The role of instrument compliance in normal force measurements of polymer melts |
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Authors: | Thomas Schweizer André Bardow |
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Institution: | (1) Institute of Polymers, ETH Zürich, CH-8093 Zürich, Switzerland |
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Abstract: | 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.Extended Version of a paper presented at the 2nd Annual European Rheology Conference in Grenoble, France, April 21–25, 2005. |
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