Study on the Relationship Between the Microstructure and Density of Polyacrylonitrile‐Based Pre‐Oxidized Fibers

ABSTRACT Pre‐oxidized fibers (OFs) are a critical intermediate in the manufacturing of polyacrylonitrile‐based carbon fibers (CFs). However, systematic research on the relationship between the microstructure, properties, and density of OFs remains limited. In this study, a series of OFs with density ranging from 1.29 to 1.43 g cm −3 were prepared using a pilot‐scale pre‐oxidation line. A strong linear correlation is obtained between elemental content and density, while the rates of change in other structural transformation parameters decrease with increasing density due to the development of a skin‐core structure. The thermal stability of OFs initially improves with increasing density but deteriorates beyond 1.39 g cm −3 , whereas tensile strength consistently declines as density increases. Considering the necessity of achieving an adequate degree of pre‐oxidation and maintaining a minimal skin‐core structure, a density range of 1.35–1.37 g cm −3 for OFs represents the optimal range for producing high‐quality CFs. This study establishes the relationship between the macroscopic control indicator (density), the degree of reaction and properties of OFs and provides strong theoretical guidance for the industrial pre‐oxidation process of CFs, with significant scientific and economic implications.