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Mechanism and Kinetics of Stabilization Reaction of Polyacrylonitrile and Related Copolymers II

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Mechanism and Kinetics of Stabilization Reaction of Polyacrylonitrile and Related Copolymers II Polymer Journal, Vol. 29, No.4, pp 353-357 (1997) Mechanism and Kinetics of Stabilization Reaction of Polyacrylonitrile and Related Copolymers II. Relationships between Isothermal DSC Thermograms and FT-IR Spectral Changes of Polyacrylonitrile in Comparison with the Case of Acrylonitrile/Methacrylic Acid Copolymer Hideto KAKIDA t and Kohji TASHIRO* Central Technology Research Laboratories, Mitsubishi Rayon Co., Ltd., Otake, Hiroshima 739-06, Japan *Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560, Japan (Received October 9, 1996) ABSTRACT: The relationship between thermal behavior and structural change has been studied by an organized combination of isothermal DSC thermograms and FT-IR spectra measured for the stabilization reaction of polyacrylonitrile (PAN) in comparison with the case of acrylonitrilelmethacrylic acid (ANIMAA) copolymer. The isothermal DSC exothermic thermogram of PAN is much broader and the structural changes proceed much more slowly than the case of ANIMAA copolymer. On the stabilization ofPA N, at first, some nitrile groups change into amide groups, which initiate the dehydrogenation and cyclization reactions. This dehydrogenation reaction proceeds rather faster than the cyclization. The comonomer MAA was considered to accelerate the dehydrogenation reaction more effectively than the cyclization reaction. KEY WORDS Polyacrylonitrile I Acrylonitrile-Methacrylic Acid Copolymer I Carbon Fiber I Stabilization I Differential Scanning Calorimetry I Fourier-Transform-Infrared Spectra I Polyacrylonitriles (PAN) containing acidic comono­ exothermic peak at an early stage of the reaction, mers are the most commonly used precursors for carbon followed by a broad and "large" exothermic peak. This fibers because of the excellent performance ofPAN-based behavior is quite contrast to the case of the AN/MAA carbon fibers. 1 copolymer, in which almost only a sharp and "large" Production process of PAN-based carbon fibers in­ exothermic peak can be observed at an early stage. volves the stabilization of oriented acrylic fibers and the The present study has therefore been undertaken so carbonization of them at an elevated temperature. 2 as to clarify the difference of stabilization mechanisms Stabilization is the most important process which between PAN and the AN/MAA copolymer through our converts acrylic fibers to infusible and nonflammable new method combining isothermal DSC thermograms fibers by heating at 200-300oC for about one hour in and FT-IR spectra. an oxidative atmosphere. The thus stabilized fibers can be heated up to the carbonization temperature (1000- EXPERIMENTAL 20000C) in an inert atmosphere. 2 To accelerate the stabilization reaction, several co­ Materials monomers such as methacrylic acid (MAA), acrylic acid PAN and the AN/MAA copolymer (CPl) were synthe­ (AA), methacrylate (MA), and acrylamide (AAM) are sized by an aqueous suspension polymerization method often copolymerized into PAN. But, the role of the co­ with redox initiator. The composition of AN/MAA monomers on the kinetics of the stabilization reaction copolymer was AN: MAA = 98.5: 1.5 in weight ratio and the mechanism of the stabilization reaction itself are in feed. The weight-averaged molecular weights of these not so clear yet now. 2 polymers were about 2 x 10 5 . The obtained two kined In order to solve these problems of the stabilization of polymers showed the same triad tacticity as mm = of PAN and related copolymers, we have developed a 0.27, mr=0.50, and rr=0.23 when evaluated by means new technical idea to combine the experimental data of of 13C-NMR. isothermal DSC thermograms with FT-IR spectra. In The polymer powders filtered through a 400 mesh the previous paper, 3 we revealed the intimate relationship screen were supplied to the DSC and FT-IR measure­ between the thermal behavior and the structure changes ments. by such an organized combination of isothermal DSC thermograms and FT-IR spectra measured for the DSC and FT-IR Measurements stabilization reaction of an AN/MAA copolymer, and The isothermal DSC and FT-IR measurements were also found for the first time the induction period of the carried out on the basis of the methods described cyclic structure formation. previously. 3 The infrared spectra were measured at room After that work, 3 the isothermal DSC thermo grams temperature for the samples quenched from the various of PAN homopolymer were found to be very different points of the DSC thermograms as seen in Figure 5, for from those of an AN/MAA copolymer. The isothermal an example. DSC thermogram of PAN shows a sharp but "small" t To whom correspondence should be addressed. 