Callister - An introduction - 8th edition

622 • Chapter 15 / Characteristics, Applications, and Processing of Polymers
10 4
Fracture of Polymers
Relaxation modulus (MPa)
–80.8°C
–76.7°C
10 2 –74.1°C
–70.6°C
–49.6°C
–65.4°C
–40.1°C
–58.8°C
1
10 –2
0°C
25°C
50°C
Miscellaneous Mechanical Characteristics
15.11 For thermoplastic polymers, cite five factors
that favor brittle fracture.
15.12 (a) Compare the fatigue limits for
polystyrene (Figure 15.11) and the cast iron
for which fatigue data are given in
Problem 8.20.
(b) Compare the fatigue strengths at 10 6 cycles
for poly(ethylene terephthalate) (PET,
Figure 15.11) and red brass (Figure 8.34).
Deformation of Semicrystalline Polymers
15.13 In your own words, describe the mechanisms
by which semicrystalline polymers
(a) elastically deform and (b) plastically
deform, and (c) by which elastomers elastically
deform.
10 –4
1 10 10 2 10 3 10 4 10 5 10 6
Time (s)
Figure 15.28 Logarithm of relaxation modulus versus
logarithm of time for polyisobutylene between 80 and
50ºC. (Adapted from E. Catsiff and A. V. Tobolsky,
“Stress-Relaxation of Polyisobutylene in the Transition
Region [1,2],” J. Colloid Sci., 10, 377 [1955]. Reprinted
by permission of Academic Press, Inc.)
(c) Crystalline at 230C
(d) Crosslinked at 50C
15.9 (a) Contrast the manner in which stress
relaxation and viscoelastic creep tests are
conducted.
(b) For each of these tests, cite the experimental
parameter of interest and how it is
determined.
15.10 Make two schematic plots of the logarithm
of relaxation modulus versus temperature
for an amorphous polymer (curve C in
Figure 15.8).
(a) On one of these plots demonstrate how
the behavior changes with increasing molecular
weight.
(b) On the other plot, indicate the change in
behavior with increasing crosslinking.
Factors That Influence the Mechanical Properties
of Semicrystalline Polymers
Deformation of Elastomers
15.14 Briefly explain how each of the following influences
the tensile modulus of a semicrystalline
polymer and why:
(a) Molecular weight
(b) Degree of crystallinity
(c) Deformation by drawing
(d) Annealing of an undeformed material
(e) Annealing of a drawn material
15.15 Briefly explain how each of the following influences
the tensile or yield strength of a
semicrystalline polymer and why:
(a) Molecular weight
(b) Degree of crystallinity
(c) Deformation by drawing
(d) Annealing of an undeformed material
15.16 Normal butane and isobutane have boiling
temperatures of 0.5 and 12.3C (31.1 and
9.9F), respectively. Briefly explain this
behavior on the basis of their molecular
structures, as presented in Section 14.2.
15.17 The tensile strength and number-average
molecular weight for two poly(methyl
methacrylate) materials are as follows: