Extending the measurable limits of filament stretching extensional rheometry using time-temperature superposition

Time-temperature superposition (TTS) is utilized extensively in shear rheology to extend the range of measurable shear rates. The principle of TTS is based on the premise that changes in temperature are equivalent to changes in timescale or deformation rate, effectively allowing one to shift viscosity and modulus measurements at varied temperatures to a master curve at a reference temperature. Extensional flows are critical in polymer processing, but the imposed processing extension rates often far exceed the capable mechanical limits of a filament stretching extensional rheometer (FiSER). This study investigates the use of TTS for extending the measurable limits of polymer solutions using FiSER. A Peltier environmental chamber was retrofitted onto the FiSER to systematically vary the ambient and sample temperatures from 25 down to 5 °C. Shift factors of αT ∼ 4 were observed using solutions of both high molecular weight polyacrylamide and polyvinylpyrrolidone. At low strains, in the Newtonian plateau, the TTS shift factors extracted from the extensional viscosity showed good agreement with those obtained from the shear rheology. However, at large strains, the TTS shift factors extracted from the extensional rheology data were 20% larger than those obtained from the shear rheology data for both the polymer solutions tested. We hypothesize that the increase in the maximum extensional viscosity is the result of a reduction in the solvent quality, leading to a decrease in the radius of gyration and an increase in the finite extensibility of the polymer with decreasing temperature. A decrease in the hydrodynamic radius of the polymer with decreasing temperature was confirmed by dynamic light scattering. Our data show that TTS can be used as a technique to probe higher extension rates of filament stretching experiments. However, the TTS shift factors from shear rheology can only be used to shift data from filament stretching experiments if changes to solvent quality and finite extensibility are accounted for properly.