Introduction

Room temperature ionic liquids (ILs) have attracted extensive interest in recent years as environmentally benign solvents because of their favorable properties, such as reusability, negligible vapor pressure, thermal and chemical stability, non-volatility and non-flammability. In addition, their moderate polarity due to their ionic nature makes them able to dissolve organic, inorganic, organometallic and high-molecular-weight compounds, leading to their application in a variety of reactions as solvents and/or catalysts.1, 2, 3, 4 ILs also attracted the attention of polymer chemists in recent years, and various polycondensation processes were successfully accomplished in ionic media as well.5, 6, 7, 8, 9, 10 Reports have indicated that the reaction of different diamines with diacid chlorides in ILs resulted in the formation of high-molecular-weight polyamides (PAs).5, 6

PAs are high-performance polymeric materials characterized by thermo-oxidative stability, good mechanical properties and outstanding solvent resistance.11, 12, 13, 14 However, these polymers usually exhibit a limited solubility and high melting temperatures, decreasing their processability and restricting further applications. Therefore, researchers have been focused on the development of structurally modified aromatic PAs with increased solubility and better processability.15, 16, 17 One of the approaches that has been successful is the introduction of bulky pendants into the polymer backbone.18, 19, 20 Imidazole and carbazole rings and their derivatives have been incorporated into polymeric frameworks to improve the solubility of the polymers,21, 22, 23, 24, 25, 26 and these groups have also been suggested as promising photoconductive and photorefractive materials.27, 28, 29, 30, 31 Carbazole units can also be easily functionalized with a variety of functional groups at the (3, 6), (2, 7) or N-positions, and then covalently linked into polymeric systems. Although polymers with these functional units have been studied, difficulties in their introduction to high-performance polymers with respect to thermal, solubility and photophysical properties have not been overcome.32, 33, 34

This study reports the synthesis of a series of high-performance PAs based on a new aromatic diamine in ILs. Polar side groups such as CF3 and bulky phenyl groups along the polymer backbones were expected to give rise to restricted segmental mobility, leading overall to a high glass transition temperature (Tg) and an enhancement in solubility. The PAs were also expected to exhibit increased thermal stability because of the higher C–F bond strength. Because of the chromophore-substituted imidazole and carbazole rings in their backbones, these polymers were also expected to possess photophysical properties.

Experimental procedure

Materials

All chemicals were purchased from Fluka and Merck Chemical (Darmstadt, Germany) through a local agency and were used as received without further purification. N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide and Py were purified by distillation under reduced pressure over calcium hydride and stored over 4 Å molecular sieves.

IL synthesis

All room temperature ILs were prepared using procedures reported in the literature.10, 35 Table 1 reports the structure of the ILs prepared here, all of which were highly viscous liquids at room temperature.

Table 1 The influence of IL cation and anion upon yield and inherent viscosity (ηinh) of PAEI1 and PAEI5
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Monomer synthesis

Scheme 1 outlines the synthetic method leading to the targeted diamine.

Synthesis of 4,4′-dihydroxybenzyl ( 1). Into a 100-ml round-bottomed, two-necked flask equipped with a magnetic stir bar and a reflux condenser, 4,4′-dimethoxybenzil (2 g, 0.0075 mmol), aqueous HBr (15 ml, 48%) and glacial acetic acid (15 ml) were placed. The reaction mixture was refluxed for 24 h, and after cooling to room temperature, it was poured into 100 ml of water. Ethyl acetate was added to the mixture to obtain two phases, and the organic phase containing the product was separated and dried over magnesium sulfate for 12 h. The solvent was removed under reduced pressure and the yellow precipitate was washed thoroughly with water and then dried in a vacuum oven at 80 °C. Yield: 94% (1.7 g), and melting point (m.p.): 229–231 °C. Fourier transform infrared spectroscopy (FT-IR) (KBr) at cm−1: 3400 (OH phenol); 3045 (C–H aromatic); 1646 (C=O); 1576 (C=C); and 1223 (C–O). Proton nuclear magnetic resonance spectroscopy (1H-NMR) (400 MHz, dimethyl sulfoxide (DMSO)-d6,): δ 6.91–6.95 (d, 4H, J=8.0 Hz), 7.73–7.77 (d, 4H, J=8.0 Hz) and 10.84 (s, 2H).

Synthesis of 1,2-bis(4-(4-nitro-2-(trifluoromethyl)phenoxy)phenyl)ethane-1,2-dione ( 2). Into a 100-ml round-bottomed, two-necked flask equipped with a magnetic stirrer bar and a reflux condenser, 4,4′-dihydroxybenzyl (1.21 g, 0.005 mol) and 1-chloro-4-nitro-2-(trifluoromethyl) benzene (2.25 g, 0.01 mol) were dissolved in 10 ml N,N-dimethylacetamide, and potassium carbonate (1.38 g, 0.01 mol) was added to the solution. After 30 min of stirring at room temperature, the mixture was heated at 110 °C for 6 h. The mixture was poured into 100 ml water and the yellow precipitate was collected by filtration and dried in a vacuum oven at 80 °C. Yield: 93% (2.9 g), and m.p.: 157–160 °C.

