Introduction

Polyimides are one of the most important classes of high-performance polymers used in the aerospace, microelectronics, and optoelectronics industries. The use of polyimides (PIs), in particular, can be attributed to their outstanding properties, such as excellent thermal and thermo-oxidative stability, solvent resistance, mechanical, and electrical properties [1–3]. However, despite the excellent combined properties, their widespread applications are often limited by their poor solubility and high processing temperature, which make it impossible for most PIs to be directly processed in their imidized forms. In order to overcome these drawbacks, many efforts on chemical modifications of PIs have been made, such as introduction of flexible linkages [4–6], bulky units [7, 8], bulky pendent substituents [9–11], and noncoplanar moieties [12], in the polymer backbone. Solubility of PIs can also be improved by incorporation of less symmetric structures such as ortho- or meta-linked aromatic units in the main chain [13, 14]. The incorporation of ortho- or meta-linked units into the polymer backbone may have interrupted the chain packing, thus leading to improved solubility.

However, as is well known, introduction of some groups, such as flexible ether and acyclic linkages, to the PI backbones will sacrifice their some advantageous properties, such as thermal stability and mechanical properties. For the PI materials, synthesis of newly heteroaromatic monomers and corresponding PIs that have both good processability and maintained thermal stability would be very interesting, e.g., new kinds of heteroaromatic diamine or dianhydride monomers are always focused and employed in the synthesis of PIs; in other words, newly synthesized aromatic monomers would play very important roles in the synthesis of advanced PI materials [15, 16]. It has been proven that introduction of rigid heteroaromatic pyridine ring to the polymer backbone could endow it with excellent thermal and thermo-oxidative stability, which should be useful to decrease the negative effects resulting from the introduction of flexible ether linkages in the poly(ether-imide)s backbone [17–19].

Pyridine is a heteroaromatic molecule with rigidity and polarizability. Among different heterocyclic rings, the advantage of using a pyridine nucleus is based on its high thermal stability derived from its molecular symmetry and aromaticity [20]. Also, the presence of nitrogen atom in the structure produces a polarized bond which improves the solubility of the prepared polymers due to increased dipole–dipole interactions in the polymer–solvent system. Therefore, the main advantage of using pyridine in the backbone of PIs is to increase their solubility while maintaining their thermal stability. New kinds of heteroaromatic diamine, dianhydride, or other monomers holding pyridine unit have been designed and synthesized, and the novel heteroaromatic polymers with good thermostability and processability have been obtained derived from those monomers containing pyridine nucleus structures at the same time [21].

Among the several methods, modified Chichibabin reaction is one of the best methods for the preparation of a pyridine ring. Tamami and Yeganeh [22, 23] and Liaw et al. [24, 25] have used this method for the preparation of novel pyridine-containing diamines. Related PIs were synthesized from polycondensation of these diamines with some dianhydrides.

Introduction of pyridine ring, incorporation of an ether linkage and meta-linked unit into the polymer backbone, and phenylation of the backbone have been the main structural modifications that have been used to obtain the soluble, thermally stable PIs in this article. The obtained polyimides have better solubility than the analogous polyimides reported by Wang et al. [17].

Experimental

Materials

3-Hydroxyacetophenone (98%) and palladium on activated carbon (Pd/C, 5%) were purchased from Aladdin Reagent Inc. (Shanghai, China), and p-chloronitrobenzene from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), which was used as received. Pyromellitic dianhydride (PMDA or 2a) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), 4,4′-oxydiphthalic dianhydride (ODPA or 2b) and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA or 2d) were purchased from Shanghai Research Institute of Synthetic Resins (Shanghai, China), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA or 2c) was purchased from Alfa Aesar (USA), and these aromatic tetracarboxylic dianhydrides were all recrystallized from acetic anhydride and then dried in vacuo at 150 °C for 12 h before use. N,N-dimethylformamide (DMF) and toluene were purified by distillation under reduced pressure over calcium hydride and were stored over 4-Å molecular sieves. Analytical-grade potassium carbonate was dried in vacuo at 130 °C for 12 h before use. All other solvents were obtained from various commercial sources and used without further purification.

