At a stretch

Stretch a rubber band only a little and it obeys Hooke's law well, give it more of a tug and that simplistic theory breaks down and the material's behavior becomes altogether less linear. Explaining why the theory snaps when rubber bands or stretched too far has vexed scientists for decades. Now, Paul Goldbart of the University of Illinois at Urbana-Champaign has modified the classical theory of flexible cross-links between polymer chains in rubber and similar materials. In their model, rubber's entropy comes not only from the vibration of molecules between the crosslinks, but also from movement of the crosslinks themselves, which elegantly explains stretching behavior that previous generations of scientists had simply overlooked.