Simultaneous mechanical and thermal measurements of a glassy polymer reveal significant internal (potential) energy storage during large ductile extension.

The internal energy buildup is both intrasegmental and intersegmental in origin.

Strain hardening involves considerable elastic deformation, leading to much higher energy than that associated with rubbery elasticity.

The high intrasegmental stress is plausibly due to emergence of chain tension in load-bearing strands of the chain network.