353 H. KAKIDA and K. TASHIRO RESULTS AND DISCUSSION increased. These nearly flat regions should be considered as the induction periods of the stabilization reactions, Isothermal DSC Measurements and were observed for the CPl as reported in a previous Figure 1 shows the isothermal DSC thermograms of paper. 3 In Figure 4, the Arrhenius plot is made for the the PAN and the CP1 measured at 240aC under air flow. induction periods obtained at 215, 230, 250, and 260°C, The scales of the ordinate are different between these where the induction period ti is defined as the standing two samples; the thermogram of the PAN is expanded point of the thermal evolution (see Figure 3). From this longitudinally about 100 times larger than that of the plot an activation energy was evaluated to be ca. 22 CPl. The isothermal DSC thermograms of the PAN are kcal mol- 1 . This value is about the same with that of very different from those of the CPl. The thermograms the CPl. of the CP1 show apparently one sharp exothermic peak only at the early time region. On the other hand, the Structural Changes during Exothermic Reactions thermogram of the PAN consists of a small peak ap­ The molecular structural changes of the PAN homo­ pearing at an early period and a broad peak at a later polymer which occur during the isothermal heat evolu­ period, i.e., the stabilization reactions of the PAN seem tion process in DSC were investigated by means of to be composed of the rapid and slow exothermic proc­ esses, while that of the CPl contains only the rapid ex­ 0.1 othermic processes. In Figure 2, the isothermal DSC thermogram and the PAN total amount of heat evolved during the stabilization process are compared between the PAN and the CPl. The total amount of evolved heat was evaluated by = 3290Jig drawing the base line horizontally, as indicated by a t 0 broken line, at the heat flow level of the induction period. The existence of the induction period in the stabilization reaction was found for the first time as described in the 0 250 500 previous report. 3 The ordinate of Figure 2(a), is about 200 times largely expanded compared with that of Figure TIME I min 2(b). It can be seen that the rate of heat evolution during the stabilization of the PAN is much lower than that of 20 the CPl. It is also noteworthy that almost the same amount of heat, 3290 and 3260 J g- 1 , are evolved for the PAN and the CP1 during the stabilization process, CP1 respectively, although the reaction behavior and the rate of reaction are much different from each other. Com­ 3260Jig t paring Figure 1 with Figure 2, the comonomer MAA is 0 considered to play an important role in accelerating the slow exothermic process of the PAN homopolymer without any change in the amount of heat evolution. 0 200 400 Figure 3 shows the isothermal DSC thermograms TIME I min measured at various temperatures under air flow for the PAN homopolymer. In the early regions of these Figure 2. Isothermal DSC thermograms and total amount of heat evolved during stabilization at 240°C for PAN and AN/MAA co­ thermograms there are nearly flat region and the period polymer. of the region becomes shorter as the temperature is 250'C / / """ ' PAN I "' I t CP1 215'C induction period, ti 0 150 300 0 2.5 5 7.5 10 TIME I min TIME I min Figure 1. Isothermal DSC thermograms of PAN and AN/MAA Figure 3. Isothermal DSC thermograms of PAN measured at several copolymer measured at 240oC under air flow. temperatures under air flow. 354 Polym. J., Vol. 29, No.4, 1997 Stabilization of PAN and Copolymers II. PAN original 10 ;:;-- 0 0 ,...; small peak, 14 min X ct: ,...; 1 1.9 2.0 2.1 peak, 200 min 1 I T X 103(KI) Figure 4. The Arrhenius plot of the induction period (t;) measured at 215, 230, 250, and 260°C for PAN. 6 (CHI C==C-H 1' D 200' end, 1000 min PAN E c 1000' B 4000 2000 1500 1000 500 3' 20" CP1 Wavenumber cm- 1 A Figure 6. TheIR spectra of PAN which were observed at the points indicated by 0 marks shown in Figure 5. 1' 30" 60' 400' CPl original TIME (a.u.) Figure 5. The schematic DSC thermograms of PAN and AN/MAA copolymer measured at 230"C under air. At the several points indicated v (CooN) i' 1' by marks theIR spectra were observed as shown in Figures 6 and 7. peak, 5 min FT-IR. In Figure 5, a typical isothermal DSC thermo­ gram of the PAN homopolymer is illustrated schemati­ cally in comparison with that of the CPl. The exo­ 60min thermic phenomena of the PAN are apparently more '#. complicated than those of the CPl. The thermogram of """' 6 (CHI the PAN is composed of several stages; an induction C=C-H period A, an exothermic shoulder B, a small peak C and VI\ 1' an exothermic large broad peak D as indicated in this figure.
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