Measurements

Fourier transform infrared (FTIR) spectrometer (Spectrum One, Perkin-Elmer, USA) was used to identify the structure of the monomer and intermediates. Solid samples were coated on KBr disks. The 1H-NMR analysis was conducted on a Varian INOVA-600 spectrometer at 600 MHz. DMSO-d 6 was used as solvent and tetramethylsilane as internal reference. Elemental analysis was carried out on a Vario EL III system. Solubility was determined qualitatively by placing 10 mg of polymer into 1 mL of solvent at room temperature for 24 h. The inherent viscosities were determined at a 0.5% concentration in N,N-dimethylformamide (DMF) with an Ubbelohde capillary viscometer at 25 °C. A crystallographic study of PI was performed at room temperature (about 25 °C) on a D/MAX-IIIC X-ray diffractometer (Akishima-shi, Tokyo, Japan). The X-ray diffraction (XRD) pattern was taken from 2 to 80° (2θ value) with Cu K α radiation (λ = 1.5406 Å, operating at 35 kV and 25 mA). DSC thermograms were obtained on a Perkin-Elmer DSC-7 differential scanning calorimeter; samples were performed under nitrogen atmosphere by raising the temperature from 30 to 400 °C at the rate of 20 °C/min. Thermogravimetric analysis (TGA) of the PI was performed under nitrogen atmosphere at temperatures ranging from 25 to 800 °C at the heating rate of 10 °C/min using a Perkin-Elmer TGA-7 thermogravimetric analyzer. The tensile tests were performed on an electronic universal testing machine (Shenzhen Sans testing machine Co., Ltd., China) at 20 °C with a drawing speed of 10 mm/min. Water uptake (WU) was measured by immersing completely dried PI film samples into deionized water at room temperature for 24 h, the samples were then taken out, wiped with tissue paper, and quickly weighted on a microbalance to calculate WU, using the following equation:

$$ {\text{WU}} = {\frac{{W_{\text{s}} - W_{\text{d}} }}{{W_{\text{d}} }}} \times 100\% $$

where W s and W d are the weights of swollen and dry membrane, respectively. The fluorescence spectra were recorded by Shimadzu Co. Ltd. (Japan) RF-540 fluorospectrophotometer.

Monomer synthesis

Synthesis of 3-(4-nitrophenoxy)acetophenone (m,p-NPAP)

In a 250-mL three-necked round bottom flask equipped with a nitrogen inlet, 6.81 g (0.05 mol) of 3-hydroxyacetophenone and 8.29 g (0.06 mol) of anhydrous potassium carbonate were suspended in a mixture of 50 mL of dry DMF and 20 mL of toluene. The mixture was then refluxed at 140 °C using a Dean–Stark trap to remove small amounts of water azeatropically. After most of the toluene was distilled, 7.88 g (0.05 mol) of p-chloronitrobenzene was added with the mixture being cooled to 60 °C. The mixture was then warmed to 120 °C and kept for 6 h. After the reaction mixture was cooled to room temperature, it was poured into a 300 mL solution of ethanol/water (volume ratio 1/9) to give brown precipitates. Filtrating and washing with water, the product was recrystallized from ethanol to afford 10.10 g of light yellow crystal m,p-NPAP. Yield: 78.5%. Melting point: 104.1 °C (by DSC at a scan rate of 20 °C/min).

