Lignin network structure

Conversion of Cyclic to Acyclic Structures. Upon oxidation, the aromatic rings of lignin may be converted direcdy to acycHc stmctures, eg, muconic acid derivatives, or indirectly by oxidative splitting of o-quinoid rings. Further oxidation creates carboxyUc acid fragments attached to the lignin network. 

Recent systematic studies on the relation between network structure and substituents in kraft lignin, steam exploded, have shown that the lignin containing networks can be modified in new ways, cf. e.g. (80). Also the toughening of glassy, structural thermosets can be achieved by incorporating a variety of polyether and rubber-type soft segment components in the polymer network structure. 

The network structure of lignin, which is made of phenol units, coagulates the cell wall in wood tissue, which is composed of cellulose and hemicellulose. Lignin is currently a waste product because of its complicated structure [1-4], It is produced by an oxidative polymerization of coniferyl alcohol, sinapil alcohol, and cumarol alcohol (Figure 1) catalyzed by metalloenzymes such as laccase and peroxidases. Laccase is a protein whose active center contains four coppers per one subunit [5-20],... 

The structural elements comprising lignin are linked by carbon-carbon and ether bonds. Units that are trifunctionally linked to adjacent units represent branching sites which give rise to the network structure characteristic of lignin (see Fig. 1.1, units 3, 8, and 15). As the interunit carbon-carbon linkages are difficult to rupture without excessively fragmenting the carbon skeleton of the... 

The lignin-degrading peroxidases operate by generating hydrogen peroxide, which dissociates into hydroxyl radicals that react with lignin by free radical substitution reactions to break down the network structure of the highly crosslinked, three-dimensional polymer. This reaction sequence is illustrated below for a representative section of the complex lignin macromolecule ... 

According to Erins [8, 9], the spatial structure of amorphous lignocarbohydrate matrix of wood is described by a superposition of three networks a network of hydrogen bonds (H -network), which involves both lignin and carbohydrates a network of lignocarbohydrate valence bonds (LCV network), and a lignin network. 

The concept of treating native lignin as a network structure has become popular, because this model makes interpretation easier of the complex processes that occur during the technological treatment of wood. The strongest argument in favour of the network model... 

The measurements of the branching parameter of various lignin fractions, led to conclusion that the chains of the lignin molecules in solution are densely packed or crosslinked [110, 111]. Rezanowich and co-workers [112] observed by electron microscopy that the lignosulphonates (LS) form spherical particles of a wide range of sizes. Recently, the results obtained using AFM (atomic force microscopy) had confirmed the fact that LS molecules in aqueous solutions form a crosslinked network structure, the shape of LS molecules changing with its concentration in aqueous solution [96]. 

Saraf, V., Glasser, W. G., Wilkes, G. L., and McGrath, G. E. Engineering plastics from lignin IV. Structure property relationships of PEG-containing polyurethane networks, Journal Applied of Polymer Science, 30, 2207 (1985). Sarkar, S. and Adhikari, B. Thermal stability of lignin-hydroxy-terminated polybutadiene-co-polyurethanes. Polymer Degradation and Stability, 73, 169-175 (2001). 

This multitude of properties the polymer must possess dictate that better polymer performance will be obtained from materials with complicated structures. Such polymers are complex polymers l) random copolymers, 2) block copolymers, 3) graft copolymers, 4) micellizing copolymers, and 5) network copolymers. There has been a dramatic increase in the past decade in the number and complexity of these copolymers and a sizable number of these new products have been made from natural products. The synthesis, analysis, and testing of lignin and starch, natural product copolymers, with particular emphasis on graft copolymers designed for enhanced oil recovery, will be presented. 

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