Introduction

Aromatic polyimides are characterized by high glass transition (Tg) values along with high decomposition temperatures and advantageous mechanical properties. They are used in various fields, including the aerospace and electric industries.1 Therefore, the relationships between the structures and properties of polyimides have been investigated extensively, and the factors that influence various properties of polyimides have been clarified as follows.2, 3, 4, 5, 6 The high Tg is due to the large enthalpy change and small entropy change at the Tg.2 The thermomechanical properties are influenced by conformation;2 processability and solubility are influenced by free volume;3, 4 and the dielectric properties are influenced by the polarization of imide groups and free volume.5, 6, 7 Because these factors are greatly dependent on the polyimide structure, the relationships between structures and properties of polyimides have been examined more systematically.8, 9, 10, 11, 12, 13, 14

As shown in Scheme 1, we have synthesized aromatic polyimides from p-quaterphenyltetracarboxylic dianhydride (m=2),15 p-quinquephenyltetracarboxylic dianhydride (m=3)16 and p-sexiphenyltetracarboxylic dianhydride (2) (m=4).17 The properties of these polyimides were compared with those of polyimides from 3,3′′,4,4′′-p-terphenyltetracarboxylic dianhydride (m=1) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA; m=0) and discussed on the basis of the number (m) of phenylene units.18, 19, 20, 21, 22, 23 Tg values were independent of m, while storage moduli were higher and the decrement at the Tg decreased with increasing m. The Tg values of the polyimides (PI2x) from 2 were not so high, and the polyimides PI2x were insoluble in organic solvents even upon heating.

In this study, polyimides PI1x were synthesized from a Z-type sexiphenyltetracarboxylic dianhydride 1, an isomer of 2, and their properties were compared with those of polyimides PI2x prepared from 2. Linkages along the polymer backbone, such as p- and m-linkages, are known to influence the polymer properties,4 and the effects of such linkages in the dianhydride moiety on polyimide properties have been examined in polyimides from BPDAs.24, 25, 26, 27 There are three types of isomers in BPDA: 3,3′,4,4′-BPDA (s), 2,3,3′,4′-BPDA (a), and 2,2′,3,3′-BPDA (i). The Tg values of the polyimides from these dianhydrides increased in the order of s-polyimide<a-polyimide<i-polyimide, and the order of solubility in N-methyl-2-pyrrolidone (NMP) was s-polyimide<a-polyimide<i-polyimide. These results could be explained in terms of the restriction of the internal rotation around the biphenyl linkage and the free volumes in the polyimides. As the difference in linearity from 1 and 2 is much larger than that among the three types of BPDAs, the properties of polyimides PI1x are expected to be considerably different from those of the polyimides PI2x.

Experimental Procedure

Materials

2-Methoxyphenylboronic acid (3), tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), 4,4′-diaminodiphenyl ether (a), 4,4′-diaminodiphenylmethane (b), m-phenylenediamine (c) and p-phenylenediamine (d) were purchased from Wako Pure Chemical Industries (Osaka, Japan). 4,4′-Bis(4-aminophenoxy)biphenyl (e) and 3,4′-diaminodiphenyl ether (f) were purchased from Tokyo Kasei Kogyo (Tokyo, Japan). The diamines were purified by sublimation under reduced pressure. Diethyl 4-bromophthalate (4)15 and bis-(trimethylene)-4,4′-biphenylene diboronate (5)28 were prepared as previously reported. NMP was purified by vacuum distillation over calcium hydride.

Monomer synthesis

Diethyl 4-(2-methoxyphenyl)phthalate (6)

The following were added to a flask equipped with a reflux condenser: 25.0 g (83 mmol) of 3, 12.8 g (83 mmol) of 4, 12.4 g (90 mmol) of potassium carbonate and 150 ml of deoxygenated toluene. The system was flushed with nitrogen, and 0.5 g of Pd(PPh3)4 was added to the reaction mixture. After the mixture was stirred at 90 °C for 8 h, the salts were removed by filtration, and the solvent was evaporated. The residue was distilled under reduced pressure to yield pure liquid 6. Yield: 19.9 g (73%). BP: 233–240 °C (1 Torr).

The infrared (IR) spectrum (NaCl) exhibited absorption bands at 3040–2970 (C-H) and 1720 cm−1 (C=O).

