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

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 (Tg) 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 Tg by approximately 15 °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.