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This paper deals with the properties of phthalonitrile polymers derived from three different monomers, namely 2,2'-Bis(3,4-dicyanophenoxy)biphenyl, 1,2-Bis(3,4-dicyanophenoxy)benzene and 2,2'-Bis(3,4-dicyanophenoxy) 1,3,4-oxadiazole. DSC and rheometric measurements showed that the rate of curing reaction is different for all the three monomers. Rheometric measurements and thermogravimetric analysis showed that the thermal stability of all the three polymers were comparable. The catechol and oxadiazole containing resins maintained good structural integrity upon heating to elevated temperatures and exhibited excellent thermal properties along with thermo-oxidative stability. The dynamic mechanical measurements revealed that the fully-cured oxadiazole containing resin neither exhibited glass transition temperature nor any change in storage modulus upon heating up to 500 °C.
High-temperature polymers are an important class of materials with a variety of composite uses for aerospace, marine, and microelectronic applications . The interest in polymeric composites versus metallic materials arises from the need for a reduction in weight and an enhancement in performance . Phthalonitrile-based polymers constitute a unique class of high temperature polymer having a variety of potential uses in the adhesives , electronic [4,5] industries and as a matrix resin in structural applications  .They have a number of exceptional properties such as high glass transition temperatures, outstanding thermo-oxidative stability, excellent mechanical properties, good moisture resistance and superior fire resistance [7-15]. In addition, phthalonitrile monomers and oligomers possess a low complex melt viscosity (0.01-1 Pa s) which enables facile processing by the cost-effective, non-autoclavable processing techniques such as resin transfer molding, resin infusion molding and ¬lament winding. The key to the development of high-temperature polymers is the incorporation of thermally stable structural units such as aromatic or heteroaromatic rings within the backbone of a polymeric system. Through the design of a polymeric system with these highly stable structural units and with ¬‚exible linkages, the desired thermo-oxidative stability can be achieved along with processability . The combination of good processability and excellent high-temperature properties makes the phthalonitriles attractive for many military and civilian advanced technological applications .
They polymerize through cyano groups by addition mechanism and appear to propagate through multiple reaction pathways involving polytriazine, polyimine and polyphthalocyanine, which ensures that little or no volatile by-products are evolved during polymerization leading to void-free crosslinked networks. These heterocyclic crosslinked products are known to exhibit good thermal and oxidative stability. Phthalonitrile monomers with several structural variations have been synthesized and polymerized into thermosets [18-24]. Neat polymerization is extremely sluggish requiring extended heat treatment for several days at elevated temperature to obtain a vitri¬ed product . However, it can be enhanced through the use of curing additives such as phenols , organic amines , strong organic acids , strong organic acid/amine salts , metals and their salts .The advanced composite materials based on the phthalonitriles have now reached a level of maturity that makes them a viable structural material. The phthalonitrile monomers are now commercially available. Glass and carbon ¬ber-reinforced composites are being evaluated for numerous domestic and military applications. The ¬re performance of phthalonitrile composites is superior to that of other thermoset-based composites currently being evaluated for aerospace, ship, and submarine applications . Our present work regarding high temperature phthalonitrile resins is mainly concerned with the development of new monomers that can be polymerized at moderately elevated temperatures, and yield highly cross-linked thermally stable systems suitable as matrices for long-term and high temperature composites. Replacement of metals by such polymeric composites can affect substantial weight and energy savings and improve their performance. This work is concerned with the synthesis and polymerization of some novel orthro-linked phthalonitriles, whose thermal and rheometric studies are presented.
4-Nitrophthalonitrile was purchased from Alpha (Shijiazhuang China) chemical Co., Ltd and used as received. Biphenol-2, 2'-diol, catechol, 2-hydroxy benzoic acid and hydrazine sulfate were purchased from Beijing Beihua Fine Chemicals Co., China and used as received. Dimethyl sulfoxide (DMSO) was puri¬ed by distillation under reduced pressure over CaH2. Potassium carbonate was dried at 150°C under vacuum. All other solvent and reagent used were purified by standard method.
