Highly transparent polyimides derived from 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene: preparation, characterization, and optical properties

Abstract

To investigate the structure–property relationships of polyimides containing pyrimidine or pyridine moieties, two diamine monomers, 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene, were designed and synthesized. The diamines reacted with different commercially available aromatic dianhydrides to yield a series of heterocyclic polyimides via a classical two-step polymerization method. The polyimides, PI-1 and PI-6, exhibited excellent solubility in strong polar solvents, such as N,N-dimethylacetamide, N,N-dimethylformamide and N-methyl-2-pyrrolidone. The flexible, strong and transparent polyimide films were formed with UV-visible absorption cut-off wavelengths at 344–399 nm. Glass transition temperatures (T_g) of the polyimides PI-(1–6), derived from PBAPD and BADB with dianhydrides, in the range of 212–260 °C were obtained by DSC, and the temperatures for 5% wt loss of the polyimides in nitrogen and air atmospheres were obtained from TGA in the range of 430–503 °C and 422–465 °C, respectively. Moreover, the polyimide films showed a low moisture absorption of 0.18–1.38% and outstanding mechanical properties with tensile strengths of 81–111 MPa, an elongation at break of 4.3–8.7%, and a tensile modulus of 1.3–2.4 GPa. The coefficients of thermal expansion (CTEs) of the polyimides ranged from 28 to 62 ppm °C⁻¹.

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1 Introduction

Aromatic polyimides, which are considered to be high performance polymer materials, have been applied in many fields, for example microelectronics, optical devices, integrated electronic circuits and so on, and possess fantastic thermal and chemical stability, and mechanical properties.^1–5 However, classical polyimides have some drawbacks, such as high melting and softening temperatures and being insoluble in common organic solvents. These traits make them generally intractable or difficult to process, and ultimately limit their applications.^6,7 Many efforts have been made to improve their processability while maintaining their excellent thermal and mechanical properties. For example, bulky lateral pendants, flexible aryl ether linkage and noncoplanar moieties have been used to enhance the solubility and processability. Unfortunately, bulky lateral pendants, flexible aryl ether linkage and noncoplanar moieties can enhance the solubility, but these groups generally degrade the thermal properties. An effective way to improve the thermal and mechanical properties is by incorporating heterocyclic units, pyridazine,^8–12 pyridine,¹³ pyrimidine^14–16 and so on, into the polyimide backbone since these heterocycles can increase the rigidity of the polyimide.

In pyrimidine or pyridine moieties, the presence of nitrogen atoms, which are similar to the nitro of nitrobenzene, leads to the electron cloud densities being lower than on the benzene ring in the ortho and para positions. If the ortho and para positions have a better leaving group (such as a halogen or nitro), it is very prone to a nucleophilic substitution reaction. Also, the presence of nitrogen atoms with a free electron gives an opportunity for protonation to modify optical properties.

In this study, aimed at interpreting the structure–property relationships of heterocyclic high-performance polyimides, two novel diamines, 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene, were characterized and used to synthesize PIs. Meanwhile, PIs derived from 2,4-bis(4-aminophenoxy)pyrimidine (2,4-BAPD), 4,6-bis(4-aminophenoxy)pyrimidine (4,6-BAPD) and 1,3-bis(4-aminophenoxy)benzene (BAPB) with 6FDA were prepared, and the effect of different units, which were incorporated into the polymers, was studied.

2 Experimental

2.1 Materials

The aromatic bis(ether amine)s, 2,4-bis(4-aminophenoxy)pyrimidine (2,4-BAPD), 4,6-bis(4-aminophenoxy)pyrimidine (4,6-BAPD) and 1,3-bis(4-aminophenoxy)benzene (BAPB), were prepared according to methods used in the literature.^17–19 3,3′,4,4′-Oxydiphthalic anhydride (ODPA) and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) were recrystallized from acetic anhydride before use. 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA), 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) and pyromellitic dianhydride (PMDA) were baked at 130 °C in vacuo overnight prior to use. All commercially available dianhydrides were supplied by Sinopharm Chemical Reagent Beijing Co. Ltd. Potassium carbonate (K₂CO₃) was supplied by Acros and dried in a vacuum at 130 °C for 1 h prior to use. N,N-Dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF) were purified by reduced pressure distillation over calcium hydride and stored over 4 Å molecular sieves before use. 2-Chloro-5-nitropyridine, 4,6-dichloro-2-phenylpyrimidine, m-dihydroxybenzene, p-nitrophenol, 10% palladium on activated charcoal/carbon (Pd/C), and 80% hydrazine monohydrate were purchased from Acros and used without further purification. The other commercially available reagents and solvents were also used without further purification.