IR (KBr, cm−1): 3104, 3071 (C–H), 1686 (C=O), 1515, 1342 (–NO2), 1242 (C–CO–C), 1181 (C–O–C), 851 (C–N). 1H NMR (600 MHz, DMSO-d 6, δ, ppm): 8.28 (d, J = 9.0 Hz, 2H, H1), 7.91–7.89 (m, 1H, H4), 7.69 (dt, J = 1.8 Hz, 8.4 Hz, 2H, H6, H5), 7.50 (d, J = 9.6 Hz, 1H, H3), 7.19 (d, J = 9.0 Hz, 2H, H2), 2.51 (s, 3H, –CH3). Elemental analysis: calculated for C14H11NO4: C, 65.37%; H, 4.31%; N, 5.44%; Found: C, 65.12%; H, 4.38%; N, 5.45%.

Synthesis of 4-phenyl-2,6-bis[3-(4-nitrophenoxy)phenyl]pyridine (m,p-PNPP)

A mixture of 6.43 g (0.025 mol) of m,p-NPAP, 1.33 g (0.0125 mol) of benzaldehyde, 12.53 g (0.1625 mol) of ammonium acetate, and 20 mL of glacial acetic acid were taken in a 100-mL three-necked flask equipped with a magnetic force stirrer and a reflux condenser. The mixture was refluxed with stirring for 16 h. The viscous solid precipitated was filtered off and stirred in 100 mL 50% acetic acid/water for 8 h. Then, the precipitated solid was filtered off and washed thoroughly with ethanol and water. After drying under vacuum at 40 °C, 3.40 g of yellow powder of m,p-PNPP was obtained. Yield: 46.8%. Melting point: 70.2 °C (by DSC at a scan rate of 20 °C/min).

IR (KBr, cm−1): 3105, 3070 (C–H), 1577 (C=C), 1342 (–NO2), 847 (C–N). 1H NMR (600 MHz, DMSO-d 6, δ, ppm): 8.52 (d, J = 8.4 Hz, 4H, H10), 8.34 (s, 2H, H4), 8.11 (d, J = 7.2 Hz, 2H, H8), 8.02–8.01 (m, 4H, H5, H3), 7.95–7.93 (m, 4H, H2, H6), 7.66 (d, J = 2.4 Hz, 4H, H9), 7.62–7.60 (m, 2H, H7), 7.56–7.54 (m, 1H, H1). Elemental analysis: calculated for C35N3O6H23: C, 72.28%; H, 3.98%; N, 7.23%; Found: C, 72.03%; H, 4.13%; N, 7.26%.

Synthesis of 4-phenyl-2,6-bis[3-(4-aminophenoxy)phenyl]pyridine (m,p-PAPP)

To a 250-mL three-necked flask equipped with a dropping funnel and a reflux condenser, 2.91 g (0.005 mol) of m,p-PNPP, 0.2 g of palladium on activated carbon (Pd/C, 5%), and 50 mL of anhydrous ethanol were added, and after heating to refluxing temperature with stirring, 5 mL of hydrazine monohydrate was added dropwise for 1 h. After the completion of the addition of hydrazine monohydrate, the mixture was refluxed for an additional 8 h. Then, the mixture was filtered, and the resultant solid was extracted using enough ethanol. On concentrating all the ethanol solution, yellow precipitation appeared, which was filtered off, and recrystallized from ethanol to get 2.17 g of needle crystal of m,p-PAPP. Yield: 83.2%. Melting point: 79.4 °C (by DSC at a scan rate of 20 °C/min).

IR (KBr, cm−1): 3339, 3211 (N–H), 3029 (C–H), 1578, 1505 (C=C), 1214 (C–O–C). 1H NMR (600 MHz, DMSO-d 6, δ, ppm): 8.14 (s, 2H, H4), 8.03 (d, J = 7.2 Hz, 2H, H5), 7.95–7.88 (m, 4H, H8, H3), 7.56 (t, J = 7.2 Hz, 2H, H2), 7.50–7.44 (m, 3H, H6, H1), 6.93–6.92 (m, 2H, H7), 6.85–6.82 (m, 4H, H9), 6.64–6.61 (m, 4H, H10), 4.95 (s, 4H, –NH2). Elemental analysis: calculated for C35N3O2H27: C, 80.59%; H, 5.22%; N, 8.06%; Found: C, 80.42%; H, 5.26%; N, 8.15%.