The 1H nuclear magnetic resonance (NMR) spectrum (δ in CDCl3) showed signals at 1.35 (t, 3H, J=7.0 Hz, -CH2CH3), 1.37 (t, 3H, J=7.0 Hz, -CH2CH3), 3.82 (s, 3H, -OCH3), 4.35 (q, 2H, J=7.0 Hz, -CH2CH3), 4.37 (q, 2H, J=7.0 Hz, -CH2CH3), 6.96–7.01 (m, 1H, H7), 7.02–7.07 (m, 1H, H5), 7.30–7.33 (m, 1H, H6), 7.33–7.39 (m, 1H, H4), 7.67–7.70 (dd, 1H, J=8.1 Hz, J=2.2 Hz, H3), 7.78 (d, 1H, J=8.1 Hz, H2) and 7.85 (d, 1H, J=2.2 Hz, H1).

The 13C NMR spectrum (δ in CDCl3) exhibited peaks at 14.1, 55.4, 61.6, 61.9, 115.9, 125.4, 126.8, 128.6, 128.7, 129.4, 129.7, 130.0, 131.7, 133.8, 145.8, 160.1, 167.3 and 168.2 p.p.m.

Analysis calculated for C19H20O5: C, 69.50%; H, 6.14%. Found: C, 69.42%; H, 6.20%.

4-(2-Hydroxyphenyl)phthalic acid (7)

To synthesize 4-(2-hydroxyphenyl)phthalic acid (7), 19.7 g (60 mmol) of 6 was heated together with 70 g of pyridine hydrochloride for 30 min at reflux temperature (260 °C). After the reaction mixture became homogeneous, it was poured into water (1000 ml). The precipitated solid was collected by filtration, washed thoroughly with water and dried under vacuum. The crude product was used in the next reaction without further purification. Yield: 13.9 g (90%).

The IR spectrum (KBr) exhibited absorption bands at 3400–3000 (OH) and 1700 cm−1 (C=O).

The 1H NMR spectrum (δ in DMSO-d6) showed signals at 6.85–6.90 (m, 1H, H7), 6.94–7.00 (m, 1H, H5), 7.19–7.26 (m, 1H, H6), 7.26–7.36 (m, 1H, H4), 7.69–7.78 (m, 2H, H2 and H3) and 7.81 (d, J=2.2, H1).

The 13C NMR spectrum (δ in DMSO-d6) exhibited peaks at 114.1, 125.6, 126.2, 128.2, 128.7, 129.8, 130.2, 130.8, 131.6, 132.9, 144.6, 156.9, 167.4 and 167.6 p.p.m.

Diethyl 4-(2-hydroxyphenyl)phthalate (8)

To synthesize diethyl 4-(2-hydroxyphenyl)phthalate (8), 13.7 g (53 mmol) of 7, 250 ml of absolute ethanol and 0.2 ml of sulfuric acid were added to a flask, and the mixture was heated at reflux for 60 h. After the mixture was cooled, it was washed twice with 100 ml of 10 wt% sodium carbonate solution and extracted three times with 100 ml of dichloromethane. The combined extract was dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified by silica gel column chromatography eluted by dichloromethane. Yield: 12.2 g (73%). Mp: 70–71 °C.

The IR spectrum (KBr) exhibited absorption bands at 3400–3000 (OH), 3020–2970 (C-H) and 1730 cm−1 (C=O).

The 1H NMR spectrum (δ in CDCl3) showed signals at 1.35 (t, 3 H, J=7.0 Hz, -CH2CH3), 1.37 (t, 3H, J=7.0 Hz, -CH2CH3), 4.34 (q, 2H, J=7.0 Hz, -CH2CH3), 4.37 (q, 2H, J=7.0 Hz, -CH2CH3), 5.60 (s, 1H, OH), 6.94–6.97 (m, 1H, H7), 6.95-7.01 (m, 1H, H5), 7.22-7.27 (m, 1H, H6), 7.22-7.30 (m, 1H, H4), 7.68-7.72 (dd, 1H, J=8.1 Hz, J=2.2 Hz, H3), 7.83 (d, 1H, J=8.1 Hz, H2) and 7.86 (d, 1H, J=2.2 Hz, H1).

The 13C NMR spectrum (δ in CDCl3) exhibited peaks at 14.1, 61.6, 61.9, 115.9, 124.9, 126.5, 128.5, 128.7, 129.4, 129.8, 130.5, 131.4, 134.5, 144.8, 156.4, 167.3 and 168.3 p.p.m.

