Chemistry Reference
In-Depth Information
Chapter 2
Physical Properties and Physical Chemistry of Polymers
2.1 Structure and Property Relationship in Organic Polymers
For a very large proportion of polymeric materials in commercial use, mechanical properties are of
paramount importance, because they are used as structural materials, fibers, or coatings and these
properties determine their usefulness. Properties that also determine their utilization are compressive,
tensile, and flexural strength, and impact resistance. Hardness, tear, and abrasion resistance are also of
concern. In addition, polymers may be shaped by extrusion in molten state into molds or by
deposition from solutions on various surfaces. This makes the flow behaviors in the molten state or
in solution, the melting temperatures, the amount of crystallization, as well as solubility parameters
important.
The physical properties of polymer molecules are influenced not only by their composition, but
also by their size and by the nature of their primary and secondary bond forces. They are also affected
by the amount of symmetry, by the uniformity in their molecular structures, and by the arrangements
of the macromolecules into amorphous or crystalline domains. This, in turn affects melting or
softening temperatures, solubilities, melt and solution viscosities, and other physical properties [
1
].
Due to the large sizes of polymeric molecules, the secondary bond forces assume much greater
roles in influencing physical properties than they do in small organic molecules. These secondary
bond forces are van der Waal forces and hydrogen bonding. The van der Waal forces can be
subdivided into three types: dipole-dipole interactions, induced dipoles, and time varying dipoles.
2.1.1 Effects of Dipole Interactions
Dipole interactions result from molecules carrying equal and opposite electrical charges. The
amounts of these interactions depend upon the abilities of the dipoles to align with one another.
Molecular orientations are subject to thermal agitation that tends to interfere with electrical fields. As
a result, dipole forces are strongly temperature dependent. An example of dipole interaction is an
illustration of two segments of the molecular chains of a linear polyester. Each carbonyl group in the
ester linkages sets up a weak field through polarization. The field, though weak, interacts with another
field of the same type on another chain. This results in the formation of forces of cohesion. Because
polymeric molecules are large, there are many such fields in polyesters. While each field is weak, the
net effect is strong cohesion between chains. The interactions are illustrated in Fig.
2.1
.
