(329g) Chemical Modification of Lignin: Increasing Molecular Weight and Polarity Control of Lignin Via Esterification

As a key component of lignocellulosic biomass, lignin is an abundant polymer, notable for its relatively high char yield, low cost, and its functionality making lignin an excellent candidate for downstream applications such as carbon fibers and polymeric blends. However, lignin is still underutilized due to some of its undesirable properties such as poor compatibility with polymers, low reactivity, and wide distribution of molecular weight. To improve lignin’s compatibility with commonly available polymers such as polyethylene and polystyrene, there is a need for polarity control. Furthermore, the processing and mechanical properties of lignin-based products such as carbon fibers and composites are dependent on the molecular weight (MW) and glass transition temperature (T_g) of lignin. Recent work in our group has shown that increases in MW and glass transition temperature of lignin are beneficial to the processing and mechanical properties (tensile strength and modulus) of lignin-based carbon fibers.

To improve the performance of lignin, this biopolymer has been subjected to a variety of techniques that involves physical modification such as fractionation, or chemical modification such as functionalization of its hydroxyl groups and depolymerization. Physical modification is commonly used for obtaining molecular weight control, however the desired goal set by the Department of Energy (DOE) for lignin-based carbon fibers (tensile strength and modulus of 1.7 and 172 GPa respectively) has yet to be obtained. There has been little to no work utilizing chemical modification to increase the molecular weight of lignin, but chemical modification has been shown to yield significant improvement in flowability with polymers, and to improve the durability and uniformity of the final products. Therefore, to be utilized for carbon fibers and polymeric blends, it would be of great advantage to chemically modify lignin to target increasing molecular weight, improving flowability with other polymers, and obtaining better polarity control.

This work will focus on interlinking lignin molecules by functionalization of the hydroxyl group present in lignin with di-functional or multi-functional molecules while simultaneously interlink lignin molecules. By modifying and reducing the hydroxyl group, and thus decreasing the polarity of lignin, its compatibility with non-polar polymers will also increase making it a better alternative for polymeric blends and composites. Using this technique, we will be able to improve the properties of lignin by increasing molecular weight and glass transition temperature while also reducing polarity.

This work is investigating the effects of interlinking, reaction conditions, structure of the interlinking agent, and the extent of interlinking on the performance of lignin and the subsequent products.