Abstract
We study the elastic properties of a single A/B copolymer chain with a specific sequence. We predict a rich structure in the force-extension relations which can be addressed to the sequence. The variational method is introduced to probe local minima on the path of stretching and releasing. At a given force, we find multiple configurations which are separated by energy barriers. A collapsed globular configuration consists of several domains which unravel cooperatively. Upon stretching, the unfolding path shows a stepwise pattern corresponding to the unfolding of each domain. While releasing, several cores can be created simultaneously in the middle of the chain resulting in a different path of collapse.