Polymer Journal, Vol. 24, No. 12, pp 1429-1436 (1992)

Synthesis and Properties of Novel p-Aramid Including Pyrazine

Shigeyuki UETA, Yoshihiro FuKUDA, Keiko KoGA*, and Motowo TAKAYANAGI

Department of Industrial Chemistry, Faculty of Engineering, and *Advanced Instruments Center, Kyushu Sangyo University, Matsukadai 2-3-1, Higashi-ku, Fukuoka 813, Japan

(Received June 8, 1992)

ABSTRACT: Poly[p-phenylene(2,5-di-p-carbonylphenylpyrazine)amide] (PPPA) was synthe­ sized by the polycondensation of 2,5-di-p-chloroformylphenylpyrazine with p-phenylenediamine. Dicarboxylic acid was prepared by dimerization of p-cyanophenacyl bromide. Molecular characterization was carried out by IR and NMR. PPPA is soluble in sulfuric acid, forming liquid crystals. The temperature at the lOwt% loss in TGA was 54SOC. The fiber spinning was difficult due to the strong tendency to crystallization and rapid relaxation from liquid crystalline state. From the X-ray diffraction pattern ofuniplanar oriented film ofPPPA, approximate lattice constant was evaluated as a=7.94A, b=5.23A, and c=20.6A (fiber axis). Fiber period agreed with the identity period for all trans conformation of PPPA. Tensile modulus along the molecular axis was estimated as 21(}--220 GPa by referring to the theoretical moduli of poly(p-phenylenetere­ phthalamide) and poly(p-phenylene). KEY WORDS Poly[p-phenylene(2,5-di-p-carbonylphenylpyrazine)amide] I p-Aramid I Pyrazine Ring I Decomposition Temperature I X-ray Crystal Structure I Tensile Modulus I

Poly(p-phenyleneterephthalamide) (PPT A) PPTA was found to make the polymers is a rigid rod-like molecule and gives Kevlar insoluble in sulfuric acid. fiber with ultrahigh modulus and strength Bizzarri and others1 •2 reported the solid state through liquid crystal spinning of a sulfuric polymerization of 4-amino-4' -carboxy-p-ter­ acid solution of PPTA. For improvement of phenylene and its thermal stability. Li and the functions of Kevlar fiber, new types of others3 synthesized novel aramids by low p-aramid have been explored in various ways. temperature polycondensation of p-terphenyl­ For example, to introduce p-polyphenyls such ene diamine and its dicarboxylic acid chlorides asp-biphenyl or p-terphenyl group in the main and reported their properties. According to chain of p-aramid is expected to exceed PPTA them, increase in the number of p-phenylene in the stiffness and heat endurance. The in the main chain lowered the solubility in increase of the fraction of poly(p-phenylene) sulfuric acid and the spinning of fiber from segments in the fiber period of the p-aramid sulfuric acid was impossible. Thus, so far there increases the tensile modulus along the has been no successful synthesis of p-aramids molecular axis compared with that of PPTA, composed of bi- or terphenyl groups, which since the stiffness of poly(p-phenylene) is are soluble in sulfuric acid. It is necessary to higher than that of PPTA as mentioned in later find out a new p-aramid, which includes part. In spite of these expectations, the p-terphenyl group as a stiffening component, introduction of wholly aromatic p-biphenyl or but at the same time soluble in sulfuric acid terphenyl group into the molecular skeleton of for processing.

1429 . UETA et al.

