The synthesis and characterization of a new aromatic diamine, 2,5-bis-(aminopyridine-2-yl)-1,3,4-oxadiazole (BAPO), containing pyridine and 1,3,4-oxadiazole moieties has been reported. An organophilic clay has been obtained via cation exchange reaction between the hydrochloride salt of BAPO and sodium montmorillonite (Na+-Mt‎). Basal space and thermal stability of this new modified organoclay were studied by wide-angle X-ray diffraction (XRD) and thermo gravimetric analysis (TGA) techniques. High thermal stability of BAPO-Mt in compared with conventional montmorillonite modified with aliphatic long chain surfactants is shown. A series of organoclay/polyimide nanocomposites (CPN) consisting of BAPO and benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA) were also obtained by an in situ polymerization reaction followed by thermal imidization. Structural properties and thermal stability of the obtained CPNs were studied by XRD, TGA, differential scanning calorimetry (DSC), and differential thermal analysis (DTA). The glass transition temperature (Tg) is increased with respect to pristine PI for CPNs 1-3 wt.%. At high clay concentrations, the coagulation of organoclay particles results in a decrease in Tg. Based on the obtained results, CPN 1 wt.% showed the most improved thermal properties.
1. Introduction
Polyimides (PI) have gained interests in both academia and industries due to their excellent thermal stability, good resistance toward organic solvents, and improved mechanical properties [1]. They have been have been applied widely in the areas of modern industries [2]. These super engineering plastics have found their way into aerospace, electrical/electronic applications [3], gas separation [4], cell processing, biochip design [5], coating, and composites [6-7].

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However, some difficulties come from rigidity and poor solubility in processing of most PIs. Introduction of flexible ether and ester linkages between the aromatic rings of the main chain is an effective way to make these polymers more pliable [8]. In this regard, preparation of poly(ester-amide-imide)s [9], poly(ether amide imide)s [10], poly(amine–amide–imide)s [11] and poly(amide-imide)s [12] have been reported. It was shown that, the incorporation of rigid heterocyclic rings in the main chain of a synthetic polymer could provide excellent thermal and thermo-oxidative stability, which should be useful to decrease negative effects resulting from the introduction of flexible linkages mentioned above. 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 [13]. Fujigaya et al. reported among the variety of polybenzimidazole derivatives, the pyridine-containing polymer is known to possess a better mechanical properties and significantly higher proton conductivity due to its higher acid doping ability [14]. 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 [15]. The outstanding thermal stability is ascribed to the electronic equivalency of the oxadiazole ring to the phenylene ring structure, which has high thermal-resistance [16].
Combination of inorganic materials with organic polymers is one the exciting topics that has been receiving increasing research attention during recent decades. Nanostructured hybrid materials showed wide potential applications in various areas such as in coatings [17], catalysis [18] and biotechnology [19], shape memory polymers [20], and fuel cells [21]. Organically modified sodium clay (Na+-Mt) has improved compatibility, hence higher efficiency of reinforcement, with the polymer matrix. It has been known that, the dispersion of small amounts reinforcing organoclay mineral with high-aspect ratios, such as layered silicate clays, can significantly enhance the properties of PI and its precursor poly(amic acid) (PAA). These improvements can include solvent resistance [22], ionic conductivity [23], enhanced fire retardance [24], increased corrosion protection [25], increased strength and heat resistance [26], decreased gas permeability [27], high moduli [28], and dielectric properties [29]. The enhancements in thermal and mechanical properties of polymer/clay nanocomposites (CPNs) are due to the lamellar structure of montmorillonite those results in high in-plane strength and stiffness, and a high aspect ratio [30-31]. The chemical structure of Na+-Mt consists of two fused silica tetrahedral sheets sandwich an edge-shared octahedral sheet of either aluminum or magnesium hydroxide [32]. The Ca2+ and Na+ ions adsorbed in the interlayer region are exchangeable with organic cations such as long chain alkyl ammonium [33-34]. These organoclays as compatibilizer may be suitable for polymer blends prepared with a low processing temperature. They have low thermal stability and start to decompose around 200°C, whereas the melt-processing temperatures of most polymers are typically above 200°C [35]. Furthermore, the preparation and processing of PI/organoclaynanocomposites is carried out at high temperatures, and the thermal decomposition of the long carbon chain of quaternary ammonium salts is inevitable. Thermal decomposition during processing can initiate/catalyze polymer degradation, in addition to a variety of undesirable effects during processing and in the final product [36-37]. To avoid the detrimental effects, modification of clay minerals with imidazolium [38] and phosphonium [39] salts have been noted. As another approach, using of aromatic amines and/or diamines, as swelling agents, has also been considered in the preparation of polyamide and polyimide (PI) nanocomposites [31, 36, 40-41].
