Visualizing Structure-Dependent Nanoscale Morphologies of Lignins in Miscible Polymer Blend Thin Films.

Abstract

The lignin structure is commonly discussed using local structural descriptors obtained by NMR and related chemical analyses, whereas its nanoscale appearance under aggregation-suppressed conditions remains difficult to evaluate. In this work, we developed a polymer matrix-assisted atomic force microscopy (AFM) approach to visualize lignin-rich domains in highly dilute miscible blend thin films. Cellulolytic enzyme lignins (CELs), acetylated CELs, and dehydrogenation polymer (DHP) model lignins were dispersed in a poly(N-vinylpyrrolidone)/glycerol matrix and spin-cast on mica for AFM observation. Differential scanning calorimetry supported apparent miscibility in the relevant low-lignin composition range, while dynamic light scattering showed that CELs were broadly aggregated in dimethyl sulfoxide. Matrix dilution and thin-film fixation enabled the observation of nanoscale particulate domains with low topographic heights. The apparent domain morphology depended on the botanical origin, acetylation, and DHP structure. These results demonstrate that matrix-assisted AFM provides a thin-film-based strategy for comparing aggregation-prone lignin samples in terms of their apparent nanoscale morphology and medium-dependent association behavior.