Synthesis and characterization of novel polyimides derived from 4,4’-bis(5-amino-2-pyridinoxy)benzophenone: effect of pyridine and ketone units in the main

Abstract A diamine monomer, 4,4’-bis(5-amino-2-pyridinoxy)benzophenone, was designed and synthesized, and used to react with commercially different kinds of aromatic dianhydrides to prepare a series of polyimides containing pyridine and ketone units via the classical two-step procedure. Glass transition temperatures (T g) of the resultant polyimides PI-(1–5) derived from 4,4’-bis(5-amino-2-pyridinoxy) benzophenone with various dianhydrides ranged from 201 to 310 °C measured by differential scanning calorimetry. The temperatures for 5%wt loss of the resultant polyimides in nitrogen atmosphere obtained from the thermogravimetric analysis curves fell in the range of 472–501 °C. The temperatures for 10%wt loss of the resultant polyimides in nitrogen atmosphere fell in the range of 491–537 °C. Meanwhile, the char yields at 800 °C were in the range of 55.3–60.8%. Moreover, the moisture absorption of polyimide films was measured in the range of 0.37–2.09%. The thin films showed outstanding mechanical properties with tensile strengths of 103–145 MPa, an elongation at break of 12.9–15.2%, and a tensile modulus of 1.20–1.88 Gpa, respectively. The coefficients of thermal expansion of the resultant polyimides were obtained among 26–62 ppm °C−1. To sum up, this series of polyimides had a good combination of properties applied for high-performance materials and showed promising potential applications in the fields of optoelectronic devices.


Introduction
Advanced polyimides have been extensively used in the fields of the aerospace, microelectronics, functional membranes, advanced composite for their exceptional thermal stability, outstanding mechanical properties, low dielectric constant and great chemical resistance. [1][2][3][4][5][6] However, with the rapid progress of high-tech fields, higher requirements are simultaneously put forward to the polymer material performances. In recent years, considerable attention has been devoted to synthesis novel diamines or dianhydrides possessing a special structure, such as fluoride structure, symmetrical and unsymmetrical structure, ketone-based structure, heterocyclic units, etc. [7][8][9][10][11] Ketone-based compounds have been receiving more attention as a monomer for the synthesis of polyimides due to possess the desirable properties.
In the classical polyimides, introducing ketone unit into the molecular chain can effectively change some properties of the polymer, resulting in high thermal stability, mechanical properties, but low transparency. This disadvantage of the polyimide hinders its application in the field of optoelectronic devices. It is an effective method to modify optical property of polyimide by incorporating bulkily fluorinated groups, aliphatic segments, and heterocyclic units into polymer backbone. [10,[12][13][14] The incorporation of aromatic heterocyclic structure into the backbone of the polymers results in imparting certain properties to the polymer while not deteriorating original outstanding properties. Pyridine belongs to heterocyclic units, and nitrogen atoms with a free electron give an opportunity for protonation to modify optical property. [6,15] If the ortho and para positions have better leaving group (such as halogen, nitro) in pyridine moiety, it is easier to take place nucleophilic substitution reaction. It is an easy way that incorporates pyridine ring into polymer backbones. So, the presence of ketone and pyridine groups lead to keep rigidity and improve optical property, these polyimides have potential applications in the fields of optoelectronic devices.
In this article, a new monomer was designed by incorporating two special structure units simultaneously into polyimide backbones while the desirable properties are of particular interest. A diamine containing pyridine and ketone units, 4,4'-bis(5-amino-2-pyridinoxy)

Thermogravimetric analysis (TGA)
Thermogravimetric data was obtained on TA 2050 under the nitrogen flowing condition at a heating rate of 10 °C/min.

Dynamic mechanical analysis (DMA)
DMA was performed on TA instrument (TA Q800), and T g decided by the peak temperature of loss modulus (E″).

Thermomechanical analysis (TMA)
TMA was performed on METTlER instrument (TMA/ SDTA841), and the coefficient of thermal expansion (CTE) decided by the temperature range of 50-150 °C.

Ultraviolet-visible (UV-vis) spectra
UV-vis spectra were measured with a Shimadzu UV-vis 2501 spectrometer in transmittance mode using the thin films as sample at room temperature.

Mechanical analysis
Tensile property of the polymer films was measured with a Shimadzu AG-I universal testing apparatus with crosshead speed of 5 mm/min at room temperature,and the thin film specimen sizes were at 25-30um thick, 3 mm wide and 4 cm long.

