Synthesis and characterization of novel polyimides derived from pyridine-bridged aromatic dianhydride and various diamines

European Polymer Journal 42 (2006) 1229–1239

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Synthesis and characterization of novel polyimides derived from pyridine-bridged aromatic dianhydride and various diamines Xiaolong Wang, Yanfeng Li *, Shujiang Zhang, Tao Ma, Yu Shao, Xin Zhao State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Institute of Biochemical Engineering and Environmental Technology, Lanzhou University, TianShui Road South, 222, Lanzhou, Gansu 730000, China Received 3 November 2005; received in revised form 14 December 2005; accepted 14 December 2005 Available online 6 March 2006

Abstract A new kind of pyridine-bridged aromatic dianhydride monomer, 4-phenyl-2,6-bis[4-(3,4-dicarboxyphenoxy)phenyl]pyridine dianhydride (PPDA), was successfully synthesized by modified Chichibabin reaction of benzaldehyde and substituted acetophenone, 4-(3,4-dicyanophenoxy)-acetophenone (DCAP), followed by acidic hydrolysis of the intermediate tetranitrile and cyclodehydration of the resulting tetraacid. The pyridine-bridged aromatic dianhydride was employed to synthesized a series of new pyridine-containing polyimides by polycondensation with various aromatic diamines in N-methyl-2-pyrrolidone (NMP) via the conventional two-step method, i.e. ring-opening polycondensation forming the poly(amic acid)s and further thermal or chemical imidization forming polyimides. The inherent viscosities of the resulting polyimides were in the range of 0.49–0.63 dL/g, and most of them were soluble in aprotic amide solvents and cresols, such as N,N-dimethylacetamide (DMAc), NMP, and m-cresol, etc. Meanwhile, strong and flexible polyimide films were obtained, which have good thermal stability with the glass transition temperatures (Tg) of 223–256 C, the temperature at 5% weight loss of 523–569 C, and the residue at 700 C of 52.1–62.7% in nitrogen, as well as have outstanding mechanical properties with the tensile strengths of 70.7–97.6 MPa and elongations at breakage of 7.9–9.7%. Wide-angle X-ray diffraction measurements revealed that these polyimides were predominantly amorphous.  2006 Elsevier Ltd. All rights reserved. Keywords: Pyridine-containing polyimide; Monomer synthesis; Solubility

1. Introduction Aromatic polyimides have been widely employed in the aerospace, microelectronics, optoelectronics, * Corresponding author. Tel.: +86 931 8912528; fax: +86 931 8912113. E-mail address: [email protected] (Y. Li).

composites and so on, because of their excellent balance of thermal and mechanical properties [1,2]. However, their applications have been limited in some fields because aromatic polyimides are normally insoluble in common organic solvents and have extremely high glass transition or melting temperatures. It is well known that the chemical composition and chain structure of aromatic polyimides

0014-3057/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.12.012

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were responsible for their prominent properties and responsible for their poor processibility, in other words, they lead to be infusible within processing temperature and insoluble in organic solvents. Therefore, one of the targets of polyimides chemistry is to incorporate new functionalities to make polyimides more tractable processing without decreasing their many desirable properties, such as excellent thermal stability and good mechanical resistance, etc. [3,4]. For this purpose, many efforts on chemical modifications of polyimides have been done, such as introduction of flexible linkages in their backbone, or incorporation of bulky side groups, which result in good solubility and processibility of the polyimides [5–10]. Introducing heteroaromatic rings into the main chain of a synthetic polymer would impart it certain properties expected, in general, the rigidity and polarizability held in pyridine with aromaticity should be suitable to prepare some heteroaromatic polymers. As a result, the novel heteroaromatic polymers with good thermostability and processability have been obtained based on new kinds of heteroaromatic diamine, dianhydride or other monomers holding pyridine nucleus structures [11–17]. Considering the rigidity based on symmetry and aromaticity of pyridine ring would have contributions for the thermal stability, chemical stability, retention of mechanical property of the resulting polymer at elevated temperature, as well as polarizability resulting from nitrogen atom in pyridine ring could be suitable to improve their solubility in organic solvents [18,19], especially, the polyimides with good thermostability and processability have been prepared by polycondensation of pyridine-containing diamine monomers with aromatic dianhydride monomers [20,21]. In this paper, a new kind of pyridine-containing dianhydride monomer, 4-phenyl-2,6-bis[4-(3,4dicarboxyphenoxy)phenyl]-pyridine dianhydride, had been designed and synthesized as a potentially convenient condensation monomer to form pyridine-containing polyimides, and a new series of pyridine-containing polyimides were synthesized derived from the resulting pyridine-bridged aromatic dianhydride monomer, and characterization of the resulting pyridine-containing dianhydride monomer and polyimides was described by FT-IR, 1 H NMR, 13C NMR, elemental analysis, DSC, TGA, wide-angle X-ray diffraction and Ubbelohde viscosimeter methods.

