Separation and kinetic analysis of the thermo-oxidative reactions of polyacrylonitrile upon heat treatment

Abstract

A polyacrylonitrile (PAN) homopolymer and a copolymer with methyl acrylate (MA) were analyzed by differential scanning calorimetry (DSC) in order to establish a method for the kinetic analysis of the stabilization reaction steps during heat treatment. The application of a program rarely used for the study of the chemical transformation of PAN and its copolymers resulted in five separate steps, which can be assigned with large probability to various cyclization and oxidation reactions. A more exact identification of the reactions need further measurements and considerations. Overlapping peaks appearing in the DSC traces were deconvoluted and then quantitatively analyzed to obtain kinetic parameters. The kinetics of the processes were described by the Kissinger model. The model could be fitted well to four of the five processes indicating that these belong to individual reactions of the first order. The parameters obtained agree well with the few values published in the literature up to now. The results have large practical relevance, since the kinetic parameters obtained can be used in the preparation of intermediates and for the optimization of the stabilization process.

The mechanism of thermal stabilization of polyacrylonitrile

“…This latter is considered a cross-linking reaction that requires higher temperature and possesses a longer induction period. This reaction is assumed to occur only in PAN homopolymers, as it could not be observed in a PAN copolymer containing methyl acrylate [11]. Crosslinking is made possible by the relatively high regularity and lateral order of the homopolymer [27].…”

“…4). Weight loss is regarded as a measure of the structural stability of PAN, which is determined by the length, ratio, 2D and 3D order of its cyclic and aromatic sequences, and their ability to arrange into a regular structure Unlike in the case of two-step stabilization, low-temperature exothermic heat release may not be observed on the DSC thermogram recorded with a linear heating rate during the direct oxidation of PAN [2,11] (see also Fig. 12).…”

“…On the other hand, some groups report results obtained in a non-oxidative, partial stabilization process of PAN, which excludes oxidative processes. Only a few groups studied the behavior of PAN in a two-step stabilization procedure, which consists of an inert pre-treatment and a subsequent oxidative step [7,9,11,12]. The two-step approach provides a deeper insight into the chemical changes taking place during stabilization as compared to the direct oxidation of PAN.…”

“…The two-step approach provides a deeper insight into the chemical changes taking place during stabilization as compared to the direct oxidation of PAN. A different sequence of reactions occurs in this approach as evidenced by the dissimilar exothermic heat release recorded in DSC [11,12]. Moreover, the structure of stabilized PAN obtained by this approach is also different, which might have a considerable impact on the structure, stability and properties of the carbon fibers obtained.…”

“…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.…”