Chemistry Reference
In-Depth Information
CHAPTER
4
Mechanical Properties of
Polymer Solids and Liquids
With a name like yours, you might be any shape, almost.
—Lewis Carroll, Through the Looking Glass
4.1
Introduction
Polymers are in general use because they provide good mechanical properties at
reasonable cost. The efficient application of macromolecules requires at least a
basic understanding of the mechanical behavior of such materials and the factors
that influence this behavior.
The mechanical properties of polymers are not single-valued functions of the
chemical nature of the macromolecules. They will vary also with molecular
weight, branching, cross-linking, crystallinity, plasticizers, fillers and other addi-
tives, orientation, and other consequences of processing history and sometimes
with the thermal history of the particular sample.
When all these variables are fixed for a particular specimen, it will still be
observed that the properties of the material will depend strongly on the tempera-
ture and time of testing compared, say, to metals. This dependence is a conse-
quence of the viscoelastic nature of polymers. Viscoelasticity implies that the
material has the characteristics both of a viscous liquid which cannot support a
stress without flowing and an elastic solid in which removal of the imposed stress
results in complete recovery of the imposed deformation.
Although the mechanical response of macromolecular solids is complex, it is
possible to gain an understanding of the broad principles that govern this behav-
ior. Polymeric articles can be designed rationally, and polymers can be synthe-
sized for particular applications. This chapter summarizes the salient factors that
influence some important properties of solid polymers.
4.2
Thermal Transitions
All liquids contract as their temperatures are decreased. Small, simple molecules
crystallize quickly when they are cooled to the appropriate temperatures. Larger
