Chemistry Reference
In-Depth Information
transition is reached. This transition is called
glass transition
or
second-order transition
[
15
]. The
temperature at that transition is called the
glass transition temperature
, designated by
T
g
. Beyond
stiffness, a change is manifested in specific volume, heat content, thermal conductivity, refractive
index, and dielectric loss.
Bueche illustrated glass transition as follows [
16
]. In measuring the force necessary to force a
needle into a polymer, like polystyrene, at various temperatures, there is a relationship between the
force required to insert the needle and the temperature [
16
]. As the temperature is being lowered,
maximum resistance to penetration is reached at
T
g
.
T
g
chains undergo cooperative localized motion. It is actually estimated that
As stated, above
T
g
segmental motion of anywhere between 20 and 50 chain atoms is possible. Below the
second-order transition temperature, however, there is insufficient energy available to enable
whole segments of the polymeric chains to move. The structure is now stiff and brittle and resists
deformation. When, however, sufficient amount of heat energy enters the material again and the
temperature rises above
above
T
g
larger molecule motion involving coordinated movement returns. This
requires more space, so the specific volume also increases and the polymer is in a rubbery or a plastic
state. Above
T
g
, because large elastic deformations are possible, the polymer is actually both tougher
and more pliable. Chemical structures of the polymers are the most important factors that affect the
glass transition temperatures. Molecular weights also influence
T
g
, as it increases with the molecular
weight. In addition,
T
g
also varies with the rate of cooling. Table
2.2
. shows the structures of and lists
relative
T
g
values of some common polymers.
One way to obtain
T
g
is by studying thermal expansion of polymers. It is generally observed that
the thermal coefficient of expansion is greater above the glass transition temperature than below it,
though the magnitude of the change differs from one polymer to another. By plotting volume vs.
temperature for a polymer, one can obtain
T
g
as shown in Fig.
2.3
, which illustrates obtaining
T
g
from
specific volume-cooling temperature curves [
17
].
Polymers with bulky, tightly held side groups or stiff bulky components in the backbone have
high
T
g
values. This is due to the fact that such side groups or bulky components interfere with
segmental motion. Such polymers require higher temperatures to acquire sufficient free volume
for segmental motion. This can be observed in Table
2.2
. which shows that the glass temperature
of polystyrene with stiff benzene ring side groups is much higher than that of polyethylene.
Also polymers with high attractive forces between chains will require more heat energy to go from
a glassy to a rubbery or a plastic state. On the other hand, polymeric chains with loose hanging side
chains that tend to loosen the polymer structure and increase the free volume for segmental movement
will have lower
T
g
. For instance, the glass transition temperature of poly(methyl methacrylate) is
higher than that of poly(
n
-butyl methacrylate) as can be seen from Table
2.2
.
The transition to the glassy state from an equilibrium liquid results in changes in enthalpy,
H
, and
volume,
V
. The specific heat is related to the enthalpy by definition:
C
p
¼ð@H=@TÞ
p
The glassy state is nonequilibrium in nature and exhibits a tendency to undergo structural
relaxation toward equilibrium. This tendency of the glassy state to relax structurally toward equilib-
rium is often referred to as
structural recovery
. It was observed, however, that the progress towards
structural recovery with time varies significantly between a down-quench and an up-quench. This is
referred to as
asymmetry of structural recovery
. The nonlinearity of the process is described by the
following equation [
16
]:
ðu u
0
Þ=
t ¼ðu u
0
Þ=t
d
d