FT-IR (KBr) at cm−1: 3045 (aromatic C–H); 1621 (C=O), 1482 (C=C); 1534, 1353 (NO2); and 1268 (C–O stretching). 1H-NMR (400 MHz, DMSO-d6,): δ 7.41 (d, 4H, Ar-H, J=8.0 Hz), 7.46 (d, 2H, Ar-H, J=8.0 Hz), 8.08 (d, 4H, Ar-H, J=8.0 Hz), 8.54 (dd, 2H, Ar-H, J=8.0 Hz) and 8.57 (d, 2H, Ar-H, J=2.8 Hz).

2-(4,5-bis(4-(4-nitro-2-(trifluoromethyl)phenoxy)phenyl)-1H-imidazol-2-yl)-9-ethyl-carbazole ( 3). Into a 100-ml round-bottomed, two-necked flask equipped with a condenser, a magnetic stir bar and a nitrogen gas inlet tube, a mixture of 9-ethyl-carbazole-2-carbaldehyde (1.11 g, 0.005 mol), Compound 2 (3.1 g, 0.005 mol), ammonium acetate (2.7 g, 0.035 mol) and glacial acetic acid (25 ml) was refluxed for 14 h. Upon cooling, the yellow precipitate was collected by filtration, washed with a mixture of C2H5OH/H2O (50/50 (v v−1)) and dried in a vacuum oven at 80 °C. Yield: 91% (3.75 g), and m.p.: 170–173 °C.

FT-IR (KBr) at cm−1: 3473 (NH, imidazole ring); 3062 (aromatic C–H); 2904 (aliphatic C–H); 1612 (C=N); 1586 (C=C), 1546, 1349 (NO2); 1278 (C–N), and 1237 (C–O–C). 1H-NMR (400 MHz, DMSO-d6,): δ 1.36 (t, 3H, CH3), 4.50 (q, 2H, CH2), 7.19 (d, 2H, Ar-H, J=8.0 Hz), 7.23 (d, 2H, Ar-H, J=8.0 Hz), 7.27 (t, 1H, Ar-H, J=8.0 Hz), 7.36 (d, 2H, Ar-H, J=8.0 Hz), 7.50 (t, 1H, Ar-H, J=8.0 Hz), 7.66 (d, 2H, Ar-H, J=8.0 Hz), 7.73 (d, 2H, Ar-H, J=8.0 Hz), 7.77 (d, 2H, Ar-H, J=8.0 Hz), 8.19 (d, 1H, Ar-H, J=8.0 Hz), 8.24 (dd, 1H, Ar-H, J=8.0 Hz), 8.48 (d, 2H, Ar-H, J=2.8 Hz), 8.55 (dd, 2H, Ar-H, J=8.0 Hz), 8.87 (d, 1H, Ar-H, J=1.6 Hz) and 12.57 (s, 1H, N–H imidazole ring). Anal. Calcd. for C43H27N5O6F6 (823 g mol−1): C, 62.69%; H, 3.28%; and N, 8.50%. Found: C, 62.66%; H, 3.30%; and N, 8.50%.

4,4′-(4,4′-(2-(9-ethyl-carbazol-2-yl)-1H-imidazole-4,5-diyl)bis(4,1-phenylene))bis(oxy) bis(3-(trifluoromethyl)aniline) ( 4). Into a 100-ml round-bottomed, two-necked flask equipped with a magnetic stir bar, a mixture of Compound 3 (4.10 g, 0.005 mol) and Pd/C (0.1 g, 10%) was dispersed in 50 m ethanol. The suspension was heated to reflux, and then 6 ml hydrazine monohydrate was slowly added to the mixture. After a further 8 h of reflux, the solution was filtered while hot to remove Pd/C, and then the filtrate was cooled to precipitate white crystals. The product was collected by filtration and dried in a vacuum oven at 80 °C. Yield: 86% (3.26 g), and m. p.: 155–157 °C.

