Revisiting the Molar Mass and Conformation of Derivatized Fractionated Softwood Kraft Lignin

Abstract

The limited utilization of reliable tools and standards for determination of the softwood kraft lignin molar mass and the corresponding molecular conformation hampers elucidation of the structure-property relationships of lignin. At issue, conventional size exclusion chromatography (SEC) is unable to robustly measure the molar mass because of a lack of calibration standards with a similar structure to lignin. In the present work, the determination of the absolute molar mass of acetylated technical lignin was revisited utilizing SEC combined with multi-angle light scattering with a band pass filter to suppress the fluorescence. Fractionated lignin isolated using sequential techniques of solvent and membrane methods was used to enhance the clarity of lightscattering profiles by narrowing the molar mass distribution of lignin fractions. Further information on the molecular conformation of derivatized samples was studied utilizing a differential viscometer, and chemical structures were identified by NMR spectroscopy analysis. Through the help of fractionation, intrinsic viscosity values were determined for the different fractions as a function of molecular weight cut-off membranes. The derivatized acetone-soluble lignin was found to possess a lower molecular weight and an extremely compact structure relative to the derivatized acetone-insoluble fraction based on a significantly lower "α" value in the Mark−Houwink−Sakurada plot (0.15 acetone-soluble vs 0.33 acetone-insoluble). The differences in geometry were supported by the linkage analysis from NMR showing the acetone-soluble part containing fewer native linkages. In both of these examples, kraft lignin behaved like a solid sphere, limiting the ability to provide entanglements between molecular chains. From this standpoint, macroscopic properties of lignin are justified with this knowledge of a dense and extremely compact structure.

Determination of the Structures of Lignin Subunits and Nanoparticles in Solution by Small‐Angle Neutron Scattering: Towards Improving Lignin Valorization

“…The kraft lignin subunits are described as a solid sphere with an average radius of 10.5±0.2 Å and a PDI of 0.15. This is consistent with the prior viscosity study where the acetone‐soluble softwood kraft lignin was shown to behave as a solid sphere with an average hydrodynamic radius of 21.7 Å [52] . It is noted that kraft lignin molecules were described as short cylinders in DMSO.…”

“…An intrinsic viscosity study of fractionated soft wood kraft lignin suggested that both the acetone-soluble and acetone-insoluble fractions behaved like solid spheres instead of polymer coils. [52] The lignin subunits dispersed in the NaOD solution possess a radius of 7.8 � 0.4 Å and a length of 58.0 � 4.8 Å. The lignin subunits dissolved in the EG solution have a radius of 8.8 � 0.2 Å and a length of 69.3 � 3.0 Å.…”

“…Lignin subunits were modeled as rigid cylinders with a radius polydispersity fixed at 0.15. An intrinsic viscosity study of fractionated soft wood kraft lignin suggested that both the acetone‐soluble and acetone‐insoluble fractions behaved like solid spheres instead of polymer coils [52] . The lignin subunits dispersed in the NaOD solution possess a radius of 7.8±0.4 Å and a length of 58.0±4.8 Å.…”

“…The nature of the aggregates can be discussed further using the literature work. As mentioned above, the softwood kraft lignin was fractionated into two parts using acetone [52,55] . The acetone‐insoluble fraction consists of lignin molecules with a higher molecular weight and a higher aliphatic to phenolic hydroxyl groups ratio [52,55] .…”

“…As mentioned above, the softwood kraft lignin was fractionated into two parts using acetone [52,55] . The acetone‐insoluble fraction consists of lignin molecules with a higher molecular weight and a higher aliphatic to phenolic hydroxyl groups ratio [52,55] . In contrast, the acetone‐soluble cut contains condensed oligomeric structures [52,55] .…”