2. Experimental 2.1. Materials 4-Hydroxyacetophenone (TCI) and 4-nitrophthalonitrile (TCI) were used as received. Most reagent-grade aromatic diamines were commercially available and they were purified just before polycondensation reaction. Bis(4-aminophenyl) ether (ODA), 1,3-bis(4-aminophenoxy) benzene (MAPB), and 1,4-bis(4-aminophenoxy) benzene (PAPB) were recrystallized from ethanol. 1,4-Bis(2-trifluoromethyl-4-aminophenoxy) benzene (PFAPB) was prepared according to the method reported by Xie et al. [22]. N-methylpyrrolidone (NMP), m-cresol, N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF) were distilled under reduced ˚ pressure over calcium hydride and stored over 4 A molecular sieves. Reagent-grade potassium carbonate was dried in vacuo at 120 C for 12 h before use. All other solvents were obtained from various commercial sources and used without further purification. 2.2. Measurements 1

H NMR and 13C NMR spectra were measured on a JEOL EX-300 spectrometer using tetramethylsilane as the internal reference. Fourier transform infrared (FT-IR) spectra of monomer and intermediates were obtained on a Micolet NEXUS 670 spectrometer (KBr disks). Elemental analyses were determined by a Perkin–Elmer model 2400 CHN analyzer. Differential scanning calorimetry (DSC) analyses were performed on a Perkin–Elmer differential scanning calorimeter DSC 7 at a heating rate of 20 C/min under flowing nitrogen. Tg was taken as the midpoint of the inflection observed on the curve of heat capacity versus temperature. Melting points were measured by DSC method. Thermogravimetric analysis (TGA) was conducted with a TA Instruments TGA 2050, and experiments were carried out on approximately 10 mg of samples under controlled flux of nitrogen at 20 C/min. Wide-angle X-ray diffraction measurements were performed at 25 C on a Siemens Kristalloflex D5000 X-ray diffractometer, using nickel-filtered ˚ , operating at Cu Ka, radiation (k = 1.5418 A 40 kV and 30 mA). The mechanical properties were measured on an Instron 1122 testing instrument with 100 · 5 mm specimens in accordance with GB 1040-79 at a drawing rate of 50 mm/min. The values

