Lignin is abundant and sustainable biomass polymer. However, their application is limited because of the multifunctionality, numerous reaction sites, and their complex chemical structures. Especially, excessive and poorly-defined reaction sites led to thermoset polymers due to uncontrolled crosslinking of lignin-based polymer. Recently, precise control method of lignin crosslinking gains high interest to synthesize value-added lignin based polymers which have improved processability and recyclability. Herein, we present a synthesis of succinic acid-modified lignin-graft-poly(ethylene glycol) (PEG) with different amounts of a capping agent, acetic acid. The lignin-based polymer synthesis has two main steps: 1) natural lignin modification with succinic acid and capping agent (acetic acid) and 2) graft copolymerization of PEG-OH onto the modified lignin. In the lignin modification step, the acetic acid deactivates excess numbers of reaction sites (hydroxyl groups) on natural lignin. We have prepared three different modified lignins that have carboxylic acid terminals of 17 %, 50 %, and 83 % out of entire reaction sites. Next, the carboxylic acid terminals of modified lignin were reacted with OH terminal of linear PEG via esterification. We have integrated three different molecular weights of PEG-OH having 1 kDa, 2 kDa, and 3kDa onto modified lignin to study their impact on thermal properties. Because thermal properties of polymer can be significantly governed by degree of crosslinking. We have determined melting temperatures (Tm) and glass transition temperatures (Tg) of new synthesized lignin-based polymers by DSC. The test results showed that melting temperatures and Tg are controlled by the molecular weights of PEG, the available percentage of reaction site, and weight ratios of modified lignin and PEG in graft copolymerization. The lignin-based polymer possesses 3K PEG with 1:1 weight ratio of modified lignin: PEG, 17 % of reaction sites on modified lignin showed 54 oC of Tm and -5.8 oC of the Tg. A Tm of 58.2 oC and a Tg of -3.5 oC were shown from the lignin-based polymer possess 3K PEG with 1:1 weight ratio of modified lignin: PEG, 83 % of reaction sites on modified lignin. In conclusion, precise control of available reaction sites on lignin by using a capping agent resulted in a convenient way to adjust the degree of crosslinking which led to tailor-made thermal properties of lignin-based polymers.

Precise property control method of lignin based polymer via crosslinking

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