Mechanism-Guided Design of Lignin Material for Sustainability

Lignin is the second most abundant biopolymers on earth and can serve as a low-cost and abundant feedstock for sustainable biomaterial. However, lignin remains primarily a waste stream from paper, pulping, and biorefining industries, due to the lack of fundamental understanding of structure-property relationship between lignin chemistry and material properties. Throughout the past decade, our interdisciplinary team has substantially advanced such understanding by defining how lignin molecular weight, uniformity, linkage profile, and functional groups could impact the properties of the lignin-derived sustainable materials, including carbon fibers, nanoparticles, recyclable plastic blends, and pavement materials. In particular, we have found that higher molecular weight, better uniformity, and more linear linkages could improve carbon material properties, empowering quality lignin carbon fiber for energy and environmental sustainability. Based on these fundamental understandings, we have recently advanced a new chemical design to substantially increase lignin molecular weight, ether linkage content, and hydrophobicity. The design was achieved through anhydride modification to produce a new type of lignin, high-molecular weight esterified linkage lignin (HiMWELL). The HiMWELL design achieved more uniform esterified linkages, larger molecular weight, and much lower –OH group via inter-unit linkage. These chemical characteristics perfectly overcome the inherent chemical limits of lignin, including the diverse chemical linkages, heterogenous functional groups, variable molecular weights, and amphiphilic structures. As a result, HiMWELL lignin has exhibited superior properties as a precursor for various functional materials. First, HiMWELL-based carbon fiber achieved record tensile strength and >25% enhancement of mechanical properties as compared to polyacrylonitrile (PAN) carbon fiber. Mechanism study revealed the substantially increased crystal content in HiMWELL-based carbon fibers, as compared to those of carbon fibers from PAN and untreated lignin. The results highlighted that HiMWELL has the potential to serve as an additive to enhance biomaterial performance. Furthermore, we explored HiMWELL lignin as a guest polymer for plastics blends to improve recyclability. Unlike most lignin to decrease plastic blend performance, HiMWELL uniquely improved the mechanical properties of the recyclable polymer blends and achieved UV-shading effects. Overall, HiMWELL design overcame the inherent chemical structure limits of lignin and turned lignin into an enhancer for biomaterial performance for broad applications.