Chemistry Reference
In-Depth Information
Fig. 2.3
The relationship
of the specific volume
of a polymer to the
temperature
This equation, however, is limited in scope. It is actually neither valid for very high molecular
weight polymers nor is it applicable to low molecular weight polymers.
The transition in a polymer from a molten state to a glassy one actually occurs over a temperature
range. This range also includes
T
g
. At the glass transition temperature, however, the change in
viscosity is rapid, from very viscous to a glassy one. Polymeric materials that undergo rearrangements
in response to outside stimulus, like light, are becoming increasingly important in various industrial
application (see Chap.
10
). Urban and coworkers [
19
] studied stimuli-responsive (
T
SR
) transitions and
correlated them to the glass transition temperatures (
T
g
). Based on their empirical data obtained from
a copolymer, they concluded that the relationship between
T
g
and
T
SR
is
log
ðV
1
=V
2
Þ¼ðP
1
ðT
SR
T
g
ÞÞ=ðP
2
ðT
SR
T
g
ÞÞ
V
1
and
V
2
are the copolymer's total volumes below and above the
T
SR
, respectively,
T
g
is
where the
P
1
and
P
2
are the fraction of the free volume
the glass transition temperature of the copolymer, and
(
P
2
), respectively. They feel
that this relationship can be utilized to predict the total volume changes as a function of
f
free
)at
T
g
(
P
1
) and (
T
g, midpoint
T
SR
)
50/50
for each random copolymer (
T
SR
T
g
for
different copolymer compositions.
2.2.2 Elasticity
The phenomenon of elasticity of rubber and other elastomers is a result of a tendency of large and very
flexible amorphous polymeric chains to form random, thermodynamically favorable, conformations
[
18
]. If a certain amount of crosslinking is also present, then these random conformations occur between
the crosslinks. In a vulcanized (crosslinked) rubber elastomer, the crosslinks may occur at every five
hundred to a thousand carbon atoms. The distance between the ends in such polymers is much shorter
than when these elastomers are fully stretched. Deformation or stretching of rubber straightens out the
various conformations in the molecules. They tend to return to the original state, however, when
the forces of deformation are removed. So each segment behaves in a manner that resembles a spring.
Some elastomeric materials are capable of high elongation and yet still capable of returning to the
original conformation. Some soft rubbers, for instance, can be extended as high as 800% and even
higher with full recovery. There is a preference for
conformation, a planar zigzag at high
extension. Rigidity of the chains, however, or crystallinity would hinder extension and, particularly,
the recovery. High viscosity and a glassy state would do the same.
The high degree of elasticity of rubbers is due in part to the effects of thermal motions upon the
long polymeric chains. These motions tend to restrict vibrational and rotational motions about the
single bonds in the main chain. Such restrictive forces in the lateral direction, however, are much
trans
