Investigation of Nanostructural Characteristics of Lignin-ABA Block Copolymer Aggregates Leading to Use of Micelles As Nanoreactors for Lignin Conversion

Nanostructural self-assembly of lignin derivatives incorporated in polyethylene oxide-block-polypropylene oxide-block-polyethylene oxide (PEO-b-PPO-b-PEO) micelles in aqueous alkaline solution has been investigated. Hypotheses for copolymer micelle incorporation of lignaceous species include the "hydrophobic-hydrophilic interaction model" in which lignin derivatives with apolar-polar characteristics, known to exist in the effluent of alkaline pretreatment of various lignocellulosic materials, have an affinity for the micelles, and tend to form relatively organized structures within them. Here we show how the concentration and chemical/structural features of lignin monomers and small molecule lignin models determine their solubilization (guest) behavior in nanoscopic micelles.

Using (¹H) NMR, we have found that increasing concentrations (0.01-0.2 mmol) of lignin-based model compounds guaiacol, eugenol, and phenol in Pluronic^® F68-D₂O solutions results in significant decreases of the PEO/PPO blocks’ proton chemical shifts, and at a critical concentration, dramatic shifts due to gross structural transitions in the micelles. Although lignin also affects copolymer chemical shifts significantly in these micellar systems, large structural transitions seen in the models are seemingly not available to lignin-micellelar structures for several reasons. We present evidence that polymer micelle-guest interactions depend on both chemical functional group characteristics of guests (i.e. polarity, H-bonding ability) and pi-orbital stacking in ordered aromatic/conjugated groups.

Our results demonstrate how the loci of incorporated guests in A-B-A block-co-polymer are affected by these features. The current study is a starting point for utilizing micelles as a nanoreactor platform for converting partially-depolymerized lignin biopolymer to value-added monomers such as the antioxidants guaiacol and eugenol.