Stretching polymers in poor and bad solvents: Pullout peaks and an unraveling transition

Using analytic theory, numerical calculation and Langevin dynamics simulation we demonstrate the existence of a first-order unraveling transition in the stretching of a polymer chain in a poor solvent. The chain suddenly unravels from a "tadpole" or "ball and chain" configuration, to one where the ball shrinks to zero size. In the force curve this appears as a discontinuous drop in the force. This transition occurs under the conditions of most relevance for atomic-force microscope experiments, where the extension is the independent variable. Our simulations show marked hysteresis as well as many local peaks associated with the pullout of small portions of the chain.