Globular Polymers under Oscillatory Forces

We study the mechanical response of a single polymer globule in a poor solvent subjected to oscillatory stretching forces using scaling theory and coarse-grained molecular dynamics simulations. Under constant loading, increasing the applied force induces a first-order globule-stretching transition accompanied by the formation of tadpole conformations at a temperature-dependent characteristic force. Under periodic loading, the delayed response of the polymer gives rise to extension hysteresis, and the associated energy dissipation is quantified by the hysteresis area. Below the unfolding threshold, the dissipative response is controlled by the intrinsic relaxation of the compact globule, and the data collapse onto a universal master curve when the driving frequency is rescaled by the equilibrium relaxation time. At and above the unfolding threshold, a force-induced dynamic phase transition between compact and stretched states gives rise to an additional low-frequency dissipation peak associated with repeated unfolding and refolding of the globule. The crossover between the driven phase transition and the relaxation of the globular state is remarkably sharp near the force-induced globule-stretching transition, where small changes in the driving frequency can increase the dissipated energy by an order of magnitude. These results establish a physical framework for understanding the mechanics of polymer globules subjected to time-dependent forces.