1H and 13C NMR spectra were measured on a Bruker Avance 400 NMR spectrometer with DMSO-d6 as the solvent and tetramethylsilane as the internal reference. The infrared spectra of monomers and products were obtained on a Bruker 27 FT-IR spectrometer using KBr disks. Di¬€erential scanning calorimetric (DSC) analyses were performed on Mettler Toledo 822e di¬€erential scanning calorimeter at a heating rate of 10°C/min under ¬‚owing nitrogen. The glass transition temperature (Tg) of the polymers was taken as the peak point temperature of tan delta curve from DMA analysis performed on a DMA 242 C (NETZSCH, Germany) with a driving frequency of 1.0 Hz, single cantilever mode and a scanning rate of 5°C/min in nitrogen. Melting points were measured using DSC method. Thermogravimetric analysis (TGA) was conducted with a Netzsch STA 409PC instrument (Germany), and experiments were carried out on approximately 10 mg of samples under controlled ¬‚ux of nitrogen/air at 10°C /min. Melt viscosity measurements were performed on a Physica MCR-300 mechanical spectrometer (Germany) at a ramp rate of 4°C/min in air and the top parallel plate was oscillated at a fixed strain of 10% and a fixed angular frequency of 100 rad/s. Sample specimen discs of 2.5 cm diameter and 1 mm thickness were prepared by press molding oligomer powder at room temperature under high pressure.
2.3. Monomer syntheses
2.3.1 2,2'-Bis(3,4-dicyanophenoxy)biphenyl (3a)
2,2'-Bis(3,4-dicyanophenoxy)biphenyl (3a) was synthesized according to the method of literature .In a 250 mL ¬‚ask equipped with a nitrogen inlet, a Dean-Stark trap, and a condenser, were placed 9.31 g (0.05 mol) of biphenyl-2,2'-diol and 15.0 g (0.108 mol) of K2CO3 with 100 mL of DMSO and 100 mL of benzene. The mixture was heated with stirring at 120°C for 6 h under a thin stream of nitrogen to remove water by azeotropic distillation with benzene. After the benzene had been removed, the mixture was cooled to room temperature and 17.66 g (0.10 mol) of 4-nitrophthalonitrile (2) added. The reaction was continued with stirring at 40°C for 40 h under nitrogen, after which it was poured into1L of cold water. The precipitates were collected by ¬ltration and dried. The product was puri¬ed by recrystallization from ethanol to afford 18.09 g (82.52%) of pale-yellow needles, m.p.156.08°C (DSC) (lit 23 155-156°C).
FTIR (KBr, cm -1): 2233 cm-1 () and 1254 cm-1(C-O-C).
1H NMR (400 MHz, DMSO-d6): 7.96-7.92 (d, 2H, Ar-H); 7.50 (d, 2H, Ar-H); 7.41-7.47 (t, 2H, Ar-H); 7.41-7.45 (d, 2H, Ar-H); 7.29-7.35 (t, 2H, Ar-H); 7.15-7.19 (d, 2H, Ar-H); 7.13-7.17 (d, 2H, Ar-H).
13C NMR (400 MHz, DMSO-d6): 161.05, 151.53, 136.58, 132.86, 130.87, 129.76, 126.47, 122.86, 122.38, 12.96, 116.99, 116.26, 115.72, and 108.53 ppm.
2.3.2 1,2-Bis(3,4-dicyanophenoxy)benzene (3b)
1,2-Bis(3,4-dicyanophenoxy)benzene (3b) was synthesized according to the method of literature . In a 300-mL flask, equipped with a nitrogen inlet, 11.02 g (0.1 mol) of catechol, 34.62 g (0.2 mol) of 4-nitrophthalonitrile (2), and 27.6 g (0.1 mol) of K2CO3 were suspended in 300 mL of DMF. The suspension was stirred at room temperature for about 24 h. The reaction mixture was poured into 1 L of water; the precipitated solid was collected and washed thoroughly with methanol and water. The yield of the product was 34.0 g (93.92%). The crude product was purified by recrystallization from acetonitrile/methanol (1:1) to afford 3b as colorless needle. The yield of the purified product was 25.9 g (70.95%); mp 188.74 °C (lit.7190.1-190.6 °C). FTIR (KBr, cm -1): 2232 cm-1 () and 1246 cm-1(C-O-C).
