Chemistry Reference
In-Depth Information
the sample causes the scale of molecular motions to increase in the time scale of
the experiment. Whole molecules will begin to slip their entanglements and flow
during the several seconds required for this modulus experiment. The sample will
flow in a rubbery manner. When the stress is released, the specimen will not con-
tract completely back to its initial dimensions. With higher testing temperatures,
the flow rate and the amount of permanent deformation observed will continue to
increase.
If the macromolecules in a sample are cross-linked, rather than just entangled,
the intermolecular linkages do not slip and the rubbery plateau region persists
until the temperature is warm enough to cause chemical degradation of the
macromolecules. The effects of cross-linking are illustrated in
Fig. 4.9
. A lightly
cross-linked specimen would correspond to the vulcanized rubber in an automo-
bile tire. The modulus of the material in the rubbery region is shown as increasing
with temperature because the rubber is an entropy spring (cf. Fig. 1.3a and
Section 4.5.2
). The modulus also rises with increased density of cross-linking in
accordance with
Eq. (4-31)
. At high cross-link densities, the intermolecular link-
ages will be spaced so closely as to eliminate the mobility of segments of the size
(
B
50 main chain bonds)
involved in motions
that are unlocked in the
glass
rubber transition region. Then the material remains glassy at all usage tem-
peratures. Such behavior is typical of tight network structures such as in cured
phenolics (Fig. 8.1).
Tightly cross-linked
10
8
Lightly cross-linked
6
Not
cross-linked
4
Temperature (
°
C)
FIGURE 4.9
Effect of cross-linking on modulus
temperature relation for an amorphous polymer.