X. Wang et al. / European Polymer Journal 42 (2006) 1229–1239

of inherent viscosity (ginh) were determined by an Ubbelohde viscosimeter at 30 C using DMAc as a solvent, and polymer solution was given a concentration of 0.5 g/dL. Solubility was determined qualitatively by placing 10 mg of polymer into 1 mL of solvent at room temperature for 24 h, or heating them until dissolution of samples. 2.3. Monomer synthesis 2.3.1. 4-(3,4-Dicyanophenoxy)-acetophenone (DCAP) In a 250-mL three-necked round bottom flask, 13.62 g (0.1 mol) of 4-hydroxyacetophenone and 29.02 g (0.21 mol) of anhydrous potassium carbonate were suspended in a mixture of 100 mL of dry DMF and 40 mL of toluene. The mixture was then refluxed at 140 C using a Dean–Stark trap to remove small amount of water azeatropically. After most of the toluene was distilled, 17.30 g (0.1 mol) of 4-nitrophthalonitrile was added when the mixture was cooled to 60 C. The mixture was then allowed to warm to 90 C and kept for 5 h. After the reaction mixture was cooled to room temperature, it was poured into 600 mL of ice/water to give brown precipitates. Filtrating and washing with water, the product was recrystallized from ethanol to afford 14.81 g of yellow crystals DCAP, the yield is 56.5%, melting point is 161 C. FT-IR spectrum exhibited absorption peaks at 2232 cm 1 (C„N stretching), 1669 cm 1 (C@O stretching). 1H NMR (300 MHz, DMSO-d6) indicates resonance signals resulting in different protons: d = 8.13–8.17 (d, J = 8.7 Hz, 1H), 8.03–8.06 (d, J = 9.0 Hz, 2H), 7.91 (s, 1H), 7.50–7.53 (d, J = 8.7 Hz, 1H), 7.26–7.29 (d, J = 8.7 Hz, 2H), 2.48 (s, 3H). 13C NMR (300 MHz, DMSO-d6): d (ppm) 197.4, 160.4, 158.8, 137.1, 134.5, 131.7, 124.6, 124.1, 120.3, 117.6, 116.2, 110.0, 27.4. Elemental analysis: calcd. for C16H10N2O2 (262.26): C 73.27, H 3.84, N 10.68, while found C 73.20, H 3.87, N 10.57. 2.3.2. 4-Phenyl-2,6-bis[4-(3,4-dicyanophenoxy) phenyl]-pyridine (PCNP) 13.11 g (0.05 mol) of DCAP, 2.65 g (0.025 mol) of benzaldehyde, 48.13 g (0.625 mol) of ammonium acetate, and 70 mL of glacial acetic acid were placed into a 250-mL three-necked flask equipped with a mechanical stirrer and a reflux condenser. The mixture was refluxed with stirring for 24 h. Then the solid precipitated was filtered off and washed thor-

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oughly with glacial acetic acid and water, respectively. After dried under vacuum at 60 C, 5.62 g of white powder 4-phenyl-2,6-bis[4-(3,4-dicyano phenoxy)phenyl]-pyridine (PCNP) was obtained, the yield is 38.0%, melting point is 273 C. FT-IR spectrum exhibited absorption peaks at 2230 cm 1 (C„N stretching), 1246 cm 1 (C–O–C stretching). 1H NMR (300 MHz, DMSO-d6) indicates resonance signals resulting in different protons: d = 8.44–8.47 (d, J = 8.7 Hz, 4H), 8.24 (s, 2H), 8.11–8.14 (d, J = 8.1 Hz, 2H), 8.04–8.07 (d, J = 7.2 Hz, 2H), 7.87 (s, 2H), 7.53–7.59 (t, J = 7.2 Hz, 2H), 7.49–7.51 (t, J = 7.5 Hz, 1H), 7.46– 7.49 (d, J = 8.7 Hz, 2H), 7.33–7.36 (d, J = 8.7 Hz, 4H). 13C NMR (300 MHz, DMSO-d6): d (ppm) 161.25, 156.04, 155.30, 150.24, 138.00, 136.80, 136.62, 129.85, 129.52, 127.83, 123.39, 122.83, 120.89, 117.24, 116.99, 116.32, 115.82, 108.94. Elemental analysis: calcd. for C39H21N5O2 (591.62): C 79.18, H 3.58, N 11.84, while found C 79.14, H 3.59, N 11.85. 2.3.3. 4-Phenyl-2,6-bis[4-(3,4-dicarboxyphenoxy) phenyl]-pyridine (PBCP) In a 250-mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser, 5.91 g (0.01 mol) of PCNP and 130 mL of 85% phosphoric acid were refluxed for 2.5 h. After cooling to room temperature, the mixture was poured into 600 mL of cold, dilute aqueous potassium hydroxide. The solution was stirred at room temperature for 6 h after modulating the pH value to 11, and then it was neutralized with 6M hydrochloric acid to pH 3–4, followed by stirring for 6 h. The product was collected by filtration and dried at room temperature under vacuum. After recrystallization from 50% of aqueous acetic acid using activated charcoal, 5.54 g of yellow powder was obtained, the yield is 83.0%, melting point is 168 C. FT-IR spectrum exhibited absorption peaks at 2500–3600 cm 1 (C(O)O–H stretching), 1710 cm 1 (C@O stretching), 1263 cm 1 (C–O–C stretching). 1 H NMR (300 MHz, DMSO-d6) indicates resonance signals resulting in different protons: d = 8.41–8.44 (d, J = 7.8 Hz, 4H), 8.21 (s, 2H), 8.04– 8.06 (d, J = 6.3 Hz, 2H), 7.86–7.89 (d, J = 7.8 Hz, 2H), 7.73 (s, 2H), 7.54–7.56 (d, J = 7.5 Hz, 2H), 7.31–7.29 (d, J = 6.6 Hz, 2H), 7.28–7.25 (t, J = 7.5 Hz, 1H), 7.21–7.23 (d, J = 7.8 Hz, 4H). 13C NMR (300 MHz, DMSO-d6): d (ppm) 169.54, 169.23, 159.14, 156.63, 153.22, 150.08, 137.93, 137.27, 135.48, 131.97, 129.76, 129.56, 129.44,