1H NMR (400 MHz, DMSO-d6): 7.99-8.03 (d, 2H, Ar-H); 7.72-7.74 (d, 2H, Ar-H); 7.38-7.46 (m, 4H, Ar-H); 7.31-7.36 (dd, 2H, Ar-H).
13C NMR (400 MHz, DMSO-d6): 160.68, 145.19, 136.72, 128.38, 123.85, 122.39, 122.07, 117.18, 116.34, 155.77, and 109.21.
2.3.3 Synthesis of 2,2'-Bis(3,4-dicyanophenoxy) 1,3,4-oxadiazole
22.214.171.124 2,5-Bis(2-hydroxyphenyl)1,3,4-oxadiazole (3c)
To a 500 mL round-bottom ¬‚ask equipped with a condenser and stirrer was charged with 10.0 g (0.078 mol) of hydrazine sulfate, and 300g of Polyphosphoric acid (PPA). The mixture was then heated up to 80 °C and 19.7g (0.156 mol) of 4-hydroxy-benzoicacid added.The temperature of the mixture was increased up to 125 °C and the reaction continued at this temperature for 8 h. The resulting mixture was kept at 140 °C for 2 h to ensure ring closure reaction. The resulting mixture was ¬nally poured into 1 L distilled water to precipitate out the product. The solids were ¬ltered and then dried in a vacuum oven. Recrystallization from methanol gave monomer 3c as white crystals. The yield of the purified product was 12.3 g (55.7 %),m.p=207°C
1HNMR (400 MHz, DMSO-d6): 10.32(s, 2H, OH); 7.86-7.90(d, 2H, Ar-H);7.46-7.51 (t, 2H, Ar-H); 7.09-7.13(d, 2H, Ar-H); 7.02-7.08(t, 2H, Ar-H).
13C NMR (400 MHz, DMSO-d6): 162.78, 156.42, 133.50, 128.82, 119.78, 117.12 and 109.46.
126.96.36.199 2,2'-Bis(3,4-dicyanophenoxy) 1,3,4-oxadiazole (4c)
In a 250 mL flask, equipped with a nitrogen inlet, 4.0 g (0.015 mol) of 3c, 5.44 g (0.03 mol) of 4-nitrophthalonitrile (2), and 4.8 g (0.03 mol) of K2CO3 were suspended in 100 mL of DMF and stirred at 60 °C for about 5 h. The reaction mixture was poured into 500 mL of water, and the precipitated solid was collected and washed thoroughly with methanol and water. The yield of the product was 7.00 g (89.8%). mp 265.70 °C.
FTIR (KBr, cm -1 ): 2232 cm-1 () and 1252 cm-1(C-O-C).
1H NMR (400 MHz, DMSO-d6): 8.02-8.09(m, 4H, Ar-H); 7.79(s, 2H, Ar-H);7.72-7.79(t, 2H, Ar-H); 7.51-7.57(t, 2H, Ar-H); 7.36-7.41(m, 4H, Ar-H).
13C NMR (400 MHz, DMSO-d6): 161.69, 161.42, 151.62, 136.79, 134.59, 130.91, 127.40, 123.42, 122.97, 122.43, 117.18, 116.93, 116.34, 116.82, and 108.96.
2.4 DSC studies and curing additive
Cure studies were performed on 3a, 3b and 4c by DSC analysis using 5 mol% p-BAPS. The compositions were thoroughly mixed in powdered form under ambient conditions. Samples were evaluated from room temperature to 400°C at a heating rate of l0°C /min under nitrogen atmosphere.