Analysis calculated for C18H18O5: C, 68.78%; H, 5.77%. Found: C, 68.51%; H, 5.92%.

3′,4′-Di(ethoxycarbonyl)biphenyl-2-triflate (9)

A solution of 12.7 g (45 mmol) of trifluoromethanesulfonyl anhydride in 70 ml of dichloromethane was added dropwise to a solution of 11.3 g (36 mmol) of 8 and 10 ml of pyridine in 150 ml of dichloromethane at 0 °C. After stirring at 20 °C for 2 h, the mixture was poured into ice water. The organic layer was collected, and the aqueous layer was extracted twice with 50 ml of dichloromethane. The combined extract was dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified by silica gel column chromatography eluted by dichloromethane. Yield: 14.3 g (89%). Mp: 66–67 °C.

The IR spectrum (KBr) exhibited absorption bands at 3400–3000 (OH), 3020–2970 (C-H), 1730 (C=O), 1420 (S=O) and 1215 cm−1 (C-F).

The 1H NMR spectrum (δ in CDCl3) showed signals at 1.35 (t, 3H, J=7.0 Hz, -CH2CH3), 1.37 (t, 3H, J=7.0 Hz, -CH2CH3), 4.34 (q, 2H, J=7.0 Hz, -CH2CH3), 4.37 (q, 2H, J=7.0 Hz, -CH2CH3), 7.40–7.46 (m, 1H, H7), 7.46–7.51 (m, 3H, H4, H5 and H6), 7.62–7.67 (dd, 1H, J=8.1 Hz, J=2.2 Hz, H3), 7.82 (d, 1H, J=2.2 Hz, H1) and 7.83 (d, 1H, J=8.1 Hz, H2).

The 13C NMR spectrum (δ in CDCl3) exhibited peaks at 14.2, 61.8, 61.9, 118.8 (q, J =320 Hz), 122.0, 127.5, 128.2, 129.1, 129.3, 129.7, 131.3, 133.4, 134.1, 139.7, 142.1, 149.6, 167.0 and 167.5 p.p.m.

Analysis calculated for C19H17O7F3S: C, 51.12%; H, 3.84%. Found: C, 50.85%; H, 3.93%.

Tetraethyl 3,3′′′′′,4,4′′′′′-1,1′:2′,1′′:4′′,1′′′:4′′′,1′′′′:2′′′′,1′′′′′-sexiphenyltetracarboxylate (10)

In total, 13.6 g (30.5 mmol) of 9, 4.83 g (15 mmol) of 5, 4.55 g (33 mmol) of potassium carbonate and 150 ml of deoxygenated N,N-dimethylformamide were added to a flask equipped with a reflux condenser. The system was flushed with nitrogen, and 0.5 g of Pd(PPh3)4 was added to the reaction mixture. After the mixture was stirred at 90 °C for 8 h, the salts were removed by filtration, and the solvent was evaporated. The residue was purified by silica gel column chromatography eluted by dichloromethane. Yield: 6.27 g (56%). Mp: 182–183 °C.

The IR spectrum (KBr) exhibited absorption bands at 3020–2970 (C-H) and 1720 cm−1 (C=O).

The 1H NMR spectrum (δ in CDCl3) showed signals at 1.35 (t, 6H, J=7.0 Hz, -CH2CH3), 1.37 (t, 6H, J=7.0 Hz, -CH2CH3), 4.35 (q, 4H, J=7.0 Hz, -CH2CH3), 4.37 (q, 4H, J=7.0 Hz, -CH2CH3), 7.18 (d, 4H, J=8.8 Hz, H8), 7.20-7.25 (m, 2H, H5), 7.42-7.52 (m, 12H, H2, H4, H6, H7 and H9), 7.52-7.57 (dd, 2H, J=8.1 Hz, J=2.2 Hz, H3) and 7.62 (d, 2H, J=2.2 Hz, H1).

The 13C NMR spectrum (δ in CDCl3) exhibited peaks at 14.2, 61.7, 61.8, 126.8, 127.2, 127.4, 127.6, 127.7, 127.8, 129.2, 129.8, 130.4, 132.1, 133.5, 138.2, 139.4, 139.9, 140.8, 143.7, 167.2 and 168.0 p.p.m.

Analysis calculated for C48H42O8: C, 77.19%; H, 5.67%. Found: C, 76.95%; H, 5.81%.

