GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51
2nd Revised Pages272 • Chapter 8 / Deformation and Strengthening MechanismsLiquids have relatively low viscosities; for example, the viscosity of water at room
temperature is about 10−^3 Pa-s. On the other hand, glasses have extremely large
viscosities at ambient temperatures, which is accounted for by strong interatomic
bonding. As the temperature is raised, the magnitude of the bonding is diminished,
the sliding motion or flow of the atoms or ions is facilitated, and subsequently there
is an attendant decrease in viscosity. A discussion of the temperature dependence of
viscosity for glasses is deferred to Section 14.7.Mechanisms of Deformation and
for Strengthening of Polymers
An understanding of deformation mechanisms of polymers is important in order for
us to be able to manage the mechanical characteristics of these materials. In this re-
gard, deformation models for two different types of polymers—semicrystalline and
elastomeric—deserve our attention. The stiffness and strength of semicrystalline
materials are often important considerations; elastic and plastic deformation mech-
anisms are treated in the succeeding section, whereas methods used to stiffen and
strengthen these materials are discussed in Section 8.18. On the other hand, elas-
tomers are utilized on the basis of their unusual elastic properties; the deformation
mechanism of elastomers is also treated.8.17 DEFORMATION OF SEMICRYSTALLINE
POLYMERS
Many semicrystalline polymers in bulk form will have the spherulitic structure de-
scribed in Section 4.12. By way of review, let us repeat here that each spherulite
consists of numerous chain-folded ribbons, or lamellae, that radiate outward from
the center. Separating these lamellae are areas of amorphous material (Figure 4.13);
adjacent lamellae are connected by tie chains that pass through these amorphous
regions.Mechanism of Elastic Deformation
As with other material types, elastic deformation of polymers occurs at relatively low
stress levels on the stress-strain curve (Figure 7.22). The onset of elastic deforma-
tion for semicrystalline polymers results from chain molecules in amorphous regions
elongating in the direction of the applied tensile stress. This process is represented
schematically for two adjacent chain-folded lamellae and the interlamellar amor-
phous material as Stage 1 in Figure 8.27. Continued deformation in the second stage
occurs by changes in both amorphous and lamellar crystalline regions. Amorphous
chains continue to align and become elongated; in addition, there is bending and
stretching of the strong chain covalent bonds within the lamellar crystallites. This
leads to a slight, reversible increase in the lamellar crystallite thickness, as indicated
by tin Figure 8.27c.
Inasmuch as semicrystalline polymers are composed of both crystalline and
amorphous regions, they may, in a sense, be considered composite materials. As such,
the elastic modulus may be taken as some combination of the moduli of crystalline
and amorphous phases.