Water uptakes
Water uptakes (WU) of the films were calculated by the following equation: WU = [(W wet − W dry )/W dry ] × 100%; where W wet refers to the weight of PI film samples after immersion in deionized water at room temperature for 24 h, and W dry is the initial weight of them.

4,4'-bis(5-nitro-2-pyridinoxy)benzophenone (BNDBP)
Under nitrogen protection, 4,4'-dihydroxybenzophenone (6.43 g, 30 mmol), 2-chloro-5-nitropyridine (10.47 g, 66 mmol), potassium carbonate (9.12 g, 66 mmol) and 75 ml of dried DMF were charged into a 250 ml reaction flask with a mechanical stirrer and reflux condenser. After 30 min of stirring at room temperature, the reaction mixture was continuously reacted at 80 °C for 6 h. After cooling to room temperature, the mixture was poured into 250 ml of deionized water. The solid power was collected by filtration and washed thoroughly with water. The crude product was then dried in vacuum at 110 °C overnight. The yellow solid was recrystallized from DMF/water, and the yield of the product (BnDBP) was 15 benzophenone (BADBP), was synthesized and the corresponding polyimides were prepared based on BADBP with five commercial dianhydrides by the classical twostep polymerization procedure. The influences of in corporating two special units, pyridine and ketone, on the thermal stability, mechanical property, optical transparency, solubility and so on were systematically investigated.

Elemental analysis
Elemental analysis was run on a Vario El cube CHn recorder elemental analysis instrument.

Inherent viscosities (ɳ inh )
ɳ inh of all the PAAs (Polyamide Acids) in DMAc solvent were measured with a content of 0.5 g/dl using an Ubbelohde viscometer at 25 °C.

Fourier transform infrared spectrometer (FT-IR)
FT-IR was recorded on a Bruker Vector22 spectrometer using KBr (potassium bromide) pellets or about 10um thick polymer films. 1 H nMR spectra of the polyimides were recorded on a BRUKER 300 MHz instrument using DMSO-d 6 as solvent.

Differential scanning calorimetric analysis (DSC)
DSC was performed on a TA Instruments (TA Q100) with a heating rate of 10 °C/min in a nitrogen flowing condition at 50 ml/min, and the results determined by the second heating cycle.

4,4'-bis(5-amino-2-pyridinoxy)benzophenone (BADBP)
Under nitrogen protection, 10.0 g (21.8 mmol) of BnDBP, and 49.2 g (218.0 mmol) of SnCl 2 ·2H 2 O were charged into a reaction flask, meanwhile 47 ml of concentrated hydrochloric acid was added slowly. After addition of hydrochloric acid was finished, the mixture was refluxed for 7 h. The reaction mixture was poured into 400 ml of distiller water when cooling to room temperature. The mixing solution was basified with 40%naOH solution to form a precipitate. The precipitate was filtrated off, washed with water, and recrystallized from ethanol to get a white product. [16] After dried under vacuum at 80 °C for overnight, 6.3 g of BADBP was obtained (63%).

Poly(amic acid) synthesis
As shown in Scheme 1, BADBP was used to react with commercially five aromatic dianhydride (6FDA, PMDA, BTDA, ODPA and BPADA, respectively) to prepare a series of heterocyclic polyimides (PI-1-PI-5) via a typical twostep polymerization procedure. In case of PI-1 derived from BADBP with 6FDA, PAA was obtained as following Scheme 1. synthesis route of the polyimides.
hydrochloric acid at refluxing temperature. The structures of BADBP and BnDBP were confirmed by elemental analysis, FT-IR spectra, and 1 H nMR spectroscopy. Figure 1 shows the FT-IR spectra of BnDBP and BADBP, the nitro group of BnDBP gave two characteristic bands at 1518 and 1391 cm −1 (-nO 2 asymmetric and symmetric stretching), respectively. After the reduction, the characteristic absorptions of the nitro group disappeared, and n-H absorption peaks of the amino group in the region of 3210-3450 cm −1 were detected. As shown in Figure 2, 1 H nMR spectrum of the diamine monomer (BADBP) illustrates that the nitro groups in BnDBP were completely reduced, and the signal of amino groups appeared at around δ5.26 as a singlet. All the spectroscopic data were in agreement with the expected structures.