3,3′′′′′,4,4′′′′′-1,1′:2′,1′′:4′′,1′′′:4′′′,1′′′′:2′′′′,1′′′′′-Sexiphenyltetracarboxylic acid (11)

At 190 °C, 5.97 g (8 mmol) of 10 was dissolved in 350 ml of 2-(2-methoxyethoxy)ethanol. A solution of 1.92 g (48 mmol) of sodium hydroxide in 20 ml of water was added, and the mixture was heated at this temperature for 2 h to yield a white precipitate. The reaction mixture was concentrated under reduced pressure, and a solution of 1.92 g (48 mmol) of sodium hydroxide in 400 ml of water was added. The system was then heated at reflux temperature for 2 h. After being cooled to room temperature, the solution was acidified by 6 M hydrochloric acid. The precipitated 11 was collected and dried under vacuum. The crude product was used in the next reaction without further purification. Yield: 4.77 g (94%).

The IR spectrum (KBr) exhibited absorption bands at 3400–3000 (OH) and 1705 cm−1 (C=O).

The 1H NMR spectrum (δ in DMSO-d6) showed signals at 7.20 (d, 4H, J=8.8 Hz, H8), 7.26–7.31 (m, 2H, H5), 7.46–7.56 (m, 12H, H3, H4, H6, H7 and H9), 7.65 (d, 2H, J=2.2 Hz, H1) and 7.70 (d, 2H, J=8.1 Hz, H2).

The 13C NMR spectrum (δ in DMSO-d6) exhibited peaks at 125.7, 126.2, 126.5, 126.7, 126.8, 127.0, 127.8, 130.5, 130.9, 132.5, 133.7, 137.2, 138.1, 139.3, 141.6, 146.8, 167.4 and 167.8 p.p.m.

3,3′′′′′,4,4′′′′′-1,1′:2′,1′′:4′′,1′′′:4′′′,1′′′′:2′′′′,1′′′′′-Sexiphenyltetracarboxylic dianhydride (1)

At reflux temperature (260 °C), 4.76 g (7.5 mmol) of 11 was heated together with 200 ml of diphenyl ether for about 1 h. The reaction mixture became homogeneous and was cooled to 150 °C. After the diphenyl ether was evaporated under reduced pressure, the residue was twice recrystallized from toluene to yield pure 1. Yield: 3.52 g (78%). Mp: 188–189 °C.

The IR spectrum (KBr) exhibited absorption bands at 1850 (C=O) and 1780 cm−1 (C=O).

The 1H NMR spectrum (δ in CDCl3) showed signals at 7.14–7.17 (d, 4 H, J=8.8 Hz, H8), 7.42–7.58 (m, 12 H, H4, H5, H6, H7 and H9), 7.60–7.64 (dd, 1 H, J=8.1 Hz, J=2.2 Hz, H3), 7.81 (d, 1 H, J=8.1 Hz, H2) and 7.88 (d, 1 H, J=2.2 Hz, H1).

The 13C NMR spectrum (δ in CDCl3) exhibited peaks at 122.5, 125.8, 126.6, 126.8, 127.0, 127.4, 127.9, 129.4, 131.4, 132.1, 134.0, 136.7, 138.2, 139.7, 141.7, 147.2, 167.2 and 168.0 p.p.m.

Analysis calculated for C40H22O6: C, 80.26%; H, 3.70%. Found: C, 80.03%; H, 3.90%.

Polymerization using a two-step procedure

Before polymerization, dianhydride 1 was treated in vacuo at 270 °C for 30 min. In a three-necked flask, 0.598 g (1.0 mmol) of 1 was added to a solution of 0.2 g (1.0 mmol) of 4,4′-oxydianiline (a) in 8.0 ml of NMP in one portion. The mixture was stirred at room temperature for 24 h under nitrogen. The NMP solution was poured into 300 ml of methanol. The precipitated polymer was filtered, washed with methanol and dried under vacuum. The inherent viscosity of polyamic acid PAA1a was 0.17 dl g−1 in NMP at 30 °C. The thermal cyclodehydration of the polyamic acid was performed by successive heating at 100 °C for 1 h, 200 °C for 1 h and finally at 300 °C for 1 h under vacuum. Yield: 0.72 g (94%).

The IR spectrum (KBr) exhibited absorption bands at 1780 (C=O), 1720 (C=O) and 1360 cm−1 (C-N).

