Chemistry

A. I. MAKLАKOV, G. G. PIMENOV

NUCLEAR MAGNETIC RESONANCE IN PYROLYZED POLYACRYLONITRILE

(Presented by Academician B. A. Arbuzov, April 13, 1964)

Recently a number of works have appeared ($^{1,2}$) devoted to the study of the process of pyrolysis of polyacrylonitrile (PAN) and the properties of the product obtained. Most investigators believe that the pyrolysis of PAN proceeds successively in two stages with the formation of structures I and II:

![reaction scheme: polyacrylonitrile chain → cyclic conjugated structure I → more conjugated structure II]

The first stage, occurring at temperatures not higher than 250–270° ($^3$) and associated with the formation of cyclic structures and conjugated —C=N bonds, should be accompanied by a decrease in the mobility of the macromolecular chains; as a result of the second stage, the number of hydrogen atoms in the polymer should decrease and conjugation along —C=C bonds should appear. Both of these processes should strongly affect the character of the nuclear magnetic resonance (NMR) spectra of the products of PAN pyrolysis. Such studies have not previously been carried out.

PAN powder with a viscosity-average molecular weight of ~21,000 was subjected to pyrolysis in a vacuum of $9 \cdot 10^{-3}$ mm Hg at temperatures of 210 and 320° for 3, 6, and 10 hr. The NMR spectra were recorded on an apparatus assembled by one of the authors in the temperature interval from −150 to +200°.

Figure 1 presents curves of the dependence of the second moment $\Delta H_2^2$ of the NMR line on the measurement temperature for the initial and pyrolyzed samples. It is seen from the figure that, with increasing temperature and, in some cases, pyrolysis time, $\Delta H_2^2$ in the low-temperature region decreases in comparison with the non-pyrolyzed sample. With a change in the pyrolysis conditions, a change is also observed in the course of the temperature dependence of the second moment.

For the initial PAN, in the region 80–100° a sharp decrease in $\Delta H_2^2$ is observed, indicating an increase in chain mobility. The glass-transition temperature, determined from measurements of the specific volume ($^4$), lies in the same region, which indicates similar molecular mechanisms of softening and of the sharp drop of $\Delta H_2^2$ in PAN.

The invariance of the second moment for the sample pyrolyzed at 210° for 3 hr and for the initial samples at −150° indicates the same arrangement of atoms in their molecules. A certain excess of $\Delta H_2^2$ for pyrolyzed PAN in the region −100°–+80° indicates an increase in the rigidity of its mole-

particles, which is associated with the appearance of cyclic structures I and, consequently, of conjugation segments —C=N in accordance with the pyrolysis mechanism. The sharp decrease in \(\Delta H_2^2\) in the region of higher temperatures makes it possible to assume that between the cyclic conjugated structures there are flexible segments with disrupted conjugation (with the original structure of the PAN molecules), the existence of which had been suggested earlier \((^3)\).

Samples pyrolyzed at \(210^\circ\) for 6 h give, in comparison with the preceding ones, smaller values of \(\Delta H_2^2\) in the temperature region below \(+80^\circ\), and a sharp drop of the second moment is absent altogether. The latter indicates still greater stiffening of the molecules due to an increase in the segments of cyclic structures, the formation of transverse intermolecular bonds \((^2)\), and shortening of the nonconjugated regions.

Fig. 1. Dependence of \(\Delta H_2^2\) of the initial and pyrolyzed PAN on the measurement temperature \(T^\circ\mathrm{C}\). \(a\)—initial sample, \(b\)—pyrolyzed at \(210^\circ\), 3 h; \(v\)—at \(210^\circ\), 6 h; \(g\)—at \(320^\circ\), 3 h; \(d\)—at \(320^\circ\), 6 h; \(e\)—at \(320^\circ\), 10 h.

PAN pyrolyzed at \(320^\circ\) for 3, 6, and 10 h has coincident \(\Delta H_2^2\) values over the entire temperature range. The absolute values of \(\Delta H_2^2\) of these samples are smaller than those of the preceding ones.

In order to clarify the reason for the decrease of \(\Delta H_2^2\) with increasing temperature, and in some cases with pyrolysis time as well, a comparison was made of the theoretical values of the intramolecular contribution \(\Delta H_2^{2*}\) to the second moment for the rigid structures of the initial PAN and of products I and II.

The structure of the PAN molecule has not yet been fully elucidated; however, most authors \((^5)\) favor its atacticity, while not excluding the existence of isotactic blocks. The proposed pyrolysis mechanism also requires an isotactic arrangement of the substituent groups. It should be noted that the arrangement of hydrogen atoms in PAN and polyvinyl chloride molecules coincides; therefore their \(\Delta H_2^{2*}\) values should be approximately equal. For a rigid planar isotactic polyvinyl chloride molecule, \(\Delta H_2^{2*}\) has been calculated and is \(12.7\ \mathrm{Oe}^2\) \((^6)\). This value may also be taken as \(\Delta H_2^{2*}\) for PAN. Assuming that, upon formation of structure I, the carbon chain is not distorted, the same value, \(12.7\ \mathrm{Oe}^2\), may also be taken for its intramolecular contribution.

The value of \(\Delta H_2^{2*}\) for structure II was calculated from the formula \(\Delta H_2^{2*} = 716\, r^{-6}\), which we obtained from the Van Vleck equation \((^6)\), under the assumption that all protons are located along one straight line at equal distance \(r\) (in Å) from one another and that only the two nearest neighbors interact, which is valid for the present case. The value of \(r\) can be determined from the structure of the pyridine ring. However, if II is formed from fused undistorted rings, its macromolecule should close upon itself, which is unlikely. If, however, II is considered to be composed of distorted rings (owing to an increase of the angle at the N atom or a decrease of the opposite angle at C), then \(\Delta H_2^{2*}\) is \(2.6\)–\(5.0\ \mathrm{Oe}^2\), depending on the nature of the distortion.

Thus, the transition from structure I to II should be accompanied by a decrease in \(\Delta H_2^2\). Comparison of the experimental values of the second moment with the theoretical ones shows that, upon prolonged heating of PAN (more than 3 h), even at a temperature of \(210^\circ\), along with the first stage there also proceeds the ...

the latter. With an increase in the pyrolysis temperature to \(320^\circ\), the fraction of structure II in the product increases. The independence of \(\Delta H_2^{0}\) from the time of thermal treatment of PAN at \(320^\circ\) indicates an unchanged hydrogen structure of the substance obtained. The increase in electrical conductivity observed in this case can apparently be associated with partial graphitization of the substance.

Kazan State University

Received
8 IV 1964

REFERENCES

\(^{1}\) A. V. Topchiev, M. A. Geiderikh et al., DAN, 128, 312 (1959).
\(^{2}\) N. Grassie, J. N. Hay, J. Polymer Sci., 56, 189 (1962).
\(^{3}\) I. A. Drabkin, L. D. Rozenstein et al., DAN, 154, 197 (1964).
\(^{4}\) R. B. Buvers, E. F. T. White, J. Polymer Sci., B1, 171 (1963).
\(^{5}\) C. Y. Liang, S. Krimm, J. Polymer Sci., 31, 513 (1958).
\(^{6}\) A. Odajima, J. Sohma, J. Phys. Soc. Japan, 12, 272 (1957).
\(^{7}\) Collection: Organic Semiconductors, ed. A. V. Topchiev, Publishing House of the USSR Academy of Sciences, 1963.