Streaming birefringence of polymer solutions

Abstract

AbstractStreaming or flow‐birefringence is the optical anisotropy induced by laminar flow in liquids. The optical anisotropy can lie described by an optical tensor which is coaxial with the stress tensor in an elastic material In a tension experiment this is obvious. In laminar flow this has been proved by investigating the distribution of the birefringence in three directions of space. The amount of anisotropy, usually described by the degree of birefringence Δn, is proportional to the difference in principal stresses of the mechanical stress tensor in a very large range of variables, therefore giving the possibility of using optical measurements for the determination of mechanical stresses. However, the relationship is valid either for concentrated solutions or for conditions where the solvent and the polymer have essentially the same index of refraction n—a condition usually termed matching solvent. The proportionality constant between the difference in principal stresses and Δn—the stress‐optical coefficient C—is, as also follows from theory, independent of concentration, shear stress temperature, molecular weight and its distribution, only determined by the structure of the monomer unit and its n. Another quantity in flow birefringence is the so‐called extinction angle which is measured between the direction of the principal optical axis and the direction of flow. A reasonable amount of experimental data has been obtained to show that this angle can be quantitatively correlated with the so‐called “recoverable shear” that is the shear component of the elastic strain tensor in laminar flow. Therefore, the experimental determination of the angle gives a measure of the elastic strain. A survey of the experimental results in the field shows the generalities of this approach in a variety of polymer and solvent systems.