Analysis calculated for C52H30N2O5: C, 81.88%; H, 3.96%; N, 3.67%. Found: C, 81.42%; H, 4.15%; N, 3.52%.

Polymerization by one-step procedure

Before polymerization, dianhydride 1 was treated in vacuo at 270 °C for 30 min. In a flask equipped with a condenser, 0.598 g (1.0 mmol) of 1, 0.2 g (1.0 mmol) of a and 3 ml of dried toluene as an azeotropic agent were added to 10 ml of m-cresol. The mixture was stirred at 130 °C for 30 min and at 190 °C for 6 h under a nitrogen atmosphere. The resulting viscous solution was poured into 300 ml of methanol. The precipitated polymer was filtered, washed with methanol and dried under vacuum. The inherent viscosity of polyimide PI1a was 1.15 dl g−1 in NMP at 30 °C. All the polyimide powders were dissolved in 12 ml of NMP, and the solution was cast onto a glass plate. After the solvent was removed at 80 °C, the residue was dried under reduced pressure at 200 °C for 6 h to yield a tough film.

The IR spectrum (KBr) exhibited absorption bands at 1780 (C=O), 1720 (C=O) and 1360 cm−1 (C-N).

Analysis calculated for C52H30N2O5: C, 81.88%; H, 3.96%; N, 3.67%. Found: C, 81.57%; H, 4.14%; N, 3.49%.

Measurements

1H and 13C NMR spectra were recorded on a JNM-GSX400 FT-NMR spectrometer (JEOL, Tokyo, Japan) and IR spectra were recorded on a Shimadzu spectrophotometer IR 435 (Shimadzu Corporation, Kyoto, Japan). X-ray diffraction was measured with a Rigaku RAD-B system (Rigaku Corporation, Tokyo, Japan), using Cu Kα radiation at room temperature in the range of 2θ=5–60° with a sampling step of 0.1° and a scan rate of 10° min−1. For differential scanning calorimetry (DSC) and thermogravimetry, a Shimadzu DSC-60 (Shimadzu Corporation) and Rigaku thermal analysis station TG 8110 (Rigaku Corporation) were used, respectively, and measurements were made at a heating rate of 10 °C min−1 in air or nitrogen. Dynamic mechanical analysis (DMA) was performed with the Advanced Rheometric Expansion System (TA Instruments—Waters LLC, New Castle, DE, USA) at 1.0 Hz at a heating rate of 5 °C min−1 from room temperature to 500 °C. Mechanical properties were measured with a Shimadzu Autograph precision universal tester (Shimadzu Corporation) at a strain rate of 10% min−1. Densities of polyimide films were measured using a density-gradient tube prepared from carbon tetrachloride and heptane. Five specimens (7 × 7 mm2) per polyimide film were measured, and the maximum and minimum values are reported in Table 4. The refractive indices of the polyimide films were measured at C-line (656 nm) on an Abbe refractometer (Atago NAR 4T, Atago Corporation, Tokyo, Japan). Transmission spectra of polyimide films (ca. 40 m) were measured with a Shimadzu UV–vis-NIR (Shimadzu Corporation) scanning spectrophotometer.

Results and discussion

Monomer synthesis

3,3′′′′′,4,4′′′′′-1,1′:2′,1′′:4′′,1′′′:4′′′,1′′′′:2′′′′,1′′′′′-Sexiphenyltetracarboxylic dianhydride (1) was synthesized with reactions involving the extension of the unit using 2-methoxyphenylboronic acid (3), as previously reported (Scheme 2).17 Cross-coupling of 3 and 4 using Pd(PPh3)4 as a catalyst yielded 6, which was treated with pyridine hydrochloride to yield 7 and then converted to diethyl ester 8. Afterwards, 8 was converted to triflate 9 using trifluoromethanesulfonyl anhydride, and cross-coupling of 9 with diboronate 5 yielded tetraethyl sexiphenyltetracarboxylate 10. Hydrolysis of 10 afforded tetracarboxylic acid 11, which was converted to tetracarboxylic dianhydride 1 by heating in diphenyl ether.

The chemical structures were confirmed by IR and NMR spectroscopy and elemental analysis.

Compound 1 exhibited IR absorption at 1850 and 1780 cm−1, which is characteristic of a carboxylic anhydride. The 13C NMR spectrum of 1 exhibited 18 peaks due to symmetry, indicating the formation of the desired tetracarboxylic dianhydride. Elemental analyses of all compounds were in good agreement with the calculated results for the proposed structures.

