The rheological properties of polypropylene melts were investigated in
oscillatory shear flow, capillary rheometry, and uniaxial elongation
at constant tensile stress as well as constant strain rate. At small s
tresses the steady-state elongational viscosity of linear conventional
polypropylene has the threefold value of the shear viscosity. With in
creasing stress both the shear and elongational viscosity decrease. Th
e transient elongational viscosity at constant strain rate is equal to
the threefold value of the linear viscoelastic stressing viscosity as
calculated from the relaxation time spectrum. In contrast, long chain
branched polypropylene shows a maximum in the steady-state elongation
al viscosity and pronounced strain hardening in experiments at constan
t strain rate above an elongation of epsilon = 1. These phenomena are
obtained by less than three branches per molecule. The description of
the strain hardening by means of the Lodge model underestimates the me
asured data at deformation rates less than epsilon = 0.2 s-1. An impro
vement is obtained by adding a rubber-like stress component to the ten
sile stress.