Kinetic Study on the Pre‑oxidation Process of Polyacrylonitrile (PAN) Based on In‑situ Infrared Spectroscopy

Abstract

This study focuses on PAN films to investigate the kinetic characteristics and infrared spectral evolution during isothermal heat treatment under nitrogen and air atmospheres. Building upon conventional models, a two-parameter kinetic model consistent with experimental data under isothermal and equal-time conditions was developed to elucidate the dual role of oxygen in the reaction. The results indicate that, compared with the nitrogen atmosphere, the optimal temperature in air shifts downward by approximately 10°C, suggesting that the presence of oxygen lowers the optimal reaction temperature and enables higher reaction efficiency at relatively lower temperatures. The apparent rate constant initially increases and then decreases with rising temperature, further corroborating the dual-action mechanism of oxygen. In both atmospheres, the apparent equilibrium concentration decreases with increasing temperature; however, the values of the apparent equilibrium concentration (y∞) in air are consistently higher than those in nitrogen at each temperature. Specifically, as the temperature increases, y∞ declines by 15% in air, whereas it drops by 42% in nitrogen, implying that oxygen may shift the reaction equilibrium toward a higher residual cyano content, thereby hindering the cyclization reaction from achieving deep conversion. Moreover, the apparent activation energy and pre-exponential factor in air are both higher than those in nitrogen, reflecting that oxygen increases the complexity of the reaction pathways and the frequency of effective molecular collisions. This work provides a quantitative kinetic basis for understanding the regulatory role of oxygen in PAN pre‑oxidation and for optimizing the stepwise temperature-rising process.