Polymer synthesis

Aromatic polyimides PI1a–PI1f were synthesized by a conventional two-step procedure that included ring-opening polyaddition of aromatic diamines a–f to 1 and thermal cyclodehydration, as shown in Scheme 3. The ring-opening polyadditions were carried out in NMP at room temperature, leading to the formation of polyamic acids with inherent viscosities of 0.11–0.17 dl g −1 in NMP. Although the polymerizations proceeded in homogeneous solution, the viscosities of the polyamic acids were rather low compared with those of polyimides PI2x from 2. Thermal conversion of the polyamic acids to polyimides was then performed by heating the precursor polymers in powder form successively at 100 °C for 1 h, 200 °C for 1 h and 300 °C for 1 h under vacuum. The polyimides PI1x were soluble in organic solvents, and the inherent viscosities of the polyimides formed were in the range of 0.14–0.49 dl g−1 in NMP (Table 1), which is higher than those of the polyamic acids. Polymerization likely proceeded during imidation. Polymerizations were also examined by a one-step procedure. The polymerizations were carried out in m-cresol at 130–190 °C, leading to the formation of polyimides. The water formed during the reaction was removed as an azeotrope to promote polymerization. The reactions proceeded homogeneously in m-cresol, and no precipitation occurred when the hot solutions were cooled to room temperature. The inherent viscosities were in the range of 0.23–1.21 dl g−1 (Table 2) and those of PI1a, PI1b, PI1d and PI1e were much higher than those of the polyimides produced by the two-step procedure. A similarly higher viscosity of the polyimides produced by the one-step procedure than by the two-step procedure was reported in the preparation of polyimides from 3,6-diphenylpyromellitic dianhydride.29 Based on molecular modeling, the authors reported some steric hindrance from the phenyl groups, restricting the formation of the tetrahedral intermediate necessary for addition of the amine. In the present polymerization, the lower viscosities of the polyimides by the two-step procedure may be due to steric hindrance of the 1,4-phenylene group. This influence was thought to be smaller in the one-step procedure, because the polymerization was carried out at a higher temperature (190 °C). Polyimides with high viscosities could not be prepared from diamines c and f. The reason for this failure is still unclear but may be partially attributed to the trend of forming cyclic oligomers in the polymerization of diamines c and f with a narrow V-shaped angle between the two amino groups. The polyimide PI1a, PI1b, PI1d and PI1e films were obtained by casting the NMP solutions onto a glass plate and removing the solvent. The obtained films were dried at 200 °C for 6 h under vacuum. The formation of polyimides was confirmed by the appearance of IR absorption bands at approximately 1780 and 1720 (C=O) and 1360 cm−1 (C-N), which is characteristic of an imide group. The values from elemental analysis were in close agreement with the calculated values, further supporting polyimide formation.

Table 1 Synthesis of polyamic acids and conversion to polyimides
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Table 2 Synthesis of polyimides by one-step procedure
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The polyimide films, PI1a, PI1b, PI1d and PI1e, were transparent and yellow in color. The transparencies of the PI1x films were compared with those of the PI2x films. The transmission spectra of the PI1a and PI2a films are shown in Figure 1, and the cutoff wavelength is defined here as the point at which the transmittance decreases below 10% in the spectrum. The cutoff wavelength of the PI1a film was 423 nm, which is shorter than that (438 nm) of the PI2a film, and the PI1a film exhibited higher transmittance in the 400–450 nm region. The cutoff wavelengths of PI1b, PI1d and PI1e films were 421, 430 and 425 nm, respectively, while those of the PI2b and PI2e films were 440 and 442 nm, respectively. The cutoff wavelengths of the PI1x films were shorter than those of PI2x, indicating that the λmax values of the PI1x films were smaller than those of the PI2x films. The calculated λmax values for the optimized model compounds (Figure 2) of PI1x and PI2x were 355 and 373 nm, respectively.

Figure 1
figure 1

Comparison of the transmission spectra of polyimide films of PI1a and PI2a.

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Figure 2
figure 2

Optimized structures of model compounds corresponding to PI1x and PI2x. A full color version of this figure is available at Polymer Journal online.

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Polymer properties

The crystallinity of the polyimides was estimated by wide-angle X-ray diffraction. All polymers were amorphous.

