GTBL042-07 GTBL042-Callister-v2 August 6, 2007 12:43
7.15 Viscoelastic Deformation • 221Temperature (°C)Temperature (°F)Relaxation modulus,Er(10) (MPa)Relaxation modulus (psi)150 100 150 200 25010102103104105106
104 150 200 250 300 350 400 45010310210110 –110 –210 –3TgABCFigure 7.29
Logarithm of the
relaxation modulus
versus temperature for
crystalline isotactic
(curveA), lightly
crosslinked atactic
(curveB), and
amorphous (curveC)
polystyrene. (From
A. V. Tobolsky,
Properties and
Structures of Polymers.
Copyright©c1960 by
John Wiley & Sons,
New York. Reprinted
by permission of John
Wiley & Sons, Inc.)independently of one another. At these temperatures, any deformation is entirely
viscous and essentially no elastic behavior occurs.
Normally, the deformation behavior of a viscous polymer is specified in terms
of viscosity, a measure of a material’s resistance to flow by shear forces. Viscosity is
discussed for the inorganic glasses in Section 8.16.
The rate of stress application also influences the viscoelastic characteristics. In-
creasing the loading rate has the same influence as lowering temperature.
The logEr(10)-versus-temperature behavior for polystyrene materials having
several molecular configurations is plotted in Figure 7.29. The curve for the amor-
phous material (curveC) is the same as in Figure 7.28. For a lightly crosslinked
atactic polystyrene (curveB), the rubbery region forms a plateau that extends to
the temperature at which the polymer decomposes; this material will not experience
melting. For increased crosslinking, the magnitude of the plateauEr(10) value will
also increase. Rubber or elastomeric materials display this type of behavior and are
ordinarily used at temperatures within this plateau range.
Also shown in Figure 7.29 is the temperature dependence for an almost totally
crystalline isotactic polystyrene (curveA). The decrease inEr(10) atTgis much
less pronounced than in the other polystyrene materials since only a small volume
fraction of this material is amorphous and experiences the glass transition. Further-
more, the relaxation modulus is maintained at a relatively high value with increasing
temperature until its melting temperatureTmis approached. From Figure 7.29, the
melting temperature of this isotactic polystyrene is about 240◦C (460◦F).Viscoelastic Creep
Many polymeric materials are susceptible to time-dependent deformation when the
stress level is maintained constant; such deformation is termedviscoelastic creep.This
type of deformation may be significant even at room temperature and under modest
stresses that lie below the yield strength of the material. For example, automobile tires
may develop flat spots on their contact surfaces when the automobile is parked for
prolonged time periods. Creep tests on polymers are conducted in the same manner