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127.98, 127.03, 120.19, 119.46, 118.05, 116.42. Elemental analysis: calcd. for C39H25NO10 (667.62): C 70.16, H 3.77, N 2.10, while found C 70.08, H 3.89, N 2.07. 2.3.4. 4-Phenyl-2,6-bis[4-(3,4-dicarboxyphenoxy) phenyl]-pyridine dianhydride (PPDA) The dianhydride was prepared by refluxing the tetraacid precursor PBCP (6.67 g, 0.01 mol) in acetic anhydride (100 mL) and acetic acid (200 mL) for 12 h. After cooling, the colorless solid was collected by filtration and dried overnight at 150 C under reduced pressure. Recrystallization of the product from acetic anhydride gave 5.17 g of white crystals PPDA, the related yield is 82%, melting point is 282 C. FT-IR spectrum exhibited absorption peaks at 1848 and 1774 (C@O stretching), 1272 cm 1 (C– O–C stretching). 1H NMR (300 MHz, DMSO-d6) indicates resonance signals resulting in different protons: d = 8.48–8.51 (d, J = 8.7 Hz, 4H), 8.40 (s, 2H), 8.23–8.27 (d, J = 10.5 Hz, 2H), 8.12 (s, 2H), 8.06– 8.09 (d, J = 9.0 Hz, 2H), 7.59–7.62 (t, J = 8.4 Hz, 1H), 7.55–7.59 (d, J = 10.8, 2H), 7.36–7.39 (d, J = 8.7 Hz, 4H), 7.21–7.26 (t, J = 8.7, 2H). 13C NMR (300 MHz, DMSO-d6): d (ppm) 168.66, 167.93, 159.34, 156.61, 156.19, 150.13, 138.13, 137.19, 135.70, 132.56, 129.76, 129.52, 129.37, 127.81, 127.24, 120.38, 119.68, 117.98, 116.73. Elemental analysis: calcd. for C39H21NO8 (631.59): C 74.17, H 3.35, N 2.22, while found C 74.03, H 3.38, N 2.21. 2.4. Synthesis of polyimides The typical two-step method was applied to polycondensation of objective polyimides. 1.263 g (2.0 mmol) of pyridine-bridged aromatic dianhydride PPDA were gradually added to a stirred solution of 0.4005 g (2.0 mmol) of diamine ODA in 9.5 mL of NMP in a 50-mL three-necked flask equipped with a nitrogen inlet at 5 C. The mixture was stirred for 24 h at room temperature under nitrogen atmosphere, forming a viscous solution of poly(amic acid) (PAA) precursor in NMP. The PAA was subsequently converted into polyimide by either a thermal or chemical imidization process. Chemical imidization was carried out by adding 3 mL of a mixture of acetic anhydride/pyridine (6/ 4, v/v) into the PAA solution with stirring at ambient temperature for 1 h, then the mixture was stirred at 100 C for 4 h to yield a homogeneous polyimide

