Elastomers are polymers that have over 200% elastic elongation. They can be thermoplastic or thermoset. Elastomers have highly random, flexible chains that allow easy movement under tension, returning to their original shape when tension is removed. High strength comes from hindered movement or strong interactions between molecules. Natural rubber is a cis-polyisoprene elastomer. Vulcanization adds crosslinks for improved properties. Elastomers are used in products requiring cushioning, resilience, or flexibility like gloves, tires, and mattresses.

1. Elastomers

2. Elastomers
 Polymers that have more than 200%
elastic elongation
 High elastic elongation is associated with
low stiffness and low strength
 Can be either thermoplastic or thermoset

3. Elastomers
 Highly random (high entropy)
 Totally amorphous
 Few strong interactions between
molecules
 Flexible aliphatic polymer chains
 When a tensile force is applied ,
the molecules can move easily
relative to each other (simple
uncoiling movement)
 When the force is removed, the
molecules will assume their
original random shape and the
entire structure will return to its
original shape
 Possible only if the molecules
have not slid past one another
and have only uncoiled

4. Elastomers
 High strength and high stiffness in
materials is associated with
Hindrance to easy movement of molecules
relative to each other which is associated with
 Crystalline regions
 Strong interactions between the molecules or
atoms
 General stiffness of the molecules

5. Thermoplastic elastomer

8. Thermoplastic and thermoset
elastomers

9. Vulcanized Rubber
Natural rubber
(isoprene)

10. Natural rubber vs. Vulcanized
rubber

11.  Trans arrangement permits the molecular
chains to fit together more easily
Greater interactions between the molecules

12. Cis/trans isomerism in Isoprene
 Trans isomer is harder
Used in golf balls and shoe soles
 Cis is highly elastomeric

13. Applications
 Excellent fatigue resistance
 Excellent resilience (linear over a broad range of
forces
 Cushioning products
 Relatively low in cost
 Used in gloves, mattresses, tires (in
combinations), rain gear, straps and rubber
bands

14. Butadiene rubber (polybutadiene)
H
CH3
Cl
Polyisoprene Polybutadiene

15. Resilience or compressive recovery
 Compressed state (low entropy and highly
ordered state)
 When compressive force is removed the
molecules tend to push against the
surface in order to return to their random
positions
 The entropy also increases as a result

16. Thermoset elastomers
 Lightly crosslinked (soft crosslinking) so
that substantial hardening does not occur
 About 100 atoms between the crosslinks
 Crosslinking is sufficient to prevent melting
after they have been shaped

17. Thermoset elastomers
 Capable of moving with freedom
 Uncoiled and coil
 Long term creep is reduced especially at elevated
temperatures
 Complete melting is prohibited
 Max extent of elongation is limited
 Max elongation can be set within the elastic region
 The crosslink density can be increased or decreased as
per requirement