Synthesis and Characterization of Novel Processable and Flexible Polyimides Containing 3,6-Di(4-carboxyphenyl)pyromellitic Dianhydride

A series of six novel polyimides containing 3,6-di(4-carboxyphenyl)pyromellitic dianhydride were synthesized via two steps condensation method. Aromatic diamines monomers, 4-(4-aminophenoxy)-N-(4-(4-aminophenoxy)benzylidene)-3-chloroaniline (DA1), 4-(4-amino-3-methylphenoxy)-N-(4-(4-amino-3-methylphenoxy)benzylidene)-3-chloroaniline (DA2), 4-(4-amino-2methylphenoxy)-N-(4-(4-amino-2-methylphenoxy)benzylidene)-3-chloroaniline (DA3) 4-(4-aminophenoxy)-N-(4-(4-aminophenoxy)benzylidene)-2-methylaniline (DA4), 4-(4-amino-3-methylphenoxy)-N-(4-(4-amino-3-methylphenoxy)benzylidene)2-methylaniline (DA5), and 4-(4-amino-2-methylphenoxy)-N-(4-(4-amino-2-methylphenoxy)benzylidene)-2-methylaniline (DA6) were prepared and used to synthesize new polyimides by reaction with resynthesized 3,6-di(4-carboxyphenyl)pyromellitic dianhydride by using two-step condensation method. The inherent viscosities of polyimides range from 0.68–1.04 dL gm and were soluble in polar solvents. Polyimides have excellent thermal stability by showing 10% weight loss temperature was above 450C. Their glass transition temperatures lie in the range of 250–335C. Wide-angle X-ray diffractometer investigations revealed the amorphous nature of polyimides. Therefore, these polymers can be a potential candidate as processable high performance polymeric materials.


Introduction
Polyimides are very interesting group of amazingly strong and marvellously heat resistant polymers [1]. Aromatic polyimides have received much attention for half a century, due to their excellent combination of properties and potential applications in aerospace, microelectronics (flexible printed boards for electronic devices), photoelectronic industry (as photoresists), and separation industry [2][3][4][5][6]. Other applications include adhesives and matrix resins for composites. However some of their properties like limited solubility, rigid chain characteristics, strong chain-chain interaction, high glass transition, and melting temperatures create difficulties in their processing. That is why applications of these rigid polyimides are restricted in technological and industrial applications. Extensive research has been carried out to improve their solubility by synthesizing soluble polyimides without disturbing their excellent properties [7][8][9][10][11][12][13][14][15]. In present days a number of ways exist to alter chemical structure of synthesizing polymeric materials while maintaining the excellent level of their thermal and mechanical properties. Several modifications have been made in their chemical structure by the introduction of bulky alkyl side substitution, noncoplanar, alicyclic structures, flexible aryl, or alkyl ether linkages and asymmetric biphenyl moieties in the back bone of rigid polyimides [16][17][18][19][20]. By the introduction of flexible linkages progress in solubility and significant processability have been achieved by altering crystallinity and intermolecular interactions. The incorporation of aliphatic segments 2 Advances in Chemistry and noncoplanar structures helps to improve solubility of polyimides but is deleterious for thermal and mechanical properties. To achieve better quality polyimides there is need to design new monomers, that is, diamines and dianhydride with structural modifications [21][22][23][24]. Researchers are doing efforts to design and synthesize new diamines and dianhydride monomers, thus, producing a wide range of polyimides with promising processability and solubility for various technological and industrial applications. These efforts make it possible to synthesize many polyimides which are soluble and easily processable without disturbing their excellent properties. Among these polyimides fluorinated monomers have also gained attention. When bulky fluorinated groups are incorporated, soluble polyimides with excellent thermal properties can be achieved [25][26][27][28]. Asymmetric introduction of bulky substituent or linkages in the backbone of polyimides is another effective approach to improve the solubility of polyimides. Another approach is the introduction of ether linkages using a nucleophilic aromatic substitution reaction. Due to the presence of flexible moieties on the polyimide backbone; there will be a decrease in rigidity of polymer chain to improve the solubility of polyimides. To improve the solubility of polyimides carboxy groups were also introduced in the backbone of polyimides and were found very helpful in improving solubility of polyimides. The carboxy functionality is desirable in developing new applications such as fabrication of nanostructure by molecular self-assembly, ion exchange membranes polymer electrolytes, or ionomers. Thus, components into the polyimide main chain are one of the most successful approaches in attaining solubility without changing their excellent properties. These structural modifications for monomers have also led to new polyimides with several improved properties. Consequently, it was of interest to investigate the thermal stability of aromatic polyimides with various linking groups in their backbone. The azomethine linkage is of distinctive significance due to its remarkable properties such as liquid crystalline property, semiconductivity, ability to form metal chelates, fibre-forming ability, fine thermal stability, and nonlinear optical activity [26][27][28][29]. A variety of polymers with a Schiff-base structure have been synthesized, characterized, and investigated with respect to their properties [30]. Keeping in view the useful properties of polyimides, thus, a diamine monomer containing ether and azomethine linkage has been designed, synthesized, and exploited to prepare novel poly azomethine imides with good thermal stability and processability. The aromatic diamines containing azomethine moiety along with ether linkages were incorporated in polymer backbone to examine their structure-property relationship in terms of inherent viscosity, solubility in various solvents, thermal stability, glass transition temperature, and so forth. Introduction of azomethine moiety in the polymer backbone will incorporate features like semiconductivity [31], biomedical activity [32], as thermal stabilizers [33], and corrosion inhibition [34][35][36] in the newly synthesized polymers. The monomers and polyimides, PI (1-6), were characterized by means of elemental analyses, FTIR, 1 H-NMR spectroscopy.

