We examine flow birefringence and flow modification effects (polymer‐induced changes in the flow field) for two‐dimensional extensional flows of polystyrene solutions at two concentrations in the semidilute regime. The concentrations of the polymer solutions are 1500 and 4500 ppm. This corresponds to ∼0.33 and 1.0×c*, the critical concentration for domain overlap of coiled polymer molecules at equilibrium. Results from a dilute 100 ppm solution of the same polymer sample are also presented for comparison. From the steady flow birefringence results, it appears that polymer chain extension is inhibited due to intermolecular interactions as the concentration is increased. In addition, inception of steady flow experiments show distinctive overshoots in birefringence. The main result of the velocity gradient measurements in this study is that the semidilute polymer solutions (1500 and 4500 ppm) show significant inhibition of high strain rates. The onset of flow modification corresponds to a critical effective volume concentration φ based upon spheres circumscribing the extended polymer chain of φc∼175 for both semidilute solutions. The degree of flow modification also shows correlation with φ using different concentration polymer solutions.

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