Discussion
In an ideal rubber, the primary factor responsible for elasticity up to small strains is
Answer: C
Elasticity comes from no change in enthalpy, but change in the entropy.
In an ideal rubber the chains are freely jointed and stretching does not alter bond lengths or intermolecular energies, so the internal energy (enthalpy) does not change. What changes is the number of available chain conformations: stretching aligns the chains and lowers their entropy, and the restoring force \(f = -T(\partial S/\partial L)_{T}\) is purely entropic. That is why a stretched rubber band warms up and why its stiffness rises with temperature.
Enthalpy-driven elasticity is the behaviour of metals and crystals, where bonds are stretched; the options with an enthalpy change, or with neither change, describe that case or no elasticity at all.
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