FT-IR (KBr) at cm−1: 3478, 3373 (NH2); 3462 (N–H imidazole ring); 3051 (C–H aromatic); 2912 (aliphatic C–H); 1628 (C=N); 1597 (C=C); 1286 (C–N); and 1214 (C–O–C). 1H-NMR (400 MHz, DMSO-d6,): δ 1.35 (t, 3H, CH3), 4.47 (q, 2H, CH2), 5.47 (s, 2H, NH2), 5.52 (s, 2H, NH2), 6.82–6.99 (m, 10H, Ar-H), 7.25 (t, 1H, Ar-H), 7.47 (t, 1H, Ar-H), 7.49 (d, 2H, Ar-H, J=8.0 Hz), 7.56 (d, 2H, Ar-H, J=8.0 Hz), 7.64 (d, 1H, Ar-H, J=8.0 Hz), 7.70 (d, 1H, Ar-H, J=8.0 Hz), 8.15 (d, 1H, Ar-H, J=8.0 Hz), 8.21 (dd, 1H, Ar-H, J=8.4 Hz), 8.82 (d, 1H, Ar-H, J=1.6 Hz) and 12.51 (s, 1H, N–H imidazole ring). Carbon-13 nuclear magnetic resonance (13C-NMR) (100 MHz, DMSO-d6, δ): 1 (14.23), 2 (37.56), 3 (109.71), 4 (109.83), 5 (111.16), 6 (115.51), 7 (115.97), 8 (116.36, 117.00, 117.08 and 117.58), 9 (119.09), 10 (119.59), 11 (120.71), 12 (121.68, 121.98, 122.09 and 122.26), 13 (122.70, 125.41, 125.42 and 128.63), 14 (122.73), 15 (123.57), 16 (123.98), 17 (124.55), 18 (126.03), 19 (126.49), 20 (127.04), 21 (128.83), 22 (130.37), 23 (130.46), 24 (131.90), 25 (136.42), 26 (139.92), 27 (140.49), 28 (142.32), 29 (142.96), 30 (146.19, 146.54 and 146.90), 31 (157.47) and 32 (158.49). Distortionless Enhancement by Polarization Transfer (DEPT) technique (100 MHz, DMSO-d6, δ): 1 (14.22), 2 (37.56), 3 (109.71), 4 (109.84), 5 (111.15), 7 (115.97), 8 (116.35, 116.99, 117.08 and 117.58), 9 (119.09), 10 (119.59), 11 (120.71), 15 (123.58), 16 (123.99), 19 (126.49), 21 (128.83), 22 (130.37) and 24 (131.90).

Anal. Calcd. for C43H31N5O2F6 (763 g mol−1): C, 67.62%; H, 4.06%; and N, 9.17%. Found: C, 67.59%; H, 4.10%; and N, 9.16%.

Polymer synthesis

Method I. direct polycondensation using TPP/NMP/Py/LiCl

The following general procedure was used for the preparation of poly(amide-ether-imidazole)s (PAEIs) from the diamine and various aliphatic and aromatic dicarboxylic acids. In a three-necked, round-bottomed flask equipped with a condenser, a mechanical stirrer and a nitrogen gas inlet tube, diamine 4 (1 mmol, 0.763 g), a dicarboxylic acid (1 mmol) and LiCl (0.30 g) were dissolved in a mixture of Py (1 ml), triphenyl phosphite (TPP) (1.20 mmol) and NMP (5 ml). The mixture was heated at 120 °C under a dry N2 atmosphere for 12 h with stirring. The reaction mixture was then cooled to room temperature and the resulting polymers were precipitated in 200 ml methanol. The precipitate was filtered and washed with hot water, and was then further purified by washing with refluxing methanol for 24 h in a Soxhlet apparatus to remove the low-molecular-weight oligomes. The inherent viscosities of the polymers were measured at concentrations of 0.5 g dl−1 in NMP at 25 °C, and were in the range of 0.34–0.62 dl g−1.

Method II. direct polycondensation using TPP/ILs

The following general procedure, as illustrated in Scheme 2, was used for the preparation of PAEIs from the diamine and various aliphatic and aromatic dicarboxylic acids.

Into a 10-ml three-necked, round-bottomed flask fitted with a water-cooled condenser, a mechanical stirrer and a nitrogen gas inlet tube, a mixture of diamine 4 (1 mmol, 0.763 g), terphthalic acid (1 mmol, 0.166 g), 1,3-dipropyl imidazolium bromide ([1,3-(pr)2im]Br) (0.70 g) and TPP (1.29 mmol)) was introduced. The mixture was heated at 110 °C for 2.5 h. As the reaction proceeded, the solution became viscous. The reaction mixture was then cooled to room temperature, and the resulting polymers were precipitated in 100 ml methanol. The precipitate was filtered and washed with hot water, and was then further purified by washing with refluxing methanol for 24 h in a Soxhlet apparatus to remove the low-molecular-weight oligomers. The inherent viscosity (ηinh) of the resulting PAEIs was between 0.34 and 0.62 dl g−1, measured at concentrations of 0.5 g dl−1 in NMP at 25 °C. All other PAEIs were prepared using the above procedure.

