(142ai) Constant Elongational Stress Measurements of LD-PE and PS Using a Filament Stretching Rheometer

Traditionally, extensional rheological material parameters are characterized using uniaxial elongational deformation of polymer melts under constant strain rate. Elongational experiments performed under constant stress, i.e. elongational creep experiments, are considerably less reported, since few elongational rheometers are capable of applying a constant stress deformation. The filament stretching rheometer (FSR), which historically controls the strain rate and measures the stress on a cylindrical polymer sample (filament) undergoing uniaxial deformation at the midplane, is adapted to run in constant stress mode using an active control scheme. The details of the active control scheme for both controlled stress and controlled strain rate experiments are outlined. Constant stress experiments for both LDPE and PS are discussed and comparisons are made to the linear viscoelastic envelope (LVE) and previously reported constant strain rate experiments using the FSR for the same materials. We evaluate the viability of extensional creep experiments to measure the steady-state extensional viscosity of polymer melts.