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A detailed approach for the quantification of different lignin structures in milled wood lignin (MWL) has been suggested using a combination of NMR techniques. 1H-13C heteronuclear multiple quantum coherence and quantitative 13C NMR of nonacetylated and acetylated spruce MWL have been found to have a synergetic effect, resulting in significant progress in the characterization of lignin moieties by NMR. About 80% of side chain moieties, such as different beta-O-4, dibenzodioxocin, phenylcoumaran, pinoresinol, and others, have been identified on the structural level. The presence of appreciable amounts of alpha-O-alkyl and gamma-O-alkyl ethers has been suggested. Although the quantification of various condensed moieties was less precise than for side chain structures, reliable information can be obtained. Comparison of the calculated results with known databases on spruce MWL structure shows that the suggested approach is rather informative and comparable with the information obtained from the combination of various wet chemistry methods. Discrepancies between the results obtained in this study and those previously published are discussed.

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Twenty-one organosolv ethanol lignin samples were prepared from hybrid poplar (Populus nigra xP. maximowiczii) under varied conditions with an experimental matrix designed using response surface methodology (RSM). The lignin preparations were evaluated as potential antioxidants. Results indicated that the lignins with more phenolic hydroxyl groups, less aliphatic hydroxyl groups, low molecular weight, and narrow polydispersity showed high antioxidant activity. Processing conditions affected the functional groups and molecular weight of the extracted organosolv ethanol lignins, and consequently influenced the antioxidant activity of the lignins. In general, the lignins prepared at elevated temperature, longer reaction time, increased catalyst, and diluted ethanol showed high antioxidant activity. Regression models were developed to enable the quantitative prediction of lignin characteristics and antioxidant activity based on the processing conditions.

Hydrogen bonding plays an important role in the thermal and mechanical properties of biopolymers. To investigate hydrogen bond formation in lignin, an abundant natural polymer found in plants, Fourier transform infrared (FTIR) analysis of various lignin model compounds was performed. Four monomeric model compounds and one dimeric model compound were studied under various conditions. FTIR analysis revealed aliphatic hydroxyl groups form stronger hydrogen bonds than phenolic hydroxyl groups. Further, the dimeric biphenyl-type structure formed significantly stronger intermolecular hydrogen bonds as compared to the other monomeric model compounds. Results from the model compound studies were used to explain the observed complex hydrogen-bonding system present in both softwood and hardwood technical lignins. Together with chemical analysis, we discuss the difference in hydrogen bonding between hardwood and softwood lignin and the observed differences in the glass transition temperature.