Structural changes and molecular motion of polyacrylonitrile fibers during pyrolysis

The 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 diffractometer combined with a fiber specimen holder for heating was used to measure the activation energy of crystal transition Ec. Ec 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, and then at the boundaries of the ordered phase.