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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)