Large Step Shear Strain Experiments with Parallel‐Disk Rotational Rheometers

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Abstract
A new method is proposed and tested for step shear strain experiments with parallel‐disk rotational rheometers. The method is applicable to large strains, i.e., outside the linear viscoelastic region. The nonhomogeneity of the strain in the parallel‐disk rheometer is accounted for by a correction term which is similar to the well‐known Rabinowitsch correction in capillary rheometry. The transient shear relaxation moduli of a low‐density polyethylene (150°C) and of a polystyrene (180°C) from this method agree very well with equivalent data from a cone‐and‐plate rheometer. The two different geometries give an overlapping set of data; small‐strain data (γ=0.1−5) from cone‐and‐plate and large‐strain data (γ=0.4−25) from parallel disk. The step strain data support the separability of the relaxation modulus into time‐ and strain‐dependent functions. The strain dependence is well approximated by a sigmoidal function. The data were obtained with a Rheometrics dynamic spectrometer having a maximum angular displace...

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