solution, which was poured slowly into ethanol to give a fibrous precipitate, which was collected by filtration, washed thoroughly with methanol, and dried at 80 C in vacuum overnight. Yields were nearly quantitative for all polymerizations. For the thermal imidization, the PAA was poured into a glass substrate, which was placed overnight in a 90 C oven for the slow release of the solvent. The semidried PAA film was further dried and transformed into polyimide by sequentially heated at 120, 150, 180, 210, 230 and 280 C for 1 h each, then the fully imidized polymer film was stripped from the glass substrate by being soaked in water. 3. Results and discussion 3.1. Monomer synthesis Among the several methods for the preparation of a pyridine ring, modified Chichibabin is one of the best ones that offers advantages such as good yield, available starting materials, and the potential for introducing different substituents in the pyridine ring [14]. Therefore the method was used for the preparation of the new pyridine-bridged aromatic dianhydride according to Scheme 1. First, tetranitrile compound, 4-phenyl-2,6-bis[4-(3,4-dicyanophenoxy)phenyl]-pyridine (PCNP), was prepared by the reaction of 4-(3,4-dicyanophenoxy)-acetophenone (DCAP), which was synthesized by nucleophilic substitution reaction of 4-hydroxyacetophenone and 4-nitrophthalonitrile, with benzaldehyde through the similar pathway reported by Weiss [23]. Then it was converted into tetraacid compound, 4-phenyl-2,6-bis[4-(3,4-dicarboxyphenoxy)phenyl]pyridine (PBCP), by acid hydrolysis carried out by the method of phosphoric acid [24]. Finally, the novel pyridine-bridged aromatic dianhydride, 4phenyl-2,6-bis[4-(3,4-dicarboxyphenoxy)-phenyl]-pyridine dianhydride (PPDA), was obtained in a high yield and pure enough for polycondensation by the chemical cyclodehydration of tetraacid compound PBCP with acetic anhydride. According to the analytic results from FT-IR, 1H NMR, 13C NMR and elemental analysis in Section 2, the dianhydride PPDA and related intermediates holds structure as uniform as shown in Scheme 1. Fig. 1 compares the FT-IR spectra of the intermediate compound PCNP and PBCP and the dianhydride PPDA. The FT-IR spectrum of tetranitrile PCNP is characterized by absorption at 2230 cm 1 because of the cyano group. The most characteristic bands of tetra-

X. Wang et al. / European Polymer Journal 42 (2006) 1229–1239 O 2N

O H3CC

CN

O

N

CN

O H 3CC

CN

OH

K2CO3/Toluene NC

1233

O

CHO

CN

CH3COONH 4/HOAc

DCAP O

NC

CN

85% H3PO 4

CN

HOOC HOOC

O

COOH COOH

PCNP AcOH/Ac2O

N

O

PBCP O C O C O

N

O

O

O C O C O

PPDA

Scheme 1. Synthesis of the pyridine-bridged aromatic dianhydride PPDA.

Fig. 1. FT-IR spectra of PCNP (a), PBCP (b) and PPDA (c).

acid PBCP can be observed near 1710 cm 1 (C@O stretching) and in the region of 2500–3500 cm 1 (O–H stretching). The disappearance of the characteristic cyano stretching band on the FT-IR spectrum revealed completion of hydrolysis. The FT-IR spectrum of dianhydride PPDA shows two characteristic cyclic anhydride absorptions near

1848 and 1774 cm 1, attributed to the asymmetrical and symmetrical stretching vibrations of C@O. Fig. 2 shows the 1H NMR and 13C NMR spectra of the dianhydride PPDA, in which its chemical structure was clearly identified. All intermediate compounds and the dianhydride PPDA were also confirmed by elemental analysis, which were in good

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Fig. 2. 1H NMR and

13

C NMR spectrum of the dianhydride PPDA.

agreement with the calculated values, as reported in Section 2. The results indicate that the design and

synthesis of novel dianhydride monomer PPDA should be successful and feasible in this work.

