Mechanism and Kinetics of Stabilization Reaction of Polyacrylonitrile and Related Copolymers I. Relationship between Isothermal DSC Thermogram and FT/IR Spectral Change of an Acrylonitrile/Methacrylic Acid Copolymer

Abstract

ABSTRACT:Intimate relationship between the thermal behavior and the structural change has been clarified for the first time by an organized combination of isothermal DSC thermogram and FT/IR spectra measured for the stabilization reaction of an acrylonitrile (AN)/methacrylic acid (MAA) copolymer. In an early stage of the isothermal exothermic thermogram measured by DSC under air, a flat region or the induction period of the cyclic structure formation was found to exist, which is immediately followed by the two stages of steep heat evolution and the slow heat release. Based on the IR spectral changes observed during this thermal reaction, the induction stage was found to be associated with the reaction of methacrylic acid groups with the adjacent nitrile groups and the steep heat evolution region with the propagation of the cyclic structure and dehydrogenation of the polyacrylonitrile (PAN) chain sequences to give an unsaturated ladder structure. An activation energy for this initiation reaction of the cyclic structure formation was evaluated to be ca. 26 kcal/mo! by an Arrhenius plot.KEY WORDS Polyacrylonitrile / Methacrylic Acid / DSC / FT/IR / Precursor / Carbon Fiber / Stabilization / Induction Period / Polyacrylonitrile (PAN) contammg acidic comonomers are the most commonly used precursors for carbon fibers because of the excellent performance of PAN based carbon fibers. 1 Production process of PAN based carbon fibers involves a stabilization of oriented acrylic fibers and a carbonization of them at elevated temperature. 2 Stabilization is the most important process which converts PAN to an infusible and nonflammable fiber by heating at 200-----300°C for about one hour in an oxidative atmosphere. Thus stabilized fibers can be heated up to carbonization temperature (1000-2000°C) in an inert gas atmosphere. 2 A number of studies 5 -zs have been reported concerning the stabilization of PAN since the proposition of the fully hetero aromatic cyclic structure for the stabilized PAN in 1950. 4 The subject of stabilization was reviewed by Bashir 40 years after this first proposition. 3 PAN copolymers containing acidic comonomers work as a good precursor and gives high quality carbon fibers. The role of comonomers in the stabilization reaction was said to lower the initiation temperature of cyclization reaction. 1 Grassie and coworkers 27 · 28 studied by DTA method the effects of some comonomers on lowering the stabilization temperature. Some acidic comonomers, e.g., itaconic acid, methacrylic acid (MMA), and acrylic acid, lower the exothermic peak temperature and also decrease the peak height in the following order: itaconic acid> methacrylic acid> acrylic acid > acrylamideIn this way the degree of reduction of the exothermic peak temperature may be useful as a measure of the effect of comonomers in stabilization reaction. On the other hand, the effects of comonomers on the t To whom all correspondence should be addressed. 36 • 37 developed a kinetic model including the two rate processes so as to simulate the he...

Mechanism and Kinetics of Stabilization Reactions of Polyacrylonitrile and Related Copolymers III. Comparison among the Various Types of Copolymers as Viewed from Isothermal DSC Thermograms and FT-IR Spectral Changes

“…In Figure 10, the changes of the IR spectra during the induction period caused by annealing in the DSC cell at 200°C under air flow are compared between CP2 and CPI. As described previously, 4 in the case of the CPI, the intensity of the 1740cm-1 (MAA) band decreases and the 1700 cm -l (reacted MAA) band appears and increases in intensity. This means that some MAA units react with the adjacent AN units in the induction period.…”

“…The observation of the induction period in the stabilization reaction was made for the first time by us as described in detail in the previous report. 4 The total amount of heat evolved during isothermal stabilization of the CP2 at 240°C is nearly the same as that of the CPI and also close to that of the PAN homopolymer, ca. 3290 J g-1 .…”

“…These nearly flat regions should be considered as induction periods observed for the CPI as reported in a previous paper. 4 In Figure 4, the Arrhenius plots are made for the induction periods obtained at 215, 230, 250, and 260°C, where the induction period ti is defined as the standing point of the steep thermal evolution (see Figure 3). From the slope of the plot in Figure 4, activation energy for the initiation of the stabilization of the CP2 was evaluated as ca.…”

“…3 In order to clarify the kinetics and the/mechanism of the stabilization of PAN and related copolymers, we developed a new idea to combine the experimental data of isothermal DSC thermograms and FT-IR spectra. 4 In previous papers, 4 • 5 we found the relationship bet To whom all correspondence should be addressed.tween the thermal behavior and the structure changes through an organized combination of isothermal DSC thermograms and FT-IR spectra measured for the samples picked out during the stabilization reaction of an AN/MAA copolymer 4 and PAN. 5 The isothermal DSC thermograms of AN/AAM copolymer were found to show intermediate features between PAN and AN/MAA copolymer.…”

“…3 In order to clarify the kinetics and the/mechanism of the stabilization of PAN and related copolymers, we developed a new idea to combine the experimental data of isothermal DSC thermograms and FT-IR spectra. 4 In previous papers, 4 • 5 we found the relationship bet To whom all correspondence should be addressed.…”