2.2 Measurements

NMR was performed on a BRUKER-300 spectrometer with DMSO-d₆ or CDCl₃ as the solvent at 300 MHz for ¹H. HRLC-MS (High Resolution Liquid Chromatograph-Mass Spectrometry) was measured by an Agilent 1290-micrOTOF-QII. Inherent viscosities (η_inh) were measured with an Ubbelohde viscometer at a PAA concentration of 0.5 g dL⁻¹ at 25 °C in DMAc. Weight average molecular weights (M_w) and number-average molecular weights (M_n) were obtained via gel permeation chromatography (GPC) on the basis of polystyrene calibration on a PL-GPC 220 instrument with DMF as an eluent at a flow rate of 1.0 ml min⁻¹. FT-IR (Fourier transform infrared) spectra were recorded on a Bruker Vector22 spectrometer with KBr (potassium bromide) pellets or about 10 μm thick films. Elemental analyses were obtained by a Vario EL cube CHN recorder elemental analysis instrument with the films. Glass-transition temperatures (T_g) of the polyimides were determined on a TA Instruments DSC Q100 in a nitrogen flow of 50 ml min⁻¹ at a heating rate of 10 °C min⁻¹, and determined by the second heating cycle. Thermogravimetric analysis (TGA) was performed on a TA 2050 with a heating rate of 10 °C min⁻¹ in a nitrogen or air atmosphere. Dynamic mechanical analysis (DMA) was carried out with a TA Instruments DMA Q800, and T_g was treated as the peak temperature of loss modulus (E′′). Thermomechanical analysis (TMA) was carried out with a METTLER TOLEDO instrument (TMA/SDTA841) and the coefficient of thermal expansion (CTE) was obtained in the temperature range of 50–150 °C. Tensile properties of the films were measured by a Shimadzu AG-I universal testing apparatus with a crosshead speed of 5 mm min⁻¹ at room temperature. The film specimen sizes were 25–30 mm thick, 3 mm wide and 4 cm long. A Shimadzu UV-vis 2501 spectrometer was used in transmittance mode to obtain the UV-vis (ultraviolet-visible) absorption spectra with the thin films as the samples at room temperature. Water uptakes (WUs) of the films were calculated using the following equation:
where W_wet refers to the weight of film samples after immersion in deionized water at room temperature for 24 h, and W_dry is their initial weight.

2.3 Monomer synthesis

2.3.1 2-Phenyl-4,6-bis(4-nitrophenoxy)pyrimidine. p-Nitrophenol (12.34 g, 88.85 mmol), 4,6-dichloro-2-phenylpyrimidine (8 g, 35.54 mmol) and 60 ml of dried DMF were placed into a 250 ml three-necked round-bottom flask with a mechanical stirrer and a reflux condenser under the protection of nitrogen. After the reactants were completely dissolved, potassium carbonate (12.28 g, 88.85 mmol) was added to the mixture. After stirring at room temperature for 30 min, the mixture was continuously reacted at 120 °C for 8 h, then cooled to room temperature. The obtained mixture was poured into 350 ml of deionized water to give a precipitate, which was collected by filtration, washed with ethanol, and dried in a vacuum at 110 °C overnight. The light yellow solid was recrystallized from DMF/water to give a product yield of 14.07 g (92%). Melting point: 195–197 °C. FT-IR (KBr): 3118, 1622, 1561, 1523, 1492, 1378, 1349, 1251, 1223, 1168, 1127. ¹H NMR: 6.92 (s, 1H), 7.63 (d, 4H), 8.39 (d, 4H), 7.93 (d, 2H), 7.54–7.42 (m, 2H), 7.40 (dd, 1H). Elemental analysis calcd: C, 61.4%; H, 3.28%; N, 13.02%. Found: C, 60.99%; H, 3.42%; N, 13.24%. HRLC-MS (ESI): 430.4 (M + H)⁺, calcd 431.1 for C₂₂H₁₄N₄O₆.

