Four kinds of 13C-labeled polyacrylonitrile (PAN) samples were prepared respectively by solution polymerization of acrylonitrile (AN) with selective 13C labeling of different molecular sites. The composition and structure of the residues from the thermal treatment of PAN in argon at 250 and 350 °C were quantitatively analyzed in detail by one- and two-dimensional solid-state 13C nuclear magnetic resonance (ssNMR) experiments. Compared with the NMR spectrum of each labeled carbon in AN monomer unit, nine chemical structures created during the heat treatment process have been identified accurately. On this basis, four reaction routes were proposed. It is noted that the main chemical change for PAN started from a cyclization reaction at a relatively low temperature, then experienced an aromazation reaction to form a molecular chain basically composed of isolated pyridine units, instead of the commonly reported ladder structure. This work also shows that the combination of selectively 13C-labeled technique and a high spinning speed of 20 kHz in magic-angle spinning (MAS) NMR experiment could improve the detection sensitivity to nearly 2 orders of magnitude, and provide a clear ssNMR spectra with little peak overlaps, which will be helpful to discover the complex reaction mechanism in the manufacture of carbon fibers with high performance.

Covalently functionalized carbon nanotubes (CNTs) wrapped in intumescent flame retardant were successfully fabricated and characterized. By adjusting the ratio of CNTs and flame retardant, the diameter of the functionalized CNTs was effectively controlled to 20-90 nm. Compared with pristine CNTs, the functionalized CNTs are better dispersed in polypropylene (PP) due to the in situ compatibilization reaction between the active groups of the intumescent flame retardant on the CNT surface and the maleic anhydride groups in the compatibilizer, maleic anhydride-grafted polypropylene (PPMA). Incorporating the functionalized CNTs could confer outstanding flame retardancy on PP/ PPMA, and considerably enhance the mechanical properties of the polymeric materials due to the improved interfacial adhesion and stress transfer. Therefore, use of intumescent flame-retardantwrapped carbon nanotubes and in situ compatibilization are promising strategies for simultaneously improving the flame retardancy and mechanical properties of polymeric materials.

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