Stabilization of <i>Atactic</i>-Polyacrylonitrile
under Nitrogen and Air As Studied by Solid-State NMR
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Abstract
Solid-state (ss) NMR spectroscopy
was applied to study the stabilization
process of 30 wt % <sup>13</sup>C-labeled <i>atactic</i>-polyacrylonitrile (<i>a</i>-PAN) heat-treated at various
temperatures (<i>T</i><sub>s</sub>) under nitrogen and air.
Direct polarization magic-angle spinning (DP/MAS) <sup>13</sup>C NMR
spectra provided quantitative information about the functional groups
of stabilized <i>a</i>-PAN. Two dimensional (2D) refocused <sup>13</sup>C–<sup>13</sup>C INADEQUATE and <sup>1</sup>H–<sup>13</sup>C HETCOR NMR spectra gave through-bond and through-space
correlations, respectively, of the complex intermediates and final
structures of <i>a</i>-PAN stabilized at different <i>T</i><sub>s</sub> values. By comparing 1D and 2D NMR spectra,
it was revealed that the stabilization process of <i>a</i>-PAN under nitrogen is initiated via cyclization, while the stabilization
under air proceeds via dehydrogenation. Different initial processes
lead to the isolated aromatic ring and ladder formation of the aromatic
rings under nitrogen and air, respectively. Side reactions and intermediate
structures are also discussed in detail. Through this work, the stabilization
index (SI) was defined on the basis of the quantified C-1 and C-3
DP/MAS spectra. The former reached 0.87 at <i>T</i><sub>s</sub> = 370 °C, and further higher <i>T</i><sub>s</sub> values did not affect SI; however, the latter continuously
increased up to 0.66 at <i>T</i><sub>s</sub> = 450 °C.
All of the experimental results indicated that oxygen plays a vital
role on the whole reaction process as well as the final products of
stabilized <i>a</i>-PAN