Recently, we reported the synthesis of a new aromatic diamine, 2-(5-(3,5-diaminophenyl)-1,3,4-oxadiazole-2-yl)pyridine (POBD). Thermally stable poly(amide-imide)s [12], polyamides [42], polyimides [43], and PI/Clay nanocomposites [44] have been prepared using POBD. We noticed to the metal coordination ability of the 1,3,4-oxadiazole ring adjacent to 2-pyridyl group in designing POBD, Scheme 1. The ability of prepared hybrid materials for removal of the Co(II) ion have also been investigated [41, 45].
Thus, as part of our continuing efforts on the synthesis of polyimides with high thermal stability and metal ions coordination ability, in this work, we wish to report the synthesis and characterization of another designed aromatic monomer containing pyridine and 1,3,4-oxadiazole moieties. In this work, BAPO has been synthesized in four steps starting from 2-amino-6-methyl pyridine (1). The dihydrochloride salt of BAPO was used as a swelling agent for the modification of Na+-Mt. The novel modified organoclay (BAPO-Mt) was used in the preparation of PI/organoclay hybrids of BAPO/BTDA. Thermal stability of BAPO is higher than those for commonly used quaternary alkyl ammonium salts. Therefore, thermal degradation will be prevented during heat treatment needed for curing of poly(amic acid)s. The obtained films were studied by FT-IR spectroscopy, XRD, and SEM. The thermal properties were examined by TGA-DTA and DSC.
3. Results and discussion
3.1. Preparation of BAPO-modified organoclay
The new diamine, BAPO 5, was synthesized in four steps. 2-Amino-6-methtypyridine 1 was acetylated with acetic anhydride, oxidized with potassium permanganate, and then alkaline hydrolyzed to give 6-amino-picolinic acid 4 [47]. Cyclo-dehydration to 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).
The chemical structure of BAPO 5 was confirmed by FT-IR, 1H NMR, 13C NMR and mass spectrometry techniques. In the FT-IR spectrum, amino stretching vibrations observed at 3332 and 3202 cm-1. Vibration of C=N bonds 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 on the pyridine nucleolus.
The amino protons also merged to appear as a broad singlet centered at 6.32 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 1. Molecular ion peak was observed as base peak in the mass spectra of BAPO, Figure 2. The ‎fragmentation pattern is shown in Scheme 3.‎
To prepare the organophilic clay (BAPO-Mt) via a cation exchange reaction, the Na+-Mt was initially mixed with a hydrochloride solution of the intercalating agent, BAPO 5. Scheme 4 presents a schematic drawing of the modification step.
3.2. Characterization of BAPO-Mt organoclay
Figure 3 shows FT-IR spectra of BAPO, sodium montmorillonite, and BAPO-Mt. The spectrum of organoclay exhibits the characteristic bands of Mt and BAPO: N–H stretching at 3330 and 3206 cm-1, –C=N– stretching of the pyridine nucleolus at 1652 cm-1, –C=N– stretching of the oxadiazole ring at 1546 cm-1, stretching vibrations of the double bonds of the aromatic rings in BAPO at 1627 cm-1, and typical bands of montmorillonite at 1033 and 525 cm-1.
Figure 4 presents wide XRD of BAPO-Mt and pristine clay. A strong peak is observed at 2= 8.95° for Na+-Mt, corresponding to the (001) plane, indicating that the interlayer spacing (d001-spacing) of Na+-Mt is about 1.0 nm. The interlayer ­d001–spacing can be calculated from peak positions using Bragg’s law: nλ = 2d sin θ, where λ is the X-ray wavelength (1.5418 Å). The reflection peak of (001) in BAPO-Mt shifted to a lower diffraction angle at 6.90°, corresponding to the larger d001-spacing (1.28 nm) than Na+-Mt. The replacement of sodium ions with the ammonium ions of BAPO seems to increase the d001-spacing of layered silicate. In general, a larger d001-spacing should assist the intercalation of the polymer chains and should also lead to better clay dispersion within the polymer matrix. Table 1 summarizes the diffraction peaks and the calculated d001-spacings of Na+-Mt and organophilic clays.
