One-Pot Synthesis of Lignin Thermosets Exhibiting Widely Tunable Mechanical Properties and Shape Memory Behavior

A series of kraft lignin based thermosets were successfully synthesized by a one-pot heat curing method composed of lignin, PEG400, and citric acid through esterification reactions with water as the only produced byproduct. The polyester thermosets were prepared by varying the ratio of lignin and PEG400 in combination with citric acid as the cross-linker. Lignin and PEG400 were chosen as the rigid and soft segments, respectively, to tailor the thermal mechanical properties of the thermosets. An increase of lignin content from 20 to 40 wt % facilitated an increase in the cross-linking density and aromatic content. This was reflected in the storage modulus at 25 °C, which increased from 5.7 to 2000 MPa, and the glass transition temperature, which increased from −0.3 to 102 °C. At the same time, the tensile strength changed from 1.2 to 34.3 MPa. The mechanical properties were, thus, tunable from flexible to rigid, demonstrating a significantly high storage modulus and tensile strength for a biobased thermoset. Furthermore, a superb thermally stimulated shape memory property was illustrated. This is promising for the use of commercial kraft lignin as a building block for versatile applications.