2.3.2 1,3-Bis(5-nitro-2-pyridinoxy)benzene. Under N₂ atmosphere, m-dihydroxybenzene (3.30 g, 30 mmol), 2-chloro-5-nitropyridine (10.47 g, 66 mmol) and 70 ml of dried DMF were placed into a 250 ml three-necked round-bottom flask with a mechanical stirrer and a reflux condenser. After the reactants were completely dissolved, potassium carbonate (9.12 g, 66 mmol) was poured into the mixture. After 20 min of stirring at room temperature, the reaction mixture was continuously reacted at 65 °C for 7 h, then cooled to room temperature. The obtained mixture was poured into 250 ml of deionized water to give a solid precipitate, which was collected by filtration, washed with water, and dried in a vacuum at 80 °C overnight. The yellow solid was purified by recrystallization from DMF/water. The yield of the crude product (BNDB) was 10.1 g (95%). Melting point: 125–127 °C. FT-IR (KBr): 1605, 1572, 1518, 1465, 1388, 1349, 1253, 1163, 1118. ¹H NMR: 7.17 (d, 1H), 7.24–7.18 (m, 2H), 7.32–7.24 (m, 2H), 7.60–7.51 (m, 1H), 8.66–8.58 (m, 2H), 9.06 (dd, 2H). Elemental analysis calcd: C, 54.24%; H, 2.85%; N, 15.81%. Found: C, 54.26%; H, 2.76%; N, 15.91%. HRLC-MS (ESI): 355.1 (M + H)⁺, calcd 354.3 for C₁₆H₁₀N₄O₆.

2.3.3 2-Phenyl-4,6-bis(4-aminophenoxy)pyrimidine. To a 250 ml three-necked flask equipped with a mechanical stirrer, 10.0 g (23.2 mmol) PBNPD, 0.5 g of 10% Pd/C, and 150 ml of dioxane was added, and heated to reflux. Then 7.0 g (112.0 mmol) of hydrazine monohydrate (80%) was added dropwise for 0.5 h. The reaction mixture was heated to reflux for another 16 h, filtered to remove Pd/C, and distilled under reduced pressure to remove excess solvent to obtain the crude product. The crude product was purified by recrystallization from DMF/water. 7.3 g PBAPD was obtained. The yield was 85%. Melting point: 155–156 °C. FT-IR (KBr): 3448, 3385, 3220, 1618, 1586, 1513, 1447, 1383, 1215, 1152. ¹H NMR: 5.13 (s, 4H), 5.99 (d, 1H), 6.69–6.56 (m, 4H), 7.02–6.81 (m, 4H), 7.47–7.42 (m, 1H), 7.50–7.46 (m, 2H), 8.09 (dd, 2H). Elemental analysis calcd: C, 71.34%; H, 4.90%; N, 15.13%. Found: C, 71.44%; H, 4.93%; N, 15.06%. HRLC-MS (ESI): 371.2 (M + H)⁺, calcd 370.4 for C₂₂H₁₈N₄O₂.

2.3.4 1,3-Bis(5-amino-2-pyridinoxy)benzene. Similarly, BADB (yield: 95%) was prepared by the above-mentioned procedures. Melting point: 79–80 °C. FT-IR (KBr): 3371, 3185, 1613, 1578, 1471, 1422, 1274, 1238, 1129. ¹H NMR: 6.52 (s, 1H), 6.69–6.58 (m, 2H), 6.77 (d, 2H), 7.08 (dd, 2H), 7.25 (t, 1H), 7.57 (s, 2H), 5.14 (s, 4H). Elemental analysis calcd: C, 65.30%; H, 4.79%; N, 19.04%. Found: C, 65.24%; H, 4.86%; N, 18.96%. HRLC-MS (ESI) 295.1 (M + H)⁺, calcd 294.3 for C₁₆H₁₄N₄O₂.

2.4 Polymer synthesis

As shown in Scheme 2, the diamines were employed to react with PMDA, BTDA, ODPA, BPADA and 6FDA to prepare a series of PIs (polyimides) via a typical two-step polymerization method. In the case of PI-1, firstly, PAAs were obtained using the following procedure: 0.60 g 6FDA (1.35 mmol) was slowly added to a solution of PBAPD (0.5 g, 1.35 mmol) in 6.5 ml DMAc with a solid content of 15 wt%. Then, the reaction mixture was stirred at room temperature for 12 h under N₂ atmosphere. Finally, thin polyimide films were prepared to cast PAAs (polyamic acids) on glass plates via thermal imidization by elevated temperatures as follows: (40 °C/2 h; 60 °C/2 h; 80 °C/2 h; 100 °C/2 h; 120 °C/2 h; 150 °C/1 h) to remove the solvents in air, followed by the heating imidization procedure (200 °C/0.5 h, 250 °C/1 h, 300 °C/1 h) under vacuum. The freestanding films were obtained by soaking in water to release them from the glass plates. Similarly, PI-2–PI-9 films were obtained by the above-mentioned procedures.

