Page 88 - Basic Principles of Textile Coloration

CELLULOSE 77

for the development of crystalline regions. In these, the cellulose molecules are
held together by intermolecular hydrogen bonds between hydroxyl groups. X-ray
diffraction studies of ramie fibres show that the repeat distance in crystalline
cellulose corresponds very closely to the length of a cellobiose molecule.
Cellobiose is the dimer of glucose (Figure 5.2). The molecules in cellulose are
oriented along the fibre axis passing through regions varying from fully crystalline
to amorphous. Whenever the molecules are in proximity, hydrogen bonds will
form between pairs of adjacent hydroxyl groups.

The arrangement of a-glucose units in starch is less regular. Unlike cellulose,
molecules of amylose, one component of starch, are not flat and crystallinity is
totally suppressed. In amylopectin, the other major a-glucose polymer in starch,
the polymer molecules are branched. The oxygen atom at C-1 at the end of one
chain links to a C-6 in the middle of a neighbouring chain.

Cellulose is soluble in a solution of cuprammonium hydroxide,
Cu(NH3)4(OH)2. The viscosity of this solution depends on the average molecular
weight of cotton cellulose and can establish if any depolymerisation has occurred
during processing. The DP (degree of polymerisation) of purified cotton is around
1000–3000. The dissolution of cotton in cuprammonium hydroxide solution is
quite slow and protection of the solution from light and oxidation during the
procedure is essential. The quantity usually determined is the fluidity or inverse of
the viscosity [5]. The higher the molecular weight or DP of the cellulose, the
higher the viscosity, and the lower the fluidity of the cuprammonium solution.
Any process such as hydrolysis or oxidation that reduces the length of the
molecular chains, causes a loss in fibre strength and an increase in the fluidity.

5.3.2 Chemical properties of cellulose

Cellulosic fibres absorb large amounts of water (25–30% regain at 100% relative
humidity and 25 °C). Water molecules, however, do not penetrate into the
crystalline regions of cotton. Even though cellulose has a large number of hydroxyl
groups per molecule, it is not soluble in water, unlike glucose. Even quite large
molecules, such as those of typical cotton dyes, easily penetrate into the accessible
interfibrillar and amorphous regions of the fibres. The high molecular weight and
crystallinity of cotton cellulose are thus responsible for its good mechanical
properties, and the fibrillar structure and amorphous regions for its hydrophilic
character and absorptivity. Starch, in which the arrangement of glucose units