Mechanical unfolding of directed polymers in a poor solvent: novel critical exponents

We study the thermodynamics of an exactly solvable model of a self-interacting partially directed self-avoiding walk (DSAW) in two dimensions, when a force is applied on one end of the chain. The critical force for the unfolding is determined exactly, as a function of the temperature, below the $\Theta$-transition. The transition is second order and characterized by new critical exponents which are determined by a careful numerical analysis. The usual polymer critical index $\nu$ on the critical line, and another one, which we call $\zeta$, take a non-trivial value which is numerically close to 2/3.