The thermal properties of the polyimides were evaluated by DSC, thermogravimetry and DMA. The Tg and 10% weight loss temperatures (T10) are summarized in Table 3. The Tg and T10 of the rigid polyimides PI2x17 are also shown in Table 3 for comparison. The T10 values of PI1c and PI1f were lower due to lower inherent viscosities. None of the polyimides PI1a, PI1b, PI1d and PI1e lost weight below 480 °C in air or nitrogen. The T10 values in air and nitrogen of PI1a, PI1b, PI1d and PI1e were in the ranges of 575–590 and 635–640 °C, respectively. All PI1x, except PI1c and PI1f, had thermal stabilities similar to the corresponding polyimides PI2x.

Table 3 Comparison of thermal properties of polyimides PI1x with those for PI2x
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The polyimides PI1x showed Tg values without melting in the DSC thermograms, indicating amorphous morphology. PI1x had a Tg between 270 and 345 °C (Table 3), and the values were higher than those of the corresponding PI2x. In particular, the Tg value of PI1d was 65 °C higher than that of the corresponding PI2d. The higher Tg can be explained by the difficulty of internal rotation around the biphenyl linkages and the conformational change during rotational motion, which were both reported as causes of the higher Tg values of a-BPDA- and i-BPDA-based polyimides compared with s-BPDA-based polyimides.24, 25, 26, 27 Figure 2 shows the optimized structures of bis(N-phenylimide)s of 1 and 2, which are model compounds of PI1x and PI2x, respectively. The dihedral angles between the aromatic rings around the C1-C1′, C2′-C1′′ and C4′′-C1′′′ bonds in the model compound of PI1x are denoted as ω11, ω12 and ω13, respectively, and those around the C1-C1′, C4′-C1′′ and C4′′-C1′′′ bonds in the model compound of PI2x are denoted as ω21, ω22 and ω23, respectively. Density functional theory calculations in the gas phase were carried out as implemented in the Gaussian09W package of quantum chemical programs. The optimized ω11, ω12 and ω13 were 50, 51 and 35°, respectively, and the optimized ω21, ω22 and ω23 were all 36°. The potential energies were estimated as a function of ω, and the difference between the maximum and minimum values (the optimized structure) was defined as the energy barrier for rotation of the aromatic rings around the C-C bond. The values for ω11, ω12 and ω13 in the model compound of PI1x were 26, 27 and 2.2 kcal mol−1, respectively, and those for ω21, ω22 and ω23 in the model compound of PI2x were 2.2, 2.2 and 2.1 kcal mol−1, respectively. The energy barriers for rotation in the model compound of PI1x were much higher than those in the model compound of PI2x due to the approach of the hydrogen atom in the 6-position to the hydrogen atom in the 2′′-position. Thus, internal rotations around the biphenyl linkages in polyimide PI1x were suggested to be more restricted. Based on the optimized structures of the model compounds, a larger conformational change was also suggested to be required in the internal rotation around flexible connecting groups, such as -O- and -CH2-, in PI1x.

DMA of PI1x was performed, and Tg values were observed as tan δ peak temperatures (Table 3). The Tg values of PI1x were higher than those of the corresponding PI2x, which was in agreement with the result determined by DSC. Polyimides PI1c and PI1f were not obtained as films. Figure 3a compares the DMAs for PI1a and PI2a, and Figure 3b compares the DMAs for PI1d and the polyimide from 3,3′′′′,4,4′′′′-p-quinquephenyltetracarboxylic dianhydride (QPDA)16 and d. Because PI2d was not obtained as a film,17 the polyimide from QPDA and d was used as a rigid polyimide for comparison. The Tg value of the polyimide from QPDA and d was almost equal to that of PI2d, and polyimides from QPDA showed dynamic mechanical behaviors similar to those of PI2x, exhibiting a plateau region above 300 °C; the decrement in the storage modulus E′ at the Tg was small. In the temperature dependence of storage modulus, a high modulus glassy region was followed by a decrease of modulus at the Tg. The decrease temperatures of PI1a and PI1d were higher than those of the corresponding rigid polyimides, and the decreases of PI1a and PI1d at the Tg s were larger. The same results were observed in DMA of PI1b and PI1e. Rotation around the biphenyl linkages and flexible connecting groups in the polyimides was intense above Tg and resulted in conformational changes. The larger decreases of E′ for the Z-type polyimides were possibly due to the larger conformational change of PI1x, which is expected from the optimized structures of the model compounds. The E′ value below Tg of PI1d was much lower than that of the polyimide from QPDA and d. The higher E′ of the polyimide was possibly due to the crystallization of the rigid quinquephenylene unit.