Synthesis and characterization of polyimides
The diamine, BADBP, was used to react with commercially different aromatic dianhydrides (6FDA, PMDA, BTDA, ODPA procedure: 0.56 g 6FDA (1.25 mmol) was slowly added to the solution of BADBP (0.50 g, 1.25 mmol) in 6.0 ml DMAc at a solid content 15%wt. Then, the reaction mixture was stirred at room temperature for 12 h under n 2 protection to yield the viscous PAA solution.
Similarly, the other PAAs were obtained by the above-mentioned procedures.

Preparation of polyimide films
Polyimide film was prepared by the casting of 15%PAA onto the clean glass plate. The glass plate was placed in an oven and allowed to elevate temperatures (60 °C/2 h; 80 °C/2 h; 100 °C/2 h; 120 °C/1 h; 150 °C/1 h) to remove the solvents in air. Then the heating imidization procedure (200 °C/0.5 h, 250 °C/0.5 h, 300 °C/1 h) was performed in imine furnace under vacuum. The freestanding PI film was obtained by soaking in water to release from the glass plate. [17] Similarly, PI-2-PI-5 films were obtained by the above-mentioned procedures.

Synthesis of diamine monomer
The synthesis of the novel diamine containing heterocyclic pyridine and ketone groups, 4,4'-bis(5-amino-2-pyridinoxy)benzophenone, was shown in Scheme 2. The nitro compound was synthesized with nucleophilic substitution reaction starting from 4,4'-dihydroxybenzophenone and 2-chloro-5-nitropyridine in the presence of potassium carbonate in DMF by stirring of the mixture at 80 °C for 6 h. The chlorine atom activated by the para nitro-group and ortho nitrogen-atom was readily carried out the chlorine-displacement reaction of 2-chloro-5-nitropyridine by the phenoxide anion. The diamine was readily obtained in a good yield by the catalytic reduction of the intermediate dinitro compound with SnCl 2 ·2H 2 O and concentrated   Table 1, these found values were in agreement with the calculated ones of C, H, and n in elemental analyses of the approved molecular formulas, and the yield of polyimides was in the range of 95-97%.

Thermal properties of polyimides
As shown in Table 2, thermal properties of the polyimides were detected by DSC, TGA, DMA and TMA. In DMA curves, T g was decided by the peak temperature of loss modulus (E″). Glass transition temperatures (T g ) of the polyimides (PI-1-PI-5) derived from BADBP with various dianhydrides in the range of 201-310 °C obtained by DSC in Figure 4 and 147-291 °C obtained by DMA in Figure 6, respectively. All the polyimides exhibited outstanding thermal property. It is probably attributed to incorporate ketone and pyridine units in polymer backbones. Generally, T g values of polymers are determined by rigidity of the polymer backbones. The rigidity of dianhydrides is listed as follow: PMDA > BTDA > 6FDA > ODPA > BPADA. PI-5 derived from PMDA possessed the highest T g due to its rigidity polymer and BPADA, respectively) to yield a series of heterocyclic polyimides (PI-1-PI-5) via a typical two-step polymerization method. PAA was prepared by reacting equimolar amounts of diamine monomer with commercially aromatic dianhydride at a solid content of 15wt%, and stirred for 12 h at room temperature under n 2 atmosphere. The polyimide film was prepared by the casting of 15%PAA onto the clean glass plate. The glass plate was placed in an oven and allowed to elevate temperatures (60 °C/2 h; 80 °C/2 h; 100 °C/2 h; 120 °C/1 h; 150 °C/1 h) to remove the solvents in air. The heating imidization procedure (200 °C/0.5 h, 250 °C/0.5 h, 300 °C/1 h) was performed in imine furnace under vacuum. As shown in Table 2, the inherent viscosities of the PAAs were in the range of 0.34-0.57 dl/g measured at a content of 0.5 g/dl in DMAc at 25 °C. In FT-IR spectra of PIs, shown in Figure 3, the characteristic imide absorption   segments due to the existence of -CF 3 group in the polyimide backbone. [18] The thermal stability properties of the polyimides were obtained by TGA in nitrogen at a heating rate of 10 °C/ min, and showed in Table 2. The temperatures for 5%wt loss and 10%wt loss of polyimides in nitrogen atmosphere were obtained from Figure 5 in the range of 472-501 °C and 491-537 °C, respectively. Meanwhile, the char yields at 800 °C were in the range of 55.3-60.8%. From the 10% weight loss temperatures in nitrogen, the following relative order of thermal stability was observed: PI-1 > PI-4 > PI-5 > PI-2 > PI-3. PI-3 having more ether-connecting groups had the lowest decomposition temperature and char yield at 800 °C.
As shown in Figure 7, the TMA curves of the polyimide films were obtained at the heating rate of 10 °C/min, and listed in Table 2. The CTEs of the polyimides were among 26-62 ppm °C −1 . CTEs were influenced by the rigidity and linearity of the polymer chains. [19] The rigidity relative order of dianhydrides was listed as follow: PMDA > BTDA > 6FDA > ODPA > BPADA. All the polyimides were chain structure, and PI-3 derived from BPADA possessed the lowest T g due to its flexible polymer chain structure. However, PI-1 derived from 6FDA showed higher T g value than that of PI-4 derived from BTDA. This exception might be attributed to inhibit the free rotation of the polymer     Wide-angle X-ray diffraction analysis results showed that the X-ray diffraction curves of polyimides in Figure 8 exhibited almost amorphous patterns apart from PI-5, by which the polyimides were amorphous due to exist the flexible ether linkage loosening the chain packing of the polymer. PI-5 exhibited semi-crystalline pattern because PI-5 derived from PMDA possessed the rigidity polymer chain structure in favor of crystallization.