Scheme 1 Preparation of monomers containing pyrimidine or pyridine moieties.

Scheme 2 Synthesis route of the polyimides.

3 Results and discussion

3.1 Monomer synthesis

Two diamine monomers, 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene, were successfully prepared by an aromatic nucleophilic substitution reaction starting from p-nitrophenol and 4,6-dichloro-2-phenylpyrimidine, and m-dihydroxybenzene and 2-chloro-5-nitropyridine, respectively (shown in Scheme 1). Aromatic nucleophilic substitution reaction occurred more easily on pyrimidine or pyridine rings, especially at the ortho and para positions, owing to an electron cloud density far less than that of benzene on the pyrimidine or pyridine rings. The reaction, which was used to synthesise nitrocompounds, was carried out readily in the presence of K₂CO₃. The reduction of the nitro-compounds uses hydrazine hydrate in the presence of a Pd/C catalyst in refluxing dioxane to yield the diamine monomers. The diamine monomers were finally purified by recrystallization in DMF/water. The structures of diamines, PBAPD and BADB, were confirmed by elemental analysis, HRLC-MS, FT-IR spectra, and ¹H NMR spectroscopy. Fig. 1 shows the FT-IR spectra of PBAPD and BADB. The characteristic absorptions of the nitro group disappeared, and the N–H absorption peak of the amino group in the region of 3180–3450 cm⁻¹ was detected. As shown in Fig. 2, the ¹H NMR spectra of the diamine monomers illustrate that the nitro groups in PBNPD and BNDB were completely reduced, and the signal of amino groups appeared at around δ5.0 as a singlet.

Fig. 1 FT-IR spectra of PBAPD and BADB.

Fig. 2 ¹H NMR spectra of PBAPD and BADB.

3.2 Polymer synthesis

The diamine PBAPD was reacted with PMDA, BTDA, ODPA, BPADA and 6FDA, and the diamines, BADB, BABP, 2,4-BAPD and 4,6-BAPD, were reacted with 6FDA, to prepare a series of polyimides (PI-1–PI-9) via a typical two-step polymerization method. PAAs were obtained by reacting equimolar amounts of diamine monomer with aromatic dianhydrides with a solid content of 15 wt%, and stirred for 12 h at room temperature under a N₂ atmosphere. Thin polyimide films were prepared by casting PAAs on glass plates via thermal imidization by elevated temperatures. As shown in Table 2, the inherent viscosities of the PAAs were in the range of 0.59–1.29 dL g⁻¹ measured at a concentration of 0.5 g dL⁻¹ in DMAc at 25 °C. The molecular weights of the PIs determined by GPC in DMF, relative to polystyrene standards, were in the range of 48 309–68 127 for M_w and 31 805–45 723 for M_n, with M_w/M_n values of 1.35–1.61. In the FT-IR spectra of the PIs, shown in Fig. 3, the characteristic imide absorption bands were detected at 1782 cm⁻¹ (asymmetrical C=O stretching), 1726 cm⁻¹ (symmetrical C=O stretching) and 1374 cm⁻¹ (C–N stretching). The N–H absorption peak around 3180–3450 cm⁻¹ disappeared because the PAAs had undergone full imidization. As shown in Table 1, the molecular formulas were also supported by the elemental analyses (C, H, and N) of the PIs, because the found values are in agreement with the calculated ones. Two typical ¹H NMR spectra of the novel polyimides (PI-1 and PI-6) were measured and all the protons in the polyimides can be assigned clearly, as depicted in Fig. 3.

Fig. 3 ¹H NMR spectra of the polyimides (PI-1 and PI-6).

