Chemistry Reference
In-Depth Information
sections of macromolecules in the solid specimen. The alternative, elastic
response is characteristic of glasses, in which the components cannot flow past
each other. Such materials usually fracture in a brittle manner at small deforma-
tions, because the creation of new surfaces is the only means available for release
of the strain energy stored in the solid (window glass is an example). The glass
transition region is a temperature range in which the onset of motion on the scale
of molecular displacements can be detected in a polymer specimen. An experi-
ment will detect evidence of such motion (
Section 4.4.4
) when the rate of molecu-
lar movement is appropriate to the time scale of the experiment. Since the rate of
flow always increases with temperature, it is not surprising that techniques that
stress the specimen more quickly will register higher transition temperatures. For
a typical polymer, changing the time scale of loading by a factor of 10 shifts the
apparent
T
g
by about 7
C. In terms of more common experience, a plastic speci-
men that can be deformed in a ductile manner in a slow bend test may be glassy
and brittle if it is struck rapidly at the same temperature.
As the temperature is raised the thermal agitation becomes sufficient for seg-
mental movement and the brittle glass begins to behave in a leathery fashion. The
modulus decreases by a factor of about 10
3
over a temperature range of about
10
20
C in the glass-to-rubber transition region.
Let us imagine that measurement of the modulus involves application of a ten-
sile load to the specimen and measurement of the resulting deformation a few sec-
onds after the sample is stressed. In such an experiment a second plateau region
will be observed at temperatures greater than
T
g
. This is the rubbery plateau. In the
temperature interval of the rubbery plateau, the segmental displacements that give
rise to the glass transition are much faster than the time scale of the modulus mea-
surement, but the flow of whole macromolecules is still greatly restricted. Such
restrictions can arise from primary chemical bonds as in cross-linked elastomers
(
Section 4.5.1
) or by entanglements with other polymer chains in uncross-linked
polymers. Since the number of such entanglements will be greater the higher the
molecular weight of the polymer, it can be expected that the temperature range cor-
responding to the rubbery plateau in uncross-linked polymers will be extended to
higher values of
T
with increasing
M
. This is shown schematically in
Fig. 4.8
.A
cross-plot of the molecular weight
temperature relation is given in
Fig. 4.3a
.
The rubbery region is characterized by a short-term elastic response to the
application and removal of a stress. This is an entropy-driven elasticity phenome-
non of the type described in
Section 4.5
. Polymer molecules respond to the gross
deformation of the specimen by changing to more extended conformations. They
do not flow past each other to a significant extent, because their rate of translation
is restricted by mutual entanglements. A single entangled molecule has to drag
along its attached neighbors or slip out of its entanglement if it is to flow. The
amount of slippage will increase with the duration of the applied stress, and it is
observed that the temperature interval of the rubbery plateau is shortened as time
between the load application and strain measurement is lengthened. Also, molecu-
lar flexibility and mobility increase with temperature, and continued warming of
