Verification of the Stress-Optical Rule

Early work on the use of optical methods on the dynamics of polymeric liquids focused on establishing the validity of the stress-optical rule. A comprehensive account of this research can be found in the books by Janeschitz-Kriegl [29] and Wales [84], The majority of studies have considered simple shear flow, and the rule has been found to hold up to 

Extensive work investigating the stress-optical rule has also been performed on polymer solutions [101]. Here the rule can be successfully applied if the solvent contributions to the birefringence are properly subtracted. Care must be taken, however, to avoid form birefringence effects if there is a large refractive index contrast between the polymer and the solvent. 

Investigations of the stress-optical rule in extensional flow are fewer in number due to the difficulty in establishing this flow. Using an extrusion device, van Aken and Jan-eschitz-Kriegl [102] produced extensional flows in melts by forcing the material through a 

As discussed in section 7.1.6.4, semidilute solutions of rodlike polymers can be expected to follow the stress-optical rule as long as the concentration is sufficiently below the onset of the isotropic to nematic transition. Certainly, once such a system becomes nematic and anisotropic, the stress-optical rule cannot be expected to apply. This problem was studied in detail using an instrument capable of combined stress and birefringence measurements by Mead and Larson [109] on solutions of poly(y benzyl L-glutamate) in m-cresol. A pretransitional increase in the stress-optical coefficient was observed as the concentration approached the transition to a nematic state, in agreement of calculations based on the Doi model of polymer liquid crystals [63]. In addition to a dependence on concentration, the stress-optical coefficient was also seen to be dependent on shear rate, and on time for transient shear flows. 

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