Recently, there has been a growing interest in utilization of lignins as a precursor for carbon fibers. Proper understanding of association models of lignins during solution processing provides useful structural information that can be utilized to optimize the use of lignins in this value added application. Hence we follow the assembly of lignin molecules from a variety of sources in dimethyl sulfoxide, a good solvent for lignins, using small angle neutron scattering. In order to mimic industrial processing conditions, concentrations of lignins were kept above the overlap concentration. At small length scales, short lignin segments with ~4-10 monolignol units associate to form rigid rod-like/cylindrical building blocks, where the number of repeat units in a cylindrical segment decreases with increasing lignin concentration. These cylindrical building blocks associate to form spherical aggregates with low cross-linking densities and a random coil or network like structures for highly branched systems. The degree of branching of the base lignin molecule, which varies with source, plays a crucial role in determining their association behavior. Overall sizes of aggregates decrease with increasing concentration at low cross-linking densities as illustrates in the accompanying figure, whereas the opposite trend is observed for highly branched lignins.Figure.  SANS data of lignins with low cross-linking density. Solid lines are the best fits for cylindrical form factor and dash lines correspond to the best fits of unified model.

Self-assembly of lignin molecules in solution

245th National Meeting (2013)

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