Microsecond relaxation processes in shear and extensional flows of weakly elastic polymer solutions

In this paper, we introduce an experimental protocol to reliably determine extensional relaxation times from capillary thinning experiments of weakly elastic dilute polymer solutions. Relaxation times for polystyrene in diethyl phthalate solutions as low as 80 mu s are reported: the lowest relaxation times in uniaxial extensional flows that have been assessed so far. These data are compared to the linear viscoelastic relaxation times that are obtained from fitting the Zimm spectrum to high frequency oscillatory squeeze flow data measured with a piezo-axial vibrator (PAV). This comparison demonstrates that the extensional relaxation time reduced by the Zimm time, lambda (ext)/lambda (z), is not solely a function of the reduced concentration c/c*, as is commonly stated in the literature: an additional dependence on the molecular weight is observed.