Abstract
Lignin is a renewable biopolymer considered as a potential precursor for low-cost carbon materials. Thermal oxidative stabilization (TOS) is an important processing step to maintain fiber geometry during carbonization, yet the impact of TOS on the properties of lignin-based carbon materials has not been clearly identified in the literature. Yield, change in fiber diameter/distribution, elemental composition, and mechanical properties were explored for both stabilized and carbonized lignin fibers. Vibrational spectroscopy and solid-state (13)C nuclear magnetic resonance spectroscopy were used to analyze the changes in lignin molecular structure after exposure to various heating conditions during the TOS steps. Further, studies were focused on the effects of TOS conditions on the resulting carbon structure of fiber mats through Raman spectroscopy measurements and electrical conductivity analysis. Although TOS conditions influenced the properties of the oxidized lignin fiber mats, properties of the carbonized samples were invariant to the TOS procedures used in this study over most of the conditions. As a result, there was flexibility for the parameters (time and temperature) in the TOS process when conditioning softwood lignin materials for carbon fibers.