Novel, thermally stable polyimides (PIs) containing a 1,3,4-oxadiazole and pyridine moieties based on a new aromatic diamine 2,5-bis-(aminopyridine-2-yl)-1,3,4-oxadiazole, BAPO, were synthesized. The prepared polymers were soluble in dimethysulfoxide (DMSO) and concentrated sulfuric acid at room temperature and in polar and aprotic solvents, such as, N-methylpyrrolidone (NMP), and N,N-dimethylacetamide (DMAc) at elevated temperature. Thermal behaviors of the PIs were studied by thermogravimetric analysis/dynamic mechanical analysis (TGA-DTA) and differential scanning calorimetry (DSC). The inherent viscosities of the PI solutions were in the range of 0.38-0.61 dL/g (in DMSO with a concentration of 0.125 g/dL at 25??0.5 ??C). The removal of Co(II) and Ni(II) ions from aqueous solutions was performed using polymer 6, which was obtained from BAPO and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA). The maximum adsorption capacity was observed for Co(II) ion at pH=7.0 (110.4 mg g-1, 1.87 mmol g-1).
Keywords: Polyimide; 1,3,4-Oxadiazole; Aromatic Diamine; Metal Ion Removal
Wholly aromatic PIs are well-known as highly thermally and thermo-oxidatively stable polymers with a desirable balance of physical and chemical properties. They were developed in the early 1960s and have since been of great technological importance due to their outstanding thermal and electrical properties [1-2]. They also possess outstanding mechanical, optical and good insulation properties with low dielectric constant as well as good chemical resistance [3-4]. Because of their inherent toughness and flexibility, low density, remarkable thermal stability, radiation resistance and mechanical strength, aromatic PIs are the most useful super engineering plastics and have been applied widely in the areas of modern industries . They have found their way into biochip design , aerospace , optoelectronic applications , gas separation [9-10], coating , superhydrophobic surfaces , and photocatalyst for hydrogen generation .
However, most PIs suffer from processing difficulty due to their rigidity and poor solubility in common organic solvents. Preparation of co-polymers with the introduction of flexible linkages, such as ether and ester linkages between the aromatic rings of the main chain, is an effective way to make polymers more tractable . In this regard, preparation of poly(ester-amide-imide)s , poly(ether amide imide)s , poly(amine'amide'imide)s  and poly(amide-imide)s  have been reported.
It was shown that, the incorporation of rigid heterocyclic rings in the main chain of a synthetic polymer could also provide excellent thermal and thermo-oxidative stability. Pyridine nucleolus, as a rigid symmetric aromatic ring, would contribute to the thermal stability, chemical stability, and retention of mechanical property of the resulting polymer at elevated temperature. Furthermore, the polarizability, resulting from the nitrogen atom in the pyridine ring, could be suitable to improve their solubility in organic solvents and lower their dielectric constant . Among the variety of polybenzimidazole derivatives, the pyridine-containing polymer showed a significantly higher proton conductivity due to its higher acid doping ability and better mechanical properties .
On the other hand, it was known that, the thermal stability of polymers can be raised by the incorporation of 1,3,4-oxadiazole moieties into the polymer structure . Aromatic poly(1,3,4-oxadiazole)s are a class of heterocyclic polymers possessing chemical and thermal stability. These polymers are classified as high-performance polymers with excellent mechanical strength and stiffness. The outstanding thermal stability is ascribed to the electronic equivalency of the oxadiazole ring to the phenylene ring structure, which has high thermal-resistance . These features cause polymers containing 1,3,4-oxadiazole moieties to act as alternatives in the development of heat flame-resistant, semi-conducting, fiber-forming and thermally stable membranes for gas separation [22-26].
Recently, we reported the synthesis of a new aromatic diamine, 2-(5-(3,5-diaminophenyl)-1,3,4-oxadiazole-2-yl)pyridine (POBD), which contains an oxadiazole moiety. POBD has been used for the preparation of thermally stable poly(amide-imide)s , polyamides , polyimides , and PI/Clay nanocomposites . The metal coordination ability of the 2-pyridyl group adjacent to the 1,3,4-oxadiazole ring was considered in designing POBD, Scheme 1. The ability of prepared hybrid materials for removal of the Co(II) ion have also been investigated .
