Читать книгу Carbon Nanofibers - Группа авторов - Страница 90

Cellulose-based fibers were developed in the 1960s for aerospace applications and at that time became the first carbon fibers produced commercially. Though they are not as strong as PAN-based fibers they have many advantages, such as abundant resources of the precursor, and are a naturally occurring polysaccharide. But since they consist of D-glucose monomer units joined by 1-4 glucosidic bonds, forming an ether linkage by the elimination of water (one molecule may include up to ten thousand units), these ether linkages together with hydrogen bonds between different units make cellulose relatively stiff and hard to dissolve. They are not soluble in the most common solvents, which leads to extremely difficult processing of cellulose. On the other hand, their derivatives, such as cellulose acetates, are much easier to handle with spinning processes [54]. Cellulose acetates are common esters of cellulose. They are synthesized by reacting cellulose with acetic anhydride or acetic acid in the presence of sulfuric acid. The acetylation makes cellulose more soluble in organic solvents, making it more suitable to produce films from cellulose triacetate. The degree of substitution affects the solubility of cellulose acetate and hence determines the options for further processing for different applications. For example, cellulose acetate with degree of substitution of 2–2.5 is soluble in acetone, dioxane or methyl acetate, while celluloses with higher degree of acetylation are soluble in dichloromethane.