Chemical Reactions and Their Kinetics of atactic-Polyacrylonitrile As Revealed by Solid-State ¹³C NMR

Abstract

Inter-and intramolecular chemical reactions and their kinetics for 13 C-labeled atactic-polyacrylonitrile (aPAN) powder heat-treated at 220−290 °C under air and vacuum were investigated by various solid-state nuclear magnetic resonance (ssNMR) techniques. By applying 13 C direct polarization magic angle spinning (DPMAS) as well as through-bond and throughspace double quantum/single quantum ssNMR techniques, it was concluded that aPAN heat-treated under air at 290 °C for 300 min adopted the ladder formation, namely, conjugated six-membered aromatic rings with partially cross-linked and oxidized rings and polyene components. In contrast, aPAN heat-treated under vacuum at the same condition thermally decomposed into oligomeric chains that were mainly composed of isolated aromatic rings connected by alkyl segments. Furthermore, early stages of the chemical reactions were investigated by 13 C cross-polarization (CP) and DPMAS spectra. The latter provided quantitative information regarding the kinetics of the chemical reactions. As a result, it was shown that chemical reactions under oxygen occurred homogeneously with a higher activation energy (E a ) of 122 ± 3 kJ/mol compared to that of vacuum at 47 ± 2 kJ/mol. By comparing both chemical structures and kinetics under two different conditions, the chemical reaction mechanisms of aPAN will be discussed in detail.

Chemically Enhanced Wet‐Spinning Process to Accelerate Thermal Stabilization of Polyacrylonitrile Fibers

“…Recently Liu et al. showed the different mechanism of stabilization in air and vacuum atmosphere using 13C NMR study . As it is shown in their study, stabilization in air resulted in a higher aromatization that leads a higher degree of crosslinking in the stabilized fibers.…”

“…Three precursor fiber samples were analyzed and their thermochemical properties were compared. The stabilization reaction is considered to occur according to the mechanism presented in Figure . A study by Liu et al .…”

The mechanism of thermal stabilization of polyacrylonitrile

“…Frequently, the stereochemistry of cyclization is conventionally not discussed, and the reaction is often presented in a non-stereospecific manner instead. The authors, who consider stereochemistry, like Wang and Liu [15,16], accept a strictly isotactic concept.…”

“…The goal of our study was to answer yet unclear questions regarding the stabilization 4 of PAN. These include the stereochemistry of cyclization [3,[13][14][15][16], the requirement of a cyclized structure for oxygen inclusion [9,17], the effect of inert pre-treatment on oxygen up-take [9], the average zip-length of cyclization [7,15,16], the relationship between chemical changes and exothermic peaks observed in DSC measurements [1,[6][7][8][9]11,12] and the mechanism of the reactions occurring. The complexity of the stabilization process makes the answering of these questions extremely difficult.…”

A novel methodology for designing thermal processes in order to optimize stabilization of polyacrylonitrile (PAN) fibers

“…Thus, the question arises as to how someone could find the best treatment pathway for any given precursor material. Yet, the relevant discussion is almost nonexistent, with the research effort mainly focused on analyzing the chemistry of the process (and the various reactions taking place), as well as on modifying the CF manufacturing process in order to achieve improved results . Even though recently commercialized CFs have achieved the goal of combining high tensile strength with high modulus (such as Toray T110G fibers), relevant product datasheet hints that this result was achieved by improvements on the structure of the PAN precursor fibers (and not explicitly on the manufacturing process)…”