Page 53 - Basic Principles of Textile Coloration

42 FIBROUS POLYMERS

chemicals and it is here that dye molecules can penetrate. The amorphous regions
are those zones where the fibre is weaker but that provide flexibility and
accessibility to chemicals. The most crystalline fibres will be those with regularly
repeating structural units along the chains, aligned in proximity so that
intermolecular bonds stabilise the three-dimensional structure. Copolymers will
generally be more amorphous than homopolymers, and polymers with large
irregular branching groups will have difficulty crystallising. Fibrous polymers are
often insoluble in common solvents, or only dissolve very slowly after considerable
swelling. Their solubility depends upon the degree of crystallinity and the
molecular weight.

The extent of chain orientation along the filament axis influences the degree of
crystallinity. Orientation increases on extending the filaments under tension. The
process is called drawing. It causes the molecular chains to slide by one another as
the plastic filament extends and increases in length. This promotes chain
orientation and therefore crystallisation. Drawing is an important operation for
improving the mechanical properties of artificially made filaments.

All fibres exhibit their own characteristic morphology. They may have
crystalline and amorphous zones. They may be non-porous, or have porous
structures in which the fibres themselves are constituted of bundles of much
smaller fibre-like units called fibrils. They may have pronounced differences
between the fibre skin and core. Molecular organisation in fibres has many
different forms and fibres cannot be considered to be homogeneous materials. The
following chapters review the morphology and properties of different types of
individual fibres.

3.3 INTERMOLECULAR FORCES

In the preceding section, we saw that polymer molecules will form crystalline
structures if they arrange themselves so that the position of each type of atom has
an exact repeated pattern in three dimensions. The proximity of the molecules
allows intermolecular attractive forces to operate that stabilise the crystalline
arrangement. What is the nature of this kind of force? It is a kind of bonding. Like
covalent and ionic bonding, these forces originate from the mutual attraction of
negatively charged electrons in an atom in one molecule for positively charged
nuclei of atoms in a neighbouring molecule. These attractive forces, however, only
operate over short distances and are generally quite weak. They therefore lack the
relative stability of ionic and covalent bonds. These weak intermolecular bonding