Lignin, complexes with cellulose structure

We have seen that lignin is a highly branched amorphous pol3rmer containing an extremely complex array of structures and that when in situ in the wood, it is a graft polymer with cellulose and hemicellulose to boot. Characterization of lignins thus becomes a problem. 

The complex three-dimensional structure of these materials is determined by their carbon-based polymers (such as cellulose and lignin), and it is this backbone that gives the final carbon structure after thermal degradation. These materials, therefore, produce a very porous high-surface-area carbon solid. In addition, the carbon has to be activated so that it will interact with and physisorb (i.e., adsorb physically, without forming a chemical bond) a wide range of compounds. This activation process involves controlled oxidation of the surface to produce polar sites. 

Anhydrous ammonia also is known to cause temporary platicization of wood. The ammonia swells and plasticizes both the lignin and the cellulose, and the crystalline structure of the cellulose is converted to a different form in the process. To shape the wood, it is immersed in liquid ammonia or treated with gaseous ammonia under pressure until the cell walls have been penetrated and the wood becomes pliable and flexible. In this condition it is easily shaped and formed by hand or mechanically. The ammonia readily vaporizes and evaporates from the wood, so that the wood regains its normal stiffness but retains the new form into which it has been shaped. With this process the wood can be distorted into quite complex shapes without springing back to its original form. Treating plants have been developed on a pilot-plant scale, but the process has not been widely adopted. 

In nature, hemicelluloses are found in the cell walls of woody and annual plants, together with cellulose and lignin.33 Hemicellulose is made up of a group of complex low-molecular-weight polysaccharides that are bound to the surface of cellulose microfibrils, but their structure prevents them from forming microfibrils by themselves. 

Phenols are less involved than alcohols in fundamental metabolic processes. Three phenolic alcohols do, however, form the basic building blocks of lignins, complex polymeric substances that, together with cellulose, form the woody parts of trees and shrubs. They have very similar structures. 

Lignin is a complex macromolecule with a polyphenol structure not converted into fermentable sugars. This amorphous resin acts as a binder between the fibrils and is present in all layers of the plant cellular wall, but it is concentrated in primary and secondary layers, where it occurs in association with cellulose and hemicellulose [23,28]. 

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