PAEI1. Yield=90%, and ηinh (dl g−1)=0.62. FT-IR (KBr) at cm−1: 3432, 3202 (N–H); 3052 (aromatic C–H); 2862 (aliphatic C–H); 1663 (C=O), 1642 (C=N); and 1266 (C–N). 1H-NMR (400 MHz, DMSO-d6,): δ 1.37 (t, 3H, CH3), 4.50 (q, 2H, CH2), 7.08–7.71 (m, 15H, Ar-H), 8.17–8.62 (m, 9H, Ar-H), 8.91 (s, 1H, Ar-H), 10.76 (s, 2H, N–H amide) and 12.63 (s, 1H, N–H, imidazole ring). Anal. Calcd. for C51H33N5O4F6 (893 g mol−1): C, 68.53%; H, 3.69%; and N, 7.83%. Found: C, 68.49%; H, 3.78%; and N, 7.81%.

PAEI2. Yield=87%, and ηinh (dl g−1)=0.51. FT-IR (KBr) at cm−1: 3428, 3198 (N–H); 3046 (aromatic C–H); 2867 (aliphatic C–H); 1665 (C=O), 1642 (C=N); 1264 (C–N). 1H-NMR (400 MHz, DMSO-d6,): δ 1.36 (t, 3H, CH3), 4.48 (q, 2H, CH2), 7.06–7.25 (m, 7H, Ar-H), 7.50–7.64 (m, 8H, Ar-H), 8.16–8.34 (m, 8H, Ar-H), 8.58 (d, 1H, Ar-H), 8.85 (s, 1H, Ar-H), 10.74 (s, 2H, N–H amide) and 12.61 (s, 1H, N–H, imidazole ring). Anal. Calcd. for C51H33N5O4F6 (893 g mol−1): C, 68.53%; H, 3.69%; and N, 7.83%. Found: C, 68.50%; H, 3.73%; and N, 7.82%.

PAEI3. Yield=85%, and ηinh (dl g−1)=0.39. FT-IR (KBr) at cm−1: 3417, 3187 (N–H); 3041 (aromatic C–H); 2861 (aliphatic C–H); 1647 (C=O); 1638 (C=N); 1262 (C–N). 1H-NMR (400 MHz, DMSO-d6,): δ 1.39 (t, 3H, CH3), 4.55 (q, 2H, CH2), 7.12–7.77 (m, 14H, Ar-H), 8.20–8.68 (m, 9H, Ar-H), 8.96 (s, 1H, Ar-H), 10.80 (s, 2H, N–H amide) and 12.69 (s, 1H, N–H, imidazole ring). Anal. Calcd. for C50H32N6O4F6 (894 g−1 mol−1): C, 67.11%; H, 3.57%; and N, 9.39%. Found: C, 67.09%; H, 3.60%; and N, 9.36%.

PAEI4. Yield=91%, and ηinh (dl g−1)=0.53. FTIR (KBr) at cm−1: 3418, 3177 (N–H); 3046 (aromatic C–H); 2879 (aliphatic C–H); 1642 (C=O); 1637 (C=N); and 1262 (C–N). 1H-NMR (400 MHz, DMSO-d6,): δ 1.42 (t, 3H, CH3), 4.56 (q, 2H, CH2), 7.16–7.79 (m, 8H, Ar-H), 7.53–7.66 (m, 8H, Ar-H), 8.18–8.36 (m, 8H, Ar-H), 8.72 (4H, Ar-H), 8.99 (s, 1H, Ar-H), 10.84 (s, 2H, N–H amide) and 12.71 (s, 1H, N–H, imidazole ring). Anal. Calcd. for C57H37N5O6F6 (1001 g−1 mol−1): C, 68.33%; H, 3.69%; and N, 6.99%. Found: C, 68.30%; H, 3.69%; and N, 6.97%.

PAEI5. Yield=92%, and ηinh (dl g−1)=0.37. FTIR (KBr) at cm−1: 3455, 3217 (N–H); 3061 (aromatic C–H); 2845 (aliphatic C–H); 1673 (C=O); 1647 (C=N); and 1267 (C–N). 1H-NMR (400 MHz, DMSO-d6,): δ 1.45 (3H, CH), 1.57 (4H, CH), 2.34 (4H, CH), 4.52 (2H, CH), 6.93–7.39 (m, 7H, Ar-H), 7.55–8.31 (m, 13H, Ar-H), 9.13 (s, 1H, Ar-H), 10.37 (s, 2H, N–H amide) and 11.97 (s, 1H, N–H, imidazole ring). Anal. Calcd. for C49H37N5O4F6 (873 g mol−1): C, 67.35%; H, 4.23%; and N, 8.01%. Found: C, 67.29%; H, 4.34%; and N, 8.00%.

