Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 17

are polymerised. Thermosetting plastics are stronger than thermoplastics and have
a higher product-service temperature. Thermoplastics may have many processing
advantages, however, over thermosets. A major advantage is that thermoplastics can
be ground and recycled into other useful products.
2.2.7 Morphological Structures
Morphology is the order or arrangements of a polymer structure. The possible 'order'
between a molecule or molecule segment and its neighbours can vary from a very
ordered highly crystalline polymer structure to an amorphous structure (a structure
in random order). For example, a purely amorphous polymer is formed only by
non-crystalline or amorphous chain structures, whereas a semi-crystalline polymer
is formed by a combination of all structures. The semi-crystalline arrangement that
certain polymer molecules form during cooling is in large part due to intra-molecular
forces. As the temperature of a polymer melt is lowered, the free volume between
the molecules is reduced, causing an increase in intra-molecular forces. As the free
volume is reduced further, intermolecular forces cause the molecules to arrange in a
manner that brings them to a lower state of energy.
An amorphous polymer is defined as having a purely random structure. However,
electron microscopic observations have shown evidence of amorphous polymers that are
composed of relatively stiff chains; and which show a certain degree of macromolecular
structure and order. Even polymers with soft and flexible macromolecules such as
polyisoprene (which was the first bundle-like structure to be identified) are relatively
weak and short-lived if the material experiences stresses. The shear thinning viscosity
effect of polymers is sometimes attributed to the breaking of such macromolecular
structures.
Before the existence of macromolecules was recognised, highly crystalline structures
had been suspected. Such structures were discovered if undercooling or stretching
cellulose and natural rubber. It was found later that a crystalline order also existed
in synthetic macromolecular materials such as PA, PE and polyvinyls. Because of
the polymolecularity of macromolecular materials, a 100% degree of crystallisation
cannot be achieved. Hence, these polymers are referred to as 'semi-crystalline'. It
is usual to assume that a semi-crystalline polymer is formed by small regions of
alignment or crystallites connected by random or amorphous polymer molecules.
The speed at which crystalline structures grow is dependent upon the type of polymer
and temperature conditions. This phenomenon is mainly because at the beginning
nucleation occurs at a finite rate.