Lignin is an important biomass that can be a raw material to produce functional polymers due to its abundance, low price, sustainability, and high concentration of aromaticity. Using these features, lignin can be integrated with the concept of aliphatic polyesters to produce biomass-based biodegradable polymers. In spite of the high potential, poor understanding of well-defined chemical lignin modifications and characterizations limit the rapid growth of biomass lignin-based polymers. Excessively high and uncontrollable glass transition temperature of lignin-based polymer is another problem to produce a conveniently processible polymers. Too high and/or too low mechanical property is an additional huddle of the practical use of the lignin-based polymers. To solve these multiple problems that is led by poorly defined chemical modification of lignin and following polymerization, we developed a new and precisely defined synthetic protocol which integrates lignin and poly(ethylene brassylate) (PEB). This new synthesis method yielded a new polymer, lignin-graft-PEB, which possesses PEB as a soft segment of the copolymer and lignin as a hard segment. Among various biodegradable polyesters, PEB is a relatively unexplored cheap polymer that is derived from sustainable resource, castor oil. The new polymer shows optimal melting temperature (78 oC) that enables an easy thermal process. Mechanical properties of the new lignin-graft-PEB can be changed by altering molecular weight (Mn) of PEB (6.5 kDa, 13.1 kDa and 16.9 kDa) and mass ratios of lignin and PEB (1:1, 1:3 and 1:5). Increased Mn of PEB yields enhanced mechanical properties because of enhanced entanglement from the lengthened polymer chains. High mass ratio of lignin yields enhanced mechanical properties due to the role of lignin as a hard domain in this copolymer. The highest modulus is 471.99 MPa, which is 3-folds higher than low-density polyethylene. However, blended polymer without covalent linkage shows poor mechanical properties (Young’s modulus 220.7 MPa) due to the microlevel phase separations within the polymer matrix.

Synthesis of mechanically strong and biodegradable biomass-based polyesters

ACS Fall 2021

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