b)Explain how elastomers are able to Experience strains of up to 400% when under load and state the importance of the Glass transition temperature.
Answer 5 b)
Polymers such as rubber – very high strain Capability – in excess of 100%.
Generally thermoplastics above their Tg – permits Bond rotation.
Molecules form zig-zag configurations – allows large Amount of stretching - Resistance increases as chains straighten.
Rubbers have very low ‘E’ values (1 to 20 N/mm2) at Strains up to 400%
At very high strains a shape change takes place – Provision must be made during use
Extension or compression results in twisting action At the molecular level
Above the glass transition temperature (Tg) the polymer atoms have enough energy to allow this twisting
At temperatures below Tg, polymers tend to be Stronger & stiffer, but become extremely brittle
Amorphous – generally employed below Tg value – Elevated temperatures weaken the Van der Waals forces & reduce rigidity - Examples – polyvinyl chloride, polystyrene, acrylics, acrylonitrile butadiene styrene (ABS)