Measurements.
Melting points were determined by using capillary tube on an electrochemical melting point apparatus, model MP-D Mitamura Riken Kogyo, Japan and are uncorrected. Infrared absorption spectra were recorded as KBr disc on Bio-Rad Excalibur FT-IR Model FTS 3000 MX. Elemental analysis was performed using a Perkine Elmer 2400 CHN elemental analyzer. The 1 H spectra were recorded on a BRUKER Spectrometer operating at 300 MHz. Solvent used for analysis was deutrated dimethyl sulfoxide (DMSO-d6). Thermal and DSC analysis were carried out using Perkin Elmer TGA-7 and DSC 404C Netzsch under nitrogen atmosphere. Wide-angle diffractograms were obtained using 3040/60 X'Pert PRO diffractrometer. Viscosities were obtained by using Gilmount falling ball viscometer. Solubility was determined in different solvents. Moisture absorption values were determined by changes in weight of the dried film before and after immersion of water at 25 ∘ C.

Synthesis of 3,6-Di(4-carboxyphenyl)pyromellitic Dianhydride.
A 100 mL two-necked flask was charged with white crystal of 3,6-di(4-carboxy)phenylpyromellitic acid (4.58 g, 10 mmol) and 50 mL of acetic anhydride in inert conditions. After the reflux of 5 h, acetic anhydride was evaporated out to get yellow solids which were further recrystallized from dried DMAc

Synthesis of Polyimide (PI).
The synthesized monomers were subjected to polyimide synthesis. polyimides were synthesized by polycondensation of diamine monomers DA1, DA2, DA3, DA4, DA5, and DA6 with resynthesized dianhydride monomer 3,6-di(4-carboxyphenyl)pyromellitic dianhydride [10]. Three-necked flask equipped with nitrogen inlet and mechanical stirrer was charged with 1.2 mol of diamine in 5-10 mL of NMP. Then 1.2 moL of 3,6-di(4carboxyphenyl)pyromellitic dianhydride was added slowly in the dissolved diamine in NMP. The reaction mixture was stirred for 24 h at room temperature to produce polyamic acid in inert conditions. The resulting polyamic acid solution was converted into polyimides through chemical imidization and thermal method. For chemical imidization, in polyamic acid solution equimolar pyridine and acetic anhydride was added. The reaction mixture was stirred at room temperature for one hour and then heated at 100 ∘ C for 3 h. The resulting solution was poured into methanol and fibrous precipitates of polyimides were collected and dried as shown in Scheme 2. Advances in Chemistry

Results and Discussion
Six new different polyimides were synthesized by polycondensation method. All the synthesized diamines were characterized by IR, NMR, and elemental analysis. FT-IR and NMR spectroscopic techniques confirmed the structures of diamines monomers and polyimides.

IR Spectroscopy.
FT-IR analysis and spectral data confirmed the chemical structure of monomers diamines (DA1-DA6) and their respective polyimides. The data is presented in Table 1. The presence of imide ring was confirmed by the characteristic bands at 1788−1715 cm −1 for (CO) asym and (CO) sym stretching, respectively. The dehydration cyclization of polyamic acid (PAA) to form an imide ring was confirmed by the disappearance of the band at 1690 cm −1 (related to C=O of amic acid). Further the formation of polyimides was confirmed by the absence of N-H vibration at 3500 and 3200 cm −1 . Characteristic band at 1627-1664 cm −1 corresponding to the azomethine group (-CH=N-) indicated the presence of azomethine moiety in polyimides as shown in Figure 1. The disappearance of the amide and carbonyl bands indicated virtually a complete conversion of the imide ring in the resulting polyimide [38]. amines by high field shift of aromatic protons. Signals due to NH 2 protons were found near 5.11-6.12. Characteristic signals of C=N-at 8.15-8.66 in spectra of diamines and polymer confirmed the presence of azomethine moiety. The peaks between 2.06 and 2.20 ppm confirmed the presence of aliphatic proton structure in the polymer, that is, methyl. The aromatic proton peaks can be observed between 6.30 and 8.00 ppm regions. The characteristic signal of amino group disappears in the 1 H NMR of polyimides confirm the synthesis of polyimides and complete imidization [39].  Table 3: Thermal stability of the polyimides (PI-1-PI-6).
Polymer code ( ∘ C) Thermal stability

