Insighting Evolution of Radial Heterogeneity in Photochemically and Thermochemically Stabilized PAN Fibers from Nanomechanics and Microscopic Imaging

The radial heterostructure in thermally stabilized fibers (SFs), especially for the core with a loose structure, seriously affects the properties of carbon fibers (CFs). Hence, a new process combination of γ-irradiation and heat treatment is built to improve the microstructure of SFs. The synergistic effect of irradiation and heat treatment as well as changes in radial heterogeneity of SFs were investigated. Through nanoindentation and high-resolution transmission electron microscopy, a hierarchical model for SFs composed of outer-surface, sub-surface, inner-surface, and core parts was proposed. The hardness of each part in SFs prepared by the coordination of irradiation and heating has a significant improvement. SFs prepared by coordination of irradiation and heating possess a more compact and ordered trapezoidal structure both in the sub-surface and core parts. However, in the core of SFs (first heating and then irradiation), smaller improvement in the trapezoidal structure is found due to the lack of oxygen. Atomistic simulations jointly elucidate improvement of the structure in SFs by producing more cross-links. In a word, the combination of irradiation and heat treatment improves the cross-linking structure of each region in the SFs. It indicates that the reasonable combination of photochemistry and thermochemistry has great potential in the structural optimization of polymers.