In this paper, 4,4"-dicarboxy-p-terphenyl p-Cyanophenacyl bromide (1) was synthe­ including pyrazine ring was synthesized, with sized by bromination of 4-acetylbenzonitrile. which the p-aramid prepared by polycondensa­ Bromination was carried out in a mixed tion with p-phenylenediamine was obtained. solution of dichloromethane and methanol The pyrazine ring has in addition (10: 1) and white plate-like crystals were to a tertiary amine nature. Replacement of the obtained. mp 92°C, yield 98%. IR (KBr): 3100 aromatic ring by pyrazine ring will bring no (vCH,), 2230 (vcN), 1710 em - 1 (vc=o). 1H NMR effect on the molecular stiffness of the p-aramid. (CDC1 3) () 4.45 (s, 2, CH2), and 7.95 ppm ( q, atoms in pyrazine rings have 4, ring). nucleophilic properties for sulfuric acid, which 2,5-Di-p-cyanophenylpyrazine (2) was syn­ are expected to make new aramid molecules thesized by dimerization of 1. N,N-Dimethyl­ soluble in sulfuric acid. The aramid composed acetamide (DMAc) was cooled to -10°C and of p-terphenyl, one of which rings is pyrazine, saturated with , to which a solution has been found soluble in sulfuric acid as of 1 in DMAc was added dropwise to dimerize. expected. The spinning of fiber using this The solution was left overnight, heated at 90oC aramid was difficult at present due to rapid for 6 h, to which hydrogen peroxide and crystallization and orientation relaxation from sodium hydroxide were added. The precipitate the liquid crystalline state, but useful informa­ was recrystallized with N,N -dimethylform­ tion on this polymer was obtained from amide and white needle like crystals were uniaxially oriented film. The X-ray crystal obtained. mp 385°C, yield 22%. IR (KBr): 2230 structure of the oriented film revealed that the (vcN), 1480 em - 1 (pyrazine ring). molecular conformation in crystal was all 2,5-Di-p-carboxyphenylpyrazine (3) was pre­ trans-conformation as found in the PPTA pared by refluxing 2 in a mixed solution of crystal. This information made evaluation of H 2 SOcH2 0-CH3COOH (5: 5: 4) at 110- tensile modulus of this new polymer possible, 120oC. White crystal powder was obtained. mp which suggested higher modulus value than 480°C, yield 98%. IR (KBr): 3500 (v0H), 1690 PPTA. Other properties such as heat endurance (vc=o), 1480 em - 1 (pyrazine ring). 1H NMR and phase diagram of liquid crystal are also (D2 S04 ) () 7.96 (q, 8, benzene ring), 9.48 (s, 2, reported here. pyrazine ring), and 10.32 ppm (s, 2, COOH). 2,5-Di-p-chloroformylphenylpyrazine (4) EXPERIMENTAL was prepared by dissolving 3 in thionylchloride, adding as a catalyst and refluxing Synthesis of Aramid Monomers upon heating. White needle like crystal was Scheme 1 shows the route of synthesis. obtained. mp 295°C, yield 98%. IR (KBr): 1710

Bromination Dimerization NC--o-Q-0-cN Br2 NH3 /DIIAc 1 2

Hydrolysis Chloroformylation --o-Q-0-c HOOC --o-Q-0-cDOH _....:_..:._:__:._:_:..c..:.:.:., C I OC OC I H+ SOCI2 3 4

_L_o_w _T_em..:..p_. _P_o ...:.1y_me_r_i z_a_t _i _LH._t=\ ..H Q F\ fr=\ F\ HMPA/LiCiz, -lO"C 5 Scheme 1.

1430 Polym. 1., Vol. 24, No. 12, 1992 p-Aramid Including Pyrazine Ring

(vc=o), 1480 em - 1 (pyrazine ring). RESULTS AND DISCUSSION

Preparation of p-Aramid Results of Characterization of P P P A p-Aramid was prepared by the method of Figure 1 shows IR absorption spectra of Bair and others.4 p-Phenylenediamine was PPP A and PPTA. The N-H stretching band dissolved in a solution of hexamethylphos­ of 3350 em- 1 is found in common for both phoric triamide (HMPA) and lithium chloride PPPA and PPT A. Amide I, II, and III are in a nitrogen atmosphere, to which a solution found at 1650, 1550, and 1310 cm- 1 , acid chloride solution 4 was added at - lOoC respectively. The wave number of 850 em - 1 is with stirring. Then it was warmed to 50°C by the absorption of p-substituted phenylene. The taking 30 min. After washing the polymerizate absorption of pyrazine ring is found at 1480 with water, ethanol and acetone, and drying in em- 1 only for PPPA. vacuum, a yellow powder of poly[p-phenyl­ Figure 2 shows 13C NMR spectrum of PPPA ene(2,5-di-p-carbonylphenylpyrazine)amide] in deuterized sulfuric acid. Two absorptions (PPPA) (5) was obtained. The method of Akzo 5 was also tried.