X. Wang et al. / European Polymer Journal 42 (2006) 1229–1239

3.2. Synthesis chemistry of polyimides The new polyimides (PIs) were prepared by polycondensation of dianhydride monomer PPDA with aromatic diamine monomers through two-step methods, which usually were carried out through poly(amic acid)s (PAAs) or related derivatives, as shown in Scheme 2. In our case, the PAAs were prepared by gradually adding the resulting solid dianhydride monomer PPDA to the equimolar amounts of diamines in anhydrous NMP, and stirring for 24 h at room temperature. The either thermal or chemical imidization procedures were chosen to achieve imidization at the final stage of the reaction forming polyimides, merits of the former were ease for the synthesis, whereas the latter was suited for the preparation of soluble PI. The experimental data of the isolated PIs obtained have been summarized in Table 1. According to the data from Table 1, the resulting PIs all get high yields (96–98%), and elemental analyses show that compositions found of the repeating unit of them can agree with those calculated of that of them well. Mean-

O C O C O

N

O

while, the ginh values of these PIs ranged from 0.49 to 0.63 dL/g, and according to data reported for soluble polyimides [25], the resulting polyimides should possess moderate average molecular weights (Mw), ranged from 40,000 to 80,000. This is consistent with the fact that strong toughness films could be obtained by coating and solvent evaporation of polymer solutions. Fig. 3 compares FTIR spectra of PAA and PI based on PPDA–ODA. The complete conversion of amic acid to imide ring was shown by the disappearance of the amic acid bands at 1650–1700 and 2500–3500 cm 1, together with the appearance of characteristic imide absorption bands at 1777 (asymmetrical C@O stretch), 1721 (symmetrical C@O stretch), 1376 (C–N stretch), 1111, and 744 cm 1 (imide ring deformation). The results show that the polyimides had formed. Comparing with FT-IR spectra of polyimides, there exist similar absorption bands for the polyimide obtained either by thermal or by chemical imidization method, these indicate that the conversion from PAA to PI were basically complete by using the two kinds of imidization methods.

O C O + H2 N Ar NH2 C O

O

5 oC, NMP O HO C

N

O

O C OH

O

HN C O

C NH Ar n O

Poly(amino acid) (PAA) O C N C O

-H2O

N

O

O C N C O

O

Ar n

Polyimide (PI) Ar=

O

O

ODA

O MAPB

O

O PAPB

1235

O CF 3 PFAPB

Scheme 2. Synthesis of the polyimides.

F 3C O

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Table 1 Physical properties and elemental analysis of the polyimides Polyimide

Yield (%)

ginh (dL/g)a

Composition of repeating unit

PPDA–ODA

96

0.58

C51H29N3O7

PPDA–MAPB

96

0.55

C57H33N3O8

PPDA–PAPB

98

0.49

C57H33N3O8

PPDA–PFAPB

97

0.63

C59H31F6N3O8

a

Elemental analysis (%)

Calcd. Found Calcd. Found Calcd. Found Calcd. Found

C

H

N

76.97 76.82 77.11 77.03 77.11 77.14 69.21 69.08

3.67 3.69 3.75 3.76 3.75 3.80 3.05 3.11

5.28 5.21 4.73 4.71 4.73 4.69 4.10 4.06

Measured on 0.5% polymer concentration in DMAc, at 30.0 ± 0.1 C.

Fig. 3. FT-IR spectra of poly(amic acid) and polyimides based on PPDA–ODA.

