Impact of lignin structuration on its chemical and adhesive properties at the nanoscale: involvement with native and...

Lignin, produced from papermaking and biorefinery industrial processes, is an interesting green substitute for petroleum-based derivative products with multiple functional properties, including bioplasticizing, bioadhesive, UV resistance and antioxidant properties. Although lignin is a major wood component (20-25 wt%) available in large quantities, it is seldom used industrially because of its difficult handling in the manufacture of high-value products. Controlling the lignin structural morphology in native and composite materials remains challenging because of the wide variety of chemical functional groups influencing its spatial organization and final network physicochemical properties. In thermoplastics applications, improving the interfacial interactions between the lignin charge and polymer matrix is essential. This work reports a combination of experimental approaches to investigate the chemical and adhesive properties of lignin at the molecular level, both in preparation of atomic force microscopy probes and in measurement at a nanometric scale through single-molecule force spectroscopy (SMFS), nano-infrared spectroscopy and imaging. The application of an original Langmuir-Blodgett (LB) deposition procedure produced homogeneous ultrathin coatings with controlled thicknesses in comparison with classical chemical deposition routes. Both procedures were verified for two synthetic lignins, guaiacyl and mixed guaiacyl-syringyl dehydrogenation polymers, before being applied to pine lignin as a proof-of-concept for the development of green lignin-based (nano)composites. Functionalizing the tip via the LB approach increased the adhesion force measurement resolution and provided new insights into the molecular affinity of lignin for other materials at the nanoscale, which prefers xylan polymers, such as those in native plant cell walls, and apolar/polar polymer matrices, such as those in composite materials, albeit to a lesser extent.