Dynamic Mechanical Analysis of Plasticized and Esterified Native, Residual, and Technical Lignins: Compatibility and Glass Transition

The high glass transition temperature (T_g), stiffness, and poor flow properties of lignin are obstacles to lignin and lignocellulose utilization in thermoplastic applications. Two commonly applied methods to modify the viscoelastic properties of polymers are external plasticization, which involves physically blending them with low-molecular-weight additives, and internal plasticization, which involves covalently attaching side chains. However, most studies on lignin plasticization have focused on either technical, low-molecular-weight lignin or native, in situ lignin, with few efforts to bridge this gap. This study aims to determine if different lignin structures are susceptible to different modes of plasticization and how the plasticizer affects the phase morphology of the blends. Four lignins (softwood kraft lignin and lignin isolated from wheat straw, Norway spruce xylem, and residual softwood kraft pulp lignin) were plasticized with three external plasticizers (glycerol, triacetin, and diethyl phthalate) with different functionalities. The four lignins were in parallel internally plasticized by esterification with short-chain fatty acids (acetic, propionic, and butyric acid). The T_g and phase morphology of the modified lignins were studied by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Apart from phase separation in some lignin plasticizer blends, each plasticizer demonstrated similar efficiency (T_g depression) across all lignins, suggesting that the structure of the plasticizer, rather than the lignin structure, plays a more significant role in determining the outcome. Aprotic plasticizers were generally more efficient than protic per molar unit, and the magnitude of their mechanical dampening was also smaller over the glass transition, likely due to a decrease in the hydrogen bond density of the system. External plasticization was also found to narrow the width of the glass transition, indicating the formation of a morphologically more homogeneous material with less local T_gs than the pure lignin, whereas esterification broadened it somewhat.