3.3. Solubility of the resulting polyimides The solubility of the PIs was determined by dissolving 10 mg of polymers in 1 mL of solvent at room temperature or upon heating, as shown in Table 2. It can be seen that almost all the PIs could be dissolved in test solvents, such as m-cresol, NMP, DMAc, DMF and DMSO, even at room temperature in most cases. The good solubility should be result from both the flexibility of ether groups and polarizability of nitrogen atom in pyridine ring in the polyimide structure. The solubility varies

depending upon the diamine used. PI based on PPDA–PFAPB possesses the best solubility because of the presence of the pendent trifluoromethyl and the flexible ether groups. In contrast, PI based on PPDA–ODA possesses worse solubility because there are relative less flexible ether groups in ODA. As expected, PIs prepared via thermal imidization method have poor solubility than those prepared via chemical imidization method, which was possibly due to the presence of partial inter molecular crosslinking during the thermal imidization procedure [26,27].

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Table 2 Solubility date of the polyimidesa Solventb

Polyimides Number

Structure

m-Cresol

NMP

DMAc

DMF

DMSO

1 2 3 4 5c 6c 7c 8c

PPDA–ODA PPDA–MAPB PPDA–PAPB PPDA–PFAPB PPDA–ODA PPDA–MAPB PPDA–PAPB PPDA–PFAPB

– – + + + + + +

– – – – + + + +

– – – – + ++ ++ ++

– – – – – + + ++

– – – – – – + ++

+ + + +

+ + + +

a Qualitative solubility was determined by dissolving 10 mg of polymers in 1 mL of solvent at room temperature or on heating; (++) soluble after stirring for 6 h at room temperature, (+) soluble after stirring for 12 h at 100 C, and (–) insoluble after stirring for 12 h at 100 C. b NMP, N-methyl-2-pyrrolidone; DMAc, N,N-dimethylacetamide; DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide. c Measured by chemical cyclization polyimides.

3.4. X-ray diffraction of the polyimides The crystallinity of the polyimides was examined by wide-angle X-ray diffraction analysis with graphite monochromatized Cu Ka radiation, 2h ranging from 0 to 50, using the polyimide films obtained by thermal cyclodehydration as samples, and the results were shown in Fig. 4. The X-ray diffraction curves of the polyimides express a set of wider diffraction peaks, these should be evidences that indicate the polyimides holding heterogenous morphology, and should also be a reason that could obtain transparent films from these polyimides. The X-ray diffraction curve of the polyimide based on

PPDA-PFAPB

PPDA-PAPB

PPDA-MAPB PPDA-ODA

0

10

20

30

40

50

2θ (deg) Fig. 4. Wide-angle X-ray diffraction curves of the polyimides.

PPDA–ODA exhibits two peaks around 20 and 23, which indicate a little of crystalline morphologies in the resulting polyimides. This should be related to rigidity structure of the polymer chains. The polyimides based on PPDA–MAPB, PPDA– PAPB and PPDA–PFAPB show amorphous patterns, and this is because the presence of flexible ether link induces looser chain packing and reveals a large decrease in crystallinity. Therefore, the amorphous nature of the resulting polyimides would endow them a good solubility. 3.5. Thermal properties of the resulting polyimides The thermal properties of the resulting polyimides, which were evaluated by DSC and TGA methods, are listed in Table 3. The data in Table 3 represent that Tg values of these PIs are in the range of 223–256 C, which depended on the chemical structure of aromatic diamines component. The polyimide derived from PPDA–ODA exhibited the highest Tg value because of the effect of the rigid polymer backbone. On the contrary, polyimides based on PPDA–MAPB, PPDA–PAPB and PPDA–PFAPB showed the lower Tg values due to the relative flexible polymer chain. These results are also confirmed by X-ray diffraction patterns of these polyimides shown in Fig. 4, in which polyimide based on PPDA–ODA displayed a little of crystalline pattern, whereas other polyimides showed the amorphous patterns. Furthermore, according to Fig. 4, polyimide based on PPDA– ODA should exhibit melting peaks from the crystalline morphology in its DSC curve, but no melting

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Table 3 Thermal properties of the polyimides Polyimides

PPDA–ODA(1) PPDA–MAPB(2) PPDA–PAPB(3) PPDA–PFAPB(4)

Thermal propertiesa Tg (C)

