The possible existence of a qualitative difference on extensional steady state viscosity between polymer melts
and polymer solutions is still an open question. Recent experiments [1-4] showed the extensional viscosity of both
polymer melts and solutions decayed as a function of strain rate with an exponent of -0.5. When the strain rate
became higher than the order of inverse Rouse time, the polymer solutions showed an upturn [1, 4]. However, in
the same regime for polymer melts, the experiments were contrary: some of the experiments showed an upturn
[4, 5], while others did not [2, 3].
In order to further investigate the extensional steady state viscosity of polymer melts, we carefully synthesized two
monodisperse polystyrenes with molar masses of 248 and 484 kg/mole. The start-up and steady uniaxial
elongational viscosity have been measured for the two melts using a filament stretching rheometer. We then
compared the measurements with the bi-disperse polystyrene melts made from the above two polymers. The
influence and sensitivity of impurities were studied by adding different percentages of 484k into 248k polystyrene
melt. Furthermore a polydisperse polystyrene with weight average molecular weight 230 kg/mole was also
measured for comparison. Possible reasons for the differences shown in the previously mentioned experiments
are discussed.