Characterization Methods ofp-Aramid(PPPA) Inherent viscosity was measured by dissolv­ ing 0.5 g dl- 1 of p-aramid in 98% sulfuric acid Q) (.) and using a Ubbelohde viscometer at 25°C. c .....ctl Infrared absorption spectrum was measured on ..... a Nihonbunko A-202 infrared spectrometer by E (/) use of KBr method. A Rigaku DSC-8230 c ctl.... Differential Scanning Calorimeter was used t- with a nitrogen purge. Thermogravimetric analysis was carried out on the instrument of the same company, Model TG-8110, with 4000 2000 1500 1000 500 heating rate of lOaC min - 1 to the temperature as high as 600°C in nitrogen atmosphere. A Wave number /cm- 1 Rigaku Geiger Flex RAD-III A X-ray dif­ Figure 1. Infrared spectra of (a) PPPA and (b) PPTA. fractometer was employed for recording the diffraction intensity curves. An X-ray photo­ graph was taken using a Rigakudenki X-ray H a H c b d

(D2 S04 ). Chemical shifts were measured with tetramethylsilane as a reference. 180 160 140 120 13 C Chemical shift 5/ppm

Figure 2. 13C NMR spectrum of PPPA in D 2 S04 .

Po1ym. J., Vol. 24, No. 12, 1992 1431 S. UETA et a/. around 130 ppm are the carbon atoms in Figure 4 shows the phase diagram as a benzene ring. A comparison with the NMR function of temperature and concentration for spectrum ofPPTA and the absorption intensity PPPA in 97% sulfuric acid prepared by confirmed that the aromatic ring bonded to the observing the birefringent image under polar­ nitrogen atom of amide group denoted by (a) ization optical microscope under crossed is located at 126.9 ppm and the aromatic ring polars. PPPA dissolves in sulfuric acid very connected to the carbonyl carbon denoted by easily owing to the proton-philic properties of (b) is at 132.0ppm. The carbon atom of nitrogen atoms in pyrazine ring. Liquid crystal carbonyl group denoted by (c) is located at formation starts at room temperature at about 172.1 ppm. The 2,5-carbon atoms of pyrazine 8 wt% PPPA. The range of liquid crystal ring (d) are at 144.5 ppm and the 3,6-carbon formation has been narrowed in comparison atoms (e) at 150.7ppm. These results together with IR spectra confirm the molecular structure

of PPPA to be as designed. Elemental analysis Isotropic of PPPA showed C: 71.34 (73.46), H: 4.29 phase (4.11), and N: 14.03 (14.28), where the value p 150 in the parenthesis is theoretical. The inherent Q) -.... viscosity was 1.26--2.05 dl g- 1 in sulfuric acid. :::3 ..... 100 Figure 3 shows the TGA and DSC curves ro.... Q) measured in nitrogen atmosphere for PPPA(a) c. E and PPTA(b). The lOwt% weight loss in TGA Q) 50 curves was 545oC for PPPA and 530°C for t- PPTA. DSC curves show two endotherm phase peaks: the peak around lOOoC is ascribed to the vaporization of absorbed water and the one 0 5 10 15 20 around 560oC is to the decomposition ofPPPA. Concentration /wt% The thermal properties ofPPPA resemble those Figure 4. Phase diagram of a solution of PPPA in 97% ofPPTA. sulfuric acid.

a 0 <#e

b (/) 10 -(/) 20 0 ..... >- 30 ..... tlO b (/) Q) c .....Q) a c

0 "'C c w

0 200 400 600 15 20 25 30 Temperature rc 2 e /deg. Figure 3. TGA and DSC curves of (a) PPPA and (b) Figure 5. X-Ray diffraction intensity curves of (a) PPPA PPTA. and (b) PPT A.

