  Extensional rheometry and Brownian Dynamics simulations of flexible polymer
solutions confirm predictions based on blob concepts that coil--stretch
hysteresis in extensional flows increases with concentration, reaching a
maximum at the critical overlap concentration $c^\ast$ before progressively
vanishing in the semidilute regime. These observations demonstrate that chain
stretching strengthens intermolecular hydrodynamic screening in dilute
solutions, but weakens it in semidilute solutions. Flow can thus strongly
modify the concentration dependence of viscoelastic properties of polymer
solutions.
