
PAI8b	++	++	++	++	+	+	–	–
PAI8c	++	++	++	++	±	±	–	–
PAI8d	++	++	++	++	±	±	–	–
PAI8e	++	++	++	++	±	+	–	–
PAI8f	++	++	++	++	±	±	–	–

Note: (++) Soluble at room temperature, (+) soluble after heating, (±) partially soluble, (–) insoluble.

^a Measured at a polymer concentration of 0.05 g/ml.

3.5. XRD analysis of PAIs

XRD is the most commonly used method to study the structure of nanostructure materials. As indicated by X-ray diffractograms of PAIs in Figure 5, the weak reflection centered at a 2θ value around 20° was characteristic of the amorphous polymers with only a low degree of molecular ordering. This might have been caused by the presence of heterocyclic imide ring and wholly aromatic structures in the polymer chain, which limit the molecular mobility, decreased the intermolecular forces between the polymer chains and constrict the formation of crystals.

The PAI8f due to the presence of bulky pendent group of diamine is less crystalline than other PAIs. Furthermore, the amorphous character is also supported by the reasonable solubility of PAIs, which is in agreement with the general rule that the solubility decreases with increasing crystallinity.

3.6. Nano structure of PAIs

The development of nanostructured polymers has opened up new fundamental and rapidly growing field of research, which has attracted notable attention in recent years. Nano structured polymers have been developed for a wide range of advanced uses such as conducting wires, optoelectronic devices, material science, biotechnology, biomedicine, biochemical sensor design and drug delivery systems.[36,37] Morphological characterization of PAIs was studied by FE-SEM. In this study it is very interesting to mention that the FE-SEM images of PAIs showed that the obtained macromolecules have nano structure morphology (Figure 6). As can be seen in Figure 6, the average diameter of polymeric particles is in the range of 35–90 nm. The PAIs offered a homogeneous nanostructure, dispersed as spherical and cylindrical beads, and also have amorphous and spongy morphology.

3.7. Optical properties of PAIs

The optical properties of the PAIs are investigated by UV–vis spectroscopy. Table 5 lists some important optical data obtained from dilute solutions (0.2 g/dL) of the PAIs in NMP and from thin films. The absorption spectra of the resulting PAIs in solutions and in films were nearly identical with each other. The PAIs showed strong UV absorption with maximum at range of λ_max = 294–311 nm, which are attributed to the π–π^* transitions. Figure 7 depicts the UV–vis transmittance spectra of PAI films, the λ₀ values (absorption edge or cut off wavelength) of the resulting polymers were found to be in the range of 380–390 nm and the optical transmittance values at 700 nm are higher than 80%, indicating high degree of the film transparency.

Table 5. Optical properties of the PAIs.

Polymer	λmax (nm)		λonset (nm)		λ0 (nm)
	Solution^a	Film^b	Solution^a	Film^b
PAI8a	301	307	421	411	459
PAI8b	297	303	409	420	463
PAI8c	305	294	427	431	465
PAI8d	298	308	436	415	473
PAI8e	311	302	417	429	468
PAI8f	304	299	403	414	471

^a From UV–vis absorption spectra measured in NMP solution (0.2 g/dL) at room temperature.

^b The cut off wavelengths (λ₀) from the transmission UV–vis spectra of PAI films.

3.8. Thermal stability of PAIs

Thermal stability plays an important role in determining both technological applications and processing conditions of polymers and generally studied by TGA and derivative of thermaogravimetric (DTG). TGA and DTG analysis in a nitrogen atmosphere at a rate of 10 °C min⁻¹ were used to study the thermal properties of these PAIs. TGA and DTG curves of PAIs are shown in Figure 8 and the obtained results are given in Table 6. The initial decomposition temperature (T_i), the temperature of 5% weight loss (T₅) and 10% weight loss (T₁₀), that is an important criterion for evaluation of thermal stability, are ranged between 396–433 °C, 413–476 °C and 446–537 °C, respectively. There was a small weight loss at the temperature range from 80 to 200 °C in the TGA curves of PAIs, which can be attributed to the loss of absorbed moisture hydrogen bonded with amide linkages that was not removed in the drying process. The amount of carbonized residue (char yield or CR) of PAIs was from 53.4 to 75.9 at 800 °C under a nitrogen gas flow, depending on the structure of diamine component. The high char yields of these polymers can be attributed to their high aromatic content. An examination of data reveals that that the T₁₀ of PAI8b was slightly higher than that of PAI8a, which may be attributed to the fact that the meta-oriented architecture has less dense polymer chain packing than para-oriented, due to the smaller conformational entropy of para-oriented chain. So, the former has better thermal stability. From TGA curves it can be detected that the most stable polymer is PAI8f with T_i of 439 °C, T₅ of 510 °C, T₁₀ of 520 °C, which can be due to absence of flexible linkage such as ether, methylene, or sulfone group in the polymer backbone and also due to bulky rigid side group of diamine. As can be seen from the DTG, all PAIs decomposed in one stage weight loss process. The temperature of the maximum speed of decomposition (T_max) is situated in the range of 435–590 °C and it is due to the degradation of the amidic linkages.

