Defence of doctoral thesis: Mohammad Morsali – Site-specific reactions of softwood kraft lignin for biobased vitrimers and reactive colloidal particles

Lignin, a natural polyphenolic compound of wood, holds promise as a green alternative to fossil resources given the current environmental concerns. However, its complex structure and limited usability have impeded widespread use of lignin in biobased materials. Herein, we have explored overcoming the limitations of lignin by using a series of chemistries and techniques that enables utilization of lignin in a variety of application ranging from bulk materials to colloidal particles. Lignin-based vitrimers, developed by a one pot, catalyst-free click addition of softwood kraft lignin and formation of dynamic acetal exchange network showed excellent performance as recoverable adhesives with lab shear strength of 2.6 MPa and 6.0 MPa for wood and aluminum substrates respectively. Stabilized lignin nanoparticles synthesized by hydrothermal crosslinking of hydroxymethylated lignin nanoparticles showed an excellent colloidal stability in organic solvents such as ethanol, acetone, dimethylformamide, and tetrahydrofuran, and aqueous media (3< pH <12). These stabilized lignin nanoparticles were subjected to direct surface modification in colloidal state to develop aminated pH-responsive particles. Stabilized lignin nanoparticles with preserved redox activity showed a capacity in reducing silver ions for silver concentrations as 200 μM, forming hybrid lignin-silver nanoparticles for applications in hydrogen peroxide colloidal sensors. Interaction of silver ions and stabilized lignin nanoparticles contributed to the emergence of discrete patterns of silver in lignin nanoparticle embedded hydrogels. The location and distance of the discrete patterns can be modified by altering the particle size and concentration. Redox activity of stabilized lignin nanoparticles, hydroxymethylated lignin nanoparticles and unmodified lignin nanoparticles with different particle sizes (90 nm, 150 nm, 640 nm) were studied in charge storage applications in PEDOT:PSS based organic electrodes. Among the three, stabilized lignin nanoparticles showed the least charge capacity while addition of non-modified lignin nanoparticles increased specific capacities of the electrode up to 42.5 mAh/g at a current density of 1 A/g, showcasing their potential for Zinc-lignin battery applications. To further explore and broaden the horizon of lignin applications, propargylated lignin nanoparticles synthesized by propargylation of lignin demonstrated light induced chemical reaction. Energy-efficient light emitting diodes with 405 nm wavelength initiated the reaction in water at room temperature. Propargylated lignin nanoparticles were utilized to demonstrate the light-triggered Pickering emulsion assembly in lignin-based Pickering emulsions.

Keywords: Softwood kraft lignin, lignin nanoparticles, biobased vitrimers, redox chemistry, organic batteries.