Lignin structure is well known to be highly heterogeneous and diverse yet recalcitrant. These features limit the practical application of this biopolymer. New approaches to lignin characterization may contribute to understanding its complex structure, ultimately enabling efficient utilization. In this study, we employed a suite of methods enabling both mass balance closure and structural features’ evaluation combined with accurate MW determination. This approach revealed unexpected structural features of alkali lignin fractions obtained by preparative size exclusion chromatography (SEC). A relatively novel method based on sequential carbon evolution at varied temperatures, thermal carbon analysis (TCA) yielded quantitative temperature profiles, including the final oxidation of remaining char quantitated upon conversion to methane. Thermal desorption/pyrolysis gas chromatography-mass spectrometry (TD-Py-GC-MS) corroborated the TCA results. Combining these methods revealed the lignin structure unfolding as a result of SEC fractionation. The second run of the preparative SEC fractions through a similar column (analytical SEC) showed strong interactions between high- and low-MW components, presumably occurring in native lignin. Thus, the disruption of these interactions leads to lignin unfolding. <sup>31</sup>P NMR spectroscopy showed that such an “unraveled” lignin exposed a large fraction of its “hidden” hydroxyl groups, as the fractionated lignin featured &gt;2 times more OH groups accessible to phosphitilation. The application of TCA showed that the fractionated lignin evolves more of low-MW products without charring, i.e., showing a less recalcitrant behavior. The sum of fractionated lignin fractions produced almost no char upon pyrolysis compared to native lignin, &lt;3%. TD-Py-GC-MS corroborated this information showing that fractionated lignin featured a higher abundance of low-MW products of thermal evolution, particularly guaiacol carbonyls. These observations indicate a pronounced alteration in lignin components’ intermolecular association as a result of size exclusion fractionation. The observed phenomenon may be used for more efficient lignin processing.

Lignin structure unfolding by fractionation involving size exclusion

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