The role of instrument compliance in normal force measurements of polymer melts

Schweizer et al. [J Rheol 48(6):1345–1363, 2004] showed nonlinear step shear rate data for a polystyrene melt (M _w=200 kg/mol, M _w/M _n=1.06, T=175°C). For different rheometers, cone angles, and sample sizes, the delayed normal force rise observed therein relative to a compliance-free reference N ₁ (from a thermodynamically consistent reptation model) is shown to depend on rheometer compliance characterized by the instrument stiffness K _A. K _A can be obtained from mapping N ₁ on the measured N _1,meas. or directly from mechanical contact measurement with a mismatch of 20–30%. The ranking of the stiffnesses found is K _A(RMS 800)>K _A(MCR 300)>K _A(ARES LR2). Once K _A is known, N _1,meas.-data can be corrected by solving the ill-posed Volterra equation involved in it. The correction shown for experiments with the 0.15-rad cone angle gives very good results. The characteristic decay time of the normal force after cessation of flow scales linearly with the axial response time t _a calculated from K _A, cone angle, and sample radius. The torque decay time is practically independent of t _a.