Polymer synthesis and film preparation

The general procedure for the preparation of the PIs 4a–d (Scheme 2) was illustrated as follows. A measure of 2.0 mmol of dianhydride was gradually added to a stirred solution of 2.0 mmol of m,p-PAPP in 10 mL of DMF in a 150-mL three-necked flask equipped with a nitrogen inlet. The mixture was stirred at room temperature under nitrogen atmosphere for 24 h, forming a viscous solution of poly(amic acid) (PAA) precursor in DMF. The PAA was subsequently converted into PI by either a thermal or a chemical imidization process. Chemical imidization was carried out by adding 3 mL of a mixture of acetic anhydride/pyridine (volume ratio 2/1) into the PAA solution with stirring at room temperature for 1 h. Then, the mixture was stirred at 60 °C for 18 h to yield a homogeneous PI solution, which was poured slowly into deionized water to give a fibrous precipitate, which was collected by filtration, washed thoroughly with hot methanol, and dried at 80 °C in vacuum overnight. Yields were nearly quantitative for the polymerizations. For the thermal imidization, the PAA was poured into a ceramic trough, which was dried in an oven at 120 °C for 12 h, at 150, 200, 250, and 300 °C for 1 h each temperature. Polyimide films were stripped from the ceramic trough when the trough was cooled.

Results and discussions

Synthesis of diamine monomer

As shown in Scheme 1, the diamine monomer (m,p-PAPP) was obtained through a three-step synthetic route. First, the nucleophilic substitution reaction of 3-hydroxyacetophenone with p-chloronitrobenzene in the presence of potassium carbonate gives the intermediate compound m,p-NPAP. Then, the dinitro compound m,p-PNPP was prepared via a modified Chichibabin reaction, which is one of the best methods among the several ones for the preparation of a pyridine ring, from benzaldehyde and m,p-NPAP. Finally, the novel diamine monomer m,p-PAPP was obtained in a high yield by the reduction of m,p-PNPP using hydrazine monohydrate catalyzed by Pd/C. The new aromatic diamine monomer is stable in atmosphere at room temperature and pure enough for polymerization with commercial aromatic dianhydride monomers to prepare PIs. The detailed characterizations on the diamine monomer m,p-PAPP were done using FTIR (Fig. 1), NMR techniques (Fig. 2), and elemental analyses, which support unambiguously the structure shown in Scheme 1. The analytical data of m,p-PAPP and the intermediate compounds m,p-NPAP and m,p-PNPP are given in the experimental section. The results indicate that the design and synthesis of novel diamine monomer m,p-PAPP should be successful and feasible in this study.

Scheme 1
scheme 1

Synthesis of the diamine monomer m,p-PAPP

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Fig. 1
figure 1

FTIR spectra of m,p-NPAP, m,p-PNPP, and m,p-PAPP

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Fig. 2
figure 2

1H NMR spectra of am,p-NPAP, bm,p-PNPP, and cm,p-PAPP

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Synthesis of polyimides

Polyimides 4a–d were prepared conveniently from aromatic diamine 1 and various aromatic dianhydrides 2a–d (PMDA, ODPA, BTDA and BPADA) as shown in Scheme 2. The new PIs were synthesized using two-step methods, which were carried out via poly(amic acid)s as intermediates. First, the diamine was dissolved in a measured amount of dry DMF, and the equimolar amount of dianhydride monomer was added to the solution slowly. After completing the addition, the solution was then stirred for 24 h at room temperature, forming viscous poly(amic acid) solutions. Thermal or chemical imidization procedures were chosen to form PIs. The merit of the former was convenient for preparation of polyimide films, whereas the latter was suitable for the preparation of soluble PIs. As shown in Table 1, the resulting poly(amic acid)s and PIs all get high yields (95–98%), and the inherent viscosity (η inh) values of the poly(amic acid)s and PIs were in the range of 0.59–0.73 and 0.56–0.68 dL/g, respectively, which indicated the formation of high molar masses. This is consistent with the fact that tough and flexible films could be obtained by solvent evaporation of polymer solutions.

