Lignin hydrodynamic properties

Experimental data on hydrodynamic properties of natural and biosynthetic lignins in dilute solutions are reviewed. The results of viscometric, translational diffusion, and sedimentation studies provide evidence for the macromolecnles of lignin having a complex topological structure. The benefits and limitations of the theoretical approaches used to describe the topology of lignin macromolecular chains are discussed. The relationship between hydrodynamic characteristic and fractal properties of lignin is considered. 

Table 2.1 The hydrodynamic properties of lignin (MWL) fractions from wood Pinus silvestris) [48] ...

Thus, analysis of hydrodynamic properties of native lignins reveals that their behaviour in dilute solutions is different from that of linear polymers, both flexible- and rigid-chain, in any of the known conformations. Apparently, the macromolecules of soluble lignins are randomly branched chains. Branchings in a chain are known to reduce the hydrodynamic dimensions, (i.e., reduce [q]), and increase the diffusion mobility compared to the linear analog, theoretical value of b, in a 0-solvent is 0.25. The branching of the polymer also reduces the hydrodynamic invariant by 15-20% compared to the standard value 3.2 x 10 erg/(K mol ) and results in anomalous values of the Huggins parameter. 

In [66, 67], hydrodynamic properties of two model lignins, synthesised under identical reaction conditions but differing in the mode of monomer supply, were compared. In agreement with theoretical expectations, the end-wise DHP was found to be a linear polymer. The authors claimed that bulk-polymerisation led to a randomly branched polymer. It is noteworthy that the validity of this assignment is confirmed by the results reported by Tsvetkov and co-workers [68], who studied polymers with similar characteristics. 

O.M. Sokolov, Macromolecular Reactions of Lignin at Alkali Cookings, Polymolecular Composition and Hydrodynamic Properties of Slightly Decomposed and Industrial Lignins, Institute of Wood Chemistry, Riga, Latvia, 1988. 

The understanding of the macromolecular properties of lignins requires information on number- and weight-average molecular weights (Mn, Mw) and their distributions (MWD). These physico-chemical parameters are very useful in the study of the hydrodynamic behavior of macromolecules in solution, as well as of their conformation and size (1). They also help in the determination of some important structural properties such as functionality, average number of multifunctional monomer units per molecule (2, 3), branching coefficients and crosslink density (4,5). 

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