Page 20 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.6 CHAPTER 1

dom unordered structure termed amorphous. An example of an amorphous polymer is
polystyrene. If the structure of the polymer backbone is a regular, ordered structure, then
the polymer can tightly pack into an ordered crystalline structure, although the material
will generally be only semicrystalline. Examples are polyethylene and polypropylene. The
exact makeup and architecture of the polymer backbone will determine whether the poly-
mer is capable of crystallizing. This microstructure can be controlled by different syn-
thetic methods. As mentioned above, the Ziegler-Natta catalysts are capable of controlling
the microstructure to produce stereospecific polymers. The types of microstructure that
can be obtained for a vinyl polymer are shown in Fig. 1.6. The isotactic and syndiotactic
structures are capable of crystallizing because of their highly regular backbone, while the
atactic form would produce an amorphous material. The amount of crystallinity actually
present in the polymer depends on a number of factors, including the rate of cooling, crys-
tallization kinetics, and the crystallization temperature. Thus, the extent of crystallization
can vary greatly for a given polymer and can be controlled through processing conditions.

FIGURE 1.6 Isotactic, syndiotactic, and atactic polymer chains.

1.4 MECHANICAL PROPERTIES

The mechanical behavior of polymers is dependent on many factors, including polymer
type, molecular weight, and test procedure. Modulus values are obtained from a standard
tensile test with a given rate of crosshead separation. In the linear region, the slope of a
stress-strain curve will give the elastic or Young’s modulus, E. Typical values for Young’s
modulus are given in Table 1.2. Polymeric material behavior may be affected by other fac-

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.