Elongational dynamics of narrow molar mass distribution linear and branched polystyrene melts

The startup of uni-axial elongational flow followed by stress relaxation as well as reversed bi-axial flow has been measured for narrow molar mass distribution (NMMD) linear (Mw=145 kg/mole) and branched multi-arm polystyrene melts, using the filament stretching rheometer. The branched polystyrene melt was a multiarm A(n)-C-C-A(n) pom-pom polystyrene with an estimated average number of arms of n=2.5. The molar mass of each arm is about 28 kg/mole with an overall molar mass of Mw=280 kg/mole. The principle of time-strain separability fails completely to describe the dynamic elongation data. Similarly the Doi-Edwards model with any 'stretch evolution' equation is not capable of capturing the reversed dynamic of NMMD melts. An integral molecular stress function constitutive formulation within the 'interchain pressure' concept, seem to agree with all experiments for linear melts.