Optical properties of the polyimides
The polyimide films with about 30 μm thick as the sample were measured for optical transparency property with UV-vis spectroscopy and the UV-vis spectra were given in Figure 9. The results, percentage transmittances conformed to this feature, and PI-5 possessed the lowest CTE value due to its highest rigidity and linearity of the polymer chain. It is understandable also from no obvious glass transition of PI-5. PI-3 exhibited highest CTE value, because BPADA possesses lowest rigidity and linearity in these dianhydrides.

Mechanical properties of the polyimides
The tensile property of the polyimide films with 25-30 mm thick, 3 mm wide and 4 cm long were measured by a Shimadzu AG-I universal testing apparatus with crosshead speed of 5 mm/min at room temperature, and summarized in Table 3. The films had tensile strengths of 103-145 MPa, an elongation at break of 12.9-15.2%, and a tensile modulus of 1.20-1.88 Gpa, respectively. The mechanical properties of PI-4, 5 were better than that of others, which might be attributed to rigidity structure in the polyimide backbone.

Solubility and X-ray diffraction of the polyimides
To obtain the solubility of the polyimide films, dissolving 10 mg of polymers in 1 ml of solvent at room temperature for 24 h, and the results were listed in Table 4. The    at 450 nm and cut-off wavelengths (λ cut-off ), were listed in Table 3. The cut-off wavelengths (λ cut-off ) were in the range of 357-391 nm, and the percentage transmittances at 450 nm were in the range of 29.2-89.7%. Because the trifluoromethyl groups could inhibit the formation of the CTC (charge-transfer complex) between polymer chains through steric hindrance and the inductive effect, the PI-1 film shows higher transparency at 450 nm and lower λ cut-off than the others in Figure 9. [20]

WU of the polyimides
WU of the films were calculated by the following equation: WU = [(W wet − W dry )/W dry ] × 100%; 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 the initial weight of them. The WU of polyimides were in the range of 0.37-2.09%, and listed in Table 3. Song et al. [21] reports WU of Upilex®-75S (Ube Industries ltd, Japan) was 1.49% under the same conditions. Compare with it, these results implied that introduction of pyridine and ketone moieties did not deteriorate the water absorption behavior of the PIs. Moisture absorption behavior of polymer could be affected by chemical structure. PI-4 showed highest moisture uptake in comparison to the others owing to the presence of more ketone polar groups which formed hydrogen bonding with H 2 O. PI-3 exhibited the lowest moisture absorption (0.37%), and this result may be attributed to the fact that polyimide contained the water proofing effect of -CH 3 units and flexible ether linkages of BPADA.

Conclusions
A new diamine containing pyridine and ketone, BADBP, was designed and prepared through the nucleophilic substitution reaction of 4,4'-dihydroxybenzophenone and 2-chloro-5-nitropyridine followed by the catalytic reduction of the dinitro intermediate with SnCl 2 ·2H 2 O and concentrated hydrochloric acid at refluxing temperature. A series of polyimides were obtained from the diamine with commercially aromatic dianhydrides by thermal imidization. The resulting polyimides exhibited excellent thermal stability and T g s of these polyimides were in the range of 201-310 °C. All the polyimides showed high tensile modulus in the range of 103-145 Mpa. To sum up, this series of polyimides had a good combination of properties applied for high-performance materials and showed promising potential applications in the fields of optoelectronic devices.

Disclosure statement
no potential conflict of interest was reported by the authors.