PAEI6. Yield=95%, and ηinh (dl g−1)=0.34. FTIR (KBr) at cm−1: 3455, 3217 (N–H); 3061 (aromatic C–H); 2845 (aliphatic C–H); 1673 (C=O); 1647 (C=N); and 1267 (C–N). 1H-NMR (400 MHz, DMSO-d6,): δ 1.29–1.35 (m, 8H, CH), 1.47 (3H, CH), 1.59 (4H, CH), 2.33 (4H, CH), 4.52 (2H, CH), 6.91–7.31 (m, 7H, Ar-H), 7.55–7.88 (m, 9H, Ar-H), 8.18 (d, 2H, Ar-H), 8.27 (d, 2H, Ar-H), 9.05 (s, 1H, Ar-H), 10.34 (s, 2H, N–H amide) and 11.96 (s, 1H, N–H, imidazole ring). Anal. Calcd. for C53H43N5O4F6 (927 g−1 mol−1): C, 68.60%; H, 4.63%; and N, 7.55%. Found: C, 68.58%; H, 4.68%; and N, 7.52%.

Characterization

1H-NMR and 13C-NMR spectra were recorded on a 400 MHz Bruker Avance DRX (Bruker, Karlsruhe, Germany) instrument using DMSO-d6 as the solvent and tetramethyl silane as the internal standard. Proton resonances are designated as singlet (s), doublet (d) and multiplet (m). FTIR spectra (Bruker) were recorded on KBr pellets using a Bruker Tensor 27 spectrometer (Bruker) over the range of 400–4000 cm−1. Elemental analyses were performed by a CHN-600 Leco elemental analyzer (MI, USA). The melting point (uncorrected) was measured with a Barnstead Electrothermal engineering LTD 9200 apparatus (Southerland, UK). Inherent viscosities (at a concentration of 0.5 g−1 dl−1) were measured with an Ubbelohde suspended-level viscometer (Schott-Gerate, Hofheim, Germany) at 25 °C using NMP as the solvent. Quantitative solubility was determined using 0.05 g of the polymer in 0.5 ml of solvent. Thermogravimetric analysis (TGA) was performed with a DuPont Instruments (TGA 951) analyzer (Wilmington, NC, USA) at a heating rate of 10 °C min−1 under nitrogen atmosphere (20 cm3 min−1) over the temperature range of 30–650 °C. Differential scanning calorimetry (DSC) data were recorded on a Perkin-Elmer Pyris 6 DSC (Groningen, The Netherlands) under nitrogen atmosphere (20 cm3 min−1) at a heating rate of 10 °C min−1. Tg values were taken as the middle of the transition in heat capacity in the second heating scan after cooling from 350 °C at a rate of 20 °C min−1. Ultraviolet–visible and fluorescence emission spectra were recorded on Cecil 5503 (Cecil Instruments, Cambridge, UK) and Perkin-Elmer LS-3B spectrophotometers (Norwalk, CT, USA) (slit width 2 nm), respectively, using a dilute polymer solution (0.20 g dl−1) in DMSO. X-ray powder diffraction patterns were recorded by an X-ray diffractometer (GBC MMA Instrument, Wollongong, NSW, Australia) with a Be-filtered Cu K source (1.5418 Å) operating at 35.4 kV and 28 mA. The 2θ scanning range was set between 4° and 50° at a scan rate of 0.05° s−1.

Results and discussion

Synthesis and characterization of diamine (4)

A new diamine, 4,4′-(4,4′-(2-(9-ethyl-carbazol-2-yl)-1H-imidazole-4,5-diyl)bis(4,1-phenylene))bis(oxy)bis(3-(trifluoromethyl)aniline), was synthesized according to the synthetic route shown in Scheme 1. Compound 1 was synthesized by the hydrolysis of 4,4′-dimethoxybenzil with aqueous HBr and glacial acetic acid. Compound 2 was synthesized by a nucleophilic aromatic substitution reaction of 4, 4′-dihydroxybenzyl with 2-chloro-5-nitrobenzotrifluoride. The reaction between Compound 2- and 9-ethyl-carbazole-2-carbaldehyde with an excess of ammonium acetate, a well known synthetic method for the preparation of an imidazole ring, was used to synthesize Compound 3.36 The catalytic hydrogenation of the dinitro groups in Compound 3 into the corresponding diamine was accomplished using hydrazine hydrate in ethanol in the presence of a catalytic amount of Pd/C.

The purity of all compounds was checked by thin-layer chromatography using an ethylacetate/n-hexane mixture. The structure of diamine 4 was confirmed by elemental analysis, FT-IR and 1H- and 13C-NMR spectroscopy. The FT-IR spectrum of Compound 3 showed absorption bands at 1546 and 1349 cm−1 because of symmetric and asymmetric –NO2 stretching vibrations. After reduction, absorption peaks related to NO2 groups disappeared and new bands were observed at 3478 and 3373 cm−1 because of N–H stretching. The 1H-NMR spectrum of Compound 4 in Figure 1 and the assigned protons reported in the experimental section confirmed that the nitro groups were completely transformed into amine groups by the high field shift of the aromatic protons and by the characteristic resonance of two different protons of two amine groups at 5.47 and 5.52 p.p.m.

