Summary

In this teaching and learning package you have been familiarised with the following concepts:

• Polymer chains in rubbers are coiled up in their equilibrium state.
• When a rubber is stretched, this occurs by uncoiling of individual chain segments. Its stiffness is thus much lower than other materials, for which stretching occurs by lengthening of the inter-atomic bonds.
• The retractive force exerted by a stretched piece of rubber arises from the tendency of individual chain segments to recoil back to their equilibrium shape, thus raising the entropy and reducing the free energy. It is thus possible to predict the stiffness of a rubber solely from a knowledge of its crosslink density (which dictates the chain segment length).

You should also have read and understood the entropy spring derivation and be familiar with the idea that the retractive force a rubber exhibits under tension is caused by the lowering of the rubber's conformational entropy.

You should have seen how rubber deforms under uniaxial tension, observed the effect of heating a strip of rubber under tension and be able to explain both.

You should also have observed how a rubber membrane deforms under biaxial tension and be able to explain the three regimes