Due to its light weight and superior mechanical properties, carbon fiber has many attractive applications. However, PAN-based carbon fiber is too costly, which prevent its wide applications. On the other hand, lignin is considered as a low-cost alternative of PAN for carbon fiber production. As ever-increasing amount of lignin is produced from emerging biorefineries as a byproduct, lignin-based carbon fiber could contribute to economic sustainability of the lignin producing industries. In the first section of this work, the potential of producing carbon fiber from a high-ash containing corn stover lignin is investigated. The lignin was pretreated with methanol fractionation and partially acetylation and melt-spun at 165-170 ℃. After stabilization and carbonization, carbon fiber with tensile strength of 0.454 GPa was obtained. In following serious of study, a new concept called “depolymerization followed repolymerization” is proposed to prepare structurally modified lignin precursors in order to improve carbon fiber properties. Specifically, red oak lignin was thermally depolymerized into crude bio-oil via fast pyrolysis, and then repolymerized into carbon fiber precursors through different reaction routes. The aim of the work is to produce a lignin-based carbon fiber precursor with modified structure and molecular orientation.

In first route, the bio-oil, so called pyrolytic lignin (PL) was repolymerized in the presence of acid catalyst to prepare a precursor. The precursor had a Tg of 101 ℃, could be easily melt-spun at 115-120 ℃, and stabilized at 0.3 ℃/min. The resulting carbon fiber had smooth surface and void-free cross section. The average tensile strength of the fiber was 0.855 GPa. In the second route, PL was thermally co-treated with 5 or 20% of polyethylene terephthalate (PET), prior to the PL-PET precursor was melt-spun. Results

from FTIR, NMR, thermal, and rheological tests all indicated that PL and PET chemically bonded to improve molecular orientation of the resulting precursor. As a result, PL-PET precursor had decreased spinning temperatures and higher thermal stabilities. Also, PL-PET based carbon fiber had lower ID/IG from Raman spectra for improved crystallinity and higher mechanical properties compared to PL-based carbon fiber. In third route, PL was converted to lignin-based polymers through free radical polymerization. To convert PL into radically polymerizable material, PL was functionalized with different amounts of methacryloyls and acetyls and then polymerized by applying Reversible Addition-Fragmentation chain Transfer (RAFT) technique. Three polymers namely PLMAP1, PLMAP2, and PLMAP3 were synthesized. PLMAP1 (fully methacylated PL) was a cross-linked polymer. PLMAP2 (partially methacrylated PL) was a thermoplastic polymer with glass transition temperature (Tg) of 161 ℃ and thermal decomposition temperature (Td) of 241 ℃. PLMAP3 (partially methacrylated and the fully acetylated PL) had reduced Tg of 130 ℃, while its Td increased to 250 ℃. PLMAP3 is melt-spinnable and also demonstrated highly attractive properties as ideal carbon fiber precursor. It is carbonizable at 1000 ℃, and has much improved molecular orientation.