Thermal properties of isotropic pitch fibers characterized by thermomechanical analysis

The thermal behavior of single filament of pitch fibers has been investigated using thermomechanical analysis (TMA). Thermal characteristic parameters such as glass transition temperature (T g) and linear coefficients of thermal expansion (CLTE) below and above T g were determined with high reproducibility to probe the conditions of pitch fiber materials. Different loads and heating rates were applied to examine their effects on measurement results. A load-free CLTE below T g can be extracted from the intercept of CLTE–stress curve, showing a constant value of 63.3 μm/(m °C) for the pitch fibers spun from different spinning speeds. On the other hand, the free CLTE above T g showed a remarkable trend decreased from positive to negative for pitch fibers produced with accelerated spinning speed. The isotropic pitch fibers can be partially oriented to fiber axis under strong elongation, and the limited degree of orientation is confirmed by X-ray diffraction. Thus, the free CLTE above T g can serve as an excellent indication of degree of preferred orientation for pitch fibers.