Glassy carbons from poly(furfuryl alcohol) copolymers: structural studies by high-resolution solid-state NMR techniques

Abstract

The chemical structure of glassy carbon particles produced from poly(furfuryl alcohol) copolymers is studied by ¹³C cross-polarization/magic-angle spinning (CP-MAS) NMR and high-speed ¹H MAS NMR. In agreement with earlier proposals, ¹³C NMR spectra confirm the buildup of a highly unsaturated system at the expense of furan rings and aliphatic carbon atoms, and upon heating to 800 K this conversion is essentially complete. Successive carbonization by air oxidation or pyrolysis at temperatures up to 1600 K is reflected in a gradual decrease of the ¹³C chemical shift from ca. 130 to 115 ppm versus tetramethylsilane. ¹H MAS NMR is used to detect and quantitate the amount of residual C-bonded hydrogen species at various stages of the carbonization process. In addition, these spectra show intense, narrow resonances due to sorbed H₂O molecules, which resonate over a wide range of chemical shifts (between 2.5 and -8 ppm versus tetramethylsilane). In analogy with effects observed by Tabony and co-workers for molecules adsorbed above the basal plane of graphite, the upfield shifts observed for water sorbed in the glassy carbons of the present study are attributed to the large susceptibility anisotropy of submicroscopically ordered, turbostratic, or partially graphitized regions of the samples. The extent of this ordering is inversely correlated with the absolute content of residual C-bonded hydrogen species and depends mainly on the temperature of pyrolysis, whereas the oxygen content of the heating atmosphere and the composition of the initial polymeric material appear to be of secondary importance. The results suggest that sorbed H₂O molecules can function as sensitive NMR chemical shift probes for the initial stages of crystallization processes in glassy carbons.

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