Figure 3
figure 3

Comparison of the dynamic mechanical behaviors of polyimides PI1x and those of PI2x. (a) PI1a and PI2a and (b) PI1d and polyimide from QPDA and d.

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The tensile properties of the PI1x films were evaluated from the stress–strain curves; the typical stress–strain curves of PI1x and PI2x films are shown in Figure 4. The stress–strain curve of PI1d was compared with that of the polyimide film from QPDA and d. The tensile values, tensile strength/elongation at break/tensile modulus of the PI2a film, were approximately 180 MPa/25%/3.2 GPa, while those of the PI1a film were 144 MPa/33%/2.9 GPa. The tensile strength and tensile modulus of PI1x were lower than those of PI2x, and the elongation at break was larger.

Figure 4
figure 4

Comparison of the stress–strain curves for polyimides PI1x and those for PI2x.

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Table 4 compares the qualitative solubilities and densities for polyimides PI1x and corresponding polyimides PI2x. Solubility tests were conducted by stirring the polyimide films or powders in solvents at 30 °C for 12 h at a solid content of approximately 3 wt%. Insoluble polyimides were heated at approximately their boiling temperature for a few minutes, followed by cooling to room temperature. All polyimides PI1x except PI1b and PI1d were soluble in polar solvents such as NMP, N,N-dimethylacetamide, N,N-dimethylformamide and m-cresol at room temperature, and PI1b and PI1d were soluble in polar solvents upon heating, in contrast to polyimides PI2x,17 which were insoluble even upon heating. The Z-type structure had a significant influence on the solubility of the polyimides from sexiphenyltetracarboxylic dianhydride. The higher solubility is presumably due to the free volume. The densities of PI1x films, except PI1d, were 1.27–1.31 g cm−2, lower than those of the PI2x films, and the free volume fraction (the ratio of free volume to the sum of free volume and the van der Waals volumes of the polyimide) was suggested to be larger.

Table 4 Comparison of solubilities and densities of polyimides PI1x with those for PI2x
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The refractive indices (n) of polyimide films PI1a, PI1b, PI1d and PI1e were measured, and the dielectric constants (ɛcal) were estimated using an empirical relationship, ɛcal=1.1 × n2.7, 30 The ɛcal were compared with those of the corresponding PI2x. The values of PI1c and PI1f could not be measured, as they were not obtained as films. The values of ɛcal for PI1a, PI1b, PI1d and PI1e were 2.74, 2.73, 2.78 and 2.73, respectively, lower than those of PI2x. The values of PI2a, PI2b and PI2e were 2.90, 2.88 and 2.87, respectively. The free volume may also have an effect on the dielectric constant. The dielectric constant of polyimide from pyromellitic dianhydride and 4,4′-diaminodiphenyl ether was reported to be 3.22 (10 GHz), while that of the polyimide from pyromellitic dianhydride and 3,3′-diaminodiphenyl ether was reported to be 2.84. The lower dielectric constant was likely due to the larger free volume.7

Conclusion

We have examined the synthesis of new polyimides PI1x from a Z-type tetracarboxylic dianhydride, 3,3′′′′′,4,4′′′′′-1,1′:2′,1′′:4′′,1′′′:4′′′,1′′′′:2′′′′,1′′′′′-sexiphenyltetracarboxylic dianhydride (1) and compared their properties with those of polyimides PI2x that were previously prepared from rigid 3,3′′′′′, 4,4′′′′′-p-sexiphenyltetracarboxylic dianhydride (2). Polyimides PI1x produced by a one-step procedure had higher inherent viscosities than those produced by a two-step procedure. The Tg temperatures of PI1x were higher than those of polyimides PI2x, and the decrease in the storage modulus at the Tg was larger than that of PI2x. Polyimides PI1x showed higher solubilities and lower dielectric constants (ɛ) than PI2x, and the Z-type sexiphenylene affected the polyimide properties.

scheme 1

Preparation of polyimides with various numbers of 1,4-phenylene units.

scheme 2

Synthesis of tetracarboxylic dianhydride 1.

scheme 3

Synthesis of polyimides PI1x and structure of polyimides PI2x.