Coil-stretch transition of dilute flexible polymers under ultrahigh velocity gradients

Because the hydrodynamic interactions are reduced by stretching, a solute polymer coil should unwind abruptly when a certain critical value of the velocity gradient is reached. Depending on the details of the velocity field (``longitudinal'', or ``transverse'', or more complex gradients) this coil ↔ stretch transition may be continuous (``second order'') or discontinuous (``first order''). In the latter case hysteresis should often be observed; a qualitative discussion of the associated relaxation times is given. Simple shear flow is an exceptional case, with no sharp transition. Some expected effects of the C↔S transition on the mechanical behavior, on optical properties, and on chemical degradation in flow are briefly analyzed.