Scheme 2
scheme 2

Synthesis of the pyridine-containing polyimides

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Table 1 Yields and inherent viscosities of poly(amic acid)s and polyimides
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Spectroscopy of polyimides

The formation of PIs was confirmed by FTIR spectroscopy. Figure 3 shows the FTIR spectra of poly(amic acid) 3b, the PI 4b prepared by chemical or thermal imidization method, the latter being based on m,p-PAPP and ODPA. The complete conversion of amic acid to imide ring was proven by the disappearance of absorption bands at 1,714 cm−1 corresponding to C=O stretching of carboxylic acid and 1,660 cm−1 corresponding to C=O amide stretching, together by the appearance of absorption bands at about 1,780 cm−1 (asymmetrical C=O stretching), 1,723 cm−1 (symmetrical C=O stretching), 1,375 cm−1 (C–N stretching), and 725 cm−1 (C=O bending) corresponding to the characteristic of imide bands. Compared with FTIR spectra of PIs, there exist similar absorption bands for the PI obtained either by thermal or by chemical imidization methods. These results indicate that full imidization PI can be obtained using both kinds of imidization methods. The 1H NMR (600 MHz) spectrum of PI showed no amide and acid protons, confirming full imidization. Also, all protons in the backbone of PI can be assigned, as shown in Fig. 4.

Fig. 3
figure 3

FTIR spectra of the poly(amic acid) 3b and polyimide 4b

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Fig. 4
figure 4

1H NMR spectrum of the polyimide 4b

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Polymer solubility

The solubilities of the resulting PIs obtained by chemical imidization were investigated in different organic solvents by dissolving 10 mg of polymers in 1 mL of solvent at room temperature, as shown in Table 2. It can be seen that almost all the PIs exhibited good solubility in common organic solvents, such as m-Cresol, DMF, DMAc, DMSO, NMP, etc. even at room temperature. It can also be seen that the PIs 4b, 4c, and 4d showed good solubility in low-boiling-point solvent THF. The good solubility should be because of the introduction of the flexible ether group and the bulk pendent phenyl group as well as the meta-linked unit into the polymer backbone. The incorporation of pyridine ring in the backbone of PIs also can increase their solubility.

Table 2 Solubility behavior of polyimides in different organic solvents
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X-ray diffraction

Morphological information of the PIs were obtained by X-ray diffraction studies. In most cases, the degree of crystallinity of the PI depended on the imidization methods. In general, the chemical imidization method can yield a material with a higher degree of crystallinity than that obtained from the thermal imidization method. This implies that imidization of the poly(amic acid) in solution may allow it to obtain a more favorable conformation for packing [26]. The X-ray diffraction patterns of the PIs prepared via chemical imidization route are shown in Fig. 5. All the PIs except PI 4d showed amorphous pattern mainly because of the introduction of meta-linked structure into the polymer chain. Polyimide 4d (based on m,p-PAPP and BPADA) displayed a little of the crystalline pattern. In general, amorphous polymers have a lower softening temperature and an improved solubility with respect to their crystalline analogs, thus they had good solubility (confirmed by solubility tests in Table 2) and could be solution-cast into flexible and tough films.

Fig. 5
figure 5

X-ray diffraction patterns of the polyimides

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Thermal properties

The thermal properties of the resulting PIs were investigated by DSC and TGA methods. The T g values of the PIs were 204.5–237.4 °C (as shown in Fig. 6). The T g values of the obtained PIs except 4d (based on m,p-PAPP and BPADA) were higher than that of commercial PI, Ultem 1000 (T g, 217 °C), based on BPADA (bisphenol-A diphthalic anhydride) and MPD (m-phenylene diamine). However, the T g values of the obtained PIs were lower than that of Kapon film derived from PMDA–ODA (T g, 390 °C). The increasing order of T g generally correlated with the structure of the dianhydride component. As expected, the PI 4d derived from dianhydride BPADA showed the lowest T g value due to the increase in flexibility of polymer chain determined by the ether linkage between the phenyl rings, whereas the PI 4a obtained from dianhydride PMDA had the highest T g value because of the rigid phenyl unit.