Table 1 Elemental analysis of the polyimides

Polyimides	Formula of repeating unit	Elemental analysis (%)
		C		H		N
		Calcd	Found	Calcd	Found	Calcd	Found
PI-1	C41H20F6N4O6	63.25	62.93	2.59	2.71	7.20	7.23
PI-2	C39H20N4O7	71.34	72.21	3.07	3.38	8.53	8.73
PI-3	C32H16N4O6	69.57	69.35	2.92	3.21	10.14	10.20
PI-4	C53H34N4O8	74.47	74.14	4.01	3.97	6.55	6.57
PI-5	C38H20N4O7	70.81	70.49	3.13	3.59	8.69	8.66
PI-6	C35H16F6N4O6	59.84	59.44	2.30	2.83	7.98	7.97
PI-7	C37H18F6N2O6	63.44	63.10	2.59	2.66	4.00	3.97
PI-8	C35H16F6N4O6	59.84	59.42	2.30	2.54	7.98	8.02
PI-9	C35H16F6N4O6	59.84	60.17	2.30	2.45	7.98	8.13

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3.3 Thermal properties of the polyimides

As shown in Table 2, the thermal properties of the polyimides were detected by DSC, TGA, DMA and TMA. In the DMA curves, T_g was treated as the peak temperature of the loss modulus (E′′). Glass transition temperatures (T_g) of the polyimides (PI-1–PI-6), derived from PBAPD and BADB with dianhydrides, were in the range of 212–260 °C and 220–293 °C obtained by DSC in Fig. 4 and DMA in Fig. 7, respectively. From Fig. 4, PI-3 shows neither an obvious glass transition nor a melting behavior due to rigid-rod PIs derived from PMDA and the high packing density of polymer backbones.²⁰ Generally, the T_g values of polymers are determined by molecular packing density and rigidity of the polymer backbones. PI-4 derived from BPADA possessed the lowest T_g due to its flexible polymer chain structure. Compared to analogous polyimides, PI-1 shows lower T_g values than that of PI-8 and PI-9, which might be attributed to a decreased molecular packing density by incorporating the benzene pendant. The T_g of PI-6 was higher than that of analogous polyimide PI-7, which may be due to the introduction of the rigid pyridine units into the polymer backbone.²¹ The thermal stability properties of the polyimides were obtained by TGA in nitrogen and air at a heating rate of 10 °C min⁻¹, and are summarized in Table 2. The temperatures for 5% wt loss of polyimides (PI-1-PI-6) in nitrogen and air atmospheres were obtained from the TGA curves in the range of 429–503 °C and 422–465 °C, respectively. Similarly, temperatures for 10% wt loss of polyimides in nitrogen and air atmospheres were in the range of 454–529 °C and 450–501 °C, respectively. Meanwhile, char yields at 800 °C were in the range of 40.3–59.9%. As shown in Table 2, Fig. 5 and 6, the polyimides exhibited high thermal stability due to the rigid heterocyclic groups. However, the 5% wt and 10% wt loss temperature in nitrogen and air of PI-1 was similar to that of analogous polyimides (PI-9 and PI-8), and the residue weight % at 800 °C in nitrogen of PI-4 was higher than that of polyimides PI-9 and PI-8. This was due to the incorporation of the benzene pendant. The 5% wt and 10% wt loss temperatures in nitrogen and air of PI-8 and PI-9 were lower than that of polyimide PI-7, and the char yield at 800 °C in nitrogen of PI-8 and PI-9 was lower than that of polyimide PI-7. This was due to the incorporation of a pyrimidine unit in the polymer backbone. From Fig. 5 and 6, polymers, except for PI-6 and PI-7, showed two decomposition stages. The former, before 500 °C, is probably attributed to the thermal degradation of the pyrimidine moiety,²² and the latter was the thermal degradation of the polymer backbone.

