Investigating the Effect of Fixation Tension on the Thermal Analysis Process and Crystal Structure of Polyacrylonitrile (PAN) Fibers

Thermal analysis of polyacrylonitrile (PAN) fibers is commonly employed to assess the chemical and physical changes occurring during the pre-oxidation stage. However, in actual pre-oxidation processes, PAN fibers are stretched to a certain extent (approximately 5%) to prevent fiber shrinkage. Consequently, the pre-oxidation process of PAN fiber is influenced not only by temperature and atmosphere but also by the stretching tension. In this study, two types of PAN fiber samples with different crystal structures were utilized alongside four different processing methods (PAN powder, unfixation, physical fixation, and chemical fixation) to generate three degrees of fixed stretching. These included PAN powder and unfixed PAN fiber samples (without fixation tension), physically fixed PAN fiber with relatively weak fixation tension, and chemically fixed PAN fiber with strong fixation. Through a comparison of these samples, it was observed that the fixation tension aided in preserving the crystal structure during the heating process, significantly reducing the cyclization rate of the PAN fibers. The cyclization reaction rate of PAN fiber A, initially at 85% without any fixation, decreased to 66% after physical fixation and dropped to 56% after chemical fixation. For PAN fibers with distinct crystal morphologies, the impact of the fixation tension was more prominent for PAN fibers with larger crystal sizes and lower orientation. Additionally, the presence of crystal orientation facilitated the occurrence of the cyclization reactions.