Stress‐strain isotherms for polymer networks at very high elongations

Some polymer networks show an anomalous increase in the modulus or reduced stress at very high elongations. This behavior has now been investigated definitively by determining stress‐strain isotherms for both crystallizable and noncrystallizable networks, prepared using several curing techniques (carried out so as to yield a wide range in degree of cross‐linking). The networks were studied unfilled at a number of temperatures, and at several degrees of swelling. The results clearly implicate strain‐induced crystallization as the origin of the upturn in the modulus, and thus demonstrate that the wide spread interpretation of this upturn in terms of limited chain extensibility is incorrect.