Figure 1
figure 1

Proton nuclear magnetic resonance spectroscopy (1H-NMR) spectrum of diamine (4) in dimethyl sulfoxide (DMSO)-d6.

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The 13C-NMR spectrum of diamine 4 in Figure 2 showed 32 different carbon atoms for the aromatic segment and heterocyclic ring. The chemical shift in the upfield region (14.23 and 37.56 p.p.m.) is ascribed to the resonance of the aliphatic methyl group. Also, there are four quartet peaks because of the heteronuclear 13C–19F coupling. The large quartet centered at about 123 p.p.m. is due to the –CF3 carbons. The one-bond C–F coupling constant in this case is about 270 Hz. The CF3-attached carbon also shows a clear quartet centered at about 122 p.p.m. with a smaller coupling constant of about 32 Hz because of two-bond C–F coupling. These quartet signals corroborate the presence of the CF3 group in these compounds. Thus, all the spectroscopic data obtained by 13C-NMR and DEPT techniques were in good agreement with the expected chemical structures.

Figure 2
figure 2

Carbon-13 nuclear magnetic resonance (13C-NMR) spectra of diamine (4) in dimethyl sulfoxide (DMSO)-d6: (a) Distortionless Enhancement by Polarization Transfer (DEPT) and (b) normal.

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Polymer synthesis and characterization

As an extension of studies on the application of ILs in polycondensation, we wish to report a simple and efficient method for the polymerization of a new diamine with several readily accessible dicarboxylic acids in imidazolium-based ILs. The reaction proceeded efficiently with the combination of ILs and the activating agent TPP, enabling PA synthesis by direct polycondensation of dicarboxylic acids and diamines without using NMP, CaCl2 (or LiCl) or Py, which are necessary when using traditional organic solvents. This investigation used room temperature ILs, predominantly those based on substituted imidazolium cations, which can be prepared simply from the commercially available starting materials N-trimethylsilylimidazole and alkyl halides.35, 10 Thus, the different symmetrical 1,3-dialkylimidazolium ILs bearing Br−, PF−4 and BF−6 anions shown in Table 1 were prepared and used as polycondensation agents. These ILs are soluble in polar solvents, such as water and methanol, allowing for the complete isolation of the obtained polymers.

To determine the effect of the nature of ILs on the yield and viscosity of the polymer, the synthesis of aromatic PAEI1 and aliphatic PAEI5 were carried out in different imidazolium-type ILs at a constant reaction temperature. Table 1 compares the yields and viscosities of these two representative polymers in different ILs. The yield of aliphatic PAEI5 is higher, whereas the viscosity is somewhat lower compared with the aromatic PAEI1. IL needs to be a thermally stable liquid near the process temperature and to be a good solvent either for initial monomers or for the resulting polymers. These ILs were all very viscous liquids at room temperature. Therefore, the polycondensation temperature (110 °C) was determined by the viscosity of the ILs and the solubility of the diamine 4. Although the representative PAEI1 and PAEI5 were soluble in all ILs, as the alkyl chain length in the ILs increased, the yield and viscosity decreased. This can be explained by the decrease in the polarity of the IL, resulting in a decreased mobility of polymer chains in ILs with longer alkyl chains. The highest yield and viscosity of the polymers PAEI1 and PAEI5 were obtained when [1,3-(Pr)2im]Br was used as the reaction medium, and this IL was also selected for the synthesis of other PAEIs (Table 1). The optimum polycondensation time was determined for the preparation of PAEI1 in the selected IL and at the selected temperature by measuring the yield and viscosity of the polymer at different times. These results are shown in Figure 3. The best results were achieved in ILs with Br− as the anion. As shown by the data in Table 2, using aliphatic acids as the initial materials resulted in higher polymer yields. All of the PAEIs (1–6) were prepared in two different reaction conditions: direct polycondensation reactions using NMP/Py/LiCl/TPP and reactions with a combination of [1,3-(Pr)2im]Br/TPP. The results in Table 2 demonstrate the beneficial effects of using IL/TPP combinations in the synthesis of PAs, in addition to other useful properties, such as the non-volatility of ILs. Furthermore, removal of some chemicals (for example, NMP, LiCl and Py) that are essential for conventional direct polycondensation and increasing the reaction time (12 vs 2.5 h) increases the cost of polymerization as well as the resulting environmental pollution.

Figure 3
figure 3

Optimal conditions for the synthesis of poly(amide-ether-imidazole) (PAEI)1 in ionic liquid (IL).