Scheme 1. Proposed schematic representation for the coordination of POBD toward Co(II) ion .
Thus, as part of our continuing efforts on preparation of PIs with high thermal stability and metal ion coordination ability, in this work, we wish to report the synthesis and characterization of another designed aromatic diamine containing pyridine and 1,3,4-oxadiazole moieties. 2,5-Bis-(aminopyridine-2-yl)-1,3,4-oxadiazole, BAPO (5) has been synthesized in four steps starting from 2-amino-6-methyl pyridine (1). It was reacted with aromatic dianhydrides to give the PIs after thermal imidization of their corresponding poly(amic acid) (PAA) intermediate. The obtained polymers were characterized by FT-IR, elemental analysis (CHN), and inherent viscosity measurements. Thermal behaviors of the obtained polymers were studied by DSC and TGA-DTA methods. As a typical, polyimide BAPO/BDTA was subjected to removal of Co(II) and Ni(II) ions from aqueous solutions.
NMR spectra were obtained on a Bruker 300 MHz spectrophotometer. The FT-IR (KBr) spectra were recorded on a PerkinElmer RXI spectrophotometer (2 w/w% in KBr, resolution 4 cm-1, scan no. 6). TGA-DTA curves of the powdered samples were obtained on a Linseis STA PT 1000. Elemental analyses of the samples were performed on an Elemental Vario EL III instrument.. Inherent viscosities were measured by an Ostwald viscometer at 25??C in DMSO. The metal concentrations were assayed with a UV-vis spectrophotometer (HACH DR/5000, USA) using 1 cm cells.
All chemicals were laboratory grade and were obtained from Merck (Germany). The drying of solvents was carried out according to common methods. All solvents were of laboratory grade and dried according to procedures described in the literature . 6-Aminopicolinic acid (4) was prepared according to the method described in literature in three steps starting from 6-amino-2-methylpyridine (1) in overall 36.9% yield . Cobalt (II) chloride.6H2O was used as source of cobalt ions. Cobalt solutions were prepared according to standard methods .
2.2.1 Synthesis of 2,5-bis-(aminopyridine-2-yl)-1,3,4-oxadiazole, BAPO 5
The diamine, BAPO 5, was synthesized under anhydrous condition according to the method described in the literature . To the stirred solution of 6-aminopicolinic acid 4 (1.38 g, 0.01 mole) and hydrazine sulfate (0.65 g, 0.005 mole) in an aqueous solution of phosphoric acid (85%, 2.8 ml), P2O5 (4.26 g, 0.03 mole) was added slowly followed by POCl3 (0.9 ml, 0.01 mole). The viscous solution was heated at 160??C while magnetic stirring for 2h. After cooling to room temperature, the reaction mixture was poured over crushed ice with vigorous stirring. The mixture was neutralized with 10% sodium hydroxide solution to pH=6 and then completely neutralized with 5% solution of sodium hydrogencarbonate and then filtered in vacuum. The precipitate was then stirred in 10 ml of distilled water for 30 min, filtered in vacuum and then dried, to give 0.67 g (53.0% yield) a pale yellow solid product. FT-IR (KBr, cm-1): 3377 (m), 3332 (m), 3096 (w), 1603 (w), 1625 (w), 1547 (m). 1H NMR (300 MHz, DMSO-d6, ', ppm ): 6.80-6.10 (m, 4H); 7.70-7.15 (m, 2H), 3.45 (br, s, 4H, exchanged with D2O). 13C NMR (75 MHz, DMSO-d6, ', ppm): 164.7, 160.4, 141.2, 138.5, 111.8, 111.7. Mass: (m/e)+: 254 (100%)
2.2.3 General procedure for the preparation of PI films 6-9
To a solution of BAPO (0.1589 g, 0.625 mmol) in 2.5 ml of DMSO was added gradually bis-anhydride (0.625 mmol). The mixture was stirred at room temperature for 6 h under argon. The obtained DMSO solution of poly(amic acid) was poured onto a glass plate, and then heated from 60 to 270??C (at a heating rate of 1 ??C/min) in a furnace. The film was then peeled off from the glass plate to obtain the PI film.