1432 Polym. J., Vol. 24, No. 12, 1992 p-Aramid Including Pyrazine Ring

with PPTA. PPPA tends to crystallize more crystallization and rapid relaxation from the rapidly than PPTA. The increase in molecular liquid crystalline state. The molecular weight stiffness by introduction of terphenylene rings of PPPA was too small to permit spinnability. might accelerate the rate of crystallization of The spinning apparatus employed for PPPA PPPA, in comparison with PPT A. was the same as used for PPTA, but the dope Figure 5 shows the X-ray diffraction of PPPA was coagulated at the tip of spinneret. intensity curves of powder samples for (a) Instead of spinning fibers, uniaxially oriented PPPA and (b) PPTA. The intensity peaks thin film was prepared by shearing a 16wt% located at scattering angles at 20.2°, 22S, PPPA solution in sulfuric acid between glass and 27.8° for PPPA are comparable to the plates at sooc and rapidly throwing in excess identified scatterings of PPTA, the indices of aqueous ammonia solution as a coagulant. which are (110), (200), and (004), respectively. Figure 6 shows the polarization optical Resemblance in relative diffraction intensity micrograph with crossed polars of uniaxially suggests the packing of main chains in the unit oriented film ofPPPA prepared by the method lattice to be similar for PPPA and PPTA. mentioned above. A strongly colored bire­ fringence image can be seen. Evaluation of Crystal Lattice of P P P A The X-ray diffraction diagram of the mat of The fiber formation of PPPA from a sulfuric the PPPA films annealed at 450oC for 10 s acid solution was unsuccessful due to rapid showed only Debye rings when the X-ray beam was irradiated perpendicular to the film surface. The diffraction intensities in the diagram of edge view were concentrated on the equator and meridian as shown in Figure 7. This means that the c-axis (fiber axis of PPPA) orients parallel to the film surface, which corresponds to the uniplanar orientation. This diffraction pattern closely resembles the edge view pattern ofPPTA film. In the case ofPPTA film, it was proved that the hydrogen bonded plane is the be-plane and parallel to the film Figure 6. Polarization optical micrograph under crossed surface. 6 This gives in the edge view diagram polars of oriented PPPA film. the separated four-point reflections of (110),

X-ray

Figure 7. X-Ray diffraction pattern and schematic diagram of PPPA film annealed at 450°C. X-Ray beam is irradiated from the film edge.

Polym. 1., Vol. 24, No. 12, 1992 1433 S. VETA et al. the diffractions corresponding to which are found in the edge view diagram of PPPA. The (200) reflections are also found in common in PPTA and PPPA diagrams. Referring to the known indexing in the case of PPTA crystal, 7 estimation of crystal lattice constants of PPP A crystal was carried out on the X-ray diffraction pattern in the edge view of PPPA film. As mentioned before, the spinning of PPPA in fiber form was impossible. Precise analysis •<( using a fiber diagram was given up. The lat­ M tice constants thus evaluated on the film are a=7.94A, b=5.23A, and c(fiber axis)= 20.6± 1.6A. The number of repeating units of PPPA in unit lattice, Z, was 1.904 after density determination by the following equation.

where Vis the unit cell volume, a·b·c, M 0 is the molecular weight of the repeating unit of PPPA, NA is Avogadro's number, and pis the density. The density of PPPA film was evaluated as 1.402 gem- 3 using the floating Figure 8. Schematic diagram of conformation of PPPA method. Assuming the number of chain unit molecule in crystal. per unit lattice to be two, the theoretical density of PPPA crystal is 1.472 g em- 3 . Taking into stiffer than one phenylene ring as seen in account error in evaluation of fiber period, two terephthaloyl group in PPTA. When the same chains per unit lattice are an acceptable value. displacement of main chain is given to PPTA All trans conformation of PPPA gives the and PPPA along the molecular axis, and calculated fiber period of 21.3 A. Figure 8 conformations of PPTA and PPPA are the shows the trans-conformation of PPPA mole­ same (all trans), distortion of the bond angles cule and bond distances and bond angles used associated with the amide groups is the main in calculation of fiber period. If the calculated contributor to deformation. The same number period of 21.3 A is employed for theoretical of bond angles are found per repeating unit of calculation of crystal density, the density of PPTA and PPPA. This relation results in 1.424 gem- 3 is obtained, which is closer to the increased modulus in PPPA in comparison with observed density of 1.402 g em- 3 . PPTA. The problem is that wholly aromatic p-aramid, poly(p-phenylene-p-terphenylene­ Approximate Evaluation of Theoretical Fiber amide) is insoluble in sulfuric acid. To over­ Modulus of PPPA come this difficulty, the pyrazine ring was in­ The synthesis of PPPA was done for troduced into p-terphenylene group in this improving the fiber modulus of PPTA by paper. The problem of solubility in sulfuric introducing the p-terphenylene rings in the acid has been solved and to evaluate the mod­ main chain of p-aramid. p-Terphenylene rings ulus of PPPA in a more strict way is a mat­ form virtual bonds, which make the main chain ter of primary concern.

