  The effect of concentration on coil-stretch hysteresis in extensional flows
of polymer solutions is examined with insights from Brownian dynamics
simulations of isolated chains and scaling theory for non-dilute solutions. In
the hysteresis regime, stretched molecules pervade larger volumes than
equilibrium coils. For such chains, intermolecular overlap and hydrodynamic
screening crossover set in at concentrations much smaller than the critical
overlap concentration $c^\ast$ for equilibrium coils. The width of the
hysteresis window is consequently strongly enhanced around $c^\ast$.
