Page 91 - Mechanical Behavior of Materials

92 Chapter 3 A Survey of Engineering Materials

Figure 3.18 Effect of degree of cross-linking on the elastic modulus E of a synthetic similar
to natural rubber. (From [Ashby 06] p. 272; reprinted with permission of Elsevier, Oxford, UK;
c 2006 M. F. Ashby and D. R. H. Jones.)

orderly crystalline structure, so that the degree of crystallinity is limited to about 65%. In contrast,
the high-density variant HDPE has less branching, and the degree of crystallinity can reach 90%. As
a result of the structural differences, LDPE is quite flexible, whereas HDPE is stronger and stiffer.
An extreme variation in the properties of rubber is possible by varying the amount of
vulcanization, resulting in different degrees of cross-linking. The effect on the elastic modulus
(stiffness) of the synthetic rubber polyisoprene is shown in Fig. 3.18. Unvulcanized rubber is soft
and flows in a viscous manner. Cross-linking by sulfur at about 5% of the possible sites yields a
rubber that is useful in a variety of applications, such as automobile tires. A high degree of cross-
linking yields a hard and tough material called ebonite that is capable of only limited deformation.

3.5.6 Combining and Modifying Polymers

Polymers are seldom used in pure form, often being combined with one another or with other
substances in various ways. Alloying, also called blending, involves melting two or more polymers
together so that the resulting material contains a mixture of two or more chain types. This mixture
may be fairly uniform, or the components may separate themselves into a multiphase structure.
For example, PVC and PMMA are blended to make a tough plastic with good flame and chemical
resistance.