Fig. 6
figure 6

DSC curves of the pyridine-containing polyimides

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The thermal stabilities of the obtained PIs were evaluated by TGA. The TGA curves for PIs prepared by the thermal imidization method are shown in Fig. 7. The PI films did not exhibit obvious weight loss before the scanning temperature reached up to 500 °C in nitrogen atmosphere, indicating that no thermal decomposition occurred. As shown in Table 3, the decomposition-starting temperatures of the resulting PI were in the range of 501.6–525.5 °C, and the temperatures at 5% and 10% weight loss were 479.7–534.0 and 527.7–552.0 °C in nitrogen atmosphere, respectively. In addition, the residual weight at 800 °C for the resulting PIs were 61.6–65.0% in nitrogen atmosphere, implying that all the PIs possess excellent thermal stability. The good thermal stability should be the because of the introduction of pyridine ring into the polymer backbone.

Fig. 7
figure 7

TGA curves of the polyimide films in nitrogen atmosphere

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Table 3 Thermal properties of the polyimide films in nitrogen atmosphere
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Mechanical properties and water uptake

High-quality PI films could be prepared by casting the PAA solution in a ceramic trough followed by the thermal curing as per the following procedure: 120°C for 12 h, at 150, 200, 250, and 300°C for 1 h each temperature in air. Table 4 summarizes the tensile strength and modulus of the resulting PIs. The PI films possess tensile strength of 88.6–90.4 MPa, tensile modulus of 1.04–1.56 GPa, and elongation at break of 7.2–8.7%, which indicate that they were strong and tough polymeric materials. Meanwhile, the PI films exhibited low water uptake of 0.89–0.98%.

Table 4 Mechanical properties and water uptake of the polyimide films
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Optical properties

Figure 8 shows the emission spectra of protonated PI 4b with HCl. Calculated amount of HCl solution was added into the THF solution of the PI sample, and the solution was kept overnight before the fluorescence measurement. There was little fluorescence around 448 nm observed in neutral polymer solution excited at 360 nm. However, the stronger fluorescence about 439–464 nm was observed after protonation with different HCl concentrations. Compared to a benzene ring, pyridine is an electron-deficient aromatic heterocycle, with a localized lone pair of electrons in sp2 orbital on the nitrogen atom; consequently, the derived polymers have greater electron affinity and better electron-transporting properties, and offer the possibility of protonation or alkylation of the lone pair electrons as a way of modifying their properties [9, 19, 25, 27, 28].

Fig. 8
figure 8

Fluorescence emission spectra of polyimide 4b in THF solution as a functional of HCl (excited at 360 nm) concentrations with a polymer concentration of 1 mg/10 mL

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Conclusions

A new pyridine-containing aromatic diamine monomer, 4-phenyl-2,6-bis[3-(4-aminophenoxy)phenyl]pyridine (m,p-PAPP), was successfully synthesized and characterized, which was employed in polycondensation with various aromatic dianhydrides, to prepare a series of pyridine-containing PIs. Experimental results indicate that the resulting diamine monomer m,p-PAPP has good polymerizability, and therefore, strong and tough PI films can be obtained. The novel PIs obtained have fairly high T g values, excellent thermal stability in nitrogen, as well as good solubility not only in polar aprotic solvents but also in some low-boiling-point solvents such as THF. Meanwhile, the films also possess low water uptake of less than 1%. The protonated polymer possessed fluorescence (439–464 nm) in THF solution after being protonated with acids excited at 360 nm.