RESPONSE OF FLEXIBLE POLYMERS TO A SUDDEN ELONGATIONAL FLOW

Individual polymers at thermal equilibrium were exposed to an elongati onal flow producing a high strain rate; and their dynamics were record ed with video fluorescence microscopy. The flow was turned on suddenly so that the entire evolution of molecular conformation could be obser ved without initial perturbations. The rate of stretching of individua l molecules is highly variable and depends on the molecular conformati on that develops during stretching. This variability is due to a depen dence of the dynamics on the initial, random equilibrium conformation of the polymer coil. The increasing appearance at high strain rates of slowly unraveling hairpin folds is an example of nonergodic dynamics, which can occur when a statistical mechanical system is subjected to nonadiabatic, or ''sudden,'' external forces.