Page 94 - Mechanical Behavior of Materials

Section 3.5 Polymers 93

Figure 3.19 Microstructure of rubber modified polystyrene, in which the dark-colored
particles and networks are rubber, and all light-colored areas inside and outside of particles
are polystyrene. The originally equiaxial particles were elongated somewhat when cut to
prepare the surface. (Photo courtesy of R. P. Kambour, General Electric Co., Schenectady, NY.)

Copolymerization is another means of combining two polymers, in which the ingredients and
other details of the chemical synthesis are chosen so that the individual chains are composed of
two types of repeating units. For example, styrene–butadiene rubber is a copolymer of three parts
butadiene and one part styrene, both of which occur in most individual chain molecules. ABS plastic
is a combination of three polymers, called a terpolymer. In particular, the acrylonitrile–styrene
copolymer chain has side branches of butadiene polymer.
Among the nonpolymer substances added to modify the properties of polymers are plasticizers.
These generally have the objective of increasing toughness and flexibility, while often decreasing
strength in the process. Plasticizers are usually high-boiling-point organic liquids, the molecules
of which distribute themselves through the polymer structure. The plasticizer molecules tend
to separate the polymer chains and allow easier relative motion between them—that is, easier
deformation. For example, plasticizers are added to PVC to make flexible vinyl, which is used
as imitation leather.
Polymers are often modified or filled by adding other materials in the form of particles of
fibers. For example, carbon black, which is similar to soot, is usually added to rubber, increasing its
stiffness and strength, in addition to the effects of vulcanization. Also, rubber particles are added to
polystyrene to reduce its brittleness, with the resulting material being called high-impact polystyrene
(HIPS). The microstructure of a HIPS material is shown in Fig. 3.19. If the added substance has the
specific purpose of increasing strength, it is called a reinforcement. For example, chopped glass
fibers are added as reinforcement to various thermoplastics to increase strength and stiffness.
Reinforcement may also take the form of long fibers or woven cloth made of high-strength
fibers, such as glass, carbon in the form of graphite, or Kevlar. These are often used in a matrix of