Mechanism and kinetics of oxidation during the thermal stabilization of polyacrylonitrile fibers

Abstract

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N 2 and air were investigated using differential scanning calorimetry and solid-state 13 C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C¼ ¼O) groups could occur at 160 C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers.

Comparatively Thermal and Crystalline Study of Poly(methyl‐methacrylate)/Polyacrylonitrile Hybrids: Core–Shell Hollow Fibers, Porous Fibers, and Thin Films

“…As shown in Figure Ba,b, no obvious T m was observed in raw PMMA powders and pure PMMA fibers, displaying amorphous state of pure PMMA powders and fibers. It was well known that the conventional DSC is difficult to investigate the melting temperature of PAN because of the simultaneous occurrence of cyclization reactions and melting . A sharp exothermic peak was observed above 239 °C (Figure Bb–g), attributing to the reactions of PAN cyclization of nitrile groups, dehydrogenation, and oxidation .…”

“…It was well known that the conventional DSC is difficult to investigate the melting temperature of PAN because of the simultaneous occurrence of cyclization reactions and melting . A sharp exothermic peak was observed above 239 °C (Figure Bb–g), attributing to the reactions of PAN cyclization of nitrile groups, dehydrogenation, and oxidation . The exothermic temperatures of raw PAN powders, pure PAN fibers, and PMMA/PAN porous fibers relating to the reactions of PAN cyclization of nitrile groups, dehydrogenation, and oxidation were almost the same (Figure Be–g), demonstrating that the blending of PMMA into PAN did not affect the PAN reactions of nitrile cyclization, dehydrogenation, and oxidation.…”

Preparation and thermal stability of initiator fragments end-capped oligomers/silica nanocomposites

“…9, we can observe the relatively sharp peaks related to the parent DMAA oligomer (c), in the nanocomposites before calcination (a), and after calcination (b). Especially, the new signal has been appeared at around 7 ppm [8,9,11]. From these findings, end-capped nitrile groups in oligomer would interact with each other to form the imine units (>C=N∼) during the calcination process.…”

“…The formation of PAN precursor fibers possessing the heatresistance ability by heating to 200 ∼ 300°C is due to the change of the chemical structure from the linear molecular chain into ladder structures through a variety of chemical reactions including cyclization of nitrile groups leading to hydronaphthiridine rings, dehydrogenation leading to a certain degree of aromatization, and oxidation reaction leading mainly to acridone and some other structure [9,11,12]. From this point of view, it is of particular interest to study on the thermal stability of initiator fragments end-capped oligomers, which should be synthesized by using a variety of radical initiators containing not only the nitrile groups but also the 2-methyl-N- (2-hydroxyethyl)propionamide and the sulfate ester units.…”

“…On the other hand, it is well known that polyacrylonitriles (PANs) are widely used as precursors for fabricating high performance carbon fibers [8][9][10][11][12][13][14][15]. The formation of PAN precursor fibers possessing the heatresistance ability by heating to 200 ∼ 300°C is due to the change of the chemical structure from the linear molecular chain into ladder structures through a variety of chemical reactions including cyclization of nitrile groups leading to hydronaphthiridine rings, dehydrogenation leading to a certain degree of aromatization, and oxidation reaction leading mainly to acridone and some other structure [9,11,12].…”