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Table 2 Solubility of synthesized PAEI1–6
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The PAEIs were obtained as light brown powders with good yields, 85–95% after extraction with refluxing methanol for 24 h to remove low fraction oligomers, and their inherent viscosities were in the range of 0.34–0.62 dl g−1, indicating intermediate molecular weights. The structures of the polymers were identified by means of elemental analysis, FT-IR and 1H-NMR spectroscopy. The elemental analysis values generally agreed with the calculated values for the proposed polymer structures. FT-IR spectra of these PAEIs showed characteristic absorption peaks at 3300 cm−1 and at 1670 cm−1 related to the N–H and C=O groups of amide linkages. Figure 4 shows a typical 1H-NMR spectrum of the PAEI6 in DMSO-d6; all the peaks have been assigned to the hydrogen atoms of the repeating unit in this polymer. There are two types of N–H protons in the backbones of these polymers, N–H protons of amide groups in the region of 10.34 p.p.m. and the N–H proton of the imidazole ring, which appeared as a single peak at 11.96 p.p.m. The signals related to the aromatic and aliphatic protons in the PAEI6 structure appeared in the ranges of 6.91–9.13 and 1.29–4.56 p.p.m., respectively.

Figure 4
figure 4

Proton nuclear magnetic resonance spectroscopy (1H-NMR) spectrum of poly(amide-ether-imidazole) (PAEI)6 in dimethyl sulfoxide (DMSO)-d6.

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Wide-angle X-ray diffraction and solubility of PAEIs

The crystallinity of these polymers was evaluated by wide-angle X-ray diffraction. Figure 5 shows the X-ray diffractograms of three polymers, PAEI1–PAEI3–PAEI6. All the polymers showed completely amorphous patterns. This observation is reasonable because the presence of bulky pendants and unsymmetrical ether linkages in the polymer chain decreased the intermolecular forces between the chains, leading to a disturbance in the tight structure and indicating that the polymers were non-crystalline.

Figure 5
figure 5

X-ray diffraction patterns of poly(amide-ether-imidazole) (PAEI)1, PAEI3 and PAEI6.

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The solubilities of these PAEIs were determined at a concentration of 5% (wv) in a number of solvents, and the results are reported in Table 2. All these polymers exhibited good solubility in polar aprotic solvents such as NMP, N,N-dimethylacetamide, N,N-dimethylformamide and DMSO, and the aliphatic PAs of PAEI5 and PAEI6 were even soluble in less polar solvents like Py and m-cresol at room temperature. In general, these polymers exhibited better solubility than common aromatic PAs, likely due to the introduction of bulky and polarized (CF3, substituted imidazole and carbazole) groups and unsymmetrical linkages (ether) in the polymer backbones. This can be explained by the presence of bulky side groups such as substituted carbazole rings leading to chain separation, thus decreasing the interchain interactions between amide groups and allowing for better diffusion of solvent molecules into the chains to interact with the polar groups of the polymer backbones. Despite the fact that these polymers contain many bulky backbone groups such as phenyl groups, their solubilities are similar to those of previously reported PAs.37, 38, 39, 40 In addition, the solubility of these polymers vary depending on the dicarboxylic acid used. In general, the aliphatic PAs PAEI5 and PAEI6 dissolved faster and exhibited better solubility in less polar solvents compared with the aromatic polymers (PAEI1–4).

Ultraviolet–visible absorption and fluorescence characteristics

The ultraviolet–visible spectra of dilute NMP solutions of the diamine (2 × 10−5 M) and the corresponding PAEIs (0.2 g dl−1) are shown in Figure 6a. The diamine and the polymers exhibited strong absorption at 300 nm, which was assigned to a π–π* transition resulting from the conjugation of aromatic and heterocyclic rings. The optical properties of these PAs could also be elucidated from the cutoff wavelengths observed in the ultraviolet–visible spectra, listed in Table 3. The cutoff wavelengths of these PAs were up to 370 nm, and the transmittance in the visible region (above 400 nm) was over 100%. The diamine showed strong blue fluorescent light, and when excited at 265 nm in a 2 × 10−5 M NMP solution, its maximum emission (λmax(em)) was observed at 412 nM. The fluorescence spectra of the PAEIs at 0.2 g dl−1 in NMP exhibited emission between 400 and 600 nm, with maxima around 440 and 500 nm for the aliphatic and aromatic PAEIs, respectively.

Figure 6
figure 6

(a) Ultraviolet-visible (UV-vis) absorption and (b) fluorescence emission spectra of diamine (4) and poly(amide-ether-imidazole) (PAEI)1–6.

