Fractional Precipitation of Wheat Straw Organosolv Lignin: Macroscopic Properties and Structural Insights

Wheat straw organosolv lignin has been thoroughly characterized with respect to bulk material properties, surface properties, and structural characteristics by means of antioxidant assays and determination of the equilibrium constant in water-octanol partitioning, i.e., logP determination, optimized gel permeation chromatography, quantitative 31P NMR spectroscopy, quantitative HSQC measurements, and XPS studies. The material was subsequently fractionally precipitated based on a binary solvent system comprised of n-hexane and acetone to yield four fractions that exhibit distinct molecular mass characteristics, while displaying similar structural characteristics, as revealed by the same set of analysis techniques applied to them. Extensive correlation studies underline the versatility of the obtained fractions as higher quality starting materials for lignin valorization approaches since, for example, glass transition temperatures correlate well with number-average molecular weights, applying the Flory-Fox relation as well as its Ogawa and Loshaek variations.