The thermal treatment of pristine clay under nitrogen consists of two main stages. The first stage occurs from ambient temperature to 200°C. In this step, free water molecules physically adsorbed on the external surfaces of crystals along with the hydrating water molecules around the exchangeable cation located inside the interlayer space are removed. The second stage is attributed to the dehydroxylation of the structural silanol units of the montmorillonite in the range of 200-700°C. The temperature intervals of dehydration corresponding to these processes as well as the amount of water released depends on the nature of adsorbed cations and the hydration of the surface [50]. On the other hand, organically modified montmorillonite shows a four-step decomposition process. The vaporization of free water takes place at temperatures below 200°C, while the surfactant’s decomposition occurs in the temperature range of 200–500°C. Dehydroxylation of the structural silanol groups related to aluminosilicates occurs between 500–800°C. The last step is the decomposition associated with the combustion reaction between organic carbon and inorganic oxygen [51]. The amount of loaded diamine can be estimated by TGA measurement. Figure 5 shows the TGA curves of the Na+-Mt, the BAPO-Mt, and BAOP. Pristine Na+-Mt contains a large quantity of water due to the intercalation of hydrated sodium (Na+) and hydrated calcium (Ca2+) cations inside the clay layers. These physically adsorbed water molecules are removed in the range from ambient temperature to 230°C (ca. 3.81% weight loss). The virgin clay also undergoes a 6.36% weight loss within 230-598°C related to dehydroxylation of the structural silanol units. Whereas, under the same condition BAPO-Mt shows a weight loss of about 10.6% within 230-598°C related to surface dehydroxylation and thermal decomposition of the surfactant molecules within the organoclay galleries. The difference between weight losses of Na+-Mt and BAOP-Mt within 230-598°C (Δm = 4.24%) can be attributed to the weight of the loaded diamine. Therefore, the amount of loaded diamines (42.4 mg/g of clay) can be calculated from Eq. (1).
Surface energy of Na+-Mt is lowered by the presence of the aromatic ammonium ions within the interlayer spacing. Therefore, the hydrophilic silicate surface transforms to an organophilic one. As seen in Figure 5, the thermal decomposition of the surfactant molecules occurs in two stags at 260°C and 370°C. These temperatures are higher than those of decomposition temperature of aliphatic long chain surfactants commonly used for modification of Na+-Mt, which occurs below 200°C [52]. This study suggests that the BAPO-Mt can be used in the preparation of PI nanocomposites that need to be cured at elevated temperatures.
The images obtained by SEM demonstrate significant changes on the surface of the BAPO-Mt. The Na+-Mt particles seem to be stuck together due to moisture (Figure 6a-b), but the organoclay particles are clearly separated in organically modified clay (Figure 6c,d). It seems that the hydrophilicity of the clay is reduced after modification reaction. This study is in accordance with TGA results.
3.4. Polymer Synthesis
The present work reports the preparation and characterization of new polyimide CPNs. BAPO-Mt is used as the surfactant at different concentrations. Scheme 5 depicts the synthesis of the PI from the reaction of BAPO and BTDA through thermal dehydration of the poly(amic acid) intermediate.
The FT-IR absorptions appearing at approximately 1786, 1727, 1366, 1094 and 722 cm-1 (Figure 7) indicate the presence of imide functional groups in the polyimide film [53]. The polyimide is also characterized by elemental analysis. The observed and calculated values for CHN analyses are in good agreement. Solubility test results (Table 2) show that the obtained PI is soluble in dimethylsulfoxide (DMSO) and concentrated sulfuric acid at room temperature and in other polar aprotic solvents such as, dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP) at boiling temperature of the solvents. For this experiment, about 0.01 g of the polymer sample was examined in 1 ml of solvent at room and at boiling temperature of the solvents. The inherent viscosity of the 0.125 and 0.25 g/dL solutions of the polyimide were 0.36 and 0.39 dL/g, respectively (measured at 25± 0.5°C in DMSO).