Viscosity Measurement.
Viscosities of all the synthesised polyimides were obtained by using Gilmount falling ball viscometer in H 2 SO 4 at 30 ∘ C. The inherent viscosities of the synthesized polyimides were calculated between the ranges of 0.68 and 1.04 dL/g indicated that polymers have moderate to higher molecular weights. Most of the polyimides showed higher value of inherent viscosity than other reported polyimides having azomethine linkages [40,41].

Organosolubility of Polymers.
Solubility of the synthesized polyimides was investigated in different solvents like DMSO, DMAc, DMF, m-cresol, and THF and summarized in Table 2. Polyimides were found to be soluble in most of the polar protic solvents. The improvement in solubility is attributed to flexibility induced by incorporated moieties along with carboxyl group in polyimide structure which is related with intermolecular interaction and packing of polyimide films. Some polyimides were soluble at room temperature and some were soluble on heating. The ether linkages along with other aliphatic substitutions like methyl and chloro also helpful in increasing solubility of polyimides [10,24].

Moisture Absorption.
The polyimides were also subjected to analyse the moisture absorption capacity through change in weight of dried polyimide before and after immersion in distilled water at 25 ∘ C for 24 hrs. The moisture absorption of the polyimides was in the range from 1.02 to 1.28%. That increase in water uptake of the polymers is might be due to the incorporation of carboxyl group [10].
3.6. Thermal Behavior. The thermal properties of the synthesized polyimides were evaluated by TGA and DSC at heating rate of 10 ∘ C min −1 . The obtained data is summarized in Table 3. Thermal properties of the compounds are strongly depended on their chemical structure. Polyimides showed excellent thermal properties ( Figure 2). They were found thermally stable up to 480-525 ∘ C from the 10% weight loss, max of ca. 495-525 ∘ C and char yields of 58-65 at 600 ∘ C. The loss of weight in the first degradation step may be due to the -COOH group. So it may be predicted that first step was decarboxylation step and showed the decomposition of polyimide backbone. Dehydration behaviour of polymers was also observed by loss of 2-3% of weight. This might be due to water uptake ability of polyimides. Taking into account the results from TGA analysis, it was found that the presented polyimides possess good thermal stability without significant weight losses up to 380 ∘ C. This implies that no thermal decomposition occurs below this temperature and that the onset decomposition temperature was as high as 400 ∘ C for the all polyimides. The glass transition temperature ( ) of the PIs samples, which is one of the key parameters of polymers when considering the high-temperature devices fabrication and the long-term heat releasing environment was taken as the midpoint of the change in slope of the baseline in DSC curves. The glass transition temperature as a second order endothermic transition could be considered as the temperature at which a polymer undergoes extensive cooperative segmental motion along the backbone. The flexible linkages decreased the energy of internal rotation and lowered the [42,43]. Different intra-and intermolecular interactions including hydrogen bonding, electrostatic and ionic forces, chain packing efficiency, and chain stiffness also affected the . The of the polyimides (PI-1-PI-6) were observed in the range of 250-335 ∘ C, depending upon the structure of diamine component along with dianhydride and decreased with the decreasing of rigidity of the polymer backbone. (PI-5) exhibited the highest (335 ∘ C) owing to the highly rigid pyromellitimide unit along with azomethine backbone with methyl substituents.

X-Ray
Diffraction. X-ray diffraction analysis was also carried out for the characterization of polyimides ( Figure 3). All the polyimides showed broad peaks in wide-angle X-ray diffraction pattern indicated the amorphous nature of polyimides. It might be due to strong chain interaction between the acid groups, which could not help in building regular structure and disturb chain packing of the polyimides. The strong interaction among carboxyl group hindered the chain mobility. The bulky phenyl groups inside the polymer chain with ether linkages might have introduced the amorphous characteristic in these polyimides. The amorphous nature of polyimides is responsible for the improvement in solubility of polyimides [10,24].

Conclusion
A new series of polyimides were synthesized by condensation method of newly synthesized diamines having ether and azomethine linkages with resynthesized dianhydride. All the polyimides were found soluble in most of the polar aprotic solvents and have inherent viscosities in the range of 0.68-1.04 dL/g indicated the moderate to higher molecular weights of polyimides. All the polyimides were found thermally stable upto 498-523 ∘ C. All the polyimides have amorphous nature indicated by the wide-angle X-ray diffraction analysis. On the basis of such results it is concluded that these polyimides could be considered for new processable high performance polymeric materials.