Page 46 - Basic Principles of Textile Coloration

POLYMER STRUCTURE 35

weight of the repeating unit. This is because the number and mass of the groups at
the ends of the long polymer molecules (–OSO3– in the case of 2) are negligible in
comparison with the total number and mass of the repeating units. The end groups
in a polymer come from the monomer itself, or from the initiators used in the
polymerisation process. Polyacrylonitrile often has sulphate (–OSO3–) and
sulphonate (–SO3–) end groups derived from the sodium persulphate (Na2S2O8)
and bisulphite (NaHSO3) that initiate the polymerisation of acrylonitrile. Ionic
end groups in a polymer have an important role in ion exchange dyeing processes
(Section 1.1.2).

The repeating units and end groups define the molecular structure of a polymer.
The next structural level involves the distribution of the values of the molecular
weight or DP. Unlike most chemicals, in which all the molecules are identical, the
molecules in a polymer have varying numbers of repeating units resulting in a
distribution of molecular weights. At an even more refined level, the arrangement
of the polymer molecules in three dimensions, and the possibility that their
alignment and interaction can lead to the formation of ordered crystalline regions,
becomes relevant.

Because polymers consist of molecules with high molecular weights, they are
involatile and often insoluble in common solvents. They have poorly defined
melting points, or decompose before melting. Not all polymers form fibres. Those
polymers that do, have a high molecular weight and long, unbranched, linear
molecules with regular structures. Alignment of the long polymer molecules along
the fibre axis results in intermolecular attractive forces and the formation of
crystalline regions. This promotes fibre strength. Although intermolecular bonding
is weak, these attractive forces, between aligned neighbouring molecules along a
significant portion of their length, provide resistance to deformation and chain
slippage.

Fibrous polymers, with a minimum of chain branching, are made from
bifunctional monomers. Vinyl compounds, such as acrylonitrile, are bifunctional
since two new bonds form to each monomer unit during polymerisation. The
polymerisation of vinyl compounds can be initiated by free radicals, cations or
anions. A free radical is a reactive chemical species possessing an unpaired
electron. Radical polymerisation is a chain reaction. The initially generated free
radical rapidly adds a monomer molecule but each addition regenerates the free
radical reactive site at the end of the growing polymer molecule. Termination of
the growing chain occurs when two radicals couple together or react by transfer of
a hydrogen atom. Cationic and anionic vinyl polymerisation are similar, except the