Influence of the molecular structure of precursor pitch from different sources on the structure and properties of mesophase pitch and its derived carbon fibers

The structure and properties of mesophase pitch and its derived carbon fibers are closely related to the molecular structure of precursor pitch. Exploring the relationship between mesophase characteristics and precursors at the molecular level is of great significance. In this study, mesophase pitch was prepared from petroleum pitch (PP), coal liquefied pitch (CLP), and coal tar pitch (CTP), and the effects of the molecular structures of PP, CLP, and CTP on the structure and properties of mesophase pitch and its derived carbon fibers were systematically investigated. The results showed that PP has the highest H/C atomic ratio of 0.88 and lower aromaticity (f_a = 0.64) due to its rich aliphatic structure (H_α = 30.79 wt%) with a long alkyl side chain. The semi-rigid molecular structure endows MP-PP with a low softening point (292 ℃), wide-domain optical texture, and excellent spinnability. The relatively poor planar structure, smaller microcrystalline structure size, and the low order degree of MP-PP result in a lower tensile strength and Young's modulus of MPCF-PP, which were 1258 MPa and 146 GPa, respectively. CLP exhibits higher aromaticity (f_a=0.70) and a large number of naphthenic structures (H_N = 11.02 wt%), along with a moderate amount of alkyl side chains. The ordered arrangement of aromatic molecules and the formation of large-size microcrystalline structures (d₀₀₂ = 0.3423 nm, L_c =4.36 nm) give MP-CLP flow-domain optical texture and good thermal stability. MPCF-CLP has the highest tensile strength and Young's modulus among the three types, reaching 1364 MPa and 165 GPa, respectively. CTP had the highest aromaticity, the least amount of alkyl side chain, and the highest PI component content of 20.7 wt%, leading to the excessive condensation of MP-CTP, which has a high softening point (312 ℃), the obvious mosaic structure, the poorly ordered microcrystalline structure, and the worst spinnability, resulting in a splitting morphology of MPCF-CTP and the worst mechanical properties. This study provides theoretical support for the refinement of precursor pitch and the preparation of mesophase pitch and its derived carbon fibers.