1434 Polym. J., Vol. 24, No. 12, 1992 p-Aramid Including Pyrazine Ring

When the molecular structure of the The fiber period of PPPA, c, calculated by repeating unit of PPPA shown in Figure 8 is eq 1 is 21.5 A. The observed value is 20.6 A compared with that of PPTA, the main and the calculated value is 21.3 A. The effect difference lies between the p-phenylene ring in caused by inclination of the terphenylene group terephthaloyl group in PPTA and the 2,5- to the molecular axis is negligible at these diphenylpyrazine group in PPPA. By replacing approximations. the pyrazine ring with the phenylene ring, the If the method of polycondensation of PPPA modulus of PPP A is approximated by the is improved to give enough spinnability, the modulus of on imaginary chain composed of fiber modulus ofPPPA is expected to be around the PPTA unit and the biphenylene (BP) unit 210-220 GPa, which is higher than that of connected in series. The fiber period of PPT A PPTA, 130-180 GPa. is given in the literature 7 as 12.9 A and the In conclusion, we prepared novel p-aramid modulus ranges from 182 GPa 8 to 200 GPa. 9 including 2,5-diphenyl- pyrazine group, PPPA, The fiber period ofBP was estimated as 8.564 A which is soluble in sulfuric acid to form liquid using the bond distances and bond angles given crystals. This is important for processing in the data handbook. 10 The modulus of BP p-aramid. Its inherent viscosity remained at is the same as that ofpoly(p-phenylene) (PPP), about 2 dl g- 1 and wet spinning of PPPA from which has been calculated by Kobayashi 11 as the sulfuric acid dope was impossible. X-Ray 275GPa. diffractometry on the oriented film of PPPA The molecule of PPPA (its properties shown revealed close similarity in conformation and with no suffix) is approximated by the chain packing in the unit cell to that of PPTA. mechanical model of PPTA (suffix 1) and BP Evaluation of tensile modulus along the fiber (suffix 2) connected in series: the modulus (£), axis of PPPA amounted to 210-220GPa, fiber period (c) and its displacement (Ac) of which is higher than that of PPTA. The in­ PPPA under tensile stress (a-) are derived by troduction of a high modulus component of the following relations. poly(p-phenylene) was effective for raising the modulus. Heat-endurance of PPPA was c=c +c (1) 1 2 comparable or slightly superior to PPTA. Thus,

Ac=Ac1 +Ac2 (2) our p-aramid including p-pyrazine in terphenyl is one of the applicants of p-aramid fibers E=CJ/(Acjc)=CJ/[(Ac 1 +Ac2)/(c 1 +c2)] (3) improved in its mechanical and thermal properties, without sacrificing any merit of PPTA.

Substitution of eq 4 into eq 3 gives Acknowledgments. This work was partly supported by the Grant-in-Aid from the 1 ( c1 ) 1 ( c2 ) 1 5 E= C1 +c2 £:+ C1 +c2 £; ( ) Ministry of Education, Science, and Culture of Japan. The authors also express sincere thanks The data of c1 = 12.9 A, £ 1 = 182 GPa, c2 = to Professor H. Taniguchi, Faculty of Engi­ 8.564 A, and £ 2 = 275 GPa are substituted, the neering, Kyushu University, for his teaching modulus of PPPA, E, is estimated as 210 GPa. the synthesis of raw materials of PPPA. If £ 1 is 200GPa, E amounts to 224GPa. If £ 2 is underevaluated, E will increase to 250 GPa. REFERENCES The upper limit is at the value lower than

300 GPa which is calculated on the assumption I. P. C. Bizzarri, C. D. Case, and A. Momaco, Polymer, of no deformation of terphenylene group. 21, 1065 (1980).

Polym. J., Vol. 24, No. 12, 1992 1435 S. UETA et al.

2. P. C. Bizzarri, C. D. Case, and A. Momaco, Polymer, Appl. Polym. Sci., 23, 903 (1979). 22, 1263 (1981). 7. M. G. Northolt, Eur. Polym. J., 10, 799 (1974). 3. z. B. Li and M. Ueda, J. Polym. Sci., Polym. Chern. 8. K. Tashiro, M. Kobayashi, and H. Tadokoro, Ed., 22, 3063 (1984). Macromolecules, 10, 413 (1977). 4. T. I. Bair, P. W. Morgan, and F. L. Killian, 9. G. S. Fielding-Russell, Text. Res. J.,41, 861 (1971). Macromolecules, 10, 1396 (1977). 10. "KAGAKU-BINRAN," Kiso-Hen II, 3rd ed, 5. Akzo, Japanese Patent Specification 1976-109099. Maruzen, Tokyo 1984, 11-659. 6. K. Haraguchi, T. Kajiyama, and M. Takayanagi, J. 11. M. Kobayashi, Kobunshi, 36, 346 (1987).

1436 Polym. J., Vol. 24, No. 12, 1992