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Cellulose
is a macromolecule –– a polymer made up of a long chain of glucose molecules
linked by C-1 to C-4 oxygen bridges with elimination of water.
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After
scouring and bleaching, cotton is 99% cellulose.
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The anhydroglucose units are linked together
as beta-cellobiose; therefore, anhydro-beta-cellobiose is the repeating unit of
the polymer chain.
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The number of repeat units linked together to
form the cellulose polymer is referred to as the “degree of polymerization.”
Wood pulp, rayon and
cellophane (all three derived from wood cellulose) are also constructed of
cellulose polymers. Cotton cellulose differs from wood cellulose
primarily by having a higher degree of polymerization and crystallinity. Crystallinity indicates that
the fiber molecules are closely packed and parallel to one another). Higher
degree of polymerization and crystallinity are associated with higher fiber
strengths.
The
cellulose chains within cotton fibers tend to be held in place by hydrogen
bonding. These
hydrogen bonds occur between the hydroxyl groups of adjacent molecules and are
most prevalent between the parallel, closely packed molecules in the
crystalline areas of the fiber.
The
three hydroxyl groups, one primary and two secondary, in each repeating
cellobiose unit of cellulose are chemically reactive. These groups can undergo
substitution reactions in procedures designed to modify the cellulose fibers or
in the application of dyes and finishes for cross-linking. The hydroxyl groups
also serve as principal sorption sites for water molecules. Directly sorbed water is firmly chemisorbed on the
cellulosic hydroxyl groups by hydrogen bonding.