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

Area of Science:

• Polymer Chemistry
• Materials Science
• Physical Chemistry

Background:

• Lignin, a complex biopolymer, contains persistent free radicals (PFRs).
• These PFRs are stabilized by restricted molecular mobility within lignin's glassy state.
• Understanding PFR behavior is crucial for lignin valorization and material applications.

Purpose of the Study:

• To investigate the quenching effects of solvation and thermal treatment on PFRs in softwood kraft lignin.
• To elucidate the role of molecular mobility in PFR stabilization and reactivity.
• To characterize the chemical and physical changes induced by these treatments.

Main Methods:

• Electron Paramagnetic Resonance (EPR) spectroscopy to quantify PFR concentration and identify radical types.
• Temperature-modulated Differential Scanning Calorimetry (TM-DSC) to determine glass transition temperature (Tg) and thermal behavior.
• Solvent swelling experiments using methanol and acetone at room temperature.

Main Results:

• Room-temperature swelling with methanol or acetone significantly reduced the EPR signal intensity (to 48% and 71%, respectively).
• Heating lignin above its glass transition temperature (Tg) led to radical recombination and signal depletion.
• Both treatments shifted the radical type from carbon-centered to oxygen-centered (g-value change from 2.0016 to 2.0033).
• Thermal treatment induced cross-linking, increasing Tg by approximately 15 °C, while solvent swelling did not.

Conclusions:

• Overcoming the restricted mobility of the glassy state is the primary mechanism for quenching PFRs in kraft lignin.
• Both chemical (solvation) and thermal treatments effectively reduce PFRs by increasing molecular mobility.
• Thermal treatment additionally induces structural changes like cross-linking, impacting lignin's thermal properties.