Using a simple phenomenological mean-field model and Monte Carlo simulation, we investigate a collapsed macromolecular chain in poor solvent conditions under the action of a force to unwind it. We show that the macromolecule changes from the collapsed state to the stretched state via a first-order transition as the stretching force is increased. The phase diagram is calculated which consists of a first-order line with a tricritical end point. These analytical results are confirmed by Monte Carlo simulation using the bond-fluctuation model. The first-order transition is observed clearly in the data and a strong hysteresis effect is associated with the transition. Possible connections to molecular gateway and memory devices in chemical engineering are also explored.