2.2.4 Adsorption of Co(II) and Ni(II) ions from aqueous solution on PI 6:
The PI (0.01 g) were placed in 100 ml aqueous solutions of Co(II) and Ni(II) at pHs 6.0, and 7.0. The pHs of the solutions were adjusted to the desired value by adding 0.1M NaOH/HCl aqueous solutions. The initial concentrations of the metal ions were about 25 ppm. The vials were shaked at 25??C for 15 min and the solids were then separated by filtration. The remaining concentrations of metal ions were measured spectrophotometricaly by the analytical kits (HACH Permachem Reagents) at 620 nm. The method is based on the absorption measurement of color intensity of the complex formed by the reactions of Co(II) and Ni(II) with 1-(2-pyridylazo)-2-naphtol.
Results and Discussions
The new diamine, 2,5-bis-(aminopyridine-2-yl)-1,3,4-oxadiazole (BAPO 5), was obtained in four steps starting from 2-amino-6-methtypyridine 1 which was acetylated with acetic anhydride in CH2Cl2. The subsequent oxidation of 2-acetamido-6-methylpyridine 2 with excess amount of potassium permanganate followed by de-protection of amino group in aqueous alkaline solution gave 6-amino-picolinic acid 4. Cyclo-dehydration of 4 with hydrazine sulfate in the presence of P2O5 in the mixture of POCl3 and concentrated phosphoric acid gave BAPO 5 in overall 20.0% yield (Scheme 2).
Scheme 2. Synthesis of BAPO 5.
The chemical structure of BAPO 5 was confirmed by FT-IR, 1H NMR, 13C NMR and mass spectrometry techniques. In the FT-IR spectrum (Figure 1), amino stretching vibrations observed at 3232 and 3202 cm-1. Vibration of C=N bond of pyridine and oxadiazole rings appeared at 1575 and 1653 cm-1, respectively. The absorption band with medium intensity observed at 1273 cm-1 is related to vibration of C-N bond of the amino pyridine moiety. The amino protons also merged to appear as a broad singlet centered at 3.45 ppm in the 1H NMR spectrum, (this peak was disappeared upon addition of D2O and a new peak related to HOD was appeared at 3.90 ppm). In the 13C NMR spectrum of BAPO 5 totally 6 signals observe that it is compatible with the desired structure, Figure 2. Molecular ion peak was observed as base peak in the mass spectra of BAPO, Figure 3. The 'fragmentation pattern is shown in Scheme 3.'
Scheme 3. Mass fragmentation pattern of BAPO 5.
PAA is requisite intermediate for preparing PIs and PI nanocomposites . In this work, the new diamine BAPO 5 reacted with aromatic dianhydrides in DMSO at room temperature to give the corresponding PAAs. The PI films were obtained via thermal imidization of the PAA solutions, (Scheme 2).
Scheme 2. Synthesis of PIs using BAPO 5.
The FT-IR absorptions appearing at approximately 1786, 1727, 1366, 1094 and 722 cm-1 indicate the presence of imide functional groups in the PI film . As a typical, the FT-IR spectrum of polymer 6 is shown in Figure 4. Infrared spectroscopy data and the reaction yields of polymers are listed in Table 1.
The inherent viscosities of the PI solutions were in the range of 0.38-0.61 dL/g (measured in DMSO with a concentration of 0.125 g/dL at 25??0.5 ??C). The highest viscosity was noted with PI 7, obtained from the reaction of BAPO 5 with 4,4-(hexafluoro-isopropylidene)diphthalic anhydride.
The synthesized PIs were also subjected to elemental analysis (CHN). The calculated and result values for the CHN analyses agreed. The elemental analysis (CHN) data along with the results of inherent viscosity measurements are presented in Table 2.