Table 2 Physical properties and thermal properties of the PI films

Polyimides	ηinh of PAA^a (dL g^−1)	Mn^b	Mw^b	Mw/Mn^b	Tg (°C)		T5%^f (°C)		T10%^g (°C)		Rw^h (%)	CTE^i (ppm °C^−1)
					DSC^d	DMA^e	N2	Air	N2	Air
a Measured at PAA concentration of 0.5 g dL^−1 in DMAc at 25 °C.b Measured by GPC in DMF, polystyrene was used as a standard.c Not dissolved in DMF.d Tg obtained by DSC for the second time.e Tg measured by DMA at a heating rate of 5 °C min.f T5% (5% weight loss temperatures) measured by TGA in air and nitrogen.g T10% (10% weight loss temperatures) measured by TGA in air and nitrogen.h Char yield at 800 °C.i CTE, coefficients of thermal expansion measured at a heating rate of 10 °C min.j Not obviously observed.k Appropriate concentration in DMF could not be prepared.
PI-1	1.29	45 723	68 127	1.49	260	264	440	422	477	461	54.4	61
PI-2	0.72	—^c	—	—	255	261	430	427	465	464	59.9	39
PI-3	0.59	—	—	—	—^j	293	437	422	469	450	59.7	28
PI-4	0.88	31 805	49 114	1.58	212	220	447	426	468	452	51.9	66
PI-5	0.76	—^k	—	—	236	239	429	427	454	455	57.6	42
PI-6	1.11	40 582	65 337	1.61	245	253	503	465	529	501	40.3	62
PI-7	1.21	42 086	65 654	1.56	244	253	518	495	539	523	56.3	70
PI-8	0.97	35 785	48 309	1.35	264	274	439	439	459	478	49.7	67
PI-9	1.04	38 820	57 065	1.47	267	275	443	439	464	477	49.9	46

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Fig. 4 DSC curves of polyimide films.

Fig. 5 TGA curves of polyimide films in N₂.

Fig. 6 TGA curves of polyimide films in air.

As shown in Fig. 8, TMA curves of the polyimide films were obtained at a heating rate of 10 °C min⁻¹, and the results were listed in Table 2. The coefficients of thermal expansion (CTEs) of the polyimides range from 28 to 62 ppm °C⁻¹. CTEs are influenced by the rigidity and linearity of the polymer chains. PI-3 possessed the lowest CTE value owing to its highest rigidity and linearity of the polymer chain. This is also understandable from no obvious glass transition of PI-3. PI-1, PI-8, PI-9 and PI-6 exhibited higher CTE values than that of PI-7, which is probably attributed to them possessing higher rigidity and linearity by introducing the heterocyclic groups into the polymer chain.

Fig. 7 DMA curves of polyimide films.

Fig. 8 TMA curves of polyimide films.

3.4 Mechanical properties of the polyimides

The tensile properties of the polyimide films were analyzed by a Shimadzu AG-I universal testing apparatus with a crosshead speed of 5 mm min⁻¹ at room temperature, and the sizes of the film specimens were 25–30 mm thick, 3 mm wide and 4 cm long, as summarized in Table 3. The films had tensile strengths of 81–111 MPa, an elongation at break of 4.3–8.7%, and a tensile modulus of 1.3–2.4 GPa. In a comparison of the mechanical properties of PI-6 and PI-9 with that of analogous polyimide PI-7, PI-6 and PI-9 exhibit higher tensile strength and moduli than PI-7, which may be due to the introduction of the rigid heterocyclic rings to the polymer backbone. PI-1 possessed lower tensile moduli than that of PI-9, but PI-1 had a higher tensile strength. This may be attributed to the introduction of the benzene pendant increasing the rigidity of the polymer chain, and simultaneously, reducing intermolecular forces between the polymer chains.²³

Table 3 Mechanical properties and optical properties of PI films

Polyimides	TS^a (MPa)	TM^b (GPa)	EB^c (%)	λcut-off^d (nm)	Transmittance^e (%)	WU^f (%)
a Tensile strength (TS).b Tensile modulus (TM).c Elongation at break (EB).d Cut-off wavelength (λcut-off).e Transmittance at 450 nm.f Water uptake (WU).g 2.6, standard deviation.
PI-1	101 ± 2.6^g	2.4 ± 0.19	4.3 ± 0.2	344	90	0.18
PI-2	102 ± 3.1	2.0 ± 0.09	6.4 ± 0.3	389	43	0.73
PI-3	81 ± 1.7	1.7 ± 0.17	5.8 ± 0.3	388	42	1.21
PI-4	84 ± 2.2	1.3 ± 0.16	8.6 ± 0.4	363	91	0.20
PI-5	98 ± 2.5	1.7 ± 0.04	7.1 ± 0.2	357	93	0.23
PI-6	111 ± 3.2	1.9 ± 0.08	8.7 ± 0.3	334	94	1.38
PI-7	99 ± 2.4	1.6 ± 0.06	9.7 ± 0.4	314	94	0.28
PI-8	108 ± 3.5	1.6 ± 0.16	12 ± 0.1	302	94	0.55
PI-9	103 ± 2.8	1.8 ± 0.16	8.3 ± 0.3	312	93	0.84

