Mechanical unfolding of directed polymers in a poor solvent: Critical exponents

We study the thermodynamics of an exactly solvable model of a self-interacting, partially directed self-avoiding walk 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 of second order and is characterized by new critical exponents that are determined by a careful numerical analysis. The usual polymer critical index $\nu$ on the critical line, and another one which we call $\zeta ,$ takes a nontrivial value that is numerically close to $2/3.$