Abstract
A new cone‐plate rheometer has been constructed in order to measure torque‐induced sample volume change ΔVs as well as the usually measured total thrust F and torque M. One aim is to use ΔVs data to assess the accuracy of time‐dependent values N1(t) of the first normal stress difference calculated from F data obtained in step shear and step shear rate experiments. The cone‐plate gap has an 8° angle and 25.4 mm radius. The total axial stiffness is 31.4 MN/m (when F is 100 N); F resolution is about 1 N. ΔVs resolution is about 0.001% at 130 °C; a silicone liquid is used as a dilatometer indicator. First applications made with a low‐density polyethylene sample (“IUPAC A”) at 130 °C subjected to a step shear rate of 1 s−1 for a duration of 14 s gave no detectable change in sample volume; it follows that, under these conditions, the maximum possible error (caused by sample volume changes) in N1 values is only 1.3% in our Rheodilatometer and 4.2% in the BASF Weissenberg Rheogoniometer which was modified by Meissner. It also follows that the effects of any temperature rise (e.g., due to work done in shearing the sample) are negligible in the present measurements. This gives reason to question the reliability of certain torque data recently obtained by Y.‐H. Lin with a Rheometrics System Four rheometer.
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