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Table 3 Optical properties data of PAEI1–6
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To measure the photoluminescence quantum yields (Φf), dilute polymer solutions (0.2 g dl−1) were prepared in NMP. A 0.1 N solution of quinine in H2SO4 (Φf=0.53) was used as a standard.41 The emission spectra of the diamine and PAEIs are shown in Figure 6b. The Φf values were 35% for the diamine, 21–29% for the aliphatic PAEIs and 9–12% for the aromatic PAEIs. The blue shifts and higher fluorescence quantum yields of the aliphatic PAs compared with the aromatic PAs may have been due to the reduced conjugation and the reduced capability of the aliphatic diacids to form charge-transfer complexes with the electron-donating diamine moiety compared with the stronger electron-accepting aromatic diacids.42 Because of their lower charge-transfer capacity, PAEI5 and PAEI6 exhibited a light color and high optical transparency. Intermolecular interactions between the carbazole and triarylimidazole moieties in the polymer backbone were negligible, presumably because both chromophores have similar absorption and emission properties. Polymer chains containing excited species may become entangled, leading to intersystem crossing to some extent, but the concrete reason for this result is unclear.

Thermal properties

DSC and TGA were used to evaluate the thermal properties of the polymers. DSC was performed at a heating rate of 10 °C min−1 in a nitrogen atmosphere, and the resulting curves are shown in Figure 7. Quenching from 350 °C to room temperature yielded mainly amorphous samples without an exothermic peak related to crystallization, thus making the Tgs of these polymers easily observed in the second DSC scans. The second DSC curves again did not display endothermic peaks up to 350 °C because of the amorphous nature of the polymers. Among these polymers, PAEI1 and PAEI4 showed the highest Tg values because of the symmetrical structure of the remaining dicarboxylic acids in the polymer chain and the presence of more ordered segments that inhibited the molecular motion. The methylene units between the amide groups in PAEI5 and PAEI6 facilitated bond rotation and reduced Tg values. As a general rule, the incorporation of bulky and rigid units along a polymer backbone restrict the free rotation of the macromolecular chains and increase Tg values, but the presence of flexible ether linkages reduces the rigidity of the backbone to some extent, leading to a balance between these two opposing properties.

Figure 7
figure 7

Differential scanning calorimetry (DSC) curves of poly(amide-ether-imidazole) (PAEI)1–6 under N2 at a heating rate of 10 °C min−1.

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The thermal stability of these polymers was evaluated by TGA in N2 and O2 atmospheres, and the resulting curves are shown in Figure 8. The TGA data presented in Table 4 show that the temperatures of 10% weight loss were in the range of 421–521 °C and 388–492 °C in N2 and O2 atmospheres, respectively. All of these polymers exhibited good thermal stability with insignificant weight loss up to 388 °C in air. The residual weight retention at 800 °C for the PAEIs ranged from 33–72% in N2.

Figure 8
figure 8

Thermogravimetric analysis (TGA) curves of poly(amide-ether-imidazole) (PAEI)1, PAEI3 and PAEI5 under N2 and O2 at a heating rate of 10 °C min−1.

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Table 4 Thermal properties of PAEI1–6
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Char yield can be used to evaluate the limiting oxygen index (LOI) of the polymers in accordance with the Van Krevelen and Hoftyzer equation:43 LOI=17.5+0.4 CR, where CR=char yield. The LOI values for all the polymers were calculated based on their char yields at 800 °C. According to Table 4, the aromatic PAEIs have better thermal stabilities and higher LOIs as compared with the aliphatic PAEIs. This result is likely related to the rigid structure of aromatic diacids compared with the flexible structure of aliphatic diacids. PAEI4 and PAEI6 have the highest and lowest thermal stabilities, respectively, because of the presence of rigid phenyl and flexible aliphatic units in their backbones. Therefore, these polymers possess excellent thermal stability because of their aromaticity, the strong electron negativity of the F atom and the high polarity of the CF3 group due to the high C–F bond strength.44

Conclusions

This study reports the possibility of using imidazolium-based ILs in a simple and efficient approach for the synthesis of photoactive and organosoluble high-performance PAs. For comparison purposes, a series of PAEIs were prepared from a new diamine and various dicarboxylic acids via direct polycondensation using NMP/TPP/Py/LiCl and a combination of IL/TPP without NMP/Py/LiCl. In general, the results demonstrated the beneficial effects of using IL/TPP media in the synthesis of PAs, such as good yields, moderate viscosity and shorter reaction time, without the need for the removal of chemicals (for example, NMP, LiCl and Py) and without using volatile chemicals. These PAs have different functional groups, such as ether linkages, bulky groups such as CF3 and substituted imidazole and carbazole groups, along their backbones that give rise to restricted segmental mobility, resulting in a high Tg (up to 300 °C) and an enhancement in solubility in many organic solvents. These polymers exhibit high thermal stability (T10 up to 492 °C in O2) because of their aromatic structure and the presence of strong C–F bonds, and these polymers also emit fluorescent light at λe.max=437–484 nm with Φf up to 29%.

scheme 1

Synthesis of the target diamine.

scheme 2

Polycondensation reactions of the diamine with different dicarboxylic acids.