Simple Strategies for Enhancement of the Strength of Lignin‐Based Nanofibrous Aerogels

Solvent fractionated lignin and cellulose nanocrystals (CNCs) are used to create highly resilient nanofiber‐based aerogel materials. Two fractions of softwood kraft lignin (SKL) are combined and subsequently electrospun into nanofibers composed of 99% lignin. Additionally, 5 wt.% of CNCs is added into the fiber, based on the solid lignin weight, to enhance the physical properties of the nanofiber materials. The manufacturing process involves dispersing the fibers in water followed by freeze‐drying and subsequent heat treatment. The heat treatment process, with carefully chosen blends of fractionated lignin with specific glass transition temperatures, provides an initial thermoplastic behavior that results in the physical cross‐linking of entangled fibers upon heat treatment. This tailored morphology shows four times higher compressive strength compared to lignin nanofiber materials that only contain high molecular weight fractions. Moreover, CNC is a critical additive that helps maintain fiber geometry by reducing significant softening of lignin under elevated temperatures. Therefore, the fibers with CNC additives ensure the 3D shape after heat treatment, resulting in enhanced physical connections at fiber junctions. As a result, lignin/CNC nanofibers are transformed into 3D structured, lightweight materials that can undergo near full recovery after repeated compressive strain matching the performance of some carbonized analogs. The renewable biopolymer lignin contains fractions of low and high molar mass,with a Tg difference of 85 K for the fractions. By mixing fractions in a certain ratio, physical bonding can occur in nanofiber aerogels strengthening the material. However, nanoparticle additives like cellulose nanocrystals are required to control rheological properties to help with the formation of 3D ultralight weight materials.