Understanding Lignin Radical Dynamics: Quenching Radicals by Solvent and Thermal Induced Mobility

Journal article, 2026

Lignin contains persistent free radicals (PFRs) stabilized by restricted molecular mobility in a glassy state. Herein, we investigate quenching effects in softwood kraft lignin PFRs by solvation and thermal treatment. Electron paramagnetic resonance (EPR) spectroscopy and temperature-modulated differential scanning calorimetry (TM-DSC) were used to characterize changes. Room-temperature methanol or acetone swelling reduced the paramagnetic signal to 48% and 71% of the original intensity. Similarly, heating lignin above its glass transition temperature (T-g) mobilized trapped radicals, leading to their recombination and the depletion of the EPR signal. Both heating and solvent swelling induced a transition from carbon- to oxygen-centered radicals (g-value 2.0016 -> 2.0033), but only heating caused cross-linking that increased the T-g by approximately 15 degrees C. Ultimately, overcoming the restricted mobility of the glassy state, whether chemically or thermally, is the primary driver for quenching persistent radicals in kraft lignin.