DAY 28: ELASTOMERS AND
THERMOSETS
What’s required for elastomeric behavior
 Essentials of elastomeric behavior
 Examples of Elastomers
 Example of a Thermoset

THERMOSETS AND ELASTOMERS
Both have some primary bonds between the
chains.
 Because of the primary bonds, neither can be
reground and reused.
 Elastomers show significant ELASTIC
deformation.

THE ISOPRENE MER

Here is an important, naturally occuring mer. It
exhibits what the text calls a “geometrical
isomerism.”
The double bond on the
backbone is quite rigid
and does not allow
rotation from one form to
the other.
One of these mers will
result in a natural
“curlique” to the chain. Do
you see which?
THE CURL MAKES THE DIFFERENCE
Polyisoprene “cis” has naturally occuring very
coiled chains. It is the starting point for natural
rubber.
 Polyisoprene “trans” has straighter chains. It is
a material called gutta percha.
 Both are found in the sap of trees in Brazil and
S.E. Asia.
 Interesting uses of gutta percha.
1. One of the earliest insulators.
2. “The guttie” core golf ball revolutionized golf.
3. Also used in filling tooth cores in dental work.
4. It is not highly elastic.

MAKING RUBBER

So far we have really coiled chains that might
uncoil when loaded. They also might begin to
slide past each other, ie. deform plastically.
As the Macrogalleria notes, extending a piece of rubber decreases
its entropy.
MORE ON RUBBER
To delay the onset of plastic deformation, a
certain amount of crosslinking is created. One
way of doing this is by adding sulfur. (Goodyear
invented this process known as vulcanization.)
 What we have so far is:
1. Highly coilable chains.
2. Backbone bonds allow free rotation.
3. Crosslinking to prevent plastic deformation.
Chains are fixed together by primary bonds at
fixed locations.
4. Be above the glass temperature.

THEORY OF RUBBER ELASTICITY
Conventional elasticity. (I.e. metal spring)
elastic strain energy stored in the primary bonds
during deformation.
 Rubber elasticity. As we stretch, energy stored
thermally. This is associated with an entropy
decrease. (Heat transferred out of the rubber.
Feels hot)
 There is then a thermodynamic “force” which
drives us back towards high entropy (the original
state.) So there is an entropy increase as we
expand. (Heat transferred back into the rubber.
Feels cool)

ENTROPY SPRING
Just as the conventional spring stores and
releases energy, the rubber “entropy spring”
rejects and accepts entropy.
 Further consequences:
1. Nonlinear stress strain behavior
2. Extremely large strains possible
3. Hysteris loop in the curve on unloading

STRESS STRAIN CURVES
Notes
1. Cycle
dependence.
First loading
cycle results in
permanent
deformation.
2. 20th cycle curve
is more typical.
3. Note large
strains. The
question of how
to define strain
becomes
important.
STRESS STRAIN CALCULATIONS IN
RUBBER
Please note: these materials are elastic, they are
NOT linearly elastic.
 They fall into a very large materials class: the
hyperelastic materials. In such materials, the
stress is derived (conceptually) as follows:
 Theoretically and experimentally, we find a
strain energy density function, V. Then, derive
the following non-linear relationship for stress

V
 ij 
  ij
SOMETHING TO REMEMBER
For all practical purposes, rubber is brittle.
There is no ductility.
 This is because of the crosslinking.
 This is confusing, because when we see large
deformations in metals, we know there is
ductility / plasticity at work.
 Not true in elastomers. Fracture surfaces show
no macroscopic deformation.

OTHER ELASTOMERS
During WWII when access to the rubber trees
was less, polymer chemists came up with ways
to synthesize elastomers.
 Here is one, commonly used – polybutadiene.

Again we see the double bond
on the backbone of the
polymer and the cis
configuration. It will be coily.
Widely used in automotive: tire treads, belts, hoses, gaskets,
etc.
SBS RUBBER – A THERMOPLASTIC
ELASTOMER

We start with what is called a “block copolymer.”
By clever chemistry, we can get
Like HIPS, but this one is
a block, not a graft. In
this case what happens
is:
Can be
recycled!!!
The polystyrene
clumps
together.
These clumps
do the job of
crosslinks.
SOME USES OF SBS
ABS (ACRILONITRILE – CO –BUTADIENECO- STYRENE

Here is the mer for PAN, polyacrilonitrile.
Important source of fiber.
Copolymerize it
with styrene in
what’s called an
alternating
copolymer
There’s more!
ABS

We then copolymerize this with butadiene. Get
ABS, a very strong, tough plastic.
ABS PROPERTIES
Material Density
(g/cc)
UTS
(ksi)
Ductility
%EL
Izod
Impact
Ft-lb/in
ABS
1.07
5.8
50
2.45
Polycarbo 1.24
nate
8.2
14
2.5
It’s very light and tough.
THERMOSETTING POLYMERS -- EPOXY

We have to chemically make the polymer and
form it all at the same time.
Ingredient #2
Ingredient #1
EPOXY CHEMISTRY

We get the two chemicals to react. The chemistry
is complex. Curing, i.e. heating to promote the
final reaction may be needed.
The result is
a 3D network
solid. It will
not soften
and flow
when heated.
This is a
thermoset.
SOME PROPERTIES
Note that Epoxy is quite strong and stiff. This is,
of course, the unreinforced stuff.
1.
2.
3.
4.
5.
Polymer Density
g/cc
UTS ksi
%EL
E ksi
PVC
1.35
7.5
45
385
Epoxy
1.16
22.5
2
2500
Uses
Electrical moldings, like some plugs
Sinks
Adhesives
Coatings
Matrix material for composites.

http://www.ides.com/resinprice/resinpricingreport
.asp
POLYTETRAFLUOROETHANE (TEFLON)

This is an interesting one. Here’s the mer
It’s like PE but with the H’s replaced
by fluorines. (F). This makes for a
strange substance.
1. Secondary bonding is very strong between the highly
polarized F’s.
2. The bonding is such that no other substances are
attracted. This accounts for the no-stick.
TEFLON PROPERTIES
Very low coefficient of friction
 Dense
 No-stick
 Very good temperature resistance
 Kind of average strength good ductility

Material
Density
g/cc
UTS
Ksi
%EL
E
ksi
HDPE
0.953
4.5
200
240
Teflon
2.16
5.3
400
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