2.5 TGA studies
The thermal properties were obtained using TGA analyses of 3a and 3b and 4b prepared in a TGA chamber by heating with 5 mol% of p-BAPS at 275°C/5h, 315°C/5h, and postcured for an additional 5 h at 375°C under a ¬‚ow of air. Measurements up to 900 °C were made at a heating rate of 10°C /min under nitrogen/air atmosphere.
2.6 Rheometric studies
The complex viscosity of phthalonitrile polymers was monitored at 270 °C in air as a function of time. The complex viscosity of 4c was also studied as a function of temperature.
DMA studies were carried out the on samples of 3a, 3b and 4c containing 5 mol% of p-BAPS in nitrogen over the temperature range of 25-500 °C. An oscillatory temperature ramp of 5 °C min-1 was used to determine storage modulus (E') and damping factor (tan δ) with a fixed strain of 10% and a fixed angular frequency of 100 rad/s.
3. Results and discussion
The syntheses of aromatic/heterocyclic ether linked phthalonitrile monomers based on biphenyl, catecoal and 1,3,4-oxadiazole were obtained by base-catalyzed nitro displacement of 4-nitrophthalonitrile with 1a, 1b and 2c respectively (Scheme 1,2). The reactions were carried out in dry aprotic solvents like DMSO and DMF. In case of 3a, the water, formed as a by-product during the formation of the dialkaline salt of 1a, was removed by azeotropic distillation with benzene before the addition of 4-nitrophthalonitrile.While compounds 3b and 4c, were obtained by stirring the mixture of corresponding diols with 4-nitrophthalonitrile in the presence K2CO3 at room temperature and 60°C, respectively. The entire phthalonitrile end capped products were obtained in 70-89% yield.
Scheme 1. Synthesis of 3a and 3b.
FTIR and NMR techniques were used to identify the structures of 3a, 3b and 4c. The FTIR spectra of 3a, 3b and 4c showed absorption bands at 1200-1220 cm-1 assigned to C-O-C linkage and the disappearance of the nitro (1540 and 1355 cm-1) and hydroxyl (3430 cm-1) absorption bands attributed to the reactants.1HNMR and 13CNMR spectra further con¬rmed the structures. Assignments for all the protons were in complete agreement with the proposed molecular structures.
Scheme 2. Synthesis of 3c and 4c
The Polymerization reactions of 3a, 3b and 4c were studied by DSC analysis up to 400 °C in the presence of 5 mol% of p-BAPS [12,32] as shown in the Fig.1 and Table. In addition to endotherm for each monomer, a small exothermic transition peak is shown for each monomer attributed to melting and to the reaction with p-BAPS, respectively. However, the thermogram for 4c, showed only melting point peak (265.7°C) and no reaction peak, probably due to insufficient time or very low reactivity. For 3a, the reaction temperature is at about 264°C; whereas the reaction temperature is shifted to lower temperature at about 243°C for 3b. This might be due to the reactivity of monomers towards p-BAPS. The monomer 3a, with bulky size made it more difficult for the curing additive to find the ends and to continue the reaction compare to 3b having a small size.
Figure 1. DSC scans of phthalonitrile monomers cured with 5 mol % of p-BAPS
Complex viscosity changes during polymerization reactions were studied by performing isothermal rheometric measurements at 270°C on prepolymers formed in the presence of 5 mol% p-BAPS. Typical melt viscosity vs. time plots for prepolymers presented in Fig.2 (a).The data revealed that 3b has higher tendency to cure than 3a, and in turn higher than 4c.This order may also be related to the fact that the monomer 3a with bulky size have slow polymerization rate than 3b.There was no change in viscosity of 4c, with oxadiazole linking unit, up till more than one hour. This might be attributed to the rigid structure of 4c. Since, the rheological behavior of a material is a key factor in determining its processability. Therefore, the variation of the melt viscosity of blend 4c was determined as a function of temperature at the rate of 4°C /min. The results presented in Fig.2 (b), showed that the complex viscosity of 4c increased at 380°C.
