“…In a rubber, the entanglements (i.e., cross-links) are fixed, and no significant molecular or even segmental extension can occur; hence, the behavior of these materials is almost purely entropic, as has been verified experimentally long ago. However, in a polymer melt, there is a significant reduction in the number of entanglements under flow conditions [ 11 , 12 , 13 , 22 , 53 ], and this leads to substantial molecular and segmental extension that not only changes the intramolecular internal energy but also allows for alignment of the molecules and segments that leads to substantial changes in the intermolecular internal energy. At high flow rates, where the entanglement network is effectively degraded, these internal energy changes not only become non-negligible but can also become of comparable magnitude to the entropic changes.…”