(715c) Breakdown of Time-Temperature Superposition During the Transient Response of Entangled Melts to Startup Extension

It is well known that entangled polymeric materials obey the time-temperature superposition principle in the rheological characterization of both linear viscoelastic and nonlinear behavior in steady state.   Here we report the first observations of the violation of the time-temperature superposition principle during startup uniaxial extension of entangled polymer melts.  Specifically, we will show that at relatively low temperature, but still 20 to 40 degrees above the glass transition temperature, well entangled polymer melts would rupture at a much smaller Weissenberg number, compared to their behavior at higher temperatures.   On the other hand, the same melts would show steady capillary rheological properties consistent with the time-temperature superposition principle.  In other words, the flow curve from capillary rheometry shifts according to the WLF relation found from small amplitude oscillatory shear.  The interpretation and implication of this new violation of the tTs principle will be discussed in this work.  The phenomenon is further confirmation that polymer rheology may not be treated at the level of the classical bead-spring picture, on which all popular theoretical descriptions are based.