Solubility test results (Table 3) showed that the obtained PIs are soluble in concentrated sulfuric acid at both ambient and elevated temperatures and in polar, aprotic solvents such as DMSO, DMAc, DMF, and NMP at elevated temperatures. For this experiment, about 0.001 g of the polymer sample was examined in 1 ml of the solvent at room temperature and at elevated temperature (120??C). They are insoluble in polar, protic and less polar solvents.
Thermal analysis results of the polymers are summarized in Table 4. TGA-DTA and DSC techniques were used to study the thermal behaviors of the obtained PIs. The DSC thermograms were obtained after heating the polymer samples to 150??C and then cooling to room temperature; this removed any adsorbed water on the polymer. Based on DTA measurement, the glass transition temperature was occurred at 300.1??C for PI 6, which was obtained from BAPO and BTDA. PI 7 shows the glass transition temperature at 290.0??C, measured by DTA. Figure 5 shows a typical DSC-TGA curves of the PI 8, which was obtained from BAPO and 4,4'-oxydianiline (ODA). As seen, a clear melting endotherm observes without any thermal decomposition for PI 8 at 310.7??C. The glass transition temperature occurs at relatively low temperature for this polymer (222.6??C based on DSC and 239.9??C based on DTA measurements). This is due the introduction of flexible ether linkages into the polymer. The TGA curves of prepared PIs are shown in Figure 6. As seen, temperature of %10 weight loss, onset decomposition temperature and char yield at 599??C of PI 6 are the highest among the prepared polymers. Based on the obtained results from DSC, DTA, and TGA measurements and considering observation of the Tg and Tm values for PIs 6 and 8, it can be conclude that, among the prepared polymers, these two polymers have the most improved thermal properties.
It has been shown that, the 2-pyridyl group adjacent to the 1,3,4-oxadiazole ring has coordination ability toward metal ions, such as Cu(II), Co(II), and Cd(II) [27-28]. Therefore, it may be proposed that polymers containing BAPO units could find applications in wastewater treatment for removing toxic heavy metal ions. In order to show the metal coordination ability, as typical experiments, the adsorption of Co(II) and Ni(II) ions from the single metal aqueous solutions on the PI 6 was investigated in batch experiments. To determine the amount of adsorbed Co(II) and Ni(II) a spectrophotometric reading was performed using HACH kits. The amount of adsorbed metal ions was calculated using the following equation:
where Qt is the amount of adsorbed metal ions (mg g'1) on the polymer, C0 and Ct are the concentrations of metal ions in the initial solutions and in the aqueous phases after adsorption, respectively (ppm), Vs is the volume of the aqueous phase (lit), and m0 is the weight of the adsorbent (g).
To investigate the effect of pH on the adsorption of Co(II) and Ni(II) ions, the pH values of the initial metal ion solutions were adjusted to 6.0, and 7.0 at 25??C. This narrow pH range was used because of possible protonation of nitrogen in the pyridine ring and precipitation of Co(II) and Ni(II) at low and high pH values, respectively. The results are summarized in Table 5. As seen, the metal uptake capacity is significantly influenced by the pH of solutions. The maximum metal uptake capacity were observed at pH 7.0 for Co(II) (Qt = 110.4 mg g'1, 1.87 mmol g'1). The decrease in the Qt values for both Co(II) and Ni(II) ions was observed at pH = 6 due to the protonation of the pyridine moieties in the polymer backbone.
Novel thermally stable PIs containing 1,3,4-oxadiazole and pyridine rings in the main chain based on a new diamine BAPO 5 were synthesized. Solubility tests showed the polymers are soluble in DMSO and concentrated sulfuric acid at room temperature, as well as in NMP, and DMAc at elevated temperatures. The thermal behavior of the polymers was studied by TGA, DTA and DSC methods. Softening temperatures were observed only for polymers 6 and 8. These two polymers showed the most improved thermal properties. PI 7 possessed the maximum inherent viscosity (0.61 g/ dL in DMSO with a concentration of 0.125 dL/g at 25??0.5 ??C)
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