Structural changes and molecular motion of polyacrylonitrile fibers during pyrolysis

Abstract

SynopsisThe stabilization process of polyacrylonitrile (PAN) fiber is necessary to develop high-performance carbon fiber. This work is concerned with studies of the activation energy of PAN fibers during the stabilization process. A wide-angle X-ray diEractometer combined with a fiber specimen holder for heating was used to measure the activation energy of crystal transition E,. E, is almost the same as the activation energy of the cyclization reaction E which is measured by a differential thermal analysis (DTA). The variation of crystal size in PAN fiber and a model of the ladder polymer in stabilized fiber transformed from acrylontrile (AN) units of PAN fiber are discussed also. The crystal size of PAN fiber increases when the thermal treatment temperature is raised. When exposed to temperatures above the crystal degradation temperature Td, the molecular rods of PAN fiber are destroyed completely, and ladder polymer is formed in the ordered phase of the original PAN fiber. The transformation of ladder polymer is initiated in the disordered phase, &d then at the boundaries of the ordered phase.

Influence of tension on the oxidative stabilization process of polyacrylonitrile fibers

“…Very probably at low temperatures (i.e., 160–190°C), the ordered phase contained a great number of intermolecularly bound nitrile groups, which left fewer free nitrile groups. In contrast, the molecular chains in the amorphous phase were flexible 16. Thus the stretching and segmental mobility led the molecular chains to adjust gradually, the mutual alignment of nitrile groups to form a stronger dipolar interaction.…”

The Microstructure Characterization and the Mechanical Properties of Electrospun Polyacrylonitrile-Based Nanofibers

“…Researchers now realize that understanding and controlling the orientation structure during the precursor formation step is critical if the properties of the carbon fibers are to be optimized (Ruland 1990). Therefore, the research on the microstructure of the precursors under different processing conditions is important in the study of the final mechanical properties of the carbon fibers (Ko et al 1989). However, due to the complicated process in the manufacture of carbon fibers, it is not easy and fast to determine the orientation factor for PAN precursors, especially the less theoretical studies on PAN precursors.…”

An investigation on structure characterization of thermally stabilized polyacrylonitrile precursor fibers pretreated with guanidine carbonate prior to carbonization

“…It is already well established that X‐ray diffraction scattering is only sensitive to the highly ordered regions and no scattering is picked up from the disordered regions. It has been claimed that the transformation of ladder‐like structure is initiated initially in the disordered regions [80] and it is highly likely that the X‐ray diffraction measurements do not pick up scattering from such regions during the data collection of the thermally stabilized samples. This is the most likely reason why the X‐ray stabilization index values are relatively lower than those of values derived from the DSC and IR methods.…”