Td (C)

T5 (C)

T10 (C)

Rw (%)

256 239 238 223

609 551 561 570

569 556 523 542

581 576 588 572

52.1 60.3 62.7 60.2

a Tg: determined by DSC curve; Td: decomposition-starting temperature; T5: the temperature at 5% weight loss in air; T10: the temperature at 10% weight loss in air; Rw: residual weight retention at 700 C in air.

peaks were observed in the DSC curve presented in Fig. 5. This might be that the Tm values of the resulting polyimide were too high to have overstepped measure range of used DSC meter [28,29]. Fig. 6 compares the TGA curves of the resulting polyimides prepared by thermal imidization method. These polyimides did not exhibit obvious weight loss before the scanning temperature reached up to 500 C in nitrogen, indicating that no thermal decomposition occurred. As shown in Table 3, the decomposition-starting temperature of the resulting polyimides were in the range of 551–609 C, and the temperature at 5% and 10% weight loss was in the range of 523–569 and 572–588 C, respectively. In addition, the residual weight retentions at 700 C for the resulting polyimides were 52.1–62.7%, implying that these polyimides possess excellent thermal stability.

Fig. 6. TGA curves of PIs at a heating rate of 20 C/min in N2.

3.6. Mechanical properties of the polyimides High-quality polyimide films could be prepared by casting the PAA solution on glass plates followed by the thermal curing in the following procedure, 120, 150, 180, 210, 230 and 280 C for 1 h each in air. Table 4 summarizes the tensile strength and modulus of these resulting pyridine-containing polyimides. The polyimide films have tensile strength of 70.7–97.6 MPa, tensile modulus of 1.34–2.82 GPa, and elongation at breakage of 7.9– 9.7%, which indicate that they are strong and tough polymeric materials. It is noticed that the tensile modulus of the polyimide PPDA–ODA (2.82 GPa) and PPDA–PAPB (2.57 GPa) is two times higher Table 4 Mechanical properties of the polyimides

Fig. 5. DSC thermograms of PIs at heating rate of 20 C/min in N2.

Polyimide

Tensile strength (MPa)

Tensile modulus (GPa)

Elongation at breakage (%)

PPDA–ODA PPDA–MAPB PPDA–PAPB PPDA–PFAPB

97.6 94.1 93.4 70.7

2.82 2.46 2.57 1.34

9.2 8.6 9.7 7.9

X. Wang et al. / European Polymer Journal 42 (2006) 1229–1239

than that of polyimides PPDA–PFAPB (1.34 GPa), derived from the pyridine-containing dianhydride PPDA and fluorinated aromatic diamine PFAPB. It is obvious that the high fluorine concentration in the polymer backbone would be responsible to the reduction in the tensile modulus. 4. Conclusion A new pyridine-bridged aromatic dianhydride monomer, i.e. 4-phenyl-2,6-bis[4-(3,4- dicarboxyphenoxy)phenyl]-pyridine dianhydride (PPDA) was successfully synthesized and characterized in the present work, which was employed in polycondensation of it with various aromatic diamines to prepare a series of pyridine-containing aromatic polyimides. Experimental results indicate that the resulting dianhydride monomer PPDA holds a good polymerizability, and the novel polyimides obtained have fairly high Tg values, excellent thermal stability in nitrogen, as well as good solubility in organic solvents. Meanwhile, transparent and tough polyimide films could be obtained by coating and solvent evaporation of PAA solutions. References [1] Ghosh MK, Mittal KL, editors. Polyimides: fundamentals and applications. New York: Marcel Dekker; 1996. p. 7–48. [2] Wilson D, Stenzenberger HD, Hergenrother PM, editors Polyimides. New York: Chapman & Hall; 1990. p. 58–77. [3] Hergenrother PM, Havens SJ. Macromolecules 1994;27: 4659–64. [4] Banihashemi A, Abdolmaleki A. Eur Polym J 2004;40: 1629–35. [5] Liaw DJ, Liaw BY, Yang CM. Macromolecules 1999;32: 7248–50.

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