Figure 2. (a) Complex viscosity (n*) at 270 °C as a function of time of phthalonitrile monomers cured with 5 mol % of p-BAPS (b) Complex viscosity (n*) of 4c as a function of temperature cured with 5 mol % of p-BAPS
Many polymers containing orthro-linked units are relatively unstable to thermal decomposition . Therefore, it is of interest to learn the influences of incorporating the ether-linked, into orthro-linked unit structures that otherwise are well-known for their thermal stability. The thermal stability of thermosetting phthalonitrile polymers that had been postcured in the presence of 5 mol% of p-BAPS was detected with TGA at a heating rate of 10 °C min-1, in air/inert atmospheres between 25 and 900°C as shown in the Figure.2. All the three polymers were cured at 275°C/5h, 315°C/5h, and postcured for an additional 5 h at 375°C. All of them showed different behavior due to different skeleton structures. Polymers derived from 3a, 3b and 4c retained 95 wt % at 444, 489.6 and 511.8°C and exhibited char yields of 64.37, 67.56, and 67.86%, respectively, upon pyrolysis to 900°C under an inert atmosphere. While heating in air showed weight retention of 95% at 436.5, 488, and 500°C, respectively, with catastrophic decomposition occurring between 600- 800°C as shown in the table. The higher thermal stability of the polymer derived from 4c may be due the presence of heterocyclic i.e. oxadiazole ring. It has been shown by comparing the T5 of the polymers that the introduction of the oxadiazole is especially e¬€ective to improve the thermal stability. This would be the contributions of polarity and rigidity of oxadiazole ring. These results show that 3b and 4c possess excellent thermal stability and can be used for making heat-resistance high performance composites.
Figure 3. TGA of phthalonitrile monomers cured with 5 mol % of p-BAPS under Nitrogen and air atmospheres
Figure 4. Storage moduli and tan d of phthalonitrile monomers cured with 5 mol % of p-BAPS under Nitrogen for 5h (a) 3a at 375°C (b) 3b at 375°C (c) 4c at 375°C (d) 4c at 400°C
The dynamic mechanical analyses on 3a, 3b, and 4c cured with 5 mol% of p-BAPS to maximum temperature of 375 °C for 5 h, were evaluated up to 500 °C in a nitrogen atmosphere, to estimate the glass transition temperature of the cured polymers. The 4c was also cured at 400 °C for 5 h. The storage moduli and damping factors for the samples are presented in figure Fig.4. The measurements showed different dynamic mechanical properties of the polymers depending on different chemical structure under same condition of temperature treatment. The 3a, heated to 375 °C only, showed a peak in the tan δ curve with the decrease of storage modulus as the temperature increased (Fig.4 (a)). The peak of tan δ was identified as the glass transition temperature (Tg) as shown in the Fig.4 (b) and Table, exhibited tan δ peak for 3b cured to 375°C, approximately at 500°C with a gradual decrease in storage modulus up to 400°C.The 4c, cured to 375°C, showed different behaviour than usual as shown in Fig.4 (c),which exhibited tan δ peak approximately at 350°C with a gradual increase instead of decrease in storage modulus up to 450°C.It probably due to incomplete curing. After curing to 400°C for 5h, the storage modulus and tan δ curves appeared nearly featureless up to 500°C (Fig.4 (d))
Table. Properties of the synthesized polymers
Curing temperature (0C) (DSC)
The curing, thermal and rheometric measurement of the synthesized monomers were investigated using p-BAPS curing reagent. The rate of polymerization of 3b is higher than 3a and in turns than 4c. All the three polymers showed good thermal and thermo-oxidative stability; especially 4c.Fully cured polymers 3b and especially 4c did not exhibit glass transition temperature, almost with out any change in storage modulus and maintains structural integrity, at elevated temperatures and offer promise as matrix for composite applications. However high melting point of 4c limits its applications.