Relaxation of stress and birefringence in polymers of high molecular weight.

We report experimental evidence of a mechanical instability in high molecular weight polystyrene solutions under large shear strains, as a result of which shear strain is non-uniformly distributed in the material. If the strain were uniformly distributed, then the ratio of the first normal stress difference to the shear stress, after a step shear, would equal the imposed shear strain. We report here a deviation from this equality, which is called the Lodge- Meissner relationship; the deviation indicates the presence of a non-uniform strain distribution. Because mechanical measurements of the first normal stress difference are subject to large error from transducer compliance, this deviation from the Lodge-Meissner relationship is confirmed by compliance-free birefringence measurements.