Poly(ε-caprolactone) chains grafted from lignin, hydroxymethylated lignin and silica/lignin hybrid macroinitiators: Synthesis and characterization of lignin- based thermoplastic copolymers

Lignin is the second most abundant biopolymer after cellulose. Due to its biodegradability and availability, lignin is a suitable replacement for the conventional polyol. Modification of lignin enhances its reactivity and dispersion in the polymer- based materials in order to prepare low cost lignin- based thermoplastics as an alternative for petroleum- based thermoplastics. In this study, unmodified lignin and lignin modified with silica particles and formaldehyde were used as a polyol macroinitiator for ring opening polymerization (ROP) of epsilon-caprolactone (CL) monomer. As a determining parameter of the polymer chain length, the molar ratio of CL to hydroxyl groups (OH) of macroinitiator was set at 5.5 or 22 for all copolymers. Decrease in the OH peak intensity and the enhancement of carbonyl peak intensity were observed in the FTIR spectra, indicating progress of the reaction between lignin or modified lignins and CL. Compared to the macroinitiators, changes in the morphology of the graft copolymers were identified by SEM. The chain structure, chain length of the grafted poly-caprolactone (PCL) onto the lignin or modified lignin and percentage of the free PCL were determined using NMR analyses. Thermal properties of the synthesized lignin- based thermoplastics were investigated by DSC and TGA analyses. By decreasing the CL/OH ratio from 22 to 5.5, melt temperatures (T-m) and degree of crystallinity (X-c) descended from 57 degrees C and 70% to 55 degrees C and 57%, respectively.