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3.5 Solubility and X-ray diffraction of the polyimides

The solubility of the PIs was obtained by dissolving 10 mg of the polymers in 1 ml of solvent at room temperature for 24 h, and the results were listed in Table 4. The PIs, except for PI-2 and PI-3, exhibited good solubility in polar solvents, such as NMP, DMSO, DMAc, DMF and m-cresol, and even in low boiling point solvents, such as THF and CHCl₃ at room temperature due to the introduction of the flexible –O– and –C(CF₃)₂– groups in the polymer backbone. The former increased chain flexibility and the affinity of the polymers,²⁴ and the latter decreased the interaction between polymer chains. In a comparison of the solubility properties of PI-1 with that of analogous polyimide PI-9, PI-1 exhibited better solubility than PI-9, which may be due to the introduction of the benzene pendant into the polymer chain reducing intermolecular forces between the polymer chains. Furthermore, PI-6 shows better solubility than that of PI-7. This may be attributed to the presence of a nitrogen atom in the pyridine unit which produces a polarized bond and increases dipole–dipole interactions in the polymer–solvent system.

Table 4 Solubility behavior of the polyimides^a

Solvents	PI-1	PI-2	PI-3	PI-4	PI-5	PI-6	PI-7	PI-8	PI-9
a The solubility experiment was carried out with 10 mg of PIs in 1 ml of solvent at room temperature for 24 h.b ++, soluble; +−, partially soluble; −−, insoluble.
m-Cresol	+−^b	−−	−−	−−	−−	−−	+−	−−	−−
DMF	++	−−	−−	++	+−	++	++	++	++
DMAc	++	−−	−−	++	+−	++	++	+−	+−
NMP	++	−−	−−	+−	+−	++	++	+−	−−
DMSO	+−	−−	−−	+−	−−	+−	+−	−−	+−
THF	++	−−	−−	+−	−−	++	++	−−	−−
CHCl3	++	−−	−−	++	−−	++	++	++	++
Cyclohexanone	+−	−−	−−	−−	−−	+−	−−	−−	−−
CH3COOH	−−	−−	−−	−−	−−	+−	−−	−−	−−
Pyridine	++	−−	−−	++	−−	++	++	+−	+−

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Wide-angle X-ray diffraction analysis is shown in Fig. 9. The X-ray diffraction curves of the polyimides shows a series of wide diffraction peaks. This indicated the polyimides were amorphous due to the introduction of the flexible ether linkage and the bulky benzene pendant loosening the chain packing of the polymer.

Fig. 9 XRD curves of the polyimide films.

3.6 Optical properties and water uptake of the polyimides

Sample polyimide films with thicknesses of about 30 μm were measured for their optical transparency properties using UV-visible spectroscopy. The UV-visible spectra are given in Fig. 10, and the percentage transmittances at 450 nm and cut-off wavelengths (λ_cut-off) were listed in Table 3. The cut-off wavelengths (λ_cut-off) are in the range of 334–389 nm, and the percentage transmittance at 450 nm is in the range of 42%–94%. The data obtained from these spectra are shown in Fig. 10. Because the trifluoromethyl groups could inhibit the formation of the CTC between polymer chains through steric hindrance and the inductive effect, the PI-1 film shows higher transparency at 450 nm and a lower λ_cut-off than PI-2. However, PI-1 shows lower transparency at 450 nm and higher λ_cut-off than PI-9. This may be attributed to the introduction of the benzene pendant group.

Fig. 10 UV-visible spectra of the polyimides.

The water uptake of the polyimides is in the range of 0.18–1.38%, and listed in Table 3. PI-1 exhibited the lowest moisture absorption (0.18%), and this result may be attributed to the fact that the polyimide contained the water proofing effect of the benzene pendant and trifluoromethyl groups. Meanwhile, because of the water proofing effect of the benzene pendant group, PI-1 had significantly lower moisture absorption than PI-9.

4 Conclusions

For clarifying the structure–property relationships of heterocyclic high-performance polymers, two novel diamines containing pyrimidine or pyridine moieties were designed and characterized, which were employed to react with various commercially available aromatic dianhydrides to yield a series of heterocyclic polyimides via a classical two-step polymerization method. The characterized polyimides were not weakened and retained their fantastic thermal stability and mechanical properties, and showed high solubility, excellent optical transparency and a lower cut-off wavelength, so they are good candidates as components for advanced optical device applications.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19167j

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