Chemistry Reference
In-Depth Information
molecular mechanism that is responsible for the substantial changes in mechanical
properties over a fairly narrow temperature range. The glass transition appears to
be a second-order transition as the heat capacity and thermal expansion coeffi-
cient of the polymer undergo finite changes. However, the glass transition temper-
ature depends on the rate of measurement (see
Section 4.4.4
). Therefore, it should
not be considered as a real second-order thermodynamic transition.
4.4.1
ModulusTemperature Relations
At sufficiently low temperatures a polymer will be a hard, brittle material with a
modulus greater than 10
9
Nm
2
2
(10
10
dyn/cm
2
). This is the glassy region. The
tensile modulus is a function of the polymer temperature and is a useful guide to
mechanical behavior.
Figure 4.8
shows a typical modulus
temperature curve for
an amorphous polymer.
In the glassy region the available thermal energy (RT energy units/mol) is
insufficient to allow rotation about single bonds in the polymer backbone, and
movements of large-scale (about 50 consecutive chain atoms) segments of macro-
molecules cannot take place. When a material is stressed, it can respond by
deforming in a nonrecoverable or in an elastic manner. In the former case there
must be rearrangements of the positions of whole molecules or segments of mole-
cules that result in the dissipation of the applied work as internal heat. The mech-
anism whereby the imposed work is absorbed irreversibly involves the flow of
Glassy
10
Transition
8
Rubbery
6
Rubbery liquid
Increasing polymer molecular weight
4
Temperature (
°
C)
FIGURE 4.8
Modulus
temperature relations for an amorphous polymer.
