Stepwise unwinding of polyelectrolytes under stretching

We consider the unwinding of a tethered, flexible, and weakly charged polyelectrolyte in a salt-free poor solvent under an externally imposed strain. Using scaling arguments, we predict that the restoring force of a stretched polyelectrolyte shows a sawtooth pattern as a function of the externally imposed end-to-end distance. This nonmonotonic behavior arises from a cascade of conformational transitions between necklace-like structures with different number of beads. The transitions result from the interplay between the unscreened Coulomb repulsion of charged monomers and the short-range attraction of the backbone.