# Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-_co_-acrylamide)
## 3 Results and Discussion
### Thermal Treatment of the Copolymers in Air  
The similar experiments in dynamic and isothermal conditions have been conducted in air atmosphere. Figure 10 presents DSC thermograms of random copolymers with various compositions (a) and copolymers with various microstructures (b). The exo-effect can be observed at lower temperatures as compared with experiment in argon atmosphere and its intensity at the temperature of the maximum of heat release is several times less (see Figure 6a,b); the multiple peaks corresponding to various chemical processes are observed. Both chemical composition and chain microstructure affect the shape of the thermogram, i.e., the contribution of the chemical reactions proceeding upon thermal treatment in air atmosphere (Table 5). The increase in the number of the chemical reactions causes the rise of the value of the enthalpy of the process. Figure 9: Dependence of the stabilization index \(\mathrm{E_{s}}\) on the time of heat treatment at 200 \({}^{\circ}\)C (**a**) and 225 \({}^{\circ}\)C (**b**) for PAN and AN/AAM copolymers. Numeration of the curves corresponds to the numeration of the samples in Table 1.  
Upon heating at 200 \({}^{\circ}\)C in air atmosphere, the chemical structure of the copolymers changes. These changes can be followed by FTIR spectroscopy (Figure 11). No visible qualitative differences are observed during transformation of FTIR spectra of random (a), gradient (b), and block-gradient (c) copolymers. However, the differences are observed by comparison of Figures 7 and 11, conforming to changes in the chemical structure of the copolymers after heating during the same period in argon (Figure 7) and in air (Figure 11). In argon, the pyridine type structure becomes dominant after 30 min. In air, the number of new bands is less, and the most noticeable changes are due to the band at 1590 cm\({}^{-1}\) that stands out above the band of initial amide group at 1680 cm\({}^{-1}\) only after 90 min. Simultaneously, the bands at 816 and 956 cm\({}^{-1}\) (CH bending vibrations in olefins) become visible upon treatment in air and are absent in argon. The reduction of intensity of the nitrile group at 2244 cm\({}^{-1}\) and NH in amide group at 3480 cm\({}^{-1}\) occurs slowly. The lower conversion of nitrile groups is confirmed by Figure 12a. The rate of intramolecular cyclization reaction in air atmosphere is smaller as compared to experiment in argon atmosphere (Figure 8). Thus, cyclization is not a dominant reaction upon heating the copolymers at 200 \({}^{\circ}\)C in air. Moreover, it is less sensitive to the amount of amide groups in the copolymer, except the sample 1, containing least value of AAM units (3.4 mol. %), and to the copolymer microstructure. As a consequence, the stabilization index reflecting the development of the ladder structure is relatively low. Figure 10: DCS thermograms registered in air at heating rate 10 K-min\({}^{-1}\) of (**a**) the random copolymers 1, 2, 4; (**b**) random 2, gradient 6 and block-gradient copolymer 7 (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **F\({}_{\text{AAM}}\), mol. \%** & **T\({}_{\text{on-set}}\), \({}^{\circ}\)C** & **T\({}_{\text{pr}}\), \({}^{\circ}\)C** & **-\(\Delta\)H, J-g\({}^{-1}\)** \\ \hline
1 & 3.4 & 236 & 243, 254, 262, 311 & 895 \\ \hline
2 & 7.8 & 228 & 233, 251, 266, 302, 366 & 855 \\ \hline
4 & 8.8 & 236 & 244, 251 & 720 \\ \hline
5 & 10.2 & 236 & 242, 254, 267, 308, 340 & 740 \\ \hline
6 & 9.2 & 238 & 241, 248, 264, 315, 340 & 770 \\ \hline
7 & 6.0 & 238 & 240, 246, 266, 330 & 920 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: DSC data for the copolymers of AN and AAM registered in air. Summarizing FTIR data, it may be proposed that in oxidative conditions (upon heating in air), the formation of the conjugated C=C bonds occurs faster than that of the C=N bonds for AN/AAM copolymers (Scheme 4). As a result, the ladder structure develops slowly, at least at 200 \({}^{\circ}\)C. The important role of oxidation reaction is in accordance with [25].