Uni-axial Elongational Viscosity of Linear and Branched polymer melts

About 40 years ago interest in the measurement of elongational viscosity of polymer melts started to grow. Here we present measurements of transient (and steady) uni-axial elongational viscosity, using the FSR, of the following melts: Four narrow MMD polystyrene (PS) samples with weight-average molar mass Mw in the range of 50k to 390k. Three different bi-disperse samples, mixed from the narrow MMD PS. Two low-density polyethylene (LDPE) melts (Lupolen 1840D and 3020D). A steady-state viscosity was kept for 1-2.5 Hencky strain units in all measurements.The measurements on the bi-disperse PS melts have demonstrated that both the transient and steady elongational viscosity is quite sensitive to polydispersity. Bi-disperse PS resembles the behaviour of mono-disperse melts only at elongational rates larger then the inverse of maximal time constant of the smallest molecule. As observed in Boger fluids the elongational viscosity increases severely with increasing elongational rates, from an elongational rate larger than the inverse of maximal time constant of the largest molecule. It seems that the transient elongational viscosity is a monotone increasing function in linear melts. By contrast, in branched (LDPE) melts Raible et al. (1979) presented the first measurements concerning the possible existence of a maximum in the transient elongational viscosity. Though contemporary and more recent measurements on LDPE melts do not indicate the existence of a maximum, as steady viscosity were not attained at high elongation rates.In our measurements, at high elongation rates, a maximum in the transient elongational viscosity followed by a steady viscosity was observed for the two LDPE melts. The steady elongation viscosity was about 50% less than the maximum at high strain rates.We interpret the drop in the transient elongational viscosity as a result of a retraction of side chains from the backbone of the LDPE molecule, into the tube occupied by the backbone.