3.5. Preparation and characterization of PI/BAPO-Mt CPNs
The preparation and characterization of polyimide CPNs with different concentrations of organoclay are also investigated. Scheme 6 shows a procedure for the preparation of PI/BAPO-Mt CPNs by thermal imidization according to method described earlier [44-45].
Figure 8 presents XRD curves of BAPO-Mt, and PI films with various organoclay contents. The lack of any diffraction peak in the XRD patterns of CPNs 1 and 3 wt.%, at 2 = 2-10°, can be attributed to the possible formation of nanocomposites of exfoliated structure. A wide and week diffraction peak at 2 = 6.62° was displayed by CPN 5%, equaling a d00-spacing of 1.33 nm for the layered silicates in the CPN. The shift to higher interlayer d00–spacing with respect to BAPO-Mt (2= 6.90°, d001 = 1.28 nm) is due to the intercalation of the polymer within the organoclay galleries and the formation of an intercalated nanocomposite. Pure polyimide does not show any diffraction peak at 2 = 2-10°. The XRD data are summarized in Table 3.
The glass transition temperature of the pure PI is observed at about 271.3°C (measured by DSC), and the polymer does not show any melting endotherm. The decomposition of the polymer begins at 435.5°C, and no thermal decomposition occurs below this temperature. To remove any adsorbed water, the polymer samples were heated to 150°C and then cooled to room temperature prior DSC measurements. Figure 9 shows DSC curves of the PI and CPNs. The results are summarized in Table 4. As seen, the glass transition temperature increases dramatically from 271.3°C for pure PI to 297.0°C for CPN 1%. The restriction of the intercalated polymer chains within the clay galleries can be responsible for preventing segmental motions of the polymer chains [46]. However, further addition of organoclay up to 5 wt.% leads to a decrease in Tg. This decrease might be due to the aggregation of BAPO-Mt particles that reduces the interfacial interaction between organoclay and the PI matrix [47]. Both DSC and DTA methods show similar trends of changing in the Tg values upon increasing the organoclay content.
Figure 10 shows TGA curves of the pure PI and CPNs. The results are given in Table 4. As seen thermo-gravimetery parameters such as temperature for %10 mass loss and initial thermal decomposition (TD) are increased for CPN 1%, remained almost unchanged in CPN 3%, and then decreased. Char yields are less influenced by the BAPO-Mt content. The drop in the thermal properties at high organoclay loading may be attributed to the better miscibility of polymer and organoclay phases at low organoclay concentrations. Like a superior insulator, the obtained multilayered carbonaceous silicate structure increase the total path of evaporation for small molecules produced during pyrolysis [24]. Based on DSC, DTA and TGA studies, it can conclude that CPN 1% has the most improved thermal properties.
The morphology of the PI and CPN film surfaces was also studied be SEM. Some significant and interesting changes have been observed in the surface of CPN 1% with respect to virgin PI film, Figures 11a-d. As seen in the pictures, too many micro-cracks are observed in the background of both films, however, homogeneity of the film surface is increased in the CPN 1wt.%.
Conclusion
A new thermally stable organoclay has been prepared through the modification of Na+-Mt with BAPO. An X-ray diffraction study confirmed the intercalation of organic surfactant within the silicate layers. SEM images showed that some significant changes occurred on the surface of BAPO-Mt with respect to Na+-Mt, including a decrease in hydrophilicity. Furthermore, the high thermal stability of BAPO avoids pyrolysis during thermal imidization of poly(amic acid) intermediate. The preparation and characterization of new PI/BAPO-Mt CPNs with different contents of organoclay have also been investigated. CPNs 1-5% were prepared from the thermal imidization of a BAPO-Mt dispersion in a poly(amic acid) solution obtained from BAPO and BTDA. XRD patterns showed that exfoliated CPNs may be obtained with the organoclay content of 1-3 wt.%, but at higher clay loadings intercalated structure is significant. TGA-DTA and DSC measurements showed that Tg increases with increasing organoclay content loading to 1 wt.%, and then decrease thereafter. SEM images showed that CPNs 1% produces smoother film than that of the virgin polyimide.