Measurement of the first and second normal stress differences in a polystyrene melt with a cone and partitioned plate tool

Abstract.

Step shear rate experiments in the range of

$0.1 < \dot \gamma < 30s^{ - 1}$ (0.14<Wi<42) were performed with a PS 158K melt at 190 °C. A cone (gap angle=0.148 rad) and partitioned plate tool was used to measure the time dependent first (N₁) and second (N₂) normal stress difference. N₁ and N₂ were extracted from a series of measurements with different ratios of R/R_Stem, R being the sample radius, R_Stem the radius of the central part of the plate connected to the transducer. A very good reproducibility was found for the measured torque. Edge fracture was observed for strains ≥18, independent of shear rate. For larger samples, the onset of edge fracture as seen by the transducer was delayed to larger strains. This is due to damping of the disturbances by the melt between the rim of the sample and the stem. The steady state value of the ratio –N₂/N₁ decreases from 0.24 at 0.1 s^–1 shear rate to 0.05 at 30 s^–1. For $\dot \gamma > 10s^{ - 1}$ the steady state value of –N₂/N₁ becomes larger if smaller cone angles are used. Data for α=0.148 rad showed a better coincidence with the linear viscoelastic predictions of viscosity η and first normal stress coefficient $\psi _1 = N_1 /\dot \gamma ^2$ compared to smaller cones.