Chemistry Reference
In-Depth Information
however, because the molecules can flow past each other to relieve the stress, and
the shape of the article will be deformed by this creep process. (Alternatives to
cross-linking are mentioned in Sections 1.5.4 and 11.2.6)
Polybutadiene with no substituent groups larger than hydrogen has greater resil-
ience than natural rubber, in which a methyl group is contained in each isoprene
repeating unit. Polychloroprenes (neoprenes) have superior oil resistance but lose
their elasticity more readily at low temperatures since the substituent is a bulky,
polar chlorine atom. (The structures of these monomers are given in Fig. 1.4.)
4.5.2
Rubber as an Entropy Spring
Disorder makes nothing at all, but unmakes everything.
—John Stuart Blackie
Bond rotations and segmental jumps occur in a piece of rubber at high speed at
room temperature. A segmental movement changes the overall conformation of
the molecule. There will be a very great number of equi-energetic conformations
available to a long chain molecule. Most of these will involve compact rather
than extended contours. There are billions of compact conformations but only one
fully extended one. Thus, when the ends of the molecule are far apart because of
uncoiling in response to an applied force, bond rotations after release of the force
will turn the molecule into a compact, more shortened state just by chance. About
1000 individual C
C bonds in a typical hydrocarbon elastomer must change con-
formation when a sample of fully extended material retracts to its shortest state at
room temperature
[5]
. There need not be any energy changes involved in this
change. It arises simply because of the very high probability of compact com-
pared to extended conformations.
An elastomer is essentially an
entropy spring
. This is in contrast to a steel wire,
which is an
energy spring
. When the steel spring is distorted, its constituent atoms are
displaced from their equilibrium lowest energy positions. Release of the applied force
causes a retraction because of the net gain in energy on recovering the original shape.
An energy spring warms on retraction. An ideal energy spring is a crystalline solid
with Young's modulus about 10
11
10
11
N/m
2
). It has a very
small ultimate elongation. The force required to hold the energy spring at constant
length is inversely proportional to temperature. In thermodynamic terms (
10
12
dyn/cm (10
10
@
U/
@
l)
T
is
large and positive, where
U
is the internal energy thermodynamic state function.
An ideal elastomer has Young's modulus about 10
6
10
7
dyn/cm
2
(10
5
10
6
N/m
2
) and reversible elasticity of hundreds of percent elongation. The force
required to hold this entropy spring at fixed length falls as the temperature is low-
ered. This implies that (
@
U
/
@
l
)
T
5
0.
4.5.2.1
Ideal Elastomer and Ideal Gas
An ideal gas and an ideal elastomer are both entropy springs.
The molecules of an ideal gas are independent agents. By definition, there is
no intermolecular attraction. The pressure of the gas on the walls of its container