Table 6. Thermal characteristic data of PAIs.

Polymer	Decomposition temperature (°C)				CR^c	LOI^d	∆Hcomb(kJ/g)^e
	Ti ^a	T5 ^a	T10 ^a	Tmax ^b
PAI8a	412	428	503	536	71.3	46.0	17.4
PAI8b	407	421	499	521	63.1	42.7	18.7
PAI8c	413	431	461	483	57.4	40.4	19.8
PAI8d	417	437	453	492	60.0	41.5	19.3
PAI8e	396	413	446	481	53.4	38.9	20.5
PAI8f	433	476	537	539	75.9	47.9	16.7

^a The initial decomposition temperature and temperature at which 5 and 10% weight loss was recorded by TGA at a heating rate of 20 °C/min in a nitrogen atmosphere.

^b The temperature of the maximum speed of decomposition at a heating rate of 20 °C/min in a nitrogen atmosphere.

^c Percentage weight of material left undecomposed after TGA analysis at 800 °C in a nitrogen atmosphere.

^d LOI evaluating at char yield at 800 °C.

^e The specific heat of combustion evaluating at LOI.

Interesting relationships have been found between the limiting oxygen index (LOI), CR and heat of combustion as the parameters of combustion process. The amount of CR can be applied as decisive factor for evaluating of the halogen free polymers in agreement with Van Krevelen and Hoftyzer equation: LOI = 17.5 + ‏0.4 CR.[38] For all PAIs the LOI values calculated based on their char yield at 800 °C and were in the range of 38.9–47.9. From this equation, a higher char yield will enhance flame retardancy of polymers and based on the LOI values, these PAIs can be classified as self-extinguishing polymers.

According to Johnson [39] the LOI values of many common materials can be logically well predicted by the expression, provided the atomic C/O ratio is larger than 6: ∆H_comb = 8000/LOI, where ∆H_comb is the specific heat of combustion in kJ g⁻¹. The ∆H_comb values of the PAIs were in the range 16.7–20.5 kJ g⁻¹. It is concluded from Table 6 data that these nano structure PAIs established high thermal stability that could be ascribed to the incorporation of high rigid imide ring, heteroaromatic pyridine moieties and amide linkages.

4. Conclusions

A series of novel nano structure PAIs containing hetroaromatic pyridine ring and different linkages in the main chains were successfully synthesized from a new symmetrical diimide-dicarboxylic acids monomer and various commercial aromatic diamine via direct phosphorylation polycondensation reaction. The synthesized PAIs, with inherent viscosities of η_inh = 0.37–0.51 dL g⁻¹, were obtained in high yields. The structures of monomer and PAIs were confirmed by different techniques. Due to the presence of amide and imide linkages and pyridine groups in main chain of polymers, these macromolecules exhibited interesting properties such as thermal stability, solubility in many polar aprotic organic solvents and capability of forming colorless flexible thin films through solution casting. The thin films of PAIs showed high optical transparency in the UV–vis light region. With XRD, it was found that all of these polymers were amorphous. Morphological studies showed that the synthesized macromolecules have nano structure and according to FE-SEM results, the particle size of PAIs is around 35–90 nm. On the basis of TGA/DTG data, these PAIs are thermally stable and can be classified as self-extinguishing polymers. In sum, these nano structure PAIs displayed excellent solubility, good film forming capability and reasonable thermal stability suitable for thermoforming processing.

Acknowledgements

We wish to express our gratitude to the Payame Noor University of Estahban, for partial financial support.